Session PP11: Poster Session VI: HEDP II; Compression and Burn; NSTX-U & Boundary; Direct, Indirect, and Polar-drive; Hydrodynamic Instability; Basic Plasmas

Development of a compact 30 T magnetic field system for OMEGA

Aiming at conducting studies of magnetized high-energy density plasmas in a high magnetic field, we are developing a compact system capable of creating a pulsed magnetic field of about 30T in a volume of several cubic centimeters. The system prototype will be tested at the University of Michigan and will be adopted afterwards for use at the OMEGA facility of the Laboratory for Laser Energetics (LLE) of the University of Rochester, NY. The system consists of a pulsed power supply situated outside of the Omega vacuum chamber and a magnetic coil inserted into the chamber with a diagnostic inserter. The power supply is based on a 50$\mu$F/20kV storage capacitor and is capable of driving a pulse of current of up to 50kA through the coil. The power supply is connected with the coil via a low-inductive chain of power cables and a strip transmission line. The system electrical, magnetic, and thermal analysis will be presented along with the results of initial testing.   

Solenoid for Laser Induced Plasma Experiments at Janus

Creating invariant magnetic fields for experiments involving laser induced plasmas is particularly challenging due to the high voltages at which the solenoid must be pulsed. Creating a solenoid resilient enough to survive through large numbers of voltage discharges, enabling it to endure a campaign lasting several weeks, is exceptionally difficult. Here we present a solenoid that is robust through 40 $\mu$s pulses at a $\sim$13 kV potential. This solenoid is a vast improvement over our previously fielded designs in peak magnetic field capabilities and robustness. Designed to be operated at small-scale laser facilities, the solenoid housing allows for versatility of experimental set-ups among diagnostic and target positions. Within the perpendicular field axis at the center there is 300 degrees of clearance which can be easily modified to meet the needs of a specific experiment, as well as an $\sim$f/3 cone for transmitted or backscattered light. After initial design efforts, these solenoids are relatively inexpensive to manufacture.   

Characterization of cylindrically imploded magnetized plasma by spectroscopy and proton probing

Understanding the role of magnetic field in relativistic electron beam transport and energy deposition is important for several applications including fast ignition inertial confinement fusion. We report the development of a cylindrically compressed target platform with externally applied magnetic fields on OMEGA. As a first step, we performed an experiment to characterize the imploded plasma and compressed field condition. The implosion of the target was performed using 36 UV beams (400 J per beam, 1.5 ns square pulse), and the magnetic field was measured by proton deflection using mono-energetic protons produced from D3He capsule implosion. The target was a CH foam cylinder doped with 1{\%} chlorine in order to detect the time-resolved 1s-2p Cl absorption structures, using a gold foil as a broad band backlighter source. A Cu foil at the beginning of the foam cylinder and a Zn foil at the end, allowed us to measure the K$\alpha $ and the 1s-2p transitions of He-like and Li-like ions for both elements. The emission and absorption spectroscopic data are compared to atomic physics codes to determine the plasma temperature and density under the influence of the magnetic field.   

Hybrid Fluid/Kinetic Modeling Of Magnetized High Energy Density Plasmas

MHD modeling with an equation of state (EOS) of the Rayleigh-Taylor (RT) instabily in Z indicates that it is seeded by the electro-thermal instability. Large thermodynamic drives associated with gradients at the interface between the liner and the coronal regions distort distribution functions and likely lead to non-local transport effects in a plasma which varies from weakly to strongly coupled. In this work, we discuss using effective potential theory \footnote{S. D. Baalrud and J. Daligault \em{PRL}, \bf{110}, 235001 (2013)} along with a Chapman-Ensksog-like (CEL) formalism to develop hybrid fluid/kinetic modeling capabilities for these plasmas. Effective potential theory addresses the role of Coulomb collisions on transport across coupling regimes and the CEL approach bridges the gap between full-blow kinetic simulations and the EOS tables, which only depend locally on density and temperature. Quantitative results on the Spitzer problem across coupling coupling regimes will be presented as a first step.   

Enhancing Understanding of Magnetized High Energy Density Plasmas from Solid Liner Implosions Using Fluid Modeling with Kinetic Closures

Recent results from experiments and simulations of magnetically driven pulsed power liners have explored the role of early-time electrothermal instability in the evolution of the MRT (magneto-Rayleigh-Taylor) instability. Understanding the development of these instabilities can lead to potential stabilization mechanisms; thereby providing a significant role in the success of fusion concepts such as MagLIF (Magnetized Liner Inertial Fusion). For MagLIF the MRT instability is the most detrimental instability toward achieving fusion energy production. Experiments of high-energy density plasmas from wire-array implosions have shown the requirement for more advanced physics modeling than that of ideal magnetohydrodynamics. The overall focus of this project is on using a multi-fluid extended-MHD model with kinetic closures for thermal conductivity, resistivity, and viscosity. The extended-MHD model has been updated to include the SESAME equation-of-state tables and numerical benchmarks with this implementation will be presented. Simulations of MRT growth and evolution for MagLIF-relevant parameters will be presented using this extended-MHD model with the SESAME equation-of-state tables.   

Kinetic Simulations of Laser Parametric Amplification in Magnetized Plasmas

Laser pulse compression using magnetized resonance near the upper-hybrid frequency is promising for achieving higher output intensity in regimes previously thought impossible using unmagnetized plasmas. Using one-dimensional particle-in-cell simulations, we verify that, by partially replacing plasma with an external transverse magnetic field of megagauss scale, the output pulse can be intensified by a factor of a few, due to the increased allowable amplification time despite a decreased growth rate. Further improvement is impeded by the generation of an electromagnetic wakefield, to which the amplified pulse loses more energy than it does in the unmagnetized case. This limitation can however be circumvented by the use of a stronger pump. In contrast to unmagnetized compression, the magnetized amplification remains efficient when the pump intensity is well above the wavebreaking threshold, until a higher phase-mixing threshold is exceeded. This surprising resilience to wavebreaking in magnetized plasma is of great benefit for magnetized compression.   

On the Crossover from Classical to Fermi Liquid Behavior in Dense Plasmas

We explore the crossover from classical plasma to quantum Fermi liquid behavior of electrons in dense plasmas. To this end, we analyze the evolution with density and temperature of the momentum lifetime of a test electron introduced in a dense electron gas. This allows us 1) to determine the boundaries of the crossover region in the temperature-density plane and to shed light on the evolution of scattering properties across it, 2) to quantify the role of the fermionic nature of electrons on electronic collisions across the crossover region, and 3) to explain how the concept of Coulomb logarithm emerges at high enough temperature but disappears at low enough temperature.   

Thermal conductivity study of warm dense matter by differential heating on LCLS and Titan

A differential heating platform has been developed for thermal conduction study, where a temperature gradient is induced and subsequent heat flow is probed by time-resolved diagnostics. Multiple experiment using this platform have been carried out at LCLS-MEC and Titan laser facilities for warm dense Al, Fe, amorphous carbon and diamond. Two single-shot time-resolved diagnostics are employed, SOP (streaked optical pyrometry) for surface temperature and FDI (Fourier Domain Interferometry) for surface expansion. Both diagnostics provided excellent data to constrain release equation-of-state (EOS) and thermal conductivity. Data sets with varying target thickness and comparison between simulations with different thermal conductivity models are presented.   

Proton Beam Driven Isochoric Heating to Warm Dense Matter Conditions on Texas Petawatt

Isochoric heating of solids and gases to warm dense matter conditions is relevant to the study of equation of state as well as laboratory astrophysics, specifically heating of hydrogen gas (\textasciitilde 10$^{\mathrm{17}}$-10$^{\mathrm{19}}$ cm$^{\mathrm{3}})$ to 0.5-3eV for the study of white dwarf atmospheres. In a series of experiments on Texas Petawatt, we have built a platform using the petawatt laser focused softly to a large focal spot (60-70um) to generate large numbers of intermediate energy protons via TNSA, ideal for isochoric heating. We have previously used the proton beam to isochorically heat 10um aluminum foils to 20eV. This poster presents results of experiments in which low Z materials such as methane gas, carbon foams, and hydrogen are heated using this platform. We are measuring the surface brightness temperature and heating with a streaked optical pyrometer, and XUV emissions using an XUV spectrometer.   

Single pass density diagnostic for expanded warm dense plasmas

Warm dense plasmas are opaque to visible light. However, the density gradient of the expanded, less dense plasma surrounding the warm dense matter (WDM) can be optically accessed. This paper describes the development and implementation of Moir\'{e} deflectometry and Nomarski interferometry techniques for analysis of the expanded WDM produced from an intense electron beam on a thin foil. The 20 MeV beam incident on copper or titanium targets produce plasmas where the densities are \textless 8 x 10$^{\mathrm{22\thinspace }}$cm$^{\mathrm{-3}}$. The measurements rely on a probe laser of wavelength 405 nm in which the critical density is \textasciitilde 7 x 10$^{\mathrm{21}}$ cm$^{\mathrm{-3}}$, meaning a large portion of the plasma is accessible. Preliminary maps of the density gradient obtained by Moir\'{e} deflectometry and the density by Nomarski interferometry will be presented. In addition, the characterization, development, and implementation of these techniques are applied to atmospheric plasma sources.   

Soft X-ray Spectrometer for Characterization of Electron Beam Driven WDM

A preliminary design study is being performed on a soft X-ray spectrometer to measure K-shell spectra emitted by a warm dense plasma generated by an intense, relativistic electron beam interacting with a thin, low-Z metal foil. A 100-ns-long electron pulse with a beam current of 1.7 kA and energy of 19.8 MeV deposits energy into the thin metal foil heating it to a warm dense plasma. The collisional ionization of the target by the electron beam produces an anisotropic angular distribution of K-shell radiation and a continuum of both scattered electrons and Bremsstrahlung up to the beam energy of 19.8 MeV. A proof-of-principle Bragg-type spectrometer has been built to measure the Ti K-$\alpha $ and K-$\beta $ lines. The goal of the spectrometer is to measure the temperature and density of this warm dense plasma for the first time with this heating technique.   

Extreme ultraviolet capillary discharge lasers

An extreme ultraviolet capillary discharge laser has recently been installed at the University of York. The laser produces EUV radiation of wavelength 46.9nm, with pulse durations of approximately 1.2ns and energies of up to 50$\mu $J. A population inversion is produced by a high voltage electrical discharge passing through an argon filled capillary tube. Within the capillary, radial pinching of the argon plasma through \textbf{J}x\textbf{B} force causes the pressure and temperature of the plasma to increase which causes amplification between 3p -3s (J$=$ 0-1) transitions producing EUV radiation. Laser optimisation, calibration of detectors and designs for initial experiments to produce warm dense matter by focusing onto solid targets are presented. The plasmas formed by the EUV laser irradiation of solid targets can be shown to produce warm dense matter in a regime where the ionization equilibrium is dominated by radiative ionization.   

Prediction of scaling physics laws for proton acceleration with extended parameter space of the NIF ARC

The Advanced Radiographic Capability (ARC) laser at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the world's most energetic short-pulse laser. It comprises four beamlets, each of substantial energy (\textasciitilde 1.5 kJ), extended short-pulse duration (10-30 ps), and large focal spot ($\ge $50{\%} of energy in 150 \textmu m spot). This allows ARC to achieve proton and light ion acceleration via the Target Normal Sheath Acceleration (TNSA) mechanism, but it is yet unknown how proton beam characteristics scale with ARC-regime laser parameters. As theory has also not yet been validated for laser-generated protons at ARC-regime laser parameters, we attempt to formulate the scaling physics of proton beam characteristics as a function of laser energy, intensity, focal spot size, pulse length, target geometry, etc. through a review of relevant proton acceleration experiments from laser facilities across the world. These predicted scaling laws should then guide target design and future diagnostics for desired proton beam experiments on the NIF ARC.   

Measurements of plasma mirror reflectivity and focal spot quality for tens of picosecond laser pulses~

The Advanced Radiographic Capability (ARC) laser at the NIF (LLNL) is high-energy (\textasciitilde 4 kJ) with a pulse length of 30ps, and is capable of focusing to an intensity of 10$^{\mathrm{18}}$W/cm$^{\mathrm{2}}$ with a \textasciitilde 100 $\mu $m focal spot.~The ARC laser is at an intensity which can be used to produce proton beams.~ However, for applications such as radiography and warm dense matter creation, a higher laser intensity may be desired to generate more energetic proton beams. One possibility to increase the intensity is to decrease the focused spot size by employing a smaller f-number optic. But it is difficult to implement such an optic or to bring the final focusing parabola closer to the target within the complicated NIF chamber geometry. A proposal is to use ellipsoidal plasma mirrors (PM) for fast focusing of the ARC laser light, thereby increasing the peak intensity. There is uncertainty, however, in the survivability and reflectivity of PM at such long pulse durations.~ Here, we show experimental results from the Titan laser to study the reflectivity of flat PM as a function of laser pulse length. A calorimeter was used to measure the PM reflectivity. We also observed degradation of the far and near field energy distribution of the laser after the reflection by the PM for pulse-lengths beyond 10ps.   

Plasma formation and target preheating by prepulse of PW laser light

An intense short pulse laser with intensity over $10^{21}$ W/cm$^2$ has become available, i.e. J-KAREN-P at QST. Although the contrast of the short pulse is improved to be of the order of $10^{-11}$, there is an unavoidable prepulse, which has multiple spikes (~ ps) on top of an exponential profile with intensity greater than $10^{14}$ W/cm$^2$ about 50 ps in front of the main pulse. The prepulse preheats the target and also produces tenuous plasmas in front of a target before the main pulse arrives. It is critical to understand such preheating of the target, where the nonlocal heat transport is essential at intensity $> 10^{14}$ W/cm$^2$, since the target condition might totally change before the interaction with the main pulse. Using a hydro code, FLASH, and a collisional particle-in-cell code, PICLS, we study the preplasma formation and target preheating over tens of picoseconds timescale, and discuss the prepulse effects on the main pulse interaction.   

High order harmonic generation with femtosecond mid-infrared laser

There has been growing interest in high order harmonic generation (HHG) from laser-solid interactions as a compact source of coherent x-rays. The ponderomotive potential in laser-plasma interactions increases with longer laser wavelength, so there may be significant differences in the physics of harmonic generation and other phenomena when experiments are conducted with mid-infrared lasers. Previous experiments, however, have been done almost exclusively with near-infrared lasers. In this work, we report the results of experiments performed with millijoule, 40 fs, 2 \textmu m laser pulses generated by an optical parametric amplifier (OPA) which are focused onto solid targets such as silicon and glass. The HHG efficiency, polarization dependence, and x-ray emission are studied and compared to measurements with near-infrared lasers.   

Enhanced Laser-to-Electron Energy Conversion Efficiency using Micro-Plasma Waveguide (MPW) Targets

We present experiments from the Scarlet laser facility and 3D Particle in Cell (PIC) simulations detailing the improved hot electron spectrum of MPW targets over flat targets. We observe an increase in the electron cutoff energy by a factor of 3 and a 10x enhancement of the total signal of electrons above 5MeV. From PIC simulations, we see strong transverse electric fields extract electron bunches from the MPW walls with the laser period, which are then accelerated by the usual \textbf{v}x\textbf{B} force. In addition, quasi-static longitudinal electric fields arise and are observed to increase the acceleration length of electrons along the tube walls. In this way, the micro-engineered structures provide a geometry more conducive to efficient direct laser acceleration and offer a new dimension in target design. We present evidence that by varying the structure's geometry we can alter the laser plasma interactions with applications in high field science, laser based proton therapy and relativistic nonlinear optics. In particular, the relationship between the MPW tube and laser-electron dephasing length is examined.   

Modeling Transport of Relativistic Electrons through Warm-Dense Matter Using Collisional PIC

In electron transport experiments performed on the OMEGA EP laser system, a relativistic electron beam was created by focusing a high intensity ($eA/m_ec > 1$) laser onto a gold (Au) foil. Behind the Au foil was a layer of plastic (CH) foam, with an initial density of $200mg/cm^3$. Before the high intensity laser was switched on, this foam was either left unperturbed; or it was shocked using a lower intensity laser ($eA/m_ec \sim 10^{-4}$) with beam path perpendicular to the high intensity laser, which left the CH layer in a warm dense matter (WDM) state with temperature of 40 eV and density of $30mg/cm^3$. The electron beam was imaged by observing the k-$\alpha$ signal from a copper foil on the far side from the Au. The result was that transport was decreased by an order of magnitude in the WDM compared to the cold foam. We have modeled this experiment using the PIC code OSIRIS, with also a Monte Carlo Coulomb collision package. Our simulations indicate that the main cause of the differences in transport is a collimating magnetic field in the higher density, cold foam, created by collisional resistivity. The plasma density of the Au layer, difficult to model fully in PIC, appears to effect the heat capacity and therefore temperature and resistivity of the target.   

Backward Raman amplification of laser pulses in plasma with random inhomogeneities.

Backward Raman amplification of laser pulses in plasmas to nearly relativistic intensities was already observed experimentally, but complete pump depletion by amplified pulse still was not achieved. We argue here that the complete pump depletion may be more readily achieved experimentally in plasmas having noticeable random density inhomogeneities, rather than in highly uniform plasmas which are difficult to produce. Random inhomogeneities can naturally suppress such major parasitic effects as the transverse filamentation instability and Raman back- and side-scatterings of laser pulses by plasma noise, thus enabling useful amplification through longer distances.   

