U.S. patent number 3,866,132 [Application Number 05/474,556] was granted by the patent office on 1975-02-11 for moving foil stripper for a particle accelerator.
This patent grant is currently assigned to The United States of America as represented by the United States Atomic. Invention is credited to Andrew J. Gorka, Jr..
United States Patent |
3,866,132 |
Gorka, Jr. |
February 11, 1975 |
MOVING FOIL STRIPPER FOR A PARTICLE ACCELERATOR
Abstract
Thin foils for stripping a particle beam are stored on the edge
of a disk spinning in the accelerator vacuum. Cutting a foil at one
edge releases the foil to project beyond the disk for insertion
into the beam at a time determined by controlling the phase of the
disk. A wiper removes a spent foil from the disk. The foil release
and wiper are operable from a remote location.
Inventors: |
Gorka, Jr.; Andrew J.
(Naperville, IL) |
Assignee: |
The United States of America as
represented by the United States Atomic (Washington,
DC)
|
Family
ID: |
23884044 |
Appl.
No.: |
05/474,556 |
Filed: |
May 30, 1974 |
Current U.S.
Class: |
315/505; 313/149;
313/359.1; 976/DIG.437 |
Current CPC
Class: |
G21K
1/14 (20130101) |
Current International
Class: |
G21K
1/00 (20060101); G21K 1/14 (20060101); H05h
007/00 () |
Field of
Search: |
;328/228,233
;313/359,363,146,149 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Demeo; Palmer C.
Attorney, Agent or Firm: Horan; John A. Churm; Arthur A.
Reynolds; Donald P.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An apparatus for timed insertion of thin stripping foils into
the beam of a particle accelerator comprising:
a circular disk disposed in a vacuum environment in a plane
perpendicular to the beam and substantially tangent to the
beam;
a rotating drive connected to the circular disk to rotate said
disk, said rotating drive including means for adjusting the phase
of rotation of said circular disk relative to the beam;
a stripping foil connected to the circular disk at a first edge of
said foil near the circumference of the disk and at a second edge
of said foil radially inward from said first edge; and
means for releasing said foil near said second edge while rotating
said disk,
whereby release of said foil and adjustment of said phase of
rotation effects timed insertion of said foil into said beam.
2. The apparatus of claim 1 wherein the rotating drive
comprises:
a disk shaft connected to the disk on the axis of the disk;
a motor shaft coaxial with the disk shaft and disposed outside the
vacuum environment;
means connected to the disk shaft and the motor shaft to couple
said shafts in a rotating linkage;
a synchronous a-c motor coupled to said motor shaft;
means connected to said synchronous a-c motor to vary the phase of
rotation thereof.
3. The apparatus of claim 1 wherein said means for releasing said
foil comprise:
an electrical heater wire connected to said disk and to said
stripping foil at said second edge;
a source of a-c electrical energy;
an electrical transformer having a primary winding connected
electrically to said source and a secondary winding connected
electrically to said electrical heater wire to heat said heater
wire and thereby release said foil.
4. The apparatus of claim 1 wherein said disk includes a plurality
of stripping foils, each of said foils having a first edge
connected to said disk near the circumference thereof and a second
edge connected to said disk at a point radially inward from said
first edge, further wherein said apparatus includes means for
releasing each of said plurality of stripping foils near said
second edges while rotating said disk.
5. The apparatus of claim 4 wherein said means for releasing said
foil sections one at a time comprise:
a plurality of electrical heater wires each connected to said disk
and each connected to one of said foil sections at a second edge
thereof;
a source of a-c electrical energy;
a source of d-c electrical energy;
an electrical transformer having a secondary winding that is
rotatable about an axis while maintaining transformer coupling with
a primary winding, said primary winding secured in a stationary
position, said secondary winding connected by ratcheting means to
said rotating disk to rotate with said disk and to move axially
with respect to said primary winding in response to a d-c current
from said source of d-c electrical energy to operate said
ratcheting means and index said secondary winding with respect to
said disk to connect to a particular one of said plurality of
heater wires to establish a path therethrough for a-c current from
said source of a-c electrical energy to heat and melt through said
one of said foil sections and thereby release said foil.
