U.S. patent application number 16/662434 was filed with the patent office on 2020-10-29 for broad band tunable energy electron beam pulser.
This patent application is currently assigned to Euclid Techlabs, LLC. The applicant listed for this patent is Euclid Techlabs, LLC. Invention is credited to Chunguang Jing, Alexei Kanareykin, Roman Kostin, Ao Liu, Eric John Montgomery, Jiaqi Qiu, Wade Rush, Yubin Zhao.
Application Number | 20200343013 16/662434 |
Document ID | / |
Family ID | 1000005147448 |
Filed Date | 2020-10-29 |
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United States Patent
Application |
20200343013 |
Kind Code |
A1 |
Jing; Chunguang ; et
al. |
October 29, 2020 |
BROAD BAND TUNABLE ENERGY ELECTRON BEAM PULSER
Abstract
An electromagnetic mechanical pulser implements a transverse
wave metallic comb stripline TWMCS kicker having inwardly opposing
teeth that retards a phase velocity of an RF traveling wave to
match the kinetic velocity of a continuous electron beam, causing
the beam to oscillate before being chopped into pulses by an
aperture. The RF phase velocity is substantially independent of RF
frequency and amplitude, thereby enabling independent tuning of the
electron pulse widths and repetition rate. The TWMCS further
comprises an electron pulse picker (EPP) that applies a pulsed
transverse electric field across the TWMCS to deflect electrons out
of the beam, allowing only selected electrons and/or groups of
electrons to pass through. The EPP pulses can be synchronized with
the RF traveling wave and/or with a pumping trigger of a transverse
electron microscope (TEM), for example to obtain dynamic TEM images
in real time.
Inventors: |
Jing; Chunguang;
(Naperville, IL) ; Qiu; Jiaqi; (ShaoXing, CN)
; Liu; Ao; (Naperville, IL) ; Montgomery; Eric
John; (Oak Park, IL) ; Zhao; Yubin;
(Naperville, IL) ; Rush; Wade; (Lawrence, KS)
; Kostin; Roman; (Oak Park, IL) ; Kanareykin;
Alexei; (Bethesda, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Euclid Techlabs, LLC |
Solon |
OH |
US |
|
|
Assignee: |
Euclid Techlabs, LLC
Solon
OH
|
Family ID: |
1000005147448 |
Appl. No.: |
16/662434 |
Filed: |
October 24, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
16393469 |
Apr 24, 2019 |
10515733 |
|
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16662434 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/261 20130101;
G21K 1/043 20130101 |
International
Class: |
G21K 1/04 20060101
G21K001/04; H01J 37/26 20060101 H01J037/26 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0002] Portions of the present invention may have been made in
conjunction with Government funding under Department of Energy SBIR
Phase IIA Grant #DE-SC0013121, and there may be certain rights to
the Government.
Claims
1. An ElectroMagnetic Mechanical Pulser ("EMMP") comprising: an
input configured to accept a continuous input electron beam; a
Traveling Wave Metallic Comb Stripline kicker ("TWMCS" kicker)
located downstream of the input and having an internal passage
through which the electron beam passes, the TWMCS kicker being
configured to impose an oscillatory transverse deflection on the
electron beam according to at least one of a transverse
time-varying electric field and a transverse time-varying magnetic
field generated within the TWMCS kicker by a first RF traveling
wave propagated through the TWMCS kicker; a Chopping Collimating
Aperture ("CCA") located downstream of the TWMCS kicker and
configured to block the electron beam when its deflection exceeds a
threshold maximum or minimum, thereby chopping the electron beam
into a chopped stream of electron pulses having an electron pulse
repetition rate and duty cycle; an output configured to allow
electron pulses to emerge from the EMMP as an output stream of
electron pulses having a pulse repetition rate and a pulse duty
cycle; and a vacuum chamber surrounding all elements of the EMMP
and configured to provide a vacuum that is sufficient to allow the
electron beam to pass through the EMMP without significant
attenuation thereof by residual gasses, wherein: the TWMCS kicker
includes at least one pair of opposing combs; each of said opposing
combs of said pair of combs comprises a strip from which a
plurality of substantially identical, equally spaced-apart blocks
extend as teeth; the combs of the pair of combs are spaced apart
with teeth facing inward such that the internal passage through
which the electron beam passes is between the teeth of the pair of
combs; the pair of combs includes an RF energy input proximal to a
first end thereof and an RF energy output proximal to an opposite,
second end thereof; the teeth of the pair of combs are configured
to control a phase velocity of a traveling RF wave propagating from
the first end to the second end so that it is matched to an
electron velocity of the electron beam; and all exposed surfaces of
the pair of combs are electrically conductive; and wherein: the
TWMCS kicker further includes an Electron Pulse Picker (EPP)
configured to apply EPP pulses to the TWMCS, wherein each EPP pulse
creates a transverse electric field across at least one of the pair
of opposing combs of the TWMCS, said transverse electric field
being configured to deflect electrons that are within the TWMCS
during an EPP pulse so that the deflected electrons are removed
from the electron beam.
2. The EMMP of claim 1, wherein the EPP is configured to maintain
an electric bias on a first comb of a first pair of opposing combs
of the TWMCS, and wherein each of the EPP pulses applies an equal
electric bias to the other, second comb of the first pair of
opposing combs, so that during each of the EPP pulses both combs of
the first pair of opposing combs carry an equal electric bias,
thereby nullifying the electric field across the first pair of
opposing combs.
3. The EMPP of claim 2, wherein each of the combs of the first pair
of combs includes an RF energy input proximal to a first end
thereof and an RF energy output proximal to an opposite, second end
thereof; and wherein the RF energy inputs of the first and second
combs of the first pair of combs include series capacitors that
isolate the RF energy inputs from the DC bias and the EPP pulses,
respectively.
4. The EMPP of claim 3, wherein the RF energy output of the first
comb of the first pair of combs is directed through an intervening
series capacitor to a resistive terminating load, so that
application of the DC bias does not require application of a DC
current to the first comb of the first pair of combs, while the
second comb of the first pair of combs is terminated by a resistive
load without an intervening series capacitor, so that the second
comb is maintained at zero electric charge between EPP pulses.
5. The EMMP of claim 1, wherein the pulse repetition rate of the
electron pulses in the output stream is tunable from 0.1 GHz to 20
GHz.
6. The EMMP of claim 1, wherein a pulse length of the electron
pulses in the output stream is tunable from 100 fs to 10 ps.
7. The EMMP of claim 1, wherein the duty cycle of the electron
pulses in the output stream is tunable from 1% to 10%.
8. The EMMP of claim 1, wherein the pulse repetition rate and the
duty cycle of the electron pulses in the output stream are
independently tunable.
9. The EMPP of claim 1, wherein a pulse width of each of the EPP
pulses is adjustable over a range from 100 picosecond to 10
microseconds.
10. The EMPP of claim 1, wherein a pulse repetition rate of the EPP
pulses is adjustable over a range from 1 kHz to 10 MHz.
11. The EMMP of claim 1, further comprising a dispersion
suppressing section downstream of the CCA, the dispersion
suppressing section being configured to suppress a residual
dispersion of the stream of electron pulses arising from the
deflection imposed by the TWMCS kicker.
12. The EMMP of claim 11, wherein the dispersion suppressing
section includes a demodulating mirror TWMCS having an internal
passage through which the electron beam passes downstream of the
CCA the mirror TWMCS having a physical configuration that causes a
phase velocity of a second RF traveling wave propagated through the
mirror TWMCS to be matched to a velocity of the electron beam, the
mirror TWMCS being configured to demodulate the oscillatory
transverse deflection imposed on the electron beam by the TWMCS
kicker.
