High power experimental studies of hybrid photonic band gap accelerator structures

Abstract

This paper reports the first high power tests of hybrid photonic band gap (PBG) accelerator structures. Three hybrid PBG (HPBG) structures were designed, built and tested at 17.14 GHz. Each structure had a triangular lattice array with 60 inner sapphire rods and 24 outer copper rods sandwiched between copper disks. The dielectric PBG band gap map allows the unique feature of overmoded operation in a TM 02 mode, with suppression of both lower order modes, such as the TM 11 mode, as well as higher order modes. The use of sapphire rods, which have negligible dielectric loss, required inclusion of the dielectric birefringence in the design. The three structures were designed to sequentially reduce the peak surface electric field. Simulations showed relatively high surface fields at the triple point as well as in any gaps between components in the clamped assembly. The third structure used sapphire rods with small pin extensions at each end and obtained the highest gradient of 19 MV=m, corresponding to a surface electric field of 78 MV=m, with a breakdown probability of 5 × 10 −1 per pulse per meter for a 100-ns input power pulse. Operation at a gradient above 20 MV=m led to runaway breakdowns with extensive light emission and eventual damage. For all three structures, multipactor light emission was observed at gradients well below the breakdown threshold. This research indicated that multipactor triggered at the triple point limited the operational gradient of the hybrid structure.

Fabrication and cold test of dielectric assist accelerating structure

“…Since the basic theory of DLA structures were first proposed in the 1940s [16][17][18], numerous studies have examined the use of dielectric materials in accelerating structures [19][20][21][22][23][24][25][26][27]. Recently, new concepts of dielectric based accelerating structures with improved performance have come along, such as a dual-layered DLA structure [21], a hybrid dielectric and iris loaded periodic accelerating structure [22], a multicell disk-and-ring tapered structure [23,24], an over-mode hybrid photonic band gap accelerating structure [25], and a multilayered DLA structure [26,27] which is operated by a higher order mode. Thus, the possibilities of dielectric based accelerating structures are gradually expanding.…”

Multipactor suppression in dielectric-assist accelerating structures via diamond-like carbon coatings

“…[8] The study of photonic bandgap cavities using sapphire rods shows a maximum electric field of 19.1 MV/m, which is limited by breakdowns in triple junctions. [12] In both examples, short RF pulses of 100 [ns] to 500 [ns] in the X-band are fed in. In contrast, in a DAA cavity, relatively long RF pulses of several µs are fed in at a lower frequency, the C-band.…”