Observation of multipactor in an 11.424-GHz dielectric-loaded accelerating structure

Summary form only given. A joint Argonne National Laboratory/Naval Research Laboratory program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. The purpose of the high-power tests is to find the rf breakdown limits of these structures and to test their ability to produce high accelerating gradients. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating a strong multipactor effect in the tube. The multipactor was found to absorb an increasing fraction of the incident microwave power, as the power level was increased, ultimately absorbing half of the incident power. This behavior is unlike that observed in multipactor on rf windows, which typically saturates at /spl sim/1% power absorbed, independent of incident power. We present a simple model that explains these observations, and discuss means to eliminate the effect.