6. The apparatus of claim 1 comprising in addition means for
removing said released foil from said disk while rotating said
disk.
7. The apparatus of claim 6 wherein said means for removing said
released foil comprise:
a stationary solenoid disposed at the periphery of said disk;
and
a plunger connected to the solenoid and extendable by actuating the
solenoid to project into the path of said rotating released foil
and thereby catch said foil and tear said foil from said rotating
disk.
8. The apparatus of claim 1 wherein the stripping foil is a
vapor-deposited polymer of p-xylene (polyparaxylylene) having a
nominal thickness less than 4000 Angstroms and having a surface
cross-section larger than the cross-section of the beam.
9. An apparatus for making timed insertion of stripping foils into
the beam of a particle accelerator comprising:
a disk disposed in the vacuum environment of the particle
accelerator in a plane perpendicular to the beam and tangent to the
beam;
a rotary coupling connected to said disk to couple into the vacuum
environment;
a shaft connected to said rotary coupling;
a d-c motor located outside the vacuum environment and connected to
said shaft to effect thereon rotation of said disk;
a synchronous motor connected to said shaft to effect synchronous
rotation of said shaft and hence of said disk;
a phasing motor connected to said synchronous motor to vary the
phase thereof;
a plurality of foils of poly-paraxylylene, each attached to the
disk at an outer edge near the circumference of the disk and at an
inner edge located radially inward on the disk from the outer
edge;
a plurality of heating wires connected to the disk, each heating
wire touching one of said plurality of foils near the inner edge of
the foil;
a source of electrical energy;
remotely controllable indexing means for establishing an electrical
connection to a selected one of said plurality of heater wires and
to said source of electrical energy and releasing one of said
plurality of heater wires; and
remotely controllable wiping means disposed in the vacuum
environment to wipe the circumference of the disk and thereby
remove a foil from the disk.
Description
CONTRACTUAL ORIGIN OF THE INVENTION
The invention described herein was made in the course of, or under,
a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.
BACKGROUND OF THE INVENTION
This invention relates to the means of charge exchange known as
stripping. It provides improved operation of various kinds of
accelerators of charged particles.
Stripping is a well-known technique in the field of
charged-particle accelerators. As a part of the process of
acceleration, it is often desirable to change the state of charge
of the particles in a beam. This is done by causing the particles
to collide with other particles. One way of stripping is to
introduce molecules of gas into the path of the beam. Another way
is to place a thin foil in the path of the beam. The objective is
to assure that a high percentage of the number of particles in the
beam makes a predetermined number of collisions with stripping
molecules, and that a small percentage collides with a different
number of stripping molecules. The latter objective requires that,
if a foil is used, it be very thin.
Various materials are used to strip. In an example of the instant
invention, the stripping atom is carbon which is disposed in the
beam path as a very thin foil of an organic polymer. The particular
foil used in the practice of the present invention is a thermally
stabilized polymer of p-xylene, approximately 3500 A in thickness,
formed by vapor deposition. Such foils are usable in thicknesses up
to 6000 A. Criteria for foil selection included the following. A
material for stripping should comprise atoms of low atomic numbers,
preferably at and below the atomic number of carbon. Such a
material must withstand the energy of the beam without melting or
decomposing. It must have sufficient tensile strength to support
itself in pieces that span a cross section larger than the cross
section of the beam to be stripped. It must maintain this tensile
strength long enough to provide a reasonable period of accelerator
operation, of the order of at least an hour, before requiring
replacement. It must be self-supporting without a backing and
without a framework at all edges. It must withstand the vacuum
environment in the interior of a particle accelerator and it must
not produce or release particles to comprise a virtual leak.
