Session IW5: Poster Session II; Exhibition & Coffee (4:00pm-6:00pm)

Temperature Relaxation Rates for Strongly Magnetized Plasmas in Antimatter Traps

Antimatter appears in nature only in tiny quantities relative to matter for reasons not yet fully understood. To address this open question, a better understanding of the fundamental physics governing antimatter is needed.  The ALPHA experiment at CERN is working to improve our understanding of antimatter by trapping and studying antihydrogen. The plasmas used to form the antihydrogen fall into the very strongly magnetized regime, meaning that the gyrofrequency exceeds the plasma frequency and in the moderate Coulomb coupling regime denoting that the potential energy of interaction is on the order of the kinetic energy. These regimes modify the Coulomb collision frequency making traditional theories invalid. Our goal is to develop the theoretical plasma physics models in these regimes to better understand the dynamics of these non-neutral plasmas, which may in turn aid in antihydrogen production.

Using a recently developed generalized Boltzmann kinetic theory for strongly magnetized plasmas, the electron-ion temperature relaxation rates in both parallel and perpendicular directions are calculated. In this regime, the particles will emit radiation in the perpendicular direction producing an anisotropy between the parallel and perpendicular temperatures which is transferred to the other species via Coulomb collisions. These results have relevance in several processes used in the ALPHA experiment, including antiproton cooling via Coulomb collisions with electrons, the thermal equilibration between antiprotons and positrons during recombination, and the sympathetic cooling of positrons on laser-cooled Beryllium. The improved understanding of temperature relaxation presented here will contribute to a more complete fluid model in this novel plasma regime.   

Examination of OH and H2O2 production by uniform and non-uniform modes of dielectric barrier discharge in He/air mixture

In this work we have carried out a parametrical study of hydroxyl radical (OH) generation in nanosecond dielectric barrier discharge (DBD) in He/air mixture using a laser-induced fluorescence approach. The foci of the study are the investigation of differences between uniform and non-uniform modes of the discharges and the difference in production of OH and H₂O₂ Nanosecond-time scale imaging of the discharge shows transition from streamer to diffuse mode when applied electric field to the discharge gap approaches∼ 90 kV/cm. The results show that both OH production in the gas phase and downstream H₂O₂ delivery rates to liquid depend on the discharge mode operation and are respectively 30% and 3 times higher for the non-uniform DBD compared to the diffuse discharge.   

Advancing the Efficiency of Rotating Magnetic Field Thrusters through Surrogate Model Optimization

The rotating magnetic field (RMF) thruster is an emerging inductive pulsed plasma thruster (IPPT) concept. In general, IPPT thrusters possess a significant advantage over conventional electric propulsion architectures through their propellant agnosticism due to their inductive drive [1]. RMF thrusters offer a distinct advantage over other IPPTs in that they indirectly couple the driven plasma currents from the inductive antennas, reducing stress on the driving circuit and enabling lower voltages and steady-state operation. However, the low efficiencies of RMF thrusters remain a key challenge [2-6]. Due to the numerous parameters for these devices (such as RMF frequency, RMF magnitude, pulse length, duty cycle, flow rate, flow injection location, magnetic field shape, ...), manually optimizing them is impractical in experimental settings. Therefore, this study presents the results of utilizing surrogate model optimization in conjunction with experimental performance measurements of an RMF thruster to intelligently select test points. The primary goals are to enhance the efficiency of these devices and uncover trends in the underlying physics. The implications of these findings are discussed within the context of future RMF thruster operation and design.

[1] Polzin, K., et al., Aerospace, Vol. 7, 2020, p. 105

[2] Gill, T. M., et al., PSST, 2023. Submitted

[3] Sercel, C. L., et al. AIAA P&E, 2021

[4] Weber, T. E. Thesis, 2010

[5] Furukawa, T., et al., AIAA P&E, 2020

[6] Sun, Y., et al. PSST, Vol. 30, 2021   

Determination of Ionization Frequency in Microwave Discharges at Microgaps

The continuing reduction of device size in electronics increases the importance of accurately predicting microwave breakdown voltage at microscale¹, where the ionization frequency ν differs from the ionization coefficient used to assess DC breakdown². Previous studies determined ν at microwave frequencies by modifying the DC equation³; however, this is inaccurate. A comprehensive assessment of ν with gap distance, electric field, and frequency is required to improve the accuracy of AC breakdown theories³, particularly at smaller gaps where the breakdown electric fields can be strong². Here, we use particle-in-cell simulations to characterize ν for atmospheric pressure argon plasmas in 2-10 µm gaps at 1-1000 GHz. For f<f_cr (critical frequency), ν/f scales with the reduced electric field. For f >f_cr, ν becomes a function of the reduced electric field, which agrees with previous results at larger gaps. For f<f_cr, the electrodes collect the electrons during each cycle, causing the electron number to oscillate over time. For f >f_cr electrons are confined in the gap, leading to exponential growth in the electron number.

1. Y. Fu, et al., Plasma Res. Express 2, 013001 (2002).

2. H. Wang, et al., J. Appl. Phys. 132, 073302 (2022).

3. M. U. Lee, et al., Eur. Phys. J. D 71, 94 (2017).   

Resonant processes and their impact in transport properties

Resonant exchange is a general process playing a key role in many-body dynamics and transport phenomena, such as spin, charge, or excitation diffusion. The underlying process is described by the resonant exchange cross section σ_exc. A prime example is the diffusion of an ion A⁺ in its parent neutral gas A. In fact, the charge actually behaves as a hole (h) at ultralow tem peratures, hopping from atom to atom instead of staying on its heavy center (the ion) [1]. We have predicted a faster diffusion coefficient for the hole (D_h) than if the charge was diffusing via collision (D_coll). In this work, we show that the exchange symmetry for identical (homonuclear) atom-ion system leads to special outcomes for ion transport in ultracold experiments. We compute the two body charge hopping probabilities and rates, which are used to model charge hopping in the dynamics of an ultracold ^6/7Li⁺ ion immersed within an ultracold gas of ^6/7Li atoms at micro-Kelvin temperatures [2]. We show that the charge hopping and collisional diffusion compete, giving unique results leading to charge trapping in regions of high atomic density gradient, leading to a region of "negative" diffusion. As mentioned above, the dynamics is dictated by σ_exc. In previous work [3], we showed that the locking of s-wave phase shifts could be used to explain the behavior of exc at ultracold tem peratures. Moreover, we found an unexpected consequence of phase-shift locking; namely, the behavior of the resonant-exchange cross section over a broad range of energies is largely dictated by s-wave scattering, whose inuence extends high above the s-wave Wigner regime. We now generalize our treatment to higher energies and derive an analytical expression for the resonant-exchange cross section which accounts not only for the locking of phase shifts, but also for their gradual unlocking as the energy increases. We find good agreement between the computed (fully quantal) cross section and our newly obtained result, which we illustrate for resonant charge-transfer in ion-atom collisions.

[1] R. Côté, PRL 85, 5316 (2000).

[2] N. Joshi, M. Niranjan, A. Pandey, O. Dulieu, R. Côté., S.A. Rangwala, PRA 105, 063311 (2022).

[3] R. Côté and I. Simbotin, PRL 121, 173401 (2018).   

Calculating Transport Coefficients in Warm Dense Matter

The tin used in Extreme Ultraviolet Lithography (EUVL) exists in a state of plasma known as

warm dense matter (WDM), where the average thermal, potential, and Fermi energies are all roughly

equal. In this process, energy from a laser is deposited to the tin atoms by first exciting electrons

and then through electron ion collisions, thus transport in WDM is important to understand. Here,

the traditional Boltzmann description of a plasma begins to break down, so the Mean Force Kinetic

Theory (MFKT) [S. D. Baalrud and J. Daligault, Phys. Plasmas 26, 082106 (2019)] can be used to

calculate transport properites instead. A code is presented to calculate transport coefficients using

the MFKT, and results show transport properties calculated for plasmas in the WDM regime. These

plasmas contain degenerate electrons, whose screening effect is modeled by the potential of mean

force which is obtained using the Quantum Hyper Netted Chain Model (QHNC) [C. E. Starrett

and D. Saumon, High Energy Density Phys. 10, 35 (2014)] developed by Starrett and Saumon.

Future work intends to develop an independent version of the QHNC code, a module to expand

transport coefficients to an arbitrary order in the Chapman-Enskog expansion, and a model for

electron transport properties.   

The Ernst Ratz Analytical Solution in a High Speed Rotating Cylinder Revisited

  In this study the Ernst Ratz differential equation governing the concentration field in a high speed rotating cylinder with and without axial back diffusion was revisited, and the solution of the differential equation was developed to study the effect of product baffle opening on the optimum feed flow rate and on the optimum enrichment for a wide range of normalized counter-current (L/F in the range 1.2 to 8) with unit cut (P/F) equal to 0.4, 0.5 and 0.6. Here, L is the counter-current circulation rate, F is the feed flow rate, and P is the product flow rate. The analysis shows that at a given unit cut and normalized counter-current, the optimum feed flow rate can be reduced by lowering the product baffle opening, and the effect is significant for normalized counter-current L/F up to 5, and beyond that point the influence is not as much of important, whereas in the case of optimum enrichment (N_P – N_W)_opt , as the product baffle opening is lowered, the optimum enrichment increases monotonically with normalized counter-current. Here, N_P and N_W are the concentration of the product and waste stream respectively. Next, the flow profile efficiency (E_F) and mass flow efficiency (E_M) have been studied for wall pressure in the range 20 to 100 m-bar based on the mass flow rate in the inner stream (m_i), and the analysis indicates that the product of flow profile and mass flow efficiency (E_F x E_M ) has an optimum for each wall pressure, and the separative power (δu) attains its maximum value at that m_i value. The comparison between axial back diffusion and without axial back diffusion reveals that at a given unit cut, the rectifier has an additional length due to axial back diffusion effect, and the influence can be reduced by increasing the aspect ratio (Z/R_w) of the cylinder ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). Here, Z is the effective length of axial counter-current, and R_w is the radius of the cylinder. An important finding is that there is a cross over point for normalized counter-current beyond which the optimum feed flow rate is higher with axial back diffusion compared to without axial back diffusion, and the cross over point shifted to smaller values with the lowering of product baffle opening.   

Reducing Starting Current of Smith-Purcell Radiation with a Two-Layer Grating Structure

Smith–Purcell radiation (SPR) is generated when an electron beam passes near a periodic structure  [1]. Two-layer gratings are proposed to enhance SPR by improving electromagnetic coupling [2] [3]. Using particle-in-cell (PIC) simulations [4], two-layer gratings have been shown to enhance SPR growth rates and reduce Smith-Purcell backward wave oscillators' start currents.

This study presents a novel approach to enhance the coupling of evanescent waves with an electron beam in SPR by utilizing a two-layer grating structure. The operating frequency of SPR with the two-layer grating is determined through the derivation of a cold-tube dispersion relation, which is further validated by CST and PIC simulations [4]. PIC simulations reveal that the two-layer gratings exhibit significantly higher radiation intensity. Moreover, the two-layer metallic gratings demonstrate the generation of coherent SPR with a reduced beam starting current [3]. This reduction in beam current yields several advantageous effects for particle accelerators, such as minimizing beam loss, prolonging cathode lifetime, and mitigating the heating of beam-carrying components.

REFERENCE

[1]    S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953).

[2]    J. Shu, P. Zhang, M. Liang, S.-P. Yang, S.-M. Wang, and Y.-B. Gong, Chin. Phys. B 31, 044103 (2022).

[3]    M. Arifuzzaman Faisal and P. Zhang, IEEE Trans. Electron Devices 70, 2860 (2023).

[4]    J. P. Verboncoeur, A. B. Langdon, and N. T. Gladd, Comput. Phys. Commun. 87, 199 (1995).   

