US3873839A

US3873839A - High speed linac-beam analyzer

Info

Publication number: US3873839A
Application number: US459824A
Authority: US
Inventors: Kenneth W Johnson
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1974-04-10
Filing date: 1974-04-10
Publication date: 1975-03-25
Anticipated expiration: 1992-03-25

Abstract

A device for analyzing a charged particle beam developed by an accelerator device having an RF driving signal is provided. The particle beam passes through a transparent medium, developing light of intensity proportional to the intensity of the particle beam. A photocathode is aligned to detect the light and thereby generate an electron beam of intensity proportional to the intensity of the light. The RF driving signal is coupled via a phase-varying network to an X-axis deflection system and, after a phase shift of 90*, to a Y-axis deflection system. The electron beam is directed through the X-axis and Y-axis deflection system, thereby causing the electron beam to precess about an axis and describe a circular trace in a plane perpendicular to the axis. Means are provided to measure the intensity of the beam along a particular narrow arc of the circular trace as the phase of the RF signal applied to the X and Y deflection systems is varied from 0* to 360*.

Description

United States Patent 91 Johnson [4 1 Mar. 25, 1975 HIGH SPEED LINAC-BEAM ANALYZER [75] Inventor: Kenneth W. Johnson, Lockport, Ill.

[21] Appl. No.: 459,824

Primary E.\'aminer.lames W. Lawrence Assistant Examiner-T. N. Grigsby Attorney, Agent, or Firm-Dean E. Carlson; Arthur A. Churm; Paul A. Gottlieb [57] ABSTRACT 52 us. Cl 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 [51] Int CL Golj 39/18, G01" 21/16 G0 H20 the intensity of the light. The RF driving signal is cou- 581 Field of Search 250/369 36i 362 379 PM Via a Phasevarying network to X-aXiS deflec- 6 tion system and, after a phase shift of 90, to a Y-axis deflection system. The electron beam is directed [56] References Cited through the X-axis and Y-axis deflection system, thereby causing the electron beam to precess about an UNITED STATES PATENTS axis and describe a circular trace in a plane perpendicular to the axis. Means are provided to measure the Cover e intensity of the beam along a particular narrow arc of gay s the circular trace as the phase of the RF signal applied 4/1974 l 750/369 to the X and Y deflection system is varied from 0 to 7 Claims, 4 Drawing Figures OSCILLATOR I 4Z0 flMPL/F/Ef? j il it M F1 7 V/YlP/HBLE Had/4 f0"; PMEE REC/ORDER I SHIFTER ea I l i 90 //60 55???? {-25 l l l/ I 26 PATENTEDHARZSISYS SHEET 1 2 tmmkgmt mmhmim w w m HIGH SPEED LINAC-BEAM ANALYZER 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 A particle beam analyzer is a tool for examining the frequency and amplitude characteristics of a beam of charged particles developed by an accelerator. For example, the ideal beam developed by a linear accelerator consists ofa series of substantially identical bunches or pulses of charged particles traveling at a speed approaching the speed of light, with the frequency of the generation of each bunch by the accelerator corresponding to the frequency of an RF driving signal. An analyzer should detect the presence of these pulses and provide means for determining how closely coincident in time the particles are generated, which provides a measure of the quality of the bunching together of the particles.

Certain types of linear accelerators produce particle beams having a frequency of bunch generation greater than other types requiring a particle beam analyzer capable of sensitivity to the higher frequency accelerators. Thus the analyzer must be able to respond more quickly to the pulses which comprise the beam than for the lower frequency accelerator analyzer. Prior art analyzers have a time resolution, which is the smallest measurable time interval over which the analyzer can distinguish between the intensity, i.e., the varying number of charged particles, at adjacent points along the beam, on the order of 30 picoseconds. The modern highcurrent linear accelerators require an analyzer which is capable of operating at approximately 2 picoseconds time resolution.

In addition to the inadequacies of the prior art analyzer in terms of time resolution, transmission of highspeed signals, associated with the beam, from the accelerator to the analyzer by conventional means is extremely difficult. Transmission of these high-speed sig- 1 nals for distances on the order of 50 feet or more by coaxial cable is subject to great induced error due to the difficulties of handling wire carried high frequency signals.

