Session VP11: Poster Session: Fundamental Plasmas: Turbulence and Transport (2:00pm - 5:00pm)

Overview of Experiments from the Wisconsin Plasma Physics Laboratory (WiPPL)

WiPPL is a multi-machine, collaborative research facility directed toward fundamental topics in discovery plasma science: dynamos, reconnection, turbulence, particle acceleration, coherent structures, and plasma systems. Users from outside institutions have led and are leading WiPPL experimental projects on basic, astrophysical, and fusion plasma studies. We present an overview of recent WiPPL projects. On the Big Red Ball collisionless reconnection is studied with unprecedented spatial and temporal resolution; shock formation is studied with a novel theta pinch configuration; compact toroids are injected that create collisionless shocks for particle energization and heating studies. Enormous magnetic field amplification is observed in high beta Hall dominated Couette flow and a novel instability driven by differentially rotating electrons has been observed. On the Madison Symmetric Torus, tokamak plasmas are routinely created for a variety of studies: disruptions caused by loss of equilibrium in low q plasmas are studied; whistler waves driven by runaway electrons have been observed. Nonlinear behavior of tearing mode fluctuations versus Lundquist number are being studied in self-organizing reversed-field pinch plasmas.   

Development of a rotating magnetic field experiment for the Big Red Ball

We are developing a rotating magnetic field (RMF) experiment to emulate a pulsar wind in the Big Red Ball (BRB) device at the Wisconsin Plasma Physics Laboratory. In the design, two coils are arranged in a quasi-Helmholtz configuration, and a second coil pair is orthogonally placed to form a square array, situated at the BRB core with coil axes in the equatorial plane. The application of quadrature-phased coil currents produces the RMF, which is to be the plasma source and driver. We present design development work, in particular on the power amplifier that is to source the RMF. Target field strengths at a 6~kHz drive frequency demand induction levels of $\sim$ 30~kA-turns, or power levels of $\sim$1.5~MVA per channel. We estimate that an optimized high-$Q$ resonant configuration can sustain the large circulating currents with only $\sim$150--200~kW of circuit losses. Simulations suggest a loose-coupled double-resonant tuned circuit can be used to match the nominal 200~$\Omega$ class-C amplifier plate impedance to the 50~m$\Omega$ drive coil impedance. Simulation studies are to be compared with experimental measurements of prototype operation.   

High $\beta$ Plasma Turbulence Studies on the Big Red Ball

Recent and upcoming experiments on the Big Red Ball (BRB) explore the regime of $\beta \gt 1$ for collisionless turbulence. High $\beta$ plasmas are an area of active study critical to understanding astrophysical phenomena like the solar wind. Prior experiments on BRB examined the magnetic fluctuations between magnetized high-$\beta$ flux ropes. Measurements of the turbulence power spectrum suggested preferential ion heating, which will be investigated further using a local ion doppler spectroscopy probe. Experimental parameters will be expanded to examine energy partitioning between the magnetic field and electron and ion species as a function of $\beta$. A second upcoming turbulence experiment involves the collision of a fast compact toroid with a conducting grid intended to shred the CT into smaller eddies. Preliminary measurements and correlations between density, temperature, and magnetic fields will be presented. The design and construction of probes capable of making these measurements in BRB-specific plasmas will also be shown.   

Exploring Transition Regions of Collisionality, Criticality, and Magnetization in Laboratory Parallel Shock Experiment

A series of parallel shock experiments were performed on the Big Red Ball at the Wisconsin Plasma Physics Lab. An upgraded compact toroid injector produced a supersonic piston (60 - 100 km/s) that collided with a stationary background plasma. Target plasma densities ($0.5 < n_e < 50 \cdot 10^{18}\ m^{-3}$) were adjusted by varying the number of plasma washer guns, with the experiment length (L = 3 m) always much larger than the ion skin depth ($3 < d_i < 30$ cm). The variable magnetic field ($0 - 10$ mT), aligned in the direction of piston propagation, allowed for target plasma betas of $0.1 < \beta < 10$. Together, this amounts to a substantial parameter space in magnetosonic Mach number ($M_{\rm{ms}} \sim 0.5 - 5$), in collisionality ($\nu_e / \omega_{ce} \sim 0.1 - 5$), and in magnetization ($\rho_i / L \sim 0.1 - 2$). Results and analysis to identify the transitions from subcritical to supercritical, from collisional to collisionless, and from Hall-MHD to MHD, will be presented.   

