Session GP12: Poster Session III: Low Temperature PlasmasFundamental Plasma Physics I: computation, boundaries Fundamental Plasma Physics II: dusty, diagnostics MFE Measurement and Diagnostics Techniques, Technology, and Edge and Pedestal Physics 9:30 AM - 12:30 PM

Spectral characterization of the BHT-200 thruster operation on argon propellant

With this study we complete non-invasive passive emission characterization of the BHT-200 Hall Effect Thruster operation on common noble gases [1,2]. First, vacuum EUV spectra of the HET’s running on Ar propellant are collected and the emission lines of prime plasma species are identified. Next, FUV-MUV spectrum is examined for notorious boron nitride lining erosion products as a function of the discharge parameters. Finally, the upgraded high-resolution Doppler spectroscopy system [3] is used to measure the axial and azimuthal velocities of Ar⁺ ions along the plasma plume. Results are compared to the similar Xe and Kr data [1,2].

[1] O.Batishchev, A.Hyde, J.Szabo, Spectral Characterization of Hall Effect Thruster Exhaust, 62nd Annual APS DPP Meeting, NP14.00013, November 2020.

[2] O.Batishchev, A.Hyde, J.Szabo, Spectral characterization of the Hall Effect thruster operating on Kr propellant, 63rd Annual APS DPP Meeting, PP11.00028, November 2021.

[3] A. Hyde and O. Batishchev, Review of Sci. Instr. 91(063502), June 2020.   

Experimental study of the ionization region of the highly magnetized RF discharge in argon gas

We use high-resolution spectroscopy to investigate ionization region of the mini-helicon plasma (MHT) thruster [1]. The ionization region was shown to be just several centimeters long in the Ar discharge [2]. We are using a similar high-resolution MUV-VIS system with improved spectral resolution of about 0.1pm per pixel. It allows capturing Doppler spectra with ~100m/s accuracy for VIS emission lines. The ratios of emission intensities of close ionic and neutral atomic spectral lines are used to determine the ionization region axial structure. Several strong Ar⁺ ion line shapes are collected by angled optical collimators with narrow cones of light collection and numerically analyzed to quantify plasma flow velocities along the MHT discharge.

 

[1] O. Batishchev, Minihelicon Plasma Thruster, IEEE Trans. Plasma Science, 37 (8) 1563 - 1571, August 2009.

[2] T. Matlock, Emission Spectroscopy for the Study of Electric Propulsion Plasmas, MS Thesis, MIT, June 2009.

[3] A. Hyde, O. Batishchev, A high-resolution spectroscopic system for remote measurement of plasma parameters, Review of Sci. Instr. 91(063502), June 2020.   

Particle Balance Measurements by Laser Induced Fluorescence in a Two Antenna Helicon for Plasma Wakefield Acceleration

A high plasma density (10²⁰ - 10²¹ m^-3) with very high axial uniformity of under 0.25% is needed to achieve beam-driven wakefield acceleration of electrons in the GV/m range in AWAKE plasmas. Helicon plasmas have been known for several years to be capable of reaching these high densities. To determine whether helicon plasmas can achieve the necessary uniformity, it is helpful to understand the helicon particle balance. Using laser induced fluorescence, it is possible to measure the 2D particle balance in a helicon plasma. We present these measurements for various relevant RF configurations. We utilize two identical antennas, each capable of delivering up to 10 kW of RF power to the plasma. We investigate the effects of adding the second antenna, and the effects of increasing RF power. Understanding how these two inputs influence the particle balance is key to understanding how helicon plasmas can be used to sustain uniform, high, on-axis plasma densities for use in AWAKE.   

Power and gas injection design for plasmoid magnetic reconnection thruster

With increasing interest in space travel and multiple companies and countries looking towards future missions to Mars, efficient and high-power propulsion systems are needed. Towards this end, this paper looks at the preliminary design of a thruster that utilizes plasmoid ejection through fast magnetic reconnection for propulsion [F. Ebrahimi, Journal of Plasma Physics, 86(6), 2020]. This thruster system can potentially fill the gap between high-efficiency Hall and Ion thrusters and the high thrust of chemical rockets needed for such missions. The design of the capacitor bank system for plasma generation and the gas injection system of this thruster will be examined in detail. These designs show the requirements of such a thruster and how they can be achieved. The method of data analysis for the prototype will be discussed, how the prototype's performance will be verified, the limitations of the prototype, and ways to remove these limitations in later iterations.    

Analysis of scattering diagnostic information from magnetized thruster plasmas

The adaptation of well-established fusion diagnostics (coherent and incoherent Thomson scattering, in particular) has provided key insights into low-temperature magnetized devices such as plasma thrusters. Such diagnostics have been used to provide evidence of instabilities believed to be relevant to anomalous transport, such as the electron cyclotron drift instability and ion-ion two-stream instability, and important spatially-resolved information on the electron properties and drifts. In this work, we study the spectral information from such diagnostics using simulated and experimental data for magnetized thrusters. Such approaches have the potential to provide guidance for the optimization of propulsion devices and deeper physical insights into their operation which are currently lacking.   

Development of PIC simulations and plume diagnostics for a magnetoplasmadynamic thruster

With the increasing number of satellite launches, the use of electric propulsion systems in space has expanded rapidly in recent years. There is significant interest in electric propulsion technologies offering high specific impulse and thrust for deep space and manned missions, and as a result, devices with different thrust ranges are currently under research and development. These devices include the magnetoplasmadynamic thruster (MPDT). This device, while promising for high thrust applications and known to be efficient at powers in the hundreds of kW range, has significant limitations, such as erosion and unstable operating regimes. The operation of an MPDT involves a substantial mass flow and typically high back pressure, resulting in more facility effects compared to other electric propulsion devices. Specifically, in the plume region, interactions such as charge exchanges with neutral particles in the vacuum chamber can lead to erroneous performance assessments during diagnostics investigations. This study aims to construct a plume simulation for a kW-scale MPDT using particle-in-cell methods to evaluate the facility effects present in experimental settings.   

Two-dimensional MHD simulations and development of an applied-field magnetoplasmadynamic thruster

Plasma propulsion devices such as magnetoplasmadynamic thrusters, while possessing simple coaxial architectures, exhibit complex particle dynamics which impact thruster performance and optimization strategies. In this study, a two-dimensional magnetohydrodynamic simulation of an applied-field magnetoplasmadynamic thruster prototype is conducted to analyze the expected characteristics of the flow field in axial-radial dimensions and key performance parameters. The investigation of the applied conditions of magnetic field, flow rate, voltage, and thruster geometry are used to determine features such as the localization of acceleration and ionization. Features of the prototype from initial testing are also discussed. These results reveal the dynamics of the flow field and provide insights into the physics of prototypes currently under development.   

Estimation of the sublimation of the propellant after the pulse discharge in a parallel-plate pulsed plasma thruster

One of the electric propulsion systems suitable for microsatellites is the pulsed plasma thruster (PPT). In this study, the thrust characteristics and efficiency are evaluated when the input energy to the PPT is changed. Generally, the mass of solid propellant (Teflon) consumed is used to calculate the specific impulse. However, sublimation from the Teflon propellant surface continues even after the pulse discharge ends, and the gas generated after that is not accelerated and does not contribute to the thrust. In other words, the mass of the gas that is not accelerated is also included in the estimation of the specific impulse. The proportion of gas generated after the discharge is not yet well understood. Here, the specific impulse can also be estimated using the plasma velocity. The plasma velocity was measured using a double probe. When the plasma flows into the probe, a current corresponding to the time change of the plasma density flows, and the plasma velocity is measured from the time delay between the upstream and downstream currents. The plasma velocity was estimated to be about 35 km/s, and the propellant loss rate was about 45 %.   

Extending the Lifetime of Microwave-Driven Ion Sources

Microwave-driven plasma sources can be utilized to generate deuterons for use in neutron production. At low power these types of sources have very long lifetimes, on the order of 1000s of hours, but at high power that lifetime can drop by 50% or more. This becomes especially important when tritium is utilized due to radiological concerns when executing a source swap. To better understand source performance and lifetime at high power, including the underlying causes and mechanisms, a SHINE ion source is being retrofitted with an array of diagnostic equipment. A combination of Langmuir probes, spectrometry, calorimetry, and magnetic field measurements will allow characterization of important plasma parameters both internally and emitted from the source as well as the ideal operational conditions and parameters. Using this information, we aim to identify the mechanisms which enable high-power standard operation as well as those that drive towards instability and failure. Correctly identifying these mechanism and mitigating those that lead to failure could enable high-power microwave-driven plasma sources to operate with greater ease-of-operation and extended lifetimes like their low-powered bretheren.   

A Comprehensive Particle-In-Cell Model for Microwave-Based Ion Source Optimization

SHINE Technologies, LLC currently utilizes beam-target based fusion devices to produce neutrons. The components that constitute these neutron generators are purpose-built for long-duration operation, and significant investments have been made to ensure their high reliability. To minimize maintenance of system components, further improvements are desired. Achieving these improvements requires a deep understanding of the microwave-driven plasma that acts as an ion source. In this work, we present a comprehensive Particle-In-Cell (PiC) model of SHINE's microwave-based ion source, aimed at investigating the coupling dynamics between the plasma and the chamber wall under various conditions. Our study includes detailed comparisons between the model's predictions and empirical data, such as total output current, crosswire scanner data, and emittance measurements obtained from a low energy beam transport test stand. The primary objective of both the measurements and the modeling campaign is to understand the heating mechanisms, spatial distribution of the plasma in the steady state, and its stability in response to perturbations. The modeling campaign includes a systematic exploration of system parameters, providing valuable insights into the behavior and optimization of microwave-based ion sources.   

Reproduction of Plasma Conditions during Atmospheric Re-entry by Injecting Gun-Generated Plasmoids into a Gas Atmosphere

Reentry capsules of space probes and reusable launch vehicles (RLVs) generate shock waves when they re-enter the atmosphere at supersonic velocity, and plasma is generated due to aerodynamic heating in front of the fuselage. Plasma can affect the radar's radio-reflective properties and communication with ground stations during radar tracking. However, especially in Japan, actual flight data accumulation is not enough, and investigations are mainly conducted through simulations and experiments with test vehicle launches. The goal of this study is to reproduce the plasma conditions during atmospheric re-entry in the laboratory and measure the interaction between the radar and the plasma under these conditions. The first step is to reproduce the plasma conditions. The experimental device consists of a magnetized coaxial plasma gun (MCPG) connected to a quartz chamber. When a hydrogen plasmoid is injected into a vacuum space using this MCPG, the plasma density is 10¹⁹ m^-3 and the velocity is 50 km/s. To reproduce the plasma conditions during atmospheric re-entry, the density and velocity should be 10¹⁸–10²⁰ m^-3 and 2–6 km/s, respectively. To approach this, a high-speed plasmoid is ejected into a vacuum chamber filled with neutral gas. By ionizing the neutral gas, it is expected that the plasmoid decelerates. Plasma parameters will be measured using a triple probe, interferometer, and a high-speed camera.   

Riverside Research Plasma Lab: preliminary lab capabilities and results

Riverside research, a 501c(3) not for profit research institution, has been developing a flexible plasma chamber for the past several years to study various plasma phenomena, ranging from ionospheric plasmas, hypersonic sheath plasmas, and plasma thruster exhaust. The chamber currently uses an 8 inch wide Lanthanum Hexaboride (LaB₆) square tile plasma source and five internal water cooled magnets to produce and control either a DC or pulsed plasma with electron densities on the order of 10¹⁵-10¹⁹ particles/m³ and magnetic field on axis of up to 0.3 T. The lab employs various diagnostics, including 2D triple Langmuir probes (LTP), microwave interferometry, and spectroscopy to diagnose plasmas, and has several diagnostics under development including Laser Induced Fluorescence, Terahertz interferometry, and residual field analyzer probes to further diagnose the plasma. Initial 2D LTP measurements for a one inch thick, 8 inch wide slab plasma in both DC and pulsed regimes will be presented.   

Preliminary Results from the Riverside Research Plasma-Materials Interaction Experiment

Hypersonic vehicles are often exposed to extreme environments where heat fluxes to the vehicle’s surface can pose several challenges for the integrity of the shielding material. At hypersonic speeds the temperature surrounding the vessel can exceed 3000 °C and the local environment becomes partially ionized. Under these conditions oxidation effects become a significant concern. Despite the importance of understanding the role oxygen plays for materials testing, facilities capable of creating controlled oxygen plasma with sufficient plasma and material diagnostics and that can achieve sample temperatures of interest (i.e., ℃) are scarce. Recently, our team began to develop a facility capable of heating small material samples to high temperatures in a controlled cold gas environment. Compact samples (i.e., ) are heated to  °C using a simple heating element. The heated sample is exposed to gas mixtures of various known concentrations and operating pressures to study oxidation effects in a controlled environment. The current suite of diagnostics available to our laboratory can measure electromagnetic properties of the heated samples in the X band (8 – 12 GHz) and mid infrared (IR) range ( THz) as well as monitor global environment characteristics. Here we present an update on the progress of this facility and preliminary results of materials characterization after a controlled gas exposure to a heated sample. Planned plasma source and diagnostic upgrades are also shown.   

Kinetic treatment of hypersonic plasmas during re-entry

A prominent application of Hypersonics is the re-entry of spacecraft vehicles in the atmosphere. During re-entry, thermal plasma is created by shock compression of the gas between the bow shock front and the surface of the vehicle. The electron kinetics of such plasmas is modeled by solving the time-dependent electron Boltzmann equation for the electron energy distribution function. The energy source for the electrons is elastic and second-kind collisions from vibrationally excited states of N₂. Numerical simulations of air plasma showed that the electron energy distribution function deviates from Maxwellian and the collisional rates differ by orders of magnitude from those computed with a Maxwellian distribution.   

Non-equilibrium chemistry model of an air plasma flow behind strong normal shock waves

A non-equilibrium air chemistry model is developed to study the shock layer around fast moving objects. It is based on an augmented Dunn-Kang model [1] that includes elastic scattering, vibrational relaxation, species diffusion, transport model and surface reactions. The air chemistry is coupled to a normal shock wave model and a one-dimensional stagnation line model. It provides the electron and gas temperatures, species densities, species and thermal fluxes. At temperatures below about 5,000 K the leading species and power fluxes are due to vibrationally excited nitrogen molecules, while at higher temperatures the nitrogen and oxygen molecules are dissociated and the fluxes of atomic species (oxygen and nitrogen) dominate. The adiabatic parameter γ decreases with gas temperature increasing and approaches its limiting value of 1 at high gas temperatures [2].

[1].  M. G. Dunn and S. Kang, “Theoretical and experimental studies of reentry plasmas”, NASA-CR-2232, NASA Langley Research Center (1973)

[2]. Tz. B. Petrova, G. M. Petrov, and J. R. Peñano, “Transport properties of high Mach number hypersonic air plasmas”, submitted for publication in Plasma Sources Sci. Technol.   

High-resolution electric field mapping in RF plasma using an optically trapped single particle.

The electric field is a key plasma parameter affecting charged particle behavior in RF plasma and sheath kinetics. It is often measured with a Langmuir probe, which can disturb plasma parameters by depleting energetic electrons and altering the global discharge. An alternative involves using micron-sized particles, which become negatively charged and can be levitated by the plasma's electric force. By observing their motion, the electric field E can be determined if the charge Q is known, or vice versa. However, this method is limited to fixed particle positions unless plasma conditions are changed.

An optically trapped particle provides a non-invasive probe for three-dimensional plasma measurements. It can be moved freely without altering plasma parameters. In this study, an optically trapped particle was used to map the electric field at the plasma sheath edge with high spatial resolution. Once the optical force is removed, the particle moves under gravity and the plasma's electric force. By analyzing its trajectory, the electric field is measured, assuming a constant charge Q. While accuracy depends on the Q approximation, precision is high due to careful particle positioning. This method measured electric fields affected by different electrode configurations, effectively comparing modified and unmodified fields. The optically trapped particle proves to be a reliable, high-precision, non-intrusive tool for mapping electric fields in RF plasma.   

A Faraday Cup Design for High-Current, High-Energy Ion Beams

A high-current, high-energy Faraday cup was designed and built for the Stable Isotope Program at Oak Ridge National Laboratory (ORNL). Faraday cups are commonly utilized in the current characterization of charged particle beams. In ion beam applications, the Faraday cup must be able to mitigate the effects of secondary electron emission (SEE) induced by the positive ions, which can be a challenge as the size, current, and energy of the ion beam increases. ORNL’s Faraday cup is capable of probing high-current (tested up to 100 mA), high-energy (tested up to 45 kV) ion beams up to 10 cm in diameter. However, preliminary testing of the Faraday cup suggest that the effectiveness of its electron suppression system begins to decrease as the ion beam parameters approach the upper tested limits. In this presentation, we will discuss the current capabilities and limitations of the Faraday cup design, and some of the ongoing research efforts to improve this ion beam diagnostic system.   

Sheath Measurements for a Trapezoidal Wafer Bias Waveform using LIF

Nonsinusoidal bias waveforms are being used on plasma etch tools to positive effect. This work reports on a particular version consisting of a short positive voltage spike to draw electrons, and a longer negative-going ramp to provide approximately constant current for the ion etch species.  Investigations of the sheath during the various phases of the applied bias waveform are performed in a low pressure Argon plasma using Laser Induced Fluorescence (LIF). Additional diagnostics employing hairpin and Langmuir probes mounted on a 3D probe drive system will be used to see changes in the bulk plasma immediately above the RF sheath.  We expect that the approximately linear dV/dt of the ramp can be adjusted to create an approximately constant sheath. Forthcoming results will be reported.   

LIF measurement of Ar+ motion above a biased wafer in a plasma etching reactor

We studied the motions and energy distribution of argon ions in an inductively coupled plasma (ICP) etching tool, by the method of laser induced fluorescence (LIF). The silicon wafer clamped to a chuck at the bottom of the chamber was biased with a 1 MHz 1–1.2 kV peak-to-peak sinusoidal voltage. The plasma is formed with a 2 MHz ICP coil pulsed at 10 Hz. Sheath thickness was measured at different phases of the bias waveform. Ion energy distribution functions and the two-dimensional flow pattern were studied near the center and edge of the wafer, with / without wafer bias, at different switch-on time of wafer bias. Significant vortex flows were observed near the wafer edge. Experiments in which the wafer was biased in the plasma afterglow resulted in a narrow distribution of ion energy close to the bias voltage at the vicinity of the wafer, and the ion incident angle on the wafer was the smallest.   

An Experimental Investigation into the Relationship Between Plasma Spot Sizes and Wavelengths in Laser-Produced Plasma-Based EUV

Xenon is a potential light source for extreme ultraviolet (EUV) lithography due to its engineering simplicity and advanced wavelength of 11.2 nm. The characterization of its emission spectra has been thoroughly studied.^1,2 In addition to the spectral research, the optical imaging investigations regarding the plasma spot images in the visible light range, deep ultraviolet (DUV) range, and EUV range were useful information for understanding its expansion dynamics. The presented work was examined using Nd: YAG laser-driven cryogenic Xenon target. This imaging study was conducted by varying the focal positions of the pulsed laser and the applied bandpass filters. In the experiments, the plasma spot sizes were measured and interpreted as dimensions in x- and y-directions with Gaussian curve fitting applied. The minimum spot sizes at the focal position were obtained for the visible, DUV, and EUV ranges. Interestingly, an engineering formula was derived for evaluating spot size across different wavelengths after we integrated our previous research on Xenon plasma within the 1-6 nm wavelength range. The analysis revealed a general trend of decreasing spot sizes logarithmically with shorter wavelengths. These findings provide a valuable reference for simulations and experimental studies related to plasma formation.

¹              V. P. Belik, S. G. Kalmykov, A. M. Mozharov, M. V. Petrenko, and M. E. Sasin,  Technical Physics Letters 43 (11), 1001 (2017).

²              S. C. Bott-Suzuki, A. Bykanov, O. Khodykin, M. Tillack, S. Cordaro, and C. McGuffey,  Journal of Applied Physics 126 (11) (2019).   

Plasma Physics Kinetics and Experiments to Optimize Extreme Ultraviolet Sources

Laser-produced tin plasmas which emit extreme ultraviolet (EUV) radiation are used as the state-of-the-art short wavelength source for semiconductor lithography[1]. The properties of the laser-produced plasma can greatly affect the efficiency of the conversion of laser energy into EUV photons. Debris from the tin plasma, particularly energetic ions, can also disrupt the reliability of the light source by damaging collection optics. Current efforts aim to understand and model how plasma physics behavior affects both the conversion efficiency and debris production. We will detail approaches to tune the laser-generated plasma to balance EUV production against debris creation using a combination of particle-in-cell modeling [2] and experiments. 

