High average beam power linac waveguides

The constant gradient condition with an intended beam loading is investigated. The motive for the investigation is the observation that when a method of heat removal from a length of waveguide has been developed, the whole waveguide can be operated advantageously at the given rate of RF heat deposition. The heat transfer process implies a maximum power attenuation per unit length (2IP), where 2I is the power attenuation coefficient (nepers/m). Achievement of high beam power linacs implies that this power attenuation ought to be used along the entire waveguide length. As a consequence, the waveguide design will automatically be constant gradient, since the field strength E²=2IPr and the shunt impedance per unit length is nearly constant. As an application, the author considers the design of a continuous-wave (CW) traveling wave linear accelerator intended for operation at 1 A, having a 4-MW, L-band RF power source, and where what is wanted is 80% beam power conversion efficiency (η-0.8), which therefore implies an energy gain per section of 3.2 MeV