Session PP11: Poster Session VI: Relativistic Laser Plasma Interaction and Beam Physics; Boundary; MHD and Stability, Transients; FRC; Dusty Plasmas; Basic Studies; Computational and Diagnostic Methods (2:00pm-5:00pm)

Laser induced electron bunch generation using Deuterium clusters

Study of dynamics and energetics of laser-cluster interaction has led to various applications, including the production of mono-energetic electron bunches as a result of outer ionization of the cluster. These electron bunches can be further accelerated to higher energies via laser wakefield accelerators. In this work, we propose to study the interaction of intense, sub-cycle and few-cycles laser pulses with Deuterium clusters for production of mono-energetic electron bunches using 3D, relativistic molecular dynamics simulation. The pulse model used for study of laser-cluster interaction has been modified to generate two mutually perpendicularly polarized pulses separated by a finite temporal phase delay. The first pulse is responsible for creating electrons by field ionization of the cluster, while the second pulse tends to accelerate these electrons as a bunch. This electron bunch gets separated from the ionized cluster and can further undergo wakefield acceleration. The complete isolation of the bunch is possible due to the two mutually perpendicularly polarized pulses acting on it. We have also studied the effect of phase delay, pulse duration and peak laser intensity on generation of energetic electron bunches to find the optimum conditions for electron bunch production.   

Characterization of Femtosecond Laser Filament Conductivity with Varying Pressure

Our experiments examine the effect of changing gas pressure on the conductivity of an ultra short pulse laser filament plasma and how the conductivity varies longitudinally in the focusing region. By comparing digital images of the plasma fluorescence to its conductivity, we are able to characterize the plasma decay.  To determine filament conductivity, we measure microwave scattering along the laser propagation direction. We use a WR284 rectangular waveguide with a 1.5 cm hole that allows the beam through. A 3.2 GHz microwave signal is emitted in the waveguide, and signals are received through a waveguide-to-coax antenna connected to a HP8470B Schottky diode. In addition, a series of glass vacuum tubes is used with a 1.2 cm diameter section encompassing the focusing region. This glass tube is passed through the waveguide so measurements can be obtained along the plasma while air pressure is varied from 760 to 0.10 Torr. Filament conductivity is determined by comparing the signal attenuation during filamentation to COMSOL simulations. Our research provides insight into the extent a visual inspection of a filament plasma is representative of its true extent.   

Four-photon amplification of powerful laser pulses in plasma

A new scheme for amplification of laser pulses in plasma is proposed, which may overcome limitations of the backward Raman amplification (BRA) scheme. Those limitations are associated primarily with the fragility of the short-wavelength Langmuir waves mediating the resonant energy transfer, which limits the allowed laser-to-plasma frequency ratios. The largest frequency ratio allowed decreases with the laser wavelengths and completely disappears at for hard x-rays. The BRA limitation on laser-to-plasma frequency ratio can be overcome by beating two auxiliary laser pulses, rather than a Langmuir wave, for mediating the energy transfer. This leads to a 4-photon amplification scheme, which may be used, for example, to amplify optical laser pulses in very rarefied plasmas, or to amplify x-ray pulses in dense plasmas. In the 4-photon scheme, a pump and an auxiliary laser pulse-1 are scattered into an amplified pulse and an auxiliary laser pulse-2. The energy can keep flowing from the pump to the amplified pulse even for amplified pulse intensity much exceeding the pump intensity. The opposite process can be excluded by absence of the auxiliary laser pulse-2 in freshly encountered plasma layers.    

Scaling Self-Phase Modulation in fused silica to MIR Wavelengths

Decreasing pulse durations to the single cycle limit enable new laser interactions, idealized probe beams, and can increase intensity. Self-phase modulation is a nonlinear effect that broadens the spectrum of an intense laser pulse. With sufficient broadening, pulse durations could be compressed to a single cycle pulse duration if the phase is properly controlled. We demonstrate SPM in a variety of targets at wavelengths spanning the near infrared, with measurements of pulse duration, spectral phase, compressibility, and wavefront. Numerical modeling of the nonlinear propagation has also been used to verify nonlinear indices.   

Attosecond-scale absorption at extreme intensities

The Zero Vector Potential (ZVP) absorption mechanism, a non-ponderomotive absorption mechanism, is expected to dominate the interactions of ultra-intense laser pulses with critically over-dense plasmas such as those that are expected with the Extreme Light Infrastructure laser systems. We extend the mechanism through the non-ponderomotive regime up to QED relevant intensities, showing that at the onset of electron-positron pair production in our numerical simulations the scaling relation of electron energy vs. laser intensity deviates significantly from that predicted by the ZVP model. This offers future experiments the opportunity to test for a distinctive signature in electron energy scalings with laser intensity, thereby providing the opportunity to validate the assumptions of current QED modules in particle-in-cell simulations.
  

