Status of X-band standing wave structure studies at SLAC

Accelerating gradient is one of the major parameters of a linear accelerator. It determines the length of the accelerator and its power consumption. The SLAC two-mile linear accelerator uses 3 meter long S-band traveling wave (TW) accelerating structures. The average gradient in the linac is about 20 MV/m. This gradient corresponds to a maximum surface electric field of about 40 MV/m. An operational gradient of 40 MV/m was reported for a 1.5 m constant impedance TW structure for the SLC positron injector. This corresponds to a maximum surface field of 80 MV/m. A typical operational gradient for standing wave (SW) structures of a medical linear accelerator is 30 MV/m, with surface electric fields of 130 MV/m at a pulse width of several microseconds (longer than the working pulse width for SLAC TW structures). SW structures for S-band rf guns routinely operate at maximum surface fields of 130 MV/m (∼2 μs pulse width). We emphasize an operational gradient with a very low fault rate in comparison to much higher gradients obtained in dedicated high gradient test structures. The operational surface fields in the above mentioned SW structures are obviously higher than in TW, S-band structures. Design considerations, results of high power tests and future plans are discussed in this paper.