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High Speed Linac-Beam Analyzer

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High Speed Linac-Beam Analyzer United States Patent im mi 3,873,839 Johnson [45] Mar. 25, 1975 [54] HIGH SPEED LIN AC-BEAM ANALYZER Primary Examiner—James W. Lawrence [75] Inventor: Kenneth W. Johnson, Lockport, 111. Assistant Examiner—T. N. Grigsby Attorney, Agent, or Firm—Dean E. Carlson; Arthur A. [73 ] Assignee: The United States of America as Churm; Paul A. Gottlieb represented by the United States Atomic Energy Commission, [57] ABSTRACT Washington, D.C. A device for analyzing a charged particle beam devel- [22] Filed: Apr. 10, 1974 oped by an accelerator device having an RF driving signal is provided. The particle beam passes through a [21] Appl. No.: 459,824 transparent medium, developing light of intensity pro- portional to the intensity of the particle beam. A pho- [52] U.S. CI 250/369, 250/361, 250/362, tocathode is aligned to detect the light and thereby 250/396 generate an electron beam of intensity porportional to the intensity of the light. The RF driving signal is cou- [51] Int. CI GOlj 39/18, GO In 21/16, GOlt 1/20 pled via a phasevarying network to an X-axis deflec- [58] Field of Search 250/369, 361, 362, 379, tion system and, after a phase shift of 90°, to a Y-axis 250/396 deflection system. The electron beam is directed through the X-axis and Y-axis deflection system, [56] References Cited thereby causing the electron beam to precess about an UNITED STATES PATENTS axis and describe a circular trace in a plane perpendic- 2,912,646 11/1959 Pilny 250/396 ular to the axis. Means are provided to measure the 2,954,473 9/1970 Hoover et al 250/362 intensity of the beam along a particular narrow arc of 3,049,619 8/1962 Genovese, Jr 250/362 the circular trace as the phase of the RF signal applied 3,170,116 2/1965 Farrington 250/396 to the X and Y deflection system is varied from 0° to 3,805,075 4/1974 Roberts 250/369 360°. 7 Claims, 4 Drawing Figures £ PATENTED FEB I 81875 3,867,634 SHEET 1 OF 8 PATENTED MAR2 5.1375 3,873,839 SHEET 2 OF 2 f x x-input(time)— 3,873,839 1 HIGH SPEED LINAC-BEAM ANALYZER The electron beam is directed to pass through the X and Y deflection systems and thereby precesses about CONTRACTUAL ORIGIN OF THE INVENTION the longitudinal axis to describe a circular pattern in a The invention described herein was made in the plane perpendicular to the longitudinal axis. The pre- course of, or under, a contract with the UNITED 5 cessing beam may be focused and the point of conver- STATES ATOMIC ENERGY COMMISSION. gence made to impinge on a fluorescent screen, thereby illuminating a circular trace of particular diam- BACKGROUND OF THE INVENTION eter. A mask having a slit therethrough is coupled to A particle beam analyzer is a tool for examining the the fluorescent screen so that an arc of the circular frequency and amplitude characteristics of a beam of 10 trace traverses the slit. A photomultiplier tube is posi- charged particles developed by an accelerator. For ex- tioned directly opposite the slit so that it detects the in- ample, the ideal beam developed by a linear accelera- tensity of that portion of the beam traversing the slit. tor consists of a series of substantially identical bunches As the phase of RF driving signal applied to the deflect- or pulses of charged particles traveling at a speed ap- ing electrode is varied from 0° to 360°, the photomulti- proaching the speed of light, with the frequency of the 15 plier tube develops a signal representative of the inten- generation of each bunch by the accelerator corre- sity of the trace at each phase variation. An alternate sponding to the frequency of an RF driving signal. An embodiment of the invention provides a slit which is analyzer should detect the presence of these pulses and positioned to intersect the circular trace of the electron provide means for determining how closely coincident beam. The slit is aligned so that the electron beam pass- in time the particles are generated, which provides a 20 ing through the slit is directed to and the point of con- measure of the quality of the bunching together of the vergence impinges on the dynode of an electron multi- particles. plier tube which, as the phase of the RF signal applied Certain types of linear accelerators produce particle to the deflecting electrodes is varied from 0° to 360°, beams having a frequency of bunch generation greater develops a signal corresponding to the intensity of the than other types requiring a particle beam analyzer ca- 25 electron beam at each variation of phase. pable of sensitivity to the higher frequency accelera- tors. Thus the