Direct ion heating in overdense plasmas through the Brillouin instability driven by relativistic whistler waves

Strong magnetic fields over kilo-Tesla have been available in the laboratory by the use of ultra-intense lasers. It would be interesting to apply those strong fields to other laser experiments such as the inertial confinement fusion and laboratory astrophysics. The characteristics of laser-plasma interactions could be modified significantly by the presence of such strong magnetic fields. We investigate electromagnetic wave propagation in overdense plasmas along the magnetic field for a right-hand circularly polarized wave by PIC simulations. Since the whistler mode has no cutoff density, it can penetrate into overdense plasmas and interact directly with charged particles there. When the external field strength is near a critical value defined by that the cyclotron frequency is equal to the laser one, it is reported that electrons are accelerated efficiently by the cyclotron resonance. However, if the field strength is far beyond the critical value, the cyclotron resonance is inefficient, while the ions gain a large amount of energy directly from the laser light owning to the Brillouin scattering. As the result, only ions are heated up selectively. We will discuss about the application of this ion heating in dense plasmas.   

Exploring Ultrahigh-Intensity Laser-Plasma Interaction Physics with QED Particle-in-Cell Simulations

Next generation high-intensity lasers are reaching intensity regimes where new physics---quantum electrodynamics (QED) corrections to otherwise classical plasma dynamics---becomes important. Modeling laser-plasma interactions in these extreme settings presents a challenge to traditional particle-in-cell (PIC) codes, which either do not have radiation reaction or include only classical radiation reaction. We discuss a semi-classical approach to adding quantum radiation reaction and photon production to the PIC code VPIC. We explore these intensity regimes with VPIC, compare with results from the PIC code PSC, and report on ongoing work to expand the capability of VPIC in these regimes.   

Generation mechanism of power law energy distribution in an expanding thin-foil plasma irradiated by intense lasers

Power law energy spectra consisting of high energy particles have been observed ubiquitously in nature such as cosmic rays in astrophysical plasmas, and are considered to be generated via multiple-scattering processes in electric/magnetic/electromagnetic fields. However, the critical details of the acceleration, diffusion and relaxation processes that lead to the observed superthermal distributions have not understood completely. In intense laser-produced plasmas, the strong laser field in the intensity level exceeding $10^{18}$ W/cm$^{2}$ and self-generated fields play a role in stochastic multiple-scattering which dominates the electron acceleration and heating [1, 2]. In this study, by using the particle-in-cell simulation, we found that the high energy tail of the electron energy spectrum becomes power law distribution, so called the ‘kappa distribution’ [3], in the interaction between a thin-foil plasma and a multi-picosecond high intensity laser. We discuss the generation mechanism of the power law tail relating to the multiple-scattering of electrons in the expanding foil plasma in details. [1] Y. Sentoku et al., Phys. Rev. Lett. 90, 155001 (2003). [2] N. Iwata et al., Phys. Plasmas 24, 073111 (2017). [3] A. Hasegawa et al., Phys. Rev. Lett. 54, 2608 (1985).   

Effects of Soft-Core Potentials and Coulombic Potentials on Bremsstrahlung Radiation during Laser Matter Interaction

An intense, short laser pulse incident on rare-gas clusters can produce nano-plasmas containing energetic electrons. As these electrons undergo scattering, both from phonons and ions, they emit bremsstrahlung radiation. Here we compare a theory of Bremsstrahlung emission appropriate for the interaction of intense lasers with matter using soft-core potentials and coulombic potential. A new scaling for the radiation cross-section and Emissivity via bremsstrahlung are derived for soft-core potential which depends on the potential depth, used to avoid coulomb singularity and for coulombic potential and implemented in a particle in cell code (PICLS). The radiation cross-section and emissivity via bremsstrahlung is found to increase rapidly with increases in potential depth up to 100 eV and then becomes mostly saturated for larger depths of a soft-core potential. For both cases, the radiation cross-section and emissivity of Bremsstrahlung increases with increases in laser wavelength. The bremsstrahlung emission may provide a broadband light source for diagnostics.   

Studying Electromagnetic Beam Instabilities in Laser Plasmas for Alfv´enic Parallel Shock Formation

We present measurements of the collisionless interaction between an exploding laser-produced plasma (LPP) and a large, magnetized ambient plasma. The LPP is created by focusing a high energy laser on a target embedded in the ambient Large Plasma Device (LAPD) plasma at the University of California, Los Angeles. The resulting super-Alfv´enic (MA = 5) ablated material moves parallel to the background magnetic field (300 G) through 12m (80 δi) of the LAPD, interacting with the ambient Helium plasma (ni = 9×1012 cm−3) through electromagnetic beam instabilities. The debris is characterized by Langmuir probes and a time-resolved fluorescence monochromator. Waves in the magnetic field produced by the instabilities are diagnosed by an array of 3-axis ‘bdot’ magnetic field probes. Measurements are compared to hybrid simulations of both the experiment and of parallel shocks.   

Magnetically-Driven Radiative Shock Experiments for Laboratory Astrophysics

We present results from new experiments, aimed at producing radiative shocks, using an ``inverse liner'' configuration on the MAGPIE pulsed power facility (1.4 MA in 240 ns) at Imperial College London in the UK. In these experiments current passes through a thin walled metal tube and is returned through a central rod on the axis, generating a strong (40 Tesla) toroidal magnetic field. This drives a shock through the tube which launches a cylindrically symmetric, radially expanding radiative shock in to gas surrounding the tube. Unlike previous converging shock experiments [1], where the shock is located within the imploding liner and thus only permits end on probing, this experimental setup is much more open for diagnostic access and allows shocks to propagate further instead of colliding of axis. Multi-frame self-emission imaging, laser interferometry, emission spectrometry and magnetic probes were used to provide a better understanding of the shock dynamics. Results are shown from experiments performed in a variety of gases (Ne, Ar, Kr, Xe 1-50 mbar).~ In addition, methods for seeding perturbations are discussed which may allow for the study of several shock instabilities such as the Vishniac instability. ~ [1] G. Burdiak et al. Journal of Plasma Physics,~\textbf{81(3)}, 365810301 (2015)   

Weibel instability mediated collisionless shocks using intense laser-driven plasmas.

The origin of cosmic rays remains a long-standing challenge in astrophysics and continues to fascinate physicists. It is believed that ``collisionless shocks'' -- where the particle Coulomb mean free path is much larger that the shock transition -- are a dominant source of energetic cosmic rays. These shocks are ubiquitous in astrophysical environments such as gamma-ray bursts, supernova remnants, pulsar wind nebula and coronal mass ejections from the sun. A particular type of electromagnetic plasma instability known as Weibel instability is believed to be the dominant mechanism behind the formation of these collisionless shocks in the cosmos. The understanding of the microphysics behind collisionless shocks and their particle acceleration is tightly related with nonlinear basic plasma processes and remains a grand challenge. In this poster, we will present results from recent experiments at the LANL Trident laser facility studying collisionless shocks using intense ps laser (80J, 650 fs -- peak intensity of \textasciitilde 10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}})$ driven near-critical plasmas using carbon nanotube foam targets. A second short pulse laser driven protons from few microns thick gold foil is used to radiograph the main laser-driven plasma.   

Multiscale Models for the Two-Stream Instability

Interpenetrating streams of plasma found in many important scenarios in nature and in the laboratory can develop kinetic two-stream instabilities that exchange momentum and energy between the streams. A quasilinear model for the electrostatic two-stream instability is under development as a component of a multiscale model that couples fluid simulations to kinetic theory. Parameters of the model will be validated with comparison to full kinetic simulations using LOKI [1] and efficient strategies for numerical solution of the quasilinear model and for coupling to the fluid model will be discussed. Extending the kinetic models into the collisional regime requires an efficient treatment of the collision operator. Useful reductions of the collision operator relative to the full multi-species Landau-Fokker-Plank operator are being explored. These are further motivated both by careful consideration of the parameter orderings relevant to two-stream scenarios and by the particular 2D+2V phase space used in the LOKI code. [1] J. W. Banks and J. A. F. Hittinger, IEEE Trans. Plasma Sci. 38 (2010) 2198.   

Exploring EBW conversion physics with applications to NSTX-U

Radiofrequency waves are commonly used on traditional tokamaks to assist plasma formation and to provide non-inductive heating and current drive (NI H{\&}CD). Their applicability to spherical tokamaks (STs), however, is complicated by the latter's comparatively high densities and low field strengths. Electron Bernstein waves (EBW) are attractive for NI H{\&}CD on STs because they do not experience a density cutoff and they damp strongly in the vicinity of cyclotron harmonics, even at low temperatures typical of startup. The excitation of EBWs using vacuum-launched electromagnetic waves requires a mode conversion that is highly sensitive to the choice of launch polarization and to local plasma parameters. Common theoretical models employ a 1D slab geometry to study such conversion processes; however, these models may be insufficient to describe the EBW conversion physics in STs, in which equilibria are typically strongly-shaped with large magnetic shear. We report our progress on a theoretical study of EBW conversion physics that emphasizes the inherent idiosyncrasies of the ST equilibrium. Additionally, using a recently developed OD2V kinetic model along with GENRAY simulations, we assess the EBW NI H{\&}CD on NSTX-U using the OXB startup technique that has been developed on MAST. We then make recommendations regarding its implementation in future experimental campaigns.   

Turbulent Dynamo Amplification of Magnetic Fields in Laser-Produced Plasmas: Simulations and Experiments

The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model for cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo. We have conceived experiments to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here, we describe the design of these experiments through large-scale 3D FLASH simulations on the Mira supercomputer at ANL, and the laser-driven experiments we conducted with the OMEGA laser at LLE. Our results indicate that turbulence is capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions.   

NIF Discovery Science Eagle Nebula

The University of Maryland and and LLNL are investigating the origin and dynamics of the famous Pillars of the Eagle Nebula and similar parsec-scale structures at the boundaries of HII regions in molecular hydrogen clouds. The National Ignition Facility (NIF) Discovery Science program Eagle Nebula has performed NIF shots to study models of pillar formation. The shots feature a new long-duration x-ray source, in which multiple hohlraums mimicking a cluster of stars are driven with UV light in series for 10 to 15 ns each to create a 30 to 60 ns output x-ray pulse. The source generates deeply nonlinear hydrodynamics in the Eagle science package, a structure of dense plastic and foam mocking up a molecular cloud containing a dense core. Omega EP and NIF shots have validated the source concept, showing that earlier hohlraums do not compromise later ones by preheat or by ejecting ablated plumes that deflect later beams. The NIF shots generated radiographs of shadowing-model pillars, and also showed evidence that cometary structures can be generated. The velocity and column density profiles of the NIF shadowing and cometary pillars have been compared with observations of the Eagle Pillars made at the millimeter-wave BIMA and CARMA observatories.   

Experiment to measure oxygen opacity at high density and temperature

In recent years, there has been a debate over the abundances of heavy elements (Z \textgreater 2) in the solar interior. Recent solar atmosphere models [Asplund 2009] find a significantly lower abundance for C, N, and O compared to models used roughly a decade ago. Recent opacity measurements of iron disagree with opacity model predictions [Bailey et al, 2015]. Repeated scrutiny of the experiment and data has not produced a conclusive reason for the discrepancy. New models have been implemented in the ATOMIC opacity code for low-Z elements [Colgan, 2013, Armstrong 2014] ], however no data currently exists to test the low-Z material models in the regime relevant to the solar convection zone. We present an experimental design using the opacity platform developed at the National Ignition Facility to study the oxygen opacity at densities and temperatures near the solar convection zone conditions.   

Simulation and Preliminary Design of a Cold Stream Experiment on Omega EP

Galaxies form within dark matter halos, accreting gas that may clump and eventually form stars. Infalling matter gradually increases the density of the halo, and, if cooling is insufficient, rising pressure forms a shock that slows the infalling gas, reducing star formation. However, galaxies with sufficient cooling become prolific star formers. A recent theory suggests that so called ``stream fed galaxies" are able to acquire steady streams of cold gas via galactic ``filaments" that penetrate the halo. The cold, dense filament flowing into a hot, less dense environment is potentially Kelvin-Helmholtz unstable. This instability may hinder the ability of the stream to deliver gas deeply enough into the halo. To study this process, we have begun preliminary design of a well-scaled laser experiment on Omega EP. We present here early simulation results and the physics involved. \newline \newline This work is funded by the U.S. Department of Energy, through the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-NA0002956, and the National Laser User Facility Program, grant number DE-NA0002719, and through the Laboratory for Laser Energetics, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-NA0001944.   

Analysis of BigFoot HDC SymCap experiment N161205 on NIF

Analysis of NIF implosion experiment N161205 provides insight into both hohlraum and capsule performance. This experiment used an undoped High Density Carbon (HDC) ablator driven by a BigFoot x-ray profile in a Au hohlraum. Observations from this experiment include DT fusion yield, bang time, DSR, Tion and time-resolved x-ray emission images around bang time. These observations are all consistent with an x-ray spectrum having significantly reduced Au m-band emission that is present in a standard hohlraum simulation. Attempts to justify the observations using several other simulation modifications will be presented.   

Enhanced electron/fuel-ion equilibration through impurity ions: Studies applicable to NIF and Omega.

In shock-driven exploding-pushers, a platform used extensively to study multi-species and kinetic effects, electrons and fuel ions are far out of equilibrium, as reflected by very different temperatures. However, impurity ions, even in small quantities, can couple effectively to the electrons, because of a Z$^{\mathrm{2}}$ dependence, and in turn, impurity ions can then strongly couple to the fuel ions. Through this mechanism, electrons and fuel-ions can equilibrate much faster than they otherwise would. This is a quantitative issue, depending upon the amount and Z of the impurity. For NIF and Omega, we consider the role of this process. Coupled non-linear equations, reflecting the temperatures of the three species, are solved for a range of conditions. Consideration is also given to ablatively driven implosions, since impurities can similarly affect the equilibration. This work was supported in part by DOE/NNSA DE-NA0002949 and DE-NA0002726.   

Reducing the effects of X-ray pre-heat in double shell NIF capsules by over-coating the high Z shell

Hohlraum generated X-rays will penetrate the ablator of a double shell capsule and be absorbed in the outer surface of the inner capsule. The ablative pressure this generates drives a shock into the central fuel, and a reflected shock that reaches the inner high-Z shell surface before the main shock even enters the fuel. With a beryllium over-coat preheat X-rays deposit just inside the beryllium/high z interface. The beryllium tamps the preheat expansion, eliminating ablation, and dramatically reducing pressure. The slow shock or pressure wave it generates is then overtaken by the main shock, avoiding an early shock in the fuel and increasing capsule yield.   

Applying the Braginskii Ion Fluid Model to Reaction Yields and Product Energy Spectra

Collisional plasmas can be described using the Braginskii ion fluid model. This can be used calculate transport fluxes (e.g. ion heat flux and viscosity) as a result of driving thermodynamic forces (temperature gradients and the rate of strain tensor, respectively) by the derivation of a set of transport equations from the kinetic equations. The solutions to these transport equations are perturbations to the Maxwellian distribution function. In this work we investigate how nuclear reaction yields and the energy spectra of reaction products are affected by the driving thermodynamic forces. The transport equations for a plasma with multiple ion species are solved using a set of associated Laguerre polynomials which define the perturbation to the distribution function. The set of polynomials is then used to calculate reaction yields and energy spectra. The model is applied to the plasma conditions found in ICF and MagLIF hotspots. It is shown that the temperature and fluid velocity gradients present in these plasmas can cause significant broadening of the neutron spectra. This broadening can cause the ion temperature inferred from the spectral width to be higher than the burn-averaged ion temperature of the plasma.   

Discharge start-up and ramp-up development for NSTX-U and MAST-U

A collaborative modeling effort is underway to develop robust inductive start-up and ramp-up scenarios for NSTX-U and MAST-U. These complementary spherical tokamak devices aim to generate the physics basis for achieving steady-state, high-beta and high-confinement plasma discharges with a self-consistent solution for managing the divertor heat flux. High-performance discharges in these devices require sufficient plasma elongation ($\kappa \quad =$ 2.4 -- 2.8) to maximize the bootstrap and beam-driven current drive, increase MHD stability at high I$_{\mathrm{p}}$ and high $\beta_{\mathrm{N}}$, and realize advanced divertor geometries such as the snowflake and super-X. Achieving the target elongation on NSTX-U is enabled by an L-H transition in the current ramp-up that slows the current diffusion and maintains a low internal inductance (l$_{\mathrm{i}}$ $\le $ 0.8). Modeling focuses on developing scenarios that achieve a suitable field null for breakdown and discharge conditions conducive to an early L-H transition while maintaining vertical and MHD stability, with appropriate margin for variation in experimental conditions. The toroidal currents induced in conducting structures and the specifications of the real-time control and power supply systems are unique constraints for the two devices.   

Analysis of vertical stability limits and vertical displacement event behavior on NSTX-U

The National Spherical Torus Experiment Upgrade (NSTX-U) completed its first run campaign in 2016, including commissioning a larger center-stack and three new tangentially aimed neutral beam sources. NSTX-U operates at increased aspect ratio due to the larger center-stack, making vertical stabilization more challenging. Since ST performance is improved at high elongation, improvements to the vertical control system were made, including use of multiple up-down-symmetric flux loop pairs for real-time estimation, and filtering to remove noise. Similar operating limits to those on NSTX (in terms of elongation and internal inductance) were achieved, now at higher aspect ratio. To better understand the observed limits and project to future operating points, a database of vertical displacement events and vertical oscillations observed during the plasma current ramp-up on NSTX/NSTX-U has been generated. Shots were clustered based on the characteristics of the VDEs/oscillations, and the plasma parameter regimes associated with the classes of behavior were studied. Results provide guidance for scenario development during ramp-up to avoid large oscillations at the time of diverting, and provide the means to assess stability of target scenarios for the next campaign. Results will also guide plans for improvements to the vertical control system.   