13. A method of generating electron pulses, the method comprising:
providing an EMMP according to claim 1; causing a continuous
electron beam to pass through the TWMCS kicker; while the electron
beam is passing through the TWMCS kicker: applying RF energy to the
RF energy input of the TWMCS kicker, said RF energy causing a
traveling RF wave to propagate through the TWMCS kicker, said
traveling RF wave having a phase velocity that is substantially
equal to an electron velocity of the electron beam, thereby
imposing a spatial oscillation on the continuous electron beam; and
applying EPP pulses to the EPP of the TWMCS, thereby allowing only
desired electrons or desired groups of electrons to remain in the
electron beam; causing the spatially oscillating electron beam to
impact the CCA, so that the CCA blocks the electron beam when the
electron beam deflection exceeds a threshold maximum or minimum,
thereby chopping the electron beam into a stream of electron pulses
having a desired electron pulse repetition rate; adjusting an
amplitude of the applied RF energy so as to adjust widths of the
electron pulses to be equal to a desired electron pulse width;
adjusting a frequency of the applied RF energy so that it is equal
to one half of a desired electron pulse repetition rate; and
adjusting a pulse width and pulse timing of each of the EPP pulses
so that only desired electrons and/or groups of electrons remain in
the electron pulses.
14. The method of claim 13, further comprising maintaining an
electric bias on a first comb of a first pair of opposing combs of
the TWMCS, and wherein applying the EPP pulses includes, for each
of the EPP pulses, applying an equal electric bias to the other,
second comb of the first pair of opposing combs, so that during
each of the EPP pulses both combs of the first pair of opposing
combs carry an equal electric bias, thereby nullifying the electric
field across the first pair of opposing combs.
15. The method of claim 13, wherein the desired electron pulse
repetition rate is between 100 MHz and 50 GHz, and the desired
electron pulse width is in a range 100 fs to 10 ps.
16. The method of claiml3, wherein the specified electron pulse
energy is between 100 keV and 500 keV.
17. The method of claim 13, wherein adjusting the pulse width of
each of the EPP pulses includes adjusting the pulse width of each
of the EPP pulses over a range from 100 picosecond to 10
microseconds.
18. The method of claim 13, wherein adjusting the pulse timing of
the EPP pulses includes adjusting a pulse repetition rate of the
EPP pulses over a range from 1 kHz to 10 MHz.
19. The method of claim 13, wherein applying the EPP pulses
includes synchronizing the EPP pulses with the RF energy that is
applied to the input of the TWMCS kicker.
20. The method of claim 13, wherein applying the EPP pulses
includes synchronizing the EPP pulses with a signal that triggers
pumping of a transverse electron microscopy (TEM) sample.
Description
RELATED APPLICATIONS
[0001] This application is a continuation in part of U.S. patent
application Ser. No. 16/393,469, which was filed on Apr. 24, 2019.
Both this application and also application Ser. No. 16/393,469 are
related to U.S. Pat. No. 9,697,982, issued on Jul. 4, 2017, and
U.S. patent application Ser. No. 15/368,051 filed Dec. 2, 2016. All
of these applications are herein incorporated by reference in their
entirety for all purposes.
FIELD OF THE INVENTION
[0003] The invention relates to apparatus and methods for
generating pulsed electron beams, and more particularly, to
apparatus and methods for generating and controlling low and medium
energy pulsed electron beams at very high rates.
BACKGROUND OF THE INVENTION
[0004] Generation and precise control of low and medium energy
pulsed electron beams is required for many industrial, medical, and
research applications, including scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and
horizontal/vertical accelerator-based beamlines (HAB/VAB), as well
as relevant experimental analytical methods that use electron beams
in SEM or TEM, or in HAB/VAB as probes.
[0005] In research, pulsed electron beams with ultrashort pulse
durations are used for investigating dynamic processes in a variety
of materials. Frequently, the electron beams are combined with
other primary excitation probes such as laser beams or other
photon-based probes such as X-ray beams. An example would be the
"pump-probe" class of experiments.
[0006] One approach for generating electron beam pulses of a
specific length and charge (i.e. intensity) in a periodic sequence
is to create electron pulses directly on the surface of an electron
source (cathode) by exciting the electrons using either a laser or
heat combined with an external electric field.
[0007] If a laser is used as the excitation method, the sequences
of electron pulses are controlled by adjusting the wavelength,
power, and/or temporal structure (pulse length and repetition
frequency) of the laser photon pulses. For example, if a
combination of femtosecond lasers and photocathode electron
emitters is used, the electron pulse lengths are strictly
determined by the pulse lengths of the fs-laser and the response
time of the photocathode. Using this approach, it is possible to
routinely obtain pulse lengths as short as 100 femtoseconds ("fs")
or less.
[0008] However, high repetition rates, defined herein as being
repetition rates of at least 1 GHz or higher, are simply not
available for laser-excited electron beams, because modern lasers
are only capable of repetition rates on the order of 100 MHz or
less. For example, the commercially available UTEM (Ultrafast TEM)
is a stroboscopic pump-probe method, where the pump and the probe
signals are both laser-actuated. Most laser-driven processes,
including many processes that are driven electrically,
magnetically, or both, can be cycled indefinitely at frequencies
above 1 GHz, thus enabling truly in operando microscopy, with the
most notable example being switching in a semiconductor device.
However, in UTEM, data are repeatedly collected over extended
periods of time, and thermal load from the pump laser must be
managed so that a process under study is not irreversibly damaged.
Therefore, even though lasers with higher repetition rates are
available, UTEM systems typically operate at much less than 0.1
GHz, and sometimes even at about 0.1 MHz, depending on the
experiment.
[0009] In addition, it is often important in experimental systems
to provide flexible and simple solutions for switching between
continuous and pulsed beam modes. If the combination of a
photocathode and an fs-laser is used for pulsed beam generation,
then the required continuous beam must be generated using a
separate thermionic or field emission source.
[0010] On the other hand, if heat combined with an external
electric field is used as the excitation method, then the sequences
of electron pulses are controlled by the electric field strengths
and the temporal structure (pulse length and repetition frequency)
of the electric field pulses.
[0011] Still another approach for generating pulsed electron beams
is to mechanically or electromagnetically block and unblock (i.e.
"chop") a continuous electron beam at a desired periodicity,
according to the desired electron pulse timing. Typically, a
transverse oscillation is imposed onto the beam, and then an
aperture is used to chop the oscillating beam into pulses.
Approaches that use deflecting cavity technology for chopping
electron beams of tens of kV in the GHz frequency range have been
known since the 1970's.
[0012] According to this approach, a continuous beam of electrons
is directed through a device, referred to herein as an
electro-magnetic mechanical pulser (EMMP), that operates to chop
the beam and collimate the output. The EMMP includes a "kicker"
that uses radio frequency energy to impose transverse oscillations
onto the beam according to at least one of a time-varying electric
field and a time-varying magnetic field generated within the
kicker, after which an aperture "chops" the laterally oscillated
beam into pulses. The RF is generated in the kicker in a
"transverse" mode, meaning that its electric and magnetic field
components oscillate transverse to the beam propagation direction.
More specifically, the electric and magnetic components of the RF
wave propagate in orthogonal planes that contain the long axis of
the EMMP along which the electrons propagate.
[0013] One possibility for implementing an EMMP kicker is to use a
"stripline." As is generally known, in a metallic traveling wave
stripline, which in its simplest form comprises two flat metallic
parallel slabs, if the medium between two slabs is vacuum or air,
then the phase velocity of the RF electromagnetic wave will travel
along the stripline at the speed of light. However, for many
applications the electrons travel much more slowly. For example, in
TEM applications the energy of the electrons is typically in the
range of 100 keV to 300 keV, whereby the electron beam speed is
around 2.1.times.108 m/s, which is only about 70% of the speed of
light.
[0014] In many EMMP applications, it is therefore necessary to
limit the interaction time between the RF wave and the electrons,
because otherwise the electrons will experience a phase slippage,
which will mean that the overall applied kicking force will be
greatly reduced or even canceled. For this reason, among others,
current approaches often employ just one single-cell deflecting
cavity, and are typically limited to pulse lengths of 1 picosecond
("ps") at best and repetition rates of 1 GHz or less, which cannot
be changed or tuned. Furthermore, these approaches are only
applicable for generating low energy electron beams having energies
of less than 100 kilo-electron Volts ("keV"). Perhaps even more
importantly, these approaches typically result in very extensive
electron beam quality deterioration in both the transverse
direction (beam diameter and divergence) and longitudinal direction
(temporal coherence).
