High Power Testing of Dielectric-Loaded Accelerating Structures at 11.424 GHz

Summary form only given. A joint Naval Research Laboratory/Argonne National Laboratory study is under way to investigate the performance of 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. We have recently tested DLA structures employing cylindrical ceramic liners fabricated from two different materials, high purity alumina (Al₂O₃), dielectric constant 9.4, and high-index magnesium calcium titanate (Mg_xCa_1-xTiO₃), dielectric constant 20. For alumina, we have seen no evidence of RF breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, strong multipactor effects were found to absorb an increasing fraction of the incident microwave power, as the power level was increased. These effects could be mitigated by use of a TiN coating on the alumina. For magnesium calcium titanate, the multipactor effects were smaller, but a problem of breakdown at dielectric joints between separate ceramic sections limited the achievable gradients. In this case, gradients of ~6 MV/m were achieved at ~1 MW drive power. Detailed experimental results will be presented