Beam density modulation during emission under RF and laser fields

        Free-electron beam-based devices are vital in numerous applications, such as telecommunication systems, satellite-based transmitters, radar, communication data links, and electronic countermeasures. These devices utilize the collective interaction between an electron beam and a circuit structure, to convert the energy of the electron beam into electromagnetic radiation. If we can generate an electron beam with direct density modulation during its emission, rather than modifying a continuous beam in an RF interaction region to achieve beam modulation, the performance of the devices for power amplification can be significantly enhanced [D.R. Whaley, et al, IEEE Trans. Plasma Sci. 30(3), 998–1008 (2002].

        This work demonstrates various pulse shapes of pre-bunched electron beams emitted from a radio frequency (RF) cold cathode under different combinations of RF field and optical field (continuous-wave or pulsed), using an exact quantum model for photo-/field- emission [Y. Zhou and P. Zhang, J. Appl. Phys. 130(6), 064902 (2021)]. The profile of the produced emission current can be modulated by varying RF field (amplitude and frequency) and optical field (laser wavelength, laser intensity, pulse length). The emission current pulse amplitude, beam width, electron numbers per pulse, as well as the harmonic spectrum of the current pulse, are investigated in detail under various combinations of RF and optical fields.   

Measurements of DC breakdown in hydrofluoroolefine C3H2F4

Hydrofluoroolefin C₃H₂F₄ belongs to a new class of refrigerants and gaining attention, especially in the automotive industry where it is used as a replacement for R-134a. This novel gas is also considered a promising alternative in gaseous particle detectors (RPC systems) and high-voltage insulation devices due to its low global warming potential. In this study, we present the results obtained from our Steady-State Townsend experiment (SST). The discharge was ignited in a plan-parallel electrode system placed inside a tight quartz tube with electrodes spaced 1.1 cm apart. The diameter (2r) of the electrodes was 5.4 cm. The design of the electrical circuit enabled the stable operation of the discharge near the breakdown conditions. These measurements yielded breakdown voltages (Paschen curve), critical electrical fields, spatially and spectrally resolved distribution of discharge emission, and effective ionization coefficients. Our research aims to understand the fundamental processes that occur in fluorocarbon gases when they are subjected to electric fields. These processes determine the characteristics and behavior of breakdown itself and gas discharges and can be used in modeling various gas discharge applications. The available cross-section data were used to analyze the collisional processes occurring in this gas. Additionally, we obtained effective ionization coefficients as transport coefficients that can be used for normalizing existing ionization cross-sections for electron scattering on all gases, including the HFO-1234yf.   

Studies of electron transport and propagation of negative ionization fronts in C2F6 and its mixtures with N2 and CO2

A multi-term solution of the Boltzmann equation is used to calculate the transport coefficients of electron swarms under the influence of an electric field. The hierarchy resulting from a spherical harmonic decomposition of the Boltzmann equation in the hydrodynamic regime is solved numerically by representing the speed dependence of the phase-space distribution function in terms of an expansion in Sonine polynomials about a Maxwellian weighted function. Electron transport coefficients are calculated over a range of reduced electric fields with significant differences in behavior between the pure C₂F₆ and its mixtures with N₂ and CO₂ being found. Values of mean energy, ionization rate, drift velocity, and diffusion tensor are reported here. In order to simulate the propagation of negative ionization fronts, we employ the classical fluid model which combines the equation of continuity for electrons and ions as well as the drift-diffusion approximation for electrons, and Poisson’s equation for the calculation of the space charge electric field.   

Ultrafast Ionization, Heating, Thermalization and Constriction of High-Pressure Nanosecond Pulsed Discharge Plasmas

We present our approach for investigating mechanisms of the fast transition of partially ionized atmospheric pressure nonequilibrium discharges to fully ionized thermal spark discharges on nanosecond time scales. A suite of ultrafast optical diagnostics including streaked optical emission spectroscopy (OES), picosecond Thomson scattering, streaked absolute continuum emission, coherent anti-Stokes Raman scattering (CARS) and coherent Rayleigh-Brillouin scattering (CRBS) will be performed. This study is expected to produce new insights in the underpinning plasma physics of nanosecond repetitively pulsed (NRPs) discharges. In addition, we will leverage the huge range of ionization degrees and gas temperatures encountered in NRPs to assess the applicability range of several optical diagnostics in the partially and fully ionized regimes as well as during the transition phase between these two regimes. The outcomes of this study are expected to elucidate new insights in electron kinetics, gas heating and thermalization mechanisms responsible for the occurrence of instabilities in atmospheric pressure plasmas.   

Experimental Studies of the difference between electron and ion densities measured by Langmuir probes in the presheath

 It has recently been shown[1] that Langmuir Probes (LPs) measure an unphysically positive plasma potential in the presheath of low temperature plasma, near conducting boundaries at which ion rich sheaths form, compared with the plasma potential measured by emissive probes in the region.  We present[2] experimental evidence that concomittant with this, a similar and related difference occurs between electron and ion densities Langmuir probes in the presheath. A negatively biased plate (-100V) is immersed in a weakly collisional (λ_mpf  «λ_D), low pressure (P_n ≤1 mTorr), low temperature (kT_e ~ 1eV), single ion species plasma formed in a hot-filament DC discharge, where the feedstock gas is argon, helium, or krypton. Here we present details of the electron and ion density  profile measurements, which differed in the presheath, that were designed to test whether the ion flow in the presheath, quantified by the calculate Bohm speed, V_B = √(kT_e/m_i ), affects the difference between electron and ion densities.

[1] P. Li et al. PSST {f 29} (2020) 025015}   

Anode-Initiated Vacuum Insulator Flashover Experiments and Modeling

Vacuum insulator flashover is a breakdown process occurring along a dielectric surface separating electrode surfaces. Here, the dielectric is a separation barrier between vacuum and water-containing (or oil-containing) sections in pulsed power systems. While one kind of breakdown process, cathode-initiated flashover, is somewhat understood (an electron cascade resulting in surface charge saturation), other processes are hypothesized for anode-initiated breakdowns. One hypothesized process involves plasma initiation due to high fields at the anode triple junction and subsequent advancement and growth of a breakdown “spot” along the insulator towards the cathode. This process has some phenomenological overlap with positive streamers. This work will describe recent efforts to better understand anode-initiated flashover, including experimental investigation of the location and timing of various species emissions and plasma growth. A high-fidelity PIC-DSMC modeling approach using Sandia’s massively parallel plasma simulation code, Aleph, will be described, including multiple neutral, excited state, and ion species. Challenges with various emission models will be explored.   

Monitoring the non-equilibrium electronic response of surfaces during plasma exposure

Plasmas have long been used for the synthesis and modification of materials because of their unique ability to deliver both energetic and chemically active species to surfaces, which enables the engineering of a material’s surface properties without modifying the material’s bulk properties. Yet despite its widespread use, many details of the effect of plasma treatment on a surface, including the energy transfer of the species making up a plasma and their effects on the material’s electronic and vibrational structure, are still missing in literature. For this reason, understanding the energy transfer at the surface of a material upon plasma exposure is vital in optimizing its use in surface treatment.

In this work, we investigate the optical response of metals upon plasma treatment using a tunable-wavelength pulsed laser. Our approach first utilizes a plasma jet to excite the sample; then, laser pulses are used to probe the carrier dynamics of the irradiated region. By using a tunable-wavelength laser as our probe, we selectively monitor the changes in the optical response of different electronic states, giving direct insight into how plasma excitations influence individual states in a material. This study furthers the understanding of how plasma modifies electronic states under non-equilibrium conditions, allowing for a more precise optimization of surface treatments depending on plasma-surface interaction.   

Micro electric fields detection improvements: Steps toward tailoring cold atmospheric pressure plasma

From microfabrication to medicine, agriculture to surface decontamination and many more, applications of cold plasma technology seem limitless. The ability of cold atmospheric pressure plasma to generate highly reactive species through the plasma's electric field is very relevant for plasma tailoring applications. An efficient, sensitive, and high-resolution detection techniques to determine the electric field is needed for time and spatial resolved diagnostics. We proposed to use electric field-induced second harmonic, a well-established nonperturbative technique for measuring the amplitude and orientations of cold atmospheric plasma electric fields. Although E-FISH allows for a good and tunable time resolution, it has been shown that E-FISH presents some issues with spatial resolution and sensitivity. Work on enhancing these two characteristics of E-FISH have been made by our team and collaborators. Using a femtosecond laser, we have developed an amplified E-FISH technique that will allow cross beam measurements for high spatial resolution. The presented results confirmed the improvement of the electric field detection technique, the E-FISH, and will certainly deepen our knowledge on the spatio-temporal electric field distribution of cold atmospheric plasma.   

Development of the patch-type flat-cutoff sensor for real-time plasma density monitoring

In industrial plasma processing, monitoring electron density is significantly important because it is closely related to discharge characteristics that affect the processing yield. Among various methods for measuring plasma parameters in plasma processing, the flat-cutoff sensor^1,2 has gained attention because it allows non-intrusive and real-time electron density measurements. In this study, a patch-type flat cutoff sensor (P-FCS) with an antenna-dielectric-metal structure was introduced to simplify and thin the structure. The electromagnetic (EM) simulation was utilized to optimize the P-FCS compared to the previously researched flat-cutoff sensor. Moreover, it exhibits the characteristic of deeper penetration of EM waves into the plasma region. These results indicate that the P-FCS, with its improved directionality compared to the conventional flat-cutoff sensor, is capable of measuring electron density closer to the plasma bulk when compared to the conventional flat-cutoff sensor. Through experimental verification, it was confirmed that P-FCS measures plasma density adjacent to the plasma bulk compared to the conventional flat-cutoff sensor.   

Measurements of the current evolution on a load of X-pinch system using the Faraday rotation with spun fibers

A measurement system based on optic fibers is developed to measure currents on a load of the SNU X-pinch device [1]. As a high and fast current pulse is driven, electric circuit-based sensors are likely to be damaged by high electric fields. In addition, distortion of the measured signals is expected due to broadband electromagnetic waves induced by the pulsed high voltage sources. Hence, we construct the current measurement system using optic fibers such that the sensor part around the load is made with electrically insulated materials and the required electronic devices, e.g., a laser source, photo-detectors and oscilloscopes, can be shielded in a Faraday cage. The optic fiber is wound around the load to detect the Faraday rotation of the polarization state induced by the load current. We present configuration of the current measurement system using the spun fibers, which is better for maintaining an arbitrary polarization state compared to other types of optic fibers. We, then, compare temporal evolutions of the measured currents for the cases of a short circuit, a single wire and an X-wire load.   

Single-shot, non-resonant, four-wave mixing laser diagnostics for low temperature plasmas.

Single-shot coherent Rayleigh-Brillouin scattering (CRBS) is experimentally demonstrated for the measurement of the velocity distribution function (VDF) of neutral species in a glow discharge, from which macroscopic quantities, such as the flow velocity, density, and translational temperature, can be extracted. In CRBS, a four-wave mixing technique, the resulting single shot (~200 ns) CRBS lineshape is a direct mapping of the medium's VDF. CRBS has already been demonstrated to be the coherent analogue of spontaneous Rayleigh-Brillouin scattering and for measurement of nanoparticles in an arc discharge¹. In this study, single-shot CRBS is applied to measure simultaneously the temperature and density of neutral species in a weakly ionized DC glow discharge plasma (xenon gas, 15 Torr). For this application, we employ a newly developed dual-color CRBS scheme^2,3 .

The temperature and density of neutral xenon particles inside the DC glow discharge is evaluated simultaneously by analyzing the resolved single-shot CRBS lineshapes and is characterized as a function of the discharge current and radial position, successfully demonstrating the use of CRBS in a partially ionized plasma environment. A fluid model of the glow discharge is also developed and there is good agreement between the simulation and the experimental measurements in the glow discharge.

1) 10.1103/PhysRevApplied.9.014031

2) 10.1088/1361-6463/acb275

3) 10.1364/OE.470764   

Electric-field-induced coherent anti-Stokes Raman scattering in visible region (E-CARSv) generation from nitrogen in air

Plasmas generated in relatively high-pressure environments, especially in open air, are expected to play an active role in a wide range of applications, including medical and agricultural applications. Theoretical understanding of plasmas generated in relatively high-pressure environments is required to accelerate the development of such plasma applications. In particular, understanding the electric field, one of the most fundamental parameters of plasmas, is important to understand the dynamics occurring in plasmas. Recently, we reported a sensitive electric-field-measurement method based on electric-field-induced coherent anti-Stokes Raman scattering in the visible region (E-CARSv), which is appropriate for the relatively high-pressure of atmospheric pressure [1].