[0015] An EMMP that implements a transverse deflecting cavity
(TDC-EMMP) is disclosed in U.S. Pat. No. 9,697,982, and in an
article published in Ultramicroscopy 161 (2016) 130-136, both of
which are incorporated by reference herein in their entirety for
all purposes. The TDC-EMMP disclosed in these references avoids the
problem of phase velocity mismatch by creating a standing
electromagnetic wave within the transverse deflecting cavity,
rather than a traveling wave, as the mechanism to impose a spatial
oscillation onto a continuous input electron beam. This TDC-EMMP
approach is able to generate electron beams that can be pulsed at a
high duty cycle with pulsing rates greater than 1 GHz and with
minimal transverse and longitudinal dispersion. The spatially
oscillating beam is then applied to an adjustable Chopping
Collimating Aperture ("CCA") so as to break the beam into a series
of pulses, after which a dispersion suppressing section comprising
a plurality of cavity resonators and/or magnetic quadrupoles is
used to suppress temporal and spatial dispersion of the pulsed
beam.
[0016] FIG. 1A is a conceptual diagram that illustrates the
fundamental concepts underlying the TDC-EMMP approach. In the
illustrated example, an initially continuous, "DC" electron beam
100 is transversely modulated into a sinusoid 110 as it passes
through a vacuum-filled TDC 102 which is operated at a resonant
frequency that lies within a range between 1 GHz and 10 GHz. The
amplitude of the sinusoid 110 grows as the modulated beam
propagates, and then the beam 110 impinges upon a chopping,
collimating aperture, or "CCA" 104, having an opening 106 that is
adjustable between 10 .mu.m and 200 .mu.m. The CCA "chops" the beam
into pulses 108 that emerge from the CCA at an ultrahigh repetition
rate that is twice the TDC modulation rate, because the pulses 108
are produced by cutting the sinusoid 110 of the beam modulation on
both the up-swing and the down-swing. The aperture opening 106 and
the modulating field of the TDC work together to tune the pulse
lengths to between 100 fs and 10 ps, resulting in duty cycles of
the TDC-EMMP device of less than or equal to 20%.
[0017] After the beam 100 has been chopped into pulses 108, if
nothing further were done, both the longitudinal and lateral
divergence of the stream of pulses 108 would increase. In other
words, the pulses would get longer (temporal divergence in the
propagation or "z" direction) and would spread out (spatial
dispersion in the x and y directions). So as to avoid this, as
shown in FIG. 1A, additional components 112, 114 are included in a
divergence suppressing section downstream of the CCA 110 that
reverses and suppresses this divergence. In the example of FIG. 1A,
the divergence suppressing section includes an additional,
demodulating, TDC 114, which is identical in design to the
modulating TDC 102, as well as a magnetic quadrupole 112.
Additional details as to the features and underlying principles of
the TDC-EMMP are presented in application Ser. No. 15/091,639, and
in Ultramicroscopy 161 (2016) 130-136.
[0018] With reference to FIG. 1B, in a similar implementation the
TDC-EMMP 120 includes a first electromagnetic kicker 102, a
collimating aperture 112, a first magnetic quadrupole 112, a second
"mirror" electromagnetic kicker 114 that functions as a
demodulating element, a second magnetic quadrupole 116, and a third
magnetic quadrupole 118. The incoming longitudinal DC electron beam
100 is directed along the optical axis (dashed line in FIG. 1B). At
the entrance of the TDC-EMMP 120, a transverse sinusoidal momentum
is imparted to the DC beam 100 by the electromagnetic field
generated by the first beam kicker 102. Since the electromagnetic
field oscillates in the transverse direction, its perpendicular
electric and magnetic components vary with time, such that the
modulation force that is applied to each incoming electron 100
depends on the time at which it arrives in the TDC kicker 102.
[0019] The amplitude of the sinusoid grows in the transverse
(horizontal in FIG. 1B) direction as the modulated beam propagates
along the optical axis 100 (downward in the figure). When the beam
reaches the collimating aperture 104, which is placed on the
optical axis 100 downstream of the first kicker 102, the slit in
the aperture 104 chops the beam 100 and converts it into a pulsed
sequence. However, after passing through the aperture the beam will
expand, and both the beam size and divergence will increase. As
shown in FIG. 1B, the addition of quadrupole magnets 112, 116, 118
and a second "mirror" beam kicker 114 can demodulate the beam 100
and reduce both its emittance growth and energy spread (i.e. both
the spatial and temporal coherence of the beam).
[0020] According to the TDC-EMMP approach, the pulse length, and
therefore the duty cycle of the pulses, can be adjusted by varying
the RF amplitude. However, the wavelength of the standing wave
within the TDC kicker is fixed by the dimensions of the TDC.
Accordingly, the pulsing rate is adjustable only by varying the
electron velocity, and cannot be adjusted independently.
[0021] Another approach to EMMP electron beam pulsing is to
implement a traveling RF wave in a stripline that is configured to
reduce the phase velocity of the RF as it propagates through the
kicker. U.S. patent application Ser. No. 15/368,051, included
herein by reference in its entirety for all purposes, discloses
such an approach, whereby a traveling RF wave is generated in the
kicker, but the RF wave is propagated through a dielectric, causing
the phase velocity of the RF wave to be slower than the speed of
light, and thereby allowing the electron velocity to be matched to
the RF phase velocity. More specifically, according to this
approach the "kicker" is a Traveling Wave Transmission Stripline
(TWTS) that is terminated by an impedance load. In exemplary
implementations of this approach, the TWTS kicker is a hollow
continuous tube that is dielectric-filled. The electron beam
propagates through the hollow center of the tube, while
transverse-mode RF waves simultaneously propagate through the tube.
As noted above, the dielectric serves to reduce the phase velocity
of RF to a sub-light velocity that can be matched to a velocity of
the electrons in the beam.
[0022] The RF phase velocity in the TWTS kicker is independent of
the RF frequency, such that the modulation rate of the beam and the
resulting pulse rate can be tuned over a very wide range by
adjusting the RF frequency to a desired value. Independently, the
amplitude of the electron beam modulation, and thereby the pulse
width, and consequently the pulse duty cycle, can be varied by
varying the amplitude of the applied RF. In embodiments, the
divergence suppressing section according to this approach includes
a "mirror" dielectric TWTS that functions to suppress residual
transverse oscillation and divergence of the pulsed beam.
[0023] The TWTS-EMMP offers advantages of independent, continuous
adjustment of the pulse width and duty cycle over an ultra-broad
operating bandwidth, and also includes the advantage of fabrication
simplicity. In embodiments, this allows the radiation dose rate to
be reduced below a damage threshold level of the measurement
sample, while maintaining a high pulse repetition rate so as to
rapidly accumulate data. Important applications include electron
tomography of cellular structures over a wide range of spatial and
temporal scales. The TWTS-EMMP can also be advantageous for
enabling a high frequency stroboscopic mode in Transmission
Electron Microscope (TEM) applications, whereby the dose rate of
the TEM can be varied. Low dose rate TEM can be crucial, for
example, when examining biological samples that are vulnerable to
radiation damage caused by energetic electrons.
[0024] It should be noted that the term "duty cycle" is defined
herein as being the ratio of the electron beam pulse width divided
by the time between successive electron beam pulses. It should
further be noted that the term "continuous variation" and
derivatives thereof are used herein to refer to parameters that can
be adjusted smoothly throughout their defined ranges, without
gaps.
[0025] While the TWTS-EMMP provides many advantages over previous
approaches, under some circumstances the dielectric that is
included in the TWTS-EMMP can be subject to "electron charging,"
whereby the dielectric acquires an electric charge due to impacts
by incoming electrons. While this can be alleviated by applying a
thin film conductive coating to the dielectric, the application of
such a conducting film can result in loss of electromagnetic energy
and reduced system efficiency.
[0026] What is needed, therefore, is an alternative to the
TWTS-EMMP that provides many of the advantages of the TWTS-EMMP,
including production of a pulsed electron beam that is
independently and continuously tunable over a wide range of pulse
repetition rates and pulse duty cycles, but is not subject to
electron charging.
