Jim Browning

We report the development of a large area cold atmospheric plasma array. The device consists of a parallel stack of 43 linear-discharge plasma elements that create a $10\times 10$ cm treatment area. The cold atmopsheric pressure plasma (CAP) device is fabricated using low-temperature co-fired ceramic (LTCC) layers to create 10-cm-long linear-discharge channels (1.1-mm discharge gap) with embedded opposing silver metal electrodes. A 21-kHz ac voltage of 1.55 kVrms applied to the electrodes generates an Ar plasma between the plates, with the gas flow directing the reactive species toward the intended biological sample (bacteria biofilms and so on) to affect the antimicrobial treatment. Internal ballast resistors (20 k $\Omega $ ) were used on each side of the two electrode elements to improve discharge uniformity and to prevent large filamentary discharges. Typical element discharge currents were 3.5–4 mA with the total array current tested at 178-mA root mean square (rms) to provide optimal device uniformity at a 1.55 kV (rms), and an argon flow rate of 130 lpm. Furthermore, the gas flow system was optimized to obtain a uniform plasma. Treatment of Pseudomonas fluorescens bacterial biofilms on stainless steel coupons demonstrated a 91% decrease in colony-forming units (CFUs) after 150 s of treatment with a 1.5-cm gap.

Crossed-field amplifiers are known for being noisy; however, the mechanism of noise generation remains poorly understood. This article identifies the spurious emission noise generation mechanism by using simulation and spatial–temporal, spatial–spectral, and spatial–tonal visualizations of the electron population. The VSim simulation model is based on the L-4953 crossed-field amplifier (CFA) from Stellant Systems, a $\approx {5}$ -MW peak power, $\approx {11}.{2}$ -dB, L-band, re-entrant, backward-wave device. The electron spokes are characterized for low-noise, typical stable, and unstable operation, where the spurious emission amplitudes are $-{~47}$ , $-{~37}$ , and $-{~15}$ dB, respectively. The simulation spectrum for typical stable operation matches the available physical device spectrum data, demonstrating the ability of simulation to model the spurious emission mechanism. Simulations show high sensitivity of the spectrum to operation parameters, where a relatively small shift in dc voltage from ${91}.{6}$ to ${93}.{1}$ kV causes a transition from typical stable to low-noise operation. The dispersion of the circuit was calculated and demonstrates the “hard” limit to bandwidth where mode competition occurs. Newly developed electron population visualizations identify noise generation mechanisms well within the band. Using these electron population visualizations, three conclusions are made: phase/frequency modulations (PMs/FMs) of the spokes cause many of the spurious emission sidebands on the output spectrum; the modulations of the spokes only couple to the output if the modulations in each spoke are in phase with each other; and the spurious emission amplitude is higher when the modulations in the spokes are a harmonic of a subharmonic of the transit frequency.

The study evaluates the efficacy of an image-guided CAP treatment method with a plasma device capable of rapid biofilm removal from chicken tissue. The plasma treatment operating configuration includes a gas mixture of Argon and H₂O at a flowrate of 1.5 lpm. An X-Y stage was used to move the chicken sample below the stationary plasma scalpel at a speed of 0.1 mm/s. The discharge voltage and current were maintained between 3.2 and 3.7 kV (AC 20 kHz), and at 3 mA, respectively. The electrode gap and sample distance were set to 0.6 mm and 4 mm. This configuration facilitated effective biofilm removal, as confirmed by CFU analysis and 3D microscopic analysis showing a >99% reduction in biofilm post treatment with an etch rate of 2.2–5.8 µm/s and an impact width of up to 300 µm. The plasma scalpel electrode temperature reached 94.7 °C, while the targeted biofilm area was heated to 36.3 °C, suggesting non-thermal biofilm disruption. Three-dimensional microscopic analysis revealed biofilm thickness on chicken tissues ranging from 20 to 180 µm, comparable to biofilm loads on mammalian tissues. In conclusion, the study highlights the potential of CAP devices as a promising solution for biofilm debridement.

