U.S. patent number 3,916,246 [Application Number 05/389,915] was granted by the patent office on 1975-10-28 for electron beam electrical power transmission system.
This patent grant is currently assigned to Varian Associates. Invention is credited to Donald Henry Preist.
United States Patent |
3,916,246 |
Preist |
October 28, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electron beam electrical power transmission system
Abstract
Electrical power is transmitted from a transmitting location to
a remote receiving location by means of an electron beam injected
into an evacuated magnetically shielded pipe extending between the
transmitting location and the receiving location. The beam is
magnetically focused within the evacuated pipe. Electrical power to
be transmitted is put into the beam in the form of kinetic energy
by accelerating the beam to a high kinetic energy. The kinetic
energy is extracted from the beam at the receiving location and
converted into potential electrical energy for application to the
load. In one embodiment, the kinetic energy is extracted from the
beam by collecting the beam current at a potential substantially
equal to the potential of the source of the electrons, i.e. cathode
potential, and causing the collected beam current to flow through
the load to develop the depressed collector potential. In another
embodiment, radio frequency accelerator means are utilized for r.f.
current density modulating and accelerating the beam. The radio
frequency current modulation on the beam is extracted at the
receiving end by means of a radio frequency circuits coupled to the
beam. The extracted radio frequency energy is rectified for
application to the load. In another embodiment, AC power at
conventional AC power frequencies, as of 60 Hertz, is extracted
from the beam by sequentially directing the beam into a plurality
of depressed collectors coupled to respective primary windings of
power transformers for deriving AC output power for application to
a load.
Inventors: |
Preist; Donald Henry (Menlo
Park, CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
23540289 |
Appl.
No.: |
05/389,915 |
Filed: |
August 20, 1973 |
Current U.S.
Class: |
315/5; 315/3.6;
363/111; 976/DIG.434 |
Current CPC
Class: |
H02J
50/30 (20160201); H01J 25/02 (20130101); H02N
3/00 (20130101); G21K 1/093 (20130101); H05H
9/00 (20130101) |
Current International
Class: |
G21K
1/00 (20060101); G21K 1/093 (20060101); H01J
25/02 (20060101); H01J 25/00 (20060101); H02J
17/00 (20060101); H02N 3/00 (20060101); H05H
9/00 (20060101); H01J 025/02 () |
Field of
Search: |
;315/3.5,3.6,4,5
;328/233,227,228 ;321/8,22,32,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Cole; Stanley Z. Pressman; D. R.
Nelson; Richard B.
Claims
What is claimed is:
1. In an electron beam power transmission system for transmitting
electrical power from a transmitting location to a receiving
location remote from the transmitting location:
elongated evacuated envelope means extending from the transmitting
location to a geographically removed receiving location;
transmitter means at the transmitting location for forming,
accelerating, and projecting electrons over an elongated beam path
extending within and along said evacuated envelope from the
transmitting location to the receiving location; and
transmitter means at the transmitting location for accelerating and
projecting electrons through said evacuated envelope means from the
transmitting location to the receiving location;
receiver means at the receiving location for collecting the
electrons and converting the kinetic energy thereof to electrical
power for application to a power load;
said transmitter means including first and second electron guns
having first and second cathode emitters respectively for emitting
electrons and first and second apertured anode electrodes
respectively for drawing first and second streams of emitted
electrons from said first and second cathode emitters to form
respective first and second electron beams;
first and second converging evacuated envelope portions opening
into a common portion of said elongated envelope means which
extends from the transmitting location to the receiving location,
said first and second convergent envelope portions being disposed
to receive said first and second electron beams from said first and
second electron guns, respectively, and to direct said respective
beams via convergent first and second beam paths into a common beam
path within said common portion of said elongated envelope means,
and first and second beam focus means for focusing said first and
second beams into said first and second convergent beam paths
within said first and second envelope portions, respectively.
2. The apparatus of claim 1 wherein said means for supplying and
applying the alternating potentials between said cathodes and
anodes of said first and second guns includes, a transformer, and
wherein said inductive means is a secondary winding means of said
transformer having a centertap, and wherein said first and second
anodes are connected to said centertap of said secondary
transformer winding mean.
3. The apparatus of claim 1 wherein said means for supplying and
applying the alternating potentials between said cathodes and
anodes of said guns includes, a transformer having a core with
primary and secondary windings thereon, said secondary windings
including first and second secondary windings connected for
applying said first and second alternating potentials to said
respective first and second guns in the 180.degree. out of phase
relation, and wherein said first and second secondary windings are
balanced and bucking wound on said transformer core as connected
for conduction of cathode current drawn from said respective
secondary windings by each of said first and second guns.
4. In a power transmission system for transmitting electrical power
from a transmitting location to a remote receiving location;
elongated evacuated envelope means extending from the transmitting
location to the receiving location geographically removed from the
transmitting location;
elongated evacuated envelope means extending from the transmitting
location to a geographically removed receiving location;
transmitter means at the transmitting location for forming,
accelerating and projecting electrons over an elongated beam path
extending within and along said evacuated envelope from the
transmitting location to the receiving location; and
receiver means at the receiving location for collecting the
electrons of the beam and for converting the kinetic energy of the
beam to electrical power for application to a power load;
said transmitter means including an electron gun having a cathode
emitter for emitting electrons and an apertured anode electrode for
drawing a stream of electrons through said envelope means;
inductive means connected between said cathode emitter and said
anode electrode for supplying and applying an alternating potential
between said cathode emitter and said anode electrode for causing
said gun to produce an electron beam from said gun during only
every other half-cycle of the applied alternating potential;
control electrode means interposed between said cathode and anode
means for controlling the flow of beam current from said gun during
each beam forming half-cycle of the applied alternating potential;
and
means for supplying and applying a control potential to said
control electrode for limiting the flow of beam current from said
gun to less than the full half-cycle of the applied alternating
potential.
5. The apparatus of claim 4 wherein said first and second beam
focus means each includes at least a quadrupole magnetic beam
focusing structure.
6. In an electron beam power transmission system for transmitting
electrical power from a transmitting location to a receiving
location remote from the transmitting location;
receiver means at the receiving location for collecting the
electrons of the beam and for converting the kinetic energy of the
beam to electrical power for application to a power load;
said transmitter means including first and second electron guns
having first and second cathode emitters respectively for emitting
electrons and first and second apertured anode electrodes
respectively for drawing first and second streams of emitted
electrons from said first and second cathode emitters to form
respective first and second electron beams;
means for supplying and applying an alternating potential between
said cathode emitter and said anode electrode of each of said first
and second guns in 180.degree. out of phase relation such that the
potential of said first cathode of said first gun is negative
relative to the potential of said first anode electrode when the
potential of said second cathode of said second gun is positive
relative to the potential of said second anode electrode and vice
versa;
said means for supplying and applying the alternating potential to
said first and second guns including an inductive means connected
between said first and second cathode emitters, and said first and
second anodes being connected to said inductive means intermediate
said connections of said first and second cathode emitters.
7. The apparatus of claim l wherein said control potential
supplying and applying means supplies and applies a control
potential of a value and waveform such as to limit conduction of
beam current to that portion of the cycle of applied alternating
potential corresponding to a beam current of not less than
I.sub.max /6, where I.sub.max is the peak beam current during the
beam conductive half-cycle of the applied alternating
potential.
