Abstract
We are exploring the physics and frequency-scaling of vacuum rf breakdowns at sub-THz frequencies. We present the experimental results of rf tests performed in metallic mm-wave accelerating structures. 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. We compared the performances of metal structures made with copper and stainless steel. The rf frequency of the fundamental accelerating mode, propagating in the structures at the speed of light, varies from 115 to 140 GHz. The traveling wave structures are 0.1 m long and composed of 125 coupled cavities each. We determined the peak electric field and pulse length where the structures were not damaged by rf breakdowns. We calculated the electric and magnetic field correlated with the rf breakdowns using the FACET bunch parameters. The wakefields were calculated by a frequency domain method using periodic eigensolutions. Such a method takes into account wall losses and is applicable to a large variety of geometries. The maximum achieved accelerating gradient is $0.3\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ with a peak surface electric field of $1.5\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ and a pulse length of about 2.4 ns.
Highlights
Accelerating gradient is one of the crucial parameters affecting the design, construction and cost of the generation linear accelerators
The rf fields were excited by the FACET ultrarelativistic electron beam
During development of the Linear Collider (NLC)/ Global Linear Collider (GLC), an X-band test accelerator was built, which operated at 65 MV=m unloaded gradient [2,3]
Summary
Accelerating gradient is one of the crucial parameters affecting the design, construction and cost of the generation linear accelerators. The challenge is to develop reliable and cost-effective high-gradient accelerators. The typical working frequencies of linear accelerators are from about 1 to 12 GHz. The accelerating gradient of the long-lived SLAC S-band linac is about 17 MV=m [1]. During development of the Linear Collider (NLC)/ Global Linear Collider (GLC), an X-band test accelerator was built, which operated at 65 MV=m unloaded gradient [2,3]. The CERN based linear collider design CLIC requires 100 MV=m loaded gradient at 12 GHz in accelerating structures with heavy wakefield damping [4]. Future accelerators may need even higher gradients, such as compact synchrotron light sources or inverse Compton scattering gamma ray sources [5]
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