[1] Versolato, Oscar O. "Physics of laser-driven tin plasma sources of EUV radiation for nanolithography." Plasma Sources Science and Technology 28, no. 8 : 083001 (2019) 

[2] S. Totorica, et al, "Acceleration mechanisms of energetic ion debris in laser- driven tin plasma EUV sources” Appl. Phys. Lett. 124, 174101 (2024)   

Influence of pressure on sputter yield in DC magnetron discharge with PIC simulation

We explore the dependence of the sputter yield in planar DC magnetron discharge (DCMD) on the neutral gas pressure using 2D-RZ particle-in-cell (PIC) simulations. Magnetron discharge is the most widely used technique to deposit metallic and compound thin films. These discharges rely on an external magnetic field, applied perpendicular to the electric field, in order to reduce the necessary gas pressure and discharge voltage. The reduced pressure enables increased deposition rates due to decreased scattering of the sputtered atoms. In this work, we find that the pressure also influences the voltage profile along the discharge axis, which controls the ion bombardment energy. The ion bombardment energy is important because it determines the sputtering yield, which directly scales the deposition rate. We find that nearly all of the ionization occurs in the quasineutral plasma (outside of the cathode sheath). The ions are, therefore, accelerated by at least the sheath voltage gain, and up to the full discharge voltage. We provide a theoretical explanation of this behavior and also explore other pressure dependent phenomena in DCMD.   

Numerical thermalization in 3D particle-in-cell simulations

Numerical thermalization in multidimensional particle-in-cell (PIC) codes poses challenges for accurate simulation of low temperature plasma sources. While 1D PIC simulations often have a low rate of numerical thermalization due to some degree of kinetic blocking, multidimensional PIC simulations employing the commonly used cloud-in-cell (CIC) scheme and a well-resolved Debye length will likely have a numerical collision rate which is more rapid than true Coulomb collisions between electrons. The rate of numerical collisions in 2D PIC was empirically estimated by Hockney [1] many years ago and we have recently analytically calculated the drag and diffusion coefficients of the Fokker-Planck form of the numerical collision operator for a test electron macroparticle in a thermal electron background. [2] A similar analysis has been lacking for 3D PIC, which we have investigated here.

 

[1] R. W. Hockney, "Measurements of collision and heating times in a two-dimensional thermal computer plasma," (1971) Journal of Computational Physics 8, 19-44.

 

[2] S. Jubin, A. T. Powis, W. Villafana, D. Sydorenko, S. Rauf, A. V. Khrabrov, S. Sarwar and I. D. Kaganovich, "Numerical thermalization in 2D PIC simulations: Practical estimates for low-temperature plasma simulations," (2024) Physics of Plasmas 31, 023902.   

Measurements of two-dimensional ion velocity distributions in electron beam generated E x B plasma

Electron beam (e-beam) generated plasmas with applied crossed electric and magnetic (E x B) fields are promising for low-damage processing of materials such as graphene and single-crystal diamond. The minimal damage to these sensitive materials is typically attributed to the low energy of ions incident on the substrate surface, a result of ion confinement by E × B fields. Recently, our team observed the presence of warm ions (~1 eV), which is sufficient to produce a diffusive ion backflux towards the edge, thereby compromising the ion confinement in this system [1]. Given that our system is nearly collisionless, the radial structure of the electric potential is identified as the primary source of ion heating. However, other types of anomalous ion heating mechanisms, such as two-stream instabilities [2] at the plasma periphery, require further investigation.

We have developed a two-dimensional Laser-Induced Fluorescence (LIF) diagnostic technique using Doppler velocimetry [3] to map ion velocity distributions (IVD) across the plane perpendicular to the e-beam generated Argon plasma. A diode laser centered at 668.61 nm is used to excite metastable argon ions, and an ICCD camera is employed to collect the corresponding fluorescence signal. By combining electronic diagnostics such as Langmuir probes and emissive probes with this laser-based diagnostic method, we can extensively study ion dynamics in both the confinement and peripheral regions. This diagnostic system, equipped with an ICCD camera, enables time-efficient measurement of spatially distributed IVDs. In future work, we anticipate that our findings will enhance the understanding of ion heating mechanisms in this system, providing insights into the physics behind warm ion production in E × B plasma devices.

 

[1] Chopra, Nirbhav Singh, Ivan Romadanov, and Yevgeny Raitses. "Production of warm ions in electron beam generated E× B plasma." Applied Physics Letters 124.6 (2024).

[2] Mikellides, Ioannis G., and Alejandro Lopez Ortega. "Growth of the lower hybrid drift instability in the plume of a magnetically shielded Hall thruster." Journal of Applied Physics 129.19 (2021).

[3] Severn, G. D., D. A. Edrich, and R. McWilliams. "Argon ion laser-induced fluorescence with diode lasers." Review of Scientific Instruments 69.1 (1998): 10-15.   

Anticathode effect on electron kinetics in electron beam generated E×B plasma

Electron beam (e-beam) generated plasmas are promising for low-damage material processing applications [1-3]. In cylindrical e-beam E×B plasmas, radial confinement of electrons and ions is achieved by an axial magnetic field and radial electric field, respectively. To control the axial confinement of electrons, such e-beam generated plasma sources may incorporate a conducting boundary known as an anticathode, which is placed on the axially opposite side of the plasma from the cathode. In this work, electrostatic probe measurements demonstrate that varying the anticathode voltage bias can control the degree to which the anticathode collects or repels incident electrons, allowing control of warm electron (electron energies in 10-30 eV range) and beam electron population confinement. It is suggested that the effect of the anticathode bias on the formation of these distinct electron populations is also associated with the transition between weak turbulence and strong Langmuir turbulence.

[1]       Zhao F et al 2021 Carbon 177 244–51

[2]       Chopra N S et al 2024 Appl. Phys. Lett. 124

[3]       Chopra N S et al 2024 arXiv:2406.11642 [physics.plasm-ph]   

Particle-in-Cell Simulations of a Hollow Cathode Experiment for Molecular Spectroscopy of Metal-Containing Plasmas Using the hPIC2 Code

Understanding the sputtering yields of metals is important in fireball chemistry studies, but fireball chemistry studies have largely used Laser Induced Breakdown Spectroscopy, a short repeatable event for which its repeatability is limited by ablation of the surface. Thus, it would be desirable to have a steady source to analyze metals and metal-oxides with, the hollow cathode (HC) system allows steady state analysis of metals and metal-oxides in a steady state. Previous work by Glumac [Kautz et al. 2021] has shown interesting results in controlling the amount of metal released into the plasma in their hollow cathode system. The current study aims to constrain the uncertainty on the atomic metal released into the plasma and the amount of metal in the plasma by simulating the HC system using the hPIC2 code [Meredith et al., CPC 283, 2023]. The following question will then be answered: For the HC system can we control the metal release to the point that we can constrain the rate coefficient of molecular formation from the measured concentration of molecular species as a function of time?   

Particle-in-Cell Simulations of Multi-Peak Ion Impact Energy Distributions in RF Capacitively-Coupled Discharges using the hPIC2 code

Capacitively-Coupled Plasma (CCP) discharges exhibit complex behavior in their ion impact energy distribution function, exhibiting multiple peaks at different discharge regimes. Such multi-peak behavior of CCPs has been observed both experimentally [Wild; Koidl, Appl. Phys. Lett. 54, 1989] and numerically [Sharma et. al., J. Phys. D: Appl. Phys., 2022]. In this work we systematically analyze the occurrence of multiple ion peaks through detailed Particle-in-Cell simulations using the hPIC2 code [Meredith, Comput. Phys. Commun. 283, 2023]. We consider the case of an argon CCP operating between 5 and 200 mTorrs of neutral pressure. The energy peaks are a result of radio-frequency sheath modulation and ion-neutral interactions, dominated by inelastic charge exchange collisions. Ion-neutral collisions create low energy ions within the sheath, which cannot respond to the full sheath potential. It is observed that the number of low-energy peaks roughly scales with the ratio between the ion transit time across the sheath and the RF period.   

Measuring magnetic fields in plasma gun generated flux ropes with quantum beat spectroscopy

Accurate measurement of magnetic fields is critical in numerous plasma environments, ranging from astrophysical systems to fusion energy research. Typical approaches for measuring magnetic fields in laboratory plasmas either employ perturbative probes (i.e., magnetic sensing coils) that are prone to ambient electromagnetic noise or rely on resolving individual Zeeman-split σ-peaks from full velocity distribution function (VDF) measurements. In the latter case, transient magnetic field effects are difficult to ascertain since the measurement of the full VDF from laser spectroscopic techniques, e.g., laser induced fluorescence (LIF), is inefficient and time consuming, especially in pulsed laboratory experiments where each data point used to construct the VDF is composed of several plasma pulse discharges. In this work, an alternative laser-based technique known as quantum beat spectroscopy is employed to measure weak magnetic field strengths at near single shot acquisitions from the Zeeman-split electron energy states of the (2P^o_1/2)4s ²[1/2]^o state of neutral argon. This technique is non-intrusive and unlike LIF does not require any sweeping of the laser wavelength, making this approach ideal for transient events such as probing the reconnection magnetic field strength in laboratory magnetic reconnection experiments.   

Using Machine Learning to Locate Three-Dimensional Magnetic Reconnection within PHASMA

The PHASMA (PHAse Space MApping) facility at WVU uses pulsed plasma guns to investigate magnetic reconnection through the interaction of merging magnetic flux ropes. This study, encompassing approximately 650 shots of helium double flux rope, utilizes parameters such as the flux function and fast photodiodes to identify and locate magnetic reconnection within PHASMA through the application of machine learning techniques. Shots are clustered via having similar bias and arc currents via unsupervised machine learning. Magnetic flux evolution movies are then constructed from these grouped shots. Photodiode measurements are also correlated using unsupervised machine learning to identify similarities that exist between shots. Based on these parameters, we attempt to predict magnetic reconnection at a distant location based on a predictive neural network that uses nonlocal (edge) magnetic measurements and line-integrated fast photodiode measurements in PHASMA. This analysis will enable new studies of reconnection in highly turbulent and irreproducible systems by providing a means of localizing the time and location of reconnection to better synchronize triggered measurements, such as Thomson scattering measurements of the electron distribution function.   

Precision timing solid state pulsed power systems used for injecting a coaxial plasma gun discharge into a pulsed fan-spine magnetic field

Precision timing between two distinct pulsed power systems requires solid state switches in order to achieve high repeatability and reliability for the time constraints of this experiment. Traditional spark gaps used for pulsed power generation lack a repeatability of timing when it must be discharged at some delta T with respect to another discharge. In this case, a magnetic field is pulsed with ~1ms duration. The peak magnetic field forms after ~600us. The coaxial plasma gun (CPG) should be injected at the peak current of the pulsed magnetic field. This requires a precision repeatability of +/- 60us to be within 10% of the peak value of the magnetic field. Spark gaps do not have a reliability or repeatability within this defined window of discharge. In this study, we show how custom built solid state switch arrays can reliably and repeatedly generate a 0.5T magnetic field and inject a 6kv plasma into the magnetic field, inducing torsional magnetic reconnection. The system to charge, monitor, and time the discharges will be discussed. This work is in conjunction with University of Alabama, Huntsville with application in spacecraft propulsion and fusion energy science.    

A Complete Electrode Model for Plasma Impedance Probes

Plasma impedance probes measure the impedance spectrum of an antenna immersed in a plasma. The 1964 work of Balmain remains the standard method to interpret these data, using the peak in the magnitude at the upper hybrid frequency to infer plasma electron density. The primary limitations of Balmain's model are the assumption of a homogenous plasma and a cylindrical dipole. This work presents a numerical model applicable to inhomogeneous plasma and arbitrary antenna geometry based on the cold, fluid approximation given by Balmain. This model solves Poisson's equation using the finite element method and accounts for the effects of the dipole using the plasma complete electrode model (PCEM). The PCEM is developed in this article and accounts for the voltage shunting effects of the dipole elements, the discrete current to the dipole, and the plasma sheath surrounding the dipole. The sheath is incorporated as a contact impedance between the dipole and the plasma in a manner analogous to the complete electrode model of electrical impedance tomography. The first portion of this work presents the mathematical framework of the PCEM, starting from Maxwell's equations. The second part of the work compares the output of this numerical method to Balmain's work and to data collected by an impedance probe in the Space Physics Simulation Chamber at the U.S. Naval Research Laboratory. The PCEM results agree with both the observed data and the prior modelling done by Balmain. An additional consequence of the numerical study is the observation that of some second-order resonances not predicted by Balmain's model can be attributed to the presence of the plasma sheath.   

Impact of Faraday Shield on RF Inductive Coupling in the LUPIN Ion Source

This study presents electromagnetic simulations of the Large, Uniform Plasma for Ionizing Neutrals (LUPIN) ion source, designed as an RF inductively coupled plasma ion source for future designs of the DIII-D Neutral Beam Injection (NBI) system. LUPIN operates with 20 kW of RF power at a 2 MHz driver frequency and incorporates an RF matching network. In high-power RF plasma sources, internal Faraday shields play a vital role in safeguarding the dielectric vessel or windows from erosion caused by capacitive coupling and the resulting energetic particle fluxes. The Faraday shield in LUPIN effectively minimizes interaction between the plasma and the chamber wall, preventing damage to system components, at the expense of some power losses. Its water-cooling mechanism efficiently dissipates heat, ensuring acceptable operating temperatures and enhancing the overall reliability and efficiency of the ion source.

LUPIN is designed to direct RF power into a quartz vessel measuring 20 cm in length with a 10 cm radius, generating hydrogen plasmas with electron densities of 10¹⁸ m^-3 for a 10-second duration. This setup provides flexibility to test designs, matching the power density of a larger (100 kW, 85 A) full-scale ion source. The simulations conducted using COMSOL illustrate the effects of the Faraday shield on power coupling both with and without plasma, investigating various plasma parameters to provide detailed insights into optimizing the ion source's performance.   

ThunderBoltz: An Open-Source 0D Direct Simulation Monte-Carlo 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 particle Monte Carlo code designed to accommodate a generalized combination of species and arbitrary cross sections without the overhead of expensive field solves or statistical particle reweighting. Based on the no-time counter collision method, it can efficiently produce high-quality electron velocity distributions in external AC/DC E-field and static B-field scenarios. It is a C++ standard library implementation, also provided with a convenient Python API 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 poster, we present the capabilities of ThunderBoltz and demonstrate the dependence of electron transport on a variety of collisional parameters. We also demonstrate agreement with other 0D codes, such as Bolsig for benchmark problems.   

Investigation of Magnetic Field Curvature on Plasma-Induced Magnetic Flux Compression in a Stator Coil

     In this experiment, the effect of field curvature on magnetic flux compression in a stator coil was investigated. A laser-produced plasma (LPP) was created within an externally applied magnetic field. The magnetic field was created using various permanent magnets introducing different types of field curvature. The plasma is created by focusing a pulsed Nd: YAG laser with a wavelength of 532 nm onto a graphite target placed within the externally applied magnetic field. The stator coil used in the experiment contains 40 turns of wire and is placed in front of the graphite target with the plasma expanding inside of it. To test the effect of field curvature, three different magnetic field configurations are tested. These include a straight field geometry created by bar magnets and concave-outward and inward directed fields created by cylindrical magnets. The experiment was carried out in a vacuum chamber set to a pressure of 10^-5 Torr and a laser energy of 100 mJ was used to create the plasma. ICCD imaging data has been collected showing plasma expansion into the magnetic field and voltage measurements from the stator coil show induced currents created in stator coils. Initial data indicate slightly more induced current (and thus more field compression) in the concave-outward field geometry compared to the straight field geometry produced by the bar magnets.     

Secondary Electron Emission Yields of Imidazolium-Based Ionic Liquids

The electron-induced secondary electron emission (SEE) yield of imidazolium-based ionic liquids are presented for primary electron beam energies between 30 and 1000 eV. These results are important for plasma-synthesis of nanoparticles in DC discharges operating with an ionic liquid electrode. As a result of their low vapor pressure and high conductivity, ionic liquids can be combined with low pressure plasmas to produce metal nanoparticles through the reduction of dissolved metal salts by the reactive species in the plasma. In this work, the low vapor pressure of ionic liquids is exploited to make direct measurements of the SEE yield by bombarding the liquid with electrons and measuring the resulting currents.   

Formation of Ammonia through Meteoritic Atmospheric Shock

Here we present results from Frontiers experiments at the DIII-D tokamak, which studied ammonia formation in plasma conditions reminiscent of those in the tails of meteoroids entering the Early Earth atmosphere. The formation of ammonia in fusion plasmas has been observed in the past [1]. However, here, we argue that the range of heating conditions achievable in the DIII-D divertor, ~30MW/m^2 near the outer strike point (OSP) to ~1MW/m^2 away from the OSP, allows for the study of ammonia formation in environment like the one expected during meteoroid ablation. Here, the DIII-D’s Divertor Material Evaluation System (DiMES) was used to expose SiO2-coated graphite rods to the edge plasma. To mimic various conditions in Earth’s history, different gases were puffed in the divertor region near the rods (N2-only, N2-H2 mixture, and N2-CH4 mixture). Upon collection and chemical analysis of the rods, different amounts of ammonium (the cation of ammonia) were detected, allowing to assess ammonia formation with different seeded gases. Our experiment demonstrates that it is plausible for meteoroids ablation to generate ammonia in several different stages of the Earth’s history, which could have been used in prebiotic chemical processes.

[1] https://doi.org/10.1016/j.jnucmat.2015.01.010

Work supported by US DOE under DE-SC0022554, DE-SC0021338, DE-SC0023375, DE-SC0023367, and DE-FC02-04ER54698   

Quantum computing for plasma physics: prospects and limitations

Plasma dynamics is notoriously difficult to simulate, with state-of-the-art simulations requiring millions of CPU hours. This motivates us to explore alternative computational platforms that may speed up such simulations. Quantum computing is a potential alternative as it can solve certain problems exponentially or polynomially faster than classical computers. We will present a brief overview of quantum algorithms for scientific computation that are applicable to plasma physics. At a high level, end-to-end quantum algorithms for plasma dynamics can be broken into three separate stages: loading the initial conditions onto the quantum computer, solving the relevant equations, and extracting useful information from the solution states. We will outline recent progress in each area and highlight main limitations as well as prospects.   

Efficiency in Measurement-Based Quantum Simulations of Nonlinear Dynamics

We present a quantum algorithm to solve initial-value nonlinear ordinary differential equations (ODEs). The algorithm relies on classical evaluation of a summation over sub-Hamiltonians, weighted by the expectation values of paired observables, obtained by repeated measurement of the solution state. Standard quantum Hamiltonian simulation bridges the short times between evaluation of new Hamiltonian matrices. This algorithm requires an ensemble of quantum states, where each step consumes a subset of quantum states, which are used for measurements and are discarded from further time advance. Having demonstrated that our algorithm is capable of solving nontrivial problems, in this work, we explore the question of efficiency: For what class of problems is the algorithm efficient? For a range of problem classes, we use classical simulations to estimate how algorithmic errors scale with simulation time. For problems where the algorithm is efficient, the scaling is polynomial, rather than exponential, indicating that more accurate results is attainable with a small increase of computational resources. We present potential physical systems for the problem classes, providing further analytical analysis.   

A sparse-grids filter for structure-preserving electromagnetic PIC methods

In the past decade, the idea of using the sparse-grid recombination technique as a means to reduce noise in electrostatic particle-in-cell (PIC) methods has generated substantial interest. Likewise, structure-preserving electromagnetic (EM) PIC methods based on discretizing the Lagrangian or Hamiltonian structure of the Vlasov-Maxwell system have been a prominent direction of inquiry for their ability to automatically conserve known invariants of the continuous dynamics (e.g. energy and Gauss's laws). This work unites these two ideas to provide a structure-preserving EM PIC method with a noise-reducing sparse-grids filter. Specially designed filter matrices, which are sparse operators with a convenient Kronecker structure, are inserted into the discrete Lagrangian of a general variational electromagnetic PIC method in such a manner that the symmetries of the Lagrangian are preserved. This yields a filtered, variational EM PIC method which retains all the structure-preserving properties of a usual variational EM PIC method, which enjoys the noise-reduction properties of a sparse-grids PIC method, and whose implementation involves only a small, inexpensive modification of existing structure-preserving EM PIC methods. The strategy is general and should be amenable to a broad class of variational EM PIC methods. Numerical examples are provided using the GEMPIC method.   

