X-band photonic band-gap accelerator structure breakdown experiment

Abstract

In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV=m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV=m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV=m and a surface magnetic field of 890 kA=m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV=m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.

High power breakdown testing of a photonic band-gap accelerator structure with elliptical rods

“…The structure was designed to be directly comparable to the original high-gradient PBG structure, SLAC designation 1C-SW-A5.65-T4.6-PBG-Cu and referred to here as a PBG structure with round rods (PBG-R) [3], and a disk-loaded waveguide structure fabricated at INFNFrascati and tested at SLAC [23], SLAC designation 1C-SW-A5.65-T4.6-Cu, referred to here as disk-loaded waveguide (DLWG). The SLAC designations indicate a single high-gradient cell, 1C, standing-wave (SW) structure with an iris aperture of 5.65 mm, A5.65, and iris thickness of 4.6 mm, T4.6, made out of copper (Cu).…”

“…2. The same lattice parameter of = ¼ 0:18 is used for both the PBG-E and PBG-R structures [3]. In the elliptical-rod lattice the minor radius of the elliptical rods is kept the same as the radii of the round rods.…”

“…Photonic band-gap (PBG) structures continue to be a topic of experimental and theoretical interest in accelerator structure design [1][2][3][4]. Photonic crystals use a lattice of metallic or dielectric rods to prevent propagation of electromagnetic waves through the lattice at certain frequencies [5,6].…”

“…PBG HOMs have been simulated and the wakefields have been measured [4,10,11]. A standing-wave high-gradient PBG structure has been tested for breakdown performance at SLAC [3].…”

Origin and reduction of wakefields in photonic crystal accelerator cavities

“…Photonic crystals (PhCs) have recently attracted interest from the accelerator community [1][2][3][4][5][6][7][8][9][10][11][12] for the following reasons: (1) PhCs enable the construction of accelerator cavities using dielectric materials. (2) PhC cavities intrinsically provide a wakefield damping mechanism.…”