SUMMARY OF THE INVENTION
[0027] The present invention is an electromagnetic mechanical
pulser (EMMP) that implements a novel "Traveling Wave Metallic Comb
Stripline" kicker, and is referred to herein as a TWMCS-EMMP. The
disclosed TWMCS-EMMP is an alternative to the TWTS-EMMP, and
provides most of the advantages of the TWTS-EMMP, including
production of a pulsed electron beam that is independently and
continuously tunable over a wide range of pulse repetition rates
and pulse duty cycles. In addition, the TWMCS-EMMP is not subject
to electron charging, because the outer surface of the TWMCS kicker
is entirely metallic. In embodiments, the rate of electron pulses
produced by the disclosed TWMCS-EMMP system can be continuously
adjusted between 100 MHz and 50 GHz, and the electron dosage energy
can be also continuously adjusted by tuning the duty cycle of the
pulses within a range that, in embodiments, extends at least from
1% to 10%.
[0028] Instead of slowing the phase velocity of a traveling RF wave
by implementing a dielectric, as in the case of the TWTS kicker,
the TWMCS kicker of the present invention employs a pair of
opposing metal (or metal-coated) "combs" having RF excitation
inputs at one end and, in embodiments, a terminating load, such as
a 50 Ohm load, at the opposite end. Similar embodiments include RF
inputs at both ends, for example in embodiments where RF at more
than one frequency and/or amplitude are applied to the TWMCS.
[0029] According to the present invention, the configuration of the
kicker, and in particular the widths, spacing, shapes, offsets, and
other structural features of the "teeth" of the combs, controls and
determines the phase velocity of the RF waves propagating through
the kicker in the transverse electromagnetic mode, whereby the
phase velocity is substantially independent of the driving RF
frequency over a large frequency range. Accordingly, in each
implementation of the invention the geometry of the TWMCS kicker is
optimized so that the phase velocity of the transverse
electromagnetic wave is synchronized with the electron velocity of
the incoming electron beam, which in embodiments can be a
continuous beam having an electron kinetic energy of between 100
and 300 keV, and in some embodiments between 100 and 500 keV.
[0030] Embodiments include aspects of the TWMCS-EMMP that are
mechanically adjustable so as to provide further tuning capability
to the pulser. In various embodiments, these features can include a
CCA having an adjustable aperture, and a TWMCS having combs that
are adjustable in their lateral and/or longitudinal offset. Some
embodiments include one or more magnetic or electrostatic beam
steering features, which can be located for example at the input to
the TWMCS kicker, at the output of the CCA, and/or at the output of
the TWMCS-EMMP.
[0031] Embodiments further include one or more additional apertures
that can be located, for example, at the electron beam input to the
TWMCS kicker, to provide, for example, additional collimation of
the beam. Elements of the CCA and/or of one of the additional
apertures (if included) can be electrically isolated from each
other, such that the aperture can be used as a beam-position
monitor and/or a beam current monitor.
[0032] Embodiments of the present invention further include a
"mirror" TWMCS in the dispersion suppressing section. Some
embodiments further include a "down-selecting" TWMCS downstream of
the CCA that can be excited by RF at a second RF frequency F2 that
is a sub-harmonic of the excitation frequency F1 of the TWMCS
kicker, i.e. F1/F2=an integer. As a result, the down-selecting
kicker deflects some of the electron pulses out of the beam so that
they are blocked by a down-selecting aperture located downstream of
the down-selecting TWMCS, thereby reducing the number of pulses
that remain in the beam, such that the pulse widths are narrow, as
determined by the TWMCS kicker and CCA, while the pulse repetition
rate is slow, as determined by the down-selecting TWMCS.
[0033] The present invention further includes an Electron Pulse
Picker (EPP) that is configured to apply a transverse electric
field across at least one pair of combs in at least one TWMCS of
the apparatus. When the transverse electric field is present, it
deflects electrons as they travel through the TWMCS, so that they
are eliminated from the electron beam. By switching the picker
field on and off in synchronization with the beam, it is thereby
possible to selectively eliminate electrons and/or groups of
electrons from the beam, while allowing others to pass through.
[0034] In embodiments, a fixed DC bias is applied to one comb of a
TWMCS comb pair. When the other comb of the pair is electrically
neutral, the applied bias creates a transverse electric "picker"
field across the TWMCS of sufficient amplitude to deflect electrons
out of the beam. In some of these embodiments, the EPP apparatus is
further configured to enable DC electric pulses (EPP pulses) of
equal amplitude to the fixed bias to be applied to the other comb
of the pair, so that during each applied DC pulse the picker field
is temporarily neutralized, and electrons in the beam are able to
pass through the TWMCS. Accordingly, the EPP pulses function as
electron gating pulses, in that electrons remain in the beam only
while an EPP pulse is applied. In some of these embodiments, the
EPP pulses can be applied to the TWMCS at any repetition rate from
1 kHz or less up to 1 MHz or more.
[0035] In various embodiments, EPP pulses can have narrow widths
that only allow one electron of the beam to pass, or they can have
arbitrarily long lengths that allow an electron burst containing a
desired number of electrons to pass through the TWMCS. For example,
in embodiments the EPP pulses are variable from 100 picoseconds to
10 microseconds in length. In various embodiments, the timing of
the EPP pulses can be synchronized with an RF signal that is
driving the TWTMS, and/or with a signal that triggers pumping of a
transverse electron microscopy (TEM) sample, for example so that
dynamic images of the sample can be obtained in real time.
[0036] In embodiments, a series of EPP pulses is applied to the
TWMCS, wherein all of the EPP pulses are of equal length and are
equally spaced apart. In other embodiments, only a single EPP pulse
is applied, while in still other embodiments a series of EPP pulses
is applied having any desired pattern of EPP pulse widths and of
spacing between the EPP pulses.
[0037] A first general aspect of the present invention is an
ElectroMagnetic Mechanical Pulser ("EMMP") that includes an input
configured to accept a continuous input electron beam, a Traveling
Wave Metallic Comb Stripline kicker ("TWMCS" kicker) located
downstream of the input and having an internal passage through
which the electron beam passes, the TWMCS kicker being configured
to impose an oscillatory transverse deflection on the electron beam
according to at least one of a transverse time-varying electric
field and a transverse time-varying magnetic field generated within
the TWMCS kicker by a first RF traveling wave propagated through
the TWMCS kicker, a Chopping Collimating Aperture ("CCA") located
downstream of the TWMCS kicker and configured to block the electron
beam when its deflection exceeds a threshold maximum or minimum,
thereby chopping the electron beam into a chopped stream of
electron pulses having an electron pulse repetition rate and duty
cycle, an output configured to allow electron pulses to emerge from
the EMMP as an output stream of electron pulses having a pulse
repetition rate and a pulse duty cycle, and a vacuum chamber
surrounding all elements of the EMMP and configured to provide a
vacuum that is sufficient to allow the electron beam to pass
through the EMMP without significant attenuation thereof by
residual gasses.
[0038] The TWMCS kicker includes at least one pair of opposing
combs, each of said opposing combs of said pair of combs comprises
a strip from which a plurality of substantially identical, equally
spaced-apart blocks extend as teeth, the combs of the pair of combs
are spaced apart with teeth facing inward such that the internal
passage through which the electron beam passes is between the teeth
of the pair of combs, the pair of combs includes an RF energy input
proximal to a first end thereof and an RF energy output proximal to
an opposite, second end thereof, the teeth of the pair of combs are
configured to control a phase velocity of a traveling RF wave
propagating from the first end to the second end so that it is
matched to an electron velocity of the electron beam, and all
exposed surfaces of the pair of combs are electrically
conductive.
[0039] The TWMCS kicker further includes an Electron Pulse Picker
(EPP) configured to apply EPP pulses to the TWMCS, wherein each EPP
pulse creates a transverse electric field across at least one of
the pair of opposing combs of the TWMCS, said transverse electric
field being configured to deflect electrons that are within the
TWMCS during an EPP pulse so that the deflected electrons are
removed from the electron beam.
