Adjunct Professor of Physics, Yale University

Phone: 203-432-5428; Email: jay.hirshfield@yale.edu

Electron beam physics

Beam physics embraces study of the production, acceleration, control and use of energetic beams of elementary particles. The Yale Beam Physics group specializes in the study of electron beams, and several current theoretical and experimental programs are underway on novel means of electron acceleration. Energetic electron beams are also studied and exploited in novel interactions and devices for the efficient generation of high-power microwaves that are needed to drive contemplated future TeV-level electron-positron colliders, such as TESLA and NLC, as well as a longer-range multi-TeV collider.

Experimental and theoretical research activity is carried out using facilties within the Wright Nuclear Structure Laboratory at Yale, and in collaboration with physicists at outside laboratories such as the Accelerator Test Facility at Brookhaven National Laboratory, the Beam Physics Branch at Naval Research Laboratory, Columbia University, and the Institute of Applied Physics in Nizhny Novgorod, Russia.

Beam Physics Experimental Facilities at Yale

Facilities for beam physics experiments at Yale have been established in a 3500 sq. ft. shielded vault originally built in the early 1960's for a now-decommissioned 70 MeV electron linear accelerator. These facilities allow on-campus experimentation with energetic electron beams to be conducted without danger of human exposure to ionizing radiation. Newly installed infrastructure includes distribution of up to 2 MW of ac mains power from a dedicated substation, re-circulating chilled deionized water for disposal of at least 100 kW of waste heat from magnets, and electronics design and assembly facilities. Major installed equipment includes a 24 MW S-band klystron and its 65 MW pulse modulator that furnish rf drive power for several accelerator prototypes described below; 100 kV and 300 kV, 30 A electron guns and associated pulse modulators for injection of beams into accelerator structures; 15 cm inner diameter solenoid magnet coils totaling 4 m in length that provide axial guide fields up to 2.5 kG; a 25/35 kG two-coil superconducting magnet and closed-cycle cryostat with a 20 cm diameter room-temperature bore; and a 6 MeV rf gun and beamline to generate, control and transport psec nC beam pulses for injection into accelerator structures or a synchrotron radiation laser, as described below. The rf gun and beamline are shown in Fig. 1. Now under installation is a 500 kV, 200 A pulse modulator and 27 kG cryostat and magnet for operation of a prototype 40 MW, 34 GHz magnicon; this device is expected to be a practical candidate as the rf driver for a future multi-TeV electron-positron collider.

External Facilities

Joint projects with outside laboratories provide access for Yale personnel to specialized research facilities, including a 4-TW Nd-glass laser for vacuum beat wave accelerator research and a 500 kV, 200 A pulsed modulator for 11.4 GHz magnicon research, at Naval Research Laboratory; and beamline space at a 100 MeV electron linear accelerator and use of a 4-TW carbon-dioxide laser for dielectric wake field and optical cyclotron autoresonance accelerator experiments at Accelerator Test Facility of Brookhaven National Laboratory. These collaborations make available facilities that cannot be duplicated at Yale due to space and cost limitations, and provide opportunities for Yale personnel to work side-by-side with experienced National Laboratory scientists.

Current Research Program

Research on new acceleration mechanisms is motivated by the desire to overcome limitations of traditional rf linear accelerators. These include beam instabilities due to higher-order transverse structure modes, limited acceleration gradients due to rf breakdown and dark current, emittance growth due to wake fields, and high cost of fabrication of complex accelerating structures. Current Yale experiments that explore new interactions that could overcome one or more of these limitations include the microwave inverse free-electron laser accelerator (thesis research topic of graduate student Rodney Yoder), microwave inverse Cerenkov accelerator (shown in Fig. 2), cyclotron auto-resonance accelerator (microwave and optical versions), vacuum beat wave accelerator, dielectric wake field accelerator, and high-gradient 34 GHz linac with acceleration gradient approaching 300 MV/m. Visible in Fig. 1 is the microwave inverse free-electron laser and the beamline for the microwave inverse Cerenkov accelerator.