Here we present a generation of E-CARSv at 473 nm from nitrogen molecules [2], which are abundant in air, by applying two nanosecond-pulsed lasers at 4.3 μm and 532 nm. Details, as well as current progress, will be presented at the conference.   

Spatial Distribution of Absorbance in Laser-Induced Lithium Plasma Measured by Laser Absorption Spectroscopy

Li is currently being investigated as a material for breeding blankets in the field of nuclear fusion. To enhance the efficiency of these breeding blankets, the separation and analysis of Li isotopes is required. Generally, High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) has been used to analysis isotopes traditionally. However, this method requires expensive equipment and intricate preprocessing procedures. Laser-Induced Breakdown self-Reversal Isotopic Spectrometry (LIBRIS) is one of the alternative method to analyze Li isotopes. LIBRIS is a technique that employs the self-absorption phenomenon observed in laser-induced plasma emissions. Due to its narrower linewidth compared to emission light, self-absorption plays a crucial role in the precision of LIBRIS. In this study, understanding of the self-absorption phenomenon was focused. Li plasma was produced using an 1064 nm Nd:YAG laser with the pulse duration of 10 ns. Laser absorption spectroscopy was employed to measure the absorbance profile of Li atoms and a tunable laser with the center wavelength of 670 nm was used as the Li absorption light source. An iCCD array was used for the assessment of the intensity distribution, allowing for the precise identification of the point of maximum self-absorption.   

New method for calibration of hydrogen atoms measurement by femtosecond two-photon laser induced fluorescence

A variable pressure high voltage plasma discharge test cell was built for analysis of H-fs-TALIF capabilities for application in fusion divertors and other relevant environments. The discharge cell allows operation in the pressure range from a few mPa to tens of Pa with a gas flow through the cell that ensures a permanent gas replacement, which guarantees the absence of impurities. The plasma parameters in the discharge cell were estimated using current-voltage characteristics of the discharge. Based on the analysis of detailed kinetics, it is shown that in Xe-H₂ mixtures with low hydrogen content, complete dissociation of molecular hydrogen in the plasma is achieved in the absence of significant ionization and excitation of hydrogen atoms. This fact makes it possible to use such a system as a calibrated hydrogen atom source for various applications. Based on this H-atom source a new calibration method for H-fs-TALIF was proposed and the ratio of two-photon absorption cross-sections for H and Kr was reconstructed. The obtained estimate of the ratio of two-photon absorption cross-sections for H and Kr for broadband femtosecond laser excitation is 0.034±0.006, which is almost twenty times lower than the values obtained with the narrowband nanosecond lasers. This difference is explained by the significantly different spectral width of the excitation line and shows the need for independent calibration of the TALIF measurements in the femtosecond range.   

Bayesian Inference of the Anomalous Electron Transport in a Multi-fluid Hall Thruster Model

The lack of a first-principles understanding of the anomalous electron transport in Hall thrusters has precluded the development of fully-predictive engineering models of thruster operation. The current standard method for fluid-based simulations of Hall thrusters is to represent the electron transport as an anomalous collision frequency with a static, spatially-varying profile along channel centerline. The shape of this profile is adjusted until key quantities of interest—the ion velocity flow field and performance—match experimental measurement. The shapes of these profiles are typically hand-tuned in a process informed by user-experience and intuition. In this work, we develop an algorithm based on Bayesian Inference to rigorously determine the shape of the anomalous transport profile. We also develop a method to quantify the impacts of experimental and model-based uncertainty on our confidence in the median values of this profile. This approach is demonstrated on a multi-fluid Hall thruster code with an experimental dataset from the H9, 9-kW class magnetically-shielded Hall thruster.   

Artificial Neural Networks for Reaction Rate Prediction in ArO and UO Plasma Chemistry

Both experimental and computational approaches to the characterization of plasma-chemical reaction networks are faced with combinatorial complexity.  Plasma discharges in complex chemistries (e.g. atmospheric discharges, complex precursors) may have too many species to simulate spatially, and lifetimes for these species may be very short with signatures that are difficult to discern from each other.  Other information, principally rate expressions, may be challenging to obtain experimentally.  Application of artificial neural networks (ANNs) has shown promise towards solving these problems.

In this work we show progress towards rate expression prediction in 1) a UO laser-ablation discharge over varying laser intensities, and 2) an argon parallel-plate discharge over varying pressures and voltages. For each reaction in each chemical network, a small 2-layer 128-neuron neural network is trained to predict rate expressions from chemical species densities. Mean square percent errors in predicting out-of-training-set data were found to be below 1% for most reactions. Further, we show that ANN predictions remain robust when network inputs are restricted to 10-20% of the full species list.   

Demonstrating ThunderBoltz: An Open-Source 0D DSMC Boltzmann Solver for Plasma Transport and Chemical Kinetics

Large-scale 3D plasma codes involve a complex assembly of procedures that are not always necessary to test effects of underlying physical models. Here we present ThunderBoltz, a lightweight, publicly available 0D Direct Simulation Monte Carlo (DSMC) code designed to accommodate a generalized combination of species and arbitrary cross sections without the overhead of expensive field solves. It can efficiently produce high-quality electron velocity distributions in applied AC/DC E-field and static B-field scenarios. The code is built in the C++ standard library and includes a convenient Python interface which allows for input file generation from the LXCat data base, electron transport and reaction rate post processing, input parameter constraint satisfaction, calculation scheduling, and diagnostic plotting.

In this work we compare ThunderBoltz transport calculations against Bolsig+ calculations, benchmark test problems, and swarm experiment data, finding good agreement with all three in the appropriate field regimes. In addition to this, we present example use cases where the electron, ion, and background neutral particle species are self-consistently evolved providing an ability to model the background kinetics, a feature that is absent in fixed background Monte Carlo and n-term Boltzmann solvers.   

Two-dimensional full-fluid moment simulations of partially magnetized ExB plasmas

The state-of-the-art in fluid modeling for low temperature plasmas has long been based on the drift-diffusion approximation, in which electron inertial effects are neglected, often accompanied by a quasi-neutrality assumption. These simplifications prevent the resolution of shear and non-neutral effects, both of which are thought to play significant roles in the anomalous electron transport observed in partially magnetized plasma devices such as Hall effect thrusters. In this work, a two-dimensional, full-fluid (i.e., electron and ion fluid) code is developed which solves the continuity, momentum, and energy equations for ions and electrons, as well as Poisson’s equation. Cases are considered with an in-plane magnetic field with fully magnetized electrons and unmagnetized ions and the ExB direction out-of-plane. The effects of near-wall sheaths and oblique magnetic fields on cross-field electron transport are studied and compared to the predictions of classical collisional theory.   

Four impedance matching methods for capacitatively coupled plasma source with transmission line and lumped parameter circuit.

Capacitively coupled plasmas (CCPs) have diverse applications in fields such as plasma etching and thin-film deposition. To maximize the efficiency of plasma generation and minimize the reflection of power during transmission, an impedance matching network (IMN) and design method for capacitively coupling plasmas play a crucial role. The coupling and nonlinear interactions of the transmission line, the lumped parameter circuit and the plasma load pose great challenges to this design^[1-5]. In this study, they are described by the Lax-Wendroff method, Kirchhoff Voltage Law (KVL), and the Particle-In-Cell/Monte Carlo Collision (PIC/MCC) method respectively. Four distinct methods are utilized to design optimal impedance matching for this intricate discharge system: (1) The impedance matching design of the plasma source with two ideal transmission lines and an L-type matching network is realized by iterative calculation. Only a few iterations are required to attain a reflection coefficient below 0.01. (2) Impedance matching is achieved by employing a dichotomy approach to adjust the frequency of the RF power supply. (3) For single-frequency CCPs, two adjustable capacitors are adjusted simultaneously by the gradient descent method to minimize the reflection coefficient. (4) Machine learning techniques are leveraged to train on existing simulation data, enabling the identification of optimal circuit parameters.   

DSMC Simulation of Axial and Radial Spreading of the Feed Gas in a High Speed Rotating Cylinder.

The main focus of this work is to characterize the axial and radial spreading of the feed gas introduced into the high speed rotating cylinder at different axial and radial locations for wall pressure in the range 20 to 100 m-bar using two dimensional Direct Simulation Monte Carlo (DSMC) simulations. The feed gas is accelerated through collisions with the surrounding rotating gas molecules, and there is a slowdown of the rotating gas molecules near the feed injection point, and the slow-down (V_{θ, solid body rotation} – V_θ) decreases with the increase of axial distance away from the feed point in a very complex manner, with initial slow reduction, and then rapid decrease to zero value. An important finding is that at a given feed flow rate and feed gas temperature, the axial and radial spreading of the feed gas decreases with the increase of wall pressure, and at 100 m-bar wall pressure the axial spreading continues up to  0.141 Z, with radial spreading up to 0.733 R_wall  ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). Here, Z is the effective length of axial counter-current, R_wall is the radius of the cylinder, V_{θ, solid body rotation} is the angular velocity corresponding to solid body rotation, and V_θ is the actual angular velocity of the rotating gas after feed injection at a given axial and radial location. The DSMC simulation result indicates that the radial velocity is symmetric around the center-line of the feed point and decreases along the radial and axial directions following a polynomial function. The DSMC simulation result also reveals that with the increase of axial feed location away from the top baffle, the radial profile of the axial mass flux becomes more flatten at the axial mid plane, and effects the counter-current circulation rate to a great extent.                

Electron kinetics under AC/DC electric and magnetic fields: benchmark calculations and electron cyclotron resonance

This work explores the capabilities of the Monte Carlo open-source code LoKI-MC, aiming to investigate the electron kinetics under AC/DC electric fields and DC magnetic fields crossed at arbitrary angles. The study includes both model gases (Reid-Ramp and Lucas-Saelee) and real gases (N2 and Ar), with a thorough comparison of the code against various independent benchmark calculations available in the literature. The role of the magnetic field is examined, distinguishing between configurations with DC and AC electric fields. In the case of DC electric fields, the oscillatory motion induced by the magnetic field reduces the efficiency of electron acceleration. Conversely, in AC electric fields, when the magnetic field value is such that the cyclotron frequency is similar to the angular frequency of the electric field, electron acceleration is enhanced through the well-known phenomenon of electron cyclotron resonance. However, under conditions where the mean collision frequency significantly exceeds the cyclotron frequency, the synchronization tends to break down, and the magnetic field becomes detrimental to electron acceleration, similar to the DC electric field case. Finally, this research provides novel benchmark calculations for electron kinetics solvers under coexistent AC electric and DC magnetic fields, accounting for the effect of gas density and the angle between the fields. The current work highlights the potential of LoKI-MC, which is freely available for the community.   

Direct simulation Monte Carlo of single-shot coherent Rayleigh-Brillouin scattering

A better understanding of the gas heating mechanism of industrial plasma sources is critical to the advancement of plasma processing technology. Coherent Rayleigh Brillouin scattering (CRBS) is a non-intrusive technique that measures the translational temperature of gas particles with atomic and molecular polarizability. In order to achieve a shorter measurement time, single-shot CRBS uses a chirped laser, allowing for a variation of the optical lattice wave frequency that results in probing the entire velocity distribution function (VDF) in a single laser shot measurement (~200 ns duration). We developed a direct simulation Monte Carlo (DSMC) code that accounts for a chirped optical lattice, which is validated with single-shot CRBS measurements. Overall, the simulation results are in good agreement with experimental data. However, the CRBS signal shape obtained from the DSMC simulation shows asymmetries and skewness of the Brillouin peaks when the chirp rate is fast. We will discuss the effects of the laser chirp rate on the CRBS signal.   