[0040] In embodiments, the EPP is configured to maintain an
electric bias on a first comb of a first pair of opposing combs of
the TWMCS, and wherein each of the EPP pulses applies an equal
electric bias to the other, second comb of the first pair of
opposing combs, so that during each of the EPP pulses both combs of
the first pair of opposing combs carry an equal electric bias,
thereby nullifying the electric field across the first pair of
opposing combs. In some of these embodiments, each of the combs of
the first pair of combs includes an RF energy input proximal to a
first end thereof and an RF energy output proximal to an opposite,
second end thereof; and wherein the RF energy inputs of the first
and second combs of the first pair of combs include series
capacitors that isolate the RF energy inputs from the DC bias and
the EPP pulses, respectively. And in some of these embodiments the
RF energy output of the first comb of the first pair of combs is
directed through an intervening series capacitor to a resistive
terminating load, so that application of the DC bias does not
require application of a DC current to the first comb of the first
pair of combs, while the second comb of the first pair of combs is
terminated by a resistive load without an intervening series
capacitor, so that the second comb is maintained at zero electric
charge between EPP pulses.
[0041] In any of the above embodiments, the pulse repetition rate
of the electron pulses in the output stream can be tunable from 0.1
GHz to 20 GHz.
[0042] In any of the above embodiments, a pulse length of the
electron pulses in the output stream can be tunable from 100 fs to
10 ps.
[0043] In any of the above embodiments, the duty cycle of the
electron pulses in the output stream can be tunable from 1% to
10%.
[0044] In any of the above embodiments, the pulse repetition rate
and the duty cycle of the electron pulses in the output stream can
be independently tunable.
[0045] In any of the above embodiments, a pulse width of each of
the EPP pulses can be adjustable over a range from 100 picosecond
to 10 microseconds.
[0046] In any of the above embodiments, a pulse repetition rate of
the EPP pulses can be adjustable over a range from 1 kHz to 10
MHz.
[0047] Any of the above embodiments can further include a
dispersion suppressing section downstream of the CCA, the
dispersion suppressing section being configured to suppress a
residual dispersion of the stream of electron pulses arising from
the deflection imposed by the TWMCS kicker. In some of these
embodiments, the dispersion suppressing section includes a
demodulating mirror TWMCS having an internal passage through which
the electron beam passes downstream of the CCA the mirror TWMCS
having a physical configuration that causes a phase velocity of a
second RF traveling wave propagated through the mirror TWMCS to be
matched to a velocity of the electron beam, the mirror TWMCS being
configured to demodulate the oscillatory transverse deflection
imposed on the electron beam by the TWMCS kicker.
[0048] A second general aspect of the present invention is a method
of generating electron pulses. The method includes providing an
EMMP according to any embodiment of the first general aspect and
causing a continuous electron beam to pass through the TWMCS
kicker. The method further includes, while the electron beam is
passing through the TWMCS kicker, applying RF energy to the RF
energy input of the TWMCS kicker, said RF energy causing a
traveling RF wave to propagate through the TWMCS kicker, said
traveling RF wave having a phase velocity that is substantially
equal to an electron velocity of the electron beam, thereby
imposing a spatial oscillation on the continuous electron beam. The
method further includes applying EPP pulses to the EPP of the
TWMCS, thereby allowing only desired electrons or desired groups of
electrons to remain in the electron beam.
[0049] The method further includes, causing the spatially
oscillating electron beam to impact the CCA, so that the CCA blocks
the electron beam when the electron beam deflection exceeds a
threshold maximum or minimum, thereby chopping the electron beam
into a stream of electron pulses having a desired electron pulse
repetition rate. The method further includes adjusting an amplitude
of the applied RF energy so as to adjust widths of the electron
pulses to be equal to a desired electron pulse width, adjusting a
frequency of the applied RF energy so that it is equal to one half
of a desired electron pulse repetition rate, and adjusting a pulse
width and pulse timing of each of the EPP pulses so that only
desired electrons and/or groups of electrons remain in the electron
pulses.
[0050] Embodiments further include maintaining an electric bias on
a first comb of a first pair of opposing combs of the TWMCS, and
wherein applying the EPP pulses includes, for each of the EPP
pulses, applying an equal electric bias to the other, second comb
of the first pair of opposing combs, so that during each of the EPP
pulses both combs of the first pair of opposing combs carry an
equal electric bias, thereby nullifying the electric field across
the first pair of opposing combs.
[0051] In any of the above embodiments of the second general
aspect, the desired electron pulse repetition rate can be between
100 MHz and 50 GHz, and the desired electron pulse width is in a
range 100 fs to 10 ps.
[0052] In any of the above embodiments of the second general
aspect, the specified electron pulse energy can be between 100 keV
and 500 keV.
[0053] In any of the above embodiments of the second general
aspect, adjusting the pulse width of each of the EPP pulses can
include adjusting the pulse width of each of the EPP pulses over a
range from 100 picosecond to 10 microseconds.
[0054] In any of the above embodiments of the second general
aspect, adjusting the pulse timing of the EPP pulses can include
adjusting a pulse repetition rate of the EPP pulses over a range
from 1 kHz to 10 MHz.
[0055] In any of the above embodiments of the second general
aspect, applying the EPP pulses can include synchronizing the EPP
pulses with the RF energy that is applied to the input of the TWMCS
kicker.
[0056] And in any of the above embodiments of the second general
aspect, applying the EPP pulses can include synchronizing the EPP
pulses with a signal that triggers pumping of a transverse electron
microscopy (TEM) sample.
[0057] The features and advantages described herein are not
all-inclusive and, in particular, many additional features and
advantages will be apparent to one of ordinary skill in the art in
view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and not to limit the scope of the inventive subject
matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] FIG. 1A is a conceptual diagram that illustrates the
fundamental concepts underlying the TDC-EMMP approach of the prior
art;
[0059] FIG. 1B is a conceptual diagram that depicts an EMMP system
according to the Prior Art that includes a demodulating section
comprising a demodulating kicker and three magnetic
quadrupoles;
[0060] FIG. 2A is a simplified side view of a TWMCS kicker in an
embodiment where the TWMCS includes only one pair of "combs";
[0061] FIG. 2B is an end view of the TWMCS of FIG. 2A;
[0062] FIG. 2C is a simplified side view of a TWMCS kicker in an
embodiment where the TWMCS includes two pair of "combs";
[0063] FIG. 2D is an end view of the TWMCS of FIG. 2C;
[0064] FIG. 2E is a side view of an equivalent model of the TWMCS
of FIG. 2A that is the basis of Eqn. 1;
[0065] FIG. 2F is a graph that presents a family of dispersion
curves derived from Eqn. 1;
[0066] FIG. 3A is a conceptual diagram that illustrates the
deflection of an electron beam by a TWMCS and chopping of the beam
by a CCA;
[0067] FIG. 3B is a simple diagram illustrating the definition of
the duty cycle of the pulsed electron beam;
[0068] FIG. 4 is a block diagram of an embodiment that includes a
demodulation section comprising a mirror demodulating TWMCS and
three magnetic quadrupoles;
[0069] FIG. 5A is a side perspective view of a TWMCS in an
embodiment of the present invention;
[0070] FIG. 5B is a sectional view showing the internal structure
of the TWMCS of FIG. 5A;
[0071] FIG. 6 is a graph of the phase velocity of electromagnetic
waves propagating through a TWMCS in an embodiment of the present
invention;
[0072] FIG. 7 is a graph that presents simulated results of the
output current as a function of time for a TWMCS-EMMP in an
embodiment of the present invention;
[0073] FIG. 8 presents a Lissajous pattern traced on the aperture
plate in the embodiment of FIG. 2C where the drive frequencies of
the two pairs of combs are phase-locked at a 4:3 frequency
ratio;
[0074] FIG. 9A is a front view of a CCA that includes a plate
penetrated by a plurality of apertures of different sizes that is
slidable so as to select a desired aperture size;
[0075] FIG. 9B is a rear view of the CCA of FIG. 9A;
[0076] FIG. 10A is a front view of a CCA that includes a plurality
of overlapping, slidable elements that can be used to adjust the
aperture size of the CCA;
[0077] FIG. 10B is a rear view of the CCA of FIG. 10A;
[0078] FIG. 11A is a front view of a CCA that includes an iris that
can be mechanically actuated so as to adjust the aperture size;
[0079] FIG. 11B is a rear view of the CCA of FIG. 11A;
[0080] FIG. 12A is a side view of a TWMCS having combs that are
adjustable in their relative latitudinal offsets;
[0081] FIG. 12B is a side view of a TWMCS having combs that are
adjustable in their relative orientations;
[0082] FIG. 13 is a block diagram of a TWMCS-EMMP that includes
beam steering devices in various locations;
[0083] FIG. 14A is a simplified diagram of an embodiment of the
present invention that includes a "down-selecting" TWMCS and
aperture;
[0084] FIG. 14B presents two curves that illustrate the function of
the two kickers in the embodiment of FIG. 14A;
[0085] FIG. 15 is a block diagram of a TWMCS-EMMP that includes an
electron pulse picker (EPP);
[0086] FIG. 16A illustrates selection of a single electron from an
electron beam using a narrow EPP pulse; and
[0087] FIG. 16B illustrates selection of a contiguous pair of
electrons from an electron beam using an EPP pulse that is twice as
wide as the EPP pulse of FIG. 16A.
