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 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 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

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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 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 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 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 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 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. Therefore, when the ing electrodes is to induce the electron beam passing particle beam 23 developed by linear accelerator 10 is through deflection system 40 to precess about the Z- made to pass through quartz bead 25, light of intensity axis along which it would otherwise travel, thereby proportional to the quantity of particles in particle causing the point of convergence to describe a cone beam 23 is generated. The quartz bead is positioned at 10 about the Z-axis. Since the electron beam and the de- the focal point of parabolic reflector 28. and guide 24 flection signals ultimately derive from the same A-C is provided with an aluminum window 26 so that the source, RF oscillator 18, they are synchronized and as particle beam 23 can be extracted from guide 24 and the point of convergence traverses or traces one com- directed to the quartz bead 25 where light beam 29 is plete circle about the Z-axis it is also tracing one cycle generated and directed as shown. Note that any trans- 15 of the RF signal. parent medium having the properties described above A beam target system 65 is provided to allow the cir- will produce this Cherenkov radiation effect. cular trace of the point of convergence to be analyzed. The light beam 29 can be used to transmit the signal In this embodiment, the beam target system 65 includes associated with the charged particles over long dis- a phosphorus screen 67 positioned so that the point of tances without the distortion of the signal associated 20 convergence of the electron beam coincides with the with longdistance transmission of high-speed signals by screen at all necessary deflections. As the point of con- cable. This allows the analyzer to be positioned at rela- vergence describes a circular path it traces a circular tively larger distances from the accelerator than the an- fluorescent pattern upon phosphorus screen 67. The alyzers requiring cable feeds. light output or intensity of any region of the fluorescent Light beam 29 is directed towards an analyzer tube 25 screen 67 is proportional to the number of of 30, the elements of which may be found in a cathode the beam bombarding that region at a particular in- ray tube. The analyzer tube 30 is provided with a pho- stant. The number of electrons at any point along the tocathode 35 upon which light beam 29 is directed to beam is determined by the level of emissions from the impinge. A photocathode is an electrode which in- photocathode. Thus, it is apparent that the circular cludes a photoreactive element from which electrons trace on the screen reflects the intensity and frequency are emitted due to the incidence of radiation there- of occurrence of the bunches of the particle beam de- upon. Thus, when light beam 29 impinges upon photo- veloped by the accelerator. cathode 35, photocathode 35 serves to generate elec- As shown in FIG. 1 and FIG. 2, an opaque screen or trons in a quantity or intensity proportional to the in- mask 68, which may be of tape, is applied to phospho- tensity of incident light beam 29. These electrons are ^ rus screen 67. The mask includes a slit 69 which is posi- formed into a beam and focused by electron gun 36. tioned so that an arc of circular trace 70 on screen 67 Electron gun 36 includes those elements normally traverses slit 69. With no phase variation applied to the found in a cathode ray tube electron gun such as an ac- RF signal by variable phase shifter 50, at the beginning celerating grid 37 and focusing 38. The electron of each cycle of the RF signal the point of convergence beam formed by electron gun 36, if undeflected, can be will be at a particular point C on screen 67. As the RF considered to be traveling longitudinally along a Z-axis, signal goes through one entire cycle, the point of con- converging at a particular point along the Z-axis. After vergence will trace a circular path, always returning to the electron beam is formed and focused by electron begin each cycle at .point C. If the phase of the RF sig- gun 36, it is directed to pass through the high- nal applied to the X and Y axis deflecting electrodes is frequency deflection system 40, which acts to deflect varied a particular value between 0° and 360° from its the beam and thus the point of convergence from the initial phase by variable phase shifter 50, the point of Z-axis. Deflection system 40 includes a pair of elec- convergence