US 3757237 A
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United States Patent 1 Hendry Sept. 4, 1973 METHOD AND MEANS FOR REDUCING THE POWER DENSITY OF AN INTERNAL CYCLOTRON BEAM George 0. Hendry, Napa, Calif.
The Cyclotron Corporation, Berkeley, Calif.
Filed: Feb. 4, 1972 Appl. No.: 223,418
US. Cl. 328/234, 313/62 Int. Cl. I-IOSh 13/00 Field of Search 328/234; 313/62 References Cited UNITED STATES PATENTS 1l/l97l Hudson 328/234 Primary ExaminerPalmer C. Demeo Attorney-Carl Hoppe,James F. Mitchell et al.
[5 7 ABSTRACT A method and apparatus for reducing the power density of the internal beam in an isochronous cyclotron at its extraction radius or an internal target wherein the internal beam in its orbits near and at the extraction or target radius is spread axially by applying to it a timevarying electrostatic field tuned to the natural axial oscillation frequency of the beam in such orbits.
6 Claims, 4 Drawing Figures GENERATOR METHOD AND MEANS FOR REDUCING THE POWER DENSITY OF AN INTERNAL CYCLOTRON BEAM This invention relates generally to handling the internal beam in isochronous cyclotrons for extraction and for impingement upon internal targets. It more particularly is a method and means for reducing the internal beam power density at the extraction or target radius.
One object of this invention is to increase the amount of internal beam current that can be extracted from an isochronous cyclotron or be applied to an internal target without overheating the target or extraction system.
Another object of this invention is to provide a simple method and means for increasing the axial height of the internal beam in its orbits near and at the extraction or the internal target radius.
Other objects and advantages of this invention will become apparent from a consideration of the following description in connection with the drawings wherein FIG. 1 is a partially schematic cross-sectional view of a typical isochronous cyclotron in its median plane;
FIG. 2 is a top view of the axial beam spreader of this invention;
FIG. 3 is an elevational view of the axial beam spreader of FIG. 2; and
FIG. 4 is a schematic diagram of one oscillator circult useful in applying radio frequency voltage to drive the beam spreader of FIGS. 2 and 3.
The external beam extractable from isochronous cyclotrons or the usable beam for internal target impingement generally is limited by high power density on the extraction system or on the internal target, respectively. Any attempt to increase beam current increases the power density on these elements until they are destroyed by heat generated from accelerated particles impinging upon them. Beam power densities in isochronous cyclotrons are particularly high because the strong vertical focusing forces in the fringe field of the machine reduce the vertical beam dimension at the usual extraction or target radius.
This invention offsets that natural vertical focusing by increasing the internal beam height in its orbits near and at the extraction or target radius by applying to the beam a vertical time-varying electrostatic field tuned to the natural oscillation frequency of the internal beam about the median plane. The natural oscillation frequency of the internal beam about the median plane is a consequence of the self-focusing of the orbiting particles and is one-half the orbital frequency at a particular radius. The vertical time-varying electrostatic field is tuned to that frequency and drives the natural oscillation to greater amplitude. The increase in amplitude spreads the axial or vertical dimension of the beam in the vicinity Of the extraction or target radius.
In some of its aspects this invention relates to the electrostatic extraction method disclosed in the U. S. Pat. No. 3,582,700 entitled Cyclotron Beam Extraction System issued on June 1, 1971 to George 0. Hendry. The present mthod can be used alone or in combination with the Improved Cyclotron Beam Extraction procedures disclosed in copending application Ser. No. 118,751 filed on Feb. 25, 1971 by George 0. Hendry and Dale K. Wells.
FIG. 1 illustrates the orientation of the axial beam spreader of this invention with reSpect to the other components of a typical small isochronous cyclotron at its median plane. A main d-c electromagnet defines a magnetic guide field for the particles orbiting within the evacuated region of the cyclotron. The main d-c magnet includes a lower yoke slab 2, a pair of interconnecting iron legs 3, and two cylindrical iron pole bases 4, the lower one of which is partially shown in FIG. 1.
In the illustrated machine the lower pole tip plate 66 with sidewalls and a similar upper pole tip plate, not shown, form a vacuum tank within which charged particles are accelerated in the machine. Two sets of three shaped hill pieces 9 mount on the pole tip plates in corresponding locations to produce the azimuthally varying field necessary for isochronous operation. A pair of dees provide a radio frequency accelerating field and an ion source supplies ions for acceleration as is more fully described in U. S. Pat. No. 3,582,700. The schematically illustrated extraction system comprises electrostatic deflection means 30 as shown in U. S. Pat. 3,582,700, a magnetic channel 31 which receives and radially focuses the deflected beam, and a preseptum unit 32 adjacent to the electrostatic deflection means 30 of the type described in co-pending application Ser. No. 118,751. Instead of the described extraction system the machine may have an internal target at about the same radius for internal beam impingement as is well-known in this art.
The axial beam spreader 40 mounts as shown in FIG. 1 to embrace l5 20 turns or orbits of the internal beam near the extraction or the target radius as the case may be. Orbiting particles pass through its vertical electrostatic field prior to reaching the target or the illustrated Preseptum unit 32 and electrostatic deflection means 30 at the extraction radius. Radio frequency generator 41 applies a time-varying potential to the beam spreader. Its frequency is tuned to the natural axial oscillation frequency of the orbiting particles in the 15 20 turns or orbits at and preceding the extraction or target radius.