The stripping foil must be disposed in the beam for stripping at a
predetermined time and the means of disposing the foil must permit
remote operation. There must be a way to remove and replace foil
that has been damaged by radiation, and this way should minimize
accelerator downtime. The means for disposing foils must be
unaffected by radiation, high vacuum, and the magnetic field of the
accelerator.
It is an object of the present invention to provide an improved
means of stripping a beam of charged particles.
It is a further object of the present invention to provide a means
of disposing a thin film of poly-paraxylylene in a beam of charged
particles to effect charge exchange.
It is a further object of the present invention to provide a
remotely operable means of placing and replacing stripping foils in
a particle beam.
It is a further object of the present invention to provide a means
of placing stripping foils in a particle beam under conditions of
radiation, high vacuum, and high magnetic field.
Other objects will become apparent in the course of a detailed
description.
SUMMARY OF THE INVENTION
Thin foils of an organic polymer are mounted on the edge of a
spinning disk for timed insertion into the beam of a particle
accelerator. Collision of the beam with the foil effects stripping.
Foils are stored on the disk and are released at one end one at a
time in response to a remote signal. A released foil projects from
the disk due to centrifugal force. Spent foils are removed from the
edge of the disk, also in response to a remote signal .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional side view of an apparatus for the practice of
the present invention;
FIG. 2 is a top view of a portion of a tunnel of a particle
accelerator with the apparatus of the present invention;
FIG. 3 is a partial sectional view along lines 3--3 of FIG. 1;
FIG. 4 is a sectional view of part of the disk of FIG. 3, taken
along section lines 4--4;
FIG. 5 is a partial schematic end view of the disk of the present
invention taken along section lines 5--5 of FIG. 1;
FIG. 6 is a sectional side view of a portion of the disk of FIG. 5
taken along section lines 6--6;
FIGS. 7 and 8 are views of a ratchet attached to the disk of FIG.
5; and
FIG. 9 is a view of a portion of the apparatus of FIG. 1 showing
the removal of a foil.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a sectional side view of an apparatus for the practice of
the present invention. In FIG. 1, disk 10 is affixed to shaft 12
which is connected to d-c motor 14 and synchronous motor 16 through
rotary coupling 18. Motors 14 and 16 share a common shaft 19. Disk
10 is in the vacuum environment 20 of a particle accelerator, and
rotary coupling 18 makes driving contact into vacuum environment 20
from outside it. D-c motor 14 is used to start disk 10 rotating
with a controlled acceleration over a period of approximately one
minute and synchronous motor 16 drives disk 10 at synchronous speed
after starting. D-c motor 14 may also vary the load on synchronous
motor 16 during synchronous operation. Phasing motor 22 rotates the
frame 23 of synchronous motor 16 to adjust its phase and thus the
phase of disk 10.
FIG. 1 also shows vacuum port 24 which is used for pumping to
maintain vacuum environment 20 within walls 26. Gasketed seal 27
assists in maintaining vacuum environment 20. Heating and indexing
transformer 28 has a fixed primary coil 30 and a rotating secondary
coil 32 that is attached to disk 10. A wiper 34 is connected to a
solenoid 35 which is attached to one wall 26. Frame 36 holds d-c
motor 14 and synchronous motor 16 and also provides structural
support for wall 26. Frame 36 is mounted on table 38 which is
slidably mounted to base 40. Handwheel 42 provides a means to move
table 38 with respect to base 40 to position table 38 and hence
disk 10.
FIG. 2 is a top view of a portion of a beam tunnel of a particle
accelerator with the apparatus of the present invention in position
for use. In FIG. 2, beam tunnel 50 is perpendicular to disk 10 so
that a foil 48 is inserted into beam tunnel 50, and hence into a
beam 51, once per rotation of disk 10. Rotation and control of the
timing of foil insertion are controlled by phasing motor 22 of FIG.