Analysis of Internal Flow Phenomena in a High Speed Rotating Cylinder Using Double Parabolic Axial Flow Model

In this study we investigate the internal flow phenomena in a high speed rotating cylinder using double parabolic axial flow model for wall pressure in the range 20 to 100 m-bar. These includes the quantitative estimation of the important process parameters L₀, L, m, and (F/L).Here, L₀ is the internal circulation rate at total reflux, L is the actual internal circulation rate, m = L/ L₀ is the internal circulation parameter, (F/L) is the internal reflux ratio, and F is the external feed flow rate. The radial profile of axial mass flux  (ρv_z) is specified as a function of radial scale height  ξ   ( ξ = A² (1 - ( r² / R_w² )),  A is the stratification parameter,  A =(( M_w V_θ ²) / (2 R_g T))^1/2, M_w is the molecular weight, V_θ   is the peripheral speed of the cylinder, R_g is the universal gas constant, and T is the uniform gas temperature)  by two parabolas. One parabola represents the flow downward in the region near the rotor wall, and the second one representing the upflow adjacent to the downflow. The feed gas introduced into the rotating cylinder is considered to be associated with the upflowing stream ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). An important finding is that as the wall pressure is increased from 20 to 100 m-bar, there is a increase in L₀, L, and m. However, there are important differences. L₀ initially increases at a faster rate and then saturates at high wall pressure. On the other hand the parameters L and m increases monotonically with the wall pressure. The effect of feed flow rate on the parameters L₀, L, and m is also studied and the analysis indicates that as the feed flow rate is increased, the parameters L₀, L, and m increases, and the effect is significant at high wall pressure. The analysis also shows that as the average gas temperature is increased, the parameters L₀, L, and m increases, and the effect is more pronounced at high wall pressure. However, with the increase of wall pressure, the internal reflux ratio (F/L) decreases, initially at faster rate and then become constant at high wall pressure, which indicates that there is a strong coupling between the process parameters  L₀, L,  m, and (F/L), and the result also reveals that at high wall pressure (hence at high holdup) the separation process takes place under improved equilibrium condition.   

Two-dimensional kinetic modeling of a hollow cathode

Hollow cathodes are efficient sources of plasma, which have found extensive applications in various fields such as electric propulsion, surface processing, and plasma-material interac8on studies. In state-of-the-art fluid models [1,2], electrons are assumed to thermalize quickly, and ionization occurs in the bulk. In this work, we find that under certain operating conditions, electrons can display a non-Maxwellian energy distribution function (EEDF), which affects the ionization processes and temperature. We derive subsequent analytical models in the channel and in the plume to describe the plasma dynamics. We use the explicit Particle-In-Cell code EDIPIC-2D (hRps://github.com/PrincetonUniversity/EDIPIC-2D) to conduct our inves8ga8on.

1 G. Sary, L. Garrigues, and J.-P. Boeuf, “Hollow cathode modeling: I. A coupled plasma thermal two-dimensional model,” Plasma Sources Sci. Technol. 26(5), 055007 (2017).

2 D.M. Goebel, G. Becatti, I.G. Mikellides, and A. Lopez Ortega, “Plasma hollow cathodes,” Journal of Applied Physics 130(5), 050902 (2021).   

Analysis of Scoop System in a Mechanically Driven Gas Centrifuge.

In this study we investigate the performance of a stationary scoop system (scoop tip and its arm) in a mechanically driven gas centrifuge at wall Mach number Ma_wall in the range 4 to 8, wall pressure P_wall in the range 20 to 100 m-bar, scoop arm radius of curvature (extremity radius) R_roc in the range 5 to 30 mm, scoop wall gap d_wall in the range 5 to 20 mm, and slenderness ratio (major to minor axis) of the elliptic scoop in the range 1.2 to 6. The analytical model is formulated based on the steady state assumption where the accelerating moment exerted by the rotating wall and bottom end cap on the rotating gas contained in it, is balanced by the decelerating moment due to the aerodynamic resistance of the scoop tip and its arm, which allow us to evaluate the scoop plane Mach number as a function of wall Mach number with the parameter A Kn_axis. Here, Kn_axis is the Knudsen number at the axis of the rotating cylinder, and the factor “A”, characterized a particular scoop system (shape and size of the scoop tip and its arm). In a high speed rotating field we examine the stagnation to axis pressure ratio as a function of wall Mach number for different scoop systems, and the result indicates that scoop system having smaller dimension exhibit higher pressure recovery with modest slow-down of the circumferential velocity of the rotating gas. An important finding is that the scoop arm with higher radius of curvature exhibit less decelerating moment and generates a reduced amount of secondary radial flow ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). The analysis shows that with the increase of wall pressure from 20 to 100 m-bar the decelerating moment of the scoop-tip and its arm increases. The analysis also indicates that at a given wall Mach number, scoop tip exhibit more decelerating moment compared to the scoop-arm having same dimensions. Therefore, the required magnitude of deceleration can be achieved at a given wall pressure through proper combination of the scoop tip and its arm dimension. The effective scoop arm length that provides most of the scoop arm deceleration, is estimated with respect to the total arm length, and the result shows that arm-profile of the effective length portion with various winged shape is an important design aspect while operating at high wall pressure.   

Analysis of Feed Gas Expansion in a High-Speed Rotating Cylinder

  In this study we investigate the expansion of the feed gas from the feed nozzle into a strongly rotating gas at the pure diffusive region of the inner core in a high-speed rotating cylinder with the characteristic radial length scale equal to (ξ /A²) for peripheral speed (V_wall) in the range 450 to 700 m/sec, feed nozzle radius (R_nozzle) in the range 3 to 8 mm, expansion pressure ratio (P_o/P_∞ ) in the range 10 to 10⁵ , and  for pure UF₆ as well as for a mixture of UF₆ and Hydrogen Fluoride (HF: acts as a light gas) with HF concentration up to 60 mole % ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)).The feed gas expansion into the vacuum core is characterized by the formation of a barrel shock, and the boundary layer type flow is developed on the surface of the barrel shock. The analysis of the jet boundary is carried out with the estimation of important parameters like angular dependency of mass flux (ρV), the pressure on the jet boundary (P_∞ ), the initial longitudinal radius of curvature (R_CO), the centerline Mach number distribution (Ma_CL), and the ratio of sonic radius to nozzle exit radius (r*/R_nozzle). The analysis indicates that as the feed nozzle radius is increased from 3 to 8 mm, the pressure on the jet boundary increases monotonically, whereas, with the increase of feed nozzle radius, the ratio (R_CO / R_nozzle) initially decreases and finally saturates at a constant value. The thickness of the shock layer is also studied for the peripheral speed in the range 450 to 700 m/sec, and the result shows that with the increase of peripheral speed the shock layer becomes more and more thinner. At  (P/P_S) = 10, the thickness of the shock layer has decreased from 0.40278 mm to 0.16645 mm. Here, P_S is the pressure behind the barrel shock. An important finding is that with the increase of peripheral speed from 450 to 700 m/sec, the rate of efflux from the boundary layer increases, and at (X/R_nozzle) = 20, its normalized value ( ρV_γ (∞)/ρ_SV_γ,S (∞)) has increased from 0.4751 to 0.73918. Here, ρ_SV_γ,S (∞) indicates the properties behind the barrel shock.  It is also seen that with the increase of Reynolds number from 10^-4 to 10^3, the rate of efflux from the boundary layer decreases, and at  (X/R_nozzle) = 20,  its normalized  value ( ρV_γ (∞)/ρ_SV_γ,S (∞)) has decreased from 633.59  at Re = 10^-4  to  0.2003 at Re = 10^3.     

Effects of runaway electrons in partially ionized plasmas

Electron dynamics plays a significant role in partially and fully ionized gases. The electric field accelerates the electrons while various collisional processes, such as electron-ion and electron-neutral collisions, lead to deceleration of electrons. When taking the particle ensemble, the competition of the energy gain and loss can lead to a non-Maxwellian velocity distribution function. In this study, a zero-dimensional (0D) set up is considered where the electric field (E) and gas density (N) are imposed. Varying the ionization degree, i.e., the ratio of plasma density and gas density, the effects of the Coulomb collisions on the discharge are accounted for. In this study, we construct a theory using the equation of motion (force balance) to show that there are particles with a particular velocity range that experiences unbounded acceleration, resulting in runaway electrons. The 0D Monte Carlo collision simulation shows that the higher-order moment (e.g., the 4^th moment) indeed does not reach steady state depending on the E/N values while the lower-order moments (e.g., bulk velocity, temperature, and heat flux) can achieve steady state, confirming the observation of the theory.   

The effects of Complex Eigen modes and frequencies in a Finite-Area Plane Surface Wave Plasma

As a semiconductor manufacturing plasma source, surface wave plasma (SWP) which generates a specific wave mode between the plasma and a dielectric plate is widely used. To improve power transfer efficiency for plasma generation, it is necessary to optimize resonant frequency at dielectric surface. For this purpose, a cylindrical microwave source which uses surface wave excitation has been developed. A higher-order mode cylindrical cavity resonator is used for plasma generation. Electromagnetic waves generated by solid-state power amplifier system, which can be controlled at an operating frequency of 2.4 to 2.5 GHz, are propagated to the cylindrical resonator. Based on the electric field distribution of the cylindrical cavity, electromagnetic waves propagate through a hybrid type slot antenna composed of large aperture structures. The electric field distribution of the surface wave (TM mode) formed on the dielectric window surface varies with operating frequency control and plasma characteristics (electron density, momentum transfer frequency). This effect is experimentally and numerically confirmed in Ar and Ar/O₂ plasma at 0.1~1Torr with total injection powers from 0.6 to 1.5 kW, which shows surface wave mode is affected by complex eigen modes and frequencies.   

Characterization of Microwave Plasma Using Langmuir Probe and Imaging Techniques

A study of a microwave plasma driven at 2.45 GHz is presented. Using Langmuir probe and 2D imaging of the plasma, a mapping of plasma parameters with the size of the plasma in the overdense region is studied. Gas mixtures in the study include Ar, Ar-O₂ and Ar −He at pressures of 0.1-10 Torr. Delivered power to the plasma ranges from 25-150 W. A Langmuir probe is used to obtain electron density and electron temperature. The size of the plasma is obtained using a CCD camera with bandpass filters. The results are also compared to COMSOL and MOOSE simulations.   

α to γ transition and plasma uniformity in large area intermediate pressure N2 capacitively coupled plasma (CCP) discharges

1D particle-in-cell (PIC) simulations of a 2.5 cm gap, 1.6 Torr N₂ capacitive discharge showed an α to γ transition, characterized by a rise in density and a collapse of the sheath width when the rf sheath voltage amplitude V₁ exceeds a breakdown voltage V_B. Smaller sheath widths enhance electromagnetic (EM) effects which may negatively affect plasma uniformity and stability. We develop a 2D axisymmetric cylindrical EM fluid model including secondary emission to study the α to γ transition and plasma uniformity in a large area reactor with discharge radius R=1.8 m. For lower power and input current I_rf, the discharge is fully in the α mode with V₁<V_B for 0<r<R. At higher power and I_rf, the enhanced finite wavelength effect causes V₁>V_B (γ mode) toward the center and V₁<V_B (α mode) toward the radial edge with the transition point r=r_B approaching R. A fully γ mode discharge, in which V₁>V_B for 0<r<R, is obtainable at a frequency f=6.78 MHz but not at f=13.56MHz, due to the enhanced finite wavelength effect at the higher f. Thus, for lower f, plasma uniformity may be obtained not only at lower powers when the discharge is fully in the α mode, but also at higher powers when the discharge is fully in the γ mode.   