Advanced Plasma Shape Control to Enable High-Performance Divertor Operation on NSTX--U

This work presents the development of an advanced framework for control of the global plasma shape and its application to a variety of shape control challenges on NSTX--U. Operations in high-performance plasma scenarios will require highly-accurate and robust control of the plasma poloidal shape to accomplish such tasks as obtaining the strong-shaping required for the avoidance of MHD instabilities and mitigating heat flux through regulation of the divertor magnetic geometry. The new control system employs a high-fidelity model of the toroidal current dynamics in NSTX--U poloidal field coils and conducting structures as well as a first-principles driven calculation of the axisymmetric plasma response. The model-based nature of the control system enables real-time optimization of controller parameters in response to time-varying plasma conditions and control objectives. The new control scheme is shown to enable stable and on-demand plasma operations in complicated magnetic geometries such as the snowflake divertor. A recently-developed code that simulates the nonlinear evolution of the plasma equilibrium is used to demonstrate the capabilities of the designed shape controllers. Plans for future real-time implementations on NSTX--U and elsewhere are also presented.   

Power exhaust scenarios and control for projected high-power NSTX-U operation

An important goal of the NSTX Upgrade (NSTX-U) research program is to characterize energy confinement in the low-aspect-ratio spherical tokamak configuration over a significantly expanded range of plasma current, toroidal field, and heating power, while increasing flattop durations up to 5 seconds. However, the narrowing of the scrape-off layer at higher current combined with an improved understanding of expected halo-current loads has motivated a significant re-design of NSTX-U plasma facing components in the high-heat-flux regions of the divertor. In order to reduce the expected divertor heat flux to acceptable levels, a combination of mitigation techniques will be used: increased divertor poloidal flux expansion, increased divertor radiation, and controlled strike-point sweeping. The machine requirements for these various mitigation techniques are studied here using a newly implemented reduced heat-flux model. Systematic equilibrium scans are used to quantify the required divertor coil currents and to verify vertical stability for a range of plasma shapes. Free-boundary control schemes to constrain the strike-point location and field-line angle-of-incidence will also be discussed.   

Local compressional and global Alfv\'{e}n eigenmode structure on NSTX and their effect on core energy transport

A novel method for localized absolute reflectometer measurements of density fluctuations $\delta n$ using a synthetic diagnostic has provided new insight into CAE {\&} GAE amplitude, structure, and associated energy transport in NSTX spherical torus. The new technique is more accurate than previous analysis producing substantially different amplitudes. CAE {\&} GAE activity has been shown to correlate with core anomalous electron thermal transport in high-power beam heated NSTX plasmas [Stutman PRL09] making these measurements of significant interest. High frequency modes (17--33{\%} $f_{ci})$ are identified as GAEs {\&} CAEs in a 6 MW beam heated plasma. The synthetic diagnostic allows direct testing of HYM, a leading CAE {\&} GAE stability code that predicts substantial transport via CAE-KAW coupling [Belova PRL15]. Measured GAE structures show edge peaks, and are broad {\&} flat in the core with $\delta n$/$n$\textasciitilde 10$^{\mathrm{-5}}$--10$^{\mathrm{-4}}$. In contrast, CAEs have broad core peaks with $\delta n$/$n$\textasciitilde 10$^{\mathrm{-4}}$--10$^{\mathrm{-3}}$. The GAE measurements are used with theory for mode induced stochastization of electron drift orbits [Gorelenkov NF10] to predict the core electron thermal diffusivity ($\chi_{e})$, which shows the low amplitudes cannot explain the high $\chi_{e}$. The theory has been modified to include the CAEs, preliminarily showing negligible increase. Linear HYM simulations show GAE structures similar to those above.   

Numerical simulations of GAE stabilization in NSTX-U

Beam-driven Global Alfven Eigenmodes (GAEs) were frequently observed on NSTX and NSTX-U and have been linked with a flattening of the electron temperature profile in the plasma core. New experimental results from NSTX-U have demonstrated that neutral beam injection from the new beam sources with large tangency radii deposit beam ions with large pitch, which can very effectively stabilize all unstable GAEs. Numerical simulations using the HYM code have been performed to study the excitation and stabilization of GAEs in the NSTX-U right before and shortly after the additional off-axis beam injection. HYM simulations reproduce experimental finding, namely it is shown that off-axis neutral beam injection reliably and strongly suppresses all unstable GAEs. Before additional beam injection, the simulations show unstable counter-rotating GAEs with toroidal mode numbers n$=$7-12, and frequencies that match the experimentally observed unstable GAEs. Additional of-axis beam injection has been modeled by adding beam ions with large pitch, and about 1/3 of the total beam ion inventory. The simulations in this case show a complete stabilization of all unstable GAEs (n$=$7-12), even for the cases when the HYM calculated GAE growth rates were relatively large.   

Energetic-particle-modified global Alfv\'{e}n Eigenmodes

Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfv\'{e}n eigenmodes (GAE) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distribution function -- the normalized injection velocity $V_B/V_A$ and central pitch -- are varied in order to study their influence on the characteristics of the excited modes. It is found that the frequency of the most unstable mode changes significantly and continuously with beam parameters, depending most substantially on $V_B/V_A$. This unexpected result is present for both co- and counter-propagating GAEs, which are driven by Doppler-shifted cyclotron resonances. Large changes in frequency without clear corresponding changes in mode structure could indicate the existence of a new energetic particle mode, referred to here as an energetic-particle-modified GAE (EP-GAE). Additional simulations conducted for a fixed MHD equilibrium demonstrate that the GAE frequency shift cannot be explained by the equilibrium changes due to energetic particle effects.   

Likelihood of Alfvénic instability bifurcation in experiments

We apply a criterion for the likely nature of fast ion redistribution in tokamaks to be in the convective or diffusive nonlinear regimes. The criterion, which is shown to be rather sensitive to the relative strength of collisional or micro-turbulent scattering and drag processes, ultimately translates into a condition for the applicability of reduced quasilinear modeling for realistic tokamak eigenmodes scenarios. The criterion is tested and validated against different machines, where the chirping mode behavior is shown to be in accord with the model. It has been found that the anomalous fast ion transport is a likely mediator of the bifurcation between the fixed-frequency mode behavior and rapid chirping in tokamaks. In addition, micro-turbulence appears to resolve the disparity with respect to the ubiquitous chirping observation in spherical tokamaks and its rarer occurrence in conventional tokamaks. In NSTX, the tendency for chirping is further studied in terms of the beam beta and the plasma rotation shear. For more accurate quantitative assessment, numerical simulations of the effects of electrostatic ion temperature gradient turbulence on chirping are presently being pursued using the GTS code.   

Effect of Sawtooth crashes on fast-ion distribution in NSTX-U

During the 2016 experimental campaign of NSTX-Upgrade (NSTX-U), long L-mode and reproducible sawtoothing plasmas have been achieved that were previously not accessible on NSTX. This provides a good opportunity to investigate the conditions of sawtooth appearance and to study their effects on fast ion confinement and re-distribution in spherical tokamaks. The Fast-Ion D-alpha (FIDA) and Solid State Neutral Particle Analyzer (SSNPA) diagnostics on NSTX-U each has two subsystems with one subsystem more sensitive to passing particles and the other one more sensitive to trapped particles. It has been observed on both diagnostics that the passing particles are strongly expelled from the plasma core to the plasma edge during sawtooth crashes while trapped fast ions are weakly affected. The tangential-FIDA (t-FIDA) system which is most sensitive to passing particles saw a signal drop in the region inside the inversion radius (\textasciitilde 125cm), while an increase at the outer region. The neutron rate can drop as much as 13{\%} during sawtooth crashes. This phenomenon is similar to previous observations in DIII-D and ASDEX Upgrade conventional tokamaks. Detailed data analysis and modelling are being performed to quantity the effects of sawtooth crashes on fast-ion redistribution and to compare with the Kadomtsev sawtooth model. *\textit{Work supported by US DOE.}   

ORBIT modelling of fast particle redistribution induced by sawtooth instability

Initial tests on NSTX-U show that introducing energy selectivity for sawtooth (ST) induced fast ion redistribution improves the agreement between experimental and simulated quantities, e.g. neutron rate. Thus, it is expected that a proper description of the fast particle redistribution due to ST can improve the modelling of ST instability and interpretation of experiments using a transport code. In this work, we use ORBIT code [1] to characterise the redistribution of fast particles. In order to simulate a ST crash, a spatial and temporal displacement is implemented as $\xi (\rho, t, \theta ,\phi ) = \sum {\xi_{mn} (\rho ,t) \mathrm{cos}(m\theta +n\phi)}$ [2] to produce perturbed magnetic fields from the equilibrium field $\vec{B}$, $\delta \vec{B} =\nabla \times (\vec{\xi} \times \vec{B})$, which affect the fast particle distribution. From ORBIT simulations, we find suitable amplitudes of ξ for each ST crash to reproduce the experimental results. The comparison of the simulation and the experimental results will be discussed as well as the dependence of fast ion redistribution on fast ion phase space variables (i.e. energy, magnetic moment and toroidal angular momentum). [1] White R.B. and Chance M.S. 1984 Phys. Fluids 27 2455 [2] Farengo R. et al 2013 Nucl. Fusion 53 043012   

Destabilization of counter-propagating TAEs by off-axis, co-current Neutral Beam Injection

Neutral Beam injection (NBI) is a common tool to heat the plasma and drive current non-inductively in fusion devices. Energetic particles (EP) resulting from NBI can drive instabilities that are detrimental for the performance and the predictability of plasma discharges. A broad NBI deposition profile, e.g. by off-axis injection aiming near the plasma mid-radius, is often assumed to limit those undesired effects by reducing the radial gradient of the EP density, thus reducing the ``universal'' drive for instabilities. However, this work presents new evidence that off-axis NBI can also lead to undesired effects such as the \textit{destabilization} of Alfv\'{e}nic instabilities, as observed in NSTX-U plasmas. Experimental observations indicate that counter propagating toroidal AEs are destabilized as the radial EP density profile becomes hollow as a result of off-axis NBI. Time-dependent analysis with the TRANSP code, augmented by a reduced fast ion transport model (known as \textit{kick model}), indicates that instabilities are driven by a combination of radial and energy gradients in the EP distribution. Understanding the mechanisms for wave-particle interaction, revealed by the phase space resolved analysis, is the basis to identify strategies to mitigate or suppress the observed instabilities.   

Gyrokinetic study of electron transport in NSTX using XGC

Electron anomalous transport may play a significant role in the plasma confinement in NSTX. In such a situation it becomes important to identify the origin of the electron heat and particle transport and find ways of reducing it. Among the possible electron modes, the electron temperature gradient mode (ETG) can be important both in the core and edge pedestal plasmas. Here we aim to study the role of ETG on the anomalous loss of electrons in the NSTX tokamak with the gyrokinetic code XGC. XGC is an X-point included full-f gyrokinetic code which can also be run in the delta-f limit. We present a benchmark study of the ETG mode against those from existing flux tube gyrokinetic codes in the limit of simple circular ad hoc model similar to the cyclone base case. Simulations for actual experimental profiles and parameters corresponding to the NSTX will also be reported.   

Modeling of blob-hole correlations in GPI edge turbulence data

Gas-puff imaging (GPI) observations made on NSTX [S.J. Zweben et. al, submitted to Phys. Plasmas (2017)] have revealed two-point spatial correlation patterns in the plane perpendicular to the magnetic field. A common feature is the occurrence of dipole-like patterns with significant regions of negative correlation. In this work, we explore the possibility that these dipole patterns may be due to blob-hole pairs. Statistical methods [O.E. Garcia, et al, Phys. Plasmas 23, 052308 (2016)] are applied to determine the two-point spatial correlation that results from a model of blob-hole pair formation. It is shown that the model produces dipole correlation patterns that are qualitatively similar to the GPI data in many respects. Effects of the reference location (confined surfaces or scrape-off layer), a superimposed random background, hole velocity and lifetime, and background sheared flows are explored. The possibility of using the model to ascertain new information about edge turbulence is discussed.   

Penetration of filamentary structures in the x-point region of spherical tokamaks

ArbiTER is a flexible eigenvalue code designed for plasma physics applications. It is used here to gain insight into the spatial dependence of filamentary structures in the scrape-off layer of spherical tokamaks. In particular, observations on MAST reveal the presence of a quiescent x-point region. Observations in NSTX similarly reveal a reduction in divertor fluctuations near the separatrix and a loss of midplane correlation. We will report on the penetration of filamentary structures into the vicinity of the x-point, as well as growth rate trends, for a variety of profiles and toroidal mode numbers. This will determine whether linear properties of these structures can explain experimental observations.   

Modelling of Divertor Detachment in MAST Upgrade

MAST Upgrade will have extensive capabilities to explore the benefits of alternative divertor configurations such as the conventional, Super-X, x divertor, snowflake and variants in a single device with closed divertors. Initial experiments will concentrate on exploring the Super-X and conventional configurations, in terms of power and particle loads to divertor surfaces, access to detachment and its control. Simulations have been carried out with the SOLPS5.0 code validated against MAST experiments. The simulations predict that the Super-X configuration has significant advantages over the conventional, such as lower detachment threshold (2-3x lower in terms of upstream density and 4x higher in terms of PSOL). Synthetic spectroscopy diagnostics from these simulations have been created using the Raysect ray tracing code to produce synthetic filtered camera images, spectra and foil bolometer data. Forward modelling of the current set of divertor diagnostics will be presented, together with a discussion of future diagnostics and analysis to improve estimates of the plasma conditions.   

Radiative divertor optimization for NSTX Upgrade based on open geometry standard divertor experiments in NSTX

Recent analyses of NSTX divertor experiments suggest a way to optimize the standard open geometry divertor configuration for partial detachment with deuterium puffing and intrinsic carbon radiation. Results from the NSTX experiments and the divertor transport and radiation model obtained with the multi-fluid code UEDGE are used to show that detachment onset and properties are sensitive to 1) placing the neutral gas source in the vicinity of the strike point, 2) directing the recycling neutrals toward the separatrix by adjusting the poloidal separatrix angle, and 3) entrapping neutrals by plasma plugging via the high poloidal magnetic flux expansion configuration. These findings will be tested in NSTX Upgrade, where H-mode scenarios with 2 MA, 1 T, 10 MW NBI-heated discharges and 5 s flattop are predicted to produce unmitigated peak divertor heat fluxes above 10 MW/m$^{\mathrm{2}}$, necessitating the scrape-off layer power sharing between upper and lower divertors and inducing dissipative losses.   

Plasma-Facing Component and Materials Testing for the NSTX-U

The NSTX-U Recovery Project is developing plasma-facing components for use in the divertor of NSTX-U. The extreme conditions of the NSTX-U divertor make it possible to stress even graphite surfaces to the material limits leading to the possibility of component failures. In addition, the complex, mixed-material environment of the NSTX-U due to the use of boron and lithium wall conditioning techniques creates significant uncertainties in the monitoring of the PFCs. A testing program has been developed to inform on the material and design limitations of the NSTX-U high-heat flux components. These tests include high-heat flux testing in electron beam facilities as well as plasma-based testing. The NSTX-U components could experience perpendicular heat fluxes as high as 45 MW/m{\$}\textasciicircum 2{\$}. Parallel heat fluxes onto leading edges could reach 475 MW/m{\$}\textasciicircum 2{\$}. The testing program and material survey plan will be presented.   

Surface chemistry analysis of boronized TZM and ATJ graphite samples during and after plasma irradiation

In the National Spherical Torus Experiment Upgrade (NSTX-U) a plasma facing component diagnostic, the Material Analysis and Particle Probe (MAPP), was installed. MAPP has the capability of conducting XPS studies on materials without exposing them to atmospheric conditions. MAPP was used to conduct XPS studies of ATJ graphite (current first-wall material) and TZM alloy (99\% Mo, 0.5\% Ti, 0.08\% Zr) samples exposed to plasma operations during the 2015-2016 experimental campaign. The data shows evidence of sputtering of the boron layers following tens of plasma shots, as well as an increase in the oxygen concentration with plasma exposure. Offline depth-profile XPS analysis was performed on the TZM samples at UIUC. These studies showed layers of deposited material on the surface of the sample, including boron layers that showed significant oxygen retention, which correlates with the MAPP data. Post-mortem depth-profile XPS analysis was also performed on ATJ graphite samples from the tiles in the divertors. Additionally, deuterium irradiation studies were also conducted on the boronized TZM samples. The effects of D+ irradiation were observed in the IGNIS (Ion-Gas-Neutral Interactions with Surfaces) facility at UIUC.   

Global modeling of wall material migration following boronization in NSTX-U

NSTX-U operated in 2016 with graphite plasma facing components, periodically conditioned with boron to improve plasma performance. Following each boronization, spectroscopic diagnostics generally observed a decrease in oxygen influx from the walls, and an in-vacuo material probe (MAPP) observed a corresponding decrease in surface oxygen concentration at the lower divertor. However, oxygen levels tended to return to a pre-boronization state following repeated plasma exposure. This behavior is interpretively modeled using the WallDYN mixed-material migration code, which couples local erosion and deposition processes with plasma impurity transport in a non-iterative, self-consistent manner that maintains overall material balance. A spatially inhomogenous model of the thin films produced by the boronization process is presented. Plasma backgrounds representative of NSTX-U conditions are reconstructed from a combination of NSTX-U and NSTX datasets. Low-power NSTX-U fiducial discharges, which led to less apparent surface degradation than normal operations, are also modeled with WallDYN. Likely mechanisms driving the observed evolution of surface oxygen are examined, as well as remaining discrepancies between model and experiment and potential improvements to the model.   

