Performance of a second generation X -band rf photoinjector

Abstract

rf photoinjectors produce incredibly bright electron beams enabling advanced photon science applications such as the current generation of free electron lasers and high energy x-rays and gamma-rays via laser-Compton scattering. A second generation 5.59 cell $X$-band rf gun has been developed, installed, conditioned, commissioned, tuned, and used to produce laser-Compton x-rays and multiple electron bunches. A charge per bunch from a few pC to 500 pC has been measured, consistent with a quantum efficiency of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ using a 263 nm 10 Hz photocathode drive laser. The rf gun has operated close to design performance at high gradient, and more reliably at lower gradient achieving a root mean square normalized emittance of 0.3 mm mrad at both 80 pC at $185\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, and 40 pC at $165\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. Thermal emittance is estimated at $0.55\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}/\mathrm{mm}$. Energy spread of 0.03% has been achieved. These results agree very well with modeling predictions for the operating conditions under which the measurements were made. Unusually disruptive breakdowns were observed with an applied magnetic field of $0.5T$ used for emittance compensation.