will begin each cycle at a different point trodes 41 and 42 for deflecting the electron beam from such as point D. The shift from C to D will be equal in the Z-axis in the X direction and a pair of electrodes 44 angular degrees around the circle to the degrees of and 45 for deflecting the electron beam in the Y direc- 50 phase shift in the RF signal. Correspondingly, that por- tion. tion of the arc of trace 70 which traverses slit 69 will The output from RF oscillator 18 is amplified by am- also change a like number of degrees. Therefore, as plifier 49 and coupled to a variable phase shifter 50. variable phase shifter 50 is varied a particular value be- tween 0° and 360° from its initial phase, the locus of Variable phase shifter 50 is a device which is capable 55 of varying the phase of an A-C signal from 0° to 360° points describing trace 70 is shifted an equal amount, from its initial phase and of developing an output indi- and the portion of the arc of trace 70 traversing slit 69 cating the value of the amount of the phase shifting in is also shifted an equal amount from the previous posi- terms of a variable amplitude D-C signal. X-axis de- tion. Photomultiplier tube 75 is positioned directly fac- ing slit 69 so that light passing through slit 69 impinges flecting cathodes 41 and 42 are coupled to the RF os- 60 cillator 18 via variable phase shifter 50 by leads 55 and upon the photocathode of photomultiplier tube 75. 56 so that, as the phase of the RF signal is varied by The width "A" of slit 69 determines the length of arc phase shifter 50, the phase of the A-C signal applied to of trace 70 which will illuminate photomultiplier tube the X-axis deflecting electrodes is comparably varied. 75. Width A must be large enough to permit enough Similarly, Y-axis deflecting electrodes 44 and 45 are 65 light to produce a suitable signal from photomultiplier coupled to the RF oscillator 18 via variable phase tube 75, but small enough for high resolution to allow shifter 50. However, the signal applied to Y-axis de- examination ofthe smallest portion of the trace as pos- flecting electrodes 44 and 45 via leads 57 and 58 is sible for greater accuracy and less smearing caused by ,3,87's,83 9 5 6 light from adjacent areas of the trace. A range of 0.25 ing properly in that the pulse is spread out and not mm to 1 mm is believed suitable for width A. There- properly bunched. fore, the output of photomultiplier tube 75, opposite The embodiments of the invention in which an exclu- slit 69, will be proportional to the intensity of the fluo- sive property or privilege is claimed are defined as fol- rescent arc of trace 10 appearing at slit 69. As variable 5 lows: phase shifter 50 is varied over the entire range from 0° 1. A device for analyzing a charged particle beam de- to 360°, the output of photomultiplier tube 75 will vary veloped by an accelerator having an RF driving signal, according to the intensity of the arc over that range. comprising: An output signal is developed by variable phase a medium through which the particle beam passes at shifter 50, indicating at what phase between 0° and 10 a speed sufficient to emit light from said medium 360° the shifter is operating at that instant. This signal by Cherenkov radiation of intensity proportional to may be a D.C. signal which varies in amplitude to indi- the intensity of the particle beam; cate the applicable phase. The output signal of variable means for developing an electron beam having an in- phase shifter 50 is coupled to one input of X-Y re- tensity proportional to the intensity of said light corder 76. X-Y recorder 76 is a device which plots on 15 and including a photocathode positioned to receive some form of chart the relationship between two vari- said light and responsive thereto to emit electrons ables. The output of photomultiplier tube 75 is coupled to form said electron beam, means for directing to the other input of X-Y recorder 76. Remembering said electron beam along a Z-axis and focussing that each pulse of particles from the accelerator will be said electron beam at a point, and X-axis and Y- similar to every other pulse of the accelerator during 20 axis deflection means for deflecting said focussed one accelerator run, it can be seen that at any particu- electron beam from said Z-axis; lar phase shift the output from photomultiplier tube 75 phase varying means coupled to the accelerator and will remain substantially constant. As noted before, as to each of said deflection means and being respon- variable phase shifter 50 varies from 0° to 360°, the out- sive to the RF driving signal to develop first and put of photomultiplier tube 75 varies accordingly, so 25 second signals of equal frequency to said RF driv- that the plot display of X-Y recorder 76 will show the ing signal, said phase varying means for applying variation in the amplitude of the signal from photomul- said first signal to one of said deflection means at tiplier tube 75 compared to the corresponding varia- a particular phase shift varying between 0° and tion in the output signal from variable phase shifter 50. 