It is therefore an object of this invention to provide a particle beam analyzer operable in the 2 picosecond time resolution range.

Another object of this invention is to provide a particle beam analyzer operable at a distance from the accelerator.

SUMMARY OF THE INVENTION A device is provided for analyzing a particle beam developed by an accelerator having an RF driving signal. The particle beam is directed so that is passes through a transparent medium, thereby developing light of intensity proportional to the intensity of the particle beam. A photocathode is aligned to detect the light and generate an electron beam traveling along a longitudinal axis and of intensity proportional to the intensity of the light beam. The RF driving signal is coupled via phase-varying means to X-axis and Y-axis deflection systems, with the signal applied to the X-axis deflection system being constantly maintained 90 out of phase with the signal applied to the Y-axis system.

The electron beam is directed to pass through the X and Y deflection systems and thereby precesses about the longitudinal axis to describe a circular pattern in a plane perpendicular to the longitudinal axis. The precessing beam may be focused and the point of convergence made to impinge on a fluorescent screen, thereby illuminating a circular trace of particular diameter. A mask having a slit therethrough is coupled to the fluorescent screen so that an arc of the circular trace traverses the slit. A photomultiplier tube is positioned directly opposite the slit so that it detects the intensity of that portion of the beam traversing the slit. As the phase of RF driving signal applied to the deflecting electrode is varied from 0 to 360, the photomultiplier tube develops a signal representative of the intensity of the trace at each phase variation. An alternate embodiment of the invention provides a slit which is positioned to intersect the circular trace of the electron beam. The slit is aligned so that the electron beam passing through the slit is directed to and the point of convergence impinges on the dynode of an electron multiplier tube which, as the phase of the RF signal applied to the deflecting electrodes is varied from 0 to 360, develops a signal corresponding to the intensity of the electron beam at each variation of phase.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of the high-speed particlebeam analyzer;

FIG. 2 is a view of mask-screen-trace arrangement;

FIG. 3 is a partial schematic of an alternate embodiment of the analyzer; and

FIG. 4 is a set of curves of the output of a particle beam analyzer.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. I, there is shown a schematic of a linear accelerator and particle beam analyzer. A linear accelerator is a device in which charged particles gain in energy by the action of oscillating electromagnetic fields. A particle beam developed in the accelerator is composed of a series of particle bunches with the frequency at which the bunches are generated corresponding to the frequency of oscillation of the electromagnetic fields. For illustrative purposes, the linear accelerator l0 depicted in FIG. 1 is of the drift tube variety. Any other type of linear accelerator having an RF driving signal is appropriate to practice the invention. Another example of such an accelerator is the traveling wave linear accelerator. Linear accelerator 10 includes a series of drift tube electrodes l1, l2, l3, l4 and 15, which are coupled to an RF oscillator 18, via leads l6 and 17. The RF oscillator 18 applies an RF signal to the electrodes. Charged particles are developed by particle source 20 and are then accelerated by linear accelerator 10 with a resultant beam from the linear accelerator 10 consisting of a series of pulses or bunches of particles. Each pulse will be similar to every other pulse due to the characteristics of the accelerator so that errors will be repeated in each pulse. It is desirable to detect the presence of these bunches and how closely in time the particles are generated within each bunch to determine if the accelerator is functioning properly. It is for this purpose that a particle beam analyzer is necessary.

The charged particle beam 23 developed by linear accelerator l0 and contained in an evacuated beam guide 24 is directed to pass through a quartz bead 25 outside guide 24. When an electrically charged particle is made to pass through a transparent medium, such as a quartz bead, at a velocity in excess of the speed of light in that transparent medium, Cherenkov radiation occurs at a light intensity proportional to the quantity of particles of the incident beam. Therefore, when the particle beam 23 developed by linear accelerator is made to pass through quartz bead 25, light of intensity proportional to the quantity of particles in particle beam 23 is generated. The quartz bead is positioned at the focal point of parabolic reflector 28, and guide 24 is provided with an aluminum window 26 so that the particle beam 23 can be extracted from guide 24 and directed to the quartz bead where light beam 29 is generated and directed as shown. Note that any transparent medium having the properties described above will produce this Cherenkov radiation effect.

