Higher Harmonics in Multipactor Induced Plasma Ionization Breakdown near a Dielectric Surface.

In this Letter, the novel physics of higher harmonic (HH) generation of the normal electric field near a dielectric surface is reported in multipactor induced plasma ionization breakdown, as determined by kinetic particle-in-cell simulations. The observed HH frequency is around ten times the fundamental rf driving frequency, but lower than the electron plasma frequency. A theory is constructed which indicates that stream plasma interaction-induced instability is the mechanism of HH generation in the collisional regime. The HH frequency and its corresponding growth rate of the HH oscillation amplitude from the theory are in good agreement with kinetic particle-in-cell simulations.

Time-dependent physics of single-surface multipactor discharge with two carrier frequencies.

This work investigates the time-dependent physics of multipactor discharge on a single dielectric surface with a transverse rf electric field of two carrier frequencies using a multiparticle Monte Carlo simulation model with adaptive time steps. The effects of the relative strength and phase, and the frequency separation between the two carriers are studied. Closed Lissajous curves are obtained to describe the relationship between the rf electric field parallel to the surface and the normal surface charging field in the ac saturation state. It is found that two-frequency operation can reduce the multipactor strength compared to single-frequency operation with the same total rf power, though the effect of the frequency separation is not prominent on multipactor susceptibility. Formation of beat waves is observed in the temporal profiles of the normal electric field due to surface charging with a noninteger frequency ratio between the two carrier modes. Phase space evolution of multipactor electrons is examined, revealing a periodic bunching and debunching of electrons in the surface normal direction, but a gradual debunching effect in the direction tangential to the dielectric surface. Migration of the multipactor trajectory is also demonstrated for different configurations of the two-frequency rf fields.

Observation of Nonlinear Standing Waves Excited by Plasma-Series-Resonance-Enhanced Harmonics in Capacitive Discharges.

We report the first experimental observation of nonlinear standing waves excited by plasma-series-resonance-enhanced harmonics in low pressure, very high frequency, parallel plate, capacitively coupled plasmas. Spatial structures of the harmonics of the magnetic field, measured by a magnetic probe, are in very good agreement with simulations based on a nonlinear electromagnetics model. At relatively low pressure, the nonlinear sheath motion generates high-order harmonics that can be strongly enhanced near the series resonance frequencies. Satisfying certain conditions, such nonlinear harmonics induce radial standing waves, with voltage and current maxima on axis, resulting in center-high plasma density. Excitation of higher harmonics is suppressed at higher pressures.

A new B-dot probe circuit for magnetic diagnostics of radio frequency discharges.

Accurate magnetic measurements in radio frequency capacitively coupled plasmas (CCP) are challenging due to the presence of inherently strong electric fields and relatively weak magnetic fields. In this work, a new B-dot probe circuit is presented, comprising two variable capacitors in a tunable series resonance circuit, with a center-tapped, step-up transformer. The output characteristics of the probe are predicted using two distinct equivalent circuit models, one for the differential mode and the other for the common mode. A Helmholtz coil and a Faraday cup are used for experimental validation of the predicted probe output. By tuning the two variable capacitors in the circuit, the magnetic probe can achieve improved signal-to-noise ratio by amplifying the inductive signal, while suppressing capacitive coupling interference. Using the newly designed probe, magnetic measurements in typical CCP are presented.