Regulation of the Normalized Rate of Driven Magnetic Reconnection through Shocked Flux Pileup

Magnetic reconnection is explored on the Terrestrial Reconnection Experiment\footnote{Olson, J., \textit{et al.}, Phys. Rev. Letters, \textbf{116}, 255001 (2016).} at the Wisconsin Plasma Physics Laboratory\footnote{Forest, C.B., \textit{et al.}, J. Plasma Phys., \textbf{81}, 1 (2015)} where the absolute rate of reconnection is set by an external drive. A shock interface between the supersonically driven plasma inflow and a region of magnetic flux pileup permits the normalized reconnection rate to self regulate to a fixed value. The observations demonstrate the role of shock formation in driven reconnection and confirm previous theoretical results on the normalized rate of reconnection.   

Fast Magnetic Probes for the Terrestrial Reconnection Experiment (TREX)

Fast magnetic reconnection is studied on the Terrestrial Reconnection Experiment (TREX) at the Wisconsin Plasma Physics Laboratory (WiPPL). The experimental scenario includes narrow electron scale current layers jogged past magnetic probes at speeds approaching 100km/s, and accurate characterization requires magnetic probes with a linear frequency response up to 10MHz. A new probe design is implemented with printed circuit boards that include termination resistors located in proximity of the magnetic pick-up loops. The design provides a 3 directional magnetic field profile at finer spatial resolution than before while keeping a high-resolution sampling rate and reduced electronic noise. In addition to the technical details on the design, we will present preliminary measurements of a three-dimensional instability observed within the electron diffusion regions during fast magnetic reconnection in the TREX configuration.   

Friction and electrical conduction in strongly magnetized plasmas

Frictional drag on a particle due to its interaction with the medium through which it travels is commonly expected to act antiparallel to its velocity. Recent work has shown that a qualitatively different effect arises in strongly magnetized plasmas, whereby the friction force gains a transverse component that is perpendicular to the velocity vector in the plane formed by the velocity and magnetic field vectors [1]. The transverse force arises due to the manner in which the Lorentz force influences the dielectric polarization of the background plasma. It is large when the electron gyrofrequency exceeds the plasma frequency. It causes the gyroradius of fast particles to increase, and that of slow particles to decrease faster than in its absence. The alteration of single particle motion leads to qualitatively new macroscopic transport properties. As an example, we show that it leads to a new transverse component of electrical resistivity that alters the flow of current in response to an applied electric field. [1] Lafleur and Baalrud, PPCF 61, 125004 (2019).   

Not Participating

Electron temperature gradient (ETG) mode turbulence is considered as an experimentally relevant mechanism to the electron energy transport in tokamak plasmas. On the basis of nonlinear gyrokinetic equation, the present work analytically addresses the zonal flow generation in ETG turbulence. It is shown that, unlike previous analyses, the intermediate-scale zonal flows can be significantly excited via the modulational instability, and contribute to the regulation of ETG streamers. The threshold of the modulational instability and the associated saturation level are derived. Direct comparisons indicate a qualitative agreement between the theoretical model predictions and recent gyrokinetic simulation results [1,2,3]. \begin{thebibliography}{99} \bibitem{cit:jun:26:11:48} N. T. Howard \emph{et al}, Phys. Plasmas, 23, 056109, (2016). \bibitem{cit:jun:26:11:55} C. Holland \emph{et al}, Nucl. Fusion, 57, 066043, (2017). \bibitem{cit:apr:17:21:54} G. J. Colyer \emph{et al}, Plasma Phys. Control. Fusion, 59, 055002, (2017). \end{thebibliography}   

Towards Two-Scale Simulations of Global Tearing Coupled With Microturbulence

Various studies have highlighted the possible impact of large-scale tearing activity on microturbulence, zonal flows, and small-scale transport, with particularly striking effects in the reversed-field pinch. To predict such physics self-consistently, a multi-scale global framework is required. Here, linear and nonlinear tearing-mode physics are investigated with the gyrokinetic turbulence code GENE, using three different implementation levels for the current-gradient drive: as part of the fluctuating distribution, as a fixed background but varying on the microscale, and using shifted Maxwellians as the background distribution along with a global profile. The differences in the linear behavior of these three approaches are discussed, as are the differences in the means by which tearing modes ultimately saturate. Separately, the impact of tearing modes and associated turbulence on micro-scale modes such as ion-temperature-gradient-driven or trapped-electron mode turbulence is investigated.   