Advantages and Challenges of Radiative Liquid Lithium Divertor

Steady-state fusion power plant designs present major divertor technology challenges, including high divertor heat flux both in steady-state and during transients. In addition to these concerns, there are the unresolved technology issues of long term dust accumulation and associated tritium inventory and safety issues. The application of lithium (Li) in NSTX resulted in improved H-mode confinement, H-mode power threshold reduction, and reduction in the divertor peak heat flux while maintaining essentially Li-free core plasma operation even during H-modes. These promising results in NSTX and related modeling calculations motivated the radiative liquid Li divertor (RLLD) concept and its variant, the active liquid Li divertor concept (ARLLD), taking advantage of the enhanced Li radiation in relatively poorly confined divertor plasmas. It has been suggested that radiation-based liquid lithium (LL) divertor concepts with a modest Li-loop could provide a possible solution for the outstanding fusion reactor technology issues such as divertor heat flux mitigation and real time dust removal, while potentially improving the reactor plasma performance. Laboratory tests are also planned to investigate the Li-T recover efficiency and other relevant research topics of the RLLD.   

2D full-wave simulation of HHFW in the scrape-off layer of NSTX

The scrape-off layer (SOL) region, the region of the plasma between the last closed flux surface and the tokamak vessel, is important for radio frequency (RF) wave heating of tokamaks because significant wave power loss can occur in this region - for instance, up to 60{\%} of the coupled higher harmonic fast wave (HHFW) power can be lost in the SOL of NSTX. In this presentation, we perform simulations using a 2D full-wave (FW2D) code for HHFW in the SOL of NSTX. A recently developed FW2D code solves the cold plasma wave equations using the finite element method and has been successfully applied to describe low frequency waves in the planetary magnetospheres. Very recently, the FW2D code has been adapted to tokamak geometry to examine radio frequency waves in the SOL of tokamaks. We adopt (1) a rectangular boundary to benchmark with the AORSA results and (2) a limiter boundary to examine boundary effects on HHFW propagation. As results, we found that (1) FW2D and AORSA simulations show an excellent agreement in the rectangular boundary; and (2) FW2D results with a realistic limiter boundary are significantly different to results with the rectangular vessel boundary.   

Effects of the H species in the HHFW performance in NSTX/NSTX-U plasmas

One of the goal of NSTX-U is to operate at full field (B = 1 T). For this magnetic field, the first and second harmonics of hydrogen (H) are located at the high-field side and in the core plasma, respectively. In principle, part of the high-harmonic fast-wave (HHFW) injected power can be absorbed by the H population reducing the electron and/or the fast-ion heating. For this reason, full wave simulations results of NSTX-U scenarios with different H concentrations for wave frequencies of 30 and 60 MHz will be presented and discussed. Plasma scenarios with and without neutral beam injection (NBI) will be considered. Furthermore, the balance between the beam ion and electron power absorption will be analyzed comparing both NSTX and NSTX-U plasmas.   

Comparison of neutral density profiles measured using D_{\alpha}$ and C$^{5+}$ in NSTX-U

Edge neutral density profiles determined from two different measurements are compared on NSTX-U plasmas. Neutral density measurements were not typical on NSTX plasmas. An array of fibers dedicated to the measurement of passive emission of C$^{5+}$, used to subtract background emission for charge exchange recombination spectroscopy (CHERS), can be used to infer deuterium neutral density near the plasma edge. The line emission from C$^{5+}$ is dominated by charge exchange with neutral deuterium near the plasma edge. An edge neutral density diagnostic consisting of a camera with a D_{\alpha}$ filter was installed on NSTX-U. The line-integrated measurements from both diagnostics are inverted to obtain local emissivity profiles. Neutral density is then inferred using atomics rates from ADAS and profile measurements from Thomson scattering and CHERS. Comparing neutral density profiles from the two diagnostic measurements helps determine the utility of using the more routinely available C$^{5+}$ measurements for neutral density profiles. Initial comparisons show good agreement between the two measurements inside the separatrix.   

3D Field Modifications of Core Neutral Fueling In the EMC3-EIRENE Code

The application of 3-D magnetic field perturbations to the edge plasmas of tokamaks has long been seen as a viable way to control damaging Edge Localized Modes (ELMs). These 3-D fields have also been correlated with a density drop in the core plasmas of tokamaks; known as “pump-out”. While pump-out is typically explained as the result of enhanced outward transport, degraded fueling of the core may also play a role. By altering the temperature and density of the plasma edge, 3-D fields will impact the distribution function of high energy neutral particles produced through ion-neutral energy exchange processes. Starved of the deeply penetrating neutral source, the core density will decrease. Numerical studies carried out with the EMC3-EIRENE code on National Spherical Tokamak eXperiment- Upgrade (NSTX-U) equilibria show that this change to core fueling by high energy neutrals may be a significant contributor to the overall particle balance in the NSTX-U tokamak: deep core ($\Psi$ $<$ 0.5) fueling from neutral ionization sources is decreased by 40-60 \% with RMPs.   

CHI Research on NSTX-U

At present about 20{\%} of the total plasma current required for sustained operation has been generated by transient CHI. The present understanding suggests that it may be possible to generate all of the needed current in a ST / tokamak using transient CHI. In such a scenario, one could transition directly from a CHI produced plasma to a non-inductively sustained plasma, without the difficult intermediate step that involves non-inductive current ramp-up. STs based on this new configuration would take advantage of evolving developments in high-temperature superconductor technology to develop a simpler design ST that relies primarily on CHI for plasma current generation. Motivated by the very good results from NSTX and HIT-II, we are examining the potential application of transient CHI for reactor configurations through these studies. (1) Study of the maximum levels of start-up currents that could be generated on NSTX-U, (2) application of a single biased electrode configuration on QUEST to protect the insulator from neutron damage in a CHI reactor installation, and (3) QUEST-like, but a double biased electrode configuration for PEGASUS and NSTX-U. Results from these on-going studies will be described. This work is supported by U.S. DOE Contracts: DE-AC02-09CH11466, DE-FG02-99ER54519 AM08, and DE-SC0006757.   

Synthetic capability for the study of poloidal impurity asymmetries in NSTX-U

A new capability has been built to compute the two-dimensional mapping of impurity density asymmetries in NSTX-U. This technique relies on flux-surface quantities like electron and ion temperature ($T_{e,i}$) and rotation frequency ($\omega_{\phi}$), but finds the 2D electron, deuterium and carbon density profiles self-consistently assuming the presence of a poloidal variation due to centrifugal forces. The solution for the electrostatic potential for the measured carbon density and central toroidal rotation using NSTX data will be shown and compared with the values derived using Wesson’s formalism which assumed that the main intrinsic impurity was in the trace limit. The presence of O, Ne, Ar, Fe, Mo and W are considered at the trace limit with very small changes to quasineutrality and $Z_{eff}$. The few assumptions made considered a zero electron mass, a deuterium plasma, a trace impurity with charge ``$Z$'' given by coronal equilibrium ($Z=Z(T_{e}$)) and equilibrated ion temperatures (e.g. $T_{D}=T_{C}=T_{Z}$). This capability will help in the understanding of asymmetries before tearing modes onsets as well as aid the design of new diagnostics for NSTX-U.   

Design of tangential multi-energy SXR cameras for tokamak plasmas

A new synthetic diagnostic capability has been built to study the response of tangential multi-energy soft x-ray pin-hole cameras for arbitrary plasma densities ($n_{e,D}$), temperature ($T_{e}$) and ion concentrations ($n_{Z}$). For tokamaks and future facilities to operate safely in a high-pressure long-pulse discharge, it is imperative to address key issues associated with impurity sources, core transport and high-Z impurity accumulation. Multi-energy soft xray imaging provides a unique opportunity for measuring, simultaneously, a variety of important plasma properties (e.g. $T_{e}$, $n_{Z}$ and $\Delta Z_{eff}$). These systems are designed to sample the continuum- and line-emission from low- to high-Z impurities (e.g. C, O, Al, Si, Ar, Ca, Fe, Ni and Mo) in multiple energy-ranges. These x-ray cameras will be installed in the MST-RFP, as well as NSTX-U and DIII-D tokamaks, measuring the radial structure of the photon emissivity with a radial resolution below 1 cm at a 500 Hz frame rate and a photon-energy resolution of ~500 eV. The layout and response expected for the new systems will be shown for different plasma conditions and impurity concentrations. The effect of toroidal rotation driving poloidal asymmetries in the core radiation is also addressed for the case of NSTX-U.   

Upgrade to the MPTS Thomson scattering diagnostic in preparation for NSTX-U restart

Upgrades to Multi-Pulse Thomson Scattering (MPTS) diagnostic are in progress. An innovative laser is being added to existing the two 30-Hz Nd:YAG lasers. The new laser also has 30-Hz base operation, but differs notably in its capacity of generating rapid bursts of nominally 50 pulses at either 1 KHz or 10 KHz. This Pulsed-Bursting Laser System (PBLS) is described elsewhere [1]. The current laser delivery optics, which supports two paraxial beam paths, is maintained. One beam path will be occupied by PBLS. The other two laser beams will be actively combined coaxially and will occupy the second beam path. The new laser arrangement will result in a 90-Hz baseline operation, plus the PBLS burst capability. While the existing sample-and-hold electronics is expected to track a 1-KHz sequence, it will not be able to follow a 10-KHz burst. For this purpose, ten radial channels, dedicated to the pedestal region, will be instrumented with 250-MHz digitizers. The NSTX-U longer plasma duration and increased heating power will be conducive to situations with sustained high background light, a condition exacerbated by the absence of viewing dump necessitated by machine geometry. Additional work is slated to study the behavior of the fast signal detection in presence of strong background light. [1] A. Diallo \textit{et al.,} HTPD 2016, Madison, WI   

The real time multi point Thomson scattering diagnostic at NSTX-U

This contribution presents the upgrade of the multi point Thomson scattering (MPTS) diagnostic for real time application. As a key diagnostic at NSTX-U, the MPTS diagnostic simultaneously measures the electron density ($n_e$) and electron temperature ($T_e$) profiles of a plasma discharge. Therefore, this powerful diagnostic can directly access the electron pressure of the plasma. Currently, only post-discharge evaluation of the data is available, however, since the plasma pressure is one important drive for instabilities, real time measurements of this quantities would be beneficial for plasma control. In a first step, ten MPTS channels were equipped with real time electronics, which improve the data acquisition rate by five orders of magnitude. The commissioning of the system is ongoing and first benchmarks of the real time evaluation routines against the standard, post-discharge evaluation show promising results: The $T_e$ as well as $n_e$ profiles of both types of analyses agree within their uncertainties.   

Overview of High-k Scattering Diagnostics on NSTX and NSTX-U

Electron-gyro scale turbulence, e.g. driven by the Electron Temperature Gradient (ETG), has been proposed as a potential candidate for driving anomalous electron thermal transport, a major problem for magnetic confinement fusion. NSTX and NSTX-U provide a unique laboratory for studying electron-scale turbulence and its relation to electron thermal transport due to their low toroidal field, high plasma beta, low aspect ratio and large ExB flow shear. Electron-gyro scale turbulence has been successfully measured in NSTX using a unique high-k$_r$ microwave scattering system, providing the first direct evidence of ETG turbulence in STs and detailed studies of parametric dependence of electron-scale turbulence. However, the high-k$_r$ microwave scattering system could not capture the predicted ETG spectral peak. Thus a new high-k$_\theta$ FIR scattering system is being implemented for NSTX-U. We will present an overview of the scattering systems on NSTX and NSTX-U, including physics designs, capabilities and recent physics results. We will also discuss methods to achieve radially localized scattering measurements.   

Optimization of time-averaged power flux of RMP footprints in ITER

Plasma-facing components (PFCs) in the ITER tokamak have engineering limits of the incident heat flux ($\sim$10 MWm$^{-2}$). These limits may be exceeded for example by Edge Localized Modes (ELMs) or by Resonant Magnetic Perturbations (RMPs). The time-averaged power flux can be reduced by a toroidal rotation of the ITER ELM coils (IECs) current waveform. However, such a rigid rotation results in large mechanical loads to IECs, which can significantly decrease their lifetime. We evaluate various options to decrease the required variations in the IECs currents while keeping the time-averaged power flux on the ITER divertor below the engineering limit. We use the Bayesian optimization algorithm to seek the optimum configuration. This method works efficiently even for a moderately large dimensionality, in our case up to several tens. For the analysis of a particular waveform we use the tangle distance method [Cahyna et al. Nucl. Fusion 2014], which is, due to its semi-analytical nature, fast enough to evaluate a wide range of options and plasma scenarios.   

Fundamental physics behind the divertor heat-flux width in the present tokamaks and ITER

Electrostatic gyrokinetic simulation using XGC1 recovers the empirical scaling for the divertor heat-load width $\lambda_q$ in the present tokamaks ($\lambda_q \propto 1/B_p^\gamma$, with $\gamma \sim 1$). $\lambda_q$ is dominated by the neoclassical magnetic drift of ions. However, XGC1 predicts that $\lambda_q$ in ITER is much larger than the value predicted by the empirical scaling. An in-depth study shows that the edge turbulence characteristics in ITER is highly different from that in the present tokamaks. In the present tokamaks, the edge turbulence in an H-mode plasma is “blobby,” with most of the convective blob motion in the poloidal direction yielding little radial transport. Blobby electron radial transport is passive, only keeping the quasi-neutrality with ion magnetic drift. However, in ITER, the edge turbulence is found to be “streamer-like,” giving rise to active radial particle and thermal transport. There appears to be a bifurcation of the edge turbulence characteristics from blobs to streamers between JET and ITER, most likely due to the size effect, in the XGC simulation. Fundamental physics behind this turbulence bifurcation will be discussed, in relation to the sheared ExB flow, and the Kelvin-Helmholtz, TEM and ITG turbulence.   

Drift-kinetic simulations of axisymmetric plasma transport at the edge of a divertor tokamak

Eulerian kinetic calculations are presented for the axisymmetric cross-separatrix transport of plasma at the edge of a tokamak. The simulations are performed with a high-order finite-volume code COGENT that solves the full-F drift-kinetic equation for the ion species including the effects of fully-nonlinear Fokker-Plank ion-ion collisions. The ion kinetic response is coupled to two-dimensional self-consistent electrostatic potential variations, which are obtained from the vorticity equation with the isothermal fluid electron model. The paper also presents recent progress toward the full-edge turbulence code. The slab-geometry 5D version has recently become available and is successfully verified in simulations of the collisionless drift-wave instability.   

On pressure balance in a low collisionality tokamak scrape-off layer

Understanding the physics governing the scrape-off layer is necessary in order to reliably predict machine and operation critical quantities, such as the heat flux width at the divertor, plasma-wall interaction, material migration, effect of divertor condition on the pedestal profile, detachment of the divertor plasma, etc. Recent simulation results using the axisymmetric gyrokinetic code XGCa suggest that in a lower ion collisionality near scrape-off layer, where the plasma is highly non-Maxwellian, the fluid form of the momentum equation is not conserved between the low-field side (LFS) midplane and divertor. Taking care to include neutral friction and a Chew-Goldberger-Low (CGL) form of the pressure tensor (i.e. only the dominant diagonal terms) does not resolve the imbalance. Using the full kinetic distribution function in the XGC gyrokinetic code, we explore the effect of off-diagonal pressure tensor terms, to determine their effect in the momentum balance in the scrape-off layer. We also explore other simulations with higher ion collisionality, to begin to study the effect of ion collisionality versus proximity to the separatrix (flux surfaces closer to the separatrix can be more influenced by e.g. X-point loss).   

Evolution of pressure drop during detachment in the TCV tokamak

To ensure safe power exhaust in future fusion reactors will require, at the least, partially detached divertor operation. To further understand the dynamics of this process, we investigate detachment on TCV in a lower single-null geometry by examining the evolution of the profiles of plasma density, temperature, and pressure at the outboard midplane (``upstream'') and at the outer strikepoint (``target''). A fast reciprocating probe is plunged at different times of reproducible discharges throughout the detachment process. Its measurements are compared with target measurements from probes and infrared thermography. As expected, the roll-over of the ion flux, often used experimentally to identify detachment, coincides with a pressure drop along the field lines. This is compared quantitatively with expectations from an extended two-point model where radiation and momentum losses are evaluated from bolometry and probe data. The roll-over coincides with the onset of `` density shoulders '' at the outer midplane. These are observed in the upstream density profiles and in bolometric and AXUV measurements, that show a radiation increase at the outer midplane. The application of these findings to detachment in advanced divertor geometries will be discussed.   

3D nonlinear numerical simulation of the current-convective instability in detached diverter plasma.

One of the possible mechanisms responsible for strong radiation fluctuations observed in the recent experiments with detached plasmas at ASDEX Upgrade [Potzel et al., Nuclear Fusion, 2014] can be related to the onset of the current-convective instability (CCI) driven by strong asymmetry of detachment in the inner and outer tokamak divertors [Krasheninnikov and Smolyakov, PoP, 2016]. In this study we present the first results of 3D nonlinear numerical simulations of the CCI in divertor plasma for the conditions relevant to the AUG experiment. The general physical model used to simulate the CCI, qualitative estimates for the instability characteristic growth rate and transverse wavelengths derived for plasma, which is spatially inhomogeneous both across and along the magnetic field lines, are presented. The simulation results, demonstrating nonlinear dynamics of the CCI, provide the frequency spectra of turbulent divertor plasma fluctuations showing good agreement with the available experimental data.   

