Valery Dolgashev

Geometric dependence of radio-frequency breakdown in normal conducting accelerating structures

We present the experimental results of a systematic study of rf breakdown phenomenon in high vacuum accelerator structures. In this study, the surface processing, geometry, and materials of the structures have been varied, one parameter at a time. The breakdown rate or alternatively, the probability of breakdown/pulse/meter has been recorded for different operating parameters. These statistical data reveal a strong dependence of breakdown probability on surface magnetic field, or alternatively on surface pulsed heating. This is in contrast to the classical view of electric field dependence. We will present our experimental methodology and results showing this remarkable correlation.

Progress Toward Externally Powered X-Band Dielectric-Loaded Accelerating Structures

We summarize recent progress in a program to develop externally powered dielectric-loaded accelerating (DLA) structures that can sustain high accelerating gradients. High-power RF tests of earlier structures showed strong multipactor loading. In addition, arcing at dielectric joints between the uniform DLA structure and matching sections at either end limited the achievable gradient. In this paper, we study the onset of multipactor in a DLA structure. We also study the effect of thin-film TiN coatings applied by atomic layer deposition and the effect of a reduction in the inner diameter of the structure. Test results of these structures show significant decreases in multipactor loading. We also test new structure designs that eliminate separate dielectric matching sections and, thus, the requirement for dielectric joints, including a DLA structure using a coaxial coupler and a clamped DLA structure. The clamped structure demonstrated a significantly improved gradient without breakdown.

Research and Development for Ultra-High Gradient Accelerator Structures

Research on the basic physics of high‐gradient, high frequency accelerator structures and the associated RF/microwave technology are essential for the future of discovery science, medicine and biology, energy and environment, and national security. We will review the state‐of‐the‐art for the development of high gradient linear accelerators. We will present the research activities aimed at exploring the basic physics phenomenon of RF breakdown. We present the experimental results of a true systematic study in which the surface processing, geometry, and materials of the structures have been varied, one parameter at a time. The breakdown rate or alternatively, the probability of breakdown/pulse/meter has been recorded for different operating parameters. These statistical data reveal a strong dependence of breakdown probability on surface magnetic field, or alternatively on surface pulsed heating. This is in contrast to the classical view of electric field dependence.

High power tests of an electroforming cavity operating at 11.424 GHz

The achievement of ultra high accelerating gradients is mandatory in order to fabricate compact accelerators at 11.424 GHz for scientific and industrial applications. An extensive experimental and theoretical program to determine a reliable ultra high gradient operation of the future linear accelerators is under way in many laboratories. In particular, systematic studies on the 11.424 GHz frequency accelerator structures, R&D on new materials and the associated microwave technology are in progress to achieve accelerating gradients well above 120 MeV/m. Among the many, the electroforming procedure is a promising approach to manufacture high performance RF devices in order to avoid the high temperature brazing and to produce precise RF structures. We report here the characterization of a hard high gradient RF accelerating structure at 11.424 GHz fabricated using the electroforming technique. Low-level RF measurements and high power RF tests carried out at the SLAC National Accelerator Laboratory on this prototype are presented and discussed. In addition, we present also a possible layout where the water-cooling of irises based on the electroforming process has been considered for the first time.

Recent Studies of RF Breakdown Physics in Normal Conducting Cavities

The operating accelerating gradient in normal conducting accelerating structures is often limited by rf breakdown. The behavior of the rf breakdown depends on multiple parameters, including the input rf power, rf circuit, cavity shape and material. Here we discuss recent experimental data and theoretical studies of rf breakdown physics.

Prototyping high-gradient mm-wave accelerating structures

We present single-cell accelerating structures designed for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures are π-mode standing-wave cavities fed with a TM 01 circular waveguide. The structures are fabricated using precision milling out of two metal blocks, and the blocks are joined with diffusion bonding and brazing. The impact of fabrication and joining techniques on the cell geometry and RF performance will be discussed. First prototypes had a measured Q 0 of 2800, approaching the theoretical design value of 3300. The geometry of these accelerating structures are as close as practical to singlecell standing-wave X-band accelerating structures more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. Furthermore, the structures will be powered with short pulses from a MW gyrotron oscillator. RF power of 1 MW may allow an accelerating gradient of 400 MeV/m to be reached.

Experimental demonstration of a 5th harmonic mm-wave frequency multiplying vacuum tube

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.

High power S-band window optimized to minimize electric and magnetic field on the surface

RF windows are used to separate high and low vacuum regions in high power microwave systems, such as klystrons and RF distribution. RF breakdowns in megawatt environments could damage the window. An Sband RF window was designed to reduce electric and magnetic fields in the waveguide joints and the ceramic.

Ultra-high brightness electron beams from very-high field cryogenic radiofrequency photocathode sources

Abstract Recent investigations of RF copper structures operated at cryogenic temperatures performed by a SLAC-UCLA collaboration have shown a dramatic increase in the maximum surface electric field, to 500 MV/m. We examine use of these fields to enable very high field cryogenic photoinjectors that can attain over an order of magnitude increase in peak electron beam brightness. We present beam dynamics studies relevant to X-ray FEL injectors, using start-to-end simulations that show the high brightness and low emittance of this source enables operation of a compact FEL reaching a photon energy of 80 keV. The preservation of beam brightness in compression, exploiting micro-bunching techniques is discussed. While the gain in brightness at high field is due to increase of the emission current density, further increases in brightness due to lowering of the intrinsic cathode emittance in cryogenic operation are also enabled. While the original proposal for this type of cryogenic, ultra-high field photoinjector has emphasized S-band designs, there are numerous potential advantages that may be conferred by operation in C-band. We examine issues related to experimental implementation in C-band, and expected performance of this type of device in a future hard X-ray FEL such as MaRIE.

Results of high power tests of dual mode accelerating structure

We report recent results of high power tests of dual mode accelerating structure. This test is a part of an experimental and theoretical study of rf breakdown in normal conducting structures at 11.4 GHz. The goal of this study is to determine the accelerating gradient capability of normal-conducting rf powered particle accelerators. This specific experiment studies effect of rf magnetic fields on breakdown probability independently on rf electric fields.