The axial beam spreader 40 is simply a pair of conductive copper deflection plates 43, 44 embracing the internal beam, one on either side of the median plane of the cyclotron. The deflection plates mount upon non-conductive mounting block 45, for example, adjustably fixed to lower pole tip plate 66 in the location shown in FIG. 1. Separate copper cooling traces 46, 47 cool each of the deflection plates and are used as an electrical connection supplying rf voltage across the deflection plates. Conductive shield 48 shields the deflection plates from the rf accelerating field of the adjacent dees.
A tuned-grid tuned-plate oscillator as illustrated in FIG. 4 applies a time-varying voltage across deflection plates 43, 44 at its output terminals 50, 51. The circuit includes a type 304-TL triode 52 the filaments of which are supplied through transformer 53 from 117V. line voltage. Coil 54 and variable capacitor 55 in the tuned-grid circuit connect the grid through grid capacitor 56 and grid resistor 57 to system ground, terminal 51.
The tuned-plate circuit includes coil 58 and variable capacitor 59 and capacitors 60 and 61. The plate sup ply in the described circuit is 0.4 amperes at 2,500 volts supplied through choke coil 62 with by-pass capacitor 63 to system ground.
The axial beat spreader and method have been applied to the portion of the internal beam of a small isochronous cyclotron covering about one inch inside of an approximately fifteen inch extraction radius or some l5 turns. The internal beam was observed to spread axially by a factor of two when 2,500 volts zero to peak at 12.5 MHz was applied across deflection plates 43, 44.
Thus, for the same degree of radial focusing, the power density at the extraction or target radius is substantially reduced by a factor of two. This enables a substantial increase in beam current and is particularly important for economical production of radioisotopes and fast neutrons.
It should be understood that the specific embodiments described herein are for illustrative purposes only and that it will be apparent to those skilled in the art that various modifications may be practiced and equivalents substituted for those specific elements described which are within the scope of the invention defined in the appended claims.
1. A method for reducing the power density of the internal beam of an isochronous cyclotron in its orbits near and at an internal target or extraction radius comprising the steps of accelerating the beam of charged particles within the guiding magnetic field and evacuated region of the cyclotron through said orbits;
increasing the axial height of said beam in said orbits by applying to the beam a vertical time-varying electrostatic field; and
tuning the frequency of said time-varying electrostatic field to the natural axial oscillation frequency of the internal beam in said orbits.
2. A method for extracting a portion of the internal beam of chaRged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region comprising the steps of increasing the axial height of that portion of the internal beam of particles to be extracted near an extraction radius;
continuously segregating that portion of the internal beam of particles to be extracted at said extraction radius in the absence of any electrostatic deflection field; and
then deflecting the segregated particles from orbit at said extration radius by exposure to an electrostatic deflection field, the location of which increases in radius with respect to the center of the cyclotron.
3. Apparatus for extracting a portion of the internal beam of charged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region including means increasing the axial height of that portion of the internal beam to be extracted in its orbits near and at an extraction radius;
preseptum means segregating that portion of said orbiting particles to be extracted at an extraction radius near the fringe of said magnetic field in the absence of any electrostatic field; and
electrostatic deflection means receiving said segregated particles and defining at the extraction radius an electrostatic deflection field, the location of which increases in radius with respect to the center of the cyclotron, to deflect the segregated particles from orbit.
4. The apparatus of claim 3 wherein the means increasing the axial height of the beam comprises means applying a vertical time-varying electrostatic field to said orbits of the beam at the natural oscillation frequency of the beam in said orbits.
5. The apparatus of claim 3 wherein the means increasing the axial height of the beam comprises I a pair of conductive deflection plates embracing said orbits, one on each side of the median plane of the cyclotron; and
oscillator means generating and supplying to said plates a radio-frequency voltage at the natural oscillation frequency of the beam in said orbits.
6. Apparatus for reducing the power density of the internal beam of an isochronous cyclotron in its orbits near and at an internal target or extraction radius including a pair of conductive deflection plates emracing said orbits, one on each side of the median plane of the cyclotron; and
oscillator means generating and supplying to said plates a radio-frequency voltage at the naJural oscillation frequency of the beam in said orbits.
UNlTED STATES PATENT OFFICE @EETlFlCATE 0F QGRRECTKON Patent No D t d September 4:,
Inventor(s) George 0. Hendry It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 1, line 66, change "re S pect" to respect Column 2, line 31, change Eresepcum" to greseptum Column 2, line 65, change "beai" co -bea1 r n Column 3, line 33', change "chaliiged" to "charged Column 4, line 38, add 'b' to change 'emracing" to em l racing Column 4, line 42, change "na lura to -nat ural Signed and sealed this 15th day of January 197L (SEAL) Attest:
EDWARD M. FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM PO-1OS0 (10-69) uscoMM-Dc 60376-P69 U45. GOVERNMENT PRINTING OFFICE: 9G9 0-366-334