Two-Frequency RF Fields Induced Multipactor in Coaxial Transmission Lines

This work investigates multipactor discharge in coaxial geometry with a two-frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$rf$</tex> electric field. We find that multipactor susceptibility depends strongly on carrier strength and phase of the second carrier mode, using Monte Carlo simulations. For various combinations of two-frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$rf$</tex> fields, in general, the multipactor susceptibility band shrinks in comparison to single-frequency operation. Mixed Multipactor modes, consisting of single-surface impacts as well as two-surface impacts are observed under two-frequency operation, which are otherwise absent, for certain gap distances and frequencies. Mode boundaries are also outlined for the single and two-frequency cases in the multipactor susceptibility chart. Our multipactor susceptibility chart for a single frequency matches Woo's (J. Appl. Phys. 39(3), 1528–1533 (1968)) results. As the phase of the second carrier mode increases relative to the first, the susceptibility chart shrinks.