Role of Energy-Transfer Resonance in the Dimits Shift for a Threshold ITG Model/

The role of resonant energy transfer to large-scale stable modes in the upshift of the critical gradient for ion temperature gradient turbulence is examined. Measurements of spectral energy transfer in gyrokinetic simulations above and below the nonlinear critical gradient show that the saturation mechanism is the same in both regimes, differing primarily in magnitude of transfer rate. Consequently the nonlinear saturation theory for transfer from the unstable mode through the zonal flow to a nearly conjugate stable mode, which has been shown to work well away from the threshold, can be extended to include the threshold by consistent treatment of magnetic-drift physics. Prior estimates of saturation level are expanded to include the levels of the stable mode and the cross correlation of the unstable and stable mode amplitudes, as well as less restrictive treatments of wavenumber summations. Resonance in the triplet correlation time is shown to produce efficient energy transfer and a low heat flux above the linear critical gradient. Resonance broadening by the ion polarization drift uncovers the gradient dependence of the triplet correlation time. This and the nonlinear coupling coefficient allow the flux to rise sharply at a higher gradient value than the linear threshold.   

Kinetic Ballooning Mode turbulence in small-average-magnetic-shear equilibria

Kinetic ballooning mode (KBM) turbulence is studied in small-average-magnetic-shear equilibria, namely HSX, Heliotron-J, and a circular tokamak, to understand stellarator transport at finite $\beta$ and to identify configurations with improved confinement. Electromagnetic flux-tube simulations of HSX using the gyrokinetic turbulence code GENE show that the onset of KBM instability at low $k_y$ occurs at a value of normalized plasma pressure $\beta$ that is an order of magnitude smaller than the MHD ballooning limit $\beta^\mathrm{MHD}$. This small $\beta^\mathrm{KBM}$ is sensitive to modifications of the magnetic shear. Heliotron-J and an axisymmetric geometry exhibit behavior similar to HSX. Regardless, saturation of nonlinear simulations of HSX with $\beta^\mathrm{MHD} > \beta > \beta^\mathrm{KBM}$ is achievable and results in lower heat fluxes than the electrostatic case. A fluid model which expands upon an electrostatic model [C.C. Hegna, 2018] by including finite-$\beta$ effects is introduced; it allows for ITG-KBM saturation in stellarators to be dominated by the transfer of energy from unstable to stable modes at similar scales via nonlinear coupling and will be used to build a physical understanding for the relationship between geometry and $\beta^\mathrm{KBM}$.   

Gyrokinetic Dimits-Shift Analysis and Quasilinear Model Building

The quasilinear heat flux estimate is widely in used reduced models because its need for nonlinear inputs is minimal. However, standard quasilinear models fail to predict certain features of turbulence scalings, including the Dimits shift, in which the onset of finite heat flux is shifted to a higher critical gradient relative to the linear growth rate. Gyrokinetic simulations for the Madison Symmetric Torus reversed field pinch and the Cyclone Base Case, which demonstrate Dimits shifts in plasmas unstable to trapped electron and ion temperature gradient instabilities respectively, are obtained and used to produce a quasilinear estimate that incorporates more complete saturation physics. In particular, the triplet correlation time, which is shown to be important in saturation for fluid and kinetic models, is introduced into quasilinear estimates near the critical gradient threshold. It additionally shows that strong resonance between modes can lead to suppression of the turbulence. Therefore the coherency of modes within and above the threshold is investigated. Also, wavenumber dependencies within the saturation theory for zonal-flow-catalyzed energy transfer to stable modes are investigated to inform how to handle wavenumber summations in flux calculations.   