Detachment studies in the Magnum-PSI linear device

Divertor detachment experiments on the Magnum-PSI linear device have been performed to investigate the relevant volume and surface processes responsible for detachment in tokamaks. The interaction of the plasma with a neutral background plays a crucial role in achieving a detached plasma regime. Detachment in Magnum-PSI is achieved by raising the H$_{\mathrm{2}}$ background pressure by seeding near the target. Increasing the pressure up to 16 Pa showed a significant decrease in plasma pressure, heat flux and ion flux to the target. Radially resolved spectroscopy showed large deviations of the Balmer line ratios from pLTE, and will provide detailed information on the volume processes, comparing the experimentally observed line intensities to collisional radiative modelling. To study the plasma chemistry in Nitrogen seeded scenarios, the H$_{\mathrm{2}}+$N$_{\mathrm{2}}$ chemistry has been implemented in the PLASIMO code. This model suggests a modified molecular-activated-recombination (MAR) scheme in which N$_{\mathrm{2}}$H$^{\mathrm{+}}$ and NH play key roles. The strongly reduced set of primary reaction mechanisms suggested by PLASIMO is being implemented into B2.5-Eunomia. Experiments in Magnum-PSI using a mixed H$_{\mathrm{2}}$/N$_{\mathrm{2}}$ neutral background showed strong NH radiation from the plasma and significant fractions of N$_{\mathrm{2}}$H in the residual gas analyser, qualitatively confirming the chemical processes proposed by PLASIMO.   

Time-dependent modeling of dust injection in semi-detached ITER divertor plasma

At present, it is generally understood that dust related issues will play important role in operation of the next step fusion devices, i.e. ITER, and in the development of future fusion reactors. Recent progress in research on dust in magnetic fusion devises has outlined several topics of particular concern: a) degradation of fusion plasma performance; b) impairment of in-vessel diagnostic instruments; and c) safety issues related to dust reactivity and tritium retention. In addition, observed dust events in fusion edge plasmas are highly irregular and require consideration of temporal evolution of both the dust and the fusion plasma. In order to address the dust-related fusion performance issues, we have coupled the dust transport code DUSTT and the edge plasma transport code UEDGE in time-dependent manner, allowing modeling of transient dust-induced phenomena in fusion edge plasmas. Using the coupled codes we simulate burst-like injection of tungsten dust into ITER divertor plasma in semi-detached regime, which is considered as preferable ITER divertor operational mode based on the plasma and heat load control restrictions. Analysis of transport of the dust and the dust-produced impurities, and of dynamics of the ITER divertor and edge plasma in response to the dust injection will be presented.   

Analysis of filament statistics in fast camera data on MAST

Coherent filamentary structures have been shown to play a dominant role in turbulent cross-field particle transport [D'Ippolito 2011]. An improved understanding of filaments is vital in order to control scrape off layer (SOL) density profiles and thus control first wall erosion, impurity flushing and coupling of radio frequency heating in future devices. The Elzar code [T. Farley, 2017 in prep.] is applied to MAST data. The code uses information about the magnetic equilibrium to calculate the intensity of light emission along field lines as seen in the camera images, as a function of the field lines' radial and toroidal locations at the mid-plane. In this way a `pseudo-inversion' of the intensity profiles in the camera images is achieved from which filaments can be identified and measured. In this work, a statistical analysis of the intensity fluctuations along field lines in the camera field of view is performed using techniques similar to those typically applied in standard Langmuir probe analyses. These filament statistics are interpreted in terms of the theoretical ergodic framework presented by F. Militello {\&} J.T. Omotani, 2016, in order to better understand how time averaged filament dynamics produce the more familiar SOL density profiles.   

SOLPS simulations of X-divertor in NSTX-U

The X-divertor (XD) geometry in NSTX-U has demonstrated, in SOLPS simulations, a better performance than the standard divertor (SD) regarding detachment: achieving detachment with a lower upstream density and stabilizing the detachment front near the target. The benefits of such a localized front is that the power exhaust requirement can be satisfied without the radiation front encroaching on the core plasma. It is also found by our simulations that at similar states of detachment the XD outperforms the SD by reducing the heat fluxes to the target and maintaining higher upstream temperatures. These advantages are attributed to the unique geometric characteristics of XD - poloidal flaring near the target. The detailed physical mechanisms behind the better XD performance that is found in the simulations will be examined.   

Efficient Coupling of Fluid-Plasma and Monte-Carlo-Neutrals Models for Edge Plasma Transport

UEDGE [1] has been valuable for modeling transport in the tokamak edge and scrape-off layer due in part to its efficient \textit{fully} \textit{implicit} solution of coupled \textit{fluid} neutrals and plasma models. We are developing an implicit coupling of the kinetic Monte-Carlo (MC) code DEGAS-2 [2], as the neutrals model component, to the UEDGE plasma component, based on an extension of the Jacobian-free Newton-Krylov (JFNK) method to MC residuals [3]. The coupling components build on the methods and coding already present in UEDGE. For the linear Krylov iterations, a procedure has been developed to ``extract'' a good preconditioner from that of UEDGE. This preconditioner may also be used to greatly accelerate the convergence rate of a relaxed fixed-point iteration, which may provide a useful ``intermediate'' algorithm. The JFNK method also requires calculation of Jacobian-vector products, for which any finite-difference procedure is inaccurate when a MC component is present [3]. A semi-analytical procedure that retains the standard MC accuracy and fully kinetic neutrals physics is therefore being developed. [1] T.D. Rognlien et al., Contrib. Plasma Phys. \textbf{34}, 362. (1994). [2] D. Stotler {\&} C. Karney, Contr. Plas. Phys., 34, 392 (1994). [3] J. Willert, et al., SIAM J. Numer. Anal. \textbf{53}, 1738 (2015).   

A numerical study of neutral-plasma interaction in magnetically confined plasmas

Interactions between plasma and neutral species can have a large effect on the dynamic behavior of magnetically confined plasma devices, such as the edge region of tokamaks and the plasma formation of Z-pinches. The presence of neutrals can affect the stability of the pinch and change the dynamics of the pinch collapse, and they can lead to deposition of high energy particles on the first wall. However, plasma-neutral interactions can also have beneficial effects such as quenching the disruptions in tokamaks. In this research a reacting plasma-neutral model, which combines a magnetohydrodynamic (MHD) plasma model with a gas dynamic neutral fluid model [Meier \& Shumlak, POP 19 (2012)], is used to study the interaction between plasma and neutral gas. Incorporating this model into NIMROD allows the study of electron-impact ionization, radiative recombination, and resonant charge-exchange in plasma-neutral systems. An accelerated plasma moving through a neutral gas background is modeled in both a parallel plate and a coaxial electrode configuration to explore the effect of neutral gas in pinch-like devices.   

GTC Turbulence Simulations near H-mode Pedestal with Resonant Magnetic Perturbations

Full plasma responses to Resonant Magnetic Perturbations (RMPs) as provided by the resistive MHD code M3D-C$^\text{1}$ are implemented into Gyrokinetic Toroidal Code (GTC) to study the effect of magnetic islands and stochastic field regions on microturbulence in realistic DIII-D geometry. Electrostatic turbulence simulations with adiabatic electrons show no significant increase of the saturated ion heat conductivity in the presence of RMP-induced islands. However, electron response to zonal flow in the presence of magnetic islands and stochastic fields can drastically increase zonal flow dielectric constant for long wavelength fluctuations. Zonal flow generation can then be reduced and the microturbulence can be enhanced greatly. Furthermore, because the RMP magnetic island size is comparable to the ion banana width, electron and ion responses to these islands may be fundamentally different, which could drive non-ambipolar particles fluxes leading to changes of the radial electric field shear.   

Linear instabilities near the DIII-D edge simulated in fluid models

The linear instability spectrum is reported near the DIII-D edge (within the separatrix) for L-mode and H-mode shots using the new eigenvalue solver FluTES (Fluid Toroidal Eigenvalue Solver). FluTES circumvents difficulties with convergence to clean linear eigenmodes (required for diagnosis of nonlinear simulations in codes such as BOUT++ [1]) often encountered with fluid initial-value solvers. FluTES is well-verified in analytic cases and against a BOUT++/ELITE benchmark toroidal case. We report results for both a 3-field, one-fluid model (the well-known “elm-pb” model) and a 5-field, two-fluid model. For the peeling-ballooning-dominated H-mode, the two solutions are qualitatively the same. In the driftwave-dominated L-mode edge, only the two-fluid solution gives robust instabilities which occur primarily at $n>50$. FluTES is optimized for this regime (near-flutelike limit, toroidally spectral). Cross-separatrix, coupled fluid and drift instabilities may play a role in explaining the gyrokinetic L-mode edge transport shortfall [2]. Extension of FluTES into the open-field-line region is underway. [1] Dudson et al., Comp. Phys. Comm. V.180 (2009) 1467. [2] C. Holland et al, Phys. Plasmas 16, 052301 (2009).   

Simulating the DIII-D grassy ELM regime with BOUT$++$

In order to develop a steady-state regime for the ITER Phase-II mission, a fully non-inductive hybrid regime with the effective ELM control using weak 3D fields is studied on DIII-D. The 2-fluid modules in BOUT$++$ are used to study the dynamics of ELMs in this regime, especially for grassy ELMs in DIII-D shot {\#}161414. Linear analysis shows that the grassy ELMs in the hybrid regime occur close to the ideal peeling boundary, which is quite different from the conclusion of high-n ballooning modes on JT60U (Oyama, NF2010). However, the inclusion of the measured edge electric field Er, which can alter the peeling-ballooning (PB) instability boundary (J.G. Chen, PoP2017), predicts wide spectrum PB modes, and all the high-n (n\textgreater 30) modes can be stabilized by ion diamagnetic effects. Therefore, linear instability of grassy ELMs is driven by ideal peeling modes and low-n ballooning modes due to the boundary changing effects of Er. Nonlinear simulations show that the ELM pressure profile crash at the outer mid-plane is enhanced by Er, but the poloidal extent of the crash is limited to the low-field side and the total energy loss is just \textasciitilde 1{\%}. Detailed nonlinear simulation results will be reported in this talk.   

Self-consistent calculation of the radial electric field with ion orbit loss mechanism by using BOUT$+$

The steady state radial electric field (Er) can be self-consistently calculated by coupling a plasma transport model with a quasi-neutrality constraint and the vorticity formulation within the BOUT$++$ framework. Based on the experimentally measured plasma density and temperature profiles inside the separatrix, the effective particle and heat diffusivities can be interpreted from the set of plasma transport equations. The effective diffusivities are then extended into the scrape off layer (SOL) to calculate the plasma density, temperature and flow profiles across the separatrix into the SOL. With plasma quantities defined in both the pedestal and SOL regions, the electric field can be calculated across the separatrix from the vorticity equations with a sheath boundary condition, and the cross-field drifts are shown to play a significant role by inducing a net flow in both the edge and the SOL region. The sheath boundary condition acts to generate a large, positive Er in the SOL, which is consistent with experimental measurements. Furthermore, the particle, momentum, and energy ion-orbit losses are incorporated into the transport equations and shown to impact intrinsic rotation, and therefore the self-consistent Er calculation.   

Ideal MHD Stability and Characteristics of Edge Localized Modes on CFETR

Investigation on the equilibrium operation regime, its ideal magnetohydrodynamics (MHD) stability and edge localized modes (ELM) characteristics is performed for China Fusion Engineering Test Reactor (CFETR). The CFETR operation regime study starts with a baseline scenario derived from multi-code integrated modeling, with key parameters varied to build a systematic database. These parameters, under profile and pedestal constraints, provide the foundation for engineering design. The linear stabilities of low-n and intermediate-n peeling-ballooning modes for CFETR baseline scenario are analyzed. Multi-code benchmarking, including GATO, ELITE, BOUT$++$ and NIMROD, demonstrated good agreement in predicting instabilities. Nonlinear behavior of ELMs for the baseline scenario is simulated using BOUT$++$. Instabilities are found both at the pedestal top and inside the pedestal region, which lead to a mix of grassy and type I ELMs. Pedestal structures extending inward beyond the pedestal top are also varied to study the influence on ELM characteristic. Preliminary results on the dependence of the Type-I ELM divertor heat load scaling on machine size and pedestal pressure will also be presented.   

Simulation of the ELMs triggering by lithium pellet on EAST tokamak using BOUT$++$

A new lithium granule injector (LGI) was developed on EAST. Using the LGI, lithium granules can be efficiently injected into EAST tokamak with the granule radius 0.2- 1 mm and the granules velocity \textasciitilde 30-110 m/s. ELM pacing was realized during EAST shot {\#}70123 at time window from 4.4-4.7s, the average velocity of the pellet was \textasciitilde 75 m/s and the average injection rate is at \textasciitilde 99Hz. The BOUT$++$ 6-field electromagnetic turbulence code has been used to simulate the ELM pacing process. A neutral gas shielding (NGS) model has been implemented during the pellet ablation process. The neutral transport code is used to evaluate the ionized electron and Li ion densities with the charge exchange as a dominant factor in the neutral cloud diffusion process. The snapshot plasma profiles during the pellet ablation and toroidal symmetrization process are used in the 6-field turbulence code to evaluate the impact of the pellets on ELMs. Destabilizing effects of the peeling-ballooning modes are found with lithium pellet injection, which is consistent with the experimental results. A scan of the pellet size, shape and the injection velocity will be conducted, which will benefit the pellet injection design in both the present and future devices.   

Simulation comparison of EHO state and broadband MHD phase in near-zero torque QH-mode on DIII-D

A DIII-D QH-mode discharge ({\#}163518), with NBI torque reduced to near zero, exhibits a spontaneous transition from coherent edge-harmonic oscillation (EHO) phase to broadband MHD turbulence state with improved pedestal conditions recently. Simulations are carried out to study the features and the mechanism of both EHO state and broadband MHD phase in QH-mode by using the 6-field two-fluid model in BOUT$++$ framework. The double null equilibriums and plasma profiles from the DIII-D low-torque QH-mode discharge {\#}163518 at t$=$2350ms (EHO state) and t$=$3130ms (broadband MHD state) are adopted in the simulations. The linear simulation results demonstrate that the ExB shear flow is the main driving factor of the low-n MHD instabilities and can destabilize the low-n modes which are dominant during this QH-mode discharge. In nonlinear simulation, the intermediate-n (6\textasciitilde 12) modes are excited first in the early linear stage and then the low-n modes develop by inverse cascade. The toroidal mode n$=$1 becomes dominant in the nonlinear saturated phase. The analysis of heat and particle fluxes and frequency spectra in nonlinear simulations is also presented.   

Impurity migration pattern under RF sheath potential in tokamak and the response of Plasma to RMP

The migration pattern of impurity sputtered from RF guarder limiter, is simulated by a test particle module. The electric potential with RF sheath boundary condition on the guard limiter and the thermal sheath boundary condition on the divertor surface are used. The turbulence transport is implemented by random walk model. It is found the RF sheath potential enhances the impurity percentage lost at low filed side middle plane, and decreases impurity percentage drifting into core region. This beneficial effect is stronger when sheath potential is large. When turbulence transport is strong enough, their migration pattern will be dominated by transport, not by sheath potential. The Resonant Magnetic field Perturbation (RMP) is successfully applied in EAST experiment and the suppression and mitigation effect on ELM is obtained. A two field fluid model is used to simulate the plasma response to RMP in EAST geometry. The current sheet on the resonance surface is obtained initially and the resonant component of radial magnetic field is suppressed there. With plasma rotation, the Alfven resonance occurs and the current is separated into two current sheets. The simulation result will be integrated with the ELM simulations to study the effects of RMP on ELM.   

Magnetic perturbations at the plasma edge due to Scrape Off Layer currents

While most of fusion community which considers the temperature pedestal region as a mysterious ``Edge Transport Barrier'', the author dismisses this notion and its ``explanations''. Instead, the temperature of the pedestal represents the boundary condition determined by the ratio of the total heat flux from a plasma component to the particle flux. One of the straightforward results of this understanding is that the level of the pedestal temperature is unshakable by perturbations of transport properties at the edge, what was confirmed unambiguously by RMP experiments on DIII-D in 2005 and in all following experiments. In the other hand, I consider the width of the pedestal determined by destruction of 2-D magnetic configuration at the plasma edge, and the Scrape Off Layer (SOL) currents considered as a primary source of perturbation. The numerical simulations of their effect are presented.   

FLIT: Flowing LIquid metal Torus*

The design and construction of FLIT, Flowing LIquid Torus, at PPPL is presented. FLIT focuses on a liquid metal divertor system suitable for implementation and testing in present-day fusion systems, such as NSTX-U. It is designed as a proof-of-concept fast-flowing liquid metal divertor that can handle heat flux of 10 MW/m2 without an additional cooling system. The 72 cm wide by 107 cm tall torus system consisting of 12 rectangular coils that give 1 Tesla magnetic field in the center and it can operate for greater than 10 seconds at this field. Initially, 30 gallons Galinstan (Ga-In-Sn) will be recirculated using 6 jxB pumps and flow velocities of up to 10 m/s will be achieved on the fully annular divertor plate. FLIT is designed as a flexible machine that will allow experimental testing of various liquid metal injection techniques, study of flow instabilities, and their control in order to prove the feasibility of liquid metal divertor concept for fusion reactors. *This work is supported by the US DOE Contract No. DE-AC02-09CH11466.   