Mid-Infrared High-order Laser Plasma Interactions in Solids

Relativistic laser-solid interactions are capable of accelerating high energy electron, ion, and x-ray beams. Most solid high-order harmonic generation experiments to date have been with near-infrared lasers despite scalings which are strongly wavelength dependent. With the advent of high energy optical parametric amplifiers, relativistically intense short pulse mid-infrared experiments are now possible. We report the first results of radiation production from these interactions, with supporting Particle-in-Cell simulations.   

Simulation Study of Ionization and Laser Beam Quality in CO2-Laser Plasma Interaction

3D numerical simulations of the interaction of a powerful CO₂laser with hydrogen jets demonstrating the role of ionization and laser beam quality on the characteristics of induced wakes are presented. Simulations are performed in support of the plasma wakefield accelerator experiments being conducted at the BNL Accelerator Test Facility (ATF). SPACE, a parallel relativistic Particle-in-Cell code, developed at SBU and BNL, has been used in these studies. A novelty of the code is its set of efficient atomic physics algorithms that compute ionization and recombination rates on the grid and transfer them to particles. The influence of ionization and laser beam quality on the spectrum of pump laser has been studied for a range of gas densities. Simulations accurately explain the variations in Stokes/anti-Stokes signal intensities observed in the experiments for different gas-reservoir pressures.    

Simulation and modelling of diamond emitters in compact sources for high brightness beams

Diamond emitters manufactured from the semiconductor process is a candidate as an electron beam source for advanced compact accelerators and electron microscopy. The micron-scale pyramid structure of the emitter allows enhancement of the external field leading to emission with small beam size. We investigate the dependence of field enhancement due to the shape of the emitter and the resulting emission characteristic. The beam dynamics are simulated with the LSP PIC code to characterize the beam size and divergence. To account for electron transport in the bulk material and the tunnelling through the surface, a semi-classical Monte-Carlo (MC) emission model is developed for the diamond pyramid. We apply such emission model in our simulations and compare with experiments. The presence of a nano-scale tip from its fabrication process can introduces electronic structure size quantization affecting its transport and tunnelling processes which are accounted for in our nanowire emission model.   

Development of Unconventional Edge and Scrape-off Layer Measurements Using Compact Diagnostic Neutral and Ion Beams

We are developing diagnostic beam techniques to enable equilibrium and fluctuation measurements of density (n_e) and magnetic field (B) in edge and scrape-off layers of magnetically confined plasmas. Simultaneous, localized measurements of these parameters will provide unique insight into edge localized modes, current density, turbulence, and transport. A beam of neutral or singly ionized particles is injected into the plasma and undergoes ionization due to electron impact and charge exchange collisions. Analysis of the resultant charged particles yields information about B and n_e at the ionization location within the plasma. The diagnostic is similar to a heavy ion beam probe but has notable differences: the detector is located within the magnetic field of the confinement device, beam energy (and accelerator voltage) requirements are lower, and the system footprint is smaller. We will discuss two areas of innovation: (1) development of a detector and technique that could lead to determination of the magnetic vector potential in the plasma, and (2) simulations of 10-100 keV neutral alkali beams that could be used to probe the edge, scrape-off layer, and regions near the divertor X-point of a large tokamak.
  

Performance simulation of divertor neutral baffles in the TCV tokamak with the SolEdge2D-EIRENE code

A gas baffle will soon be inserted in the vessel of the tokamak à configuration variable (TCV) [Reimerdes, Nucl. Mat. and Energy 2017]. This upgrade aims at achieving more favorable conditions for the onset of detachment. In this work, we simulate the effect of the baffle on the TCV edge plasma with the SolEdge2D-EIRENE code [Bufferand, Nucl. Fusion 2015], which couples a fluid plasma model to a kinetic model for neutrals and impurities. Firstly, a specific TCV shot with a baffle-compatible shape is simulated. This comparison allows to tune perpendicular transport coefficients in order to match upstream experimental profiles, and results in a good agreement with the experiment at the targets. The same simulation is then carried out including the baffle. The neutral compression ratio, namely the ratio between divertor and upstream neutral pressure, is shown to improve by a factor of order 10, resulting in bigger power and momentum losses in the divertor plasma. Next, we perform a scan in upstream density to access different divertor regimes, revealing that the neutral compression increases as we approach detachment. Finally, in view of possible future optimizations, the level of baffle closure is varied in the simulations and the feedback on plasma properties is discussed.   

