Session JO8: Low Temperature Plasmas

Experiments with argon-fueled Hall thruster

Argon-fueled thruster is attractive because of the low cost of argon. However, because argon is hard to ionize, a thruster that uses argon has low efficiency. We examine the use of argon in a Hall thruster. The thrust, the thrust to power ratio, the specific impulse, and the efficiency are presented for varying magnetic field, discharge voltage, and gas flow rates. Measurements in a configuration of crossed electric and magnetic fields, in which there is no closed-drift trajectory for the electrons, yield, as expected, a much lower performance. The use of rotating magnetic field together with a DC electric field and without such a DC electric field is examined theoretically. Preliminary experimental results with a rotating magnetic field are shown.   

Driving Low Frequency Oscillations in Hall Thrusters.

Hall thruster technology has become the most mature in the field of spacecraft propulsion, however, its performance capabilities are still far from its technological and fundamental limits. Further performance improvements may require new designs and operating regimes. One such potential reserve for improvements may be in controlling inherent low-frequency (10-30 kHz) oscillatory modes, so-called breathing mode and spoke mode, which are often observed in Hall thrusters [1]. The breathing mode is the axial mode that propagates in the direction of the external electric field, is the most powerful mode observed in Hall thrusters, revealing itself in oscillations of the discharge current, often reaching amplitudes comparable to the mean discharge current itself. The spoke mode manifests itself as strong perturbations in plasma density that propagate in the azimuthal ExB direction, generating substantial components in electric field in this direction. In this work, we demonstrate that both breathing and spoke modes can be amplified or suppressed depending on the amplitude and frequency of the anode voltage modulation. We compare and analyze the effect of modulations on plasma and thruster performance measured in experiments and obtained from simulations.   

Probe measurements of electron distribution function in a large-volume glow discharge device with coaxial gridded hollow electrodes.

In this study the electron distribution function (EDF) in a weakly ionized plasma of a large-volume glow discharge device with coaxial gridded hollow electrodes has been measured with a single electric probe. The diameter and the length of the discharge chamber are 50 cm and 40 cm, respectively. The discharge is created by an ac power supply with a frequency of 20 kHz and a maximum power of 2,000 W, which can be varied from 500 W and up. Because of the bipolar diffusion, charged particles are generated between the two poles and spread rapidly to the central region of the discharge chamber, thus forming a homogeneous plasma. The discharge device has been described in details in [1]. The discharge has been created in helium gas at pressure from 40 to 60 Pa. The EDF for different discharge power in the range from 1,200 to 1,600 W and different pressures have been studied. The experimental results have been compared with modeled EDFs. [1] Yuan C., Kudryavtsev A. A., Saifutdinov A. I. et al., IEEE Transactions on Plasma Science, 45, 3110-3113, 2017.   

Dynamics of transient low-pressure plasmas in the vicinity of high-voltage electrodes

Extreme ultraviolet (EUV) lithography scanners operate in a hydrogen atmosphere with a pressure of several Pascal. Pulses of ionizing EUV radiation lead to the formation and accumulation of plasma throughout the entire optical system of the scanners. We report on the experimental and theoretical studies of the plasma dynamics in the vicinity of biased (and possibly shielded by dielectric) electrodes. For that we designed a model experiment on the interaction of low-pressure hydrogen plasma with remote electrodes and conducted numerical simulations of such a model situation. The results show that in an improperly designed system, high current pulses can occur on positively charged electrodes resulting in potential surface erosion and/or contamination within EUV scanner. We discuss ways to prevent such phenomena.   

Two RF measurement techniques with microsecond resolution of laser induced atmospheric-pressure helium plasmas

Plasmas are produced in atmospheric-pressure helium by a focused 300 mJ infrared laser pulse and evolve on microsecond time scales from a 3 mm elongated shape to a torus which grows to 10 mm diameter. The plasma is visible on a ICCD camera with exponentially decaying luminosity for up to 70 microseconds. The challenge is to measure its electron density at high time resolution over several decades. Two RF techniques are used to measure the complex permittivity. Complex permittivity measurements allow to determine both the electron density and the electron-neutrals collision frequency using the Drude model. At 57 GHz, the complex transmission coefficient of the plasma placed between a transmit and receive antenna is measured at microsecond time scale. Full wave transmission simulations with a dielectric torus are performed. The simulation results are matched to the plasma measurements by varying the complex permittivity. This yields the complex plasma permittivity as a function of time. Around 2.45 GHz, cavity perturbation measurements on the plasma in an iris coupled rectangular cavity are performed. Combining a series of single frequency reflection measurements allows to determine the complex plasma permittivity over a multidecade range at microsecond time resolution.   