The light beam 29 can be used to transmit the signal associated with the charged particles over long distances without the distortion of the signal associated with longdistance transmission of high-speed signals by cable. This allows the analyzer to be positioned at relatively larger distances from the accelerator than the analyzers requiring cable feeds.

Light beam 29 is directed towards an analyzer tube 30, the elements of which may be found in a cathode ray tube. The analyzer tube 30 is provided with a photocathode upon which light beam 29 is directed to impinge. A photocathode is an electrode which includes a photoreactive element from which electrons are emitted due to the incidence of radiation thereupon. Thus, when light beam 29 impinges upon photocathode 35, photocathode 35 serves to generate electrons in a quantity or intensity proportional to the intensity of incident light beam 29. These electrons are formed into a beam and focused by electron gun 36. Electron gun 36 includes those elements normally found in a cathode ray tube electron gun such as an accelerating grid 37 and focusing anode 38. The electron beam formed by electron gun 36, if undeflected, can be considered to be traveling longitudinally along a Z-axis, converging at a particular point along the Z-axis. After the electron beam is formed and focused by electron gun 36, it is directed to pass through the highfrequency deflection system 40, which acts to deflect the beam and thus the point of convergence from the Z-axis. Deflection system 40 includes a pair of

electrodes

41 and 42 for deflecting the electron beam from the Z-axis in the X direction and a pair of

electrodes

44 and 45 for deflecting the electron beam in the Y direction.

The output from RF oscillator 18 is amplified by amplifier 49 and coupled to a variable phase shifter 50. Variable phase shifter 50 is a device which is capable of varying the phase of an A-C signal from 0 to 360 from its initial phase and of developing an output indicating the value of the amount of the phase shifting in terms of a variable amplitude D-C signal.

X-axis deflecting cathodes

41 and 42 are coupled to the RF oscillator 18 via variable phase shifter 50 by leads and 56 so that, as the phase of the RF signal is varied by phase shifter 50, the phase of the A-C signal applied to the X-axis deflecting electrodes is comparably varied. Similarly, Y-

axis deflecting electrodes

44 and 45 are coupled to the RF oscillator 18 via variable phase shifter 50. However, the signal applied to Y-

axis defleeting electrodes

44 and 45 via leads 57 and 58 is maintained constantly 90 out of phase with the signal applied to the

X-axis deflecting electrodes

41 and 42 by 90 phase shifter 60. The effect of applying identical A-C signals 90out of phase to a deflection system including X-axis deflecting electrodes and Y-axis deflecting electrodes is to induce the electron beam passing through deflection system 40 to precess about the Z- axis along which it would otherwise travel, thereby causing the point of convergence to describe a cone about the Z-axis. Since the electron beam and the deflection signals ultimately derive from the same A-C source, RF oscillator 18, they are synchronized and as the point of convergence traverses or traces one complete circle about the Z-axis it is also tracing one cycle of the RF signal.

A beam target system is provided to allow the cir cular trace of the point of convergence to be analyzed. In this embodiment, the beam target system 65 includes a phosphorus screen 67 positioned so that the point of convergence of the electron beam coincides with the screen at all necessary deflections. As the point of convergence describes a circular path it traces a circular fluorescent pattern upon phosphorus screen 67. The light output or intensity of any region of the fluorescent screen 67 is proportional to the number of electrons of the beam bombarding that region at a particular instant. The number of electrons at any point along the beam is determined by the level of emissions from the photocathode. Thus, it is apparent that the circular trace on the screen reflects the intensity and frequency of occurrence of the bunches of the particle beam developed by the accelerator.

As shown in FIG. 1 and FIG. 2, an opaque screen or mask 68, which may be of tape, is applied to phosphorus screen 67. The mask includes a slit 69 which is positioned so that an arc of circular trace 70 on screen 67 traverses slit 69. With no phase variation applied to the RF signal by variable phase shifter 50, at the beginning of each cycle of the RF signal the point of convergence will be at a particular point C on screen 67. As the RF signal goes through one entire cycle, the point of convergence will trace a circular path, always returning to begin each cycle at .point C. If the phase of the RF signal applied to the X and Y axis deflecting electrodes is varied a particular value between 0 and 360 from its initial phase by variable phase shifter 50, the point of convergence will begin each cycle at a different point such as point D. The shift from C to D will be equal in angular degrees around the circle to the degrees of phase shift in the RF signal. Correspondingly, that portion of the arc of trace 70 which traverses slit 69 will also change a like number of degrees. Therefore, as variable phase shifter 50 is varied a particular value between 0 and 360 from its initial phase, the locus of points describing trace 70 is shifted an equal amount, and the portion of the arc of trace 70 traversing slit 69 is also shifted an equal amount from the previous position. Photomultiplier tube is positioned directly facing slit 69 so that light passing through slit 69 impinges upon the photocathode of photomultiplier tube 75.

