Abstract
Reducing the operating temperature of normal conducting particle accelerators substantially increases their efficiency. Low-temperature operation increases the yield strength of the accelerator material and reduces surface resistance, hence a great reduction in cyclic fatigue could be achieved resulting in a large reduction in breakdown rates compared to room-temperature operation. Furthermore, temperature reduction increases the intrinsic quality factor of the accelerating cavities, and consequently, the shunt impedance leading to increased system efficiency and beam loading capabilities. In this paper, we present an experimental demonstration of the high-gradient operation of an X-band, 11.424 GHz, 20-cells linear accelerator (linac) operating at a liquid nitrogen temperature of 77 K. The tested linac was previously processed and tested at room temperature. We verified the enhanced accelerating parameters of the tested accelerator at cryogenic temperature using different measurements including electron beam acceleration up to a gradient of 150 MV/m, corresponding to a peak surface electric field of 375 MV/m. We also measured the breakdown rates in the tested structure showing a reduction of two orders of magnitude, x100, compared to their values at room temperature for the same accelerating gradient.
Highlights
The operation of particle accelerators at high-gradient levels is essential for future discovery machines including particle colliders and free-electron lasers [1,2,3]
There has been an effort to minimize the breakdown rates in highgradient accelerators by building accelerator structures from harder materials to reduce the cyclic fatigue on the surface, and lower breakdown rates compared to the ones built from softer materials [14,15]. Another approach that was experimentally investigated to enhance the operation of Normal conducting (NC) accelerators is the operation at cryogenic temperatures which reduces the surface resistance compared to room-temperature operation
We should emphasize that the increase of the coupling coefficient, β 1⁄4 Q0=Qext, for the accelerating structure from 1 at 300 K to 2.25 at 77 K represents a large increase in the beam-loading capability for the same accelerator structure just by cooling it down to cryogenic temperature
Summary
The operation of particle accelerators at high-gradient levels is essential for future discovery machines including particle colliders and free-electron lasers [1,2,3]. Another approach that was experimentally investigated to enhance the operation of NC accelerators is the operation at cryogenic temperatures which reduces the surface resistance compared to room-temperature operation This reduction increases the shunt impedance and internal quality factor of the accelerating cavities, leading to increased rf-to-beam efficiency and beam loading capabilities [16]. The authors performed an experimental investigation on a single-cell accelerating structure at 11.424 GHz and 45 K They reported the higher operation field at the same breakdown rate at cryogenic temperatures compared to room-temperature operation. The experimental effort in [18,19,20,22,23] showed a large promise in the operation of NC accelerators at cryogenic temperatures for enhanced shunt impedance and reduced breakdown rates This effort was performed, for single-cell testing and/or low-gradient operation, below 50 MV=m. Combining the enhanced shunt impedance using the distributed-coupling technology with the boosted performance at cryogenic temperature is capable of providing new frontiers in NC accelerating structures at high-gradient operation
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