Analysis of Edge Turbulent Transport and Divertor Heat Load for ITER Hybrid Scenario using BOUT $+ \quad + \quad

BOUT$++$ six-field two-fluid turbulence code is used to simulate ITER scenarios. The pedestal structures in different scenarios are found to be unstable to different peeling-ballooning instabilities. In linear stage, the most unstable modes are n$=$15-20, n$=$40, n$=$60-80 in steady-state operation (SSO), hybrid and baseline scenarios respectively. In nonlinear stage, the energy loss fractions in the baseline and hybrid scenarios are large (10-20{\%}) while the one in the SSO scenario is dramatically smaller (1{\%}), which are consistent with the features of type-I ELMs and grassy ELMs correspondingly. Broadened by the strong turbulence because of ELMs, the divertor heat flux widths in the three scenarios given by the simulations are 10 times larger than the predictions based on Goldston’s drift model, while fit the estimations of the critical ballooning model. The toroidal gap edge melting limit of tungsten monoblocks imposes constrains on ELM energy loss, giving that the ELM energy loss fraction should be smaller than 2.18{\%}, 5.76{\%}, and 10.44{\%} for ITER baseline, hybrid and SSO scenarios, correspondingly. The simulation shows that only the SSO scenarios with grassy ELMs may satisfy the constraint.   

Trapped Electron Mode Turbulence Optimization in HSX

The Helically Symmetric eXperiment (HSX) is a neoclassically optimized stellarator that uses quasi-helical symmetry. Anomalous heat losses, attributed to turbulent fluxes, dominate outside the mid-radius, and may be reduced by changing the currents in individual coils to produce favorable magnetic geometries. The present configuration traps plasma particles in toroidally linked magnetic wells. However, these magnetic wells strongly overlap with regions of bad magnetic curvature, which has been linked to density gradient driven Trapped Electron Mode turbulence, and is a candidate process for explaining the anomalous heat transport. Several configurations have been identified with altered magnetic field structures that shift magnetic wells outside regions of bad curvature while maintaining good neo-classical confinement. Detailed modelling of the neoclassical transport using SFINCS, as well as results from linear gyrokinetic simulations from GENE will be presented and used to identify the most promising of these coil-current configurations. These configurations can then be studied using quasilinear and nonlinear gyrokinetic approaches to identify a few configurations that will be investigated in future experiments.   

Supersymmetry Breaking and Hydrodynamic Turbulence

Supersymmetry has been previously established to be pertinent to all stochastic and deterministic differential equations (SDEs). This supersymmetry represents the preservation of continuity of the phase space by continuous-time evolution and, from the technical point of view, is the De Rham operator (or exterior derivative) that commutes with any SDE-defined stochastic evolution operator. In its turn, the spontaneous breakdown of this supersymmetry is associated with the emergence of the exponentially growing eigenstates that signal the onset of chaos and underlie the long range dynamical behavior in the spirit of the Goldstone theorem. In this work, we approach the problem of hydrodynamical turbulence and the associated power law statistics within this supersymmetric picture of continuous-time dynamics and aim to describe these phenomena using path integral representation of hydrodynamics. More specifically, the work consists of analysis of correlators that may unambiguously demonstrate that the theoretical essence of turbulence is indeed spontaneous supersymmetry breaking.   

Microtearing instabilities driving anomalous heat loss in the pedestal region of DIII-D discharges

Advances in gyrokinetic codes, along with techniques for mode identification based on a "fingerprints" method have found the significance of micro-tearing Modes (MTM) and electron temperature gradient (ETG) modes in causing the energy losses within the pedestals of fusion experiments operating in the ELMy H-mode regime. Gyrokinetic simulations using the GENE code are performed, with equilibrium EFIT profiles constructed from DIII-D data. Nonlinear local simulations have shown that electron heat flux has only minor contributions from ETG turbulence, while MTM's and neoclassical effects account for significant electron and ion heat losses, respectively, in the pedestal. MTM's found in global simulations are consistent with observed magnetic fluctuations, having a frequency in the electron diamagnetic direction and in the expected range, given the equilibrium gradients. Classifying these modes using the physical characteristics of the resulting transport gives insight into the mechanisms driving pedestal transport. Simulations of kinetic ballooning modes are shown to have "fingerprints" inconsistent with observations, ruling them out as the main cause of observed transport.   