8. In a power transmission system for transmitting electrical power
from a transmitting location to a remote receiving location:
elongated evacuated envelope means extending from the transmitting
location to the geographically removed receiving location;
transmitter means at the transmitting location for accelerating and
projecting electrons through said evacuated envelope means from the
transmitting location to the receiving location;
receiver means at the receiving location for collecting the
electrons and for converting the kinetic energy thereof to
electrical power for application to a power load;
said receiver means including first and second electron collectors
for collecting the electrons, electrical insulator means for
insulating said first and second collectors from said evacuated
envelope means to permit independent potentials to be established
on said collectors relative to said envelope means, means for
causing electrons to be collected in an alternating sequence in
said first and second electron collectors, in 180.degree. phase
relation at the fundamental frequency of a power frequency for the
electrical power to be delivered to the load first and second
bucking wound balanced inductive winding means connected to said
first and second collectors for flow of collected beam current
therethrough in 180.degree. out of phase relation at the power
frequency.
9. The apparatus of claim 8, wherein said receiver means includes,
an output power transformer means and said first and second
inductive winding means comprises primary winding means of said
transformer.
10. The apparatus of claim 8 wherein the power transmission system
is a three phase system and said receiver means includes an output
power transformer means, said first and second inductive winding
means comprises primary winding means of said transformer, and
wherein said transformer includes secondary winding means, said
secondary winding means being connected in a delta
configuration.
11. The apparatus of claim 8 including first and second series
resonant circuits connected in shunt with respective ones of said
first and second winding means, said series tuned circuits each
being resonant for an odd harmonic of the power frequency.
12. The apparatus of claim 11 wherein said series tuned circuits
are each resonant at the fifth harmonic of the power frequency.
13. In a power transmission system for transmitting electrical
power from a transmitting location to a receiving location remote
from said transmitting location:
elongated evacuated envelope means extending from the transmitting
location to the receiving location geographically removed from the
transmitting location;
transmitter means at the transmitting, location for accelerating
and projecting electrons through said evacuated envelope means from
the transmitting location to the receiving location;
receiver means at the receiving location for collecting the
electrons and for converting the kinetic energy thereof to
electrical power for application to a power load;
said transmitter means including an electron gun having a cathode
emitter for emitting electrons and an apertured anode for drawing
the stream of electrons from said cathode emitter;
inductive means connected between said cathode emitter and said
anode electrode for supplying and applying an alternating potential
between said cathode emitter and said anode electrode at a power
frequency for causing said gun to produce an electron beam from
said gun during only every other half-cycle of the applied
alternating potential;
control electrode means interposed between said cathode emitter and
said anode electrode means for controlling the flow of beam current
from said gun during the beam conductive half-cycle of the applied
alternating potential; and
means for supplying and applying a train of control potential
pulses to said control electrode for shaping the flow of beam
current from said gun during the beam conductive half-cycle of the
applied alternating potential to suppress at least one odd order
harmonic of the power frequency of the collected beam current.
14. The apparatus of claim 13 wherein said control potential is
shaped by said supplying means to provide a beam conductive phase
angle to suppress the odd order harmonics of the beam current.
15. In a power transmission system for transmitting electrical
power from a transmitting location to a remote receiving
location:
elongated evacuated envelope means extending from the transmitting
location to the receiving location geographically removed from the
transmitting location;
transmitter means at the transmitting location for accelerating and
projecting electrons through said evacuated envelope means from the
transmitting location to the receiving location;
receiver means at the receiving location for collecting the
electrons and for converting the kinetic energy thereof to
electrical power for application to a power load;
said receiver means including electron collector means for
collecting the electrons, electrical insulator means for insulating
said collector means from said evacuated envelope to permit
independent potentials to be established on said collector means
relative to said envelope, means for causing electrons to be
collected in a sequence of pulses of beam current in said collector
means, output transformer means coupled to said collector means for
supplying a current to the power load in response to collected beam
current, said transformer means including a primary winding means
connected to said collector means for conduction of collected beam
current therethrough;
means for sensing an output proportional to the collected beam
current, means for sensing an output proportional to the collector
potential;
means for comparing the phase of the collected current signal with
the phase of the collector voltage signal to derive a power factor
load correction signal; and
means connected to the secondary of said power transformer means
for controlling the power factor of the current delivered to the
load in response to the error signal derived from said comparator
means, whereby the phase of the collected beam current is brought
into coincidence with the phase of the collector potential
established on said beam collector means.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to electrical power
transmission systems utilizing an electron beam as the power
transmitting medium and, particularly, to improved means for
imparting kinetic energy to an electron beam and for extracting the
kinetic energy from the beam at a remote location.
DESCRIPTION OF THE PRIOR ART
Heretofore, it has been proposed to transmit vast quantities of
electrical power from a transmitting location to a remote receiving
location by means of an electron beam traveling within an evacuated
magnetically shielded pipe or cable employing "strong" magnetic
focusing along the pipe to prevent unwanted beam interception by
the walls of the pipe. Such a system is disclosed in U.S. Pat. No.
2,953,750 issued Sept. 20, 1960.
In this prior proposed scheme, it was contemplated that kinetic
energy would be imparted to the beam at the sending end by
accelerating the beam to a high kinetic energy, as of 100 MeV, by a
modified betatron type induction accelerator machine. In the
modified betatron, the beam, while magnetically contained within a
helical magnetic cable (pipe), was caused to pass through a
plurality of accelerating gaps for increasing the kinetic energy of
the beam in a steplike fashion. That is, the accelerating
electrical field, produced across the respective gaps by an AC
potential applied in synchronism with pulses of the beam, caused
the beam to be accelerated to the high output kinetic energy. It
was contemplated that the high energy pulses of beam current could
be at AC power frequencies of 25 or 60 Hertz or, as an alternative,
the pulse repetition rate could be in the radio frequency range by
utilizing a radio frequency cavity resonator at each of the
accelerating gaps in the helical cable.
The kinetic energy of the high energy pulses of beam current was
extracted at the receiving end by means of a reverse type
accelerator which decelerated the beam pulses in accordance with
the amount of power demanded by the load. The decelerated (unused)
pulses of beam current were returned to the sending end by means of
return magnetic cables or pipes connected back to appropriate ones
of the electron beam accelerating machines. The returning beam
pulses were 180.degree. out of phase with the transmitted pulses of
beam current leaving the machine. In this manner the unused energy
of the beam was returned to the betatron accelerating machine.
It was concluded, in the above cited prior patent, that the radio
frequency alternative was not feasible for transmitting relatively
large amounts of power. On the other hand, a problem with the use
of magnetic induction accelerators operating even at conventional
power frequencies is that a vast amount of iron must be used
causing attendant iron losses due to hysteresis effects.
SUMMARY OF THE PRESENT INVENTION
The principal object of the present invention is the provision of
an improved electrical power transmission system employing an
electron beam as the power transmission medium.
In one feature of the present invention, first and second electron
guns at the transmitting end of the power transmission system are
energized with beam voltage in 180.degree. out of phase relation
such that alternate guns conduct during each half-cycle of the
applied A.C. power, whereby a full wave rectification effect is
obtained by the self-rectifying action of the electron guns at the
transmitting end of the system.