1 and synchronous motor 16, as described above.
The disk 10 of FIG. 1 is shown from an end in FIG. 3, which is a
sectional view along section line 3--3 of FIG. 1. In FIG. 3, disk
10 is seen along its axis of rotation 44. Several foils 46 are
attached at both their ends to disk 10. One foil 48 is attached to
disk 10 at one end only, and is maintained in a radially extended
position by the centrifugal force associated with the spinning disk
10. The speed and phase of disk 10 are adjusted to place the
extended foil 48 in front of beam tunnel 50 for timed insertion
into a beam passing through beam tunnel 50. A control signal for
maintaining this speed and phase is obtained from reluctance pickup
52, which rotates with disk 10. FIG. 3 also shows wiper 34, which
can be moved externally by energizing solenoid 35 of FIG. 1 to
graze disk 10 and thus remove a foil 48 for replacement by another
foil 46. Thus, one foil 48 is extended at a time for insertion into
beam tunnel 50 to strip a beam therein. When it is desired to
replace this foil because of radiation damage or any other reason,
wiper 34 is moved to the edge of disk 10 to scrape foil 48 free and
remove it from disk 10. Another foil 46 is then selected and
released at one end to extend beyond the end of disk 10 and thus
become a foil 48. The phase of disk 10 is adjusted to place the
newly selected foil 48 into the beam at a proper time for
stripping.
The means for selecting and releasing a new foil are shown in FIG.
4, which is a sectional view of the disk and wall of the present
invention, taken along section lines 4--4 of FIG. 3. In FIG. 4,
disk 10 is attached to shaft 12, which is connected to secondary
coil 32 of transformer 28. Secondary coil 32 rotates with disk 10,
and it is free to move axially on shaft 12. Transformer 28 is
completed by primary coil 30, which is attached to wall 26 by
supports 54. Wires 56 are connected to feedthrough 58 to make an
external connection 60 to electrical source 62, which serves a dual
function. When electrical source 62 supplies a d-c voltage through
connection 60, feedthrough 58, and wires 56 to primary coil 30, an
attractive magnetic force is generated along shaft 12. This force
causes motion of secondary coil 32 toward primary coil 30, which
steps ratchet mechanism 64 by one step to select a particular foil
46 for release. An a-c voltage of a smaller value, insufficient to
operate ratchet mechanism 64, is obtained from electrical source 62
to effect the release of the foil selected by heating it at one
end.
Stepping and releasing are better understood by referring to FIG.
5, which is a partial schematic end view of disk 10 taken along
section lines 5--5 of FIG. 1. In FIG. 5, secondary coil 32 is shown
schematically, together with its terminals 66, which are connected
to a pair of wires 68 that lead to a heating element 70. When
primary coil 30 of FIG. 4 is excited by the application of an a-c
voltage, secondary coil 32 then serves as a secondary source of
electrical energy that heats heating element 70. Each foil 46 has
previously been secured at an inner end 72 and an outer end 74.
Heating element 70 is placed in contact with a foil 46 so that,
when heated, it melts through the foil 46. The centrifugal force
associated with rotation of disk 10 then flips a foil 46 into the
position of foil 48, attached to disk 10 only by the outer end 74.
Each foil 46 has its heating element 70. A particular foil 46 is
selected for release by stepping the terminals 66 of secondary coil
32 to make contact with the pair of wires 68 serving the particular
foil 46. In this way the disk 10 serves as a magazine for foils
46.
The means of storing and displaying foils are better seen in FIG.
6, which is a partial sectional view of the disk 10 of FIG. 5,
taken along section line 6--6. In FIG. 6, foil 46 is secured to
disk 10 by tape 76 at outer end 74 and tape 78 at inner end 72.
Wires 68 comprise part of an electrical circuit through heating
element 70, which touches foil 46 near inner end 72. When heating
element 70 is energized while disk 10 is spinning, foil 46 is
melted through and released to extend beyond outer end 74, thus
projecting as a foil 48.