Research on new means for efficient generation of high-power microwaves is motivated by the limited acceleration gradient (up to 25 MV/m) that can be achieved using existing 2.8 GHz 65-MW klystrons, as employed at Stanford Linear Collider. Plans for a 0.5-1 TeV Next Linear Collider with a gradient of 75 MV/m are based on use of newly-developed SLAC 11.4 GHz 60-MW klystrons. But no sources exist at higher frequencies, as would be needed to allow design of either a multi-TeV collider or a shorter TeV collider. The Yale Beam Physics group is developing a range of magnicons, scanning-beam amplifiers that were invented at Budker Institute of Nuclear Physics, Novosibirsk, Russia; these magnicons include an 11.4 GHz version (jointly with Naval Research Laboratory), a 1.3 GHz version (for the super-conducting collider TESLA under development at Deutsches Electronen-Synchrotron in Hamburg, Germany), and a 34 GHz, 40-MW version that is anticipated to allow up to 300 MV/m acceleration gradients in a modified rf linac structure. Microwave pulse compression in time, with attendant multiplication in magnitude, must be used in accelerator applications to realize the necessary peak powers. Yale, in a colloboration with Institute of Applied Physics in Nizhny Novgorod, Russia, is exploring active compression as a means of realizing compression ratios greater than 10:1, with acceptable efficiency. Other rf source studies at Yale include gyroharmonic conversion experiments, in which coherent radiation at a harmonic of the accelerator frequency is generated from a beam gyrating in a guide magnetic field. Fig. 3a shows gyroharmonic super-radiant emission at harmonics 2-6 from a 350 kV beam that had been spun up using 2.8 GHz radiation; measured line widths are below 400 kHz, i.e., at the Fourier-transform limit for a 2 msec pulse. When a cavity is tuned to coincide with one of the harmonics, higher coherent power at a single frequency is observed, as shown in Fig. 3b. Experiments are underway (and planned) on 7th- and 8th-gyroharmonic conversion at 20 GHz (and 91 GHz). Gyrating 6 MeV beams in a 25 kG magnetic field are also being used in a synchrotron radiation laser experiment (thesis research topic of graduate student Mei Wang), with the objective of generating tunable high-power far-infrared radiation.

Fig. 3a. Spectrum of super-radiant emission for beam not interacting with resonant structure.

Fig. 3b. Spectrum of emission for beam interacting with TE-311 cavity at 8.567 GHz.

References

"Experimental Demonstration of High Efficiency Electron Cyclotron Autoresonance Acceleration," M.A. LaPointe, R.B. Yoder, Changbiao Wang, A.K. Ganguly, and J. L. Hirshfield, Physical Review Letters, 76, 2718 (1996).

"Efficient Co-generation of Seventh-Harmonic Radiation in Cyclotron Autoresonance Acceleration," Changbiao Wang, J.L. Hirshfield, and A.K. Ganguly, Physical Review Letters, 77, 3819 (1996).

"Vacuum Beat Wave Acceleration," B. Hafizi, A. Ting, E. Esarey, P. Sprangle, and J. Krall, Physical Review E 55, 5924 (1997).

"Microwave Inverse Cerenkov Accelerator," by T. B. Zhang, T. C. Marshall, M. A. LaPointe, J. L. Hirshfield and Amiram Ron, Physical Review E 54, 1918 (1996).

"A Microwave Inverse Free-Electron-Laser Accelerator," by J. L. Hirshfield, T. C. Marshall, T. B. Zhang, A. K. Ganguly and P. A. Sprangle, Nuclear Instruments & Methods in Physics Research A 358, 129 (1995).

"X-Band Magnicon Amplifier for the Next Linear Collider," S.H. Gold, A.W. Fliflet, A.K. Kinkead, B. Hafizi, O.A. Nezhevenko, V.P. Yakovlev, J.L. Hirshfield, and R.B. True, Physics of Plasmas, 4, 1900 (1997).

"Stimulated Dielectric Wakefield Accelerator," T-B. Zhang, J.L. Hirshfield, T.C. Marshall, and B. Hafizi, Physical Review E, 56, 4647 (1997).

"A Cerenkov Source of High-Power Picosecond Pulsed Microwaves," T-B. Zhang, T.C. Marshall, and J.L. Hirshfield, IEEE Transactions on Plasma Science 26, 787 (1998).

"Multistage Cyclotron Autoresonance Accelerator," Changbiao Wang and J.L. Hirshfield, Physical Review E, 57, 7184 (1998).

"Cyclotron Autoresonance Acceleration with Seventh Harmonic Co-generation," J.L. Hirshfield, Changbiao Wang, and A.K. Ganguly, IEEE Transactions on Plasma Science, 26, 567 (1998).