In another feature of the present invention, first and second
convergent evacuated envelope portions are disposed to receive
first and second electron beams from first and second electron guns
and to direct the respective beam via convergent beam paths into a
common beam path and wherein magnetic focusing is employed for
focusing the beams within the first and second convergent envelope
portions.
In another feature of the present invention, an alternating
potential is applied between the cathode and anode of an electron
gun at the transmitting end of the system for generating beam
current from that gun during only alternate half cycles of the
applied beam potential, a beam current control electrode is
provided in the electron gun and energized with a control potential
such that the current from the gun is limited to less than a full
half cycle of the applied alternating potential, whereby beam
conduction is limited to periods of the power cycle wherein the
beam voltage has substantial amplitude to avoid undesired transit
time effects and to reduce the requirements of the beam focus
system to accommodate beams of widely varying current and
velocity.
In another feature of the present invention, the collected beam
current at the receiving location is directed, in sequential
half-cycles of the power frequency through bucking connected halves
of balanced winding means of an output transformer to avoid
undesired D.C. saturation effects of the transformer core and to
eliminate certain undesired harmonics.
In another feature of the present invention, the secondary windings
of the output power transformer of a three phase electron beam
power transmission system are connected in the delta configuration
to eliminate output power at the third harmonic of the power
frequency and multiples thereof.
In another feature of the present invention, current is
sequentially directed into various primary windings of the output
transformer for obtaining A.C. output power in the load and wherein
a series tuned odd harmonic filter is connected across each of the
primary windings for by-passing odd harmonic content of the
collected beam current.
In another feature of the present invention, pulses of beam current
are sequentially transmitted from the transmitting location to the
receiving location and the pulses are sequentially directed through
primary windings of a transformer in such a way as to produce A.C.
output power in the load. A control electrode is provided for
shaping the pulses of beam current to eliminate or reduce certain
undesired odd-order harmonic beam current content therein such as
the fifth, seventh, eleventh, etc.
In another feature of the present invention, pulses of beam current
are transmitted from the transmitting location to the receiving
location wherein they are collected sequentially in different
collecting structures. The collected current is directed through
primary windings of an output transformer. The phase of the
collected current is compared with the phase of the collector
potential to derive an error signal for correcting the power factor
of the load.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic longitudinal sectional foreshortened view,
partly in block diagram form, of an electron beam power
transmission system incorporating features of the present
invention.
FIG. 1A is a sectional view of the structure of FIG. 1 taken along
line 1A--1A in the direction of the arrows,
FIG. 2 is an enlarged sectional view of a portion of the structure
of FIG. 1 taken along line 2--2 in the direction of the arrows.
FIG. 2A is a view similar to that of FIG. 2 showing an alternative
embodiment of the present invention,
FIG. 3 is an enlarged schematic detail view of a portion of the
structure of FIG. 1 delineated by line 3--3,
FIG. 4 is a schematic longitudinal sectional view of a portion of
FIG. 1 delineated by line 4--4,
FIG. 5 is a view similar to that of FIG. 1 depicting an alternative
embodiment of the present invention,
FIG. 6 is a schematic circuit diagram of the winding portion of the
transformers of the circuit of FIG. 5 delineated by line 6--6,
FIG. 7 is a schematic circuit diagram of the winding portions of
the transformers of FIG. 5 delineated by lines 7--7,
FIG. 8 is a transverse sectional view of a portion of the structure
of FIG. 5 taken along line 8--8 in the direction of the arrows,
FIG. 9 is a schematic diagram, partly in block diagram form, of an
electron beam power transmission system incorporating alternative
embodiments of the present invention,
FIG. 10 are waveforms of beam voltage, beam current, and grid
voltage for an electron beam power transmission system employing an
interrupted beam,
FIG. 11 is a schematic line diagram of an electron beam power
transmission system employing alternative features of the present
invention,
FIG. 12 is a schematic line diagram of a power transmission system
employing alternative embodiments of the present invention,
FIG. 13 is a schematic line diagram of an alternative embodiment to
a portion of the structure of FIG. 11 delineated by line
13--13,
FIG. 14 is a schematic circuit diagram for an electrical power
transmission system incorporating features of the present invention
and depicting an alternative embodiment,
FIG. 15 is a plot of current and voltage waveforms for one phase of
the power transmission system of FIG. 14,
FIG. 16 is a view similar to that of FIG. 1 depicting an
alternative embodiment of the present invention,
FIG. 17 is a schematic circuit diagram, partly in block diagram
form, of a control circuit useful in a power transmission system of
the present invention,
FIG. 18 is a view similar to that of FIG. 17 depicting an
alternative embodiment of the present invention, and
FIG. 19 is a schematic circuit diagram, in block form, of the
comparator circuit useful in the embodiment of FIGS. 17 and 18.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 show an electron beam power transmission system 1
incorporating features of the present invention. System 1 includes
an electron gun 12 and a DC beam accelerator section 13 at the
power transmitting end 14 of an elongated evacuated envelope 15
which extends for a substantial distance, e.g. 0.1 to 1,000 miles,
to a remote power receiving location 16 at the receiving end of the
transmission system 1. At the receiving location 16, an electron
beam collector structure 17 is connected to the evacuated envelope
15 by a DC beam decelerating section 18.
Briefly, electron gun 12 serves to form, accelerate and project a
beam of electrons 19 into the beam accelerator section 13 which
accelerates the beam to a very high energy, as of in excess of 0.1
million electron volts MeV, and preferably in excess of 0.5 MeV. In
this manner, substantial kinetic energy is imparted to the electron
beam.
Although a separate accelerator section 13 is employed in the
embodiment of FIG. 1, this is not a requirement. If the beam
voltage is below 0.25 MeV, the anode of the gun 12 may be operated
at 0.25 MeV for accelerating the beam up to 0.25 MeV. Actually the
accelerating section 13 can be considered as part of the anode
structure of the gun 12.
The beam 19 is then projected axially into the elongated envelope
15. The beam is magnetically confined in envelope 15 by a
quadrupole magnetic beam focus structure 22 to avoid substantial
interception on the walls of the evacuated envelope. An astigmatic
magnetic lens system 21 (See FIGS. 1 and 1A) is provided between
the accelerating section 13 and the entrance to the elongated
envelope 15 to provide a smooth magnetic focusing transition of the
electron stream from the accelerator into the quadrupole beam focus
field in envelope 15.
The magnetic lens system 21 comprises one or more quadrupole
lenses. Each quadrupole lens includes four magnetic poles of
alternating polarity around the envelope 15 which are energized by
electric coils 10 wound around the poles 9. A tubular magnetic
shield 11 surrounds the poles 9.
The quadrupole magnetic beam focus structure 22, more fully
described below, spirals around envelope 15 to provide "strong"
magnetic beam focusing of the type described in the aforecited U.S.
Pat. No. 2,953,750. The envelope 15 is evacuated by means of a
plurality of glow discharge getter-ion vacuum pumps 23 or any other
suitable vacuum pump disposed at suitable intervals along the pipe
15 in gas communication therewith for evacuating same to a
relatively low pressure as of 10.sup.+.sup.7 torr. Thus a beam can
be transmitted for hundreds of miles through tubular envelope 15
without substantial loss of energy.
As an alternative to the use of the quadrupole lens 21, the
quadrupole beam focus structure 22 is extended into a transition
region between the cathode 31 of the gun 12 and the entrance to
pipe 15. In this transition region, the quadrupole beam focus
magnetic field intensity in the beam path gradually increases in
strength from zero to its full value at the entrance to the main
portion of the pipe 15 (see FIG. 13).