Effects of Triangularity on Ion Temperature Gradient Turbulence Saturation

Transport driven by ion temperature gradient (ITG) turbulence is an important loss channel in tokamaks. In this work, we model how triangularity (both positive and negative values) of an axisymmetric flux surface affects ITG linear growths, turbulent saturation and~turbulent transport. This is accomplished for each geometry by combining analysis from the gyrokinetics code GENE, a reduced fluid model for evaluating turbulence saturation by unstable-stable mode coupling through three-wave interactions[1], and a quasilinear transport model. Quasilinear scaling predicts ion heat fluxes increase at low and moderate positive and negative triangularities and sharply decrease with strong positive and negative triangularity. Using quasilinear scaling, geometries with negative triangularity are predicted to have lower ion heat fluxes than the corresponding positive counterpart. [1] Hegna et al. PoP, 25, 022511 (2018)   

Influence of Stable Modes on Momentum Transport in Freely-evolving Shear Layers in MHD

In shear flows that are unstable to the Kelvin-Helmholtz (KH) instability, linearly stable, inviscid modes are known to co-exist at the same large scales as unstable modes. While unstable modes draw energy from the shear flow and transport momentum down the gradient via turbulent stresses, stable modes transfer energy back and transport momentum up the gradient [A.E.~Fraser, PhD Thesis, UW-Madison 2020]. Here we study the role these modes play in KH-driven turbulence in MHD using 2D, incompressible simulations of an unforced shear layer, so the flow gradient flattens as turbulent stresses broaden the layer. A uniform, flow-aligned magnetic field is wound up by large-scale vortices, generating small-scale fluctuations that enhance dissipation and layer broadening, even for very weak initial fields. We vary field strength and resistivity to study how these dynamics relate to stable mode excitation, finding the enhanced dissipation reduces the importance of stable modes and their up-gradient momentum transport. While the Reynolds stress is well-described by a truncated eigenmode expansion if stable modes are included, the Maxwell stress is not, reflecting the complexity of the field fluctuations relative to the KH-dominated flow fluctuations.   

Generation of Geodesic Acoustic Modes and Zonal Flows by transport modulations

It is shown that Geodesic Acoustic Modes can be generated by modulations of the anomalous plasma transport. This mechanism is related to the Stringer spin-up and occurs as a result of the compressibility of the anomalous fluxes inducing pressure perturbation and subsequent radial current. The contribution of the transport modulations to the GAM growth rate is compared with the Reynolds stress drive. The generation of Zonal Flows due to the Reynolds stress and transport modulations mechanisms is also discussed..   

Cross-Scale Interactions in Multi-scale Turbulence Explored Through a Reduced Model

Multiscale gyrokinetic simulations using kinetic ions and electrons while resolving both ion- and electron-scale turbulence require tens of millions of CPU-hours\footnote{S. Maeyama et al., Phys. Rev. Lett. {\bf 114}, 255002 (2015).}$^,$\footnote{N. T. Howard, et al., Nucl. Fusion {\bf 56}, 014004 (2016).}. This cost is prohibitive for physics studies and parameter scans, and severely limits box sizes for ion scales. We have developed a 2D reduced fluid model based on the gyrokinetic equations for multiscale turbulence, including Bessel functions and featuring nonlinear terms of the same Poisson bracket form as large gyrokinetic codes. We have implemented several versions of the model in a new 2D pseudo-spectral turbulence code with periodic boundary conditions, and verified it against earlier ion-scale fluid simulations.\footnote{S. Smith and G. W. Hammett, Phys. Plasmas {\bf 4}, 978 (1997).} The new code is serving as a test-bed for multi-scale and multi-rate algorithms that exploit the scale separation between ion and electron scales. Using the reduced model, we are presently exploring cross-scale interactions including the previously reported impact of electron scale turbulence on ion scale zonal flows.   

