Demonstration of low emittance in the Cornell energy recovery linac injector prototype

We present a detailed study of the six-dimensional phase space of the electron beam produced by the Cornell Energy Recovery Linac Photoinjector, a high-brightness, high repetition rate (1.3 GHz) DC photoemission source designed to drive a hard x-ray energy recovery linac (ERL). A complete simulation model of the injector has been constructed, verified by measurement, and optimized. Both the horizontal and vertical 2D transverse phase spaces, as well as the time-resolved (sliced) horizontal phase space, were simulated and directly measured at the end of the injector for 19 and 77 pC bunches at roughly 8 MeV. These bunch charges were chosen because they correspond to 25 and 100 mA average current if operating at the full 1.3 GHz repetition rate. The resulting 90% normalized transverse emittances for $19\text{ }(77)\text{ }\text{ }\mathrm{pC}/\mathrm{bunch}$ were $0.23\ifmmode\pm\else\textpm\fi{}0.02$ $(0.51\ifmmode\pm\else\textpm\fi{}0.04)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in the horizontal plane, and $0.14\ifmmode\pm\else\textpm\fi{}0.01$ $(0.29\ifmmode\pm\else\textpm\fi{}0.02)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in the vertical plane, respectively. These emittances were measured with a corresponding bunch length of $2.1\ifmmode\pm\else\textpm\fi{}0.1$ $(3.0\ifmmode\pm\else\textpm\fi{}0.2)\text{ }\text{ }\mathrm{ps}$, respectively. In each case the rms momentum spread was determined to be on the order of ${10}^{\ensuremath{-}3}$. Excellent overall agreement between measurement and simulation has been demonstrated. Using the emittances and bunch length measured at $19\text{ }\text{ }\mathrm{pC}/\mathrm{bunch}$, we estimate the electron beam quality in a 1.3 GHz, 5 GeV hard x-ray ERL to be at least a factor of 20 times better than that of existing storage rings when the rms energy spread of each device is considered. These results represent a milestone for the field of high-brightness, high-current photoinjectors.