At the power receiving location 16, the beam is decelerated by a
beam decelerator section 18 to a potential as close as possible to
the potential of the source of electrons within the gun 12, thereby
converting the kinetic energy in the beam to potential electrical
energy. The electrons of the beam, having low velocity (i.e.,
within 5% of the collector potential), are collected on the
interior walls of the beam collector structure 17, which operates
close to the potential of the source of electrons within the gun
12, whereby the beam energy converted to heat in beam collector 17
is minimized.
The collected beam current is caused to flow through an inverter
load 24 to the vacuum envelope 15, typically at local ground
potential. The collected beam current is returned to the
transmitting location 14 via the electrically conductive walls of
the envelope 15. The inverter 24 inverts the DC power to three
phase AC output power which is supplied on output lines 25, 26 and
27. In a typical example, the beam 19 has a current of 1,000 amps
and is accelerated by the accelerator section 13 to a potential of
one million volts, such that the power transmitted by the beam for
transmitting location 14 to receiving location 16 is 1
gigawatt.
A control circuit 28, as more fully disclosed below with regard to
FIGS. 17-19, monitors the potential V.sub.C of the beam collector
17 and compares it with the potential V.sub.k of the source of
electrons to derive an error signal for controlling the beam
current via a control electrode 29 in the electron gun 12 or
adjustment of V.sub.k, such that the power transmitted to the
receiving end 16 is regulated to match the demand for power at the
receiving end 16.
The electron gun 12 includes a spherically concave thermionic
cathode emitter 31 of sufficient area to emit the required electron
current, such as 1,000 amps. Cathode emitter 31 is heated to
thermionic emission temperature by means of a filiamentary heater
32. Operating power is supplied to the filamentary heater 32 from a
power supply 33. A focus electrode 34 surrounds the peripheral edge
of cathode emitter 31 to aid in shaping the electron beam in the
region of cathode emitter 31.
The control grid 29 is preferably of the type protected by a shadow
grid wherein the shadow grid structure, operating at substantially
cathode potential, is disposed immediately adjacent the surface of
the cathode emitter 31 and the control grid has apertures in the
shadow grid. As an alternative to a control grid, a modulating
anode may be employed as the control electrode 29.
The cathode emitter 31 is preferably dimpled, with the dimples
having a lesser radius of curvature than that of the composite
cathode emitter surface. The individual dimples in the cathode
serve as separate cathode emitters for individual electron beamlets
passing through the aligned openings in the shadow and control grid
structures in a substantially non-intercepting manner. Such an
electron gun and control grid structure is disclosed and claimed in
U.S. Pat. No. 3,558,967, issued Jan. 26, 1971, and assigned to same
assignee as the present invention.
In a typical example, thermionic cathode emitter 31 comprises an
impregnated tungsten matrix cathode approximately 15 cm. in
diameter and having an area of approximately 350 cm.sup.2. At 1,000
amperes, such a cathode would operate at a current density of
approximately 3 amperes per square centimeter. Tunsten matrix
cathodes, impregnated with barium aluminate, can operate
continuously for 5 years at this current density.
The beam accelerator section 13 is preferably of the type used with
the Van de Graaffor Cockroft-Walton generators and is disclosed in
an article entitled "Electrostatic Generators for the Acceleration
of Charged Particles.sbsp.y appearing in Reports on Progress in
Physics, Vol. 11:1-18 (1948). Briefly, this type of accelerator
includes a sequence of generally planar centrally apertured plate
shaped electrodes 35 wherein the accelerating potential, as of 0.1
to 5 million volts, is evenly distributed among a number of the
accelerating electrodes 35. A potential divider 36 employs a string
of resistors to divide the beam potential V.sub.b for application
of respectively increasing potentials to the respective ones of the
accelerating electrodes 35. The potential difference between
successive electrodes 35 in the accelerator section 13 is
preferably less than 200 kilovolts to prevent arcing between the
adjacent electrodes 35.
Electrodes 35 serve to provide a uniform beam accelerating electric
field within the beam path 19; the first few ones of electrodes 35
near the upstream end of the beam path 19 serve to focus and to
converge the electron stream. In a typical example, the beam would
be converted from a diameter of 15 cm. at cathode emitter 31 to
approximately a diameter of 5 cm. at the output end of accelerator
section 13.
A three phase rectifier 38 receives the three phase input power
from a power generator or the like and rectifies this three phase
input to produce direct output current at a high negative cathode
voltage V.sub.k, as of -0.1 million volts to -5 million volts.
FIG. 2 shows, in section, the evacuated envelope 15 and beam focus
structure 22. More particularly, evacuated envelope 15 comprises a
pipe made of an electrically conductive material, as of aluminum.
In a preferred embodiment, the pipe also serves as the return
electrical conductor of the power transmission system 1 such that
the collected beam current flows back to the power supply of the
electron gun 12 through a return path which is symmetrical relative
to the beam 19. In this manner, undesired magnetic defocusing of
the beam by the magnetic field of the beam current loop is avoided.
In the preferred embodiment, the pipe 15 is electrically insulated
from earth such as by the ferrite permanent magnets 22. In a
typical example, aluminum pipe 15 has a diameter of approximately
10 cm. and a wall thickness of approximately 3 mm.
The beam focus permanent magnets 22 are disposed around pipe 15 at
90.degree. spacing for a quadrupole type of "strong" magnetic
focusing. In strong magnetic beam focusing a quadrupole magnetic
field is provided which has flux lines which lie in planes almost
perpendicular to the direction of the beam. The flux lines are made
to rotate about the beam by spiraling permanent magnets 22 around
pipe 15. The permanent magnets are radially polarized, with the
magnetic poles alternating in polarity in the circumferential
direction around the pipe 15. Although the preferred embodiment
utilizes a quadrupole magnetic structure, other multiple pole
structures may also be used, such as 6, 8, 10 or 12 poles etc. Also
other types of magnetic focusing may be employed such as a series
of discrete magnetic lenses or a confining magnetic field.
As an alternative to the use of permanent magnets for producing the
beam focus magnetic field, electromagnets are employed. The
electromagnetic equivalent is useful where operating temperatures
are encountered which are outside of the rated operating
temperature range of the permanent magnet material. A quadrupole
electromagnetic beam focus structure comprises four conductors 40
spiraling around the pipe 15 in 90.degree. circumferentially spaced
relation (see FIG. 2A), and energized with direct current of
opposite direction in adjacent conductors 40.
An electron traveling parallel to the beam path 19 will interact
with a magnetic vector at right angles to its direction of travel.
The field has a strength proportional to the distance between the
electron and the axis of the beam path. The magnetic vector rotates
in a direction around the pipe at twice the rate of the quadrupole
rotation. The magnetic vector will cause the electrons to follow a
helical path. The magnetic focusing force of the quadrupole field
on the electron, when the electron is far from the axis, is larger
than the defocusing force when the electron is near the axis. As a
result, there is a net time averaged inward focusing force.
In this kind of focusing, the magnetic field need only provide a
focusing force that is sufficient to compensate for the difference
between the space charge forces, which tend to defocus the beam,
and the beam self focusing magnetic field. The problem of focusing
the beam is less severe at high beam voltage since the space charge
forces are reduced at a given current flow and the self magnetic
field of the beam tends to compensate the space charge repulsion.