The width A of slit 69 determines the length of arc of trace 70 which will illuminate photomultiplier tube 75. Width A must be large enough to permit enough light to produce a suitable signal from photomultiplier tube 75, but small enough for high resolution to allow examination of the smallest portion of the trace as possible for greater accuracy and less smearing caused by light from adjacent areas of the trace. A range of 0.25 mm to l mm is believed suitable for width A. Therefore, the output of photomultiplier tube 75, opposite slit 69, will be proportional to the intensity of the fluorescent arc of trace l0 appearing at slit 69. As variable phase shifter 50 is varied over the entire range from 0 to 360, the output of photomultiplier tube 75 will vary according to the intensity of the are over that range.

An output signal is developed by variable phase shifter 50, indicating at what phase between 0 and 360 the shifter is operating at that instant. This signal may be a DC signal which varies in amplitude to indicate the applicable phase. The output signal of variable phase shifter 50 is coupled to one input of X-Y recorder 76. X-Y recorder 76 is a device which plots on some form of chart the relationship between two variables. The output of photomultiplier tube 75 is coupled to the other input of X-Y recorder 76. Remembering that each pulse of particles from the accelerator will be similar to every other pulse of the accelerator during one accelerator run, it can be seen that at any particular phase shift the output from photomultiplier tube 75 will remain substantially constant. As noted before, as variable phase shifter 50 varies from 0 to 360, the output of photomultiplier tube 75 varies accordingly, so that the plot display of X-Y recorder 76 will show the variation in the amplitude of the signal from photomultiplier tube 75 compared to the corresponding variation in the output signal from variable phase shifter 50. In effect, such a plot will represent the varying number of particles in a pulse from the accelerator over one cycle or timed period of the applied RF driving signal which at 1300 megahertz means one cycle takes approximately 770 picoseconds. Thus, a change of 1 caused by variable phase shifter 50 will correspond to about 2 picoseconds on the X-Y plotter. The output signal of the variable phase shifter 50 thus serves as the time base for the X-Y recorder display.

Referring to FIG. 3, there is shown an alternate embodiment for target system 65. The system of FIG. 1 re quires that the electron beam developed by electron gun 36 be converted to a light beam before it is detected by photomultiplier tube 75. It is the purpose of the embodiment of FIG. 3 to avoid this transformation and to directly detect the electron beam. This is accomplished by allowing the electron beam to strike the first dynode 77 of an electron multiplier tube 78 after having passed through a slit 80 of mask 81, thus eliminating the second conversion of electrons to light. Mask 81 is mounted on the casing 82 which supports the electron gun and deflecting system and is aligned so that, as the point of convergence of the electron beam describes a circle, that circle will traverse slit 80 with the point of convergence impinging upon the first dynode 77 of electron multiplier tube 78. The width B of slit 80 is subject to the same size limitations as width A in FIG. 2. The signal developed by electron multiplier 78 is then coupled to the X-Y recorder by lead 83 as in the embodiment of FIG. 1.

Referring to FIG. 4, there is shown a representative set of plots which might be developed by X-Y recorder 76. Curve 85 represents the ideal electron beam pulse developed by a high-current accelerator having a sharp rise and decay time and distinct amplitude peak indicating generation of particleswithin each bunch closely together at one point in time. Curve 86 represents an electron beam pulse from an accelerator not functioning properly in that the pulse is spread out and not properly bunched.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A device for analyzing a charged particle beam developed by an accelerator having an RF driving signal, comprising:

a medium through which the particle beam passes at a speed sufficient to emit light from said medium by Cherenkov radiation of intensity proportional to the intensity of the particle beam;