Numerical Simulation of an RF Capacitive Discharge in CO2 Using the Hydrodynamic Code SIGLO-rf

We investigated the processes occurring in an RF capacitive discharge in carbon dioxide using the SIGLO-rf hydrodynamic code, which is the 1D user-friendly simulation software of capacitively coupled RF discharges developed by JP Boeuf and LC Pitchford at the University of Toulouse and based on the same physics as in [Phys. Rev. E 51 1376 (1995)]. The dependences of the coefficients of ionization and attachment of electrons on the reduced electric field, as well as the coefficients of ion-ion, ion-electron recombination and detachment of electrons in carbon dioxide, were added for calculations in the gas parameters file. The calculated RF discharge extinction curve is in good agreement with our measured curve, which confirms the suitability of both the code and the gas parameters introduced into it for describing processes in plasma. It is shown that at low CO2 pressures (below 0.1 Torr), stochastic heating of electrons in expanding near-electrode sheaths dominates. At higher gas pressures, stochastic heating is pronounced only at high RF voltages (several hundred volts), and the main heating of electrons occurs in double layers, which are formed both in the anode phase of the sheath and in the plasma volume. The maximum number of double layers appearing in the discharge gap during one RF half-cycle reached 3.   

The Effect of Secondary Electron Emissions and Bias Voltage on Electron Heating in RF Hollow Cathode Capacitive Discharges

The change in electron heating by the secondary electron (SE) emission and the DC bias on the RF hollow cathode (HC) capacitive discharge was investigated using a 2D PIC simulation. Without SE emissions, a positive DC bias enhances the plasma density more than a negative DC bias because the electron heating near the ground electrode is dominant. However, with an asymmetric SE only from the HC, the ion-induced SE emission from the HC is the most prevalent source for electron heating. Thus, the ionization collision rate increases near the HC electrode where the SE emission occurs and forms a narrow plasma sheath. The plasma can penetrate more quickly into the hole, and the ionization collision rate also increases in the hole. With a positive bias voltage and the SE emission from the HC, the SE causes the plasma to penetrate more into the hole, resulting in an overall increase in plasma density. With a negative bias and a SE emission, differently from the no SE emission case, the plasma density inside the HC dramatically increases. Therefore, the HC effect is dominant with the ion-induced SE emission.   

Modulating Capacitively Coupled Plasma Characteristics using F- Ion Beam Injection

The characteristics of capacitively coupled plasma (CCP) not only can be modulated not only through the traditional method of using a radio frequency (RF) source but also by employing electron beam and positive ion beam injection^[1-5]. In this study, a negative ion beam (IB) source is employed to inject into the discharge region of pure CF₄ plasma. The implicit PIC/MCC method is used to simulate the stable discharge after IB injection. The results demonstrate that IB injection effectively enhances plasma density and ion. Additionally, the heating mode of the plasma shifts from the drift-ambipolar (DA) mode to a hybrid α-γ-DA mode with IB injection. Furthermore, an increase in IB current leads to a corresponding increase in plasma density. After the injection of ion beams, a significant variation in the self-bias voltage on the power electrode is observed under different gas pressure conditions. This research contributes to the understanding of CCP modulation and its potential application in thin film deposition and material surface cleaning processes.   

Advancing the Performance of Water Vapor Propellants in an ECR Plasma Cathode

Electron cyclotron resonance (ECR) plasma cathodes are microwave-driven electron sources that can be utilized in a variety of applications, including space propulsion and plasma etching. Due to their use of microwaves to generate the seed plasma, a wide range of gases may be used in the system, including complicated molecular species such as water vapor and carbon dioxide. Operating plasma thrusters on molecular species, especially water vapor, is an attractive proposition due to its availability and storability for long missions; however, previous experiments have shown discharge losses too large for practical applications. In this work, we investigate the capacity for improvement of a wave-heated ECR plasma source operating on water vapor propellant for plasma cathode applications, with the overarching goal of achieving discharge losses sub-100 W/A. We present findings that depict both the efficacy and the efficiency of the plasma cathode at operating on water vapor in the form of macroscopic parameters such as discharge losses [W/A] and extraction current [A]. The plasma composition and characteristics are assessed using electrostatic probes and optical emission spectroscopy.   

Spectroscopic measurements of plasma parameter in a low-pressure magnetized capacitively coupled nitrogen plasma

Over the past few decades, the control of charged particle flux-energy through the dc self-bias has attracted substantial attention from researchers and industries, for high-aspect-ratio etching or ion implantation. However, the conventional geometric asymmetry method is hard to control the ion flux-energy. To address this limitation, a magnetized capacitively coupled plasma (MCCP) has been investigated.

The asymmetry effect can be effectively controlled in low pressure by adjusting the magnetic field strength. The magnetic field confines electrons near the electrode and generates the dc self-bias to balance the ion-electron flux. M. Oberberg et al. have reported the magnetized asymmetry effect (MAE) such as changes in the heating mechanism, and ionization rate, in an argon MCCP [1,2]. In order to adopt the MCCP in the semiconductor industry, however, it is imperative to investigate the MAE of various gas plasmas.

In this study, we investigate the magnetic asymmetry effect on the properties of magnetized nitrogen plasma using optical emission spectroscopy. The neodymium magnets are arranged on the grounded electrode and show 60 G of magnetic field strength on the surface of the electrode. The relative ionization rate and dissociation rate are analyzed by comparing the intensity of N₂ second positive system, N₂⁺ first negative system, and N I. In addition, plasma temperatures are determined by simulating the molecular spectra and fitting them to the observed emission bands of the molecule.   

Influence of High Voltage Microsecond Pulse Polarities on a Helium Atmospheric Pressure Plasma Jet

Atmospheric pressure plasma jets (APPJs) are plasma sources having plenty of applications in surface modification, environment protection, biological and medical applications. In this experiment, APPJ was simply consist of a quartz tube, a powered electrode, and an electrically grounded electrode. A 5~10 kV of high voltage (HV) pulsed DC was applied to the APPJ to produce helium gas discharge. Positive and negative polarities of the HV pulses were separately supplied to the APPJ. To characterize the APPJ, line intensities of He and other species such as O, N₂, N₂⁺ and OH of plasma spectra measured by optical emission spectroscopy. He 2³S metastable atom density was measured by laser absorption spectroscopy. Plasma properties such as electron density and electron temperature were deduced from the optical experiment. Speed of gas discharges propagating in the way of gas flow was dramatically changed depend on the polarities of HV. Finally, the different physics of plasma discharges in the APPJ depended on the polarities of the applied HV pulsed DC was discussed.   

Study of atmospheric-pressure dielectric barrier discharges in Ar-TMS and Ar-HMDS mixtures

The use of dielectric barrier discharges (DBDs) as a source for plasma-enhanced chemical vapour deposition (PECVD) processes has increased significantly over the past two decades. An investigation of DBDs in argon with small additions of tetramethylsilane (TMS) or hexamethyldisilane (HMDS), carried out by fluid modelling and experimental diagnostics, is presented here. DBD plasma sources with plane-parallel and single-filament configurations driven by sinusoidal voltages at frequencies of 86 and 19 kHz are used in the study. The modelling is carried out using a spatially one-dimensional fluid-Poisson model with complex reaction kinetics involving precursor species. The analysis shows that Penning ionization (PI) of precursor molecules by excited argon atoms significantly impacts the discharge characteristics. It is found that electron production is dominantly determined by these processes, which consequently affect the ignition voltage of the gas. Furthermore, the analysis of the boundary fluxes reveals that the fluxes of cations generated in PI and subsequent ion-molecule processes are predominant, making them the main candidates responsible for film formation under the investigated conditions.   

Investigation of Key Intermediate Species in Non-Thermal Plasma-Activated Carbon-Nitrogen Coupling on a Catalytic Surface from Methane and Nitrogen

The integration of reactive non-thermal plasmas (NTPs) with heterogeneous catalysis can promote novel chemical transformations that neither plasma nor catalysis could deliver individually, such as direct carbon-nitrogen (C-N) coupling from methane (CH₄) and nitrogen (N₂). To unveil the full potential of plasma-catalyst coupling, it is important to understand the interaction between the NTP-activated species and a catalytic surface at the interface. In this work, we utilized multi-modal spectroscopy combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS), optical emission spectroscopy (OES), and mass spectrometry (MS) to investigate the key intermediate species that lead to C-N coupling on model catalytic surfaces (Ni, Pd, Cu, Ag, and Au). Our system consists of an argon (Ar) plasma jet propagating through a controlled CH₄ and N₂ environment and impinging upon a substrate. We investigated C-N coupling on a catalytic surface exposed to (1) 1:1 CH₄:N₂, (2) CH₄-N₂ in sequence, where pure CH₄ feed followed by pure N₂ feed, and (3) N₂-CH₄ in sequence, where pure N₂ feed followed by pure CH₄ feed. Our results show that surface CH_x and plasma-phase CN species correspond to C-N coupling on a catalytic surface. The effects of (1) different surfaces and (2) different procedures on the nature of surface-adsorbed C-N coupled products are further investigated with post-experiment X-ray photoelectron spectroscopy (XPS) and liquid chromatography-mass spectrometry (LC-MS).   

Contribution to Bead Width in Feedback Control of Cathode Torch Using Image Processing

In the industrial field, processing technology for joining metal materials is an indispensable technology. TIG arc welding has the advantages of welding without spatter and adaptability to any joint shape. TIG arc welding is widely applied in fields where high reliability and safety are required. TIG arc welding is technically challenging, and the number of welding technicians has been decreasing. In recent years, the introduction of robots and de-skilling in welding has been progressing, and it is needed to realize automatic welding without welding defects under various conditions. Automatic welding that can respond to various situations can be realized by grasping the current situation and controlling the welding process based on that information. Weld defects are caused by changes in heat input. Heat input is caused by changes in arc voltage. Arc voltage is caused by changes in the inter-electrodes distance. In this research, the bead width was calculated for feedback control of the cathode torch using image processing. In this method, images of the arc state during welding were processed and the inter-electrodes distance was measured. The cathode torch was driven using feedback control to keep the inter-electrode distance constant.   

Analysis of High-Temperature Gas Behavior Reflected to Sidewall in Sealed Arc Extinguishing Chamber

The high-temperature gas should be dissipated in the sealed arc extinguishing chamber. It is possible to lead to diffuse and reflect the high-temperature gas toward another direction from heat convective transfer after extending the arc with external magnetic field. However, the high-temperature gas reflected to the sidewall directs to inter-electrode may cause re-ignition because of its flow velocity even if the arc was extinguished. In addition to that, prediction of the phenomena is desired before the re-ignition because it takes much time to analyze complex phenomenon with experiment and simulation. The distributions must be obtained under the various conditions of shapes as a parameter. Such a method requires many results with finely varied calculation area using simulation. The goal of this research is to predict another distribution shaped the sidewall as a parameter using a result of the distribution. The method finds the results of the phenomena with the conditions of all shapes of the sidewall using a result, thus it shortens the term of development. The first objective is lowering the temperature in inter-electrodes by diffusing the high-temperature gas that move to the sidewalls in all directions. Then, the charged particle density in inter-electrode from inflow after reflection decrease. Therefore, in this research, high-temperature gas behavior reflected to the sidewall in the sealed arc extinguishing chamber was analyzed by three-dimensional electromagnetic thermal fluid simulation.   

Contribution to Arc Voltage by Automatic TIG Arc Welding Using AI Discrimination in Uneven Base Metal

TIG arc welding is used in various places because of its ability to weld for long periods of time, precision welding, and lack of spatter, but problems such as the aging of the population in recent years have led to calls for automation. To establish an automated TIG arc welding method, feedback control has been used to measure arc voltage and arc length and to control the distance between electrodes, but since these are ex-post controls, the control lags behind the phenomenon. Therefore, in this research, automatic TIG arc welding using AI discrimination was performed as a pre-control method. Specifically, the base metal was identified by having YOLOV5 learn the shape of the base metal. Measurements of the discriminated base metal were taken using iPad. Through this research, automatic TIG arc welding method will be developed that enables optimal control for various shapes by understanding the base metal shape using AI discrimination.   