This means that smaller focusing fields can be used to confine the
beam. For example, at one million volts, eight-ninths of the space
charge force is neutralized by the self-magnetic field of the beam
current. Since the focusing force is proportional to the distance
of the electron from the axis of the pipe 15, the beam will tend to
follow the center of pipe 15 even if the pipe has curvature.
Magnetic focusing results in ion trapping, which leads to plasma
instability. Residual gas molecules within the pipe, when struck by
electrons produce positive ions which are attracted toward the
center of the beam. The positive ions in the center of the beam
tend to neutralize the space charge repulsion, causing the beam to
condense toward the center. Therefore, ion drainage or
neutralization is required. The simplest way to obtain ion
neutralization is to turn off the beam periodically for a few
microseconds to allow the ions to move to the wall of the pipe 15,
where their charge will be lost. The time required for ion
neutralization of the beam at a pressure of 10.sup.-.sup.7 torr at
an electron energy of 1 megavolt is about 5 milliseconds. Thus, the
beam is turned off by means of grid 29 every few hundred
microseconds, for a few microseconds, causing any ions that have
formed to mutually repel each other and drain to the wall. As an
alternative, (shown in FIG. 3) insulated negative electrodes may be
provided at suitable spacings along and within the pipe 15 for
generation of periodic electric fields (potential wells) for
drawing the ions to the electrodes. More particularly, each ion
drain may include an enlarged diameter section of the pipe 15' to
provide an annular recess to receive a metallic ring shaped drain
electrode 1 supported from a conductive post 2 via a feedthrough
insulator 3. The post 2 is connected to source of negative
potential 4, as of -100 to -1,000 V, for collecting and draining
positive ions from within the pipe 15. Various suitable ion
draining electrodes and schemes are disclosed in U.S. Pat. No.
2,963,605 issued Dec. 6, 1970.
In a preferred embodiment, the permanent magnets 22 are made of
grain oriented ferrite particles with a BH product of nearly 4
million gauss-oersteds. Such a material is commercially available
at 2,000 gauss and 2,000 oersteds in which the ferrite particles
are bonded in a flexible plastic so that half of the energy product
of the oriented ferrite is sacrificed for the convenience of
flexible plastic bonding. Such a material has sufficient magntic
energy for this application. This material is also relatively
inexpensive in large quantity. Any other permanent magnetic
material might be substituted.
For a permanent magnet focus system capable of focusing, for
example, a one megavolt, 1,000 ampere beam, the energy stored in
the focusing field is 0.69 joule per meter of length; therefore,
16,000 cubic inches of one million gauss-oersted magnetic material
per mile of line length would be required. The magnets 22 are
placed external to the vacuum envelope 15 so as not to contaminate
the vacuum. A tubular magnetic shield 41 surrounds magnets 22. In a
typical example, the magnetic shield 41 may comprise a spiral wound
soft iron tape 0.010 inch thick. The focusing magnetic field
required inside of the pipe 15 for focusing the beam is in the
range of 100 to 200 gauss.
The vacuum envelope 15 is evacuated by a plurality of glow
discharge getter ion vacuum pumps, such as VacIon pumps
commercially available from Varian Associates of Palo Alto, Cal.
These pumps have no moving parts, produce extremely clean vacuums
free from any oil contamination and consume very small amounts of
power when pumping on a closed system. In addition, these pumps
have very long life under these conditions. Approximately 18
8-liter per second vacuum pumps 23 are required for each mile of
length of the pipe 15. Such a vacuum system would consume
approximately 0.54 watt per mile (3,000 volts at 180
microamperes).
Under certain operating values of beam voltage and current and as a
function of the diameter and length of the pipe 15, microwave
electromagnetic interaction may be obtained between space charge
waves of the beam 19 and microwave energy propagating within the
pipe 15. This results in undesired velocity and current modulation
of the beam as well as the generation of undesired amounts of
microwave energy within the pipe 15. Accordingly (See FIG. 4), wave
traps 5 or other means of coupling a lossy material to the
microwave electromagnetic fields within the pipe 15 are located
along the pipe for absorbing the undesired microwave energy to damp
out undesired microwave oscillations. In a typical mode trap 5, an
evacuated chamber 6 containing an array of resistive card wave
energy absorbers 7 is coupled to the microwave fields of the pipe
15 via a suitable coupling slot or hole 8. As an alternative, the
inside wall of the pipe 15 may be coated with a lossy coating of a
lossy allow of Al, Fe and Co, such as Kanthal.
As another alternative, the pitch of the spiraling quadrupole beam
focus magnet structure is varied by, for example, .+-. 20% in a
random way to avoid cumulative fast wave beam-field interactions
and their resulting oscillations.
At the receiving location 16 the beam decelerating section 18,
similar to the beam accelerator section 13 except turned
end-for-end, serves to decelerate the beam to a beam voltage as
close as possible to the voltage of the cathode emitter 31, namely,
V.sub.k without reflecting beam current to the decelerator section
18.
The decelerated beam is received within the depressed beam
collector structure 17; the collector operates at a potential
V.sub.c approximately equal to the decelerated beam or source
potential V.sub.k. In a preferred embodiment, the depressed
collector structure 17 is of the type disclosed and claimed in U.S.
Pat. No. 3,453,482 issued 1 July 1969 and preferably includes the
improvement of the center spike as disclosed and claimed in
copending U.S. Pat. No. application Ser. No. 283,433 filed 24 Aug.
1972, both assigned to the same assignee as the present invention.
The depressed collectors of this type are very efficient and
operate with beam collecting efficiencies of 98%, i.e., only 2% of
the transmitted power is lost in the collector 17. However, in one
gigawatt transmission system with 98% beam recovery, there is still
20 megawatts of power which must be dissipated in the collector
17.
The collector 17 is preferably of the water or liquid cooled type
disclosed in U.S. Pat. Nos. 3,374,523 issued 25 Mar. 1968 and
3,414,757 issued 3 Dec 1968 and assigned to the same assignee as
the present invention. The collector should be scaled in size such
that the power dissipation on the interior surfaces thereof results
in a power density of below 1 kilowatt per square centimeter.
One main advantage of the electrical power transmission system 11
of the present invention is that it provides means for transmitting
a gigawatt quantity of electrical power at relatively low cost per
mile and low loss. This is because of the elimination of high
voltage, the source of most of the problems in conventional
transmission lines from the main portion of the line. Energy is
transmitted instead of kinetic form by means of a beam of
electrons. The high energy electron beam is launched, transmitted
through evacuated pipe 15 and recovered with losses low enough to
be competitive with conventional overhead high voltage transmission
lines. The economic savings in right-of-way cost and ecological
advantages of less ozone generation and elimination of unsightly
towers excavations in either underground or above ground
installations justify its use. Pipe 15 may be installed underground
in a ditch or, for above ground systems, can simply lie on the
surface or be supported by bents, a catenary, or existing bridge
structures.
FIGS. 5-8 show a polyphase electric power transmission system 42
similar to that previously described with regard to FIGS. 1-2 with
the exception that the rectification and inversion functions have
been combined with the transmission system. More particularly, six
separate electron guns 12 and their respective beam accelerator
sections 13 are disposed at the transmitting location 14 for
projecting six separate electron beams into respective pipes 15'.