Effects of externally applied Lorentz force on liquid metal flow

This work looks at methods of controlling liquid metal flows using externally induced Lorentz forces. Large fusion reactors face an unsolved issue of heat fluxes at the divertor causing reactor damage. Fast-flowing liquid metal divertors can solve the heat flux problem, but to be viable there are various unfavorable flow phenomena that need to be suppressed and controlled. Some of those studied here are hydraulic jumps and surface waves. Externally induced Lorentz forces may be created by injecting electric currents into a liquid metal flow immersed within a magnetic field. Uniform Lorentz forces aligned with gravity work nearly analogously to changing gravity, and as such any flow features driven or affected by gravity may experience changes. As Lorentz force is dependent on current density which can be highly variant as cross-sectional flow depth changes, a non-uniform force field is created that is mostly unique to these types of flows; non-uniform magnetic fields yield similar effects. Lorentz force has been historically used as a driving force in pump applications, but little has been done in the way of flow control. The experiments in this work are galinstan channel flows that investigate the effects that Lorentz force has on hydraulic jump features and surface waves.   

Study of Lithium Vapor Flow In a Detached Divertor using DSMC code

A detached divertor is predicted to be necessary to handle the heat fluxes of a demonstration fusion power plant [1]. The lithium vapor box divertor has poloidal baffles to form distinct chambers and contains dense lithium vapor to cause detachment. These chambers would be differentially pumped via condensation, resulting in flow at Knudsen numbers 0.01-0.5 and densities $10^{19}$-$10^{23}$ $m^{-3}$. This divertor geometry is predicted to handle the estimated heat flux while also localizing the vapor in the divertor [2]. We provide a simulation of the divertor’s lithium vapor flow using the SPARTA Direct Simulation Monte Carlo (DSMC) code [3]. Lithium mass flow, vapor pressures, and temperatures within each chamber are given. Preliminary simulations of a vapor box divertor similarity experiment are within 30$\%$ of an ideal-gas choked nozzle flow calculation.\\ \\ 1. R.J Goldston, J. Nucl. Mat. (2015)\newline http://dx.doi.org/10.1016/j.jnucmat.2014.10.080\newline 2. R.J Goldston et al. Phys. Sc. T167 (2016) doi:10.1088/0031-8949/T167/1/014017\newline 3. M.A Gallis et al., AIP Conference Proceedings 1628, 27 (2014); doi: 10.1063/1.4902571   

Design and construction of a lithium vapor box divertor similarity experiment

Future fusion devices will require handling extreme heat fluxes. The lithium vapor box divertor is a concept to manage this heat flux. The divertor plasma impinges on a dense cloud of lithium vapor, leading to volumetric cooling, radiation, and recombination. The vapor is localized by baffles and condensation on the divertor slot walls upstream of the target, limiting the lithium reaching the main chamber. A series of test stand experiments will study vapor confinement and plasma plugging in a simplified baffled-pipe geometry. A first experiment without plasma will validate a DSMC model for evaporation, flow, and condensation of lithium vapor. Three stainless steel cylindrical cans will be heated to 550C, 600C, and 650C respectively inside a vacuum chamber. Lithium flow will be measured by weighing the cans before and after heating and by calorimetry of the latent heat of the vapor. Progress on the experiment will be presented.   

3D Laser Imprint Using a Smoother Ray-Traced Power Deposition Method

Imprinting of laser nonuniformities in directly-driven icf targets is a challenging problem to accurately simulate with large radiation-hydro codes. One of the most challenging aspects is the proper construction of the complex and rapidly changing laser interference structure driving the imprint using the reduced laser propagation models (usually ray-tracing) found in these codes. We have upgraded the modelling capability in our massively-parallel \textsc{fastrad3d} code by adding a more realistic EM-wave interference structure. This interference model adds an axial laser speckle to the previous transverse-only laser structure, and can be impressed on our improved smoothed 3D raytrace package. This latter package, which connects rays to form bundles and performs power deposition calculations on the bundles, is intended to decrease ray-trace noise (which can mask or add to imprint) while using fewer rays. We apply this improved model to 3D simulations of recent imprint experiments performed on the Omega-EP laser and the Nike laser that examined the reduction of imprinting due to very thin high-Z target coatings. We report on the conditions in which this new model makes a significant impact on the development of laser imprint.   

Re-examining the effect of low and intermediate mode number perturbations on Ignition Metrics Scaling Laws

We re-examine the way 2/3D effects on scaling laws for ignition metrics, such as the generalized Lawson Criterion (GLC) and the Ignition Threshold Factor (ITF). These scaling laws were derived for 1D symmetrical case and 2/3D perturbations [Hann et al. PoP 2010; Lindl et al., PoP 2014; Betti et al., PoP 2010]. The main cause for the difference between the 1D and the 2/3D scaling laws in those works, is heat conduction losses from the hot-spot bubbles to the cold shell [Kishony and Shvarts, PoP 2001]. This ``dry out'' of the bubbles is the dominant mechanism for intermediate mode number perturbations (6\textless l\textless 40) and can be described as an effective 1D implosion. However, for low mode number perturbations (l$\le $6), heat conduction loss does not fully ``dry out'' the bubbles and an additional mechanism- residual kinetic energy (RKE) [Kirtcher PoP 2014; Gu et al., PoP 2014] does reduce the hydrodynamic coupling efficiency from the imploding cold shell to the hot spot. These two effects do not have an effective 1D analogue and therefore needs a more complicated model. A consistent extension of the ignition metrics for l$\le $6, accounting for both energy loss mechanisms, will be presented and compared with previous models and results.   

Hybrid transport and diffusion modeling using electron thermal transport Monte Carlo SNB in DRACO

The iSNB (implicit Schurtz Nicolai Busquet$^{\mathrm{1,2}})$ multigroup diffusion electron thermal transport method is adapted into an Electron Thermal Transport Monte Carlo (ETTMC) transport method to better model angular and long mean free path non-local effects. Previously, the ETTMC model had been implemented in the 2D DRACO multiphysics code and found to produce consistent results with the iSNB method$^{\mathrm{3}}$. Current work is focused on a hybridization of the computationally slower but higher fidelity ETTMC transport method with the computationally faster iSNB diffusion method in order to maximize computational efficiency. Furthermore, effects on the energy distribution of the heat flux divergence are studied. Work to date on the hybrid method will be presented. This work was supported by Sandia National Laboratories and the Univ. of Rochester Laboratory for Laser Energetics. $^{\mathrm{1}}$Schurtz et. al. Phys. Plasmas 7, 4238 (2000) $^{\mathrm{2}}$Cao et. al. Phys. Plasmas 22, 082308 (2015) $^{\mathrm{3}}$Chenhall et.al. BAPS DPP16 CP10.17 (2015)   

Radiation Damage in Si Diodes from Short, Intense Ion Pulses

The Neutralized Drift Compression Experiment (NDCX-II) at Berkeley Lab is an induction accelerator studying the effects that concentrated ion beams have on various materials. Charged particle radiation damage was the focus of this research -- we have characterized a series of Si diodes using an electrometer and calibrated the diodes response using an $^{\mathrm{241}}$Am alpha source, both before and after exposing the diodes to 1 MeV He ions in the accelerator. The key part here is that the high intensity pulses from NDCX-II (\textgreater 10$^{\mathrm{10\thinspace }}$ions/cm$^{\mathrm{2}}$ per pulse in \textless 20 ns) enabled a systematic study of dose-rate effects. An example of a dose-rate effect in Si diodes is increased accumulation of defects due to damage from ions that bombard them in a short pulse. This accumulated damage leads to a reduction in the charge collection efficiency and an increase in leakage current. Testing dose-rate effects in Si diodes and other semiconductors is a crucial step in designing sustainable instruments that can encounter high doses of radiation, such as high intensity accelerators, fusion energy experiments and space applications and results from short pulses can inform models of radiation damage evolution.   

Instability growth seeded by ablator material inhomogeneity in indirect drive implosions on the National Ignition Facility

NIF indirect drive ablators (CH, Be, and high density carbon HDC) show hydrodynamic irregularity beyond that expected from surface features. Characterizing these seeds and estimating their growth is important in projecting performance. The resulting modulations can be measured in x-ray backlit implosions on NIF called Hydro Growth Radiography [Phys. Plasmas 24, 042706 (2017)], and on Omega with 2D velocimetry [S. Ali, invited talk, DPP2017]. This presentation summarizes the experiments for the three ablators, along with simulations thereof and projections of the significance for NIF. For CH, dominant seeds are photo-induced oxidation, which might be mitigated with alumina coating. For Be, perturbations result from Ar and O contamination. For HDC, perturbations are seeded by shock propagation around melt, depend on shock strength, and may constrain the adiabat of future HDC implosions. *Work performed under the auspices of the U.S. D.O.E. by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.   

Preventing or exploiting turbulence during plasma compression

Inertial confinement fusion (ICF) and Z-pinch compressions may be turbulent to varying degrees. The turbulent kinetic energy (TKE) in turbulence undergoing compression can be amplified, if the compression time is fast compared to the dissipation time of the turbulence. By studying the behavior of plasma turbulence under compression, we propose ways of avoiding or exploiting turbulent growth. One possibility for exploiting turbulence during a compression occurs because of a recently discovered ``sudden viscous dissipation'' mechanism. The TKE, having been amplified during the compression, is suddenly dissipated by viscosity into thermal energy. This is possible in plasmas because of the strong dependence of the viscosity on the temperature, which grows during compression. By intentionally storing energy from the compression partly in TKE, which has different loss mechanisms than the thermal energy, and only converting the TKE into thermal energy late in the compression, energy losses may be reduced. Understanding the behavior of TKE under compression also allows us to predict compression trajectories, in rho-R vs. temperature space, where (bulk) turbulent growth will be suppressed. Contamination of hydrogen plasma with higher Z materials is shown to enhance turbulent growth.   

Kelvin-Helmholtz evolution in subsonic cold streams feeding galaxies

The most prolific star formers in cosmological history lie in a regime where dense filament structures carried substantial mass into the galaxy to sustain star formation without producing a shock. However, hydrodynamic instabilities present on the filament surface limit the ability of such structures to deliver dense matter deeply enough to sustain star formation. Simulations lack the finite resolution necessary to allow fair treatment of the instabilities present at the stream boundary. Using the Omega EP laser, we simulate this mode of galaxy formation with a cold, dense, filament structure within a hotter, subsonic flow and observe the interface evolution. Machined surface perturbations stimulate the development of the Kelvin-Helmholtz (KH) instability due to the resultant shear between the two media. A spherical crystal imaging system produces high-resolution radiographs of the KH structures along the filament surface. The results from the first experiments of this kind, using a rod with single-mode, long-wavelength modulations, will be discussed.   

Designing cylindrical implosion experiments on NIF to study deceleration phase of Rayleigh-Taylor

The Rayleigh-Taylor (RT) hydrodynamic instability occurs when a lower density fluid pushes on a higher density fluid. This occurs in inertial confinement fusion (ICF) implosions at each of the capsule interfaces during the initial acceleration and the deceleration as it stagnates. The RT instabilities mix capsule material into the fusion fuel degrading the Deuterium-Tritium reactivity and ultimately play a key role in limiting target performance. While significant effort has focused on understanding RT at the outer capsule surface, little work has gone into understanding the inner surface RT instability growth during the deceleration phase. Direct measurements of the RT instability are difficult to make at high convergence in a spherical implosion. Here we present the design of a cylindrical implosion system for the National Ignition Facility for studying deceleration phase RT. We will discuss the experimental design, the estimated instability growth, and our outstanding concerns.   

Single-Mode Deceleration Stage Rayleigh-Taylor Instability Growth in Cylindrical Implosions

We present design calculations demonstrating the feasibility of measuring single-mode deceleration stage Rayleigh-Taylor instability (RTI) growth at a factor of four convergence. RTI growth rates are modified as a result of convergence [Bell LA-1321, 1951], and cylindrical targets are considered here, as they allow direct diagnostic access along the interface. The 2D computations, performed with the radiation-hydrodynamics code xRAGE [Gittings et al., CSD 2008] utilizing a new laser ray-tracing package, predict growth factors of 6 to 10 for mode 10 and 4 to 6 for mode 4, both of high interest in evaluating inertial confinement fusion capsule degradation mechanisms [Bose et al., this conference]. These results compare favorably to a linear theory [Epstein, PoP 2004] and to a buoyancy-drag model [Srebro et al., LPB 2003], which accounts for the linear and non-linear stages. Synthetic radiographs, produced by combining 2D computations of axial and transverse cross-sections, indicate this growth will be observable, and these will be compared to experimental data obtained at the OMEGA laser facility. Work performed by Los Alamos National Laboratory under contract DE-AC52-06NA25396 for the National Nuclear Security Administration of the U.S. Department of Energy. (LA-UR-17-25608)   

Modeling ICF With RAGE, BHR, And The New Laser Package

Inertial Confinement Fusion (ICF) is one method used to obtain thermonuclear burn through the either direct or indirect ablation of a millimeter-scale capsule with several lasers. Although progress has been made in theory, experiment, and diagnostics, the community has yet to reach ignition. A way of investigating this is through the use of high performance computer simulations of the implosion. RAGE is an advanced 1D, 2D, and 3D radiation adaptive grid Eulerian code used to simulate hydrodynamics of a system. Due to the unstable nature of two unequal densities accelerating into one another, it is important to include a turbulence model. BHR is a turbulence model which uses Reynolds-averaged Navier-Stokes (RANS) equations to model the mixing that occurs between the shell and fusion fuel material. Until recently, it was still difficult to model direct drive experiments because there was no laser energy deposition model in RAGE. Recently, a new laser energy deposition model has been implemented using the same ray tracing method as the Mazinisin laser package used at the OMEGA laser facility at the Laboratory for Laser Energetics (LLE) in Rochester, New York. Using the new laser package along with BHR for mixing allows us to more accurately simulate ICF implosions and obtain spatially and temporally resolved information (e.g. position, temperature, density, and mix concentrations) to give insight into what is happening inside the implosion.   

Observation of the effects of stronger magnetic fields on warm, higher energy electrons and ion beams transiting a double layer in a helicon plasma

Fast, two-temperature electrons ($>$80 eV, Te$=$13 eV tail, 4 eV bulk) with substantial tail density fractions are created at low ($<=$1.7 mtorr) Ar pressure @ 340 G in the antenna region with nozzle mirror ratio of 1.4 on MadHeX @ 900W. These distributions including a fast tail are observed upstream of a double layer. The fast, untrapped tail electrons measured downstream of the double layer have a higher temperature of 13 eV than the trapped, upstream electrons of 4 eV temperature. Upstream plasma potential fluctuations of $+-$30 percent are observed. An RF-compensated Langmuir probe is used to measure the electron temperatures and densities and OES, mm wave IF and an RPA for the IEDF are also utilized. As the magnetic field is increased to 1020 G, an increase in the electron temperature and density upstream of the double layer is observed with Te= 15-25 eV with a primarily single temperature mode. Accelerated ion beam energies in the range of 65-120 eV are observed as the magnetic field is increased from 340 to 850 G. The role of the nozzle, plasma double layer and helicon wave coupling on the EEDF and ion acceleration will be discussed. Y.-T. Sung, Y. Li, J. E. Scharer, Phys. Plasmas 23, 092113 (2016)   

Linear analysis of obliquely propagating longitudinal waves in partially spin polarized degenerate magnetized plasma

Linear analysis of obliquely propagating longitudinal waves in partially spin polarized degenerate magnetized plasma Linear analysis of low frequency obliquely propagating electrostatic waves in a partially spin polarized degenerate magnetized plasma is presented. Using Fourier analysis, a general linear dispersion relation is derived for low frequency electrostatic lower hybrid (LH) wave, ion acoustic (IA) wave and ion cyclotron (IC) wave in the presence of electron spin polarization. It is found that the electron spin polarization gives birth to a new spin- dependent wave (spin electron acoustic wave) in the spectrum of these waves. Further, the electron spin polarization also causes drastic shifts in the frequency spectrum of these waves. These effects would have a strong bearing on wave phenomena in degenerate astrophysi   

New Frontier Science Campaign on DIII-D launched in 2017

The DIII-D Frontier Science Experiments initiative explores the potential to use the DIII-D tokamak facility to investigate questions of value beyond the usual fusion-energy science mission of DIII-D. The campaign is unique within DOE-SC-FES because the DIII-D tokamak supplied a multi-day-shot platform for non-fusion-energy-motivated research for the first time. All selected FSE campaign projects competed on the basis of potential intellectual impact and on the degree to which the ability to achieve success as a transformational advance relied on the capabilities of DIII-D. The motivation of the following FSE projects, as well as the selection process, will be summarized (1) Self-organization of Unstable Flux Ropes: Universal Structures in Space/Astrophysical Plasmas (2) Impact of Magnetic Perturbations on Turbulence: Zonal Flow Interactions and Saturation (3) Interaction of Alfv\'{e}n/whistler fluctuations and Runaway Electrons (4) Self-consistent chaos in magnetic field dynamics These basic-plasma experiments, conducted in collaboration with the DIII-D team, were carried out during 5 shot days in FY2017. Additional days are earmarked in FY2018. Future studies with additional FSE-community members are envisioned. Opportunities exist to piggy back with DIII-D research A proper solicitation and peer review would be appropriate going forward if this activity on DIII-D continues Funding from U.S. DOE is gratefully acknowledged.   