A low-frequency (561 MHz), injected beam, and linear format crossed-field amplifier (CFA) using gated field emission arrays (GFEAs) has been experimentally studied and compared with simulation. The CFA uses a copper wire on Teflon meander line circuit with retardation of ~21. Eight silicon tip GFEA dies were used as the injected electron source to provide up to 160 mA. A segmented end-collector system (nine electrodes) was used to measure the spatial variation of the beam current with and without gain. A gain of ~5.5 dB was measured for a sole-circuit voltage of −2.9 kV, an injected beam current of ~160 mA, an applied magnetic field of 0.0125 T, a radio frequency (RF) input power of 15 W, and a sole-circuit gap of 2 cm. A CST particle in-cell model shows a high gain (~1–2 dB) than the experiment, but the gain variation versus injected current, voltage, and magnetic field matches well. Variation with RF input power shows a significant decrease in gain above 15 W in the experiment with the decrease seen in simulation observed after 25 W. Analysis of the end-collector current shows a rapid decrease after 12 W in the experiment and 25 W in the simulation. This result occurs because the highly cycloidal electrons are close to the CFA circuit and get collected on the circuit before providing amplification energy. This observation is confirmed in simulation, which shows that the current going to the circuit rapidly increases and the end-collector current rapidly decreases. This effect also accounts for the higher gain observed in simulation. These experiments provide a basis for using gated field emitters to study beam–wave interactions in microwave vacuum electron devices.

The electron spokes for stable operation in the L-4953 crossed-field amplifier (CFA) from Stellant Systems are characterized via particle-in-cell (PIC) code simulations in VSim (TechX). The CFA is a pulsed, re-entrant, backward wave device with a peak output power ${P}_{\text {out}}^{\text {peak}}\approx {5}~\text {MW}$ , gain ${G}\approx {11.2}~\text {dB}$ , and a bandwidth from ${f}={1.28}~\text {to}~{1.35}~\text {GHz}$ . Spatial, spatial-temporal, spatial-spectral, and spatial-tonal visualizations of the electron density in both the lab and rotating frames of reference were developed to characterize the spokes. The spatial visualizations for simulations at the operation frequency ${f}_{\text {op}}={1.3}~\text {GHz}$ identify the Brillouin and cycloidal electron flow within the spoke for space-charge limited emission (SCLE) and non-SCLE, respectively. The spatial-temporal/spectral visualizations identify an oscillation (transit wobble) within the spoke at a third of the operation frequency $\text {(}{f}_{\text {transit}}={f}_{\text {op}}/{3}={0.43}~\text {GHz}\text {)}$ caused by the spoke transiting 360° in the re-entrant design. This fundamental spoke oscillation, though, does not directly appear on the output. However, a much lower amplitude oscillation at ${f}_{\text {sideband}}={0.18}~\text {GHz}$ identified on the front of the spoke directly appears as sidebands on the output. While the mechanisms that initiate sideband oscillations within the spoke and generate noise on the output are relatively unknown, spatial-tonal visualizations show that the phase difference of the oscillations between spokes is critical to this process. The ability of the visualizations to directly connect oscillations within the spoke to the output is demonstrated, but strategies are still needed to interpret the visualizations to identify and characterize noise generation mechanisms.

This paper describes the principles behind the radio-frequency (RF) sensing of bacterial biofilms in pipes and heat exchangers in a dairy processing plant using an electromagnetic simulation. Biofilm formation in dairy processing plants is a common issue where the absence of timely detection and subsequent cleaning can cause serious illness. Biofilms are known for causing health issues and cleaning requires a large volume of water and harsh chemicals. In this work, milk transportation pipes are considered circular waveguides, and pasteurizers/heat exchangers are considered resonant cavities. Simulations were carried out using the CST studio suite high-frequency solver to determine the effectiveness of the real-time RF sensing. The respective dielectric constants and loss tangents were applied to milk and biofilm. In our simulation, it was observed that a 1 µm thick layer of biofilm in a milk-filled pipe shifted the reflection coefficient of a 10.16 cm diameter stainless steel circular waveguide from 0.229 GHz to 0.19 GHz. Further sensitivity analysis revealed a shift in frequency from 0.8 GHz to 1.2 GHz for a film thickness of 5 µm to 10 µm with the highest wave reflection (S11) peak of ≈−120 dB for a 6 µm thick biofilm. A dielectric patch antenna to launch the waves into the waveguide through a dielectric window was also designed and simulated. Simulation using the antenna demonstrated a similar S11 response, where a shift in reflection coefficient from 0.229 GHz to 0.19 GHz was observed for a 1 µm thick biofilm. For the case of the resonant cavity, the same antenna approach was used to excite the modes in a 0.751 m × 0.321 m × 170 m rectangular cavity with heat exchange fins and filled with milk and biofilm. The simulated resonance frequency shifted from 1.52 GHz to 1.54 GHz, for a film thickness varying from 1 µm to 10 µm. This result demonstrated the sensitivity of the microwave detection method. Overall, these results suggest that microwave sensing has promise in the rapid, non-invasive, and real-time detection of biofilm formation in dairy processing plants.