Each pipe is magnetically shielded and provides strong magnetic
focusing and converges toward the common magnetically shielded and
magnetic focused pipe 15 leading to the receiving location 16. A
magnetic deflection yoke 43 is provided at the confluence of the
respective beam input pipes 15', at the transmitting location 14,
for sequentially and selectively deflecting the electron beams from
respective ones of the electron guns 12 into common pipe 15.
Similarly, at the receiving location 16 the main transmission pipe
15 splits into six separate pipes 15' each leading to a respective
beam decelerator section 18 and a depressed collector 17. A
magnetic deflection yoke 44 is provided for sequentially deflecting
the output electron beam into respective ones of the output pipes
15'. Deflection yokes 43 and 44 are of the conventional type used
in cathode-ray tubes or in accelerator-to-target deflection systems
of high energy particle accelerating machines such as at the
Stanford Linear Accelerator Center at Stanford, Cal.
Input power to be transmitted to the receiving location 16 is
supplied to the transmitting location 14 from a suitable generator,
not shown. The three phase input power is applied to the primary
windings 45 of an input transformer 46. The primary windings 45 are
connected in the delta configuration as shown in FIG. 6 and the
secondary windings 47 of the input transformer 46, as shown in FIG.
7, are each center-tapped at ground or V.sub.b potential and wound
in a 6 phase configuration. The opposite ends of the center-tapped
windings 47 are coupled to the cathode emitters of each of the pair
of guns 12 for a respective phase of the three phase transmission
system. For example, for the A phase, one end of the center-tapped
winding 47 is coupled to one gun and the other end of the
center-tapped winding 47 is coupled to the other gun. Due to the
self rectification characteristic of the thermoionic diodes, each
of the guns of a particular phase would, in the absence of a
control electrode 29, conduct only during one-half of the cycle,
such conduction halves being 180.degree. out of phase with respect
to each other.
Control signals are applied to the control grids 29 via modulators
48 for limiting the beam conduction phase angle for each gun 12.
More particularly, the conduction phase angle is limited to a first
approximation to 2p/360.degree. where p is the number of phases for
the polyphase transmission system 42. In the case of a three phase
system utilizing six electron guns, the beam conduction angle for
each of the guns is limited to 60.degree., and would normally be
centered on the time when the applied voltage is a maximum. The
operation of the magnetic deflection yoke 43 is synchronized with
the potentials applied to each of the respective guns via leads 49
which feed into a deflection control circuit 51 and which serve to
synchronize the input and output beam deflectors 43 and 44,
respectively, such that the beam current is directed into the
proper beam collector 17.
At the power receiving location 16, each phase of the three phase
system has its respective pair of collectors connected to opposite
ends of one of three center tapped primary windings 52 of an output
transformer 53. The secondary windings 54 of the output transformer
53 are connected in the delta configuration as shown in FIG. 6. An
output voltage Vc is sensed across each of the respective phases of
the output primary windings 52 via voltage sensors 55 (See FIG. 9)
and these voltages are fed back to the gun modulators 48 to control
the amount of the beam current drawn from each of the respective
guns such that the power delivered to the load is equal to the
power demanded by the load as more fully described below with
regard to FIGS. 17-19.
Due to the relatively short conduction phase angle for each of the
electron guns and therefore the short phase angle for current
delivered to each of the respective collectors, the current pulses
delivered to the primary windings 52 of the output transformer 53
will be rich in harmonics of the power frequency.
However, the connection of the collectors 17 of each phase (three
phase system) to opposite ends of the respective output primary
windings 52 serves to cancel out the even harmonics of the power
frequency (i.e. 60 Hz). In addition, the balanced connection of the
collectors 17 relative to the centertap in the output primary
windings 52 serves to prevent undesired saturation effects of the
transformer 53 due to the DC component of current flowing through
each primary winding 52. The third harmonic and multiples thereof
are effectively cancelled by using the delta connected secondary
windings. The 5th, 7th, 11th, 13th, etc. odd harmonics are bypassed
by means of multiplicity of series resonant filters, such as filter
56, tuned for each respective odd harmonic and connected in shunt
with the respective primary output winding 52.
The delta winding connection for cancelling of the third harmonic
and multiples thereof is only operative in a three phase system or
multiples of a three phase system. Accordingly, in a single phase
or two phase system series resonant bypass filters 56 are employed
for each third harmonic or odd multiples thereof, such as 3rd, 9th,
15th, etc.
Typically, the lowest harmonic will have the largest amplitude.
Thus, a fifth harmonic filter 56 may suffice dependent upon the
shape of the beam current pulse. Also the beam current pulse is
preferably shaped by a waveform shaping circuit which shapes the
control electrode potential to reduce the fifth, seventh, eleventh,
thirteeth, etc. harmonics of beam current. Such a wave shaping
circuit is contained within modulator circuit 48 and is operative
upon the shape of the signal fed to control electrode 29.
For example, for a beam current pulse train as shown in FIG. 10,
there is a certain value of beam conduction angle which will reduce
any given harmonic of the pulse repetition frequency of beam
current to zero. Thus, the wave shaping circuit controls the beam
conduction angle to minimize the certain harmonic. Each of the
primary windings 42 has a bypass capacitor 57 connected in parallel
with the inductance of each of the primary windings 52 for
bypassing harmonics of higher order than those filtered by filters
56.
Referring now to FIg. 9, there is shown an alternative electron
beampower transmission system similar to that previously described
with regard to FIG. 5 with the exception that a pair of
magnetically shielded and magnetically focused pipes 15 are
employed for each phase of the AC power transmission system. For
example, in a single phase system two pipes 15 are employed. The
pair of electron guns 12 for each phase of the AC power
transmission system are connected in 180.degree. out of phase
relation by connection of the cathode emitters 31 to opposite ends
of a center tapped secondary winding 47 of the input transformer
46.
The advantage of the AC power transmission system of FIG. 9 is that
the conduction phase angle can be increased to a value
substantially in excess of the 60.degree. conduction phase angle
for the three phase system of FIG. 5. This reduces the harmonic
content in the beam current supplied to the output transformer
53.
However, it is generally undesirable to employ the whole
180.degree. beam conduction phase angle during each conductive half
cycle of the applied alternating beam potential. The reason for
this is that at relatively low values of beam voltage V.sub.b, the
electrons have relatively low velocities and thus correspondingly
relatively long transit times through the pipe 15. In such a case,
some overtaking of the slow electrons may be obtained by subsequent
fast electrons. This has a deleterious effect upon the beam
collector efficiency since, for high collector efficiency, all the
electrons at a given instant in time should have the same velocity.
Also, such overtaking will cause distortion of the current
waveform, usually increasing the unwanted harmonic content thereof.
Also, the beam focus system, depending upon the particular magnetic
focusing scheme employed, may not properly focus electrons over
wide ranges of electron velocities.
Therefore, it is preferred to limit the conduction of beam current
to only a portion of the cycle of applied beam voltage
corresponding to a value of bean current greater than one-sixth of
the peak or maximum beam current, i.e. I.sub.MAX /6.
