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To produce a compact and efficient particle accelerator, several different piezoelectric transformer configurations are evaluated for their applicability in this demanding application area. Using a derived 1-D model for three different electroding and mounting schemes, accelerator efficiency and gradient are optimized. It is shown that with some advancements in material yield strength and advanced bonding techniques, reaching 4 MeV/m with over 40% efficiency is possible.

We are developing a high-gradient, THz-driven electron gun. THz-frequency accelerator structures can enable the development of compact, bright, and efficient particle sources. Our two cell standing-wave gun is powered by a 110 GHz gyrotron and will produce 350 keV electrons with 500 k $W$ of input power. The field emission cathode is a copper tip located in the first cell. High power measurements will include diagnostics to measure the energy spread, charge, and size of the resulting bunch.

We report the experimental demonstration of a mm-wave electron accelerating structure powered by a high-power rf source. We demonstrate reliable coupling of an unprecedented rf power—up to 575 kW into the mm-wave accelerator structure using a quasi-optical setup. This standing wave accelerating structure consists of a single-cell copper cavity and a Gaussian to TM01 mode converter. The accelerator structure is powered by 110 GHz, 10-ns long rf pulses. These pulses are chopped from 3 ms pulses from a gyrotron oscillator using a laser-driven silicon switch. We show an unprecedented high gradient up to 230 MV/m that corresponds to a peak surface electric field of more than 520 MV/m. We have achieved these results after conditioning the cavity with more than 105 pulses. We also report preliminary measurements of rf breakdown rates, which are important for understanding rf breakdown physics in the millimeter-wave regime. These results open up many frontiers for applications not only limited to the next generation particle accelerators but also x-ray generation, probing material dynamics, and nonlinear light-matter interactions at mm-wave frequency.

A prototype 11.994 GHz, traveling-wave accelerating structure for the Compact Linear Collider has been built, using the novel technique of assembling the structure from milled halves. The use of milled halves has many advantages when compared to a structure made from individual disks. These include the potential for a reduction in cost, because there are fewer parts, as well as a greater freedom in choice of joining technology because there are no rf currents across the halves’ joint. Here we present the rf design and fabrication of the prototype structure, followed by the results of the high-power test and post-test surface analysis. During high-power testing the structure reached an unloaded gradient of 100 MV/m at a rf breakdown rate of less than $1.5 \times 10^{-5}$ breakdowns/pulse/m with a 200 ns pulse. This structure has been designed for the CLIC testing program but construction from halves can be advantageous in a wide variety of applications.

Ionization cooling is the preferred method for producing bright muon beams. This cooling technique requires the operation of normal conducting, radio-frequency (RF) accelerating cavities within the multi-tesla fields of DC solenoid magnets. Under these conditions, cavities exhibit increased susceptibility to RF breakdown, which can damage channel components and imposes limits on channel length and transmission efficiency. We present a solution to the problem of breakdown in strong magnetic fields. We report, for the first time, stable high-vacuum, copper cavity operation at gradients above 50 MV/m and in an external magnetic field of three tesla. This eliminates a significant technical risk that has previously been inherent in ionization cooling channel designs., Comment: 7 pages, 7 figures

We have successfully designed, fabricated, installed, and tested a super compact $X$-band SLAC Energy Doubler system at SLAC. It is composed of an elegant 3 dB coupler--mode converter--polarizer coupled to a single spherical energy storage cavity with high ${Q}_{0}$ of 94000 and a diameter less than 12 cm. The available rf peak power of 50 MW can be compressed to a peak average power of more than 200 MW in order to double the kick for the electron bunches in a rf transverse deflector system and greatly improve the measurement resolution of both the electron bunches and the x-ray free-electron laser pulses. The design physics and fabrication as well as the measurement results will be presented in detail. High-power operation has demonstrated the excellent performance of this rf compression system without rf breakdown, sign of pulse heating, and rf radiation.

