J. P. Verboncoeur’s research while affiliated with Michigan State University and other places

We experimentally demonstrate similarity laws for capacitive radio-frequency (rf) plasmas, showing that two rf discharges are scale-invariant in geometrically similar systems in which the gas pressure, gap dimension, and driving frequency are proportionally tuned. Spatiotemporal distributions of the excitation rate are measured based on phase-resolved optical emission spectroscopy, and the tendencies of the excitation dynamics scaling with control parameters are presented and agree well with particle-in-cell simulations. Furthermore, similarity-based scaling networks are established, which extensively correlate the discharge states (i.e., the initial, intermediate, and similarity states), enabling an effective strategy for determining scaling relations with fewer experiments. The framework of the scaling networks is interpreted based on the kinetic Boltzmann equation coupled with Poisson's equation. The present work reveals the nature of discharge similarity and provides an additional knob for the exploration of upscaled rf plasma sources for industrial applications, such as large-area etching facilities.

Plasma is the optimal choice for acquiring and modulating the extremely high visible and near-infrared light. However, few attempts have been made to apply this strategy for the terahertz (THz) wave modulation in an ex situ manner. Here, we show a laser-driven plasma-based THz modulator (PTM) to ex situ control the incident THz waves. The presented PTM allows for the amplification or extinction of the incident THz waves covering 0.1–2.0 THz within a few picoseconds, simply by adjusting its dipole phase. This modulation is a result of the interaction between the PTM’s dipole and THz wave, which can be accurately reproduced by the spectral analysis method. Our technique offers promising opportunities to explore the plasma-based THz optics and potential applications across different disciplines, such as THz-sensing and near-field THz technology.

In general, the radio frequency (rf) electric field within a sheath points toward the metal electrode in low pressure, unmagnetized rf electropositive capacitively coupled plasma (CCP) glow discharges. This is due to the large ratio of electron to ion mobility and the formation of an ion sheath. In this work, we studied, using fully kinetic particle-in-cell simulations, a reversed electric field induced by the strong secondary electron emission during the phase of sheath collapse in a high-voltage rf-driven low pressure CCP glow discharge. We explored the transition behavior of the formation of field reversal as a function of driving voltage amplitude and found that field reversal starts to form at around 750 V, for a discharge with an electrode spacing of 4 cm at 10 mTorr argon pressure driven at 13.56 MHz. Accordingly, the energy distribution function of electrons incident on the electrode shows peaks from around 3 to 10 eV while varying the driving voltage from 150 to 2000 V, showing potentially beneficial effects in plasma material processing where relatively directional electrons are preferred to solely thermal diffusion electrons.

Two-surface multipactor with a Gaussian-type waveform of rf electric fields is investigated by employing Monte Carlo simulations and 3D electromagnetic particle-in-cell simulations. The effects of the full width at half maximum (FWHM) of the Gaussian profile on multipactor susceptibility and the time dependent dynamics are studied. The threshold peak rf voltage, as well as the threshold time-averaged rf power per unit area for multipactor development, increases with a Gaussian-type electric field compared to that with a sinusoidal electric field. The threshold peak rf voltage and rf power for multipactor susceptibility increase as the FWHM of the Gaussian profile decreases. Compared to sinusoidal RF operation, the expansion of multipactor susceptibility bands is observed. In the presence of space charge, a high initial seed current density can shrink the multipactor susceptibility bands. The effect of space charge on multipactor susceptibility decreases as the FWHM of the Gaussian profile decreases. Decreasing the FWHM of the Gaussian electric field can reduce the electron population corresponding to the strength of the multipactor at saturation, at fixed time-averaged input power.

Spatial discrepancy of secondary emission yield (SEY) is probably exacerbated by unexpected surface contamination or imperfect surface treatments for SEY suppression, which accordingly provokes increased multipactor risk in microwave devices. In this paper, an improved 2D2V nonstationary statistical modeling for multipactor of parallel plates capable of regarding all electron impacts and electron exchange at the periodic boundaries is developed to investigate the effect of this spatial SEY discrepancy on multipactor formation in microwave devices. The comparison with the 1D2V statistical modeling results, which is valid for the parallel-plate multipactor, proves the accuracy of this improved 2D2V statistical modeling and the necessity of appropriate boundary setting in multipactor analysis with spatial SEY variation. The modeling results also reveal that the multipactor establishment is dominated by the electron multiplication and the electron overflow in the high-SEY region, thus making the multipactor threshold strongly dependent on both the SEY property and the dimension of the region. Electron multiplication can be fully sustained in the high-SEY region when its size exceeds a critical dimension, which satisfies a scaling law (the sustaining dimension is proportional to the gap with the product of the RF frequency and the gap kept constant) and varies with the involved multipactor mode. This research will help in evaluating accidental multipactor risk caused by the surface contamination and the efficiency of multipactor prevention via applying surface treatments for SEY suppression to accessible regions.