Referring now to FIG. 10, there is shown the waveforms for beam
current I.sub.b, beam voltage V.sub.b and control electrode voltage
V.sub.g1 and V.sub.g2. The beam conduction angle is readily
controlled by applying a fixed DC negative bias voltage V.sub.a to
each of the respective control electrodes 29 such that the beam
conduction angle is limited to that portion of the cycle
corresponding to a respective grid voltage exceeding the cathode
voltage V.sub.k. The beam conduction angle can then be varied and
controlled as desired by increasing or decreasing th magnitude of
the dc negative grid bias voltage V.sub.a.
Returning again to FIG. 9, it is desirable to control the power
factor of the load as reflected into each of the primary windings
52 of the output transformer 53 such that the collected beam
current I.sub.c is in phase with the respective collector potential
V.sub.c. Accordingly, a continuously variable reactance, such as
that provided by a synchronous condenser 58, is preferably
connected across each of the respective output delta connected
secondary windings 54. A voltage is derived which is proportional
to the collected beam current I.sub.c. This voltage is derived from
a current transformer 59 connected between the centertap of the
primary winding 52 and the pipe 15. This voltage is fed to one
input of a phase comparator 60 for comparison with the phase of the
respective collector voltage V.sub.c as derieved from the sensor 55
to derive an error output which is fed to the field control of the
synchronous condenser 58 for causing the condenser 58 to take the
proper value of reactance to bring the beam collector current
I.sub.c into phase with the collector voltage V.sub.c. The power
factor control circuitry of FIG. 9 is also utilized to advantage in
the system of FIG. 5.
Referring now to FIG. 11, there is shown an AC power transmission
system similar to that of FIG. 9 wherein a common magnetically
focused and magnetically shielded pipe 15 is employed for each
phase of the AC power transmission system. More particularly,
convergent and divergent pipes 15', as previously described with
regard to FIG. 5, are employed at the trasmitting and receiving
locations, respectively, for feeding the electron beams from the
guns of each phase into the common pipe 15 and out of the common
pipe to respective collectors of each phase. The deflection of the
beams at the transmitting and receiving locations is obtained by a
deflection control circuit driving each of the deflecting magnets
43 and 44 is response to inputs derived from the respective guns.
The advantage of the system of FIG. 11 over that previously
described with regard to FIG. 9 is that only one pipe 15 is
required for each phase of the AC power transmission system.
Referring now to FIG. 13, there is shown an alternative embodiment
to that portion of the structure of FIG. 11 delineated by line
13--13. More particularly, the input deflection magnet 43 is
replaced by strong magnetically focused and convergent input pipes
15". However, magnetic deflection is still required at the
receiving location.
Referring now to FIG. 12, there is shown an alternative three phase
power transmission system 59 incorporating alternative embodiments
of the present invention. More particularly, the system is similar
to that previously described with regard to FIG. 5 with the
exception that only one pipe 15 is provided for each phase of a
three phase system and collection of beam current through each
phase of the three phase system occurs only once per cycle at the
power frequency, as contrasted with twice per cycle at the power
frequency as indicated in FIG. 10. The windings 47 and 52 have only
three phases as contrasted with six phases as shown in FIG. 7.
The power transmission system 59 of FIG. 12 has the advantage of
simplicity in that it provides only one pipe 15 per phase of the
three phase system but it has the disadvantage that the harmonic
content of the current delivered to the primary windings 52 of the
output transformer is greater than that obtained in the system of
FIg. 9.
Referring now to FIGS. 14 and 15, there is shown an alternative
multiple pipe polyphase A.C. transmission system 62. Transmission
system 62 is similar to that of FIG. 5 with the exception that a
pair of pipes 15 is provided for each phase of the polyphase
system, as shown in the system of FIG. 9. In this system 62, the
beam current through each of the pipes 15 does not have to be
limited to 360/2p.degree. of phase angle where p is the number of
phases. In the case of three phase system, the conduction of
current is preferably limited to a phase angle such that the
current conducted has a value greater than one-sixth Imax, where
Imax is the peak beam current for each phase. This means that
current is conducted from each gun for approximately
120.degree.-140.degree. of phase angle of the input voltage
waveform, as shown in FIg. 10. This reduces the harmonic content in
the current flowing in the primary windngs 52 of the output
transformer 53. One advantage of the system of FIG. 14 is that the
filtering of undesired harmonics in the output of the transformer
53 is simplified at the expense of additional pipes 15. A second
advantage is the elimination of the magnetic deflectors 43 and
44.
A further advantage of the system of FIG. 14 is that the beam
current return paths are separate for each beam to prevent cross
flow of beam return current flow with attendant potential
differences between the various pipes 15 as encountered with
unbalanced loads. However, the windings 47 and 52 are balanced in
the transformers 46 and 53 to avoid D.C. saturation of the
transformer cores, i.e., bucking connected for beam current
flow.
As an alternative to the system 62 of FIG. 14, the number of pipes
15 can be reduced to one per phase of the polyphase A.C. system by
using a common pipe 15 per phase and employing the magnetic
deflection system of FIG. 11 for sequentially deflecting beams from
respective pairs of guns 12 into and out of the respective common
pipe 15. In this latter system, the input magnetic deflection can
be eliminated since the input magnetic pipes 15' will focus the
respective beams sufficiently to allow the individual beams to
negotiate the bends in the pipes 15' at the confluence of the pipes
15 with the common pipe 15 as shown in FIG. 13.
Referring now to FIG. 16, there is shown an alternative
transmission system 72 of the present invention. The power
transmission system 72 of FIG. 16 is similar to that of the system
of FIG. 1 with the exception that RF accelerator means 73 are
employed for bunching and accelerating the bean to relatively high
energies, for example, 100 MeV.
The radio frequency accelerator 73 comprises a plurality of
individual cavity resonators 74, as of 250 such cavities,
sequentially arranged along the beam path 19 for successive
electromagnetic interaction with the beam for velocity modulating
the beam with RF energy at the resonant frequency of the resonators
74. In a typical example, the resonators 74 are of the folded half
wavelength type as used in the accelerator at the National
Accelerator Laboratory of Batavia, Ill. The resonators are tuned to
a suitable radio frequency, as of 30 megahertz, and are driven in
the proper phase relation from a 30 megahertz oscillator 75 via
phase shifters 76 and power amplifiers 77.
The power amplifiers 77 are preferably conventional tetrodes
providing high efficiency, i.e., greater than 90% in class C
operation with small angle of current flow and possibly third
harmonic squaring of the plate voltage. Phase-locked magnetron
oscillators could also be utilized as a source of microwave energy
for driving the cavities 74, but a lower frequency has the further
advantage of decreasing debunching effects due to both velocity
spread and space charge effects.
For a finite beam of small diameter in pipe 15, the longitudinal
plasma frequency is proportional to the driving frequency. The
plasma frequency for a 30 megahertz driving frequency is about
2,000 hertz for a 10 ampere, 100 megavolt beam. The corresponding
debunching wavelength is 150 kilometers.
It is desired to maintain the electron bunches for more efficient
RF energy extraction at the receiving location 16. Accordingly, an
inductive cavity, i.e. a cavity resonant slightly above the
frequency of the RF driving energy, is placed on the beam 19 every
few kilometers, such as 10. These rebunching cavities 78 are
excited by the bunched beam entering the cavity and the fields of
the cavity interact back on the electron beam to velocity-modulate
the bunched beam in such a manner as to cause a rebunching of the
electron. Thus, the electron beam 19 is received at the receiving
location 16 as a well bunched current density modulated beam of
high velocity, as of 100 MeV. The beam is bunched at the frequency
of the radio frequency accelerator, such as 30 meghertz.