Formation and Evolution of Target Patterns in Cahn-Hilliard Flows: An Extension of the Flux Expulsion Studies in MHD

Spinodal decomposition is a second order phase transition for a binary liquid mixture to evolve from a miscible phase (e.g., water + alcohol) to two co-existing phases (e.g., water + oil). The Cahn-Hilliard model for spinodal decomposition is analogous to 2D MHD. We study the evolution of the concentration field in a single eddy in the 2D Cahn-Hilliard system to better understand scalar mixing processes in that system. This study extends investigations of the classic studies of flux expulsion in 2D MHD and homogenization of potential vorticity in 2D fluids. Simulation results show that there are three stages in the evolution: (A) formation of a ‘jelly roll’ pattern, for which the concentration field is constant along spirals; (B) a change in isoconcentration contour topology; and (C) formation of a target pattern, for which the isoconcentration contours follow concentric annuli. In the final target pattern stage, the isoconcentration bands align with stream lines. The results indicate that the target pattern is a metastable state. Band merger process continues on a time scale exponentially long relative to the eddy turnover time. The band merger process resembles step merger in drift-ZF staircases.   

Harmonics generation near ion-cyclotron frequency of ECR plasma.

Wave excitation at different frequency regime is employed in the MaPLE device ECR plasma [Review of scientific instruments,~81(7), 073507, (2010)] for varied excitation amplitude. At very low amplitude excitation, mainly fundamental frequency mode of the exciter signal frequency comes into play. With the increase in amplitude of applied perturbation, harmonics are generated and dominant over the fundamental frequency mode. There is a fixed critical amplitude of exciter to yield the harmonics and is independent of applied frequency. Observed harmonics and the main frequency mode has propagation characteristics and are discussed here. Exact mode number and propagation nature are also tried to measure in the experiment. Detailed experimental results will be presented.   

Ion Acceleration by Double Layers with Multi-Component Ion Species

Current-free double layers (CFDL) models have been proposed to explain observations of magnetic field-aligned ion acceleration in plasmas expanding into divergent magnetic field regions. More recently, experimental studies of the Bohm sheath criterion in multiple ion species plasma reveal an equilibration of Bohm speeds at the sheath-presheath boundary for a grounded plate in a multipole-confined filament discharge.$^{\mathrm{1}}$ We aim to test this ion velocity effect for CFDL acceleration. We report high resolution ion velocity distribution function (IVDF) measurements using laser induced fluorescence downstream of a CFDL in a helicon plasma. Combinations of argon-helium, argon-krypton, and argon-xenon gases are ionized and measurements of argon or xenon IVDFs are investigated to determine whether ion acceleration is enhanced (or diminished) by the presence of lighter (or heavier) ions in the mix. We find that the predominant effect is a reduction of ion acceleration consistent with increased drag arising from increased gas pressure under all conditions, including constant total gas pressure, equal plasma densities of different ions, and very different plasma densities of different ions. These results suggest that the physics responsible for acceleration of multiple ion species in simple sheaths is not responsible for the ion acceleration observed in these expanding plasmas. * Department of Physics, Gettysburg College. 1. G. Severn et. al., Plasma Sources Sci. Technol. \textbf{26} (2017).   

Gauge-free gyrokinetic theory

Test-particle gyrocenter equations of motion play an essential role in the diagnosis of turbulent strongly-magnetized plasmas, and are playing an increasingly-important role in the formulation of kinetic-gyrokinetic hybrid models. Previous gyrocenter models required the knowledge of the perturbed electromagnetic potentials, which are not directly observable quantities (since they are gauge-dependent). A new gauge-free formulation of gyrocenter motion is presented, which enables gyrocenter trajectories to be determined using only measured values of the directly-observable electromagnetic field. Our gauge-free gyrokinetic theory is general enough to allow for gyroradius-scale fluctuations in both the electric and magnetic field. In addition, we provide gauge-free expressions for the charge and current densities produced by a distribution of gyrocenters, which explicitly include guiding-center and gyrocenter polarization and magnetization effects.   

Magnetic pumping as a source of particle heating

Magnetic pumping is a means of heating plasmas for both fusion and astrophysical applications. In this study a magnetic pumping model is developed as a possible explanation for the heating and the generation of power-law distribution functions observed in the solar wind plasma. In most previous studies turbulent energy is only dissipated at microscopic kinetic scales. In contrast, magnetic pumping energizes the particles through the largest scale turbulent fluctuations, thus bypassing the energy cascade. Kinetic simulations are applied to verify these analytic predictions. Previous results for the one-dimensional model, as well as initial results for a two-dimensional model which includes the effects of trapped and passing particles are presented. Preliminary results of the presence of this mechanism in the bow shock region, using spacecraft data from the Magnetospheric Multiscale mission, are presented as well.   

Vlasov simulation of high energy electron tail formation in the presence of Langmuir solitons

By an electrostatic Vlasov simulation,\footnote{Y.H.Chen, Y.Nishimura, and C.Z.Cheng, Terr. Atmos. Ocean. Sci. {\bf 24}, 273 (2013).} generation mechanism of high energy electron tails in the presence of Langmuir solitons is studied. The resultant electron distribution function resembles that of the Lorentzian type observed in solar wind plasmas. The particle acceleration is discussed as a transport process toward high energy side due to overlapping of multiple resonant islands in the phase space. After the damping of specific Fourier modes (by the wave-particle interaction), and thus shrinkage of the resonant islands, the transport across the islands is prohibited. Role of inhomogeneous background density \footnote{Y.A.Chen, Y.Nishimura, Y.Nishida, and C.Z.Cheng, Phys. Rev. E {\bf 95}, 033205 (2017).} which gives rise to acceleration toward the low density side is discussed employing the kinetic model.   

Route to Chaos due to ion sheath oscillations observed in plasma bubble

The report is intended to investigate experimentally nonlinear behavior of fluctuations in current carrying unstable plasma and compared with the theory that describes ion dynamics in the sheath and pre-sheath region. Plasma bubbles are created in bulk plasma by negatively biased spherical mesh grid of 80{\%} optical transparency inserted in bulk plasma of the system. Argon plasma is produced in cylindrical chamber of 350 mm in length and 400 mm in diameter by hot cathode filament discharge method. The spherical mesh grid can congregate the particles from the plasma radially in presence or absence of biasing. A virtual anode structure has formed around the bubble when all electrons are reflected. A radially movable Langmuir and emissive probe are used to measure basic parameters. Sheath instability inside the bubble has observed, there appears regime of quasi-periodicity with various frequencies. Scanning has done throughout the bubble to understand fluctuations and its associated instabilities. These instabilities are leading to chaos through a region of quasi-period to period doubling at different positions inside the bubble. Experimentally observed ion sheath oscillations are confirmed with some theoretical analysis   

Inverse Bremsstrahlung momentum absorption and current drive

The generation of the plasma current resulting from Bremsstrahlung absorption is considered. It is shown that the electric current is higher than the naive estimates assuming that electrons absorb only the photon momentum and using the Spitzer conductivity would suggest, both because electrons get the recoil momentum from the Coulomb field of ions during the absorption and because electrons absorb power asymmetrically, which leads to the current drive effect.   

Asymmetry Velocity Distribution Function and Its Higher Order Moments in an Inhomogeneous ECR Plasma

Laser induced fluorescence spectroscopy (LIF) has been recognized to be a powerful diagnostic tool to directly obtain velocity distribution function. A neutral depletion structure with strong inhomogeneity of density has been observed in an ECR plasma of the HYPER-I device at NIFS. A high-resolution LIF system, which consists of an external cavity diode laser, has been used to measure the LIF spectrum (velocity distribution function) with velocity component parallel and perpendicular to the density gradient. It is found that the distribution function of the radial velocity (parallel to the density gradient) is asymmetry in the inhomogeneous density region. To quantitatively characterize the asymmetry of distribution function, the higher order velocity moments, i.e., skewness (third order moment) and kurtosis (fourth order moment), are evaluated. It is found that the skewness of distribution function is proportional to the inhomogeneity induced flow, and a simple relation between the skewness and the normalized flow velocity, $u_\text{flow}/v_\text{th}$=-S/3, is obtained [K. Terasaka \textit{et al}., Phys. Plasmas {\bf 23}, 112120 (2016)]. The experimental results indicate the skewness has is a fundamental quantity to characterize the flow induced by inhomogeneity of system.   

A Particle-in-Cell simulation of temporal plasma echo in the presence of Coulomb collisions

Particle-in-Cell simulation is developed to study temporal plasma echo of electron plasma wave. By imposing two external pulse electric fields to the plasma (pulse-like in time) \footnote{R.D.Sydora, Advanced Methods for Space Simulations, pp.47-60 (2007).} the echo signal is observed. Coulomb collisional effect manifests itself as a shift of the echo peak and the damping of the peak amplitude,\footnote{C.H.Su and C.Oberman, Phys. Rev. Lett. 20, 427 (1968).} which can be seen by adding (rather phenomenological) frictional force to the electron equation of motion. A first principle based binary collision model \footnote{T.Takizuka and H.Abe, J. Comput. Phys. 25, 205 (1978).} is incorporated into the numerical simulation.   

Self-consistent Simulation of Microparticle and Ion Wakefield Configuration

In a complex plasma, positively charged ions often have a directed flow with respect to the negatively charged dust grains. The resulting interaction between the dust and the flowing plasma creates an ion wakefield downstream from the dust particles, with the resulting positive space region modifying the interaction between the grains and contributing to the observed dynamics and equilibrium structure of the system. Here we present a proof of concept method that uses a molecular dynamics simulation to model the ion wakefield allowing the dynamics of the dust particles to be determined self-consistently. The trajectory of each ion is calculated including the forces from all other ions, which are treated as ``Yukawa particles'' and shielded from thermal electrons and the forces of the charged dust particles. Both the dust grain charge and the wakefield structure are also self-consistently determined for various particle configurations. The resultant wakefield potentials are then used to provide dynamic simulations of dust particle pairs. These results will be employed to analyze the formation and dynamics of field-aligned chains in CASPER's PK4 experiment onboard the International Space Station, allowing examination of extended dust chains without the masking force of gravity.   

Characterization of quasi-free-space microwave-driven argon plasmas

The Air Force Research Laboratory is interested in studying the interaction of high power electromagnetic waves with plasmas. A multi-kW, 5GHz microwave system is used for generating quasi-free-space microwave-driven argon plasma at pressures ranging from 150 to 200 mTorr. In previous experiments, two general configurations of sustainable quasi-free-space plasma discharges were observed using this system but were never fully characterized. Using a Triple Langmuir Probe (TLP) system, the electron temperature and density of these two observed configurations are measured as they change through time. In addition, a translation stage allows for TLP measurements to be taken in different regions of the generated plasma.   

Impact of Gas Chemistry on Free-Space Mircowave Driven Plasmas

The United States Air Force is studying the properties of plasmas sustained using focused microwave beams in free-space. While we have previously demonstrated the possibility to sustain these plasmas indefinitely the plasmas have been found to be unstable across a wide parameter space. The plasma stability has been shown to depend critically on the compositions of the background gas, however to date these results have been poorly quantified. A new precision gas flow system enables us to control the composition of the background gas. We will report on our efforts to quantify the effect of background gas composition on the plasma stability.   

Constructing current singularity in a 3D line-tied plasma

We revisit Parker's conjecture of current singularity formation in 3D line-tied plasmas, using a recently developed numerical method, variational integration for ideal magnetohydrodynamics in Lagrangian labeling. With the frozen-in equation built-in, the method is free of artificial reconnection, hence arguably an optimal tool for studying current singularity formation. Using this method, the formation of current singularity has previously been confirmed in the Hahm-Kulsrud-Taylor problem in 2D. In this paper, we extend this problem to 3D line-tied geometry. The linear solution, which is singular in 2D, is found to be smooth for all system lengths. However, with finite amplitude, the linear solution can become pathological when the system is sufficiently long. The nonlinear solutions turn out to be smooth for short systems. Nonetheless, the scaling of peak current density vs. system length suggests that the nonlinear solution may become singular at a finite length. With the results in hand, we can neither confirm nor rule out this possibility conclusively, since we cannot obtain solutions with system length near the extrapolated critical value.   

D-D fusion neutron generation and detection in laser-plasma interaction with free-flowing D$_{\mathrm{2}}$O stream .

Due to increasing demand for fast neutrons, there have been many efforts to generate neutrons using Laser Plasma Interactions (LPI). LPI can generate keV to MeV ions, which can undergo fusion reactions. Here, we use a 5-15mJ, 35fs laser operating at \textonehalf kHz, to accelerate deuterons from a 20 $\mu$m D$_{\mathrm{2}}$O stream. These deuterons collide with cold deuterons in the heavy water stream and the low density D$_{\mathrm{2}}$O vapor yielding 2.45MeV fusion neutrons. From the hydrogen capture peak (2.22MeV) recorded by a HPGe detector, we calculate a flux of 2x10$^{\mathrm{5}}$ n/s. In addition, the $^{\mathrm{73}}$Ge(n, $\gamma )$ peak on the HPGe detector and nToF analysis confirm the generation of neutrons. Plasma expansion generated by intentional laser pre-pulses boosts the laser absorption efficiency, giving 10 times higher neutron flux compared to `clean' interactions. 2D particle-in-cell simulations show that deuterons are accelerated forward, in the laser propagation direction, and backward in comparable numbers. But, the backward moving deuterons interacting with the low-density gas/plasma are the main contributors to fusion neutrons. Further experiments with background helium should isolate the region of fusion reactions by stopping backward traveling ions.   

Modeling of flow-dominated MHD instabilities at WiPPAL using NIMROD

Using the NIMROD (non-ideal MHD with rotation - open discussion) code developed at UW-Madison, we model two different flow scenarios to study the onset of MHD instabilities in flow-dominated plasmas in the Big Red Ball (BRB) and the Plasma Couette Experiment (PCX). Both flows rely on volumetric current drive, where a large current is drawn through the plasma across a weak magnetic field, injecting ${\bf J}\times{\bf B}$ torque across the whole volume. The first scenario uses a vertical applied magnetic field and a mostly radial injected current to create Couette-like flows which may excite the magnetorotational instability (MRI). In the other scenario, a quadrupolar field is applied to create counter-rotating von Karman-like flow that demonstrates a dynamo-like instability. For both scenarios, the differences between Hall and MHD Ohm’s laws are explored. The implementation of BRB geometry in NIMROD, details of the observed flows, and instability results are shown.   

Equilibrium and stability of flow-dominated Plasmas in the Big Red Ball

The equilibrium and linear stability of flow-dominated plasmas are studied numerically using a spectral techniques to model MRI and dynamo experiments in the Big Red Ball device. The equilibrium code solves for steady-state magnetic fields and plasma flows subject to boundary conditions in a spherical domain. It has been benchmarked with NIMROD (non-ideal MHD with rotation - open discussion), Two different flow scenarios are studied. The first scenario creates a differentially rotating toroidal flow that is peaked at the center. This is done to explore the onset of the magnetorotational instability (MRI) in a spherical geometry. The second scenario creates a counter-rotating von Karman-like flow in the presence of a weak magnetic field. This is done to explore the plasma dynamo instability in the limit of a weak applied field. Both scenarios are numerically modeled as axisymmetric flow to create a steady-state equilibrium solution, the stability and normal modes are studied in the lowest toroidal mode number. The details of the observed flow, and the structure of the fastest growing modes will be shown.   

Classical Impurity Transport: New Effects in High-Beta, Anisotropic, and Rotating 1D Systems

The classical impurity pinch arises from the Braginskii and diamagnetic frictional forces between high-Z impurities and low-Z ions, and leads to the well-known result that peaked temperature profiles can flush impurities that will otherwise accumulate in the plasma core [S. Hirshman and D. Sigmar, Nuclear Fusion 21, 1079 (1981)]. However, in high-beta systems, or systems with field line curvature, grad-B and curvature drifts will also influence the impurity transport. We analyze the impurity pinch with these drifts added, in the simple context of a screw pinch with constant rotational transform. We find that high plasma beta tends to help flush impurities, while a large rotational transform tends to cause impurities to accumulate in the plasma core. Extensions to anisotropic temperature distributions and the rotating screw pinch are discussed. The results are relevant for tokamaks at large aspect ratio, magnetized liner fusion, and the newly-proposed wave-driven rotating torus (WDRT) fusion concept [J. Rax, R. Gueroult, and N. Fisch, Physics of Plasmas 24, 032504 (2017)]. This work is supported by DOE Grants~DE-SC0016072 and DE-FG02-97ER25308.   

Investigating the Formation and Sub-Structure of Unmagnetized Collisionless Shocks

Collisionless shocks, where the shock thickness is much smaller than the collisional mean free path, are ubiquitous astrophysical phenomena. In all shocks, the Rankine-Hugoniot jump conditions are satisfied through entropy generation at the interface; the shock propagation angle with respect to the magnetic field affects the mechanism by which this entropy is generated. Two experiments on the Big Red Ball (BRB) at UW-Madison explored the formation mechanisms of parallel and perpendicular, unmagnetized and magnetized collisionless shocks with large ($1-3\ m$) system sizes. In the first experiment, a $1\ m$ diameter theta-pinch drove a supersonic ($3 < M < 4$) compressive flow perpendicular to the background magnetic field. In the second, a compact toroid ([cite TriAlpha]) was fired supersonically ($4 < M < 5$) parallel to the background magnetic field. Triple, Langmuir, emissive, and magnetic probes were used to measure electron density, temperature, plasma potential, and fluctuations in magnetic fields. Results showing the transition from above to below $M_{A} = 1$, measurements of electron precursors, exploration of subshock structure, evidence of instabilities in the shock formation process, and future work will be presented.   

