Mitigation of multipacting in 113 MHz superconducting rf photoinjector

Abstract

Superconducting RF (SRF) photo-injectors are one of the most promising devices for generating continuous wave (CW) electron beams with record high brightness. Ultra-high vacuum of SRF guns provides for long lifetime of the high quantum efficiency (QE) photocathodes, while SRF technology provides for high accelerating gradients exceeding 10 MV/m. It is especially true for low frequency SRF guns where electrons are generated at photocathodes at the crest of accelerating voltage. Two main physics challenges of SRF guns are their compatibility with high QE photocathodes and multipacting. The first is related to a possibility of deposition of photocathode materials (such as Cs) on the walls of the SRF cavity, which can result in increased dark current via reduction of the bulk Nb work function and in enhancing of a secondary electron emission yield (SEY). SEY plays critical role in multipacting (e.g. an exponential growth of the multipactor discharge), which could both spoil the gun vacuum and speed up the deposition of the cathode material on the walls of the SRF cavity. In short, the multipactor behavior in superconducting accelerating units must be well understood for successful operation of an SRF photo-injector. In this paper we present our studies of 1.2 MV 113 MHz quarter-wave SRF photo-injector serving as a source of electron beam for the Coherent electron Cooling experiment (CeC) at BNL. During three years of operating our SRF gun we encountered a number of multipacting zones. We also observed that presence of CsK2Sb photocathode in the gun could create additional multipacting barriers. We had conducted a comprehensive numerical and experimental study of the multipactor discharge in our SRF gun. We had developed a process of crossing the multipacting barriers from zero to the operational voltage without affecting the lifetime of our photocathode and enhancing the strength of multipacting barriers. We found a good agreement between the results of simulations and our experimental data.

Long lifetime of bialkali photocathodes operating in high gradient superconducting radio frequency gun

“…We observed a number of QE degrading events during the gun commissioning in 2016, which mostly occurred because of the low voltage multipacting during the turn-on processes of the gun. After detailed studies and understanding of the problem, we developed a low level RF control script to automatically detect and to avoid multipacting zones 31 . With mitigated multipacting, the cathode operational lifetime increased typically to a month, and sometimes even longer.…”

Minimum thickness of carbon coating for multipacting suppression

“…Multipacting (MP) [1,2] is a resonant electron discharge, plaguing an extremely vast range of devices, produced by the synchronization of emitted electrons with the radio-frequency (rf) field (or with a passing beam, in accelerators) and by the electron multiplication at the impact point with the surface. An extremely vast range of research fields, spanning from Hall effect thrusters [3,4], divertors in tokamaks [5], highpower microwave systems for satellite applications [6][7][8], radio-frequency cavities [9][10][11] and antennas [12][13][14], optics for extreme ultraviolet (EUV) lithography [15], to particle accelerators [2,16,17] base some of their essential functionalities on the control and mitigation of multipacting. In particle accelerators, multipacting is better known as the "e − cloud effect" [2].…”

“…Any solution that will decrease SEY around unity or below will grant full control of any detrimental MP effects and will become an essential ingredient in the design of many devices. Different approaches have been proposed to this end, one being to treat the device surfaces by coating them with intrinsically low SEY materials [2][3][4][5][6][7][9][10][11][12][13]16,17,[19][20][21][22][23][24][25][26]. In satellite rf structures, Alodine (chromate conversion coating) has been the reference antimultipactor coating for the space industry for decades because it presents a relatively low SEY, a good electrical conductivity, and it is very stable in air.…”

First experience with He conditioning of an SRF photoinjector

“…The early stages of operation were challenged by the presence of persistent low-level MP barriers, but the issue was methodically evaluated and successfully resolved resulting in complete elimination of MP as an obstacle for the cavity operation. The detailed description of our MP studies, and the dedicated start-up procedure developed to avoid the MP issue can be found in [12].…”