DETAILED DESCRIPTION
[0088] The present invention is an electromagnetic mechanical
pulser ("EMMP") that implements a novel "Traveling Wave Metallic
Comb Stripline" kicker ("TWMCS"), and is referred to herein as a
TWMCS-EMMP. The disclosed TWMCS-EMMP is an alternative to the
TWTS-EMMP, and provides most of the advantages of the TWTS-EMMP,
including production of a pulsed electron beam that is
independently and continuously tunable over a wide range of pulse
repetition rates and pulse duty cycles. In addition, the TWMCS is
not subject to electron charging because the outer surface of the
TWMCS is entirely metallic. In embodiments, the rate of electron
pulses produced by the disclosed TWMCS-EMMP system can be
continuously adjusted between 100 MHz and 50 GHz, and the electron
dosage energy can be independently, continuously adjusted by tuning
the amplitude of the RF, and thereby the duty cycle of the pulses,
within a range that, in embodiments, extends at least from 1% to
10%.
[0089] With reference to FIGS. 2A through 2D, the TWMCS 210
includes at least one pair of opposing combs 200 (FIGS. 2A and 2B),
and in embodiments two orthogonal pair of opposing combs 200 (FIGS.
2C and 2D). As can be seen in the figures, each comb 200 of the
opposing pair of combs comprises a strip 206 from which a plurality
of substantially identical, equally spaced-apart blocks extend
inward as teeth 202, such that the internal passage through which
the electron beam 204 passes is between the combs 200 of the pair
of combs 200. At least the exposed surfaces of the combs 200 are
metallic. In the embodiments of FIGS. 2A through 2D, the combs are
all parallel to each other, and the teeth of opposing combs are
aligned with each other. In other embodiments, the combs are not
parallel to each other, and/or the teeth are misaligned, so as to
excite desired electromagnetic wave modes and implement desired
beam modulations. In various embodiments, the teeth 202 and/or
strip 206 are not square in cross section, but are shaped and/or
angled in either or both of the longitudinal and transverse
direction according to the requirements of the application.
[0090] According to the present invention, as electromagnetic
traveling waves (not shown) propagate longitudinally through the
kicker 210 along and between the combs 200, the phase velocity is
slowed by the teeth 202. With reference to FIG. 2E, for any given
electron beam 204 input kinetic energy within a wide range, which
in embodiments can be between 100 and 300 keV, a TWMCS kicker 210
can be prepared having a configuration of the width t, spacing d,
repetition interval P, and height h of the teeth 202 such that the
phase velocity of the electromagnetic traveling waves that are
propagated through the TWMCS 210 will be synchronized with the
non-relativistic speed of the electrons in the beam 204.
[0091] Taking the simple example of FIG. 2E, and assuming that the
TWMCS kicker 210 is driven by a differential RF signal, i.e. that
the two combs 200 of the TWMCS 210 are driven by an RF signal
having the same amplitude but out of phase by 180 degrees, then the
RF propagation of the embodiment of FIG. 2A will be equivalent to
the physical model, i.e. equivalent structure that is illustrated
in FIG. 2E, wherein a hypothetical, infinitely thin conducting
plane 214 has been inserted in the center between the combs 200 of
the TWMCS 212.
[0092] According to the equivalent structure of FIG. 2E, the
dispersion relation is then given by
d P n = - .infin. .infin. { 1 .tau. n h tan ( .tau. n b ) ( sinc
.beta. n d 2 ) 2 } = 1 kh tan ( kh ) ( 1 ) ##EQU00001##
where k is the wavenumber of the RF, .tau..sub.n and .beta..sub.n
are the order of propagation constants in the x (lateral) and z
(longitudinal) direction, respectively, and the other variables are
as defined in FIG. 2E. Equation 1 can be used to numerically
calculate a family of dispersion relationship curves, such as is
illustrated in FIG. 2F, whereby appropriate values of d, t, b, and
h can be selected so as to match the phase velocity of the RF with
the kinetic velocity of the electrons. Embodiments implement the
lowest mode, .beta.=0, as illustrated by the bottom curve of the
figure, in which neither the electric field nor the magnetic field
has any component in the wave propagation direction z. This mode is
ideal because it ensures that kinetic energy of the electrons will
not be affected as they traverse through the TWMCS kicker 210. A
more complete field analysis of the embodiment of FIG. 2E can be
found in Keqian Zhang and Dejie Li, Electromagnetic Theory for
Microwaves and Optoelectronics, 2.sup.nd Edition, ISBN
978-3-540-74295-1 Springer Berlin Heidelberg New York, Chapter 7,
which is incorporated herein by reference in its entirety for all
purposes.
[0093] The action of the TWMCS kicker 210 on the electrons is
therefore similar to a TWTS kicker, but without the use of
dielectric materials, and therefore without electron charging of
the kicker.
[0094] FIG. 3A is a simplified model of a TWMCS-EMMP in an
embodiment of the present invention that illustrates how the
incoming continuous electron beam 100 is converted into a beam 300
of electron pulses. As each electron passes through the TWMCS
kicker 200, it is deflected by a kicking angle .theta..sub.k. After
traveling a distance L to the aperture 104, if
.theta..sub.k<.theta. (.theta. being indicated in the figure),
the electron passes through the aperture 104. Otherwise, it is
blocked by the aperture 104. Therefore, only a fraction of the
electrons pass through the aperture 104, and as such the average
current of the emerging pulsed electron beam is a fraction of the
incoming DC current, as determined by the duty cycle (D=.DELTA.t/T,
as illustrated in FIG. 3B).
[0095] With reference to tunneling electron microscope (TEM)
applications of the present invention, a typical TEM bio-sample
grid has a mesh of between 200 and 400. A 200-mesh, for example,
has a mesh size of 127 .mu.m. Assuming that the electrons are
focused through a single mesh hole that is roughly
1.6.times.10.sup.4 square microns, and if it is assumed that the
dose rate for bio-samples is required to be less than 10 electrons
per square Angstrom per second, then the low-dose-rate limit in
that case will be 1.6.times.10.sup.13 electrons per second, i.e.
2.5 micro-Amperes of average pulsed beam current. For embodiments
of the TWMCS-EMMP of the present invention, the duty cycle is
variable between 1% and 10%, with continuously tunable pulse rates
between 100 MHz and 10 GHz, which means that embodiments can
provide a continuously variable beam current of 0.25 to 2.5 .mu.A
(i.e. between one and ten electrons per square Angstrom per
second). In various applications to TEM, the independent control of
the electron pulse width and duty cycle, and of the intensity of
the incoming electron beam, that is provided by the present
invention can be used to study the effect of two or more different
low-dose-rate regimes, even when the total radiation dose rate is
the same, i.e. fewer electrons per pulse at a higher repetition
rate, vs more electrons per pulse at a lower repetition rate.
[0096] With reference to FIG. 4, embodiments of the present
invention are configured in a similar manner to the TWC-EMMP
illustrated in FIG. 1B, except that in the embodiment of FIG. 4 two
metallic comb kickers 210 are included in place of the two TWC
cavities 102 of FIG. 1B. The dimensions of the teeth 202 and the
spacing between them, as well as the gap between the combs 200, are
selected so as to achieve the following: [0097] 1) provide a
required transverse kicking force to the electrons traversing the
gap between two combs 200; [0098] 2) retard the RF wave phase
velocity so that it matches the speed of the electrons; and [0099]
3) match the impendence of the transmission line 210 to a
termination impedance (such as 50 ohms) so as to form a
substantially pure traveling wave over a frequency range that is as
wide as possible.