Dust growth under different plasma conditions in protoplanetary disks

Coagulation of dust aggregates plays an important role in the formation of planets and the evolution of protoplanetary disks. As cosmic dust becomes charged in the radiative plasma environment, the trajectories of colliding dust grains can be altered by the electrostatic force acting between them, affecting their coagulation probability. This study compares the dust growth in protoplanetary disks with different turbulence strengths and different plasma conditions, i.e. the ratio of free electrons to free ions. A Monte Carlo approach with a simple kernel based on the radius of the grains is used to choose potential colliding pairs and calculate the elapsed time between collisions. The actual collision outcome is determined using a detailed model of the collision which takes into account the aggregate morphology, trajectory, orientation, and all forces acting on the colliding grains. A statistical analysis of the collision outcomes is used to determine collision probability as well as the physical characteristics of the resulting aggregates for both charged and uncharged grains. Preliminary results show that charged aggregates tend to be more porous than neutral particles, and more highly charged particles experience less restructuring as a result of gentler collisions. In regions with weak turbulence, both the collision rate and the number of bouncing collisions are lower for highly charged grains, and the probability of hit-and-stick collisions leading to aggregate growth is a balance of the collision and bouncing rates.   

Heat capacity of spinning plasma

Equilibrium thermodynamics properties, such as heat capacity and adiabatic axial and radial compressibility of a rotating plasma column are studied. These properties depend on rotation speed, charge density, external magnetic field strength and electron-ion mass ratio. Plasma rotation serves as an additional energy storage, hence, yields to increased heat capacity. It also leads to charge separation that changes plasma density distribution due to electrostatic interaction and Lorentz force and therefore modifies thermodynamic properties. The obtained results can provide limits and optimal regimes for radial compression of z-pinch type structures and optimize energy deposition profile.   

The breakdown of the weakly-nonlinear regime for kinetic instabilities

The evolution of marginally-unstable waves that interact resonantly with populations of energetic particles is governed by a well-known cubic integro-differential equation for the mode amplitude. One of the outcomes predicted by the equation is the so-called “explosive” regime, where the amplitude grows indefinitely, eventually taking the equation outside of its domain of validity. Beyond this point, only full Vlasov simulations will accurately describe the evolution of the mode amplitude. In this work, we study the breakdown of the cubic equation in detail. We find that, while the cubic equation is still valid, the distribution function of the energetic particles locally flattens or “folds” in phase space. This feature is unexpected in view of the assumptions of the theory that are given in. We also derive fifth-order terms in the wave equation, which not only give us a more accurate description of the marginally-unstable modes, but they also allow us to predict the breakdown of the cubic equation. Our findings allow us to better understand the transition between weakly-nonlinear modes and the long-term chirping modes that ultimately emerge.   

Viscous heating in $E \times B$ type devices

In a variety of cylindrical plasma devices with axial magnetic fields, a radial electric field gives rise to plasma rotation. This E x B rotation also heats the plasma through viscous effects. In the recently proposed wave-driven rotating torus (WDRT), this viscous heating is thought to be manageable in creating, in principle, economical fusion power [1]. Here, we explore viscous heating both in the WDRT and, more generally, in devices where the primary dynamics is governed by the E x B rotation of plasma. In particular, we explore which species are primarily heated, in both cylindrical and toroidal geometry. We discuss the dependence of the heating on a variety of parameters, such as collisionality, speed of rotation, temperature and ion mix. [1] J. M. Rax, R. Gueroult, and N. J. Fisch, Physics of Plasmas 24, 032504 (2017)   

Development of modular scalable pulsed power systems for high power magnetized plasma experiments

New pulsed power switches and trigger drivers are being developed in order to explore higher energy regimes in the Magnetic Shock Experiment (MSX) at Los Alamos National Laboratory. To achieve the required plasma velocities, high-power (approx. 100 kV, 100s of kA), high charge transfer (approx. 1 C), low-jitter (few ns) gas switches are needed. A study has been conducted on the effects of various electrode geometries and materials, dielectric media, and triggering strategies; resulting in the design of a low-inductance annular field-distortion switch, optimized for use with dry air at 90 psig, and triggered by a low-jitter, rapid rise-time solid-state Linear Transformer Driver. The switch geometry and electrical characteristics are designed to be compatible with Syllac style capacitors, and are intended to be deployed in modular configurations. The scalable nature of this approach will enable the rapid design and implementation of a wide variety of high-power magnetized plasma experiments.   

Shock accelerated particles in inertial fusion

We consider both electrostatic and magnetized shocks in inertial fusion. It is known that strong localized electric fields exist in laser compressed pellets that are associated with shock like structures and these are responsible for DT fuel separation. Here we propose to excite strong shocks in the low density corona to accelerate ions as an alternative to the ion fast ignition schemes where a separate target is used. We also consider shock accelerated particles in magnetized liner inertial fusion where the Z pinch drives a strong shock. We present theory and simulations of both electrostatic and magnetized shock accelerated particles.   

Ion Heating and Flows in a High Power Helicon Source

We report experimental measurements of ion temperatures and flows in a high power, linear, magnetized, helicon plasma device, the Resonant Antenna Ion Device (RAID). RAID is equipped with a high power helicon source. Parallel and perpendicular ion temperatures on the order of 0.6 eV are observed for an rf power of 4 kW, suggesting that higher power helicon sources should attain ion temperatures in excess of 1 eV. The unique RAID antenna design produces broad, uniform plasma density and perpendicular ion temperature radial profiles. Measurements of the azimuthal flow indicate rigid body rotation of the plasma column of a few kHz. When configured with an expanding magnetic field, modest parallel ion flows are observed in the expansion region. The ion flows and temperatures are derived from laser induced fluorescence measurements of the Doppler resolved velocity distribution functions of argon ions.   

Studies on ion heating of the GAMMA 10/PDX plasma in a higher density regime toward a future divertor simulating linear device

Linear plasma devices offer flexible plasma conditions, good control and measurement accessibility, contributing to the necessary physical understanding of boundary plasma and technology development for DEMO. Among many linear devices operating in the world, one missing parameter is ion temperature. On the other hand, end-loss plasmas with ion temperature more than 100 eV are achievable in a standard discharge of GAMMA 10/PDX while the electron density significantly falls below the demand for above studies. Recently ion heating experiments in a higher density regime has been started on GAMMA 10/PDX to bridge a gap between our experiences and the knowledge required for ion heating of a future and operating divertor simulating linear plasmas. We will report on the recent trials, that were performed taking advantage of multi ICRF heating systems on GAMMA 10/PDX; (i) increase of the density of ICRF produced plasma, (ii) experimental investigation on the slow wave excitation in a higher density regime, (iii) development of ion heating methods applicable to a higher density regime.   

Optimization of the Magnetic Field Structure for Sustained Plasma Gun Helicity Injection for Magnetic Turbulence Studies at the Bryn Mawr Plasma Laboratory

A long-pulsed magnetic coaxial plasma gun is being implemented and characterized at the Bryn Mawr Plasma Laboratory (BMPL). A cold cathode discharged between the cylindrical electrodes generates and launches plasma into a 24cm diameter, 2m long chamber. Three separately pulsed magnetic coils are carefully positioned to generate radial magnetic field between the electrodes at the gun edge in order to provide stuffing field. Magnetic helicity is continuously injected into the flux-conserving vacuum chamber in a process akin to sustained slow-formation of spheromaks. The aim of this source, however, is to supply long pulses of turbulent magnetized plasma for measurement rather than for sustained spheromak production. The work shown here details the optimization of the magnetic field structure for this sustained helicity injection.   

Recent Progress on the magnetic turbulence experiment at the Bryn Mawr Plasma Laboratory

Recent progress is reported on the construction, implementation and testing of the magnetic turbulence experiment at the Bryn Mawr Plasma Laboratory (BMPL). The experiment at the BMPL consists of an ($\approx 300~\mu s$) long coaxial plasma gun discharge that injects magnetic helicity into a flux-conserving chamber in a process akin to sustained slow-formation of spheromaks. A 24cm by 2m cylindrical chamber has been constructed with a high density axial port array to enable detailed simultaneous spatial measurements of magnetic and plasma fluctuations. Careful positioning of the magnetic structure produced by the three separately pulsed coils (one internal, two external) are preformed to optimize for continuous injection of turbulent magnetized plasma. High frequency calibration of magnetic probes is also underway using a power amplifier.   

Iterative Addition of Kinetic Effects to Cold Plasma RF Wave Solvers

The hot nature of fusion plasmas requires a wave vector dependent conductivity tensor for accurate calculation of wave heating and current drive. Traditional methods for calculating the linear, kinetic full-wave plasma response rely on a spectral method such that the wave vector dependent conductivity fits naturally within the numerical method. These methods have seen much success for application to the well-confined core plasma of tokamaks. However, quantitative prediction of high power RF antenna designs for fusion applications has meant a requirement of resolving the geometric details of the antenna and other plasma facing surfaces for which the Fourier spectral method is ill-suited. An approach to enabling the addition of kinetic effects to the more versatile finite-difference and finite-element cold-plasma full-wave solvers was presented by [1] where an operator-split iterative method was outlined. Here we expand on this approach, examine convergence and present a simplified kinetic current estimator for rapidly updating the right-hand side of the wave equation with kinetic corrections. [1] D. L. Green, L. A., Comp. Phys. Comm. 185(3), 736-743 (2014)   

Analysis of plasma jets produced by a small railgun-based acclelerator

We report results of an experimental effort to characterize temperature, velocity, electron density, and composition of plasma jets generated at the Virginia Tech Center for Space Science and Engineering Research. The linear railgun, which features a 0.5 x 0.32 cm rectangular bore and 10.2 cm long rails, is fed gas from a 700 kPa manifold by a puff valve capable of opening for pulses of several milliseconds. The rails are powered by an LC pulse-forming network (PFN) designed to deliver $\sim$100 kA during a pulse of approximately 10 microsecond duration. A modular accelerator design allows rails and insulators fabricated with different materials and geometries to be swapped out with ease. To characterize the resulting plasma jet, a full suite of diagnostics is utilized including a single-chord Mach Zehnder interferometer, photodiode array, spectrometer, image intensified CCD camera, and Rogowski coil. Initial results obtained while charging the PFN to half its design voltage suggest jet velocities of $\sim$15-25 km/s are obtained consistently. Results from this device will provide groundwork for the design of future jet sources and experiments to study topics ranging from plasma-material interactions to plasma shocks.   

Thin liquid sheet target capabilities for ultra-intense laser acceleration of ions at a kHz repetition rate

The success of laser-accelerated ion experiments depends crucially on a number of factors including how thin the targets can be created. We present experimental results demonstrating extremely thin (under 200 nm) glycol sheet targets that can be used for ultra-intense laser-accelerated ion experiments conducted at the Air Force Research Laboratory at Wright-Patterson Air Force Base. Importantly, these experiments operate at a kHz repetition rate and the recovery time of the liquid targets is fast enough to allow the laser to interact with a refreshed, thin target on every shot. These thin targets can be used to produce energetic electrons, light ions, and neutrons as well as x-rays, we present results from liquid glycol targets which are useful for proton acceleration experiments via the mechanism of Target Normal Sheath Acceleration (TNSA). In future work, we will create thin sheets from deuterated water in order to perform laser-accelerated deuteron experiments.   

Effect of external forcing on the coupled state of two inductively coupled glow discharge plasma sources

The effect of an external forcing on the in-phase synchronized state of the anode glow oscillations of two inductively coupled glow discharge plasma sources is studied. The parameters of the two plasma sources are initially so adjusted that their anode glow oscillations achieve an in-phase synchronized state with an entrained frequency of 110 kHz. The system is then subjected to an external harmonic forcing from a function generator. It is observed that for a low amplitude forcing (500 mVpp to 800 mVpp ) and with a progressive increase in the frequency of the driver from 105 kHz to 112 kHz, the in-phase state changes successively to an anti-phase low frequency state (105 - 108) kHz, to a frequency pulling state (108 - 109 kHz) and finally to an in-phase high frequency state (109 - 112) kHz. When the forcing signal is of a high amplitude ($>$ 800mVpp) the transition from an anti-phase state (105 - 109) kHz to an in-phase state (109 -112) kHz is seen to occur without any intermediate frequency pulling state. These experimental observations are well reproduced in numerical solutions of a theoretical model consisting of two Van der Pol oscillators that are environmentally coupled to each other with one of them driven by an external oscillatory source.   

Magnetic helicity balance at Taylor relaxed states sustained by AC helicity injection

Magnitudes of Taylor relaxed states that are sustained by AC magnetic helicity injection (also known as oscillating field current drive, OFCD) [F. Ebrahimi et al. Phys. Plasmas 10, 999 (2003), K. J. McCollam et al. Phys. Plasmas 17, 082506 (2010)] are investigated numerically in a cylindrical geometry. Compared with the amplitude of the oscillating magnetic field at the skin layer (which is normalized to 1), the strength of the axial guide field $B_{z0}$ is shown to be an important parameter. The relaxation process seems to be active only when $B_{z0} < 1$. Moreover, in the case of weak guide field $B_{z0} < 0.2$, a helically-symmetric relaxed state is self-generated instead of the axisymmetric reversed-field pinch. As a theoretical model, the helicity balance is considered in a similar way to R. G. O'Neill et al. Phys. Plasmas 14, 112304 (2007), where the helicity injection rate is directly equated with the dissipation rate at the Taylor states. Then, the bifurcation to the helical Taylor state is predicted theoretically and the estimated magnitudes of the relaxed states reasonably agree with numerical results as far as $B_{z0} < 1$.   

Observations and modeling of magnetized plasma jets and bubbles launched into a transverse B-field

Hot, dense, plasma structures launched from a coaxial plasma gun on the HelCat dual-source plasma device at the University of New Mexico drag frozen-in magnetic flux into the chamber’s background magnetic field providing a rich set of dynamics to study magnetic turbulence, force-free magnetic spheromaks, shocks, as well as CME-like dynamics possibly relevant to the solar corona. Vector magnetic field data from an eleven-tipped B-dot rake probe and images from an ultra-fast camera will be presented in comparison with ongoing MHD modeling using the 3-D MHD BATS-R-US code developed at the University of Michigan. BATS-R-US employs an adaptive mesh refinement grid (AMR) that enables the capture and resolution of shock structures and current sheets and is uniquely suited for flux-rope expansion modeling. Recent experiments show a possible magnetic Rayleigh-Taylor (MRT) instability that appears asymmetrically at the interface between launched spheromaks (bubbles) and their entraining background magnetic field. Efforts to understand this instability using in situ measurements, new chamber boundary conditions, and ultra-fast camera data will be presented.   

Measurements of the canonical helicity evolution of a gyrating kinked plasma column

Conversions between kinetic and magnetic energy occur over a wide range of plasma scales as exhibited in astrophysical and solar dynamos, and reconnection in the solar corona and laboratory experiments. Canonical flux tubes present the distinct advantage of reconciling all plasma regimes --- e.g. kinetic, two-fluid, and MHD --- with the topological concept of helicity: twists, writhes, and linkages. This poster presents the first visualization and analysis of the 3D dynamics of canonical flux tubes and their relative helicity evolution from experimental measurements. Ion and electron canonical flux tubes are visualized from Mach, triple, and $\dot{B}$ probe measurements at over 10,000 spatial locations of a gyrating kinked plasma column. The flux tubes co-gyrate with the peak density and electron temperature in and out of a measurement volume. The electron and ion canonical flux tubes twist with opposite handedness and the ion flux tube writhes around the electron flux tube. The relative cross helicity between the magnetic and ion flow vorticity flux tubes dominates the relative ion canonical helicity and is anticorrelated with the relative magnetic helicity. The 3D nature of the kink and a reverse eddy current affect the helicity evolution.   

Statistical study of magnetic field dynamics in a system of merging current filaments

We investigate magnetic field dynamics in a system of parallel current filaments characterized by a high degree of symmetry. The system evolves through the coalescence and reconnection of the current filaments. This process does not break the symmetry, but does generate increasingly complex patterns with a degree of self-similarity. Analysis of the magnetic and kinetic energy spectra of the system as a function of time shows spectral behavior that is indistinguishable from fully developed turbulence, with the interesting difference that here the spectra show ``gaps'', as may be expected of fractal-like patterns. We attempt to characterize pattern complexity by different measures borrowed from general theory of complex systems.   

Results from the Mochi.Labjet Experiment

Magnetized plasma jets are generally modeled as magnetic flux tubes filled with flowing plasma governed by magnetohydrodynamics (MHD). Recent theoretical work has outlined a more fundamental approach based on flux tubes of canonical vorticity, where canonical vorticity is defined as the circulation of a species' canonical momentum. This approach extends the concept of magnetic flux tube evolution to include the effects of finite particle momentum and enables visualization of the topology of plasma jets in regimes beyond MHD. Under the appropriate conditions this framework suggests how to form and drive stable, collimated plasma jets with very long aspect-ratios. To explore this possibility, a triple electrode planar plasma gun (Mochi.LabJet) has been designed to produce helical shear flows inside a driven magnetized plasma jet. High speed video confirms the experiment can produce long ($\sim $1m), collimated, stable jets with core plasma currents of 60 - 80 kA, skin currents of 100 - 120 kA and axial velocities on the order of 40 -- 80 km/s (for hydrogen). Presented here are magnetic and ion flow velocity measurements as well as stability space analysis that suggests the jets are stable to kink instabilities over many Alfv\'{e}n times.   

Abstract Withdrawn

Collisionless shocks, which are held responsible for generating nonthermal particles and radiation in high-energy astrophysical objects, are widely believed to originate from micro-instabilities triggered in colliding flows. Recently, rapid theoretical developments have gone hand in hand with experimental efforts to generate collisionless shocks using powerful lasers. We have investigated both theoretically and numerically the possibility to use such lasers to study plasma collisions in strong large volume pulsed external magnetic fields. The presence of an external magnetic field can speed up the development of a collisionless shock that would otherwise be outside the reach of the largest laser systems available. The external magnetic field compression and the binary collisions between charged particles can also strongly affect the shock formation and the subsequent particle energization.