rf breakdown measurements in electron beam driven 200 GHz copper and copper-silver accelerating structures

Abstract

This paper explores the physics of vacuum rf breakdowns in subterahertz high-gradient traveling-wave accelerating structures. We present the experimental results of rf tests of 200 GHz metallic accelerating structures, made of copper and copper-silver. These experiments were carried out at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory. The rf fields were excited by the FACET ultrarelativistic electron beam. The traveling-wave structure is an open geometry, 10 cm long, composed of two halves separated by a gap. The rf frequency of the fundamental accelerating mode depends on the gap size and can be changed from 160 to 235 GHz. When the beam travels off axis, a deflecting field is induced in addition to the longitudinal field. We measure the deflecting forces by observing the displacement of the electron bunch and use this measurement to verify the expected accelerating gradient. Furthermore, we present the first quantitative measurement of rf breakdown rates in 200 GHz metallic accelerating structures. The breakdown rate of the copper structure is 10 −2 per pulse, with a peak surface electric field of 500 MV=m and a rf pulse length of 0.3 ns, which at a relatively large gap of 1.5 mm, or one wavelength, corresponds to an accelerating gradient of 56 MV=m. For the same breakdown rate, the copper-silver structure has a peak electric field of 320 MV=m at a pulse length of 0.5 ns. For a gap of 1.1 mm, or 0.74 wavelengths, this corresponds to an accelerating gradient of 50 MV=m.

Design of the Compact Linear Collider main linac accelerating structure made from two halves

“…Both of these advantages promise great cost reduction in comparison to the standard assembly. Holding these benefits, the open structure is attractive for designs of high-frequency corrugate structures and has several useful applications [11][12][13][14][15].…”

Use of a corrugated beam pipe as a passive deflector for bunch length measurements

“…The transverse kick induced in this process makes it possible to use a dechirper as a passive deflector [7,8] or a fast kicker [9]. We also note that the deflecting forces in open accelerating structures [10,11] have similar features as the transverse kicks in the dechirper.…”

Measurements of electron beam deflection and rf breakdown rate from a surface wave guided in metallic mm-wave accelerating structures

“…As part of research on the physics of vacuum rf breakdowns we performed experiments with open mm-wave metallic accelerating structures, made with hard copper, hard copper-silver alloy and stainless steel. We reported the result of our previous experiments in [1][2][3][4]. The field in the structures were excited by an ultrarelativistic electron beam.…”

“…We reported in the previous papers [1][2][3][4] the rf breakdown rate measurements performed with small gaps and with the electron beam travelling along the central axis between the two structure halves. This regime corresponds to the typical operation of particle accelerators.…”