Traveling Wave Section Research Articles

6 MeV novel hybrid (standing wave - traveling wave) photo-cathode electron gun for a THz superradiant FEL

A novel 6 MeV hybrid photo-injector has been designed and constructed, based on a smaller-scale prototype previously built in UCLA’s Particle Beam Physics Laboratory. It has been commissioned at Ariel University in Israel as an on-going collaboration between the two universities. This unique, new generation design provides a radically simpler approach to RF feeding of a gun/buncher system, leading to a much shorter beam via velocity bunching owed to an attached traveling wave section of the photo-injector. This design offers better performance in beam parameters, providing a high quality electron beam, with energy of 6 MeV. The hybrid photo-injector was designed to provide emittance of approximately 3 μm, and pulse duration of 150 fs at up to 1 nC per pulse. The hybrid gun is driven by a SLAC XK5 klystron as the high power RF source, and third harmonic of a ∼35 fs IR Laser amplifier (266 nm) to extract electrons from the photo-cathode. The unique electron gun will produce a bunched electron pulse to drive a THz FEL, which will operate at the super-radiant regime, and therefore requires ambitious beam properties. This paper describes the gun and presents experimental results from the gun and its sub-systems, including energy and charge measurements, compared with the design simulations.

High-power RF Conditioning of a Hybrid e-Gun at Ariel University

A novel hybrid photo injector is being commissioned at the Schlesinger center for compact accelerators in Ariel University. The hybrid gun is composed of a standing wave section (3.5 cells) and a traveling wave section (9 cells) in a single structure, and therefore has a much larger inner surface than a typical 1.5 cell electron gun. This gun will be driving a THz Free Electron Laser (FEL), operating at the super-radiance regime, and therefore requires extraordinary beam properties. First operation of the RF gun was in January 2018. High-power RF field entering a metallic cavity can severely damage the RF structure or the RF window. For this reason, in order to achieve the desired high-field gradients in the RF gun, a careful and extensive conditioning work on the gun is required. This is done by applying slowly increasing power levels into the structure, while monitoring the response constantly. To increase the efficiency and safety aspects of the conditioning work, we developed in-house routines and interlocks, controlled by a dedicated software. Methods and information of diagnostics elements in the RF system are presented, and the necessary requirements to simplify the conditioning work are outlined.

Tuning of 2.998 GHz S-band hybrid buncher for injector upgrade of LINAC II at DESY

The injector upgrade of LINAC II at DESY aims to improve its reliability and mitigate the radiological activation of components due to electron loss at relatively high energy of hundreds of MeV. Therefore, a 2.998GHz hybrid buncher has been developed and will be installed in between an existing 2.998GHz pre-buncher and LINAC II. It comprises a 1-cell standing-wave (SW) section for rapid electron acceleration and a 13-cells traveling-wave (TW) section for further beam bunching and acceleration. This paper focuses on its radio-frequency tuning procedure. The tuning strategy combines a non-resonant bead-pull measurement of complex electric field and a linear model for local reflection coefficient calculation. It is demonstrated that imaginary part of the local reflection coefficient represents the field distribution straightforwardly, based on which the structure can be tuned from cell to cell. During tuning, special attention has been paid to the field enhancement in the SW section to ensure its beam-capturing capability. Field amplitude and phase, global and local reflection coefficients have been analyzed for two different frequencies simultaneously, i.e. the intrinsic frequency of the structure and the target frequency, to avoid over-tuning. The tuning result is satisfying. For the target frequency, field unflatness of the TW section has been reduced from ±9% to ±4%, and field in the SW section has been enhanced significantly. Meanwhile, in the TW section, the deviation of phase advances between adjacent cells from the nominal value 120° has been reduced from the range ±5° to ±2°. By using ASTRA simulation, it has been verified that the residual detuning of the structure is acceptable in view of the beam dynamics performance.

Design and applications of an X-band hybrid photoinjector

Abstract An INFN-LNF/UCLA/URLS collaboration is developing a hybrid photoinjector in X-band. This device is an integrated structure consisting of initial standing wave gun cells connected at the input coupler to a traveling wave section. This design nearly eliminates RF reflections from the SW section; further, a 90° phase shift in the accelerating field at the coupling cell gives strong velocity bunching. The current initiative in X-band follows an S-band hybrid, now proceeding to construction at LNF and high power testing/beam production measurements at UCLA. This S-band hybrid has 1.5 cell SW and 9 cell TW sections, and produces strongly compressed 3.5 MeV beam. It can be used for novel applications; here we discuss the production of an exponential energy spectrum extending from 1 to 12 MeV to simulate the effects of radiation belt environments on space-craft. It can be optionally used with a 3 m TW linac fed from RF output of the hybrid, to boost the energy to 22 MeV. While scaling the design from S-band to X-band is conceptually simple, practical limits require changes in both RF and magnetostatic designs. As the field is limited by RF breakdown to 200 MV/m peak field, the SW section must be expanded to 2.5 cells to reach 3.5 MeV; this permits flexibility in the solenoid design. We present beam dynamics simulations that show 6D phase space compensation at 7 pC: sub-0.1 mm mrad at the emittance minimum that occurs simultaneously with a longitudinal focus of

Performance of a wideband, three-stage, mixed geometry gyrotwystron amplifier

A three-stage, mixed geometry gyrotwystron amplifier has recently been operated and characterized at 4.5 GHz. Gyrotwystrons are hybrids of the gyroklystron and gyro-traveling-wave tube (gyro-TWT) amplifiers. In the device which was studied, the first two stages were tunable rectangular resonator cavities, and the third (output) stage was a circular cylindrical waveguide traveling wave section. All three stages operate in fundamental mode, and are separated by cut-off drift regions. This experiment was conceived as an extension of a previous three-cavity gyroklystron experiment, the motivation being to improve upon the gyroklystron instantaneous bandwidth of 0.4% while not sacrificing efficiency or gain. Measurements of gyrotwystron performance showed a 1.5% instantaneous bandwidth, a fourfold increase over that of the gyroklystron. The gain-bandwidth product of the gyrotwystron exceeded that of the gyroklystron by a factor of 6. >

A High Current Injector for the Boeing Radiation Laboratory FEL Experiment

A two-stage subharmonic injector has been designed and installed to accelerate 150 ampere micropulses with the Boeing 20 MeV, S-band linac. The accelerator will provide microbunches with interpulse time of 50 nanoseconds throughout the RF macropulse of 20 microseconds. The electron source is a 5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> triode electron gun which is gated to produce 5A, 3ns pulses. Two standing wave prebuncher cavities are used, the first at 119 MHz, the 24th subharmonic of the linac frequency, and the second at 476 MHz, the sixth subharmonic. A short tapered phase velocity traveling wave structure provides the final bunching. The solenoidal containment field is adjustable for optimization using injector profile diagnostics. Computer analysis of bunching and radial trajectories predict beams of 2.2 nanocouloumbs with radius of 0.2 cm and phase width of π/2 of the fundamental at the entrance to the traveling wave section.