Hamiltonian structure of the guiding-center Vlasov-Maxwell equations with polarization and magnetization

The Hamiltonian formulation of guiding-center Vlasov-Maxwell equations, which contain dipole contributions to the guiding-center polarization and magnetization, is presented in terms of a guiding-center Hamiltonian functional that is derived from the exact guiding-center Vlasov-Maxwell energy conservation law, and an antisymmetric functional bracket that satisfies the Jacobi property. Exact energy-momentum and angular momentum conservation laws are expressed in Hamiltonian form and the guiding-center Vlasov-Maxwell entropy functional is shown to be a Casimir functional.   

Kinetic spectral simulations with local-implicit global-explicit approach

An implicit-explicit (IMEX) temporal integration approach for kinetic spectral models is presented where fast/stiff operators are treated implicitly. We solve the Boltzmann equation with the Bhatnagar–Gross–Krook (BGK) collision operator for a gas dynamics problem (GD), and the Vlasov-Ampere (VA) system for collisionless plasma physics applications. Kinetic spectral models help to manage the curse of dimensionality present in kinetic problems, however

retain drastic time scale separation which is common for plasma and fluid applications.

Thus, for GD problems, the BGK operator introduces a stiff collisional term, while for the VA system, the stiff parts are the particle acceleration term and current source in Ampere's equation.

The purpose of this presentation is to show speed-up by stepping over fast scale dynamics. This procedure ensures locality in physical space for the implicit part, making it particularly efficient where standard physical space domain decomposition is used.

The local property makes the parallel implementation and its preconditioning significantly easier than for a

fully implicit methods which require global nonlinear iterations.

Both GD and VA systems use a spectral expansion in velocity space that leverages Hermite basis, and finite differences for spatial discretization.

To illustrate the method, we present a Sedov problem, where an initial pressure gradient leads to shock-wave formation and its propagation with IMEX time stepping exceeds the explicit time step by ∼10⁴ times yet recovers the correct dynamics accurately. For plasma applications, the evolution of a large magnetohydrodynamics scale ion-acoustic wave can be evolved with IMEX time stepping of over ∼800 the fastest scale of the problem, which is the electron plasma period.   

Asymptotic and numerical analysis of perpendicular transport in low-beta plasmas

Kinetic physics, including finite Larmor radius (FLR) effects, are known to affect the physics of magnetized plasma fluid phenomena such as the Kelvin-Helmholtz and Rayleigh-Taylor instabilities. Accurately incorporating FLR effects into fluid simulations requires moment closures for the heat flux and stress tensor, including the gyroviscous stress in collisionless magnetized plasmas. However, the most commonly used gyroviscous stress tensor closure (Braginskii Rev. Plasma Phys., 1965) is based on a strongly collisional assumption for the asymptotic expansion of the kinetic equation in the so-called fast-dynamics ordering. A formal asymptotic analysis of the Vlasov equation in the slow-dynamics or drift ordering is performed in a new ``semi-fluid'' formalism, which integrates in v_⊥ to obtain a five-moment system which requires a heat flux and stress tensor closure. The leading-order perpendicular transport physics is determined via a Hilbert expansion of the kinetic equation, and the stress tensor is found to be a second-order quantity in the expansion parameter. A numerically affordable approximation to the stress tensor closure is proposed which adjusts the Braginskii closure to account for temperature gradient-driven stress. Continuum kinetic simulations of a family of sheared-flow configurations with variable magnetization and temperature gradients are used to validate the predictions of the drift ordering semi-fluid expansion. The expected convergence with magnetization is observed, and residuals are examined and discussed in terms of their relationship to higher-order terms in the expansion. The adjusted Braginskii closure is found to accurately correct for the over- and under-estimation committed by the Braginskii gyroviscous stress tensor closure in the presence of temperature gradients.   

Grid heating analysis based on energy conserving and momentum conserving PIC formulations

To study the instability responsible for grid heating, we start with a

meshfree formulation and then discretize on a grid. We then study the ef-

fects of different discretizations. Direct discretization of the meshfree equa-

tions leads to the well-known momentum conserving (MC) algorithm. A

modification leads to an energy conserving (EC) discretization. For a cold

non-drifting plasma the EC discretization involves a symmetric positive def-

inite (SPD) kernel matrix, showing stability, in agreement with PIC tests.

For this cold plasma case the MC discretization leads to a non-symmetric

kernel matrix, and in some cases this matrix is even non-normal. This matrix

is found to have no eigenvalues with positive real part, implying stability,

also in agreement with PIC tests. Results and analysis of the warm plasma

and the cold beam cases will be presented.   

Lokta-Volterra simulation on quantum computers

Simulating non-linearity with a quantum computer is a longstanding goal, with significant efforts from both the plasma computational physics community and the quantum computing community. Identifying which non-linearities can be simulated by a quantum computer is crucial for developing new computational tools for plasma physics simulations and understanding the capabilities of quantum computers. The Koopman-von Neumann (KvN) mapping proposes a straightforward method to address some non-linear PDEs through phase space simulation. The advantage of this scheme lies in the fact that, in many cases, the quantum numerical scheme requires only the Quantum Fourier Transform and diagonal operators that are efficiently implementable on quantum computers. In this work, we present a general and efficient algorithm to simulate the Lokta-Volterra system using the KvN mapping. The Lotka-Volterra system: a predator-prey model originally developed to understand population dynamics. In plasma physics, predator-prey models such as the Kim-Diamond model are used to investigate shear-flow-driven turbulence. In the absence of external sources or damping, the Kim-Diamond model reduces exactly to the Lokta-Volterra equations. Consequently, this work is a step toward simulating nonlinear plasma physics phenomena on quantum computers.   

Modeling neutral particle dynamics using kinetic discontinuous Galerkin method

In this work, we present a discontinuous Galerkin code to model the Boltzmann equation for neutral particle dynamics. Neutral atoms and molecules are crucial in fusion devices as they determine fueling efficiency and mitigate heat fluxes to the divertor [1, 2]. Kinetic Monte Carlo codes capture the full dynamics but suffer from statistical noise and high computational cost — linked to the number of particles needed — in large devices and/or regimes with large variations in neutral densities, affecting scalability [3, 4]. We propose a continuum kinetic neutral model using the discontinuous Galerkin method, which offers high-order accuracy, parallel scalability and the ability to model complex geometries [5]. This allows our code to overcome noise limitations, improve scalability for large-scale simulations and tackle complex divertor geometries. The poster discusses the theoretical background and test cases showing conservation properties in solving the Boltzmann equation in 1X1V. The method is also validated against analytic theory for ionization and charge exchange in 1X1V. Ongoing work on unstructured grids and utilizing GPUs is discussed.

References

[1] S. Mordijck, Nucl. Fusion 60, 082006 (2020)

[2] P. C. Stangeby, The Plasma Boundary of Magnetic Fusion Devices (2000)

[3] I. Josepch et al, Nucl. Mater. Energy 12, 813-81 (2017)

[4] D. V. Borodin et al, Nucl. Fusion 62 086051 (2022)

[5] J. Hesthaven, Nodal Discontinuous Galerkin Methods (2008)   

Gauge Invariance and Conservation Laws in the Variational Formulation of Macro-Particle Plasma Models

Recently there has been significant interest in formulations of macro-particle models using variational methods. This is attractive because many of the inherent pathologies of traditional PIC methods are avoided. In reducing the Low Lagrangian to a grid for numerical computation it is generally the case that translational and gauge symmetries are lost, leading to the loss of both momentum and charge conservation. Using a macro-particle reduction of the charge distribution function we introduce a method to maintain these conservation laws in a gridded domain. By representing the vector and scalar potentials in different ways, we define charge and current densities which satisfy the discrete continuity equation. The continuity equation leads immediately to momentum conservation. If a convenient choice of representing the vector potential is made it is likely to lead to a corresponding scalar field representation which is unsatisfactory for computational use. Applying the Weyl gauge, however, can eliminate the need to ever perform these computations; knowing that a suitable basis for the scalar potential and thus a definition of the charge density exists is enough to maintain the conservation laws.   

Multi-GPU acceleration of a high-order multi-species Vlasov-Poisson solver

Microphysics plays a critical role in collisionless plasma transport and affects macroscopic properties like resistivity. A kinetic description is required to model these anomalous transport phenomena. The challenge of using kinetic simulations is the computational cost associated with tracking the species distribution functions in phase space. By utilizing GPU accelerated supercomputers the scope of kinetic simulations accessible can be increased to handle realistic proton-electron mass ratios. To achieve a meaningful scaling speedup on GPUs requires reevaluating the core design of kinetic codes. Components such as the Poisson solver and inter-node data communication which accounted for an insignificant fraction of execution time on CPU codes require significant changes to avoid being major bottlenecks for scaling performance in GPU accelerated codes. We present theoretical bounds for how multi-GPU parallel algorithms scale for a multi-species fourth-order accurate finite-volume Vlasov-Poisson solver. These bounds are then used to inform the design and implementation of the VCK-GPU code. VCK-GPU is able to achieve up to 54x speedup as well has increasing simulation throughput by 341x over the CPU code. The new capabilities enabled by GPU acceleration are leveraged to characterize collisionless resistivity induced by the lower hybrid drift instability in pulsed power inertial confinement fusion experiments.   

Adaptive one-step discontinuous Galerkin method for Vlasov systems

We present an adaptive high order one-step discontinuous Galerkin (DG) method for Vlasov-Poisson and relativistic Vlasov-Maxwell systems, which describe single-species and multi-species collisionless plasma. This method is formulated in prediction-correction style, where the prediction is a regionally implicit reconstruction of mixed spacetime derivatives, inspired by Guthrey and Rossmanith (2019), and the correction is an explicit update, following Gassner et al. (2011). Different from conventional DG methods for Vlasov systems, this is an unsplit timestepping scheme with linearized electromagnetic field, which permits an efficient parallel implementation without computationally expensive nonlinear solvers. Further, a mass-conserving, p-adaptive DG discretization is developed for the distribution functions such that the solutions provide more subcell details in regions of interest.

We have implemented this method in distributed-shared memory C++ code, powered by MPI and OpenMP. We present numerical studies such as two stream instability, Landau damping, ion-acoustic shock wave, and Weibel instability.   

Development of a moment-conditioned, asymptotic preserving and mass, momentum and energy-conserving Particle-in-Cell solver for the Vlasov-Ampére system

The coupled Vlasov-Ampére (V-A) set of equations describes the temporal evolution of a collisionless electrostatic plasma, and the accurate solution of these equations is key to the computational study of plasmas. However, the V-A system has a number of properties which are difficult to satisfy discretely including: conservation of  mass, momentum, and energy, preservation of the asymptotic quasi-neutral limit, satisfaction of the Gauss's law, and maintaining positivity of the velocity distribution function (VDF). A numerical scheme which robustly preserves all of the discrete requirements have not yet been developed.

In this work, we discuss a new formulation of the kinetic equation and implement it using PIC. In this formulation, the VDF is transformed through a change of variables and coordinates to separate particle motion from the evolution of the conserved hydrodynamic moments, which are evolved using the fluid moment equations and self-consistently closed from the kinetic solution. In this way, the constraints on the numerical method set above are divorced from the particle kinetics and instead are segregated into the moment equations, where the constraints are easier to satisfy. We will demonstrate the properties and advantages of this formulation on classic electrostatic test cases, including nonlinear Landau damping and a multi-scale ion acoustic shock.   

Development of a Monte Carlo collisions model for simulating laser-matter interaction, magnetically confined plasma, and inertially confined plasma

The TriForce Institute for Multiphysics Modeling is developing a hybrid fluid-kinetic simulation framework for modeling fluids and plasmas in a wide range of environments, such as laser-matter interactions, inertial confinement fusion, and magnetic confinement fusion. In this study, we present a kinetic model for simulating collisional processes within the particle-in-cell framework. Advanced techniques are used such as energy-conserving second-order particle pushing and elastic and inelastic collisions between charged, neutral, and photon macroparticles. After verifying the model through a series of tests, we apply the model to study several different plasma physics problems: (1) hydrogen plasma and closed field line formation in a field-reversed configuration (FRC), (2) the propagation of fusion burn waves in compressed deuterium-tritium and proton-boron, (3) stimulated Raman backscattering to amplify a seed laser pulse in the presence of plasma and a pump field, and (4) power delivery in magnetically-insulated transmission lines. The FRC is found to have unusual stability, in agreement with experiments, and is a candidate for an innovative fusion reactor. The model is then applied to proton-boron problems to identify a burning regime capable of achieving gain. Raman laser amplification in plasma is compared to experiments wherein high efficiencies and significant gain have been achieved. Lastly, we present improvements in high-voltage power flow modeling for pulsed-power science.   

The 1D Vlasov-Poisson System Driven by an Externally Applied Time-dependent Electric Field

The two commonly recognized standard methods of solving the 1D Vlasov-Poisson system, the Jackson and van Kampen-Case methods, have not been formulated to account for an externally applied time-dependent electric field. The Jackson method assumes the integrand decays in the complex lower half-plane at infinity and the contour portion of the Bromwich integral is discarded while the poles are encircled and evaluated as residues. If an externally applied electric field is included in the Jackson method, the Laplace transform of the externally applied field can produce divergences in the lower-half plane at infinity and the method breaks down. It is unclear how the inclusion of an externally applied electric field affects the eigenvalue problem of the van Kampen-Case method, but even if it is incorporated its time evolution would not be immediately clear as the solution would be left as an opaque integral due to the van Kampen continuum. On the other hand, the Cauchy-type integral method recently introduced by the authors is well suited to incorporate an externally applied electric field as the divergences in the lower-half plane are properly accounted for in the Cauchy splitting procedure and the time evolution is explicitly found. We show examples of externally applied electric fields in the form of a Heaviside function, a Gaussian, and a sinusoid, and propose a method for determining the Landau frequency and damping rate for a plasma at equilibrium using an external field and the corresponding plasma response.   

Quantum Computing for Plasma Physics via Second Quantization

Quantum computers promise to transform computational plasma physics via efficient state encoding and the potential of quantum supremacy for certain classes of problems. However, simulating plasma phenomena on quantum computers poses special challenges. The most fundamental equations of plasma physics, the Vlasov-Maxwell equations, are nonlinear and admit many instabilities, but quantum computers can only simulate linear, unitary dynamics. We address this disparity by deriving inherently quantum equations for plasma physics using a second quantization method.

In this poster, I discuss the second quantization of classical plasma physics equations, including the trivial linear two-wave interaction (shown to be equivalent to the finite-dimensional discrete quantum harmonic oscillator [1]), the homogenous quantum three-wave interaction [2] (first described in [3]), and the inhomogeneous quantum three-wave interaction. I numerically show that the unitary, quantum systems I describe limit to the behavior of the classical, nonlinear and potentially unstable equations at high dimensions [4], and I explore the potential advantages of the second quantization method over other methods for linearizing nonlinear equations for quantum computing including Koopman embedding.

[1] M. Q. May and H. Qin, ArXiV preprint (2024).

[2] M. Q. May and H. Qin, Phys. Rev. A 107, 062204 (2023).

[3] Y. Shi, H. Qin, and N. Fisch, Phys. Plamsas 28, 042104 (2021).

[4] M. Q. May and H. Qin, J. Plasma Physics, in press (2024)   

A class of particle-based Hamiltonian reductions for the Vlasov equation--What a PIC code is really solving

The efficacy of the particle-in-cell method as a discretization scheme for the Vlasov equation implicitly relies on the fact that the representation of the phase-space distribution in terms of a weighted sum of delta functions constitutes an exact Hamiltonian reduction of the continuous dynamics. In an effort to reduce statistical noise, some form of filtering is frequently added to smooth out the source terms that are needed for the field solvers (e.g. charge and current density). This generally takes the form of convolution with some kernel function. This work considers, from a general perspective, the incorporation of smoothing into finite-dimensional Hamiltonian reductions of the Vlasov equation and related models and the subtle ways that the continuous dynamics must be modified in order to admit a smoothed-particle finite-dimensional reduction. In particular, smoothed PIC methods generically are not exact reductions of the Vlasov-Poisson or Vlasov-Maxwell equations, but rather approximations to these models in which the Hamiltonian has been regularized at small scales via a smoothing convolution operator. This work demonstrates exactly how such smoothed PIC methods come from regularized continuum theories thus providing an analytical tool to study the impacts of smoothing in particle-based discretizations. Of particular note is the manner in which these smoothed continuum theories may be interpreted as Lie-Poisson Hamiltonian field theories built from the inner product structure of a suitably defined reproducing kernel Hilbert space.   

Non-linear quantum computing for plasma physics

For a specific class of non-linear problems, we propose a quantum simulation process where the non-linear evolution is managed by a two-component Bose-Einstein condensate coupled with the qubit space.

We demonstrate the prowess of the algorithm by applying it into the Hydrodynamic Schrödinger equation¹ and consider its implementation to other non-linear problems pertinent to plasma physics.

¹H. Meng and Y. Yang, Phys. Rev. Research 5, (2023).   

Electromagnetic simulations of Raman amplification and magnetically-insulated transmission lines

Raman Amplification is a novel method of amplifying a laser seed pulse in the presence of a specifically tuned pump pulse and background plasma generated from a laser-heated gas jet. The short-pulse laser amplification occurs in plasma which acts as an optical medium that provides gain but does not suffer from material degradation at high laser energies. The pump pulse energy is coupled to the seed pulse through electron plasma waves, resulting in much higher electric fields and laser intensities in the seed pulse. We report our efforts to simulate these novel amplification techniques in order to better explore the parameter space and find optimal setups for experimental designs. Additionally, we cover work related to simulating magnetically-insulated transmission lines (MITLs) used in high-energy Z-pinch machines, specifically for proving the numerical origin of non-physical electron vortices in the cathode emission region. These non-physical vortices are a product of the emission models used in most plasma software. The use of a smoothed-trigger emission model in addition to methods such as conformal geometry corrections for reducing staircase artifacts are explored to demonstrate the absence of the vortices usually found in simulations of Z-pinch MITLs.   

A complete quasilinear model for the acceleration-driven lower hybrid drift instability and nonlinear Vlasov simulations that test its applicability

Pulsed power inertial confinement fusion experiments are subject to anomalous transport physics, which leads to parasitic currents and undermines predictive modeling. Characterizing anomalous resistivity and heating is challenging because existing theoretical models are not predictive and kinetic simulations are computationally costly. To address this challenge, a complete multi-species quasilinear model is developed for microturbulence driven by the lower hybrid drift instability in a collisionless accelerating magnetized plasma, like that found in pulsed power ExB systems. Unlike many applications of quasilinear theory, the model is complete in that it accounts for resonant and non-resonant interactions, and has self-consistent and numerically-solvable evolution equations for distribution functions, growth rates, and velocity-space diffusion coefficients. These important generalizations are facilitated by performing the analysis in a non-inertial frame of reference and deriving integral-form expressions for the dispersion relation and velocity space diffusion coefficients. The model is solved numerically and validated using fully nonlinear noise-free fourth-order accurate continuum kinetic simulations. The model is shown to capture aspects of nonlinear state conditions, including anomalous resistivity and heating, to within a scale factor of order unity. The results provide much needed vetting of quasilinear theory and set bounds on the theory's applicability.   

Data-driven, psuedo-spectral closures of the moment hierarchy for Vlasov-Poisson turbulence using machine learning

A common approach to studying turbulence in magnetic fusion plasmas is to simulate microscale dynamics along magnetic field lines using gyrokinetic codes.  Several of these codes, such as GENE, CGYRO, and GX, employ pseudo-spectral methods, combining the accuracy of spectral methods with efficiency gained by avoiding convolutions in nonlinear terms in the spectral domain.  One potential avenue to accelerate these codes would be to increase their accuracy when operated at coarse resolution in phase space. To that end, we are exploring the potential to integrate machine-learning models of small-scale dynamics into coarse-resolution simulations.  We implemented a pseudo-spectral Eulerian code to solve the one-dimensional Vlasov-Poisson system on a basis of Fourier modes in configuration space and Hermite polynomials in velocity space.  When cast onto the Hermite basis, the Vlasov equation becomes an infinitely coupled hierarchy of fluid moments, presenting a closure problem.  In the linear limit of the system, we developed a machine-learning closure using a reservoir-computing architecture, leveraging its temporal memory.  When the kinetic Fourier-Hermite code is augmented with the reservoir closure, we report that the closure permits a reduction of the velocity resolution by a factor of ten, with a relative error within two percent for the moment at which the reservoir closes the hierarchy.  In our initial nonlinear studies, we report errors within ten percent for the moments containing the most energy.   