[0100] In various embodiments of the present invention, the outer
surface of the TWMCS 210 can be any combination of a variety of low
resistance metals, such as copper. The combs 200 of the TWMCS can
be made entirely from the low resistance metal, or the comb can be
made from another material and coated by the low resistance metal.
For example, the combs can be made from solid aluminum to which a
copper surface coating has been applied. Because electromagnetic
waves of 100 MHz and above propagates only on the conducting
surface "skins" of the TWMCS combs 200 (shallow "skin depth"), any
material can be used for the interiors of the TWMCS combs 200, even
for example 3D printed plastic, so long as the combs are coated
with a low resistance metal. The low resistance coating can be
applied, for example, by electroplating, atomic layer deposition
(ALD), physical vapor deposition (PVD), chemical vapor deposition
(CVD), sputtering, or molecular beam epitaxy (MBE). In some
embodiments, the TWMCS 210 includes a metallic base material that
is coated with a thin layer of a dissimilar metal so as to minimize
vacuum outgassing.
[0101] With continuing reference to FIG. 4, in embodiments two
TWMCS units 210, 220 are included in the TWMCS-EMMP device 420. The
first TWMCS 210 is used as the kicker that modulates the DC
electron beam 100 so that it can be chopped by the CCA aperture
104. The second TWMCS 220 is included in the dispersion suppressing
section, and is used as a "mirror" TWMCS to suppress the transverse
momentum and dispersion of the pulsed beam (beam artifacts) that
results from the modulating action of the first TWMCS 210. The
mirror kicker 220 suppresses these artifacts by means of applying
transverse forces to the electrons at the same frequency and
amplitude, but in an opposite direction, as compared to the kicker
TWMCS 210. Both of the TWMCS devices 210, 220 are driven by an
external RF source (not shown), which in embodiments operates over
a broad frequency range, so that the repetition rate of electron
pulses produced can be continuously adjusted over a very wide
range. Embodiments include a phase shifter that can shift a
relative phase of the RF applied to each of the TWMCS devices 210,
220. In embodiments, the RF source is also variable in amplitude,
thereby enabling separate adjustment of the electron pulse duty
cycle. In other embodiments, the two TWMCS devices 210, 220 are
drive by separate RF sources that are phase-locked to each other,
but can apply RF at different frequencies.
[0102] It should be noted that all elements of the TWMCS-EMMP 420
are enclosed within a vacuum chamber that is configured to provide
a sufficiently high vacuum to enable electrons to pass through the
TWMCS-EMMP 420 without significant attenuation due to residual
gasses.
[0103] FIG. 5A is a perspective view of a TWMCS 210 in an
embodiment of the present invention. FIG. 5B is a perspective
sectional view of the embodiment of FIG. 5A taken along a central
plane of the TWMCS 210. This more detailed view of the TWMCS 210
includes a vacuum chamber 500 that surrounds the combs 200 of the
kicker 210, as well as a first vacuum feedthrough 502 at the
proximal end through which the RF energy enters the TWMCS and a
second vacuum feedthrough 504 at the distal end through which the
traveling RF waves are terminated by a resistive load (not shown)
or sent out as a feedback readout.
[0104] FIG. 6 is a graph that illustrates the dispersion curve
(circles) 600 of an electromagnetic wave propagating through a
TWMCS for which the phase velocity (slope of the dispersion curve)
has been optimized to match a 200 keV input electron beam in an
embodiment of the present invention. It can be seen from the
figures that the phase velocity of the RF electromagnetic wave is
matched to an electron kinetic energy of 200 keV electrons (dashed
line in the lower left corner of the graph 602) for frequencies up
to about 5 GHz. In the example shown in the figure, the phase
velocity is 2.14.times.10.sup.8 m/s, i.e. 69.1% of the speed of
light, which corresponds to an electron beam kinetic energy of 200
keV.
[0105] FIG. 7 is a graph that presents simulated results of the
output current as a function of time for a TWMCS-EMMP in an
embodiment of the present invention. The current amplitude 700 is
presented in arbitrary units as a function of time, whereby the
input continuous beam is constant, while the output beam is pulsed
at a rate of 6 GHz, as determined by the frequencies of the RF (the
CCA "chopping collimating aperture" is assumed to be fixed) that is
applied to the TWMCS 210. The repetition rate of electron pulses is
twice the frequency of the RF applied to the TWMCS, as described
above.
[0106] FIG. 8 presents a Lissajous pattern traced over time onto
the aperture plate 104 of a CCA 104 in an embodiment, wherein the
TWMCS 210 includes two orthogonal pairs of combs 200, as shown in
FIGS. 2C and 2D, and where the drive frequencies of the two pairs
of combs 200 are not equal, but instead are phase-locked at a 4:3
frequency ratio. The resulting Lissajous pattern created on the CCA
aperture plate 104 thereby results in a pulse train passing through
the center opening of the CCA aperture 104 having a pulse
repetition rate that is an integer fraction of the two drive
frequencies. The broadband nature of the TWMCS in this Lissajous
configuration allows selection of any integer ratio of drive
frequencies, thereby enabling a continuously tunable range of pulse
widths and pulse train frequencies to be independently
selected.
[0107] Embodiments of the present invention are mechanically
adjustable so as to provide further tuning capability to the
pulser. In various embodiments, these features can include a CCA
having an adjustable aperture. In its simplest form, the aperture
104 is a barrier that includes an opening through which electrons
can pass when they are not deflected. The opening can be of any
size, and of any shape, such as round, rectangular, or
slit-shaped.
[0108] Embodiments of the present invention include more
sophisticated apertures. For example, FIGS. 9A and 9B are front and
rear views, respectively, of a CCA in an embodiment where the CCA
includes a sliding strip 900 penetrated by a plurality of openings
of different sizes. The strip 900 can be moved up and down so as to
place a selected opening in front of a larger hole 902 provided in
the underlying plate, thereby varying the size of the aperture.
[0109] FIGS. 10A and 10B are front and rear views, respectively, of
a CCA that includes two orthogonal pair of sliding panels 1000 that
are positioned over the large opening 902 in the CCA plate and can
be moved toward and away from each other so as to continuously vary
the size of the aperture. In similar embodiment, only a single pair
of sliding panels 1000 is provided, such that the aperture is
shaped as a slit. In some of these embodiments, adjusting a
relative degree of deflection of the electron beam in the vertical
and horizontal directions, for example as indicated in FIG. 8,
results in adjustment of the pulse widths for a given aperture slit
width. In some of these embodiments the panels 1000 are
electrically isolated from the remainder of the EMMP, such that the
CCA 104 can be used as a beam current monitor and/or as a beam
position monitor.
[0110] FIGS. 11A and 11B are front and rear views, respectively, of
a CCA that includes an iris 1100 that can be rotated to provide a
continuously variable aperture.
[0111] With reference to FIG. 12A, in some embodiments at least one
of the combs 1200 of the TWMCS 1210 can be shifted in position
relative to the other, for example by a mechanical actuator, so as
to vary the space between the combs, thereby enabling the traveling
wave phase velocity to be tuned so as to match a variable kinetic
energy of the input electrons 204, for example over a kinetic
energy range of +/-500 keV. Embodiments combine this feature with
combs 1200 having teeth 1202 that are stepped or shaped as smoothly
tapered triangles in cross section, so that adjustment of the gap
between the combs 1200 effectively adjusts the depth of the teeth
1202 that is presented to the traveling wave.
[0112] Similarly, as illustrated in FIG. 12B, in embodiments the
combs 1204 of the TWMCS 1212 are not parallel. In some of these
embodiments, the combs 1204 are variable in orientation, so that
the angle formed between the two combs 1204, and thereby the width
of the passage between the combs 1204, is variable so as to excite
special electromagnetic wave modes and implement special beam
modulation patterns. In embodiments, the teeth 1202 are not
rectangular, but can be concave (as shown in FIG. 12A), convex (as
shown in FIG. 12B), pointed, beveled, or shaped in any way that
provides a desired effect on the propagating RF wave. Also, the
cross sectional shapes of the combs 200 transverse to the beam
direction can be any shape as best suits a given application.