Importance of gas dissolution on droplet ejection from liquid gallium irradiated with inductively coupled plasma

We observed the ejection of droplets from liquid Gallium interacting with an inductively coupled plasma. The droplet election was observed at the timing of the bubble rupture on the liquid gallium surface. We examined the frequency of droplet ejection by varying the ion flux ((0.2 - 7.1) × 10¹⁴ cm^-2 s^-1), ion energy (15 - 215 eV), and the liquid gallium temperature (370 - 500 K). In the case of hydrogen plasma, the frequency of the droplet ejection increased with the ion flux and the liquid gallium temperature. On the other hand, it decreased with the ion energy. When irradiating argon, helium, and nitrogen plasmas, the frequency of the droplet ejection was lower (0 - 0.004 s^-1) compared with that in hydrogen plasma (0.01 - 0.035 s^-1). We estimated the amount of dissolved hydrogen by measuring the partial pressure of molecular hydrogen desorbed from the liquid gallium using a mass spectrometer. The amount of molecular hydrogen increased with the ion flux, and it decreased with the liquid gallium temperature. In addition, a spiky increase in the hydrogen partial pressure was observed at the timing of the bubble rupture or the droplet ejection. This result indicates the importance of dissolved hydrogen in the liquid gallium. The amount of hydrogen in the liquid gallium can reach the supersaturated state by the ion irradiation. The supersaturation results in the formation of a bubble, and droplets are ejected at the bubble rupture.   

The absorption spectrum of solvated electrons in non-aqueous media at the plasma liquid interface

Non-equilibrium plasmas in contact with a liquid can produce a myriad of highly reactive free radicals such as electrons, hydroxyl ions, and reactive oxygen species. These radicals can drive useful solution-phase chemistry but will also recombine with themselves and the solvent in aqueous solutions, inhibiting efficiency of the target reaction. Non-aqueous solvents offer an alternative media for solution phase chemistry, where recombination of solvated electrons with the solvent are slow or non-existent. The interfacial behavior of plasma electrons when delivered into non-aqueous solutions has been so far unexplored. In this work, we aim to map out the absorption peaks of solvated electrons in alcohol/water solutions over percentages ranging from no alcohol (0%) to pure alcohol (100%) using an in-situ diagnostic technique to interrogate solvated electrons at a plasma-liquid interface known as total internal reflection absorption spectroscopy (TIRAS). Using glycerol and ethylene glycol as model alcohols, we measure the absorption behavior of plasma-solvated electrons at different wavelengths (532, 635, and 710 nm) and alcohol concentrations (0%, 7%, 14%, 30% 51%, 75%, 85%, 100% by volume). These measurements are compared to absorption behavior of pulse radiolysis-produced solvated electrons to assess the overall behavior of plasma-solvated electrons in alcohol solutions.

A transport-limited theory for the coalescence/de-coalescence plasma patterns on a plasma-liquid interface

The fundamental mechanism of self-organized anode patterns in plasma electrolysis is as puzzling as their beauty. Yet, a comprehensive understanding of pattern formation and self-organization remains elusive. In this work, we propose a transport-limited theory for an unexpected plasma pattern consisting of coalescence and de-coalescence oscillations (CDO) when operating a cathodic DC glow discharge at a moderate current of ~-26 mA. Time-resolved characterization yields a liquid conductivity- and viscosity-dependent CDO frequency of ~200 Hz, indicating a fluid transport process as the frequency is far smaller than any reaction timescales inherent to plasma processes. We therefore attribute the observed CDO dynamics to the advective transport of liquid phase cations due to capillary waves that arise from the electrostatic Maxwell pressure on the plasma-liquid interface. A Maxwell stress-modified dispersion relation of viscous capillary waves is developed. The derived analytical solution of the capillary wave frequency is comparable to the experimental data, suggesting a strong correlation between the induced capillary waves and the measured CDO frequency. Laser-assisted visualization further resolves the existence of the capillary waves only when the CDO pattern is being generated, confirming the hypothesized connection between the dynamics of the plasma and the dynamic liquid behavior. Ultimately, the finding of this work reflects the intrinsic electrostatic coupling between the liquid Debye layer and the plasma sheath at the plasma-liquid interface.   

Hydrogen production from water vapor in atmospheric DBD plasmas

Hydrogen as a sustainable energy carrier provides one of the promising solutions for meeting future energy demands. In this study, we consider the production of hydrogen from water using plasma splitting,  i.e., a means of water splitting driven by nonequilibrium plasmas in a dielectric barrier discharge. We have developed a customized flow reactor, which is able to transport water vapor (steam) under conditions of varying pressures and temperatures and different carrier gases. The concentration of H₂O, H₂, and O₂ is monitored ex situ, with a residual gas analyzer. We report on measurements of the hydrogen production rate under pressures ranging from 600 Torr – 900 Torr, temperatures from 300 K- 450 K, flow rates of 5 sccm to 100 sccm, and in carrier gases such as Ar, N₂, and Air. Our experiments are complemented by the ongoing development of plasma kinetic mechanisms of the H₂/O₂/H₂O system for understanding key chemical reaction pathways and factors influencing the hydrogen production rate.   

Imaging Measurements of the Spatial Distributions of OH, H₂O₂, and HO₂ in the Effluent of the COST Reference Source

Non-thermal plasmas (NTP) for medical applications have been gaining interest in the scientific community due to their controlled delivery of reactive oxygen and nitrogen species (RONS) that have been shown to produce immunostimulatory effects. However, the complexity of possible chemical pathways has made it difficult to differentiate the importance of specific RONS for this positive effect. It is therefore of interest to not only determine the overall concentration of RONS in the effluent, but also their spatial distributions. We combine laser-induced fluorescence (LIF) and photofragmentation laser-induced fluorescence (PF-LIF) techniques to image the spatial distributions of three key reactive species - the hydroxyl radical (OH), hydrogen peroxide (H₂O₂), and the hydroperoxyl radical (HO₂) - in the open effluent of the Capacitively Coupled Atmospheric Pressure Microplasma Jet (COST-jet). Approaches to quantifying the LIF and PF-LIF signals and differentiating between PF-LIF signals from H₂O₂ and HO₂ will be discussed. We consider several helium-based admixtures (He, He+O₂, He+H₂O, and He+N₂+O₂) that have been used in previous studies of the COST jet, and we compare results with the jet issuing into nitrogen and air atmospheres to separate possible chemical pathways. We observe the formation of chemical reaction fronts that indicate a significant increase in HO₂ production where the effluent of the jet mixes with oxygen from the surrounding air. This research contributes to our understanding of the formation, consumption, and transport of OH, H₂O₂, HO₂ in the jet effluent. Ultimately, a detailed account of these complex plasma chemistry interactions is needed to advance medical applications and tailor the deliverable densities of key reactive species.  

    

Characterization of a He-H2-N2 jet plasma by means of a plug-flow model

A global plasma kinetics model of atmospheric pressure jet plasma in He with admixture of H₂ and N₂ is taylored to a plug flow model. The composition and velocity of the feed gas as well the spatial profile of the power density are used as input parameters. The global model solves rate equations for the species occurring in the most important reactions including electron impact with heavy species and reactions between heavy species along with the energy balance of electrons and heavy species. The model involves 21 species and 120 reactions. The flow conditions in the active zone and in the effluent of the RF discharge are considered in terms of a pseudo-one-dimensional plug flow model. The latter accounts for gas expansion at constant pressure. In this way, the temporal evolution of a volume element flowing with the gas is converted into a spatial dependence of all quantities considered. The models allows one to get insight into the plasma chemistry and to study the influence of power and flow rate. Selected results are compared with experimental findings available from literature.   

Characterization of PZT driven plasma discharge operating in atmospheric pressure air.

In this work, a PZT-driven atmospheric pressure plasma discharge is characterized. Experiments are conducted for two electrode configurations: powered PZT–grounded copper and powered PZT–grounded PZT. The piezo material is composed of lead zirconate, lead titanate, and barium titanates. The experiments are conducted from an interelectrode separation distance of ~ 50 um operating in atmospheric pressure air. For both configurations, VI curves are obtained using rms voltage and current data by varying the input voltage from 50 VPP to 150 VPP. The temporal voltage and current waveforms show the current to lead by ~ 10 degrees for the PZT-CU and by ~ 40 degrees for the PZT-PZT configuration. This suggests that the PZT-PZT configuration is more capacitive in nature. The PZT-PZT experiments are conducted to identify the response of the piezo material in the presence of the plasma charge. For this purpose, the voltage and current in the primary and secondary sections of the grounded piezo were measured. In the PZT-PZT configuration, multiple filaments appear, unlike the PZT-CU. Additionally, the grounded PZT has a varying voltage dictated by the plasma charges striking the electrode. Optical emission spectroscopy was conducted to determine the plasma temperature. High-speed imaging of the discharge dynamics reveals distinct plasma surface interaction on the piezo electrode.   

Plasma Reactor for Liquid Fuel Reforming

Typical conversion of hydrocarbon fuels into lighter hydrocarbons is usually performed thermally, involving high energy input and elevated temperature. In this work, we report a new approach and system for multi-phase fuel reforming utilizing atmospheric pressure plasma discharge operating in the non-thermal regime. A plasma reactor system, capable of producing large volume discharge, has been developed. In the reactor, a plate-to-plate electrode configuration is used maintaining a separation distance of 1.5 mm. The plasma is driven by a nanosecond pulser with a variable repetition frequency in the range of 1-5 KHz. The reactor receives liquid fuel driven by a fuel pump. The liquid goes through phase-change when passing through a cavitation venturi. The formations of bubbles produced in the venture section is controlled by varying the liquid flow rate. The presence of a multiphase environment allows the plasma discharge to treat a larger volume. As a preliminary study ethanol is used as a test fuel. Characterization of the reformed product indicates the formations of lighter hydrocarbons such as ethylene and acetylene. The conversion of ethanol was significantly affected by the repetition frequency and applied voltage, which can be attributed to the energy density of the discharge. Further characterizations are underway to provide valuable insight into understanding the conditions that govern the production of the desired hydrocarbon products, gas and liquid.   

VSim benchmarking and improvements for high-aspect-ratio etch modeling

High-aspect-ratio (HAR) plasma etching processes can be optimized for uniformity and regularity through the use of kinetic (particle-in-cell) modeling tools, which capture the motion of charged particles and their deposition on the substrate on length scales comparable to the desired critical dimensions. Such kinetic models are able to resolve detailed physics processes that cannot be resolved with fluid codes, e.g. the response of incident ions to charge accumulation on trench walls. Ongoing developments in VSim [1] (a finite-difference time-domain/PIC-MCC-DSMC code) that focus on such modeling are presented. VSim’s previously-shown ability to rigorously pass low-temperature modeling benchmarks posed by Turner et al. [2] has recently been augmented.  As well, new user examples demonstrating the use of the code for geometrically complex HAR etching applications have been developed, and novel boundary conditions that mimic the behavior of bulk plasma far from the substrate have been formulated. In these boundary conditions, the disparity between electron and ion mobilities is exploited to balance and smooth incident particle fluxes in the presheath region.

[1] www.txcorp.com/vsim

[2] M. M. Turner et al., Phys. Plasmas 20, 013507 (2013).   

Orientation dependent etching of silicon by F2

Orientation dependent etching of silicon by F₂ gas is used for nano-scale texturing of silicon surface during photovoltaic  solar cell manufacturing. For modeling of dry etch chemistry we used several methods, such as transition state theory (TST) and classical molecular dynamics (MD). TST under DFT (density functional theory) approach is powerful method for calculation of rate constants for the elementary steps of plasma-surface interaction. This allows insight into the etching mechanism and perform chemical kinetics modeling to predict the etch rate as a function of gas phase etchants fluxes and surface temperature, predict evolution of the structure and composition of the surface during the etching process. We established that F2 dissociative chemisorption is the rate-limiting step in the etching that determines the overall rate of the whole etching process. Here we present simulation results explaining the orientation dependence of silicon surface etching by F₂ molecules using the TST approach. Namely, we show that the Si etching in the (111) direction is much slower than in the (100) and (110) directions; therefore, F₂ can be used for the anisotropic etching process to produce black silicon.