Limitations of approximate Boltzmann solutions for atmospheric combustion with high-frequency electric fields

Efforts to enhance combustion with non-equilibrium plasma have become increasingly diverse, with a variety of electric field configurations and flame environments relying on a network of transport, kinetic, and thermal enhancement pathways. Out of necessity, models of these effects typically make broad simplifications, avoiding detailed solutions of the Boltzmann equation in favor of two-fluid models or two-term expansions. For the case of high-frequency fields in atmospheric combustion, these approximations often fail; the ratio of collision to field frequency becomes velocity dependent and varies throughout the density function. This incoherence results in nuanced electron transport. Additionally, electron attachment can introduce anisotropy that further alters transport behavior. To assess the impact of these effects, we review plasma-flame experiments and identify the range of gas mixtures, reduced frequencies and fields of greatest relevance. We then employ a custom Monte Carlo program alongside BOLSIG$+$ to solve the Boltzmann equation across the parameter space. The results are then detailed, including deviations in electron mobility, diffusion, and kinetics incurred across models. Finally, we compare the electron density functions and discuss options for future models.   

\textbf{Tunability and Scalability of Low-temperature Microplasmas}

Low-temperature microplasma, with emerging importance in device and sensor miniaturization, has received growing attention from both academia and industry in recent decades. Understanding the discharge characteristics is of fundamental importance for engineering devices across microplasma parameter regimes. In this work, the tunability of microplasma ignition and scaling laws are investigated with theoretical and computational models. Firstly, with engineered electrode surface morphology, Paschen's curve for the microgap can be flattened in either left or right branch, enabling control of the breakdown voltage and current characteristics across a wide range of gas pressure [Appl. Phys. Lett. 113, 054102 (2018)]. Secondly, classical similarity laws are found to be valid into the microplasma regime, with small scaling factors. At high ionization degree a transition from low to high density scaling characteristics is observed. The breakdown scaling is also examined based on the transition of microplasma voltage-current characteristics, including Townsend discharge, subnormal glow, normal glow, and abnormal glow discharge regimes [Appl. Phys. Lett. 114, 014102 (2019)]. Thirdly, using a hybrid plasma hydrodynamics model with electron Monte Carlo collisions, atmospheric pressure microplasmas with multiple electric field-enhanced thermionic emitters, and their scaling with temperature, are investigated. Finally, challenges for the tunability and scalability of low-temperature microplasmas are discussed in different discharge regimes.   

Constraining a Uranium Reaction Mechanism Using Stochastic Optimization and Plasma Flow Reactor Measurements

The chemical processes governing the formation of nuclear debris in nuclear fireballs has been the subject of great interest for decades. However, despite years of study, no experimentally validated reaction mechanism has yet been developed for the uranium plasma-chemical system. In this work, we utilize emission measurements from a uranium plasma flow reactor to constrain the reaction rates of a previously unverified uranium reaction mechanism by performing stochastic optimization of the problem parameter space. The plasma flow reactor is particularly suitable for this task due to its inherent correlation between residence time and axial distance. This correlation enables the reactor to be approximated using a 0D kinetic model, drastically reducing the computational complexity of reaction mechanism calibration. This reduction in model complexity allows for a robust optimization of the dozens of uranium reaction channels over the entire parameter space, producing the first experimentally constrained uranium reaction mechanism.   

Modeling of a microwave plasma enhanced chemical vapor deposition system based on finite element method

Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) films have excellent electrical properties and good substrate adhesion. It is one of the promising candidates for synthetic CNTs due to vertical growth, low temperature and large area growth. The plasma consisting of ionized gas species and electrons is ignited and sustained by applying high electromagnetic power at microwave frequencies so a thin film can be deposited at lower temperature but higher efficiency. It also gained popularity in diamond fabrication. This paper discussed the design and modeling of MPECVD chamber operated at 2.45 GHz frequency using finite element method simulation. The design consists of a coaxial waveguide and a cylindrical chamber at the center connected with 4 identical slots in each direction. The placement of slot affects the resonant mode in the chamber. Both TE$_{\mathrm{111}}$ and TM$_{\mathrm{011}}$ modes in the inner chamber are employed to generate the plasma at 2.45GHz and the corresponding analysis of the MPECVD operated at different pressure and input power has been performed for industrial applications.   

Electric Field Filamentation and Higher Harmonic Generation in Very High Frequency Capacitive Discharges

The effects of the discharge voltage on the formation and nature of electric field transients in a symmetric, collisionless, very high frequency, capacitively coupled plasma are studied using a self-consistent particle-in-cell (PIC) simulation code. At a driving frequency of 60 MHz and 5 mTorr of argon gas pressure, the discharge voltage is varied from 10 V to 150 V for a fixed discharge gap. It is observed that an increase in the discharge voltage causes filamentation in the electric field transients and to create multiple higher harmonics in the bulk plasma. Correspondingly, higher harmonics, up to $7^{th}$ harmonic, in the discharge current are also observed. The power in the higher harmonics increases with a rise in the discharge voltage. The plasma density continues to increase with the discharge voltage but in a non-linear manner, whereas, the bulk electron temperature decreases. Meanwhile, the electron energy distribution function (EEDF) evolves from a Maxwellian at lower discharge voltages to a bi-Maxwellian at higher discharge voltages.   