[0113] With reference to FIG. 13, embodiments further include one
or more additional apertures 1300 that can be located, for example,
at the electron beam input to the kicker 210. Some embodiments
include one or more magnetic or electrostatic beam steering
features 1302, which can be located for example at the input to the
TWMCS kicker 210, at the output of the CCA 104, and/or at the
output of the pulser 420.
[0114] With reference to FIG. 14, some embodiments further include
a "down-selecting" TWMCS 1400 and down-selecting aperture 1402
located downstream of the CCA 104. For simplicity of illustration,
the artifact suppressing elements of the EMMP such as a mirror
kicker 220 and quadrupoles 112, 116, 118 are not included in the
figure. In the illustrated embodiment, the first kicker 210 is
excited by RF at a first RF frequency F1, and the aperture 104
converts the transversely modulated beam into a pulsed beam as
described above. The down-selecting kicker 1400 is excited by RF at
a second RF frequency F2 that is a sub-harmonic of F1, i.e.
F1/F2=an integer. As a result, the down-selecting kicker 1400
deflects some of the electron pulses out of the beam so that they
are blocked by the down-selecting aperture 1402, thereby reducing
the number of pulses that remain, such that the pulse widths are
narrow, as determined by the TWMCS kicker 210 and CCA 104, while
the pulse repetition rate is slow, as determined by the
down-selecting TWMCS 1400.
[0115] FIG. 14B presents two curves 1404, 1406 that illustrate the
function of the two kickers 210, 1400 and associated apertures 104,
1402 in the embodiment of FIG. 14A. In the illustrated embodiment,
F1/F2=4. For example, if F1=2 GHz, then the first kicker 210 and
aperture 104 will chop the incoming electron beam 204 into a 4 GHz
pulse train (solid circles in curve 1404). And if a phase-locked
sub-harmonic waveform is applied at 500 MHz to the down-selecting
TWMCS 1400, then the electron pulses will passes undisturbed
through the down-selecting TWMCS 1400 and aperture 1402 when the RF
within the down-selecting TWMCS 1400 is at a "zero-crossing," as
shown by the solid circles in the lower curve 1406. However, when
the RF waveform in the down-selecting TWMCS 1400 has nonzero
amplitude, then the electron pulses will be transversely deflected
out of the beam and will be blocked by the down-selecting aperture
1402, as indicated by the open circles in the lower curve 1406 of
FIG. 14B. The result will be a down-selected pulse train,
downstream of the down-selecting TWMCS 1400, having a repetition
rate that is an integer fraction of the pulse repetition rate of
the beam as it emerges from the aperture 104, but having the short
pulse widths that result from the higher RF F1 frequency that is
applied to the TWMCS kicker 210. Accordingly, FIGS. 14A and 14B
illustrate an embodiment that can be used to generate pulsed beams
having low repetition rates combined with short pulse lengths.
[0116] For example, if the pulses generated by the first kicker 210
and the aperture 104, when excited at 2 GHz, and depending on the
dimensions of the aperture 104, are 10 ps in width, and if RF is
applied at 500 MHz to the down-selecting TWMCS 1400, then the
result will be a 1 GHz "down-selected" pulse train of 10 ps pulses,
as compared to the 4 GHz pulse train that would result if the
down-selecting TWMCS 1400 were not present.
[0117] With reference to FIG. 15, the present invention further
includes an Electron Pulse Picker (EPP) that is configured to apply
a transverse electric field across at least one pair of combs 200a,
200b in at least one TWMCS 210 of the apparatus. When the
transverse electric field is present, it deflects electrons as they
travel through the TWMCS 210, so that they are eliminated from the
electron beam 204. By switching the picker field on and off in
synchronization with the beam 204, e.g. in synchronization with the
applied RF and therefore with the consequent EM traveling wave, it
is thereby possible to eliminate selected electrons or groups of
electrons from the beam, while allowing others to pass through.
[0118] In the embodiment of FIG. 15, a fixed DC bias is applied to
one comb 200a of a TWMCS comb pair. A simple circuit referred to in
the drawing as a "DC bias T" 1500 is used to introduce the DC bias.
The bias T 1500 includes a series capacitor that prevents the DC
bias from entering the RF amplifier 1502 that applies in phase RF
to that comb 200a, while the DC bias is applied to the comb 200a
via a coil that isolates the DC bias source from the in phase RF.
The in-phase RF is terminated by a 50 Ohm load 1504 that is coupled
to the comb 200a through a capacitor 1506, so that the comb 200a is
maintained at the DC bias voltage without drawing any current from
the DC bias source.
[0119] The other comb 200b of the pair 210 is directly connected to
a 50 ohm load 1508, so that it is normally held at zero bias.
However, the RF amplifier 1510 that applies the out of phase RF to
the second comb 200b is also coupled to the comb 200b via a DC bias
T 1512, whereby DC pulses (EPP pulses) can be applied to the second
comb 200b. Because there is no DC block between the 50 ohm load
1508 and the second comb 200b, some current is drawn from the EPP
pulse source, but only during the EPP pulses.
[0120] Accordingly, in the embodiment of FIG. 15, when an EPP pulse
is not present, the DC bias that is applied via the first bias T
1500 creates a transverse DC field between the combs 200a, 200b of
the TWMCS 210 that deflects electrons 1514 and removes them from
the beam 204. However, during an EPP pulse, which is set to the
same amplitude as the DC bias, both of the combs 200a, 200b of the
TWMCS 210 are at the same electrical potential, such that the DC
field between the combs 200a, 200b is temporarily neutralized, and
the electrons in the beam 204 are able to pass through 1516.
[0121] Accordingly, the EPP pulses 1512 function as electron gating
pulses, in that electrons remain in the beam 204 only while an EPP
pulse is applied. In embodiments, the EPP pulses can be applied to
the TWMCS at any repetition rate from 1 kHz or less up to 1 MHz or
more.
[0122] With reference to FIG. 16A, in various embodiments EPP
pulses 1600 can have narrow widths that only allow one electron of
the beam 1602 to pass, or, with reference to FIG. 16B, EPP pulses
1604 can have arbitrarily long lengths that allow an electron burst
1606 containing a desired number of electrons to pass through the
TWMCS. In FIG. 16B, the EPP pulse 1604 is twice as long as the EPP
pulse 1600 in FIG. 16A. As a result, a "burst" 1606 of two
electrons is allowed to pass through the TWMCS in FIG. 16B, while
only one electron 1602 is allowed to pass through in FIG. 16A. In
embodiments the EPP pulses are variable from 100 picoseconds to 10
microseconds in length. In various embodiments, the timing of the
EPP pulses can be synchronized with an RF signal that is driving
the TWTMS 210, and/or with a signal that triggers pumping of a
transverse electron microscopy (TEM) sample, for example so that
dynamic images of a TEM sample can be obtained in real time.
[0123] In some embodiments, a series of EPP pulses is applied to
the TWMCS, wherein all of the EPP pulses are of equal length and
are equally spaced apart. In other embodiments, only a single EPP
pulse is applied, while in still other embodiments a series of EPP
pulses is applied having any desired pattern of EPP pulse widths
and of spacing between the EPP pulses.
[0124] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. Each and every page of this submission, and all
contents thereon, however characterized, identified, or numbered,
is considered a substantive part of this application for all
purposes, irrespective of form or placement within the
application.
[0125] This specification is not intended to be exhaustive.
Although the present application is shown in a limited number of
forms, the scope of the invention is not limited to just these
forms, but is amenable to various changes and modifications without
departing from the spirit thereof. One or ordinary skill in the art
should appreciate after learning the teachings related to the
claimed subject matter contained in the foregoing description that
many modifications and variations are possible in light of this
disclosure. Accordingly, the claimed subject matter includes any
combination of the above-described elements in all possible
variations thereof, unless otherwise indicated herein or otherwise
clearly contradicted by context. In particular, the limitations
presented in dependent claims below can be combined with their
corresponding independent claims in any number and in any order
without departing from the scope of this disclosure, unless the
dependent claims are logically incompatible with each other.
* * * * *