Coupled modeling of dust and plasma dynamics in tokamaks with DUSTT-UEDGE code

Dust events are commonly observed in present tokamaks and are expected to substantially contribute to impurity contamination of plasmas in future fusion devices. Dust  formation and mobilization in tokamaks are often associated with transient plasma phenomena, leading to intermittent ejection of large amounts of dust into the plasmas. To study the impact of the intermittent dust events on fusion plasmas, we developed coupled DUSTT-UEDGE code package simulating self-consistent dynamics of dust and plasma in tokamaks. Due to very large perturbations introduced by dust injection into plasma the self-consistent modeling presents significant challenges. We report on progress in the development of the coupled modeling capabilities and present latest results on simulations of the impact of transient ejection of large tungsten dust grains on fusion edge plasmas. In particular, improvements in the code convergence were achieved, which allowed to simulate detachment dynamics during tungsten dust ejection events in ITER-like divertor plasmas.   

Hydronitrogen-enhanced MAR process in D2-N2 plasmas to induce plasma detachment

The Molecular Assisted Recombination (MAR) process is thought to be a main channel of volumetric recombination to induce the plasma detachment operation. Authors have suggested a new plasma recombination process supported by hydronitrogen molecules such as ammonia, which will be formed by impurity seeding of N2 for controlling divertor plasma temperature and heat loads in ITER. This hydronitrogen-enhanced MAR (HN-MAR) process would occur throughout two steps: 1) ion exchange reactions between hydrogen ions and ammonia molecules, 2) recombination reactions between NH3/4+ ions and electrons. In this study, the HN-MAR process is investigated in plasmas ne ~ 1016-1018 m-3 fueled by D2 and ND3 in the PISCES-E RF plasma device. Ion densities are calculated and directly measured by using a rate equation model and a calibrated Electrostatic Quadrupole Plasma (EQP). The EQP detected ND3/4+ ions (10-75% of whole ion densities) which is the production of the step 1 reaction according to the model estimation. When ne > 1017 m-3, the volumetric recombination process (step 2) cannot be ignored by comparing with the surface loss reaction. In the plasmas ne~ 1017-1018 m-3, the experimental result shows good agreement with the model including the step 2 recombination reactions.   

X-divertor study with SOLPS-ITER

We will study the X-divertor (XD) geometry, in comparison to the standard divertgor (SD), in JET and MAST-Upgrade using the latest SOLPS-ITER simulation code. According to our previous experience with DIII-D and NSTX-U we expect the XD to mitigate the exhaust power better than the SD in JET and MAST-U as well. We hope to show that the scrape-off-layer can detach at a lower upstream density in the XD cases, and can keep a localized detachment front near the target, away from the core plasma. We will examine the the physical mechanisms of the flaring effect in details.   

TITLE: Numerical Studies of Sheath at Tokamak Edge

Plasma-surface interaction has been considered to be of great importance during a tokamak disruption event. Plasma particles interact with solid surface within the sheath layer which has a strong influence on the particle and energy flux towards the wall. In classical sheath model, many assumptions are made in order to obtain a simplified analytical model. However, at the tokamak edge, where sharp temperature gradient, strong inelastic collisions, and high recycling regime exists, some of the assumptions may not be valid. Thus, a kinetic study of divertor sheath will be helpful for understanding the physics. In present work, we develop a Monte Carlo Collision (MCC) package for vector particle in cell (VPIC) code, where elastic and inelastic interactions between charged particles and neutrals are included. Simulation results of plasma distribution profile and sheath parameters will be presented.   

A numerical study of plasma-neutral interaction in a coaxial electrode configuration

The flow Z-pinch concept magnetically confines a high-temperature, high-density plasma, while using sheared axial flows to provide stability. The Z-pinch has a simple, linear configuration in which the self-field generated by the axial current compresses plasma up to 1000 eV and confines it for 20-60 microseconds. Interactions between plasma and neutral species can have a large effect on the dynamic behavior both during plasma acceleration and when the Z-pinch plasma is magnetically confined. In this research, a reacting plasma-neutral model [Meier \& Shumlak, POP 19 (2012)] is incorporated into the NIMROD plasma simulation code to explore the effects of neutral gas in the acceleration phase of a z-pinch device. This combines a magnetohydrodynamic (MHD) plasma model with a gas dynamic neutral fluid model allowing for the study of electron-impact ionization, radiative recombination, and resonant charge-exchange in plasma-neutral systems. The acceleration section of the flow Z-pinch is modeled as a coaxial electrode accelerator, and neutral gas effects on current sheet propagation and spreading are investigated. Simulations with experimentally relevant current injection lead to current sheet propagation at about 40 km/s.   

Kinetic Physics in Tokamak Boundary Plasma

Unlike the core plasma, tokamak boundary plasma is in a non-equilibrium or a far-from-equilibrium thermodynamic state.  Individual particle orbits can significantly deviate from the usual equilibrium statistical behaviors, transport may not obey Fick’s law, the transport time-scale may not be diffusive, plasma can easily become non-Maxwellian, the conventional fluid closures based on the near-equilibrium thermodynamics –including the well-utilized CGL closure and Braginskii viscosity – may not be valid, and plasma turbulence may not be of usual type seen in the core plasma. A kinetic simulation may be needed for higher fidelity understanding of the boundary physics and for improvement of fluid closures.  This talk will introduce the kinetic aspect of the boundary plasma that may not be easily captured by fluid moments equations and that may help improve fluid simulations through collaborative effort. The spatial region of discussion will include the H-mode pedestal, the magnetic separatrix and the X-point, and the scrape-off region that is in contact with the material wall.  Neutral particle kinetics will also be discussed.  Update on L-H transition dynamics, and SOL and divertor heat-load physics will be discussed as examples.   