Application of data-driven system identification algorithms to nonlinear MHD simulations of astrophysical accretion flows

First principles models of plasmas lead to high-dimensional nonlinear systems that require complex MHD or kinetic simulations. Projection-based and data-driven modeling algorithms, such as dynamic mode decomposition (DMD) and sparse identification of nonlinear dynamics (SINDy), provide a potentially powerful approach to build low-dimensional reduced models of plasma systems. Recent advances in the application of these data-driven approaches, including the development of the “trapping SINDy” algorithm [1, 2], open the door to models accurate and small enough to be applied to real-time analysis and control. This poster will present progress on benchmarking these algorithms in magnetized plasmas, with a database of MHD simulations, using the NIMROD [3] code, of astrophysical accretion flows, which exhibit multiscale turbulent dynamics through the onset of the magneto-rotational and magneto-curvature instabilities [4]. System identification algorithms will then be tested on the datasets to determine the quality of the models that can be produced using these techniques. Plans to investigate related tools such as resolvent analysis will also be presented.

[1] - Kaptanoglu, Phys. Rev. E 104, 015206 (2021)

[2] - Kaptanoglu, Phys. Rev. Fluids 6, 094401 (2021)

[3] - A H Glasser et al 1999 Plasma Phys. Control. Fusion 41 A747

[4] - Ebrahimi and Pharr, The Astrophysical Journal, 936:145 (2022)   

An open dataset from the Large Plasma Device for machine learning and profile prediction

An open, machine learning (ML)-oriented dataset has been created using the flexible configurability and high repetition rate of the Large Plasma Device (LAPD). Often, plasma-ML work uses closed tokamak datasets that are biased towards the science goals at a particular time. This dataset attempts to minimize that bias and create a balanced dataset via latin-hypercube sampling of machine configuration space. Over 100,000 shots were collected spanning over 30 different LAPD configurations by varying the axial magnetic field profile, gas puff flow, gas puff duration, and discharge voltage. Spatial Langmuir probe data were collected for local density, potential, and electron temperature measurements at several axial locations. Additional diagnostics were also collected, such as Thomson scattering, interferometers, spectrometers, a diamagnetic loop, visible light diodes, and a fast framing camera. Time-averaged signals and -series of ion saturation current (isat) can be accurately estimated using a neural network which performs dramatically better than a linear-like baseline. This model is evaluated immediately after a shot is taken to assist with real-time machine operation. Results from input ablation studies and predication via generative modeling will also be presented. The dataset developed for this study is available and open, which will be useful for pedagogy and testing plasma-oriented ML models. Current work suggests many promising avenues for ML modeling and inference using this dataset.   

Transfer Learning and Ensemble Methods for Analyzing Thomson Scattering Spectra

Thomson scattering diagnostics are powerful methods to obtain measurements of electron temperature (T_e) and electron density (n_e).  Current methods for analyzing Thomson scattering spectra, such as forward-fitting with analytical models, are computationally expensive, hindering real-time T_e and n_e  measurements.  We present a multilayer perceptron to calculate T_e and n_e from Thomson scattering spectra in the non-collective and collective regimes.  The model uses a transfer learning technique by first training a base model with 10,000 synthetic spectra.  Then, some hidden layers are retrained with experimental data.  Our T_e and n_e  measurements are the ensemble average of multiple model outputs.  We take a second approach with a Bayesian neural network, which outputs probability distributions for T_e and n_e.  We use these models to work towards real-time T_e and n_e measurements for high-repetition-rate experiments at the Phoenix Laser Laboratory and the Large Plasma Device.   

Data-Driven Recovery of Hammett-Perkins Closure from Particle Data

Finding fluid approximations for particle behavior is useful for both simplifying simulation complexity and finding interpretable ways to describe behavior. Data-driven model identification is similarly useful as it can identify fluid equations from particle simulations where otherwise the best fluid approximation may be ambiguous. When moving from particle models to fluid approximations there is also a question of what behavior is significant at the spatial and temporal scales of concern. In this work we examine using one such model identification method, WSINDy, to learn the Hammett-Perkins closure from particle simulations of landau damping. We examine how the simulation parameters and test function hyperparameters affect the resultant identified model as well as modifications to the WSINDy algorithm to account for terms that do not fit into the weak form.   

Dust Neural nEtworks Technology (DustNET) for multiscale plasma physics

Ubiquitous in the observable universe, dusty plasmas provide fertile ground to studying and testing multiscale plasma physics, as well as to developing technologies of broad interest, including, for example, microelectronics and carbon-neutral energy. Advances in high-resolution measurement of dust grains [1], neural network algorithms, and a growing number of dusty plasma image datasets in diverse settings motivate Dust Neural nEtworks Technology (DustNET), mirroring the well-known ImageNet, which has been instrumental in advancing computer vision and deep learning research. Here we describe examples of existing datasets and neural network architectures, which form the initial building blocks of DustNET.  Data fusion of experimental data, numerical data, and other synthetic and meta data, such as ones from generative artificial intelligence (AI), offer additional options for DustNET construction. Besides applications in experimental data processing, data interpretation, predictions (inferences) and uncertainty quantification, DustNET-enabled deep neural networks may also be used for real-time experimental dusty plasma controls and optimization [2], and searching for new physics beyond the Standard Model of physics. This work is supported in part by the DoE Fusion Energy Sciences. LANL release number LA-UR- 24-25332.

 

[1] Zhehui Wang, Jiayi Xu, Yao E Kovach, Bradley T Wolfe, Edward Thomas, Hanqi Guo, John E Foster, Han-Wei Shen, Phys. Plasma. 27 (2020) 033703.

 

[2] Zhehui Wang, Shanny Lin, Miles Teng-Levy, Pinghan Chu, Bradley T Wolfe, Chun-Shang Wong, Christopher S Campbell, Xin Yue, Liyuan Zhang, Derek Aberle, Mariana Alvarado Alvarez, David Broughton, Ray T Chen, Baolian Cheng, Feng Chu, Eric R Fossum, Mark A Foster, Chengkun Huang, Velat Kilic, Karl Krushelnick, Wenting Li, Eric Loomis, Thomas Schmidt Jr, Sky K Sjue, Chris Tomkins, Dmitry A Yarotski, Renyuan Zhu, arXiv preprint arXiv:2401.08390 (2024); https://arxiv.org/abs/2401.08390.   

Benchmarking the Qubit Lattice Algorithm against CFD simulations for Classical Nonlinear Physics

The Madelung transformation maps a general kinetic quantum wave function ψ evolution to its moment closures representation, with ψ = √ρ exp[iθ]. ρ is the fluid density and u = ∇θ the fluid velocity. For a simple scalar evolution equation for ψ, quantum vortices arise when the Madelung transformation becomes singular with the density ρ → 0 at the vortex core. Meng & Yang have considered quantum computing of classical fluid dynamics by restricting oneself to incompressible flows. A more general kinetic equation for ψ than the Gross-Pitaevskii BEC evolution was consid- ered so that one could eliminate terms like the quantum pressure from the momentum equation. In particular, Meng & Yang considered a generalized Gross-Pitaevskii equation for a quaternion wave function which included the gradient of a vector spin function. The algorithm is fully unitary. A qubit lattice algorithm (QLA) of this quantum evolution equation naturally introduces a 2-qubit representation. We determine the appropriate unitary collide and stream operators which yield a second order accurate representation of the Meng-Yang fluid equations. In particular we solve the 2D Taylor-Green vortex problem and benchmark our QLA with state of the art CFD computations.

Meng & Yang, Phys. Rev. Res. 5, 033182 (2023)   

Cross-field sheath physics in multi-ion species plasmas

In low-temperature plasmas, transport processes in sheaths and presheaths can be significantly influenced by kinetic instabilities and can also be modified by the presence of external magnetic fields. Previous work has shown that in unmagnetized multi-ion species plasma sheaths, the ion-ion two-stream instability can enhance ion-ion friction due to wave-particle scattering as well as enhance ion heating as indicated by an increase in the energy exchange rate in the second moment of the kinetic equation. In weakly collisional plasmas, these mechanisms may have an important role in cross-field transport where the magnetic field suppresses the electron mobility. In this work, 1D3V particle-in-cell simulations are used to model cross-field multispecies sheaths, and a modified kinetic Bohm criterion is developed to account for the presence of the magnetic field. Using linear response theory, the dielectric response function is modified to account for magnetization and instability-enhanced phenomena are investigated.   

Continuum-kinetic studies of the influence of magnetic fields and wall emission on the plasma sheath.

Plasma-material interactions are phenomena found across a wide range of applications, from numerous fusion reactors to electric propulsion concepts. These interactions are primarily dominated by the structure and behavior of the plasma sheath, which can be modified substantially by particle emissions from the material wall and background magnetic fields. Hence, in order to accurately model the interactions and the sheath dynamics in these applications, the impact of these processes must be carefully considered. In this work, continuum kinetic simulations of magnetized plasma sheaths are conducted using the Gkeyll code, which discretizes and evolves the Vlasov-Maxwell-Fokker-Planck equation using the discontinuous Galerkin method. The influence of different magnetic field strengths and angles on the structure of the sheath is illustrated and benchmarked against existing literature for the physically relevant cases and regimes.   

Leveraging Physics-informed Machine Learning to Develop a Rapid Surrogate Model for Plasma Sheaths

When plasma makes contact with a material, a plasma sheath forms where strong electric potentials moderate particle and heat fluxes from the plasma into the wall. The present study utilizes a physics-informed neural network (PINN) to evaluate a hierarchy of models of plasma sheaths. Unlike traditional deep learning methods, PINNs use the governing PDEs to constrain the predictions of a neural network, and thus do not require any experimental or simulation data to train. As a first application, we utilize a PINN to identify the parametric solution to a fluid model of the plasma sheath. While the offline training time of the PINN is far longer than a traditional solver, once trained, the PINN is able to predict the sheath profiles across a broad range of parameter regimes in a matter of milliseconds, thus yielding an effective surrogate of the plasma sheath. Ongoing work is focused on extending this sheath model to incorporate a fully kinetic ion distribution, using a recently developed Vlasov-Fokker-Planck PINN [1]. This kinetic extension of the sheath model will enable the rapid prediction of the ion distribution function at the plasma-material interface, and thus offer an efficient, but also high physics fidelity description of the plasma sheath.

[1] C.J. McDevitt and X.-Z. Tang, PoP 31, 062701 (2024)   

Measurements of Strong Shock-Front Structure in Multi-Ion Species Plasmas on OMEGA

Plasma shocks are ubiquitous in the context of astrophysical plasmas, such as supernova blast-waves and planetary ionospheres, as well as in laboratory plasmas such as laser-driven plasma experiments and inertial confinement fusion. In most cases, the shock front is considered to be negligibly thin compared to other scale lengths, simplifying modeling using hydrodynamic techniques. Shock fronts, however, feature a finite structure that is established by the kinetic motion of ions and electrons within several ion mean free paths of the discontinuity. We present the results of experiments to measure the structure of strong collisional shock fronts in two-species plasmas. The OMEGA laser was used to drive a strong shock into a gas with a mixture of ion species (H₂ and He), and Thomson-scattering images were collected from a probe beam incident along the axis of the shock. We present analyses of this dataset measuring the multispecies structure of the collisional shock front. Spatial variations in density, temperature, and flow velocity are obtained for the electrons and multiple ion species, demonstrating the dynamics of a shock propagating through multiple ion species. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number DE-NA0004144 and Department of Energy under Award Number DE-SC0020431.   

Experimental Study of Plasma Bubble Dynamics in a Magnetized Plasma Environment

The compact coaxial gun is employed to experimentally study the expansion of magnetized plasma bubbles into a magnetized background plasma, simulating the behavior of extragalactic radio lobes in the interstellar medium. To facilitate detailed examination on a laboratory scale, the high-pressure gas system of the plasma gun has been enhanced for more precise control of bubble expansion. Utilizing a multi-tip Langmuir probe array, a high-speed camera, and a B-dot probe array, we investigate the plasma bubble's evolution as it interacts with the pre-existing magnetized background plasma. Furthermore, modifications to the experimental setup allow for the study of the Hall effect, with experimental data being compared to simulation results.   

3D extended Magnetohydrodynamic simulation for the propagation of coaxial-gun-formed plasmas transverse to external magnetic field

Understanding the dynamics of magnetized plasmas propagating through a background magnetic field or magnetized plasma is important for elucidating the mechanisms underlying the formation and evolution of astrophysical or magnetospheric jets such as coronal mass ejections (CME's). To investigate these phenomena, the Plasma Bubble Expansion Experiment (PBEX) aims to simulate the interaction between high-speed plasma flows and magnetic fields under laboratory conditions. In this study, we conducted a 3D Extended Magnetohydrodynamic (xMHD) simulations to examine the evolution of PBEX coaxial-gun-formed plasmas in a weak external magnetic field (0.05 T) oriented perpendicular to the flow direction. Utilizing the PERSEUS code, which includes the Hall and electron inertial terms in the generalized Ohm's law (GOL), we compared our xMHD simulation results with ideal and resistive magnetohydrodynamic (MHD) simulations, as well as experimental results. This allowed us to assess the significance of the Hall term in shock formation and instabilities.   

Imaging Temporal Variation of Metastable Helium Distribution using Ghost Imaging Absorption Spectroscopy

We are developing ghost imaging absorption spectroscopy by integrating computational ghost imaging (CGI) with plasma absorption spectroscopy. Random structured light is generated using a digital micromirror device (DMD) as a probe beam for absorption spectroscopy. The structured light absorbed by the plasma is focused on a photodiode, and the integrated intensity is measured. In the ghost imaging method, the correlation between the output of this photodiode and the structured light provides an image of the absorber. Obtaining images requires time, as correlations must be calculated using tens of thousands of differently structured light patterns. However, as no camera is used, image measurements can be made with photodiode time resolution for repetitive phenomena. We plan to use this measurement method to visualize the response of atoms to fluctuations in recombining plasmas. This study applies the method to pulsed discharging helicon wave plasmas to image the time variation of metastable helium atom density with high temporal resolution. We will report on the details of this measurement method and the time evolution of metastable atoms in the helicon wave plasma.   

Diagnostics of electromagnetic waves in a spontaneously impulsive plasma jet at ERAU

Plasma diagnostics play a crucial role in understanding the behavior and properties of plasmas. We present the setup and utilization of a magnetic probe array (MPA) and loop antenna to study low-frequency electromagnetic waves in a MHD driven plasma jet experiment. The ERAU plasma jet experiment is spontaneously impulsive, demonstrating plasma instabilities and magnetic reconnection, therefore being reasonably relevant to solar corona. In such conditions, the propagation of electromagnetic waves is an important area of study due to its fundamental role in energy and momentum transport within magnetized plasmas. We look at an experimental setup designed to measure electromagnetic waves within a plasma jet, using diagnostic techniques to capture real-time wave propagation and interactions. The setup utilizes a magnetic probe array comprised of 45 inductors across the x, y, and z axis with a high-precision data acquisition system that records voltage levels from each of the inductors, allowing for detailed spatial and temporal analysis of the wave phenomena. The experimental parameters are selected to cover a range of magnetic field strengths, facilitating a study of magnetic wave behavior in relation to plasma density and temperature profiles. By correlating these measurements with theoretical models, the experiment aims to provide deeper insights into the mechanisms of wave-particles interactions, such as anomalous resistive heating in the context of energy transfer in a magnetic reconnection.   

Spatial Profile Measurements of Te and ne using a Reciprocating Langmuir Probe.

The Hybrid Illinois Device for Research and Applications (HIDRA) has previously been reported to operate with electron temperatures as high as 20 eV and electron densities up to 1.0×10¹⁸ m^-3, and the existence of hollow plasmas and fast electron populations has been noted. For proper usage of the mentioned stellarator to conduct future Plasma Facing Components (PFC) with liquid lithium experiments, the Reciprocating Langmuir Probe (RLP) is used to verify the spatial plasma parameters created in HIDRA. The process is performed using He and H₂ plasmas at 20%, 50%, and 90% of 6 kW magnetron heating (at 2.45 GHz), under pressures of 4.0×10^-5, 3.5×10^-5, and 2.9×10^-5 Torr. As the data analysis has proceeded so far, He plasmas in general and H₂ plasmas at higher power rates have illustrated IV-curves (current vs. voltage curves) with an invisible “knee”, which poses a problem in finding the plasma potential necessary to obtain electron temperatures and densities. Spectroscopy is a second diagnostic used to obtain the plasma’s spatial profile, where its derived electron temperatures and electron densities are used to confirm the measurements made by the RLP. T_e and n_e profiles are critical in understanding plasma behavior and impurity transport. This paper will present the latest temperature and density profile results from HIDRA.   

Development of a Plasma Sensor to Study Multispecies Interactions in Hypervelocity Impact Plasmas

Hypervelocity impacts (HVIs) generate plasmas that are complex and time varying through multiple regimes of expansion.  Understanding the bulk properties thereof will prove critical to the development of theories linking HVIs to the generation of RF radiation, hypothesized to be responsible for spacecraft damage.  Existing models of these expansions ignore the complex interactions between various ion species, meaning that multi-ion instabilities are not well understood.  Such instabilities would lead to the divergence of properties between different plasma species which would present themselves in the energy distribution functions (EDFs) thereof. 

Here, a novel plasma sensor is proposed to study the EDFs by species in an expanding HVI plasma.  The sensor design is based on a transient plasma array (TPA), designed to capture the EDF of all species using a series of electrostatic wells with walls charged to varying voltages to bin particles of specific energy ranges.  The application of a time varying potential and the introduction of a magnetic field are considered to select for particles by energy and mass to probe the plasma temperatures of individual species; ion-ion streaming instabilities, the Weibel instability, and more will leave observable signatures.  The ideal responses of the wells are explored computationally, and suites of wells then designed.  Sensitivity to nonidealities is then explored, and a prototype sensor fabricated.  Testing with thermionically emitted electrons is conducted to calibrate.   

Testing the Validity of Using Langmuir Probe in Non-Stationary Plasma Generated from Hypervelocity Impact

Langmuir probe measurement relies on the understanding of plasma sheath theory. Plasma sheath is a thin region that forms between bulk plasma and a physical boundary such as the probe surface. The behavior of this sheath has only been studied in literature for a limited range of cases, none of which can be applied to hypervelocity impact (HVI) plasma without violating some assumptions. Theoretical studies of plasma sheath are being conducted while relaxing many of the assumptions made in current literature. Preliminary results show that the data interpretation model used for Langmuir probe in many HVI research are indeed not perfectly valid and can cause discrepancies in the final measurement. In this work, 1-D fluid simulation is performed using fluid ions and Boltzmann electrons to study the temporal and long-term behavior of the plasma sheath formed between a bulk, flowing plasma and a current absorbing boundary. Plasma sheath theory for stationary and non-stationary plasma were also derived and compared to the simulation result. This work shows that the stationary plasma sheath theory is not adequate when applied to non-stationary plasma, and further research is needed to justify using Langmuir probe in HVI plasma. This work also shows that transient formation of the plasma sheath is not an issue when it comes to making transient measurements, as sheath formation occurs on the order 10-100 times the inverse of the plasma frequency.   

Real-Time Transverse Flow Velocity Measurement using Optical Vortex

We are developing optical vortex laser absorption spectroscopy (OVLAS), which replaces the probe beam of tunable diode laser absorption spectroscopy (TDLAS) with an optical vortex beam. OVLAS can measure the velocity of metastable atoms across the beam, which is challenging with conventional TDLAS. So far, we have measured the transverse flow velocity of metastable atoms by analyzing the azimuthal Doppler shift distribution on the beam cross-section. Since the measurement requires hundreds of 2D absorption images taken during laser frequency sweeping, the time resolution is limited by data transfer from the camera to a PC. Observing the temporal variation of the velocity distribution is crucial for understanding plasma dynamics, so improving OVLAS's time resolution is important. To reduce data acquisition time, we replace the camera with a quadrant photodiode (QPD). Additionally, the laser frequency is slightly modulated during the sweeping of the probe laser's wavelength, and resulting differentiated signals from the QPD are obtained using lock-in detection synchronized to the frequency modulation. The transverse flow velocity is instantly obtained by analyzing the difference in the zero-crossing frequency of the differentiated signals for each channel of the QPD. The details of the real-time transverse flow velocity measurement will be reported in this presentation.   

