High magnetic fields in couplers of X-band accelerating structures

Abstract

Recent high power tests of x-band accelerating structures at SLAC have shown that their performance is limited by breakdowns in couplers. The behavior of these breakdowns is consistent with a model of pulsed heating due to high magnetic fields. The observed damage is located in the region of high magnetic fields. To calculate these magnetic fields, 3D electrodynamics models of the couplers were built and matched. Coupler models were matched with an automated procedure that uses the commercial code HFSS. This matching procedure is based on calculation of reflection from the coupler cell using the electric field distribution on the axis of the structure. After matching, the magnetic and electric fields in the couplers were calculated for typical operation parameters. Highest magnetic fields on the order of 1 MA/m were found on sharp (80 micron radius) edges of the waveguide-to-coupler-cell irises. For some input couplers, electric fields on these edges are found be as high as 10 MV/m. To reduce the high magnetic fields new couplers were designed. These couplers have 3 mm radius waveguide-to-coupler-cell iris and maximum magnetic field below 0.5 MA/m for 70 MV/m average accelerating gradient. The matching procedures , and typical results for couplers with sharp and rounded irises are discussed in this paper as well as possible causes of breakdowns in the couplers.

Progress on high-gradient structures

“…These findings prompted a dedicated study of rf pulse heating [23] and experiments with hard materials [22]. Note that previously high rf magnetic-field and moderate electric fields were found to be source of rf breakdowns in the couplers of NLC structures [24].…”

Development of a millimeter-period rf undulator

“…The corresponding peak electric and magnetic fields on the metallic surface are 107 MV m −1 and 269 kA m −1 . For 250 ns rf pulses, the peak pulsed surface heating is 44°C [54,55], which is considered safe for copper [55]. The rf power required for K ¼ 0.15 is 3.15 MW and the peak pulsed surface heating for 250 ns rf pulses is 99°C.…”

“…For a 10 MW power flow through this mode converter, the peak magnetic field on the metal surface is 282 kA m −1 . From [54] the peak pulsed surface heating for 250 ns rf pulses is 48°C, which is considered safe for copper [55]. Figure 1 shows the entire system assembly with the undulator.…”

Design of a high power TM01 mode launcher optimized for manufacturing by milling

“…It was found that when accelerating structures are exposed to constant rf power and pulse shape, the number of rf breakdowns per pulse is nearly constant. The breakdown rate depends on pulse heating [5] and other factors, such as the peak surface magnetic field [6], the peak surface electric field, and the peak Poynting vector [7]. Presently X-band accelerating structures are the most studied in terms of rf breakdowns [2,8,9].…”