Riccati method for numerical tearing layers

We present a method for numerical solution of the tearing mode dispersion relation based on a technique for integrating stiff equations due to Forman Acton. The method is robustly stable for the tearing layers studied, and should be broadly applicable. The tearing layer equations are first Fourier transformed (FT), followed by a change of dependent variable using the Riccati-like transform w=kφ'(k)/φ(k) where φ(k) is the FT stream function. The solution is obtained via backwards integration from large k to zero. The Riccati transformation separates the `good' and `bad' solutions (as k-->∞) in the range of the Riccati variable w. The tearing layer dispersion function is related to the slope of the Riccati variable at k=0, and found by least squares fitting to a Pade approximant there.

Acton's method is generalized to higher order equations by a matrix Riccati transformation found using a Lie symmetry. We use the generalized matrix Riccati solver to study a 3rd order nonlinear system which reproduces the Glasser Effect (and real frequency tearing layers) despite neglecting classical transport and the divergence of the EXB drift in the tearing layer.   

Core magnetic shear effects on energetic ion drive to disruptive instabilities

Simulations of the onset of resistive MHD instabilities are presented where a slowing down distribution of energetic ions can either stabilize or destabilize disruptive tearing modes depending on the magnetic shear in the core.  Two cases are compared, one with monotonic shear throughout the profile and one with reversed shear in the core.  The monotonic case has a minimum q of 1.1 while the reversed case has a minimum of 1.3.   Outside of the reversal the profiles and equilibrium structure are nearly identical between the two cases.  The drive from energetic ions is stabilizing in monotonic shear and destabilizing in reversed shear, consistent with previous work.  In the reversed shear case without energetic ions a 3/2 mode is unstable, while with ions both a 2/1 and 3/2 mode are driven.  The beta limits as a function of rotation with a resistive wall are modified by the energetic ions in a reduced model depending on the magnetic shear.  Nonlinear simulations with energetic ions are also explored, and analyzed in terms of the linear results and a reduced model of the energetic ion effect on the resistive mode.   

Suppression of Tearing Modes by RF Current Condensation

Currents driven by rf (radio frequency) waves in the interior of magnetic islands can be used to stabilize deleterious tearing modes in tokamaks. Present analyses of stabilization strategies assume that the local deposition of rf power and the local rf acceleration of electrons are unaffected by the presence of an island, implying that the current required to stabilize the island increases as its width increases. It is shown here, however, that there is a threshold island width above which this assumption is significantly violated. There is a current condensation in the island, with a threshold width above which there is a decrease in the required current, and a second threshold at which the condensation effect is dramatically enhanced, allowing the stabilization of larger islands with more efficient use of the driven current. There is a hysteresis effect above the second threshold that shrinks the island to smaller width. The condensation effect also reduces the difficulty of aiming the rf current deposition for accurate radial alignment with the center of the island by concentrating the current deposition near the island center.   

Suppressing Locked Magnetic Island with Wave-driven Currents

The use of localized electron cylcotron current drive has been studied extensively as a means to control and mitigate magnetic islands in tokamaks. The use of static field perturbations to align locked neoclassical tearing modes with wave-driven currents has been demonstrated in DIII-D (F.A. Volpe et al., Phys. Rev. Letters 115, 175002, 2015), and sensitivity of tearing mode suppression to wave injection alignment has been shown numerically (D. De Lazzari, E. Westerhof, Nucl. Fusion 49, 075002, 2009). We introduce a static perturbation to a cylindrical or axisymmetric toroidal field, and investigate the effect of wave injection alignment and width on the suppression of a resonant magnetic island using a simple numerical model. We consider the impact of these parameters on the effectiveness of wave-driven currents in stabilizing the growth of locked modes. Predictions are given in parameter ranges relevant to DIII-D and ITER.   

Magnetic Presheath Boundary Conditions for NIMROD VDE Computations*

In general, VDE computations are expected to be sensitive to their boundary conditions. Axisymmetric resistive-MHD computations with the NIMROD code have shown that the computed halo current distribution depends on the temperature boundary condition and on the particle density boundary condition [Bunkers and Sovinec, BAPS 62, No. 12 (2017)]. Actual plasma contact with material surfaces leads to sheath effects that are impractical to resolve in global simulations. A practical alternative is a set of boundary conditions that represents conditions at the magnetic presheath entrance, which has been developed for plasma turbulence simulations [Loizu, et al., Phys. Plasmas 19, 122307]. Implementing these presheath-entrance conditions in NIMROD requires adapting the model for the non-reduced system of equations. The axisymmetric VDE calculations presented here compare the influence of various boundary conditions. Application of the lowest-order magnetic presheath effects produces significantly different behavior relative to the standard, mathematically simpler boundary conditions.  In particular, the flow smoothly connects to the wall when the magnetic presheath boundary conditions are applied.   