Investigation of Changes in Electron Density of a Hydrogen Plasma Over Time

The Caltech plasma jet experiment is a versatile experimental platform which generates plasma jets of various species, which can all undergo an instability cascade that ultimately leads to the emission of hard x-rays measuring 6-7keV in energy, exceeding the average plasma 2eV temperature by a factor of roughly 3000.  An optical diagnostic tool was used to simultaneously image a hydrogen plasma jet generated in the Caltech plasma jet experiment at two different frequencies near the H_β line. The near and far wings of the H_β line were captured using ultra-narrowband filters centered on 485.4nm and 484.7nm,  with pass-bands of 0.4nm and 0.6nm respectively. Preliminary data has demonstrated observable differences in image intensity in these frequency ranges, and previous spectroscopic measurements of Stark broadened H_β profiles indicate the FWHM routinely measures as wide as 1.1nm in the Caltech jet. Images of the H_β profile will be generated and processed to extract the FWHM of the Stark broadening profile of the jet, thereby estimating the local electron density over the lifetime of the jet. These images will also be compared to existing maps of neutral temperature, and a discussion placing the temperature and density changes in the context of the jet energization over time will be offered.   

Performing Argon Ions LIF Experiments under High pressures and High Magnetic fields for Dusty Plasma Relevant Conditions

The operating conditions of rf-capacitively coupled magnetized dusty plasma typically fall within a pressure range of 20-300 mTorr, and magnetic fields up to 3.5 Tesla. Pressures in this range cause dominating ion-neutral collisions, resulting in a depletion of the pumping states’ population density inferring a weak or no fluorescence signals. This effort investigates an argon ion LIF scheme, with an excitation transition of 3d⁴F_7/2 → 4p⁴D⁰_5/2 at a wavelength of 668.613 nm (vac), and fluorescence emission at 442.72 nm. We report preliminary results of LIF experiments in the dusty plasma relevant conditions. Conducting LIF experiments allows for understanding the thermal properties of ions in dust-ion interactions. Additionally, the effects of Zeeman splitting due to the high magnetic fields can be studied rigorously.   

The role of plasma chamber size and fractional ionization in spectroscopically diagnosing Argon plasmas

Optical emission spectroscopy shows promise as a non-perturbative plasma diagnostic, but the use of spectroscopy requires an understanding of the atomic physics within the plasma. Argon plasmas are widely used in low temperature plasma physics applications. This research explores the role of the chamber size and hence plasma dimensions along with the fractional ionization and their impact on the atomic physics of low temperature Argon plasmas due to the presence of metastable states. Results will be shown for the following: a torsatron with a toroidal vacuum chamber diameter of 30 cm, an electron density ranging from 1 x 10¹⁰ to 1 x 10¹² cm^-3, and a pressure of 5 x 10^-5 mTorr; a linear device with a diameter of 10 cm and length of approximately 1.2 m, an electron density ranging from 1 x 10⁸ to 1 x 10¹⁰ cm^-3 and a pressure of 0.5 mTorr; and an octagonal chamber with a height of 6 cm, an electron density ranging from 1 x 10⁸ to 1 x 10¹⁰ cm^-3, and a pressure of 20 mTorr. All plasmas have an electron temperature ranging from 1 to 10 eV. Results show that a smaller chamber results in shorter metastable lifetime, which in turn results in the need for a time dependent approach to atomic modeling.   

Progress and prospect of ion flow velocity measurement using optical vortex laser-induced fluorescence method

The ion flow measurement using the laser-induced fluorescence (LIF) method is based on the Doppler effect that modifies the absorption condition. Since the Doppler shift is given by the dot product of the phase gradient and the flow velocity, when a plane-wave-like beam with a phase gradient only in the direction of propagation is used, the measurable velocity is limited to the component projected on the beam propagation direction. This one-dimensionality can be overcome by using optical vortex beams with the azimuthal phase gradient; the Doppler shift now has two main terms, i.e., the translational Doppler shift and the azimuthal one. Optical vortex laser absorption spectroscopy has successfully used this azimuthal Doppler shift to measure the flow of neutral particles crossing the beam perpendicularly [1]. In the optical vortex LIF method, the shift of the LIF spectrum due to the azimuthal Doppler shift can be observed by asymmetrizing the intensity distribution of the optical vortex beam [2]. Progress to date and future developments of the optical vortex LIF method will be discussed in the poster.

[1] H. Minagawa et al., Sci. Rep. 13, 15400 (2023).

[2] K. Terasaka et al., Sci. Rep. 14, 2005 (2024).   

Electronic structure analysis and radiative properties of H2, H2+, CH, and OH species

High-level gas-phase electronic structure analysis are indispensable for gaining useful insight on the chemical and physical processes that occur in gas plasmas. However, the plasma models and the predictions are highly dependent on the input data. Hence the utilization of ab initio tools for data generation and property prediction is preferred. In the present work, we have performed highly accurate theoretical analysis for H₂, CH, OH, and their ions. Specifically, using multireference methods we have studied the reactions of ground and excited atomic or ionic fragments to investigate their formation. We analysed their full potential energy profiles and report a series of energy related properties and radiative properties. Furthermore, we are interested in extending our approaches to other diatomic molecules of relevance to the plasma physics community.   

Electron collisions with atoms and molecules for plasma-modeling applications

We present a summary of cross sections for electron scattering on atoms and molecules of interest in fusion and astrophysical plasma-modeling applications, calculated using the atomic and   molecular convergent close-coupling (CCC) methods. Accurate collisional-radiative modeling requires the input of cross sections for numerous processes, including elastic scattering, ionization, and excitation, considering a large number of different initial and final states. For molecules, resolution in the vibrational and rotational levels is often required. 

Over the last few decades, the CCC method has been established as one of the world's most accurate techniques for calculating collision cross sections, with its particular strength in being able to solve the scattering equations over the entire incident energy range for most processes of practical interest. The application to scattering on molecules with a focus on calculating rovibrationally-resolved cross sections has led to the largest set of collision data ever produced for any scattering system.

In this poster we showcase results for electron collisions with atomic and molecular hydrogen, as well as the H₂⁺, HeH⁺, and LiH molecules, with examples of applications in fusion and astrophysical plasma models. Comparisons are made with previous data, wherever available. However, the majority of data we have produced is the first of its kind.   

Dust Transport in a Low Temperature Plasma

Our efforts have been the realization of controlled, “boundary free” transport of plasma embedded dust (microparticles) across a horizontal surface using plasma modulations. Our work expands upon previous studies [1] which used a spatially propagating pattern of low frequency signals applied to consecutive segments of a rectangular striped electrode. Different than in the cited work, we introduced a cylindrically symmetric segmented electrode package, allowing for a continuous, boundary-free flow pattern. Using our dedicated plasma chamber as a test bed, we have iterated on a variety of electrode designs to understand and optimize the manipulation of the floating dust. We also continue our exploration of integrating the RF plasma generation with the transport manipulation system. Ultimately, we anticipate utilizing this new dust transport tool to study dust flow/shear flow kinetics on their “atomic” scale, as well as its interaction with the environment (including in the presence of an external magnetic field). Traveling plasma modulations, such as studied here, further bear a huge potential in applications: especially where dust mitigation is required such as semiconductor production or for human exploration of the moon and beyond.

[1] Li, Y. F., Jiang, K., Thomas, H. M., & Morfill, G. E. (2010, June). Dust removal in radio‐frequency plasmas by a traveling potential modulation. In AIP Conference Proceedings (Vol. 1242, No. 1, pp. 109-116). American Institute of Physics.   

Real-Time Structure Detection of Complex Plasmas Using Direct Pixel-Based Analysis

Complex (dusty) plasmas are comprised of charged macroscopic dust particles (nanometer to micrometer size) along with the background plasma components: electrons, ions, and neutral atoms. In these systems, the dust manifests various collective behaviors, including the formation of material-like phases. These phases can exhibit gaseous, fluidic (isotropic and anisotropic/hexatic), and crystal-like based on the ratio of electrostatic and kinetic energies. Complex plasmas provide a unique macroscopic, dynamic, experimental study of materials science through a soft-matter analogy.

Established characterization workflows employ spatial correlation functions, particle dynamics, and rotational invariance maps to identify different structural phases and characterize the crystal structure melting. This work employes those methods on a pixel basis for a direct image analysis, offering potential real-time analysis and improved experimental control. We present a Python library workflow, extending these metrics from 2D to 3D. The analysis framework is applied to provide preliminary structural insights from the PK4 experiment aboard the ISS.

Here we will present an update on the current analysis results, progress towards developing this system as an experimental companion/tool, and steps toward achieving real-time experimental data control.   

A STEREO View of the Distribution of Dust Near 1-AU

The first orbit of STEREO WAVES (S/WAVES) data are analyzed to provide an initial 2D map of the distribution of dust (~10-nm ≤ d ≤ 0.30-µm) in the near-ecliptic plane in the vicinity of 1-AU. Striking differences exist in the dust detections of the respective STEREO spacecraft that suggest either the presence of spacecraft-dependent observational sensitivities or the presence of heliospheric radial- and/or latitudinal-dependent structures in the near-ecliptic distribution. Amid these observational differences, high spatial correlation exists between the dust detections of STEREO-A and known heliophysics spacecraft anomalies. Surprisingly, during extended periods of S/WAVES nano-dust detection, S/WAVES data indicate the presence of fine-scale spatial-temporal structure that cannot be explained by either gravitational resonance trapping or solar wind-driven Lorentz forces arguments. This fine-scale spatial-temporal structure may lend support to the theory that dust populations near 1-AU are dominated by cometary debris.   

Dust pattern formation in magnetized low temperature plasmas

Experiments with the Magnetized Dusty Plasma eXperiment (MDPX) revealed an intriguing phenomenon known as "imposed, ordered structures" in dusty plasmas where electrons and ions are strongly magnetized (B > 1 T) (T. Hall et al., Phys. Plasmas 25, 103702, 2018). Micron-sized dust particles formed ordered structures with a 4-fold symmetry, aligning with the geometry of a square gridded conducting mesh placed above them. Fluid simulations suggest that elongated electric potential structures from the mesh may trap the dust particles (M. Menati et al., Plasma Sources Sci. Tech. 29, 085015, 2020). This differs from the 6-fold symmetry typically observed in two-dimensional dusty plasma crystals. The objective of this work is to study and quantify the transition of dust particles from a 6-fold symmetry (self-ordering) to a 4-fold symmetry (imposed-ordering) as the magnetic field increases. Preliminary results are presented from experimental studies of magnetized dusty plasmas under a mesh in the MDPX. Results from initial numerical studies of magnetized plasmas using fluid simulations will be presented, highlighting the characterization of electric potential structures formed under a mesh.   

Design and characterization of an RF plasma device for single-grain optical trapping

Complex plasmas, or plasmas with charged microparticles, are ubiquitous in astrophysical and industrial environments. They are often byproducts of semiconductor fabrication and material processing but also provide insight into fundamental physics, such as energy transport and phase transitions. However, diagnosing dusty plasma with conventional probes can be very challenging as the insertion of probes can perturb the dust. Furthermore, all ground-based laboratory experiments are restricted by gravity, which confines all dust to the plasma sheath. A means to overcome this is optically trapping dust grains, which can then be transported to any location, allowing the study of dust grain-plasma interactions outside the sheath region. This project involves the design and characterization of a rf-generated, capacitively-coupled plasma device that can be used for single-grain dust trapping and other laser-based diagnostics. This presentation will focus on the design of the plasma source, its initial characterization, and the features that make it suitable for the study of optically trapped dust grains.   

Overview of current research activities in the Magnetized Plasma Research Lab (MPRL): a collaborative research facility at Auburn University

The Magnetized Plasma Research Laboratory (MPRL) at Auburn University explores fundamental plasma and complex/dusty plasma phenomena covering a large parameter regime from unmagnetized plasmas to strongly magnetized plasmas with a mission to serve as an open access, multi-user collaborative research facility. The centerpiece of the laboratory is the Magnetized Dusty Plasma Experiment (MDPX), a highly flexible plasma device with excellent diagnostic access to study the unique regime of high magnetic fields (up to 4 T), at relatively low density (~ 10¹⁴ – 10¹⁶ m^-3) and low electron (Te < 5 eV) and ion temperature (Ti < 0.05 eV). Other instruments include ALEXIS, a linear plasma device for simulating space plasma and basic plasma experiments, and a wide variety of “tabletop” scale unmagnetized, low temperature plasma devices. Here, we will give an overview of recent studies from MPRL such as pattern formation of filamentary structures at high magnetic fields, nanoparticle growth in plasmas, laser trapping of dust particles, studies of dust clusters and dust thermodynamics, using dust as a diagnostic, controlling dust charging and dynamics by externally applied UV light, studies of dust acoustic waves at high magnetic field, development of new laser-based plasma diagnostics etc.   

The Effect of Dust Clouds on the Transmission of Electrostatic Perturbations Across Weakly Magnetized and Unmagnetized Plasmas

The presence and role of charged dust in plasmas in the space environment, such as dust structures in the tails of comets or flowing magnetized dust clouds that can affect the interplanetary magnetic field are well known yet hard to investigate. Scaled experiments allow the opportunity to investigate the physics of these exotic systems as well as provide guidance to the development of future spacecraft missions. These experiments are being pursued to investigate the impact of charged dust clouds on the background plasma in the presence of driven low frequency (f ~ 10-1500 Hz < fci) electrostatic perturbations in unmagnetized and weakly magnetized plasmas using the Magnetized Dusty Plasma eXperiment (MDPX) at Auburn University. This presentation will show preliminary results on the effects dust clouds have on the transmission of electrostatic perturbations in both unmagnetized and weakly magnetized plasmas. This is accomplished using a single tip Langmuir probe to drive low frequency perturbations while single and triple tip probes detect the fluctuations in plasma. This experiment has been conducted with various driven frequencies, magnetic fields and spatial separations between the transmitting and receiving probes.   

Investigating the cycle time of dusty plasma nanoparticles' growth during the presence of magnetic fields

Nanoparticles (NPs) spontaneously nucleate from reactive gas(es) in low temperature plasmas. They levitate in the bulk plasma due to a balance of several forces such as electric, ion drag, neutral drag, thermophoretic and gravitational. These forces depend on the radii of the NPs; thus, when they grow to > 100 nm, ion drag, and gravitational force supersede the others which cause the NPs to transport away from the central plasma and eventually escape the plasma. The departure of the particles from the plasma enables a new generation of nanoparticles to form.  This leads to a periodic process known as a “growth cycle”. In this study, NPs are grown in a capacitively coupled radiofrequency plasma. The presence of magnetic fields leads to faster cycle times. These observations will be accompanied by measurements of the plasma potential and multiphysics simulations of the background plasma.   

Transition to ion magnetization as a threshold for plasma and dusty plasma structure formation in the Magnetized Dusty Plasma Experiment (MDPX) device

The Magnetized Dusty Plasma Experiment (MDPX) device investigates capacitively coupled, rf-generated, low temperature plasma and dusty plasma configurations for magnetic fields, B > 1 Tesla. The wide variety of phenomena observed in the MDPX experiments include melting of plasma crystals in strong magnetic fields, the formation of imposed, ordered dust structures, and the formation of field-aligned plasma filamentary structures. In all these cases, the transition in the behavior of both the background plasma and the dusty plasma is observed as the ions become magnetized. Here, "ion magnetization" is defined as the condition where ions can complete one or more gyro-orbits without undergoing a collision with a neutral atom. It remains unclear how this change in the ion dynamics, which occurs at microscopic spatial scales (~10's of micrometers) influences the self-organization of the macroscopic structures (~mm to cm) that are seen in the experiments. This presentation will discuss evidence of the ion magnetization threshold for the various phenomena described above and present preliminary numerical studies that seek to gain insights into the connection between the microscale dynamics and macroscopic observations.   

Interaction of a Strongly Coupled Dusty Plasma medium with Non-Magnetic and Magnetic Obstacles

In laboratory settings, charged dust particles within plasma environments give rise to a variety of intriguing static and dynamic phenomena, including the formation of plasma crystals, shocks, voids, and vortices. By employing micron-sized dust particles, we can delve into these structures at a kinetic level and explore the diverse dusty plasma forces at play during their formation. 

Our experiment focuses on investigating strongly coupled dusty plasma featuring cylindrical and spherical obstacles immersed within it. Around this obstacle, a dust void emerges, where the outward electric field force balances with the inward ion drag force at its boundary. This void serves as an obstruction to a dusty plasma flow traveling at velocities ranging approximately from 3 to 10 cms^-1. Using a MATLAB-based open-access Particle Image Velocimetry (PIV) code, we analyze the dynamics of individual dust particles surrounding the obstacle. Complementing our experimental work, molecular dynamics simulations corroborate our observations. Furthermore, we explore the behavior of dust particles around a magnetic spherical obstacle, yielding preliminary but intriguing results that we will present.   

Langmuir Probes and Dust Particle Trajectories: Characterizing a Complex Plasma Employing Minimally Perturbative Methods

The Langmuir probe has been used for a century to diagnose plasma parameters such as electron temperature and electron density. The ability of the Langmuir probe to calculate the plasma potential at specific points enables mapping of the electric field within an experimental plasma environment. Orbital Motion Limited (OML) theory is a variation of the theory of collectors in gaseous discharges derived for instruments like the Langmuir probe. OML then applies to the dielectric particles used in usual dusty plasma experiments. As such the particles themselves also map the field-forces in a plasma environment and can also be used to diagnose other plasma parameters in the same manner as a Langmuir probe in a minimally perturbative manner when compared to the disturbances created by use of the Langmuir probe. This presentation will examine the results of an experiment comparing the potential maps derived from the Langmuir probe and from a dust drop technique.   

The demonstration and classification of photo-discharging silica particles in complex plasmas

The study of complex plasmas is critical to our understanding of how contamination by dust affects the background conditions and processing techniques of practical plasmas. Studies into complex plasmas are limited due to the direct coupling of contaminating microparticle properties, such as charge, to the surrounding plasma conditions, which makes the control of these charged microparticles difficult. Previous studies at the Auburn Magnetized Plasma Research Laboratory (MPRL) have demonstrated that charged microparticles of lanthanum hexaboride (LaB6) can be photo-discharged by a near-ultraviolet (NUV) source, with minimal perturbation of the surrounding plasma. Preliminary tests indicated that silica microparticles, a common contaminant for practical plasmas, would exhibit a similar response to the NUV source as well. Probe measurements and video footage of variably sized silica microspheres exposed to NUV pulses while suspended in an RF argon plasma were collected and are presented. Langmuir probe data and video footage is analyzed to determine the effect of the NUV source on the equilibrium charge of the silica particles. The prospect of a reproducible technique using an NUV source to photo-discharge silica, and other contaminating microparticles, is discussed.   

Comparison of dynamic behavior of LaB6 and SiO2 dusty plasmas in response to photo-discharging

Experiments with dust particles composed of either lanthanum hexaboride (LaB6) or silica (SiO2) have demonstrated that both respond when exposed to high-intensity near-ultraviolet (NUV) light in a manner consistent with photo-discharging. While some particles exhibit reproducible periodic motion in response to the NUV exposure, others exhibit more chaotic behavior. Computational investigation into the response of the dust suggests that particle geometry may play a role in determining whether particles trend towards a periodic or chaotic response. To test this hypothesis, different samples of LaB6 and SiO2 dust are suspended in low temperature plasmas and exposed to a pulsing NUV source. Video data is used to analyze the motion in phase space of several isolated particles and determine whether SiO2 microspheres exhibit a more consistently periodic response than irregularly shaped LaB6 particles.   

Learning Lagrangian dynamics to determine the anisotropic dust-dust interaction potential in a microgravity dusty plasma

Arrangements of filamentary dust structures have been observed to form under external electric fields in the microgravity dusty plasma generated in the Plasmakristall-4 (PK-4) facility on board the International Space Station. Of particular interest is the investigation of the form of the anisotropic dust-dust interaction potential, which can reveal whether the observed filamentary structures are reminiscent of an electrorheological or a liquid crystal state. We present an analysis of dust particle dynamics with the novel statistical learning algorithm, the Sparse Identification of Nonlinear Dynamics (SINDy) [1], which exploits sparse regression to deduce parsimonious and interpretable models from data. To improve SINDy’s robustness to noisy data, recent developments use SINDy to deduce the form of the Lagrangian from data. We test the “extended Lagrangian” SINDy (xL-SINDy) method [2] on a simulated two-dust-particle system which interacts via a pure Yukawa potential, and discuss different results generated from training data with different magnitudes of Gaussian-distributed random noise added. Perspectives on the application of this method to experimental PK-4 data are presented and preliminary results are shown.