    

Comparative Study of Yttrium Oxide Film Deposition: Low-Temperature Microwave-Excited Atmospheric Pressure Plasma Jet with Mist Additions

Yttrium oxide (Y₂O₃) film is one of the promising coating materials to prevent from process chamber wall erosion by the plasma process. For that reason, advanced ceramics are widely used as plasma-facing materials including Y₂O₃ due to their high resistance and chemical stability [1]. Furthermore, there is an apparent interest to investigate atmospheric pressure plasma as a coating technology due to the easy usage of industrial lines such as the semiconductor manufacturing process. Additionally, atmospheric pressure plasma allows the organic solution for deposition [2].

    To deal with corrosion-erosion issues, it is necessary to utilize a Y₂O₃ film with a thick and high density to meet the requirements of the complex structure inside the chamber for this application. Cost-effective methods are also essential in the production of semiconductors.

    We have compared the vaporizer method with the simpler cost-efficient bubbling method using the microwave-excited atmospheric pressure plasma jet (MW-APPJ) with an organic precursor solution. The deposited films and deposition processes are also confirmed by optical emission spectroscopy (OES), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and film profiler. The achieved results were a maximum deposition rate of ~0.4 um/min for the Y₂O₃ film using the vaporizer method and a maximum deposition rate of ~0.8 um/min using the bubbling method.   

Rethinking the Storage and Conversion of Fuels: Producing Green Hydrogen from Ammonia using Non-Equilibrium Plasma

Hydrogen provides a promising source of green energy; however, it presents unique energy storage challenges that require novel methods of generating hydrogen from more convenient feedstocks. A potential solution is the utilization of ammonia given its compositional percentage of hydrogen, high energy density, liquid phase at standard conditions, and the vast industrial infrastructure that exists to facilitate its production and distribution. The ammonia-to-hydrogen reaction pathway however suffers from slow kinetics given the presence of the N-H bond. Non-equilibrium plasmas, in tandem with an optimized choice of catalyst, are capable of exciting selectively the vibrational and electronic states of ammonia to reduce the activation barrier. Furthermore, within a gas temperature of 300-500 K, hydrogen and nitrogen products are favored. This work characterizes experimentally the dependence of yield and energy efficiency within an inductively coupled plasma reactor by varying temperature, pressure, flow rate, and choice and positioning of catalyst. The findings display how plasma sources can be tuned in tandem with catalytic surfaces to convert ammonia to hydrogen and provide insights into the fundamentals of plasma catalysis. The catalyst selection is informed by microkinetic modeling, and experimental results will be compared to model expectations. Furthermore, vibrational gas temperatures are measured using spectroscopic methods.   

Abatement of VOCs by Cold Atmospheric Air Plasma at Low Temperature and Ambient Pressure

Volatile organic compounds (VOCs) emitted from various industrial processes are extremely harmful pollutants. They are involved in forming ozone, photochemical smog, and fine particles (PM_2.5) in the atmosphere, which pose a considerable threat to human health and ecosystem safety. The hybrid plasma-catalytic technology is an efficient method for VOC abatement. In this work, a coaxial dielectric barrier discharge (DBD) based non-equilibrium cold plasma source powered by a bipolar pulse power source is reported for effective decomposition of VOCs in an enclosed region at low temperature (< 50°C) and ambient pressure. The decomposition efficiency of VOCs is, in general, controlled via the specific input energy, which mainly depends on the flow rates of treated VOCs, but here we report the synergistic effect of highly energetic electrons and reactive species on the decomposition process. The excited species are analyzed using optical emission spectroscopy. The electron temperature and particle density measurements have also been carried out using the recorded spectra and line intensity ratio technique. For about 60 minutes of continuous operation of the plasma source, a decomposition efficiency of 99.7% (toluene) and 74.8% (benzene) is achieved. The developed source produces powerful, immediate discharge and energetic electrons, which have been efficiently used for VOC degradation. Results are useful for applying non-equilibrium cold plasmas in an enclosed region containing numerous VOCs.   

Evaluation of a Plasma Catalytic-Membrane Dielectric-Barrier Discharge Reactor for Ammonia Synthesis

Ammonia synthesis via nonthermal plasma powered by intermittent electricity from renewable energy sources can be a potentially-transformative technology, especially for point-of-demand operations. We present results of the evaluation of a catalytic membrane Dielectric-Barrier Discharge (mDBD) reactor for the synthesis of ammonia from nitrogen and hydrogen. A porous alumina membrane (with 0.1, 1.0, or 2.0 μm pore size) is used as dielectric barrier and as H₂ gas distributor, allowing greater residence time for N₂ decomposition and greater availability of H₂. The membrane-ground electrode gap is filled with catalyst powder embedded in glass-wool. Three different catalysts are evaluated: nickel, cobalt, and bi-metallic nickel-cobalt, all loaded at 5% by weight on alumina powder (surface area ~ 200 m²/g). We assess the performance of the reactor with electrical, optical, spectroscopic, and FTIR analyses as function of driving voltage. Our results show the role of catalyst properties and design and operation parameters on ammonia production and production efficiency.   

STUDY OF LUMINESCENCE OF BIOMOLECULES IN A GAS DISCHARGE

This work is devoted to the experimental study of the luminescent characteristics of a mixture of biomolecule vapors with inert gases (argon, helium, etc.) by the method of optical spectroscopy in a low-temperature gas discharge plasma in the spectral range of 200–1000 nm. The composition of biomolecules includes amino acids - organic compounds that simultaneously contain amino (-NH₂) and carboxyl (-СООН) groups. This kind of research is being conducted for the first time.

The luminescence spectra of the mixtures based on biomolecules in the pulse-periodic discharge that we obtained should mainly be attributed to neutral compounds. Spectral lines and transitions corresponding to positive fragment of ions were not registered. In all cases, our experiments established that one of the main processes of the fragmentation of biomolecules in a low-pressure gas discharge is the separation of the carboxyl group or the R residue of the amino acid side chain, and in the discharge we observed the formation of molecular nitrogen from the amino group with the emission of the second positive system of this molecule (C→B transition) in the ultraviolet region of the spectrum. As a result, excited molecules of nitrogen and carbon monoxide appear. The formation of excited CO molecules in the gas discharge leads to the dissociation of the -СООН carboxyl group under the influence of metastable argon atoms with an excitation energy of about 11 eV (the ionization potential of -СООН molecules is about 8 eV). The maxima of the low-temperature plasma radiation obtained by us are at inert gas pressures that do not exceed 50 Torr.   

Numerical modeling on the effect of plasma-produced species on immune cells behavior

Recently, it has been found that low-temperature plasma can induce immunogenic cell death (ICD) by causing cancer cells to release damage-associated molecular patterns. Although the mechanism has not fully understood, it is suggested that the plasma-induced ICD is a result of plasma produced reactive oxygen nitrogen species (RONS). Since the collective behavior of immune cells play a critical role in ICD, it is important to examine the influence of RONS the immune cell dynamics. In this work, we developed two-dimensional agent model to simulate neutrophils dynamics, that is, migration and phagocytosis under the influence of plasma irradiation. With increasing plasma-derived hydrogen peroxide, the concentration of neutrophils migrating from blood vessels into tissues increases. As a result, neutrophils efficiently reach the target (cancer cells) and phagocyte them.   

Optimization of Flexible Surface Dielectric Barrier Discharge Electrodes for the Purpose of In-Package Deactivation of Fresh Produce

Surface Dielectric Barrier Discharges have been gaining interests in recent years due to their low operating voltage and the fact that the treatment substrate is not part of the electrical circuit. Hence, SDBDs are ideal for applications such as wound healing and in-package inactivation. [1] In this study, flexible SDBD electrodes were characterized for the purpose of disinfection of fresh produce. Different electrode geometries were investigated using optical emission spectroscopy, voltage and current analysis, and optical absorption spectroscopy for the determination of ozone concentrations, for different frequencies and voltages. Optical emission spectroscopy in conjunction with numerical simulation was used to determine gas temperature and reduced electric field of the discharges. It was found that the lattice size and structure of the electrode have a strong effect on the measured gas temperature, possibly giving another method for controlling down phase gas chemistry. Finally, voltage and current measurements were conducted on different electrode shapes and geometries and combined with an adjusted SDBD equivalent circuit model.[2] This model allows for determination of dielectric capacitance, plasma resistance, and plasma power. The results indicate that the dielectric capacitance increases linearly with voltage while plasma power increases quadratically. The obtained results taken together inform the operating conditions needed for optimal treatment of fresh produce.

 

[1] J. Xie et al, "Paper-based plasma sanitizers," Proc. Natl. Acad. Sci. U. S. A., vol. 114, (20), pp. 5119, 2017. . DOI: 10.1073/pnas.1621203114.

 

[2] D. Trosan et al.” Analysis of the Effects of Complex Electrode Geometries on the Energy Deposition and Electric Field Measurements of Surface Dielectric Barrier Discharges " (submitted)   

Optimal Experimental Design for Inferring Anomalous Electron Transport Coefficients in a Hall Thruster Model

Due to the potential of Hall thrusters for deep space propulsion applications, improvement in predictive models for the operation of these devices at flight conditions has increased in importance. One of the key concepts prohibiting the progression of these models due to the current limited understanding is the cross-field electron movement known as anomalous electron transport. The current method of approaching this problem is to tune transport model coefficients to data, such as ion velocity measurements. One problem introduced with this method is the large datasets necessary for calibration and the cost of obtaining or measuring these datasets. A solution is to apply an Optimal Experimental Design(OED). OED provides a rigorous framework for identifying conditions for the facility, thruster, or probes, for example, to conduct experiments that most improves the model calibration. In this work, a batching OED algorithm is developed and applied to iterate over sets of optimal conditions for ion velocity data that calibrate a multi-fluid Hall thruster model with incorporated anomalous transport closure models for collision frequency. As a result, it is shown that the same coefficient values can be achieved with a fraction of the typical number of experimental points that are employed. This work demonstrates an efficient approach for future Hall thruster testing that reduces the cost generated by a relatively expensive form of measurement, Laser Induced Fluorescence.   

Design and Initial Operation of an Optically Accessible ECR Magnetic Nozzle Thruster

Electron cyclotron resonance (ECR) magnetic nozzle thrusters possess several advantages over current state of the art thruster technologies. These include propellant agnosticism and the elimination of the need for a neutralizer cathode. Recent advancements have brought their performance within range of miniaturized Hall and ion thrusters, stimulating interest in further improvements. Such improvements are contingent upon an improved understanding of the electron dynamics inside the source region and the acceleration mechanisms in the nozzle region, which are difficult to study experimentally with electrostatic probes. This work presents the design and initial operation of an ECR magnetic nozzle thruster with an optically transparent source region to facilitate direct optical emission spectroscopy (OES), laser-induced fluorescence (LIF), and Thomson scattering measurements. Langmuir probes are used to estimate electron temperature and density downstream in the plume region while OES is used to determine species makeup and electron density in the source region. Thruster stability is also characterized.   

Background Pressure Effect on a Novel ECR Magnetic Nozzle Thruster

Low backing pressures are essential to accurately measure the performance of electric plasma thrusters. Too high backing pressures can increase the performance of Ion and Hall thrusters, invalidating experimental results. Conversely, the performance of Electron Cyclotron Resonance (ECR) Magnetic Nozzle Thrusters is reduced by high backing pressures.

A linear increase in thrust with a logarithmic decrease in backing pressure has been previously observed. This relationship has been experimentally verified and measured in the Surrey Space Centre’s large vacuum chamber with a novel ECR Thruster developed at the University of Surrey. Plume measurements will be taken via Optical Emission Spectroscopy as well as Faraday and Langmuir probes to determine facility pressure effects on electron temperature and plume divergence. However, with the thruster running, Surrey’s large vacuum chamber can only achieve a backing pressure of 6E-6 mbar. To test backing pressure effects at lower pressures the thruster will be tested in the University of Southampton’s large vacuum chamber. As vacuum chamber size has also been known to affect the performance of ECR Magnetic Nozzle Thrusters, performance will also be compared between the two chambers.   