On the problem of equilibrium with flow in toroidal confinement systems and intrinsic rotation

Intrinsic rotation is an important stabilizing effect of interest in toroidal confinement devices. It is a highly complex problem, in which many mechanisms are involved, such as collisional moment transport, fluctuation ion-induced Reynolds and Maxwell stress tensors, charge exchange with neutrals, orbit losses, etc. [1].  However, it is usual to base theoretical treatments on equilibria in which flow is usually ignored. When dealing with small aspect ratio configurations, taking flow into account may be necessary. This work is an attempt to modify some of the existing theories when this refinement is included.
[1] Y. Camenen et al., \textit{Plasma Physics and Controlled Fusion} \textbf{59},  034001 (2017).
[2] J.D. Callen, \textit{Physics of Plasmas} \textbf{17}, 056113 (2010).

Ideal magnetohydrodynamic equilibrium expansion about a magnetic axis in a non-symmetric torus

A variant (weitzner Phys.Plasmas 2014) equilibrium representation is employed to construct a a formal expansion of a MHD equilibrium in a topological torus with a given magnetic axis.  The lowest order terms are given explicitly and the description of the higher order terms is given. Additionally an examination of the ability to extend the expansion to all orders is presented.   The expansion is in terms of the distance from the given magnetic axis. The conditions of quasisymmetry and omnigenity are given in terms of the expanded quantities.  Comparison with results of other approaches is also given.     

Computational study of magnetohydrodynamic equilibria with emphasis on fragility of flux surfaces

NSLAB, a variant of the NSTAB code, is being developed and employed to examine non-symmetric magnetohydrodynamic equilibria in a topological torus.  The plasma domain is a cube in three space, with periodicity in the y and z coordinates.  The code employs the standard variational equilibrium minimization method to obtain the equilibrium state.  Studies of  the destruction  or preservation of flux surfaces, dependent on the choice of boundary conditions, are undertaken, with results being presented.  Comparison of cases with zero shear, low shear, and moderate shear are given.       

Investigation of core MHD activities in the high βN plasmas in EAST

Improved H-mode plasmas with high normalized beta (β_N > 1.6) have been achieved on EAST tokamak. Weak temperature and density internal transport barriers (ITB) were observed. The main focus is on the influence of the core MHD activities, including fishbones and sawteeth on plasma performance. The electron temperature and the density profile evolutions show both temperature and density ITB maintained during the fishbone activities. Detailed comparisons of the kinetic profiles show trivial changes in the core region during fishbone activities but significant decrease due to sawteeth crash, which also gives possibility to trigger neoclassical tearing modes (NTMs). The safety factor (q) profile, confined by the Faraday rotation measurements from the polarimeter/interferometer (POINT) system on EAST, represents that a flat q profile with q₀ close to unity can be maintained and a 1/1 fishbone simultaneously exist. Detailed discussions of plasma performance under the same external control parameters will be studied and discussed.   

Studies of Fast-ion-driven MHD Instabilities in Heliotron J Equilibrium by Particle-MHD Hybrid Simulation code MEGA

Understanding of fast-ion-driven MHD instabilities is crucial for realizing sustainable burning plasma. In this study, MEGA, a hybrid MHD simulation code is applied to Heliotron J, which is a low shear helical-axis heliotron.  MEGA utilizes nonlinear MHD equations for bulk plasma and a drift kinetic equation for fast-ions.   Previously, MEGA has been implemented only in Tokamak and LHD.  The objective of this study is to verify the fast-ion MHD wave resonance in Heliotron J equilibrium by MEGA. The utilized equilibrium has m/n=7/4 magnetic islands at the edge region.  This islands are located close to the experimentally observed m/n=2/1 global Alfvén eigenmode (GAE).  To simplify the calculation, the coupling between Alfvén eigenmode and magnetic island is avoided. Only eigenmodes in the core region are considered.  Two eigenmodes may exist at r/a=0.4: m/n=8/5 toroidal Alfvén eigenmode (TAE) and m/n=5/3 beta-induced Alfvén-acoustic eigenmode (BAAE).  We have found an instability with the dominant harmonic of m/n=5/3 at the BAAE gap location; however, the frequency of the mode is 1 kHz, which is less than predicted value from BAAE dispersion relation (27.4 kHz).  The properties of this instability will be discussed and other instabilites will be explored.   