[1] Brunton, S.L., Proctor, J.L. and Kutz, J.N. (2016). Proc. Natl Acad. Sci. 113 3932–7. https://doi.org/10.1073/pnas.1517384113

[2] Purnomo, A. and Hayashibe, M. (2023). Sci. Reports 19 7919. https://doi.org/10.1038/s41598-023-34931-0   

Linear and nonlinear theory of resonant drag instabilities in laboratory and astrophysical dusty plasmas

Resonant drag instabilities (RDIs) are produced by the interaction of streaming dust with waves in a background fluid when the dust velocity exceeds the phase velocity of a wave in the fluid. They have been proposed to drive pattern formation and turbulence in various astrophysical environments. We present insights connecting RDIs to other dusty plasma instabilities, and results characterizing the nonlinear saturation of RDIs. The linear RDI has been described by splitting of eigenvalues of the linearized dust/fluid system. Here, this framework is applied to other dusty plasma instabilities in experiment and astrophysics. This common mechanism motivates comparison of regimes of relevance, finding substantial overlap and suggesting laboratory study of astrophysical phenomena. RDIs saturate nonlinearly in an anisotropic turbulent state. Simulation of the acoustic RDI is used to elucidate the character of this turbulence. The saturation process is treated in terms of a balance between instability growth and turbulent turnover, resulting in a saturated state driven at an outer forcing range.   

Numerical study of anisotropic potential distribution near dust chains in the Plasmakristall-4 dusty plasma

Dust particles in the Plasmakristall-4 (PK-4) facility on board the International Space Station have been found to form extended filaments in the presence of an external polarity-switched DC electric field. These filaments are aligned in the direction of the externally applied electric field. In the direction parallel to the electric field, the dust particles are strongly coupled and behave as a crystalline-like structure. On the other hand, there is a weaker coupling among particles in the direction across filaments (i.e. perpendicular to the electric field). Due to the presence of dust particles, the charge distribution in a plasma is modified, as well as the spatial distribution of the potential. This gives rise to regions of positive and negative potential which depend on the interparticle distance, plasma pressure, dust and ion densities and other properties. The anisotropic potential distribution observed in these dusty plasmas can be compared to numerical simulations to understand anisotropies in the interparticle interaction potentials in these systems. In this project we aim to obtain a model of the electric potential distribution which captures the effects of the anisotropic ion wakes derived from an N-body numerical simulation of the dust and ions for time-averaged and time-varying plasma conditions.   

BECAA: A route to systematic production of true two-dimensional dust clouds and clusters in laboratory complex plasmas

Two-dimensional (2D) dust monolayers are ideal systems for developing and testing thermodynamic models and studying the statistical behavior of complex/dusty plasmas. However, creating a perfect 2D dust monolayer or eliminating unwanted off-plane dust grains inside an active complex plasma system is extremely challenging. There are some methods to manipulate and control the charged dust grains via electromagnetic fields, varying discharge conditions, controlling confinement boundaries, UV irradiation, etc.; however, these techniques affect the plasma parameters and disturb the dynamic equilibrium of dust grains. We present BECAA (Bidirectional Electrode Control Arm Assembly) to precisely manipulate and control the density and dimensionality of a dust cloud levitated above the RF-powered electrode in complex plasmas [3]. The developed method is independent of any discharge or plasma parameters and does not alter the dust dynamic equilibrium. BECAA can produce perfect 2D dust monolayers by eliminating off-plane particles through a lowered confinement region induced by a tilt from the electrode control arms. The method is stable and precise enough to eliminate dust particles one by one, creating the desired N-clusters, which were only possible by multiple trials until now.   

Ice grain formation from negative ions in a cryogenically-cooled laboratory plasma

Grains of ice form spontaneously when water vapor is injected into a weakly-ionized laboratory plasma with cryogenic background gas. The grains quickly grow in elongated fractal patterns, reaching tens or hundreds of microns in length [1]. The conditions for nucleation are consistent with a model for ion-catalyzed homogenous nucleation, in which anions such as OH-, created by dissociation of water vapor, attract clusters of water molecules to form stable nuclei around which grains can form [2].  The formation of clusters of water around negative ions in the plasma are measured using mass spectrometry. It is planned to compare this to observations of ice formation.

[1] Nicolov, A., Gudipati, M. S., & Bellan, P. M. (2024). Phase and morphology of water-ice grains formed in a cryogenic laboratory plasma. The Astrophysical Journal, 966(1), 66.

[2] Bellan, P. M. (2022). Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma. The Astrophysical Journal, 936(1), 52.   

Torsions effect on energy transfer in crystal lattice.

Vertical pairs of spinning dust particles, also known as torsions, are an area of active interest in dusty plasma physics. Torsions have been shown to propagate energy into the nearby surroundings of the plasma crystal structure. This presentation delves into how a torsion and energy perturbation from a laser pulse interact within the crystal lattice. Some of the energy from the laser will be transmitted past the torsion and into the surrounding structure. Some of the energy will be reflected in the direction of the laser. Some of the energy will be absorbed by the torsion pair and alter its motion. We will discuss some of the techniques used to calculate the transmission, reflection, and absorption coefficients in the system.   

Investigation of a sudden compression in a dusty plasma with increasing magnetic field

Recent experiments in the Magnetized Dusty Plasma eXperiment (MDPX) in the Magnetized Plasma Research Laboratory (MPRL) at Auburn University studying a dusty plasma in the presence of a magnetic field revealed an unexpected example of self-organization and critical behavior.  In these studies, it was observed that the dust cloud underwent a gentle compression as the magnetic field strength increased.  However, at a critical value of the magnetic field (B ∼ 0.06 T), the dust cloud underwent a sudden compression.  At the observed magnetic field strength, electrons are strongly magnetized with a Hall parameter of ∼50 and the ions are unmagnetized with a Hall parameter of ∼0.08, indicating that the electron magnetization appears to play a dominant role in modifying the plasma behavior which, in turn, significantly impacts the dusty plasma.  This presentation will focus on initial measurements aimed at beginning to understand the cause of this sudden compression.   

Viewing and Metrology measurements using the prototype In-Vessel Viewing System on a full-scale ITER Divertor IVT and damaged tungsten monoblocks

The In-Vessel Viewing and Metrology System (IVVS) is a diagnostic tool for the ITER project, designed to inspect plasma-facing surfaces within the tokamak. Following the construction and validation of a full-scale prototype of the measurement probe compatible with the final harsh environment against generic test targets in 2023, recent advancements have showcased its application on real, damaged ITER tungsten monoblock targets, including measurements on a full-scale divertor inner-vertical target (IVT). These tests have successfully validated performance of the IVVS at ITER-relevant viewing angles, distances, and focal parameters, over ITER-specific geometries, and at full cable/fiber length, confirming its suitability for in-vessel use.

The system clearly detected geometrical features such as tile gaps & edges, with the metrology channel providing precise range measurements with precision on a planar areas of 0.1 mm from 0.5-10m and angles from 0-60 degrees. Over the full IVT geometry dimensional errors are <1 mm. Viewing measurements provided high contrast images with spatial resolution limited by sampling density. The viewing channel was able to identify damage caused by heat treatment, even when the surface changes were sub-10 micron in dimension.

The metrology performance was further tested against a set of damaged tungsten monoblocks, ranging from surface roughening with no volumetric change to full melting and reshaping of the tungsten. Accurate assessment of the melting/erosion/deposition was demonstrated for damage meeting certain conditions. These findings further validate the IVVS's suitability to monitoring the condition of the ITER first wall.   

Simulating the Tritium Emission Line in a Deuterium Spectrum for Diagnostic Residual Gas Analyzer Detectability

The ITER Diagnostic Residual Gas Analyzer (DRGA) is a system that utilizes mass and optical spectroscopy to determine the neutral gas isotopic composition to a high precision [1]. This capability makes the DRGA an integral part of the fusion fuel cycle [2]. The deuterium and tritium compositions are partly determined by Optical Gas Analysis (OGA), which employs a cold plasma source to produce the optical spectra. To quantify the level of detectability and uncertainty of the tritium concentration measured by OGA, we tested two cold plasma emission sources, a Penning- and inverse magnetron- type. Due to its low availability and radioactivity, tritium was not used for lab testing. Utilizing emission spectra from known mixtures of hydrogen, deuterium, and helium, we simulated a tritium line in the spectra to determine its detectability.

[1] S. Vartanian et. al., Fusion Engineering and Design, DOI: 10.1016/j.fusengdes.2021.112511

[2] C.C. Klepper et. al., IEEE-TPS, DOI: 10.1109/TPS.2022.3223648

This work was supported under the U.S. D.O.E. contract DE-AC05-00OR22725.   

From photon-counting measurements to radiated power density estimates at WEST

Multi-energy soft and hard x-ray pinhole cameras have been designed, built and operated at the WEST tokamak to measure the plasma emission in multiple energy ranges and infer profiles of impurity densities, electron temperatures, as well as non-Maxwellian tails, including runaway electrons. This novel imaging diagnostic technique combines the best features from both PHA and multi-foil methods employing a PILATUS3 x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each sensor of their 10$^5$ to 10$^7$ pixelated systems. Described in this presentation is a new methodology to estimate also the local radiated power density profiles from photon-counting measurements in several energy bands, which could become particularly useful for machines cladded with metal plasma facing components (PFCs). This novel application is possible since the pixel responsivity is well characterized by a complementary error function while the multi-energy measurements between adjacent energy-levels is described well by a Probability Density Function of a Gaussian distribution. This methodology is being develop at the WEST tokamak for long-pulse-plasmas up to 360 s aiming for a real-time measurement capability up to 1000s for the 2024-C10 campaign.   

First Look at Plasma Operation Data from 2D Fiber Optic Bolometer Array

A 2D fiber-optic bolometer (FOB) array has been developed, and initial results demonstrated in DIII-D as a plasma radiated power diagnostic for use in challenging electromagnetic (EM) noise situations. A prototype single-channel system, which was previously installed in DIII-D, showed that the FOB is comparable to resistive bolometer performance and avoids EM interference by using a Fabry-Pérot resonator system to encode small temperature changes related to the incoming power. The 2D FOB array was installed with a tangential view into the DIII-D Tokamak. The new array consists of 8-by-8 sensors viewing near X-point and divertor regions. Several channels were damaged during the installation and baking, leading to improvements in design and procedures for future use. Surviving channels successfully measured baking temperatures of up to 300°C at the sensors. Preliminary data from plasma operations showed an unexpected increase in the baseline measurements even after plasmas have ended. This posed a problem of distinguishing the measurements of the plasma radiation to the increased background temperature. Different methods were tested to overcome the problem and showed similar trends of measurements compared to resistive bolometers that were viewing the similar regions. Initial demonstration of a 2D FOB array has led to several important improvements and continues to show promise for application in fusion plasma.   

Development of Synthetic Diagnostics for NSTX-U Resistive Bolometry

Accurate assessment of power balance in fusion devices, including NSTX-U, requires precise knowledge of the radiated power emitted by the core and edge plasma. Measurements of radiated power emission can be analyzed in conjunction with other diagnostics such as IR thermography to assess the discharge power balance. A resistive bolometry system has been proposed for use in NSTX-U for core and divertor radiated power measurements. The aim of the proposed divertor bolometer system is to characterize plasma emission in the vicinity of the X-point region and divertor strike points through the use of tomographic inversions from two crossing views. A consideration for bolometer design is to ensure that each of the foils has minimal overlap to avoid overestimating the plasma power output. We have established an NSTX-U bolometer base design and begun work on the synthetic diagnostics for the bolometer for both the midplane and lower divertor locations. This presentation will describe our current activities using Cherab to simulate the bolometer cameras and to optimize these systems to ensure adequate spatial resolution for accurate recreation of high spatial gradient emission patterns.

Identification and Reconstruction of Cut-off Regions in Electron Cyclotron Emission (ECE) Measurements

In the evolving landscape of fusion research, precise measurement and analysis of Electron Cyclotron Emission (ECE) play a pivotal role such as Alfven eigenmode detection [1]. However, the integrity of these measurements is often compromised by cut-off regions [2], posing significant challenges in data interpretation and fusion plasma diagnostics. This study introduces a novel approach utilizing Generative Adversarial Networks (GANs) to identify and reconstruct these cut-off regions in ECE measurements.

 

Our methodology leverages anomaly detection to find the cutoff channels from the dataset and uses the advanced capabilities of GANs to generate realistic, physics-consistent reconstructions of the missing or distorted data in ECE measurements. The adversarial nature of GANs aids in refining the reconstruction process, ensuring high fidelity to the underlying physical processes of fusion plasma. We demonstrate the effectiveness of our approach through extensive simulations and comparison with traditional methods. Our results show a marked improvement in accuracy and reliability, substantially enhancing the quality of ECE data interpretation.

 

This research not only addresses a critical challenge in fusion diagnostics but also opens avenues for applying advanced machine-learning techniques in the field of plasma physics. The successful application of GANs as proposed in this study promises to enhance our understanding of fusion plasma behavior, contributing significantly to the advancement of fusion energy research.   

Comparison of electron cyclotron emission models for application of ECE diagnostics on Alcator C‑Mod, SPARC, and nT-Tao devices.

Electron cyclotron emission (ECE) radiometry is a well-established diagnostic technique capable of providing high spectral and temporal resolution measurements of electron temperature profiles in toroidally confined plasmas. Modeling electron cyclotron emission and absorption is important for understanding ECE diagnostic measurement localization and resolution. This poster will compare two different formulations for ECE. The starting point is ECESIM tool [1], which uses a Vlasov-Maxwell coupled system and power transfer for calculation of optical constants [2]. This established framework is compared with a further enhanced model, which includes Coulomb collisions, wave diffusion effects in the kinetic equation and generalized electron distribution functions [3]. Calculations are compared with experimental ECE measurements from Alcator C-Mod. In addition, predictions of ECE emission and absorption are performed for two developing devices: SPARC and nT-Tao. The extension to non-thermal electrons is particularly valuable for interpretation of ECE measurements during anomalous scenarios, such as heating or instabilities, and presents a potential for the investigation of the ECE-Thomson discrepancy. 

[1] M.E. Austin, U. Texas FRC Report #534

[2] M. Bornatici et al 1983 Nucl. Fusion 23 1153 

[3] G. Giruzzi 1988 Nucl. Fusion 28 1413 

Research sponsored by DOE contract number DE-SC0014264 and by sponsored research agreements with companies nT-Tao and CFS.   

Thermonuclear Neutron Emission Profiles of Sheared-Flow Stabilized Z-pinch Plasmas in the Fusion Z-Pinch Experiment

Since its inception in the late 60’s, great progress has been made on the flow Z-pinch concept for a fusion reactor, and the most success in obtaining fusion-relevant plasmas has come from approaches using sheared-flow stabilization, such as in the Fusion Z-Pinch Experiment (FuZE). Recently, FuZE used Thomson scattering measurements to confirm electron temperatures of 1-3 keV, and from ion Doppler spectroscopy measurements, T_e ≤ T_i, demonstrate thermal equilibrium. These plasma conditions are worthy of thermonuclear neutron emissions, and indeed, neutrons have been observed to emit from the plasma at a rate of 3x10⁷ n/μs. However, not all neutrons can be considered thermonuclear due to the likely presence of unsuppressed instabilities during repeated shots in FuZE. In order to gain confidence in reporting neutronics, it is recommended to set-up instrumentation carefully, and planning of this nature requires simulating the expected response of the detectors. In these simulations, the neutron emitting volume can be approximated as a degenerate line source, but with plasma density and temperature profiles from measurements made on FuZE, more realistic profiles of the thermonuclear neutron emissions can be generated. A study was conducted to look into the neutronics on past FuZE experiments using said profiles in the neutron transport code, MCNP. With further development of the approach in this study, it is possible that plasma simulations could make more robust predictions and even suggest new diagnostics.   

Neutron Camera with Centimeter Scale Spatial Resolution

In the initial prototype of the compact fusion device called the Orbitron, deuterium ions orbit around a 100kV cathode with a co-rotating electron plasma confined by a magnetic field. Intersecting elliptical orbits of the ions near the cathode lead to high-energy ion-ion collisions and fusion events. However, neutrons are also generated on the surface of the cathode as the ion orbits decay and collide with deuterium embedded in the cathode surface. To distinguish these fusion events, we have designed a fifteen-pixel neutron camera. Each pixel consists of a pulse-shape discriminating (PSD) scintillator and PMT. Neutrons are collimated with high-density polyethylene. Neutron-producing experiments have been carried out on the Orbitron. These experiments, supported by particle-in-cell and Monte Carlo N-Particle simulations, show spatial resolution on the order of one centimeter. This poster will share our current neutron camera results and provide a detailed discussion of the initial neutron distribution within the Orbitron, as inferred through Geant4 library-based Monte Carlo simulations.   

High-wavenumber Collective Scattering Diagnostic System for EAST and NSTX-U Tokamaks and Synthetic Diagnostic System Development

A 693 GHz, 8-channel high-wavenumber scattering system is under development for NSTX-U and a 270 GHz 4-channel high-wavenumber scattering system is being installed on the EAST [1] tokamak. A synthetic diagnostic system is under development for assisting the design and data analysis of the high-k diagnostic systems on the NSTX [2] and EAST tokamaks. The high-k diagnostic system aims to study high-k electron density fluctuations, thereby providing a measurement of the -spectrum of the electron temperature gradient (ETG) [3] mode. Based on the previously developed beam tracing module, which accounts for both refraction and diffraction effects, the synthetic high-k system innovatively incorporates the 3D scattering volume; consequently, it can explore the impact of various factors (e.g. refraction, diffraction, curvature of the magnetic field, wavenumber alignment, and so on) on the scattering signal. The synthetic high-k system not only guides the design and setup of the actual high-k scattering system, aiding in better interpretation of experimental results, but it can also be adapted into other millimeter-wave/terahertz synthetic modules with beam tracing capabilities.   

Development of the cross-polarization scattering system for the measurement of internal magnetic fluctuations on HL-3 tokamak

In this work, the design, test and the first experimental result of a new cross-polarization scattering (CPS) diagnostic is presented, which is based on the scattering of an incident microwave beam into the orthogonal polarization by magnetic fluctuations [1-4]. The new CPS diagnostic focus on the measurement of local density and magnetic field fluctuations simultaneously. With the help of the rotatable polarizer, the effect of the polarization mismatch is analyzed in detail and the conditions for a well polarized matching were confirmed. The response of the CPS receiver, the isolation of the polarizer, and the isolation of electronics system were tested in the laboratory. The plasma test results of polarization purity, signal amplitude and power spectral respond indicate the new CPS diagnostic can measure the density and magnetic field fluctuations simultaneously when the polarization is well matched. In the future, the CPS diagnosis will be used to study the electrostatic and electromagnetic turbulence behavior of the high-performance plasma on the HL-3 tokamak.   

Collective high-k adjustable-radius scattering Instrument for electron scale turbulence measurement on MAST-U

Plasma turbulence on disparate spatial and temporal scales and associated cross-field particle / heat transport plays a key role in limiting the level of confinement achievable in tokamaks. The development of reduced numerical models that accurately predict cross-scale turbulent interactions is essential for understanding and maximising confinement. Such models require experimental turbulence data at both electron and ion scales to inform development. In this paper, we propose a novel, mm-wave based collective scattering diagnostic for measuring normal and binormal high-k (electron-scale) turbulence in the core and edge plasma of MAST-U. This will complement the existing ion-scale BES (beam emission spectroscopy) diagnostic, yielding core and edge measurements at both electron and ion scales whilst providing full spatial coverage under all operating conditions. We present detailed hardware specifications along with beam-tracing calculations predicting the spatial and wavenumber resolution of measurement. We also perform analysis of the instrument selectivity function computing the localisation and sensitivity of measurement accounting for both magnetic pitch rotation with radius and spatial overlap of the incident and scattered Gaussian beams. A synthetic diagnostic framework is presented combining CGYRO predictions of ETG turbulence for a sample equilibrium with beam tracing data, mapping the instrumental wavenumbers to field-aligned coordinates and predicting the scattered power spectrum. Baseline specifications of the diagnostic include an operating frequency of 376 GHz, a source power of ~100mW and a normalised turbulence wavenumber measurement range of k_⊥ρ_e = 0.1 – 0.5 where k_⊥ is the binormal turbulence wavenumber and ρ_e the electron gyroradius.   