Traditional linear interaction RF sources (klystrons and traveling wave tubes) provide limited amounts of power at the mm-wave and terahertz part of the spectrum because their critical dimensions have to be small compared to the wavelength they operate. Due to these small dimensions, the amount of current that can go through the beam apertures is limited. We designed, built and hot-tested a new gyrocon-like mm-wave source, with an output interaction circuit based on a spherical sector cavity. This spherical sector cavity provides a large beam aperture, allowing for a significant amount of current to go through. This source operates as a 5th harmonic frequency multiplier, with an output frequency of 57.12 GHz. The measured peak output power was 54.26 W at the 5th harmonic, while power at the other harmonics was at least 33.28 dB lower than the 5th harmonic.

We report the experimental demonstration of a 5th harmonic mm-wave frequency multiplying vacuum electronic device, which uses an over-moded spherical sector output cavity. In this device, a pencil electron beam is helically deflected in a transverse deflecting cavity before entering the output cavity. No magnetic field is required to focus or guide the beam. We built and tested a proof-of-principle device with an output frequency of 57.12 GHz. The measured peak power was 52.67 W at the 5th harmonic of the drive frequency. Power at the 4th, 6th, and 7th harmonics was 33.28 dB lower than that at the 5th harmonic.

Development of a 200 kW CW sheet beam klystron for the Office of Naval Research. Specifications, beam stability, and cathode bell jar testing are discussed.

2The SLAC XL5 klystron, which produces 50 MW at 12 GHz, is being used in several high gradient accelerators and test stands. The design has been transferred to industry and is currently being modified for ease of assembly and for cost reduction. Two klystrons of the new design will be manufactured and fully characterized.

Two 11.424 GHz single cell standing wave accelerating structures have been fabricated for high gradient RF breakdown studies. Both are brazed structures: one made from copper and the other from sintered molybdenum bulk. The tests results are presented and compared to those of similar devices constructed at SLAC ( Stanford Linear Accelerator Center ) and KEK ( Kō Enerugī Kasokuki Kenkyū Kikō ). The technological issues to build both sections are discussed.

High average power millimeter wave sources are being developed in a collaboration between SLAC and CPI. CPI has developed a number of high-power, broadband, millimeter-wave, CCTWT amplifiers. The coupled cavity technology is now approaching the limits for generating power at millimeter wave frequencies. To generate significant increases in power, other technologies need to be considered. At SLAC, compact, lightweight, PPM-focused, klystrons, nicknamed 'klystrinos' are being developed, using lithographic fabrication methods (LIGA). These devices will operate at 100 kW peak power and 1 kW average power at 95 GHz. Two design areas are critical to the success of the klystrino. First is the electron beam optics and beam focusing. A beam tester is being built to evaluate the electron gun and the PPM focusing system. Results of the beam tests are presented. CPI has analyzed the electron gun and is investigating the addition of a control electrode for improved modulator efficiency. The second significant design area is the modeling of the klystrino. The combination of the rectangular LIGA structure and the multiple gap output section make modeling the klystrino difficult. MAFIA and MAGIC have been used to develop an equivalent 2D model of the 3D LIGA circuit. Thermal capacity of the circuit is also investigated at CPI using the ANSYS 3D FEA code. The analysis included both the expected RF losses and beam interception. The calculations include both transient response and steady state. The thermal analysis is used to predict the maximum allowable beam interception and duty cycle that the circuit can withstand without major problems.

The Klystrode® (IOT) is a new high power electron tube which combines the features of Klystron and the Tetrode. During the past six years it has virtually taken over the UHF-TV market and has demonstrated its capabilities in high power research applications. This paper describes the principles of Klystrode IOT operation and the power and frequency ranges where it is most useful. Field experience with over fifty (50) transmitters in UHF TV service is reported as well as test results from high power scientific devices operated at 267 MHz, 250 kW (CW) and 425 MHz, 500 kW (pulse). Finally, future improvements in the Klystrode IOT to reduce cost, improve efficiency and optimize the tube for industrial applications.