Experimental high gradient testing of a 17.1 GHz photonic band-gap accelerator structure

Abstract

We report the design, fabrication, and high gradient testing of a 17.1 GHz photonic band-gap (PBG) accelerator structure. Photonic band-gap (PBG) structures are promising candidates for electron accelerators capable of high-gradient operation because they have the inherent damping of high order modes required to avoid beam breakup instabilities. The 17.1 GHz PBG structure tested was a single cell structure composed of a triangular array of round copper rods of radius 1.45 mm spaced by 8.05 mm. The test assembly consisted of the test PBG cell located between conventional (pillbox) input and output cells, with input power of up to 4 MW from a klystron supplied via a TM 01 mode launcher. Breakdown at high gradient was observed by diagnostics including reflected power, downstream and upstream current monitors and visible light emission. The testing procedure was first benchmarked with a conventional disc-loaded waveguide structure, which reached a gradient of 87 MV=m at a breakdown probability of 1.19 × 10 −1 per pulse per meter. The PBG structure was tested with 100 ns pulses at gradient levels of less than 90 MV=m in order to limit the surface temperature rise to 120 K. The PBG structure reached up to 89 MV=m at a breakdown probability of 1.09 × 10 −1 per pulse per meter. These test results show that a PBG structure can simultaneously operate at high gradients and low breakdown probability, while also providing wakefield damping.

High power experimental studies of hybrid photonic band gap accelerator structures

“…Comparing with the metallic PBG structure tested at MIT [16,17], the dark current of the HPBG-d3 structure was much higher even at a low gradient level. Two sources may be responsible: (1) the high surface E field which was enhanced by the triple point [31] and the gaps between the components introduced by the clamped assembly, and (2) multipactor which was more easily triggered by a system including a dielectric.…”

“…The procedure for the high power, high gradient testing of the HPBG structures was similar to what had been conducted with the metallic PBG structures [11,12,16,17]. The experiments were conducted at the MIT PSFC Accelerator Laboratory.…”

“…In order to achieve higher gradient, new standing wave PBG structures were designed to put into high power testing. Metallic PBG structures were high power tested to investigate breakdown phenomena at SLAC at X-Band, 11.424 GHz, and at MIT at Ku-Band, 17.14 GHz [11,12,16,17]. The gradients and breakdown probabilities achieved by the metallic PBG structures were found to be comparable with those of conventional disk-loaded waveguide (DLWG) cavities [16,17].…”

“…Metallic PBG structures were high power tested to investigate breakdown phenomena at SLAC at X-Band, 11.424 GHz, and at MIT at Ku-Band, 17.14 GHz [11,12,16,17]. The gradients and breakdown probabilities achieved by the metallic PBG structures were found to be comparable with those of conventional disk-loaded waveguide (DLWG) cavities [16,17]. Wakefield damping has been studied in simulation for metallic PBG structures including both room temperature cavities and superconducting cavities [13,18].…”

Measurement of internal dark current in a 17 GHz accelerator structure with an elliptical sidewall

“…ELLIPTICAL SIDEWALL Figure 2 compares the design of the previously studied single cell standing wave accelerator cavity having a straight sidewall [MIT-DLWG, Fig. 2(a) [32] ] with the elliptical sidewall cavity [ Fig. 2(b)] discussed in this paper.…”