The ratchet mechanism 64 is shown in two expanded isometric views
in FIGS. 7 and 8. In FIGS. 7 and 8, shaft 80 is connected to and
rotates with disk 10 of FIG. 1. Shaft 82 is coaxial with shaft 80
and is connected to and rotates with secondary coil 32 of
transformer 28 of FIG. 1. Driving pawl 84 has a driving surface 86
that is offset to form an angle with shafts 80 and 82. Ratcheting
pawl 88 has no such offset. Both driving pawl 84 and ratcheting
pawl 88 engage cog 90. Driving pawl 84 and ratcheting pawl 88 are
connected to shaft 82, and cog 90 is connected to shaft 80.
FIG. 7 shows ratchet mechanism 64 in a relaxed position, evident
from the fact that driving surface 86 engages cog 90 near the tip
92 of driving pawl 84. FIG. 8 shows ratchet mechanism 64 in the
actuated position, evident from the fact that shaft 80 is closer to
shaft 82 than in FIG. 6. Motion of shafts 80 and 82 toward each
other causes driving surface 86 to move along cog 90, rotating cog
90 with respect to driving pawl 84. Ratcheting pawl 88 steps over
one tooth of cog 90, and holds cog 90 against turning back, so that
driving pawl 84 steps back over a tooth of cog 90 when ratchet
mechanism 64 is returned to the relaxed position by spring 91 of
FIG. 4. Thus, by means of this operation of ratchet mechanism 64,
shafts 80 and 82 rotate with respect to one another by an angle
equal to the angle of one tooth on cog 90.
Ratchet mechanism 64 is actuated by means of electrical source 62
of FIG. 4. When a d-c current is supplied by electrical source 62
to primary coil 30 of transformer 28 of FIG. 4, secondary coil 32
is attracted magnetically toward primary coil 30. This attractive
force causes axial motion of secondary coil 32 toward primary coil
30. This, in turn, moves secondary coil 32 with respect to disk 10,
producing the ratcheting action described above.
FIG. 9 is a view of a portion of the apparatus of FIG. 1 showing
the operation of the wiper 34. In FIG. 9, disk 10 is rotating, thus
extending a foil 48 for stripping. When it is desired to remove
foil 48 and replace it with a foil 46 as described, solenoid 35 is
energized. This causes wiper 34 to project, striking foil 48 on the
way by and tearing foil 48 from disk 10. Foil 48 may be removed
because of radiation damage from its use in stripping, or because
it did not extend properly, or because it became wrinkled or torn,
or for any other reason chosen by an operator.
The present invention has been built and applied to the Booster
Accelerator for the Zero Gradient Synchrotron (ZGS) at Argonne
National Laboratory. The Booster Accelerator is a converted
electron synchrotron which operates at a 30-hertz frequency
synchronized to the power lines. The Booster is part of a program
to increase the number of particles accelerated per pulse in the
ZGS by producing improved conditions of injection into the ZGS. It
is desirable to inject negative hydrogen ions at energies of the
order of 50 MeV into the booster and there to strip them to
positive hydrogen ions for acceleration. The injection period is of
the order of 200 to 500 msec which necessitates placement of a
stripping medium in the beam for that length of time and then its
removal. Any particles not stripped will have the wrong sign of
electric charge for acceleration and will therefore be lost from
the beam. Thus, it is desirable to insure that foil is in the beam
during the entire injection period. However, passage through the
foil of particles already stripped produces multiple scattering
which also results in a loss of particles. These conflicting
requirements make it desirable to have a stripping foil in place
during and only during injection of negative hydrogen ions at the
beginning of the acceleration period.