Microwave backend for density profile plasma reflectometry at SPARC

Commonwealth Fusion Systems plans to install a reflectometry diagnostic in the SPARC tokamak to measure the outboard midplane radial plasma density profile [1, 2]. The reflectometry system will comprise four microwave bands from 18-90 GHz and work in O-mode and X-mode using the left-hand cutoff. The system will cover a density range of ~3x10¹⁸m^-3 to ~4x10²⁰m^-3 at a magnetic field of 12.2T on-axis. Due to limited port space, the four microwave bands will be combined in pairs, sharing the transmission line and antennas. Band combination will require quasi-optical combination and splitting units, taking inspiration from current multi-band transmission systems like in JET [3]. Band combination will require comprehensive frequency planning and system prototyping. The backend electronics must provide reliability and flexibility for the operation across all microwave bands and propagation modes while focusing on minimizing costs. The current system plan consists of four transmitter units (one per band) and eight receivers (one per band and mode) with heterodyne detection and state-of-the-art plasma reflectometry architectures. At this conference, we will present the backend design of the edge scanning reflectometry diagnostic, focusing on the microwave generation, detection, and band combination.    

Comparison of LaBr3 and LaCl3 inorganic scintillators as HXR monitors for SPARC

Tokamaks emit hard X-rays (HXR) when electrons are accelerated to relativistic energies, entering the so-called runaway regime, and undergo bremsstrahlung interaction with plasma ions and plasma facing components. This can happen at any time during a plasma discharge, but poses more concerns during plasma start-up and disruptions. SPARC¹ will be equipped with a HXR monitor system that, in case a start-up runway beam is detected, will send an alarm to the plasma control system to shut down the discharge and protect the structural integrity of the machine. The preliminary design of the HXR monitor² considers a LaBr3 inorganic scintillator operated in current mode. A new scintillator material, LaCl₃, is currently emerging as HXR detector. In this poster, we compare the performance of this material with a LaBr₃ crystal. Using semi-analytical cross sections and the MCNP code³, we present an estimate of the HXR and neutron signal levels at the diagnostic’s position. We then compare LaBr3 and LaCl3 in terms of energy resolution, total efficiency and dynamic range. Finally, we study the activation of the two materials due to DT neutrons using FISPACT⁴, from which we will estimate their neutron attenuation needs and discuss the possible application of the diagnostics for post-disruption REs monitoring.

¹A. J. Creely et al., Journal of Plasma Physics 86.5 (2020)

²E. Panontin et al. proceedings of HTPD2024 (submitted)

³C. J. Werner et al.  (2018) 10.2172/1419730

⁴J-Ch. Sublet et al., Nuclear Data Sheets 139 (2017) 77-137   

Entangled two-photon absorption to drive an atomic excited state population

Entangled two-photon absorption (ETPA) may allow for pumping an atomic excited state population with a continuous, low-intensity laser and with resulting fluorescence suitable for high-bandwidth plasma fluctuation measurements. Here, we investigate excited state populations for pumping and fluorescing transitions in an Argon plasma species to assess ETPA diagnostic capabilities. We focus on pump transitions accessible with a frequency-doubled Ti:Sapphire laser and visible fluorescence. The time-frequency entanglement of entangled photon pairs allows for the simultaneous arrival of entangled pairs at the target location and produces a narrowband sum-frequency that corresponds to the CW laser linewidth. The ETPA cross section scales linearly with the incident photon flux due to photon entanglement, in contrast to the quadratic scaling for two-photon absorption with classical light. Potential diagnostic schemes include pumping low-n transitions for low-Z impurities, high-n Rydberg transitions for high-Z impurities, and transitions in charge-exchange populations.   

Evaluating Doppler backscattering for inferring internal magnetic pitch angle of Mega Ampere Spherical Tokamak-Upgrade plasmas

Spherical tokamaks (STs), such as the Mega Ampere Spherical Tokamak-Upgrade (MAST-U), have large magnetic pitch angles that vary significantly in space and time. Measuring this pitch angle in the core, with high temporal and spatial resolution, is key for equilibrium reconstruction and controlling magnetohydrodynamic stability. Existing techniques, such as the motional Stark effect diagnostic, might not survive the harsh conditions of burning plasmas in future devices like STEP. In this work, we assess the viability of using a Doppler backscattering (DBS) system to infer the magnetic pitch angle in MAST-U plasmas. DBS is a robust microwave diagnostic typically used for measuring turbulent density fluctuations and flows. Through toroidal steering, DBS operators aim to match the beam wavevector perpendicular to the magnetic field, as mismatch leads to a lower signal and is thus generally undesirable. Conversely, this variation of power on matching has enabled DBS to infer the magnetic pitch angle in DIII-D, a conventional tokamak. We find that while this technique is more challenging to implement in STs, it is nonetheless possible to infer the magnetic pitch angle in the plasma core of up to a normalized poloidal flux of 0.2 in MAST-U. Finally, we show how to optimize a DBS system specifically for measuring the pitch angle.   

Solution-Strengthened Nb-Based Alloys for Metal Foil Pumps as Implemented for Direct Internal Recycling

General Atomics is investigating novel Nb-based alloys for low-pressure metal foil pumps, as described in the Direct Internal Recycling concept. Nb has excellent H permeability making it an attractive option for H separation, but is subject to H-induced embrittlement with successive H loadings. Here we investigate H permeability through Nb-based alloys which include various concentrations of metals, namely W, Ni, and Fe. H interactions with metal inclusions are modeled using density functional theory (DFT) to assess H solubility and diffusivity. These data are then used to assess the permeability of Nb-based alloys in comparison to pure group 5 metals, and are validated by experimental measurements of H permeation. These DFT calculations are used to parameterize mesoscale models of diffusion using kinetic Monte-Carlo techniques to accurately determine the effect of dilute-limit inclusions and assess the effect of concentrations. Results show the dependence of H permeability on inclusion species and concentrations, indicating that favorable modifications to the solubility can be attained while still obtaining the high permeability required for metal foil pumps.   

Efficient Calculation of and Coil Shape Optimization for Lorentz Forces

The Lorentz forces on electromagnetic coils play an important limiting role in the design of practical magnetic confinement fusion reactors as designs must demonstrate that the forces are within prescribed limits. Unfortunately, the Lorentz force is time-consuming to numerically evaluate due to a source point singularity in the calculation of the self-force and, as a result, is typically reserved for engineering design. A method to rapidly calculate the self-force is desirable as Lorentz forces could be more easily incorporated into the physics optimization itself, though naive attempts to treat coils as infinitesimally thin in order to reduce computational time fail as the magnetic field nonphysically diverges. Instead, we present a novel method for calculating the Lorentz self-force (in addition to similar models for the self-inductance and self-field) using non-singular integral formulae of reduced dimensions that were derived rigorously by dividing the domain of integration of the magnetic vector potential into two regions and exploiting the unique assumptions of each region. We demonstrate that these formulae show good analytic and numerical agreement to high-fidelity calculations yet evaluate ~250x faster than finite element analysis software. Taking advantage of the numerical efficiency of the reduced formulae, we optimize stellarator coils for small Lorentz forces and demonstrate reductions in the maximum force by up to ~50% with flux surfaces that qualitatively match the stage I target.   

BCTF: A High-Field Non-Nuclear User Facility for Blanket Component Testing

The preconceptual design for a non-nuclear, high-field blanket component test facility (BCTF) is presented, with the primary mission of characterizing component operation in prototypic thermal, hydraulic, and magnetic environments. The fusion blanket, a critical part of D-T fusion concepts, is currently at a low technology readiness level and will have to be demonstrated via integrated, multi-effect tests in order to verify performance. BCTF requirements have been developed with community input and include (i) magnetic field up to 10T, similar to the inboard region of toroidal device, (ii) large experimental volume of 0.75m³, (iii) liquid metal breeder loops at relevant temperatures (300-700 C), (iv) surface (~MW/m²) and simulated volumetric heating (~MW/m³), and (v) auxiliary coolant and heat rejection loops (e.g. 8 MPa helium). These requirements produce the relevant non-nuclear environment for qualifying meter-scale blanket components at the appropriate electromagnetic, thermal, and mechanical loading, as well as high MHD Hartmann, Reynolds and interaction parameter values for liquid metal breeding blanket concepts (Li, PbLi). The BCTF design incorporates significant onsite materials, infrastructure and capability developed during fabrication of the ITER central solenoid (CS): excess Nb₃Sn CS conductor in spools and wrapped as a six-layer hexapancake, a supercritical helium cryoplant, and high-current power supplies are present and integrated into the facility design.   

Status Update on the U.S. Fusion Prototypic Neutron Source Risk Reduction Activity

The U.S. Department of Energy (DOE) Office of Fusion Energy Sciences (FES) published a request for information (RFI) in March 2023 regarding the construction of a fusion prototypic neutron source (FPNS) to support the the development of advanced fusion devices, such as those proposed by commercial fusion energy companies. An FPNS has been repeatedly identified by this community as a highest-priority facility, including in the 2020 APS-DPP Community Planning Process strategic plan. The RFI asked for descriptions of concepts capable of meeting a given damage rate, neutron spectrum, sample volume, temperature range, temperature control, and flux gradient parameters by 2028 and 2032, with particular focus given to a concept's ability to match a 14 MeV fusion neutron spectrum and provide a high rate of damage to samples. 

15 concepts were submitted, which vary in conceptual approach, technical maturity, and ability to achieve a prototypical fusion neutron spectrum. A FPNS risk reduction activity (RRA) was initiated to assess these concepts. For example, the RRA team has considered the neutron spectrum, transmutant productant rates, and technical maturity of each concept, using systematic, rigorous, and unbiased approaches to perform these evaluations. A final report to DOE FES will be provided in March 2025; intermediate results were provided to FES and concept teams in June 2024. The purpose of this talk is to update the APS-DPP community on these results and the continued progress of the RRA.    

Scrape-off layer power fall-off length broadening in small ELM regimes on DIII-D

A DIII-D database study investigated divertor heat flux and its dependencies on pedestal parameters across a range of small ELM regimes: high beta poloidal, type-II, and ELMs in reduced negative triangularity H-modes [1]. It demonstrated that type-II and high beta poloidal small ELM regimes have their parallel energy fluences fall below the Eich type-I scaling from [2] and that the SOL power fall-off lengths were larger than predicted using the Eich scaling with poloidal magnetic field in [3]. Although on a whole the SOL power fall-off lengths were broadened, there were some stationary periods that had their power fall-off lengths follow the Eich scaling. Within each small ELM regime the stationary periods with the largest difference in power fall-off lengths were examined for differences in pedestal characteristics, radial electric fields, and edge fluctuation measurements. Recent work in [4], indicates the importance of E x B flow shear in causing changes in turbulence spreading that alters the broadening/narrowing of the power fall-off length. Understanding the commonalities of the broadened power fall-off lengths in the small ELM regimes informs feasibility of the small ELM regimes for effective core-edge integration in future fusion-grade tokamaks.

[1] P. Traverso et al. 2024 Nucl. Fusion (submitted)

[2] T. Eich et al. 2017 Nuclear Materials and Energy 12 84–90

[3] T. Eich et al 2013 Nucl. Fusion 53 093031

[4] N. Li et al 2024 J. Plasma Phys 90 905900117   

A Boundary Layer Theory for the Scrape Off Layer

The handling of power entering the scrape-off-layer (SOL) and reaching the plasma facing components is one of the major challenges to achieve long pulse operation in future fusion machines. Goldston’s Heuristic Drift Model [1] yields a thin SOL for laminar flow, but more recently, Chu et. al. [2] showed through scaling arguments that the SOL could be broadened beyond the HD limit via turbulence spreading from the pedestal.

The purpose of this work is to expand upon Chu et al’s scaling arguments via a full model of a “straight” turbulent SOL. This is accomplished through a boundary layer approximation of the Braginskii equations coupled to a nonlinear Fisher equation model of drift wave turbulence [3]. Magnetic shear is accounted for by varying streamwise extent as a function of penetration into the SOL. Instability is introduced to the system via a stochastic flux of turbulent kinetic energy through the LCFS and interchange instability within the SOL. Through scaling arguments, numerical simulations, and analysis of a reduced model we find a parallel-direction SOL-width profile independent of flux expansion, and that interchange effects add a small additional broadening. Even intermittent “puffs” of turbulence are capable of broadening SOL widths an interesting amount.

[1] R.J. Goldston. In: Nuclear Fusion 52.1 (Dec. 2011), p. 013009.

[2] Xu Chu et al. In: 62.6 (Apr. 2022).

[3] Ö. D. Gürcan et al., Phys. Plasmas 1 May 2006; 13 (5): 052306   

Highly radiating plasmas in negative triangularity with reactor-relevant seeding gases

The advantages of reactor-relevant, heavier noble gas seeding gases like Kr, Ar, and Ne are shown for negative triangularity (NT) plasmas at DIII-D. In this study, we combine the negative NT ELM free, high-performance scenario with a highly radiating mantle that was obtained with reactor-relevant seeding gases. We find that Kr and Ar seeding is advantageous in terms of the gas particles needed for effective divertor target heat flux mitigation, which also implies lower effects on fuel dilution and impurity accumulation. Seeding with Kr, Ar leads to a reduction of the parallel heat flux at the divertor entrance compared to N and Ne effectively reducing P_SOL the power that needs to be dissipated in the divertor. The possibility to radiate upstream to reduce P_SOL is one of the main advantages of NT. Impurity enrichment calculations demonstrate that Ar and Kr are retained more efficiently in the divertor. Thus, the use of high-Z impurities yields similar divertor conditions such as T_e and heat fluxes than low-Z gases with a reduced impurity concentration at the separatrix up to ~90% which is crucial for core-edge integration. We also show that cross-field drifts change the particle and impurity distribution. SOLPS-ITER simulations provide comprehensive insight into divertor and plasma boundary transport mechanisms and support the interpretations of the experimental results.   

Extended MHD studies of ELM-free negative-triangularity plasmas and the CETOP SciDAC-5 project

The CETOP (Center for Edge of Tokamak OPtimization) project is a SciDAC-5 project for the development of high-fidelity simulation capabilities for ELM-free design. In CETOP,  we employ a fidelity hierarchy of global models, ranging from XGC to NIMROD, M3D-C1, and MARS-K/Q. Our objectives are: 1- inclusion of non-ideals effects, plasma shaping, multi-species, nonlinear physics, coupling of extended MHD, XGC via transport coefficients, as well as performing a full nonlinear benchmark between extended MHD, gyrokinetics and NIMROD DK, 2- to develop advanced time discretizations and transition to GPU-accelerated architectures for our MHD codes,  3- to apply ML techniques for ELM-free optimization. Our extended MHD studies of negative-triangularity  (NT) plasmas in DIII-D will be presented.   We model the entire plasma, including the core, edge, SOL and vacuum regions, using NIMROD with 2-fluid and gyroviscosity effects. Our linear and nonlinear NIMROD  simulations of two strongly shaped NT plasmas (\delta =-0.54) in DIII-D show a persistent reconnecting global n=1 mode. We compare the saturated nonlinear mode amplitudes with the measurements.  The linear stability scaling of these plasmas using the MARS-F and NIMROD codes will also be presented. This work was supported by the DOE SciDAC program under Award Number DE-AC02-09CH11466 and by US DOE awards DE-SC0022270 and DE-FC02-04ER54698.   

Gyrokinetic study of neoclassical and turbulence properties in the negative triangularity edge

Tokamak discharges with negative triangularity show promising characteristics, including good energy confinement, no impurity accumulation, and the absence of detrimental edge localized modes [1]. In addition, it is reported that the H-mode transition is difficult with sufficiently strong negative triangularity. To investigate the impact of triangularity on edge plasma properties, we performed gyrokinetic simulations using the total-f, edge specialized gyrokinetic code XGC. We simulate two DIII-D-like geometries based on DIII-D negative triangularity discharge #193802 with adjustments of far scrap-off layer and private flux region: one with negative triangularity and the other with a “manufactured” positive triangularity with the separatrix and limiter/divertor shapes mirrored. Flux-surface-averaged plasma profiles are kept the same between the two shapes. The simulations show that negative triangularity gives a deeper Er well and higher ExB shearing rate than positive triangularity, and that the parallel flow direction is opposite from each other. The observed edge Er and toroidal flow from the negative triangularity simulation agree qualitatively with experimental measurements.  Differences in turbulence properties will also be discussed.   

Properties and Limits of the ELM-free Negative Triangularity Edge on DIII-D

The edge region of tokamak plasmas with strong negative triangularity (NT) shaping is fundamentally different from conventional L-mode or H-mode plasmas. Most prominently, NT plasmas are inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold [1]. Though the edge pressure gradient is therefore reduced compared to ELMy H-mode plasmas, NT plasmas are still able to support small pedestals and are typically characterized by an enhancement of edge pressure gradients beyond those found in traditional L-mode plasmas. On DIII-D, the pressure gradient inside of this small pedestal is unusually steep - sometimes maintaining similar gradients to the pedestal itself well into the core region. This allows NT configurations to access performance measures competitive with other ELM-free regimes previously achieved on DIII-D that typically have larger edge pressures. Gyrokinetic and magnetohydrodynamic modeling of the edge region are used to develop physics-informed scalings for the edge pressure of NT discharges that can be used to inform extrapolations to NT-based fusion energy systems. 

[1] A.O. NELSON, et. al, Phys. Rev. Lett. 131, 195101 (2023)   

Comparisons between Edge Turbulence in EDA and QCE Plasmas Using the CECE Diagnostic at ASDEX Upgrade

Large Type I ELM-free plasmas are imperative for sustainable operation of fusion power plants. Many Type I ELM-free regimes have been developed over the past years [Viezzer et al Nucl. Mater. Energy 2023], with promising regimes, such as enhanced D_α (EDA) and quasi-continuous exhaust (QCE), having been observed at ASDEX Upgrade. Both regimes display a ubiquitous edge fluctuation dubbed the quasi-coherent mode (QCM), and have similar access conditions. It is not clear if these regimes are fully distinct or some operational limit of the same regime. The correlation ECE (CECE) diagnostic [Creely et al RSI 2018] is used to compare properties of the QCM between each regime by investigating differences in the QCM’s coherence spectrum, electron temperature fluctuations, and correlation length. Additionally, transitions between EDA and QCE and vice versa and discharges performed in non-deuterium plasmas are investigated to understand the underlying physics of the QCM in these different scenarios.   

Active Impurity Control in ELM-absent H-mode Plasmas via On-Demand ELM Triggering with Pellet Injection

Optimal ELM control can be achieved by suppressing naturally occurring ELMs and reintroducing them as needed for impurity control. This study demonstrates a method for on-demand ELM triggering in ELM-suppressed H-mode plasmas on the EAST tokamak using submillimeter lithium and carbon granule injection from the low field side midplane and X-point.

ELM suppression was achieved with a low-recycling wall by Li coating or active boron powder injection. Robust ELM triggering was obtained with 0.9±0.1mm diameter lithium granules, achieving a triggering efficiency of ~100%. The triggered ELM frequency ranged from 5-200Hz, depending on the pellet injection frequency and plasma conditions. Core radiation from heavy impurities, dominated by W, decreased by up to 50%, and the H98 factor increased from 0.87 to 1.12 after lithium granule injection.

Initially, substantially reduced-size ELMs were triggered, followed by a transition to a mixed ELM phase with intermittent large ELMs comparable to spontaneous ELMs. The particle flux from the triggered large ELMs impacted a wide spatial region of the outer target. The W flushing efficiency for the smaller, irregular pellet-induced ELMs can be evaluated from fast bolometry and SXR measurements. No statistically significant difference in the W flushing efficiency between pellet-triggered and natural ELMs has emerged. ELM-sputtered impurity influxes, determined from visible divertor spectroscopy, can be compared among ELM types.   

Predicting and Optimizing Pedestal Structure with the EPED Model

The pressure and temperature at the top of the edge transport barrier ("pedestal height"), play a strong role in fusion performance, with fusion power density in power plant scale plasmas scaling roughly with the square of the pedestal pressure. The EPED model [1] was developed to predict the structure of the pedestal based on two calculated constraints (1) onset of stiff transport due to nearly-local kinetic ballooning modes (KBM), and (2) global constraint on the pedestal height due to peeling-ballooning (P-B) modes. Recent developments have included generalization of the KBM constraint, and coupling of EPED to SOLPS, which enables self-consistent determination of sources and introduction of non-stiff transport constraints into EPED. EPED has been used to predict the Super H Mode regime and to guide record-setting experiments on DIII-D and C-Mod exploring this regime. Here we present recent work on updates to the EPED model, coupling of the model to core and SOL models, and validation of the model in a wide variety of experimental conditions, including in Super H-Mode experiments.   