At the receiving location a plurality of decelerating cavities 79,
tuned to the frequency of the radio frequency energy imparted to
the beam, are successively coupled to the beam for extracting
kinetic energy therefrom and converting same to RF energy. The
decelerating cavites 79 are substantially identical to accerlating
cavities 74 and each cavity is capable of generating a relatively
high RF voltage thereacross on the order of 400 kV per cavity.
A rectifier load 81 is connected to each of the cavity resonators
79 for rectifying the RF energy extracted from the beam. The
rectified DC energy is applied to an output coaxial line 82. The
output DC energy on line 82 is fed to the input of an inverter 24
for producing three phase output power on lines 25-27.
Each of the resonators 79 of the output decelerator section 83
serves to extract kinetic energy from the beam. A sufficient number
of output resonators 79 is provided for extracting the kinetic
energy imparted to the beam by means of the accelerator section 73.
After the RF energy has been extracted from the beam, the beam may
be collected on the pipe 15 or in a collector 17 coupled to the end
of the pipe 15. As in the other embodiments, an output signal is
derived from the inverter which is a measure of the power demanded
by the load. This load demand signal is fed to one input ot the
control circuit 28 for controlling the beam current such that the
power delivered to the load is equal to the power demanded by the
load.
The inverter 24 may be replaced by two additional sets of
rectifiers 81 which may comprise, for example, high power triodes.
Each set of triode rectifiers is connected to a respective output
bus similar to bus 82. There is one output bus for each phase of a
three phase system. Each output bus is connected to a primary
winding of a three phase power transformer. The respective sets of
triode rectifiers are sequentially gated at the AC power frequency,
as of 60 Hertz, with 120.degree. phase shift between each of the
output power buses.
As an alternative, the gun 12 may be gated at the 60 Hertz power
frequency and the three sets of output triode rectifiers
synchronized with the 60 Hertz pulses of the beam.
An advantage to the high velocity electron beam transmission system
of FIG. 16 is that the self magnetic field of the beam almost
completely overcomes the repulsive space charge forces generated
within the beam.
As an alternative to varying the beam current in the system of FIG.
16, for matching the power delivered to the load to the power
demanded by the load, the beam current is maintained constant and
the number of resonators 74 employed for accelerating the beam
varied in accordance with the output power demanded by the
load.
Power is tapped off the beam intermediate the transmitting location
14 and the receiving location 16 by providing an output resonator
79 coupled in wave energy exchanging relation to the RF modulated
beam and excited by the beam. As with the output resonators 79, the
RF power extracted via the cavity 79 is rectified via rectifier 81
and fed to a second load 80. The load 80 may comprise an inverter
24 for providing three phase output power or three sets of
rectifiers may be employed which are sequentially switched into a
primary winding of a three phase transformer for providing
inversion of the power to the load.
The intermediate output power tap comprising the resonator 79,
rectifier 81, and load 80 is also utilized with any of the other DC
transmission systems such as 11, 42, or FIG. 9, by pulse modulating
the beam at the resonant frequency of the cavity 79 without the
necessity of the accelerating and decelerating RF structures 73 and
83.
Referring now to FIGS. 17-19, a number of circuits are shown for
controlling the beam at the transmitting end 14 to insure that the
electrons collected by the collector 17 strike the collector at
nearly zero velocity so that no large amount of power is dissipated
in the collector 17 (i.e. the system efficiency will be high). This
zero velocity condition is realized when the cathode potential
V.sub.k = V.sub.2 = I.sub.b R.sub.L, where V.sub.c is the collector
potential, I.sub.b is the electron beam current, and R.sub.L is the
load resistance presented to the collector 17.
There are two ways of accomplishing this end. First, both V.sub.k
and I.sub.b are adjusted simultaneously so that V.sub.k = v.sub.c.
Secondly the beam current I.sub.b is adjusted to be equal to
V.sub.k /R.sub.L by sensing the collector voltage V.sub.c and
setting the current I.sub.b so that V.sub.c equals V.sub.k.
All of the above methods rely upon a comparison circuit for
comparing V.sub.c and V.sub.k to derive the error control signal.
The comparison circuit comprises an analog summing amplifier or a
combination of analog-to-digital converters and digital summing
logic of the type shown in FIG. 19. More particularly, with regard
to FIG. 19, the collector voltage V.sub.c is fed to a first
analog-to-digital converter 91 to derive a first digital output
proportional to V.sub.c. The cathode potential V.sub.k is applied
to a second analog-to-digital converter 92 to derive a second
digital output proportional to V.sub.k. The two digital output are
applied to a logic subtractor 93 for substraction therein to derive
the error signal in the form of a digital output which is thence
fed to a digital-to-analog converter 94 to derive an analog control
signal (error signal).
The error signal, either analog or digital, which is proportional
to the difference between the cathode potential V.sub.k and
collector potential V.sub.c controls the system parameter selected
for control.
Referring now to FIG. 17, there is shown a control circuit for
matching the cathode potential V.sub.k to the collector potential
V.sub.c. In the circuit of FIG. 17 the cathode potential V.sub.k is
varied to be equal to the collector potential V.sub.c while
allowing the beam current to vary with variations in the cathode
potential. The beam current I.sub.b will vary as a function of the
cathode potential V.sub.k according to the relation: I.sub.b =
KV.sub.k .sup.3/2 where K is the beam perveance.
In the variable voltage cathode power supply 38, the difference
signal coming from the comparison circuitry 28 is used to vary the
mechanical position of a variable mutual inductance transformer,
such as an Inductrol, therein for varying the output potential
V.sub.k. As an alternative, the error signal derived from the
output of the comparison circuit 28 is utilized as the input signal
to a phase control regulator incorporated in the variable voltage
power supply 38 for varying V.sub.k.
Referring now to FIg. 18, there is shown the second method for
matching the cathode potential V.sub.k to the collector potential
V.sub.c. The difference signal from the output of the comparison
circuit 28 is amplified and applied to the control electrode 29 in
the proper phase to reduce the difference signal to zero. In this
case, the beam perveance K will vary up to some value which is
dependent upon the system voltage (a constant), and the maximum
system current. The beam focus system, such as the quadrupole, is
designed to handle such a range of perveance.
In a further variation of the control system of FIG. 18 the peak
current of the beam I.sub.b is maintained constant while the
average current is varied in accordance with the error signal
derived from the comparison circuit 28. The average current is
varied by creating (in a pulse modulator 96) a repetitive
rectangular pulse signal applied to the control grid 29. The duty
factor of the repetitive pulse is varied by the output of the
comparison circuit 28 in such a manner that the difference between
the collector potential V.sub.c and the cathode voltage V.sub.k
tends toward zero. When this control variation is used, there must
be enough capacitance in the collector circuit 17 and the
repetition frequency of the current pulse is high enough so that
the collector voltage V.sub.c does not follow the rapid variations
of the beam current I.sub.b. The pulsed version of the control
system of FIG. 18 imposes the most easily met requirements on the
beam focus system. Also, the method of FIG. 18 (whether or not
pulsed) is the most suitable when it is desired to deliver aa
varying amount of power to the receiving end 16 at a constant
collector voltage such as would exist in the typical power
system.
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