Simulations of SIHI Spheromak Current Drive Experiments

The HIT-SI experiment has demonstrated stable sustainment of spheromaks. Determining how the underlying physics extrapolate to larger, higher-temperature regimes is of prime importance in determining the viability of the inductively-driven spheromak. It is thus prudent to develop and validate a computational model that can be used to study current results and study the effect of possible design choices on plasma behavior. An extended MHD model has been applied to simulate the HIT-SI and its successor HIT-SI3 across a range of injector frequencies (14, 36, 53, 68 kHz), and the results of those frequency scans will be shown, detailing general agreement with most experimental trends as well as where and what differences are observed. Behavior of a proposed future design with an alternate injector configuration will also be explored via extended MHD, and comparisons to earlier configurations made.   

Preliminary measurements of electron cyclotron emission from the PFRC II

Understanding energy flows in magnetic fusion energy reactors is critical to achieving practical designs. Cyclotron radiation is expected to be an important energy loss channel in reactors utilizing advanced fuels, e. g. D-He3, due to the required core plasma temperatures. Current models for a full-scale reactor based on the Princeton Field-Reversed Configuration (PFRC) include predictions for electron cyclotron emission (ECE), though with uncertainties on the order of 10% or more. Properly characterizing the spectrum and intensity of radiation emitted by the PFRC II would improve the accuracy of these models. Results of tracing waves through FRC-like equilibria and preliminary measurements of ECE from the PFRC II are presented, along with comments on their applicability to reactor-scale FRC models.   

3D neutral distribution in C-2W determined from visible imaging and interpretive modeling

The C-2W device\footnote{M.~Binderbauer, et~al. AIP Conference Proceedings \textbf{1721}, 030003 (2016)} is equipped with eight 1.6 MW neutral beams which inject large-orbit fast ions to sustain a field-reversed configuration embedded within an axisymmetric magnetic mirror.  Collisional energy transfer from fast ions is intended to be the primary source of heating of the bulk plasma; therefore, the spatial and velocity distribution of fast ions strongly affects the overall power balance and understanding fast ion loss is critical.  At typical injected energies (15 keV) and external fields (0.7-1 kG) fast ions sample the core, scrape-off-layer, and diffuse outer-edge plasmas where they can be lost via charge-exchange with neutrals.  Careful vacuum practices and titanium gettering successfully reduced neutral recycling from the vessel walls.  As a result, warm neutrals generated from beam capture via charge-exchange with bulk plasma ions constitute a large fraction of the remaining neutrals.  Balmer-$\alpha$ emission measured with a filtered high-speed camera is used with DEGAS2 neutral particle modeling to reconstruct the strongly non-axisymmetric neutral distribution. This distribution is then used with Monte Carlo modeling to estimate the charge-exchange loss rate of fast ions.   

Langmuir Probe Characterization of the C-2W Scrape-Off-Layer Plasmas

The C-2W experiment is a field-reversed configuration (FRC) which uses neutral beam injection and an edge biasing scheme to suppress turbulence and stabilize the plasma. The FRC core is surrounded by open field lines in the edge region of the confinement vessel (CV), called the scrape-off-layer (SOL), which terminate on electrode plates located in the inner and outer divertors. Therefore, settings in the divertors can significantly affect the plasma conditions and stability in the SOL. A motorized triple Langmuir probe with 24 inches of travel has been installed on the CV, 66 cm from the mid-plane to make critical measurements of the SOL plasma parameters. Characterization of the electron density (n_e), electron temperature (T_e), floating potential, plasma potential, and edge fluctuations in the C-2W SOL are presented. Initial measurements show SOL T_e ~20 eV as the FRC is formed, after which T_e gradually decreases, and SOL n_e ~10¹⁸ m^-3, consistent with Thomson Scattering and FIR diagnostic data measured at the CV mid-plane. Finally, details of a design for a new insertable probe platform system and initial measurements in the C-2W inner divertor will be discussed.   

Onset of Tilt Instability in FRC Plasmas

Tilt instability in FRC plasmas is a well-known MHD phenomenon that has been extensively investigated over the years by different authors and an empirical scaling law of the numerical growth rate versus the plasma E/S* parameter has been discussed in the literature. FPIC, TAE Technologies’ proprietary hybrid code, was also recently deployed to study the n=1 toroidal modes as tilt, radial shift and interchange. In spite of the investigations already carried out what is the exact onset mechanism and what are the plasma structures which characterize the very early phase of the tilt instability remain largely unknown and any possible investigation requires very large computational resources. Thanks to computing resources provided at ALCF by the 2018 INCITE program it has been possible to utilize HPC capabilities and apply FPIC at unprecedented levels of resolution. Detailed plasma structures at tilt onset and subsequent times are presented and discussed.   