The stripping medium chosen to meet these requirements is a
thermally stabilized polymer of paraxylene formed by vapor
depositing this material in a vacuum environment to thicknesses of
4000 Angstroms or less. Such a foil is delicate and is very readily
damaged. The spinning disk of the present invention was found
satisfactory to store and display such foils. In operation, 56
foils were taped at equally spaced intervals along the edge of a
disk having an outer diameter of 36 inches. Each piece of foil was
taped at an outer edge of the disk and then the foil was folded
back over the tape and over a heating wire for cutoff along a
substantially circumferential line spaced radially inward on the
disk. The inner end was taped down to maintain the foil in contact
with the heating wire. The heating wire was 2-mil Nichrome disposed
circumferentially and connected by conductors to the contacts
located at the hub of the disk and connectable individually to a
heating transformer. Application of a-c power to the transformer
heated the Nichrome wire, burning through the foil in a line at the
inner end. Rotation of the disk caused the foil to fly out radially
and project from the disk. The disk was rotated by a synchronous
motor at a speed of 1800 rpm to permit a timed relationship with
the 30-Hertz accelerator frequency. The particular beam to be
stripped has an elliptical cross-section spanning approximately 2
inches in the horizontal direction and three quarters of an inch in
the vertical direction. It was convenient to place the axis of the
disk level with the beam and to insert the stripping foil from the
top of the beam, thus necessitating a foil length of at least 2
inches. The width of the foil in the direction of motion across the
beam is determined by the beam dimension in that direction, the
rotational speed of the disk, and the length of time necessary for
injection. In this case, a nominal 1 inch in the vertical direction
was adequate. However, in order to eliminate the possibility of
curling of the foil, it was found desirable to make the foil
slightly tapered in shape with the wider dimension at the disk. For
this reason, the foils used were 1 inch wide at the edge away from
the disk and tapered symmetrically to 11/2 inches in width at the
edge of the disk.
Experiments using comparable foils in stripping a proton beam
indicated that approximately 5 .times. 10.sup.18 particles passed
through the foil before destruction occurred. The pulses expected
in the present application will have a typical number of the order
of 10.sup.13 particles per pulse. When used to supply pulses for
acceleration in the ZGS, a foil should therefore be expected to
last of the order of 2 hours before requiring replacement. When
this is necessary, the solenoid is engaged to wipe the spent foil
from the edge of the disk, a d-c signal is applied to cause the
ratcheting mechanism to advance to the next foil and the next foil
is released by heating the Nichrome heater wire.
The disk was made for rapid removal from the shaft so that a
standby disk can be loaded with foils for relatively rapid
replacement when all 56 foils are spent. This is a shutdown job,
since it involves losing the vacuum environment in which the disk
is rotated. The time to change disks is of the order of an
hour.
The phase of the synchronous motor driving the disk is critical to
place the foil in the proper place at the proper time. This
placement is accomplished by rotating the frame of the synchronous
motor by a stepping motor driving a worm gear attached to the motor
frame. Two means of checking the foil placement were used. One is a
television monitor viewing an image of the foil as the foil is
illuminated by a stroboscopic light. The other is a reluctance
pickup placed on the disk and thus timed to a particular location
on the disk. Phase information is readily obtained from this signal
by use of a scaler to determine the time delay between the fixed
position and each individual foil position. The stepping motor was
used because it was believed desirable to maintain an accuracy of
the order of 10 microseconds in placing the foil. This corresponds
to a placement accuracy of 0.1.degree. or 1 part in 3600. This
accuracy is exceeded by a stepping motor making 200 discrete steps
per revolution and driving a worm gear having a reduction ratio of
80 to 1. An additional control of the accuracy of foil placement is
achieved by using the d-c starting motor as a variable loading
element after the synchronous motor has taken over the driving
function. The d-c motor can be driven to reduce the load on the
synchronous motor and facilitate maintaining synchronism.
Alternatively, the d-c motor can be used as a generator and thus a
load on the synchronous motor if necessary to damp out hunting
oscillations.
* * * * *