means for developing an electron beam having an intensity proportional to the intensity of said light and including a photocathode positioned to receive said light and responsive thereto to emit electrons to form said electron beam, means for directing said electron beam along a Z-axis and focussing said electron beam at a point, and X-axis and Y- axis deflection means for deflecting said focussed electron beam from said Z-axis;

phase varying means coupled to the accelerator and to each of said deflection means and being responsive to the RF driving signal to develop first and second signals of equal frequency to said RF driving signal, said phase varying means for applying said first signal to one of said deflection means at a particular phase shift varying between 0 and 360 from the phase of the RF driving signal and for applying said second signal to the other of said deflection means at a phase constantly 90 out of phase with said first signal so that said electron beam precesses about and said point describes a circular path about said Z-axis;

a detector responsive to the quantity of electrons incident along a particular are of said circular path to develop an output signal proportional to the intensity of said electron beam; and

recording means coupled to said phase varying means and to said detector to record the value of said output signal with respect to said particular phase shift of said first signal applied to one of said deflection means.

2. The device of claim 1 wherein said detector includes a fluorescent screen positioned so that said point impinges upon said screen to produce a circular trace upon said screen, a mask having a slit therethrough coupled to said screen so that an arc of said circular trace traverses said slit, and a photomultiplier tube facing said slit and responsive to the fluorescence of said arc of said trace passing through said slit to develop a tube output signal proportional to the intensity of said fluorescence.

3. The device of claim 2, wherein said phase varying means develops a phase output signal corresponding to the value of said particular phase shift, and said recording means includes an X-Y recorder coupled to said phase varying means and said detector, said X-Y recorder being responsive to said tube output signal and to said phase output signal to continuously display the intensity of the charged particle beam with respect to one cycle of the RF driving signal.

4. The device of claim 2, wherein said slit is between 0.25 mm and l mm wide.

5. The device of claim 1 wherein said detector includes a mask having a slit therethrough positioned so that an arc of said circle described by said point travarying means and said detector, said X-Y recorder being responsive to said tube output signal and to said phase output signal to display the intensity of the charged particle beam with respect to one cycle of the RF driving signal.

7. The device of claim 5, wherein said slit is between 0.25 mm and 1 mm wide.

Claims

1. A device for analyzing a charged particle beam developed by an accelerator having an RF driving signal, comprising: a medium through which the particle beam passes at a speed sufficient to emit light from said medium by Cherenkov radiation of intensity proportional to the intensity of the particle beam; means for developing an electron beam having an intensiTy proportional to the intensity of said light and including a photocathode positioned to receive said light and responsive thereto to emit electrons to form said electron beam, means for directing said electron beam along a Z-axis and focussing said electron beam at a point, and X-axis and Y-axis deflection means for deflecting said focussed electron beam from said Zaxis; phase varying means coupled to the accelerator and to each of said deflection means and being responsive to the RF driving signal to develop first and second signals of equal frequency to said RF driving signal, said phase varying means for applying said first signal to one of said deflection means at a particular phase shift varying between 0* and 360* from the phase of the RF driving signal and for applying said second signal to the other of said deflection means at a phase constantly 90* out of phase with said first signal so that said electron beam precesses about and said point describes a circular path about said Z-axis; a detector responsive to the quantity of electrons incident along a particular arc of said circular path to develop an output signal proportional to the intensity of said electron beam; and recording means coupled to said phase varying means and to said detector to record the value of said output signal with respect to said particular phase shift of said first signal applied to one of said deflection means.

4. The device of claim 2, wherein said slit is between 0.25 mm and 1 mm wide.

5. The device of claim 1 wherein said detector includes a mask having a slit therethrough positioned so that an arc of said circle described by said point traverses said slit, and an electron multiplier tube facing said slit and responsive to said arc of said trace to develop a tube output signal proportional to the intensity of said beam.

6. The device of claim 5, wherein said phase varying means develops a phase output signal corresponding to the value of said particular phase shift, said recorder means includes an X-Y recorder coupled to said phase varying means and said detector, said X-Y recorder being responsive to said tube output signal and to said phase output signal to display the intensity of the charged particle beam with respect to one cycle of the RF driving signal.

7. The device of claim 5, wherein said slit is between 0.25 mm and 1 mm wide.