360° from the phase of the RF driving signal and In effect, such a plot will represent the varying number 30 for applying said second signal to the other of said of particles in a pulse from the accelerator over one deflection means at a phase constantly 90° out of cycle or timed period of the applied RF driving signal phase with said first signal so that said electron which at 1300 megahertz means one cycle takes ap- beam precesses about and said point describes a proximately 770 picoseconds. Thus, a change of 1° circular path about said Z-axis; caused by variable phase shifter 50 will correspond to 35 a detector responsive to the quantity of electrons in- about 2 picoseconds on the X-Y plotter. The output cident along a particular arc of said circular path signal of the variable phase shifter 50 thus serves as the to develop an output signal proportional to the in- time base for the X-Y recorder display. tensity of said electron beam; and Referring to FIG. 3, there is shown an alternate em- recording means coupled to said phase varying bodiment for target system 65. The system of FIG. 1 re- means and to said detector to record the value of quires that the electron beam developed by electron said output signal with respect to said particular gun 36 be converted to a light beam before it is de- phase shift of said first signal applied to one of said tected by photomultiplier tube 75. It is the purpose of deflection means. the embodiment of FIG. 3 to avoid this transformation 2. The device of claim 1 wherein said detector in- and to directly detect the electron beam. This is accom- cludes a fluorescent screen positioned so that said point plished by allowing the electron beam to strike the first impinges upon said screen to produce a circular trace dynode 77 of an tube 78 after hav- upon said screen, a mask having a slit therethrough ing passed through a slit 80 of mask 81, thus eliminat- coupled to said screen so that an arc of said circular ing the second conversion of electrons to light. Mask trace traverses said slit, and a photomultiplier tube fac- 81 is mounted on the casing 82 which supports the ing said slit and responsive to the fluorescence of said electron gun and deflecting system and is aligned so arc of said trace passing through said slit to develop a that, as the point of convergence of the electron beam tube output signal proportional to the intensity of said describes a circle, that circle will traverse slit 80 with fluorescence. the point of convergence impinging upon the first dy- 3. The device of claim 2, wherein said phase varying node 77 of electron multiplier tube 78. The width B of means develops a phase output signal corresponding to slit 80 is subject to the same size limitations as width A the value of said particular phase shift, and said record- in FIG. 2. The signal developed by electron multiplier ing means includes an X-Y recorder coupled to said 78 is then coupled to the X-Y recorder by lead 83 as phase varying means and said detector, said X-Y re- in the embodiment of FIG. 1. 6Q corder being responsive to said tube output signal and Referring to FIG. 4, there is shown a representative to said phase output signal to continuously display the set of plots which might be developed by X-Y recorder intensity of the charged particle beam with respect to 76. Curve 85 represents the ideal electron beam pulse one cycle of the RF driving signal. developed by a high-current accelerator having a sharp 4. The device of claim 2, wherein said slit is between rise and decay time and distinct amplitude peak indi- ^ 0.25 mm and 1 mm wide. cating generation of particles within each bunch closely 5. The device of claim 1 wherein said detector in- together at one point in time. Curve 86 represents an cludes a mask having a slit therethrough positioned so electron beam pulse from an accelerator not function- that an arc of said circle described by said point tra- 3,8' 3,839 7 8 verses said slit, and an electron multiplier tube facing varying means and said detector, said X-Y recorder said slit and responsive to said arc of said trace to de- being responsive to said tube output signal and to said velop a tube output signal proportional to the intensity phase output signal to display the intensity of the of said beam. charged particle beam with respect to one cycle of the 6. The device of claim 5, wherein said phase varying 5 RF driving signal. means develops a phase output signal corresponding to 7. The device of claim 5, wherein said slit is between the value of said particular phase shift, said recorder 0.25 mm and 1 mm wide. means includes an X-Y recorder coupled to said phase

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