The impact of 3D magnetic perturbations on the MHD stability of the pedestal plasma in tokamaks

Experiments show that 3D magnetic perturbations (MPs) can mitigate or even eliminate edge-localized modes (ELMs) but without a rigorous theoretical model, extrapolation to future tokamaks is uncertain. ELITE has been used for two decades to assess pedestal MHD stability in axisymmetric plasmas, providing a prediction for ELM onset that agrees with observation. To efficiently understand the impact of MPs on pedestal stability a variational formalism has been developed [1]. Trial functions are constructed from the poloidal Fourier amplitudes of the displacement, calculated using ELITE for the toroidally averaged equilibrium. These are scaled relative to each other, fixing the scaling coefficients by minimizing the perturbed energy for the 3D equilibrium to derive the eigenfunction and growth rate. We extend that theory to: (1) real experimental tokamak equilibria, analyzed using a finite toroidal mode number, n, version of ELITE; (2) determine the stability threshold ([1] is restricted to unstable plasmas), and (3) explore the potential of magnetic compression to enhance stability. This will inform a robust stability code, ELITE-3D, as a new, efficient tool to assess pedestal stability in the presence of MPs.

[1] M S Anastopoulos Tzanis, et al, Nucl Fusion 60 (2020) 106003   

Global total-f gyrokinetic study of turbulence before and during RMP ELM-suppression in DIII-D

We apply the global total-f gyrokinetic code XGC to study pedestal turbulence shortly before and during RMP-ELM suppression in DIII-D.  Turbulence was found to increase with the strength of the external RMP field in KSTAR experiments. While earlier electrostatic XGC simulations with fixed RMP fields confirmed that RMPs can increase turbulent transport, the role of turbulent transport in the transition to the ELM suppressed state is not fully understood.  We investigate this question with gyrokinetic simulations of the edge and scrape-off layer plasma for two plasma equilibria from a DIII-D H-mode, taken before and during ELM suppression, respectively.  The simulations self-consistently include neoclassical and turbulent transport as well as a neutral recycling source.  Electrostatic and electromagnetic turbulent modes, from low to high toroidal mode numbers are included.  The results are compared with the RMP fields from the MHD code M3D-C1, which are used to initialize the simulations, and linear MHD stability calculations.    

3D plasma response investigation of SVR-enabled operational space expansion using a self-consistent integrated modeling framework

Several tokamaks have undergone major upgrades this year, including KSTAR with the installation of its tungsten divertor and DIII-D with its shape and volume rise (SVR), which EPED [1] predicts admits FPP-relevant plasmas endowed with enhanced pedestal pressure. ITER and FPPs will not tolerate type-I ELMs, and thus significant experimental time is devoted to developing ELM-free scenarios, the current prevailing technique for which is RMP-ELM suppression. The high sensitivity of a 3D-field-induced plasma response to shifts in operational space solicits new 3D plasma response predictions for RMP ELM control guidance. This work introduces an integrated modeling framework to self-consistently scan through presently experimentally-unrealized parts of operational space and use this to estimate the 3D plasma response, offering predictive guidance for ELM-suppression access favorability. The framework generates tightly converged equilibria satisfying a target set of plasma parameters (Ip, β, l_i) with a self-consistent set of kinetic and current profiles constrained by an EPED(NN)-computed pedestal [2], which are then input to GPEC to calculate the 3D plasma response [3]. In addition to the device upgrade application explored in this work, this tool can be versatilely used for generic scenario exploration.

  

RMP ELM Control in Dimensional and Dimensionless Matched Hydrogen Plasmas

We report on the latest attempts to achieve suppression of edge-localized modes (ELMs) via the application of resonant magnetic perturbations (RMPs) in hydrogen plasmas by matching the pedestal to those of ELM suppressed deuterium plasmas with respect to either dimensional or dimensionless parameters. Most studies investigating RMP ELM suppression have focused on deuterium plasmas. Despite meeting the known deuterium access criteria, ELM suppression has not been achieved in hydrogen. 2D peeling-ballooning stability calculations would suggest RMP ELM suppression to be present especially in the dimensionless match. In addition, the strength of the applied total (vacuum + plasma response) RMP compares favorably in terms of ideal islands widths and their overlap as well as the criterion of having a resonant flux surface overlapping with region of zero or little ExB rotation at the pedestal top. One possibly crucial difference between the hydrogen and deuterium plasmas with RMPs is the significant reduction of turbulence in the pedestal region in hydrogen as compared to deuterium. This is in line with previous observations from ASDEX Upgrade. Overall, these results challenge our understanding of RMP ELM suppression further and make predictions towards its applicability in future devices more uncertain as well as motivating the investigation of the stabilizing effect of turbulence on ELMs.   

Experiments and Modeling on the Influence of Main Ion Mass in the Pedestal and Scrape-Off Layer of DIII-D

The pedestal and divertor of H-mode hydrogen (H) plasmas have been characterized and compared to similar deuterium (D) plasmas under a range of conditions in order to assess the impact of ion mass on these regions. In H, higher electron densities are measured at the outer target despite having similar outer target temperatures as the D references. Upstream density at detachment onset is increased only 5% for H over D. Heat flux to the target is consistently measured to be 2 times higher in H with no significant reduction in peak values associated with detachment, suggesting a change in SOL energy transport mechanisms. H pedestal profiles show evidence of an increased particle source inside the confined region with the pedestal top density increasing independently of core density. Higher neutral pressure throughout the chamber in H is identified as a driving factor in this change, with divertor detachment greatly exacerbating this effect. In interpretive SOLPS-ITER modeling, H+C simulations reach detached conditions at a 10% higher upstream density than D+C, consistent with experiment. However, the experimentally observed changes to parallel heat flux are not captured. Higher neutral ionization rates are found within the LCFS in the H+C simulations.   

Constraining Neutral Transport Models using Measurements Based on Charge Exchange Neutral Spectroscopy

First measurements from a spectroscopy system that has been installed on DIII-D to provide information about the energy distribution of the atomic neutrals using the Doppler shift and broadening of passive Balmer-𝛼 emission will be presented.  The new system complements existing low field side midplane measurements of the neutral energy distribution that have demonstrated the existence of neutrals with energies above 1keV for typical DIII-D pedestals and have shown good agreement with synthetic spectra from neutral transport modeling [1]. The new measurements will be compared with and used to constrain neutral transport models such as FIDASIM, DEGAS2, and EIRENE. The system is optimized to provide details of the neutrals as they travel upwards from the lower diverter, undergoing various atomic physics processes such as charge exchange (CX). Multistep CX between ions and recycled neutrals transfers both energy and momentum, significantly affecting the neutral mean free path, allowing them to travel deeper into the confined plasma where they ultimately provide fueling.   

Adding a fluid neutrals model to the gyrokinetic code GENE-X for edge/SOL turbulence simulation in divertor geometries

Developing first-principle codes to accurately simulate the plasma edge and SOL is crucial for predicting heat and particle exhaust in future fusion devices. Significant quantities of neutrals in the SOL interact with the plasma through complex collisions, necessitating their inclusion in plasma turbulence codes.

In this work a novel coupling of a continuum gyrokinetic model with a fluid neutrals model using the GENE-X code[1] is detailed. GENE-X is a global, full-f, electromagnetic, collisional gyrokinetic turbulence code for studying the edge and SOL in realistic geometries, including magnetic X-points. Neutrals are treated as a fluid to minimize computational cost, with density evolution (1-moment model) modeled by a pressure-diffusion equation, where diffusion is caused by charge exchange collisions. Two centered finite difference schemes, a 4th order and a 2nd order direct discretization of the divergence operator, are implemented to study trade-offs between accuracy and stability. Plasma-neutrals coupling is performed using conservative Krook operators to model ionization and recombination sources and sinks. A three-operator, 2nd-order Strang splitting algorithm allows explicit integration, treating the Vlasov plasma, Coulomb collisions, and neutrals independently. To lift the time-step constraints imposed by the diffusive nature of the latter two, a 2nd order RK-Chebyshev (RKC) scheme has been implemented, also allowing the number of steps to be chosen to maximize the possible time-step in the simulations. The implementation has been verified using the method of exact solutions, obtaining the expected order of accuracy in slab, circular and toroidal geometries, and first blob simulations are performed.

References

[1] D. Michels et al., Physics of Plasmas 29, 032307 (2022)   

Sensitivity of the Particle and Heat Transport in L-mode Tokamaks to The Separatrix Density and Particle Source

The SAT2 saturation rule employed in the TGLF transport model has been successfully validated for experimental boundary condition within 98% of the separatrix for ohmic and L-mode operation regimes [1]. Predictions of the plasma density and temperature profiles depends on the separatrix density and gas puff. A series of scans of the ion source density and separatrix density has been implemented in the IPS-FASTRAN workflow [2] to match the line-average density and power balance at the edge with TGLF/SAT2 and to explore the sensitivity of the core transport prediction to the boundary density and particle source. Additionally, the 2-point model and C2 Scrape-Off Layer (SOL) transport model have been integrated in the Core-Edge-SOL (CESOL) workflow [3] to explore the heat and particle fluxes across the separatrix when coupling the core-SOL system in a set of DIII-D discharges.   

Self-consistent Neoclassical Transport Theory with Orbit Loss in a Low Collisionality Tokamak Scrape-off Layer.

Neoclassical transport theory provides the minimum level of transport in tokamaks [1]. The theory assumes closed flux surfaces and a gradient length scale across the magnetic field line in the background plasma larger than the poloidal gyro radius, and hence it is not valid in strong gradient regions such as the pedestal in H-mode plasmas [2] or regions with open field lines like the scrape-off layer. Here, we propose a self-consistent neoclassical transport model for diverted tokamak scrape-off layers at the low collisionalities that are achievable with low recycling boundaries. The strong gradient in electric potential across magnetic flux surfaces, with a characteristic radial length of the order of the orbit width, leads to orbit loss phase space regions determined by the X-point position and the radial drifts. The neoclassical transport solutions with these orbit loss phase space regions will be discussed.

 

[1] Helander, Per, and Dieter J. Sigmar., Cambridge university press, 2005.

[2] Trinczek, Silvia, et al, Journal of Plasma Physics 89.3 (2023): 905890304.   

Neoclassical transport in strong gradient regions

Regions of strong gradients in tokamaks such as the pedestal and internal transport barriers form because turbulence is somehow suppressed. As a result, neoclassical transport becomes important but standard approaches to neoclassical theory are not valid if the gradients are sufficiently large. We have previously presented a neoclassical ion formulation that allows for gradient length scales of density, temperature, and electric potential of the order of the ion poloidal gyroradius, but simultaneously keeps orbit widths small in inverse aspect ratio (Trinczek et al. 2023). The theory captures finite poloidal gyroradius effects such as strong poloidal variation. We have extended the formulation to electrons. We have also studied how the parallel flow is determined. In the limit of interest where the ion neoclassical particle flux is small, asymmetries in phase space introduced by the strong radial electric field lead to a new mechanism of parallel momentum damping that sets the mean flow. The radial electric field can then be determined self-consistently via quasineutrality.   

Recent development of GEM-X: an efficient global gyrokinetic simulation of the tokamak edge including the scrape-off layer (SOL)

Development is underway of GEM-X - a fast-time-to-solution gyrokinetic particle code for modeling the edge pedestal, including the SOL region. GEM-X has a full-f option, but then the code will have no advantage over the existing full-f codes. Equilibrium profiles in GEM-X are in a cylindrical R-Z rectangular mesh, which can be generated from other equilibrium codes, such as EFIT and SOLPS-ITER. The 2D electrical static field is going to be calculated in GEM-X 2D mode, with either fluid electron or adiabatic electron response. By changing the size of the simulating electric static potential mask in the rectangular mesh, GEM-X can simulate the core region or the entire tokamak region including SOL. We will show some results from GEM-X in the past year's development.   

The kinetic Bohm condition in collisionless and collisional plasmas

The kinetic Bohm condition [1] is an inequality that must be satisfied by integrals of the electron and the ion distribution functions at the entrance of a Debye sheath. As well as requiring that the average velocity of ions be larger than a certain threshold, this condition ensures that the number of slow ions is sufficiently small that they do not dominate Poisson’s equation near the sheath entrance. Without the kinetic Bohm condition, the sheath equations cannot be solved, and as a result, one needs it to be satisfied at the sheath entrance in quasineutral models of the plasma. Unfortunately, the kinetic Bohm condition cannot be imposed as a boundary condition because it depends only on the ions that enter in the Debye sheath and hence are completely determined by the quasineutral bulk plasma. We show that, in collisionless systems, the condition is not satisfied in general by time-independent solutions, but it can be imposed by kinetic rarefaction waves [2]. In contrast, we will show that when Fokker-Planck collisions are included, all steady state solutions must satisfy the Bohm condition.

 

[1] E.R. Harrison and W.B. Thompson, Proc. Roy. Soc. 74, 145 (1959).

[2] A.V. Gurevich, L.V. Pariiskaya and L.P. Pitaevskii, Sov. Phys. JETP 22, 449 (1966).   

Design and Analysis of an ExB Probe for Measuring Multicharge Species Distributions in Hall Thruster Plumes

An ExB probe, also known as a Wien filter, has been designed and characterized using particle tracing simulations to measure the species distribution in Hall thrusters and other ion sources. Permanent magnets are used to apply a uniform magnetic field perpendicular to the ion velocity, and a uniform electric field from two electrodes. Ions pass through a collimator and enter this ExB region. Ions whose velocities are not equal to E/B are deflected, while the undeflected ions are deposited on a metal collector and detected with a picoammeter. Using COMSOL Multiphysics, the electric and magnetic fields are simulated, and ion trajectories are traced to obtain error estimates in the velocity measurement. These estimates are made for species of ions relevant to Electric Propulsion, as multiple-charged ions are known to decrease the operating efficiency of Hall thrusters [1]. From the probe trace, the multi-charge species distribution can be estimated [2]. The ExB probe will be set up and calibrated with a 3-cm gridded ion source of known velocity distribution. 

[1]: R. Hofer and A. Gallimore. 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, (2003).

[2]: R. Shastry, et al. Review of Scientific Instruments 80.6 (2009).   

Thrust measurement experiments and planned Particle-in-Cell simulations for an electrodeless Magnetic Reconnection Thruster (e-MRT)

Advanced thrusters featuring large thrust-to-power, at sufficient specific impulse, and with long lifetime and flexibility in propellant are needed for deep space missions. To address these needs, we are exploring a new electrodeless Magnetic Reconnection Thruster (e-MRT), which will use asymmetric, partially ionized, inductively-driven reconnection outflows for net thrust. As a first step, we have built a diagnostic to measure the thrust density from reconnection outflows on the Magnetic Reconnection Experiment (MRX). The diagnostic consists of a confocal optical sensor to measure the physical displacement of a horizontal flexing arm, which will flex after impact of an outflow. Once calibrated and installed on MRX, experimental scans, varying control knobs such as neutral gas pressure, will be performed. In parallel, we have begun running VPIC simulations of an e-MRT-like geometry. These simulations will investigate the influence of neutral particles on the reconnection dynamics and test symmetry-breaking of outflow via locally strong magnetic pressure. Partial ionization may increase efficiency while maintaining high thrust via neutral-ion coupling. Preliminary results show that magnetic pressure provided by an additional coil can break outflow symmetry.   

Static and dynamic radiography using high-intensity-laser-based MeV photon sources

A recent effort led by Los Alamos National Laboratory studied the development of flexible, compact x-ray sources for static tomographic radiography and flash radiography at facilities requiring high brightness dynamic imaging with a goal of developing brilliant x-ray sources with small spot size, short pulse duration, and tunability in dose and spectrum. Results from recent experiments will be presented, evidencing the maturity of this technology for a variety of applications. Also shown will be how dose and spectra can be optimized through use of structured laser targets. Results of a simulation study of laser-target interaction using the VPIC kinetic plasma modeling code and the MCNP transport code will also be reviewed.   

Optimizing laser-accelerated electrons for laser-based X-ray radiography

In this work, we have performed a suite of kinetic simulations of relativistic laser-plasma interaction under settings relevant to recent and planned experiments on a variety of laser systems. The goal of the study is to illuminate the physics of laser-target coupling and to provide guidance for how to optimize these sources for applications. It is shown that the production of relativistic electrons is maximized when conditions of relativistic induced transparency (RIT) in dense plasmas can be achieved over a large interaction volume at the time of arrival of most intense part of the laser pulse. RIT is shown to enhance both the numbers of relativistic electrons and as well as the energies of the electrons, leading to an increased X-ray dose. A variety of approaches to enhancing laser-target coupling are considered. These include optimizing the effects of low-density, pre-plasma (arising either from finite laser pedestal or from the use of foam coatings), and of modifying the laser focusing geometry to reduce effects of filamentation and self-focusing.  Evidence of a novel approach to achieving stable laser propagation over distances of 10s of microns in a plasma gradient is also presented. These conditions coincide with plasma and laser conditions explored in recent experiments on the Omega EP laser system and compare favorably with an analytic criterion for stable laser propagation in relativistically underdense plasma obtained from a nonlinear Wentzel–Kramers–Brillouin analysis.   

Predicting performance variability of National Ignition Facility experiments

Developing truly predictive models of inertial confinement fusion experiments continues to be a challenge. High fidelity radiation hydrodynamic simulations can often do a good job at explaining the data post-shot, once uncertain input conditions like mix and drive asymmetries are tuned to match observations. However, predicting the outcome of an experiment based solely on preshot conditions requires knowing ahead of time the values of degradations an upcoming experiment is likely to experience. 

Over the last several years, we have developed techniques for inferring the distribution of simulation input conditions that are consistent with measurements for a large number of DT implosions. Recently, we’ve been using a combination of specialized sampling techniques and transfer learning with small sets of simulations to forward propagate the distribution of expected input conditions through new designs – producing predictions with quantified uncertainties.

In this talk we will present an update to this work, including our first predictions testing the extrapolation capabilities of the model from 1.9MJ to 2.2MJ laser drive experiments. We’ll also discuss how we plan to use this type of analysis to design explicitly for robust performance with exascale workflows in the coming year.

Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-865958.   

The nontrivial topologies of photons, gravitons, and electromagnetic plasma waves and their implications for splitting angular momentum into spin and orbital parts

Many physical systems, such as insulators, fluid, and plasmas, support topologically nontrivially waves. Here, we show that even the vacuum admits topologically nontrivial modes. Massless particles such as the R and L photons and gravitons form nontrivial vector bundles with Chern numbers C= [1] and C= [2]. This nontrivial topology originates from a singularity in the momentum space at k=0. By considering representations of the Poincaré group on vector bundles (rather than on vector spaces) we obtain a more rigorous version of Wigner’s little group method, showing that photons and gravitons are irreducible bundle representations of the Poincaré group with helicities h=±1 and h=±2. It is known that massless particles are characterized by helicity rather than spin. We show that this transition from spin to helicity in the massless limit occurs due to a topological singularity. A surprising implication is that this singularity obstructs the splitting of the angular momentum of massless particles into spin (SAM) and orbital (OAM) parts. We also show that EM waves in unmagnetized plasma are topologically nontrivial, and that this precludes splitting the wave angular momentum into SAM and OAM. Indeed, topological nontriviality corresponds to a twisting together of the internal and external degrees of freedom. This twisting obstructs splitting the SO(3) symmetry of the wave modes into internal (SAM) and external (OAM) SO(3) symmetries.   

Guiding Center and Gyrokinetic Theory for Strong Shear Flows in General Magnetic Field Geometry

The guiding center and gyrokinetic theory of magnetized particle motion for the regime of large electric field gradients and strong shear flows is extended to include spatial and temporal variations in the magnetic field. Performing the calculation in a reference frame that moves with the guiding center simplifies the results.  For small shear flows, the gyrofrequency is corrected by the parallel component of the guiding center vorticity. For large shear flows, the oscillation frequency depends on the gradient in the electric field in addition to the usual corrections due to the Banos drift. Moreover, the Larmor orbits deform from circular to elliptical trajectories, the drifts become anisotropic in response to external forces, and additional curvature drifts must be included. The polarization and magnetization are modified by the change in gyrofrequency, by the large drift flows, and must include self-consistent thermodynamic polarization. While several new physical effects are predicted, the theory is similar in mathematical form to the standard case and can readily be implemented within existing simulation tools.