analyzer must be able to respond more BRIEF DESCRIPTION OF THE DRAWINGS quickly to the pulses which comprise the beam than for FIG. 1 is a schematic of the high-speed particle, beam the lower frequency accelerator analyzer. Prior art ana- analyzer; lyzers have a time resolution, which is the smallest mea- 30 FIG. 2 is a view of mask-screen-trace arrangement; surable time interval over which the analyzer can dis- FIG. 3 is a partial schematic of an alternate embodi- tinguish between the intensity, i.e., the varying number ment of the analyzer; and of charged particles, at adjacent points along the beam, FIG. 4 is a set of curves of the output of a particle on the order of 30 picoseconds. The modern high- beam analyzer. current linear accelerators require an analyzer which is 35 capable of operating at approximately 2 picoseconds DETAILED DESCRIPTION OF THE INVENTION time resolution. Referring to FIG. 1, there is shown a schematic of a In addition to the inadequacies of the prior art analy- linear accelerator and particle beam analyzer. A linear zer in terms of time resolution, transmission of high- accelerator is a device in which charged particles gain speed signals, associated with the beam, from the accel- 40 in energy by the action of oscillating electromagnetic erator to the analyzer by conventional means is ex- fields. A particle beam developed in the accelerator is tremely difficult. Transmission of these high-speed sig- composed of a series of particle bunches with the fre- nals for distances on the order of 50 feet or more by co- quency at which the bunches are generated corre- axial cable is subject to great induced error due to the sponding to the frequency of oscillation of the electro- difficulties of handling wire carried high frequency sig- magnetic fields. For illustrative purposes, the linear ac- nals. celerator 10 depicted in FIG. 1 is of the drift tube vari- It is therefore an object of this invention to provide ety. Any other type of linear accelerator having an RF a particle beam analyzer operable in the 2 picosecond driving signal is appropriate to practice the invention. time resolution range. Another example of such an accelerator is the traveling Another object of this invention is to provide a parti- wave linear accelerator. Linear accelerator 10 includes cle beam analyzer operable at a distance from the ac- a series of drift tube electrodes 11, 12, 13, 14 and 15, celerator. which are coupled to an RF oscillator 18, via leads 16 and 17. The RF oscillator 18 applies an RF signal to the SUMMARY OF THE INVENTION electrodes. Charged particles are developed by particle 55 A device is provided for analyzing a particle beam source 20 and are then accelerated by linear accelera- developed by an accelerator having an RF driving sig- tor 10 with a resultant beam from the linear accelerator nal. The particle beam is directed so that is passes 10 consisting of a series of pulses or bunches of parti- through a transparent medium, thereby developing cles. Each pulse will be similar to every other pulse due light of intensity proportional to the intensity of the 6q to the characteristics of the accelerator so that errors particle beam. A photocathode is aligned to detect the will be repeated in each pulse. It is desirable to detect light and generate an electron beam traveling along a the presence of these bunches and how closely in time longitudinal axis and of intensity proportional to the in- the particles are generated within each bunch to deter- tensity of the light beam. The RF driving signal is cou- mine if the accelerator is functioning properly. It is for pled via phase-varying means to X-axis and Y-axis de- this purpose that a particle beam analyzer is necessary. flection systems, with the signal applied to the X-axis The charged particle beam 23 developed by linear deflection system being constantly maintained 90° out accelerator 10 and contained in an evacuated beam of phase with the signal applied to the Y-axis system. guide 24 is directed to pass through a quartz bead 25 3,873,839 outside guide 24. When an electrically charged particle maintained constantly 90° out of phase with the signal is made to pass through a transparent medium, such as applied to the X-axis deflecting electrodes 41 and 42 by a quartz bead, at a velocity in excess of the speed of 90° phase shifter 60. The effect of applying identical light in that transparent medium, Cherenkov radiation A-C signals 90°out of phase to a deflection system in- occurs at a light intensity proportional to the quantity 5 cluding X-axis deflecting electrodes and Y-axis deflect- of particles ofthe incident beam.
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