Anode fireball of the reversed polarity DC planar magnetron and its application

The sheath is ubiquitous in laboratory plasmas contained in a vessel. It controls the flux of the particles across the plasma solid interface. Overwhelming experiments have to deal with the ion sheath. This is most common to occur, due to large mobility of the electrons which are often restrained by negative potential of the wall. This self-sustaining process gives rise to ion sheath at most floating electrode/wall. Though, if one can set boundry conditions right, so as to extract sufficient electrons to account for its enormous flux, one can achieve an electron rich sheath. This kind of sheaths have attracted its fare share of attention\footnote{\label{Baalrud_1}S. D. Baalrud \textit{et. al.}, \textbf{Plasma Sources Sci. Technol.}, 18,035002, (2009).}$^,$\footnote{B. Scheiner \textit{et.al.} \textbf{Phys. Plasmas}, 22,123520, (2015).}. We developed a method around this concept to make dual use of the sputter magnetron setup. The electrons sheath is known to give rise to fireball\footnote{\label{Baalrud_1}S. D. Baalrud \textit{et. al.}, \textbf{Plasma Sources Sci. Technol.}, 18,035002, (2009).}$^,$\footnote{S. Chauhan \textit{et.al.} \textbf{Phys. Plasmas,} 23,013502, (2016).}, which is used for plasma immersion nanopatterning on GaSb substrate.   

Hot electrons between cold walls

We consider electron cooling in a collisionless plasma slab between two cold and freely emitting walls. Numerical calculations suggest a counterintuitive behavior of this system: ~the cooling rate slows down and eventually stops, leaving the system with a significant fraction of its initial thermal energy. Analytical treatment within the Vlasov-Poisson model reveals a set of steady-states with a two-component distribution of electrons: the primary electrons trapped within the potential wells and the secondary electrons forming the counter-streaming beams. We show that such steady-states are linearly stable with respect to one-dimensional perturbations. Establishment of a particular steady-state depends on initial conditions.   

The radiation enhancement phenomenon of a thin plasma layer covered cylindrical metallic antenna.

The radiation enhancement phenomenon of a thin plasma layer coated cylindrical antenna can be observed by carefully choosing the plasma frequency and the thickness of the plasma layer. Based on collision free, uniform plasma layer assumption and antenna-sheath-plasma layer configuration, results obtained from Multiphysics finite element method (FEM) simulations show that, when the frequency of the TE wave is equal to 300 MHz, and the thickness of the sheath is about one tenth of the thickness of the plasma layer, the radiation can be enhanced significantly by setting the plasma frequency equals to 1.5 times of the wave frequency. Furthermore, detailed investigations imply that this radiation enhancement phenomenon has a direct dependence on the radius of the cylindrical antenna, the thickness of the plasma layer and sheath thickness.   

Design and development of field emission based rising sun magnetron for industrial applications

The magnetron is widely used in radars and also well-known as a low-cost microwave source for microwave ovens, which generates microwaves based on the interaction of a stream of moving electrons under a cross electric and magnetic fields with a series of open coupled metal cavity resonators. In this work, a field emission based magnetron is investigated for industrial applications as an easier configuration and longer lifetime can be expected. The design and development are performed using a conformal finite-difference time-domain particle-in-cell simulations. The goal of this research is to design and develop magnetrons operating at a frequency of 2.45 GHz and at a working power of \textgreater 3 kW for industrial applications. A preliminary design after the optimization could achieve the required power at a high efficiency of \textgreater 78{\%}. One of the advantages is the fabrication and assembly can be much simplified compared with those of a conventional strapped magnetron based on a thermionic cathode.   

Design and development of field emission based high power magnetron for wireless power transmission

Space-based solar power is the concept of collecting solar power in outer space and distributing it to Earth. Potential advantages of collecting solar energy in space include a higher collection rate and a longer collection period without a loss to a diffusing atmosphere. The cost of the wireless power transmission (WPT) is about 30{\%} of the total cost of the space-based solar power system (SSPS). The main portion of the high cost is on the development and transportation due to the large number and mass of the magnetrons used in the WPT subsystem. In this work, we propose to design and develop a field emission based rising-sun magnetron capable of producing higher power at one or two orders of magnitude than those commercially available on the market today so that the development and installation cost of an SSPS can be dramatically reduced. The first goal of this research is to design and develop magnetrons operating at a frequency of 2.45 GHz and at a power level of \textgreater 100 kW for WPT. A preliminary design after the optimization could achieve the required power at a high efficiency of \textgreater 80{\%}. Additional advantage is the fabrication and assembly can be much simplified compared with those of a conventional strapped magnetron.