High-gradient breakdown studies of an X -band Compact Linear Collider prototype structure

Abstract

A Compact Linear Collider prototype traveling-wave accelerator structure fabricated at Tsinghua University was recently high-gradient tested at the High Energy Accelerator Research Organization (KEK). This X-band structure showed good high-gradient performance of up to 100 MV=m and obtained a breakdown rate of 1.27 × 10 −8 per pulse per meter at a pulse length of 250 ns. This performance was similar to that of previous structures tested at KEK and the test facility at the European Organization for Nuclear Research (CERN), thereby validating the assembly and bonding of the fabricated structure. Phenomena related to vacuum breakdown were investigated and are discussed in the present study. Evaluation of the breakdown timing revealed a special type of breakdown occurring in the immediately succeeding pulse after a usual breakdown. These breakdowns tended to occur at the beginning of the rf pulse, whereas usual breakdowns were uniformly distributed in the rf pulse. The high-gradient test was conducted under the international collaboration research program among Tsinghua University, CERN, and KEK.

Mechanism of vacuum breakdown in radio-frequency accelerating structures

“…Obviously, if relation (9) describes a (sharp) increase in the probability of breakdown with electric field amplitude, relation (10) should describe the dependence of E A on the delay time to breakdown. Relation (10) implies a probability of breakdown during an rf pulse if the breakdown delay time ("breakdown timing," in the terminology of [4]) is not greater than the pulse duration. For a rather large number of "normal breakdowns" (NL-BD), these times are uniformly distributed over the rf pulse.…”

“…The amplitude of the voltage U ¼ U A sinðωtÞ applied to the diode was chosen so that the maximum macroscopic field at the cathode, E A ¼ U A =d, be some hundreds of volts per meter. The breakdown delay time was calculated using relation (4) with the current density averaged over the rf field oscillation period, j av .…”

“…The initiating mechanism of vacuum breakdown in rf accelerating structures is the subject of studies aimed at improving the stability of operation of the structures and at seeking ways for enhancing the accelerating electric field. These studies have been carried out since the development of the linear collider; however, the mechanism of vacuum breakdown in the CLIC accelerating structures still remains obscure [2][3][4][5][6].…”

“…Nevertheless, recently a number of important experimental results have been obtained which, in our opinion, make a significant contribution to the understanding of the physical processes underlying the rf conditioning of the surface of an accelerating structure [4][5][6]. In particular, two types of breakdown were observed [4][5]: "normal breakdown" (NL-BD), which occurred after operation over many pulses without breakdown, and "following-pulse breakdown" (FP-BD), which occurred at the first pulse following the breakdown pulse and was spatially linked with the latter, i.e., occurred at the same place. Analysis of the conditioning of several structures tested at KEK and CERN revealed clear evidence that the conditioning progressed with number of rf pulses and not with number of breakdowns [6].…”

Cascaded Multicycle Terahertz-Driven Ultrafast Electron Acceleration and Manipulation

“…Recently, there has been significant interest in the use of THz based particle acceleration and beam manipulation as a new approach with the potential to overcome certain limitations of RF accelerators and other laser-based concepts. In particular, the picosecond time-scale of lasergenerated THz sources is expected to enable order-of-magnitude increases in sustainable field strength [9,10] to the GV/m [11][12][13][14] range, and increased precision in timing compared to RF sources while still supporting moderate charge at the picocoulomb level [15]. Proof-of-principle demonstrations with single-cycle THz pulses have resulted in multi-keV acceleration [16,17] and high-field manipulations of electron-bunch phase space [18][19][20] proving the feasibility of compact THz-based devices for future radiation sources or as components for boosting the performance of existing accelerators as well as a promising solution to produce high repetition, high energy ultrafast electron bunches, which are highly desired in ultrafast electron diffraction [21,22].…”