High Power Microwave Generation for Advanced Particle Acceleration

Abstract

This thesis presents the experimental demonstration of advanced high power microwave sources for application to high gradient particle accelerators.

The major effort was directed at the design and successful test of a metamaterial-based power extractor at the Argonne Wakefield Accelerator (AWA). We obtained up to 565 MW, 2.7 ns (FWHM) pulses at 11.7 GHz from the structure. The highest power was generated by a train of eight 65 MeV electron bunches spaced at 1.3 GHz with a total charge of 355 nC. The metamaterial structure consists of 100 copper unit cells with a total structure length of 0.2 m. Each unit cell is comprised of one “wagon-wheel” plate and one spacer plate. The 565 MW pulse generates a longitudinal on-axis wakefield of 135 MV/m that could be used to accelerate a trailing witness bunch. A surface electric field of >1 GV/m is generated on the metamaterial plates at the peak power level but no evidence of breakdown was observed during testing. Tests with trains of bunches of up to 100 nC produced output power levels in excellent agreement with simulations. At higher total bunch charge, offsets of the bunches from the axis resulted in reduced output power. Simulations indicate that a perfectly aligned bunch train would generate more than 1 GW of power.

An additional effort was directed at the design and characterization of a high power laser driven semiconductor switch (LDSS) that enabled the testing of high gradient 110 GHz accelerator structures at MIT. The LDSS, employing Si and/or GaAs wafers, was used to slice nanosecond-scale pulses from 3 microsecond pulses generated by a megawatt, 110 GHz gyrotron. An electron-hole cutoff plasma was induced in the wafers using 6 ns, 230 mJ pulses from a 532 nm laser. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS was integrated with the MIT megawatt gyrotron, enabling the testing of a SLAC W-band accelerator cavity at gradients up to 230 MV/m.