Particle Image Velocimetry Analysis for a Plasma Actuator with Serial Three-Electrode Surface Dielectric Barrier Discharges

Electro-hydrodynamics (EHD) for a plasma actuator with a surface dielectric barrier discharge (sDBD) attracted much interest for a couple of decades to decrease the drag force and increase the lift force. This research analyzes a plasma actuator with three electrodes connected in series for velocity profile and fluid behavior through particle image velocimetry (PIV) data for three cases. The first case examines the effect of the applied voltage within the same structure. The second case investigates the impact of altering the structure under constant voltage conditions. The third case explores the influence of adjusting the gap distance between the front and back electrodes while keeping the voltage and geometry constant. In addition, Lissajous curves were created for the three cases to calculate the capacitance and power consumption for the given design and the applied voltage.   

Microwave-driven plasma for miniature space propulsion

Microwave-driven Coaxial Transmission Line Resonator (μ<!--[if gte msEquation 12]> style='font-size:11.0pt;line-height:107%;font-family:"Cambria Math",serif;
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KO;mso-bidi-language:AR-SA'>μ-CTLR) produces a small-volume high-density plasma plume. When operated at low pressure, the γ discharge mode (characterized by increased electron temperature, electron density, and gas temperature) was attained even at low power levels [1]. These characteristics of the μ<!--[if gte msEquation 12]> lang=EN-US style='font-size:11.0pt;line-height:107%;font-family:"Cambria Math",serif;
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KO;mso-bidi-language:AR-SA'>μ-CTLR show promise for low-power micro-propulsion systems. In this study, we investigated the propulsion capability of the μ<!--[if gte msEquation 12]> lang=EN-US style='font-size:11.0pt;line-height:107%;font-family:"Cambria Math",serif;
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KO;mso-bidi-language:AR-SA'>μ-CTLR operating at 900 MHz and 8W power, with argon gas. At the gas flow rate of approximately 100 SCCM, the measured plasma parameters are electron density of 1×10²⁰ m^-3, electron temperature of 3.5 eV, and gas temperature of 4000 K in the middle of the plasma plume. These results suggest a minimum thrust of 1.5 mN with a power consumption of only 8 W. The corresponding thrust-power ratio (TPR) is about 190 mN/kW and specific impulse (Isp) about 51 sec. These performance metrics are comparable to or even surpass recent micro-propulsion systems [2].

[1] Kim K, Nam W, Lee S, Lee J, Shim S, & Yun G. S. (2022), Plasma Sources Science and Technology, 31(10), 105006.

[2] Tsifakis D, Charles C, & Boswell R. (2020), Frontiers in Physics, 8, 34.   

Thomson Scattering Measurements of Anomalous Electron Transport in a Hall Thruster

Hall thrusters are efficient, moderate-thrust electric propulsion devices which accelerate ions through an annular, crossed-field discharge to achieve high specific impulse (>2000 s). While these devices are commonly used as in-space propulsion systems for both Earth satellites and interplanetary missions, the rapid design and development of this technology is dependent on expensive vacuum testing. This is because the fundamental operation of Hall thrusters is governed by the transport of electrons across a magnetic field. This cross-field electron mobility is experimentally found to be far higher than predicted by classical collisions alone. Experimental and theoretical studies point to a growing consensus that high-frequency, drift-driven, azimuthal plasma turbulence is a primary cause of non-classical momentum transfer to the electrons. However, the precise nature of these wave interactions are not fully understood, precluding high-fidelity modeling of the Hall thruster without experimental inputs. The recent application of incoherent Thomson scattering (ITS) on low-density plasmas such as Hall thrusters has enabled the direct measurement of the electron velocity distribution, including information regarding the transport of energy and momentum via the electrons in the acceleration region of the Hall thruster.  This technique enables characterization of the electron transport within the Hall thruster. We carry out Thomson scattering measurements to infer the effective electron Hall parameter in a laboratory Hall thruster channel. This is accomplished using a simplified Ohm’s law electron force balance. With this result, we provide an avenue for directly informing simulations of Hall thruster physics with experimental transport profiles, paving the way for first-principles modeling investigations of the sources of anomalous electron resistivity in these devices.   

Characterizing Electrode Passivation in Air Breathing Electric Propulsion

Over time, the electrodes in air-breathing electric propulsion (ABEP) systems deteriorate and become less effective in generating thrust. This degradation is caused by the interaction between the electrodes and the surrounding air, leading to the formation of thin layers consisting of nitrides and oxides on the electrode surface. These thin layers alter the properties of the electrodes, such as how they emit light, and consequently reduce the efficiency of thrust generation. To better understand and analyze this process of electrode deterioration in ABEP systems, we carry out a series of experiments. These experiments involve real-time monitoring of the growth and roughness of the thin layers on the electrodes. We employ micro-Raman spectroscopy and optical emission spectroscopy techniques to identify the chemical composition of these thin layers. Additionally, we use scanning electron microscopy to examine the surface characteristics of the passivated electrodes and gather relevant data. Finally, we assess the performance of a pulsed plasma thruster by evaluating its thrust efficiency and discharge characteristics as the thin layers develop. This investigation provides valuable insights into the impact of these thin layers on the overall functionality of the propulsion system.   

Performance measurements and modeling for a 25W porous electrospray array thruster

Porous ionic liquid ion sources are electrohydrodynamic devices that evaporate ion beams from an ionic liquid infusing a porous substrate.  While these systems are attractive for in-space propulsion because of their high inherent thrust density, to date they have been limited to less than 5 W power.  The key barrier is that higher power requires aggregating many hundreds of emitters together.  The chance that at least one emitter in an array fails and produces a life-ending electrical short increases with the size of the system, a consequence of variability in emitter behavior.

To overcome this difficulty, we have proposed two key innovations. The first is a resilient extractor architecture whereby faults are automatically removed from the system in a process analogous to the burn-in of film capacitors.  The second is adopting a probabilistic modeling framework that accounts for uncertainty in emitter behavior, such that we can identify system designs that are robust to emitter variability.

As an illustration of these techniques, we have designed, built, and tested a 25 W electrospray system incorporating our resilient extractor design. We present the results of operating that thruster and show that it can achieve comparable performance to state of the art at smaller power. Finally, we compare the experimental data to those predicted by our model as an illustration that by accounting for manufacturing tolerances we can provide a useful tool for performing model-based design.   

Silicon surface treatment by atmospheric large remote plasma for heterogeneous material integration

Hydrophilic bonding is commonly used in semiconductor heterogeneous material integration for large-scale photonic integrated circuits. In this process, plasma irradiation is used in a vacuum apparatus for surface hydrophilization. That requires batch processing in a vacuum chamber.

To solve these problems, we have proposed inline process that integrates a hydrophilic treatment by plasma irradiation using atmospheric plasma and a temporary bonding device. Up to now, we have proven such plasma treatment showed a good processing effect. However, it required time to process large areas because of its small irradiation port.

In this study, an atmospheric large-sized remote plasma source that can hydrophilize a 12-inch semiconductor wafer in a one-dimensional scan was developed. The plasma source has an irradiation slit of 1 x 349 mm and low temperature plasma blow out from the slit. By applying a radio frequency of 27.12 MHz to the electrodes inside the device and flowing the plasma generating gas at a rate of around 20 L/min, stable helium and argon plasma could be generated.

To investigate the cleanliness of this device and the plasma jet source, the number of particles of various diameters generated in the plasma sources were measured using a particle counter. Experiments on hydrophilization of silicon surfaces were also conducted using these plasma devices.   

Studies of synthesis of ultra-fine nanoparticles using metal arc discharge

The use of single-walled carbon nanotubes (SWCNTs) in structural materials promises significant enhancement in the construction’s life span, mechanical, and even electrical characteristics, and thus—the reduction of the CO₂ emissions. One of the scalable techniques of SWCNTs production is the arc discharge method. Previous efforts mainly focused on the carbon arc, where two graphite electrodes, one of which contained catalyst powder, served as the carbon source [1]. A more scalable and sustainable method is the metal arc in a methane atmosphere in which the metal anode is continuously evaporated to produce catalyst nanoparticles, while methane is decomposed to generate carbon and H₂, with no CO₂ formation. For SWCNT synthesis, a precise control over the catalyst particle size distribution is essential [2]. Here, we start from the argon metal arc discharge to explore how to control the particle size distribution and produce ultra-fine metal nanoparticles. Apart from the process parameters, such as arc voltage, current, pressure, and temperature, we focus on understanding and improving the arc stability [3]. For that, we deploy high-speed imaging and optical emission spectroscopy.   

Control of PTFE/PFA Surface Free Energy by Atmospheric Multi-Gas Plasma Treatment

With the spread of next-generation high-speed communication systems, materials with low transmission loss are required for printed circuit boards. Fluoropolymers such as polytetrafluoroethylene (PTFE) and perfluoroalkoxyalkane (PFA) are attracting attention as low-loss materials with low relative permittivity. On the other hand, it is very difficult to bond them with other materials because of their chemically stable structure. We considered that the adhesive characteristics of PTFE and PFA could be improved by controlling the surface conditions using atmospheric multi-gas plasmas. In this study, the effects of reactive species generated in various gas plasmas on PTFE and PFA surfaces were investigated. Surface treatment was performed with an atmospheric multi-gas plasma jet developed in our laboratory. Molecular gases such as nitrogen and oxygen were used as plasma generating gases. This treatment was applied continuously by using a motor stage driven at 1 mm/s. The effect of plasma treatment was evaluated by measuring the water contact angle, which indicates the hydrophilicity of the solid surface, before and after plasma irradiation. The diiodomethane contact angle was also measured, and the surface free energy was calculated from the contact angles of these two liquids. In this presentation, in addition to these results, adhesion strength between PTFE and polyimide tapes will be reported.   

Multi-ratio method for determination of the electric field in transient argon discharges

Zero-dimensional reaction kinetic modeling in Ar is performed in order to evaluate a method for determination of the reduced electric field from intensity ratios of Ar excited states. Particular sets of Ar[2p] excited states are identified and selected based on their ability to reflect the value of the reduced electric field in the ratio of their populations. Balance equations for the selected species are combined and source terms are filtered and subsequently simplified via sensitivity analysis in order to be represented in a form that is convenient for use in the evaluation of measured line intensities. Various sources of uncertainties in the values of reaction rates and limitations of the proposed method with regards to time scales of electric field variations are discussed.   

Mapping of the electromagnetic emission from the atmospheric-pressure helium plasma discharge tube for cancer treatment

Cold atmospheric plasma contains various types of active chemical reactive species and emits broadband electromagnetic radiation. Up to date, the chemical species produced in plasma have mostly been considered as major active factors affecting living cells during cancer therapy. However, it was found recently that in case if the treated cells are isolated from plasma itself and all plasma-produced chemical species by the thin-walled dielectric vessel (so-called Plasma Discharge Tube, or PDT), electromagnetic radiation from plasma start to play the major role in affecting the cells and their sensitization making the cancer cells vulnerable for the specialized additional chemical treatment.

In this work we study the spatial distribution and frequency composition of the non-visible electromagnetic radiation emitting from PDT. Results of the spatial distribution of the amplitudes of the electromagnetic radiation are compared with the results of the survival rates of the cancer cells placed at different distances from the discharge tube, including the situation when the emission from PDT is partially or fully blocked by broadband filters.   

Production of H2 from plasma methane decomposition in Ar/CH4 mixtures at atmospheric pressure

Cold plasmas have proven their effectiveness for methane conversion with the use of several types of discharges. This study aims to contribute to the understanding of the kinetic processes involved in Ar/CH4 mixtures. An UV photo-triggered discharge is used, in which the plasma is homogeneous, the electric field uniform and the deposition of energy in the plasma controllable. CH4 and by-products concentrations are measured by chromatography. The moderate values of E/N, associated with the preceding properties, allows a 0D self-consistent modelling with local field approximation. It is then possible to propose an accurate detailed kinetic scheme by comparing experimental results to calculations, as it was performed for other molecules [1]. The experiment emphasizes that H2 is the main by-product, the concentration of detected carbon in compounds with less than 4 C being very small compared to the converted CH4. A lack of H is observed, suggesting the formation of heavier hydrocarbons and/or solid compounds. A high production rate of H2 is also predicted by the model, although kinetic data taken from the literature do not allow to explain all experimental results.