Reconstructing FRC equilibria from experimental data

The magnetic structure of FRCs cannot be measured directly by existing diagnostics. A new tool finds the structure based on field measurements at the vessel wall. The first step is data conditioning, finding smooth fits to the data, i.e. the excluded flux radius and wall magnetic flux, both as functions of the axial coordinate. Distilled from these are six parameters, three each from the excluded flux and wall flux profiles. The second step is the equilibrium finder, assuming a static equilibrium (Grad-Shafranov, “GS” equation). The inputs to GS system are the pressure vs flux function and the wall-flux boundary condition. The pressure is expressed as one function family and the wall flux as another. Both families have three adjustable parameters for a total of six. Finally, a two-level iteration finds the solution: the inner loop is a standard relaxation algorithm that updates the flux function; the outer loop iterates the six adjustables, targeting the six parameters distilled by data conditioning. The result is the 2D structure of the flux variable. Extracted from this are various properties of interest, e.g. trapped flux, fraction of plasma current flowing inside the separatrix, as well as stability indices for interchange and tearing.   

Effects of Rotation on Turbulent Transport in Field Reversed Configuration

The global field reversed configuration (FRC) geometry is incorporated in the gyrokinetic code GTC by using a field-aligned mesh in cylindrical coordinates, which enables the  global simulations coupling the FRC core and scrape-off layer (SOL) across the separatrix and from divertor to divertor. Magnetic function is used in the cylindrical coordinates to guarantee divergence-free of the magnetic field and to facilitate the construction of the global field-aligned mesh. This  global particle code has been successfully applied for linear drift wave simulation in FRC. Furthermore, a new space-charge 1D code for sheath simulation is developed, which can provide the boundary condition for the global simulation  with sheath effects and divertor biasing voltage. The effects of plasma rotation due to the external applied biasing voltage on the linear drift wave instability and transports are investigated in this work.
  

Fusion Enhancement from Bunched Ion Beams

Using a 1D PIC simulation, the potential for fusion enhancement through injection of an ion beam with density modulation (“bunching”) is demonstrated. The beam is composed of protons, and the background is composed of homogeneous deuterons and electrons in a uniform magnetic field, approximating the scrape-off-layer (β ≈ 0.1) in the FRC (field-reversed configuration) of the C-2U experiment. First, near-perpendicular angles of beam injection and wave propagation are identified as best for producing fast ions. This fast-ion tail is created by beam-driven Ion-Bernstein (IB) modes through a coherent Tajima-Dawson wakefield-like mechanism. The DD fusion reactivity is calculated from the deuteron velocity distribution and normalized with respect to the initial thermonuclear value to quantify fusion enhancement. Next, the fusion enhancement is calculated for an array of beam energies, revealing a power-law relationship. Finally, the beam is initialized with density bunching with wavelength matching that of the fastest-growing IB mode, and the scan over beam energies is repeated. The resulting fusion enhancement is found to exceed that of the un-bunched case by as much as a factor of ≈ 30 for roughly v_b < 32v_th, beyond which the mode structure evolves away from IB modes.   

Modeling of an argon fluoride single pass amplifier

The US Naval Research Laboratory is developing a repetitively pumped electron-beam ArF* laser based on Electra, which could be the ideal driver for direct and indirect drive for inertial confinement fusion. A kinetics model for the e-beam pumped ArF* laser has been initiated based on the existing Orestes model, initially developed for the KrF* laser at NRL [1]. Uncertainties in many reaction rates and unknown lasing efficiency motivate our basic research program. Here, the existing model was modified to include the Ar-F2 plasma chemistry and rates from a  Boltzmann kinetics model [2]. We report on the results for a single pass amplifier with an external laser source with different intensities. The small signal gain, non-saturable absorption, and saturation intensity of ArF* as a function of applied e-beam power, gas pressure and composition were calculated and compared with the results from experiments. This research enables understanding of power and particle balances, evaluates the extracted laser intensity and intrinsic efficiency as a function of the e-beam power deposition, and further develops the e-beam technologies.

[1] G. M. Petrov, et. al., JAP 91 2662 (2001); 92 1200 (2001).
[2] G. M. Petrov, et. al., JAP 122 133301 (2017).
  

A repetitive nanosecond pulse generator with photoconductive semiconductor and application for atmospheric plasma jets

In this paper, a compact repetitive nanosecond pulse generator is designed for exciting the atmospheric pressure plasma jets (APPJ) in noble gases. To realize the ultrashort pulse output, a novel Si photoconductive semiconductor switch (PCSS) with reflected cavity and two pulsed formed lines were used. Based on the Blumlein topology, the nanosecond pulse generator can produce repetitive pulses with output voltages of up to 15kV, pulse width of ~7ns, and pulse repetition frequencies of 1Hz~1kHz. Using the designed repetitive nanosecond pulse generator, the characteristics of the APPJ are investigated by measuring the voltages and currents and obtaining images of the discharges. Experimental results show that the nanosecnod pulsegenerator has been successfully used to sustain stable APPJs in helium. The length of plasma plume is over 2cm. Furthermore, the effects of flow rate, the applied voltage, and the pulse frequencies on the He APPJs are investigated.
  

