A Tunable Reflecting Load for Multipactor Processing of the RF Power Coupler of a Superconducting Cavity

Abstract

In recent years, superconducting RF cavities have been increasingly adopted for use in modern accelerator and light-source facilities. The microwave power is coupled into the accelerating cavity typically through a waveguide or a coaxial power coupler. The multipactor discharge in the RF power coupler constitutes one of the most serious problems to the reliable operation of superconducting cavities. Due to the lack of multipacting-free RF structures, RF processing of the RF power coupler becomes a required and routine procedure to ensure multipacting-free operation of a superconducting RF system. However, for a superconducting cavity with negligible wall losses, variations of the beam loading lead to significant magnitude and phase variations of the wave reflected from the cavity. This greatly complicates the multipacting behavior in the RF power coupler. Hence RF processing under fixed conditions, as is conventionally done, is inadequate for multipactor processing of RF power couplers used in superconducting cavities. Here we report the concept, design, and demonstration of a novel tunable reflecting load for multipactor processing of the RF power coupler. The load realistically models the RF cavity through a broad tuning range of the reflection coefficient, both in magnitude and in phase. It thus allows, for the first time, off-line multipactor processing of the RF power coupler under machine operational conditions before it is attached to the superconducting cavity. A high power prototype was assembled in-house using standard WR1800 rectangular waveguide components. Results of cold and high-power test up to 50 kW, CW were found to be in good agreement with theoretical predictions.

Analysis of the electromagnetic radiation generated by a multipactor discharge occurring within a microwave passive component

“…where CS represents the rectangular waveguide cross-section and δ p,q is the Kronecker delta. Equation (3) indicates that each i m current might excite the full set of TE and TM waveguide modes at different f m frequencies. However, in this first approach of the problem, we will consider only the first (p = 1) fundamental TE 10 mode, which is represented by the coefficient I m 1 ≡ I m .…”

“…It is often considered undesirable, since it can load the cavity [1], dissipate power, increase the noise level [2], and cause heating of the walls and outgassing. Concerning multipactor in particle accelerators, two types of effects can be distinguished: RF multipactoring in the structures [3,4] and beam-induced multipactor process driven by the field of a bunched beam [5,6]. The multipactor effect has been deeply investigated in several types of microwave waveguides with simple geometries, such as the parallel-plate guide [7,8], the rectangular waveguide [10] and the coaxial transmission line [11][12][13][14].…”

Multipactor Breakdown in Elliptical Waveguide Carrying Orthogonal Polarizations

“…The study and analysis of multipactor phenomenon has been of interest during the last years. This includes the theory and simulation of multipactor in various types of microwave devices such as high-power microwave components, operating under high-vacuum conditions for satellite communication [8][9][10][11][12], and particle accelerators systems [13,14].…”

“…Elliptical guides have been employed for the design of microwave passive components, as narrow-band dualmode filters [26,27] and polarizers implemented in circular waveguides [28]. In the research area of accelerator physics, elliptical guides have also been used for the simulation of the LHC (CERN) beam pipe, as reported in [13,14].…”