Asymmetric Bimodal Accelerator Cavity for Raising rf Breakdown Thresholds

Abstract

We consider an axisymmetric microwave cavity for an accelerator structure whose eigenfrequency for its second lowest TM-like axisymmetric mode is twice that of the lowest such mode, and for which the fields are asymmetric along its axis. In this cavity, the peak amplitude of the rf electric field that points into either longitudinal face can be smaller than the peak field which points out. Computations show that a structure using such cavities might support an accelerating gradient about 47% greater than that for a structure using similar single-mode cavities, without an increase in breakdown probability.

Multi-harmonic RF test stand for RF breakdown studies

“…That surface is expected to exhibit the greatest damage from breakdown, due to the anode-cathode field imbalance. The two input waveguide couplers and the top portion of the cavity are to be reused for a series of tests with varying power levels and phases for the two RF sources, to gauge the validity of the anode-cathode imbalance conjecture [3]. As shown in FIGURE 2, the peak electric fields are located at the center region on the flat removable side of the cavity for both frequency components; hence the RF breakdown events occur most likely there.…”

“…In particular, a bimodal test cavity that can support two modes is to be energized simultaneously in two modes, with the second mode eigenfrequency equal to twice that of the fundamental mode [3]. In this cavity, phase-locked two-frequency operation allows the electric field pointing into one wall (cathode-like) to be significantly smaller than the field pointing out of the opposing wall (anode-like); it also allows the superposition of two harmonic modes to narrow the time span of peak field exposure and delocalize the spatial distribution of the peak field during each RF cycle, which also reduces the exposure time to peak surface RF magnetic fields; thus the influence of pulsed heating might be lessened.…”

Structure-based, high transformer ratio collinear two-beam accelerator

“…A multi-harmonic cavity (MHC) concept, with the eigenfrequency of the higher mode equal to a harmonic of the fundamental TM 010 mode, is proposed by us as a means to suppress RF breakdown [2][3]. Surface field emission and surface pulse heating are two major processes that likely precede and induce RF breakdown, and thus limit accelerating gradients.…”

“…Recently, we proposed a class of novel cavity designs with unconventional spatiotemporal distributions using multi-harmonic mode superposition to suppress RF breakdown [2] and pulsed heating [3]. Multi-harmonic cavity structures can, in principle, be powered by two or more types of klystrons at two or more harmonic frequencies [4].…”

A multi-frequency RF photocathode gun

“…In a multi-frequency cavity there are effects of exposure time reduction and a so-called "anode-cathode" phenomenon [7] that may mitigate against RF breakdown. On a cavity surface, cathode fields (fields pointed out so that electron auto-emission is possible) and anode fields (such fields prohibit auto-electron emission) are not equal to one another.…”