Probe measurements at higher pressure

Existing theories and previous probe measurements of electron distribution functions (EDF) at higher gas pressure are reviewed.  An explanation of whether or not the measurements are realizable and reliable, an enumeration of the most common sources of measurement error, and an outline of proper probe-experiment design elements that inherently limit or avoid error are presented. Recent EDF-measurement developments in higher-pressure plasma conditions, including electron spectroscopy analysis with a large wall probe, are discussed. A short (without positive column) dc discharge with cold cathode and conducting walls was used in experiments at gas pressures of a few Torrs. For the investigated conditions, the plasma is collisional. It is experimentally shown that the EDF is proportional to the second derivative of electron current with respect to the probe potentials (as in collisionless theory). EDF measurements conducted in Helium-Argon gas mixtures with percent composition of Argon from 0.002% to 5% will be presented, as will the demonstration of current-voltage calibration.   

Implementation of Optical Emission Diagnostics for EEDFs on a Helicon Plasma

Optical emission spectroscopy (OES) is attractive as a non-invasive approach for measuring electron energy distribution functions (EEDF) in low-temperature plasmas. Adaptation of an approach successfully developed and implemented on an argon inductively-coupled plasma (ICP) to a helicon plasma system is the focus of this study. Trial EEDFs are an input to model calculations of excitation rates for select electronic transitions using known collision cross sections, and yielding the system EEDF as the trial that results in a best match to a set of measured emission intensities.  Preliminary helicon OES data show regimes with much stronger argon ion emissions than observed in the ICP, where argon ion lines were so weak that they could not be productively included along with neutral lines in the OES analysis.  Because of the high threshold energy for excitation of argon ion emissions, they signify the presence of energetic electrons, providing an opportunity to extend testing of the diagnostic to a new regime.  Potential for improvements in the OES diagnostic for plasmas with a significant population of relatively high energy electrons, through the addition of argon ion lines to the emission model, will also be discussed.   

Kinetic Simulations of High Power Impulse Magnetron Sputtering Process

HiPIMS is the magnetron sputtering regime which utilizes short high voltage pulses of tens of microseconds, as an alternative to the DC operation regime. HiPIMS is characterized by high density of plasma accumulated over the target, high energy of sputtered atoms, and a high ionization fraction of sputter products. Until now, optimization of magnetron devices has been done empirically or using reduced models. Reduced models, for example, may represent densities 10-100x lower than actual experimental densities. We present fully kinetic 3D simulation of HiPIMS discharge using particle-in-cell code Chicago from the developers of LSP [D. Welch, et.al, Phys. Plasmas 13, 063105 (2006)]. The model is validated with experimental measurements showing good agreement in terms of current and sputtering efficiency. Presented simulations capture the plasma formation, sputtering and transport of the sputtered material.   

Kinetic and Fluid Simulations of Collisional Plasma Shock

Thermophysics Universal Research Framework (TURF) is an in-house framework developed at the In-Space Propulsion Branch of the Air Force Research Laboratory (AFRL/RQRS). Cross verification between PIC, Vlasov, and fluid plasma models within the framework was conducted on the 1D electrostatic collisionless shock, and it was confirmed that the PIC and Vlasov models produced the same result while the fluid model did not capture the important kinetic features. This work will extend the previous work by including a collisional effect between charged species using Nanbu's method for the PIC model and solving the Fokker-Planck equation. The three models (PIC with Coulomb collision, Vlasov-Fokker-Planck, and fluid models) will be used to simulate a 1D collisional plasma shock within the same framework.   

Numerical simulation of nonlinear behavior in plasma based photonic crystals and metamaterials using a multi-fluid plasma model

Plasma photonic crystals and metamaterials (PC/MMs) have the potential to significantly expand the capabilities of current microwave filtering and switching technologies by providing high speed (μs) control of energy bandgap/pass characteristics in the GHz through low THz range.  While photonic crystals consisting of dielectric, semiconductor, and metallic matrices have been thoroughly investigated over the last several decades, plasma-based PC/MMs remain a relatively unexplored field.  Numerical modeling efforts so far have largely used the standard methods of analysis for photonic crystals (the Plane Wave Expansion Method, Finite Difference Time Domain, and Drude model simulations), which do not capture nonlinear plasma-radiation interactions.  In this study, a fully coupled Maxwell 5N-moment multi-fluid plasma model is implemented within the WARPXM finite element code to investigate the nonlinear deformation of plasma structures under intense radiation.  Transmission spectra of the deformed structures are compared with experimental data and ANSYS HFSS calculations.  It is proposed that observed bandgap broadening could be harnessed for plasma-based adjustable power modulators.