  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.
