MIT/PSFC, Cambridge, Massachusetts
Funding: Work supported by DOE HEP, under contract DE-FG02-91ER40648
The design of advanced photonic bandgap (PBG) accelerator structures is examined. PBG structures are chosen for their wakefield damping. A potential disadvantage of PBG structures, as well as damped detuned structures, is the increased wall currents at the structure surface due to the reduced surface area, leading to higher pulsed wall heating. Research is carried out to improve the pulsed heating performance of PBG structure concepts while maintaining higher order mode damping. Wakefield damping parameters are discussed and a quantitative figure of merit is expressed to evaluate and compare PBG concepts. Pulsed heating performance in PBG structures is improved by breaking perfect symmetry and allowing deformation of both rod and lattice geometry. A final design for an improved pulsed heating performance PBG structure for breakdown testing at 11.424 GHz is presented and discussed.
MIT/PSFC, Cambridge, Massachusetts
Funding: Work supported by DOE HEP, under contract DE-FG02-91ER40648
The design of a new photonic bandgap (PBG) accelerator structure based on a pentagonal array of rods is presented. The goal of this structure is to damp the higher order modes (HOMs) present in the structure. By removing the bilateral symmetry present in the four and six rod PBG structures the five rod photonic quasi-crystal (PQC) structure is able to damp the symmetric dipole mode. The field pattern and mode Q factors for the fundamental and dipole modes are presented for various values of the ratio of rod radius to rod spacing. These results are compared to the equivalent results for the six rod structure. The ratio of the Q factors is also calculated, and found to show an optimal value near a rod radius to rod spacing ratio of 0.17 in both cases.
MIT/PSFC, Cambridge, Massachusetts
SLAC, Menlo Park, California
Funding: Work supported by DoE HEP, under contracts DE-FG02-91ER40648 and DE-AC02-76-SF00515
In order to understand the performance of photonic bandgap (PBG) structures under realistic high gradient operation, an X-band (11.424 GHz) PBG structure was designed for high power testing in a standing wave breakdown experiment at SLAC. The PBG structure was hot tested to gather breakdown statistics, and achieved an accelerating gradient of 65 MV/m at a breakdown rate of two breakdowns per hour at 60 Hz, and accelerating gradients above 110 MV/m at higher breakdown rates, for a total pulse length of 320 ns. High pulsed heating occurred in the PBG structure, with many shots above 270K, and an average of 170K for 35 x 106 shots. Damage was observed in scanning electron microscope imaging. No breakdown damage was observed on the iris surface, the location of peak electric field, but pulsed heating damage was observed on the inner rods, the location of magnetic fields as high as 1 MA/m. Breakdown in accelerator structures is generally understood in terms of electric field effects. PBG structure results highlight the unexpected role of magnetic fields on breakdown. We think that relatively low electric field in combination with high magnetic field on the rod surface may trigger breakdowns.
