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

Brian J Munroe,Michael A Shapiro,Roark A Marsh,James R Lewandowski,Richard J Temkin,Sami G Tantawi,Lisa L Laurent,A Dian Yeremian,Valery A Dolgashev,Alan M Cook

doi:10.1103/physrevstab.16.012005

Abstract

An improved single-cell photonic band-gap (PBG) structure with an inner row of elliptical rods (PBG-E) was tested with high power at a 60 Hz repetition rate at X-band (11.424 GHz), achieving a gradient of $128\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown probability of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ per pulse per meter at a pulse length of 150 ns. The tested standing-wave structure was a single high-gradient cell with an inner row of elliptical rods and an outer row of round rods; the elliptical rods reduce the peak surface magnetic field by 20% and reduce the temperature rise of the rods during the pulse by several tens of degrees, while maintaining good damping and suppression of high order modes. When compared with a single-cell standing-wave undamped disk-loaded waveguide structure with the same iris geometry under test at the same conditions, the PBG-E structure yielded the same breakdown rate within measurement error. The PBG-E structure showed a greatly reduced breakdown rate compared with earlier tests of a PBG structure with round rods, presumably due to the reduced magnetic fields at the elliptical rods vs the fields at the round rods, as well as use of an improved testing methodology. A post-testing autopsy of the PBG-E structure showed some damage on the surfaces exposed to the highest surface magnetic and electric fields. Despite these changes in surface appearance, no significant change in the breakdown rate was observed in testing. These results demonstrate that PBG structures, when designed with reduced surface magnetic fields and operated to avoid extremely high pulsed heating, can operate at breakdown probabilities comparable to undamped disk-loaded waveguide structures and are thus viable for high-gradient accelerator applications.

Highlights

Photonic band-gap (PBG) structures continue to be a topic of experimental and theoretical interest in accelerator structure design [1,2,3,4]
An improved PBG structure using elliptical rods in the inner row was tested at high gradient at SLAC and achieved a maximum gradient of 128 MV=m at a breakdown probability of 3:6 Â 10À3 per pulse per meter at a pulse length of 150 ns
This is comparable to the performance of an undamped disk-loaded waveguide structure with the same iris geometry, indicating that PBG-type structures are viable for high-gradient acceleration, consistent with the goal of the revised testing methodology

Summary

Introduction

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]. In accelerator applications this allows for a drive mode to be confined by a defect region within the lattice while damping modes at both higher and lower frequencies. The initial design of a PBG accelerator structure was based on a square lattice as proposed in [7]. A triangular lattice provides better symmetry for accelerator structures, and good

Methods

Results

Conclusion