rf breakdown tests of mm-wave metallic accelerating structures

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 GV=m with a peak surface electric field of 1.5 GV=m and a pulse length of about 2.4 ns.

Prototyping high-gradient mm-wave accelerating structures

“…Recently, we have expanded our experiments on the basic physics of ultrahigh vacuum RF breakdown to the mm-wave range with beam-driven accelerators at FACET [13,14]. In this paper, we continue this study [15] using a 110 GHz MW pulsed gyrotron oscillator [16] as the RF power source.…”

Roadmap to the Future

“…Experiments have shown that shorter exposure to high-field stresses reduces both the breakdown rate and the long-term material damage [2]. Therefore, power sources capable of delivering very high peak powers in very short pulses (such as nanoseconds for microwave frequencies, or femtoseconds for optical frequencies) have an advantage in producing very high gradients.…”

Summary report of working group 3: Laser and high-gradient structure-based acceleration

“…RF structures operating in the 100-200 GHz range offer the promise of high accelerating gradients and include features that are realizable with advanced standard machining techniques [7]. M. Dal Forno of SLAC National Accelerator Laboratory provided an overview of the mm-wave accelerating structure breakdown physics research program at SLAC, and presented results of quantitative measurements of rf breakdown rates in a 200 GHz structure driven by the intense electron beam at FACET.…”

“…The experimental goal was to determine statistical properties of rf breakdown in these structures, including material tests for copper and copper-silver. The main diagnostics included arc detector to measure the field emission current and reliably detect rf breakdowns and a pyroelectric detector to measure the radiated energy [8]. The FACET beam parameters included 3.2 nC and varied bunch lengths.…”