Energy-spread compensation of a thermionic-cathode rf gun

The Advanced Photon Source (APS) ballistic bunch compression (BBC) gun was designed as a prototype injector to explore the technique of drift-space compression of a high-brightness beam [J.W. Lewellen and S.V. Milton, "Preliminary Calculations of Ballistic Bunch Compression with Thermionic Cathode RF Guns", Proc. SPIE - Int. Soc. Opt. Eng (Coherent Electron Beam X-Ray Sources: Techniques and Applications) Vol. 3154, pp. 162-171 (1997)]. It is constructed from three independently powered S-band rf cells, one cathode half-cell, and two full cells, and is designed to operate with both thermionic cathodes and photocathodes; this flexibility allows the gun to be operated in modes other than for ballistic compression. In particular, appropriate choices of rf power and phase can be used to reduce the energy spread of the beam produced by the gun. With a thermionic cathode, simulations indicate that a 1% FWFM energy spread can contain at least 90% of the emitted beam charge. This operating mode demonstrates many of the capabilities required for ballistic bunch compression, but allows verification of the basic gun performance via a much easier measurement. Also, such a beam is of interest as a source for other experiments, e.g., low-cost, compact, free-electron lasers.