Collisional Transport in Partially Magnetized Multi-Ion Species Plasma

Multi-ion species plasma that is immersed in a magnetic field features different collisional transport timescales in presence of external forces or temperature gradients, which leads to curious effects such as charge incompressibility, ion stratification, and heat pump. [1,2] Moreover, the ion-ion transport depends greatly on the strength of the magnetic field. In particular, the equilibrium state and the direction of impurity transport strongly depend on plasma magnetization, which is characterized by the Hall parameter ($\Omega_a/\nu_{ab}$ for light species $a$ and heavy species $b$). We identify the equilibrium state of the bulk plasma and impurity ions for a wide range of plasma magnetizations, as well as the parameter space where distinct collisional timescales and relevant effects can be observed. [1] ``Strategies for advantageous differential transport of ions in magnetic fusion devices" E. J. Kolmes, I. E. Ochs, and N. J. Fisch, Phys. Plasmas 25, 032508 (2018). [2] ``Heat Pump via Charge Incompressibility in a Collisional Magnetized Multi-Ion Plasma" M. E. Mlodik, E. J. Kolmes, I. E. Ochs, and N. J. Fisch, arXiv:2006.06149   

Diffusive Free Energy and Reversibility in the Continuous Limit

There are multiple ways of defining the free energy associated with a given phase space configuration. The Gardner free energy is the energy that can be extracted by exchanging pairs of elements in phase space. The diffusive free energy is defined similarly, but the elements are averaged rather than being exchanged. Both notions of free energy have been previously studied in discrete systems, in which phase space can be divided into finite blocks, and in continuous systems, in which the phase space elements being exchanged are infinitesimally small. For any discrete system, if the free energies are nonzero, it is well known that the Gardner free energy is always the larger of the two. We demonstrate here that in the continuous limit, they are the same. This is counterintuitive, since Gardner restacking operations are reversible whereas (for any discrete system) diffusive exchanges are irreversible. This result can be understood in terms of the scalings of the entropy production associated with a diffusive exchange.   

Energy dissipation in sub-ion-Larmor-radius kinetic turbulence

In a weakly collisional, low-beta plasma, the transfer of free energy to small scales in phase space by kinetic turbulence can in principle proceed via two channels: fluid-like nonlinear advection and linear phase mixing. Recent numerical studies of electrostatic drift-kinetic turbulence (Parker et al. 2016) reported the suppression of phase mixing due to "anti-phase mixing" modes caused by plasma echo effects. This result was confirmed by a recent numerical study of electromagnetic turbulence at MHD scales (Meyrand et al. 2019). However, it has also been shown that turbulent eddies can be strongly anisotropic and unstable to tearing modes (e.g., Loureiro and Boldyrev 2017), causing the turbulence to be affected by magnetic reconnection. As reconnection is known to trigger efficient phase-mixing (Loureiro et al. 2013; Numata and Loureiro 2015), it is possible that phase mixing remains an significant energy dissipation mechanism in electromagnetic sub-ion-scale turbulence. To test this conjecture, we performed numerical studies of kinetic turbulence with a reduced gyrokinetic model valid at low plasma beta. We observe strong energy dissipation at high velocity-space moments, supporting the idea that phase mixing indeed plays a key role in energy dissipation in sub-ion turbulence.   

Machine learning-based profile prediction in the Large Plasma Device

The Large Plasma Device (LAPD) currently does not have single-shot radial profile or particle transport measurements, but this information can now be predicted using a machine learning-based method. Providing accurate, single-shot transport and profile information could provide greater insight and control of all experiments conducted in the LAPD. Predictions of temperature, density, and plasma potential profiles as well as other transport-relevant quantities are produced by a neural network-based model. This model makes predictions by analyzing results from a theoretical model of particle transport [1], data from diagnostics (such as line-averaged density and fast camera frames), and information on machine state (such as fill pressure and discharge current). Results of this predictive model will be presented with comments on its accuracy and robustness. [1] J. E. Maggs, T. A. Carter, and R. J. Taylor, Phys. Plasmas 14, 052507 (2007).   

Not Participating

Tearing mode instabilities are ubiquitous in the study of plasmas, occurring both in laboratory fusion experiments as well as space plasmas. This work explores the possibility of capturing essential tearing mode physics through a reduced-model description. A growing body of literature has shown that many different flavors of instability-driven turbulence can be well-described via a truncated eigenmode decomposition, in which the nonlinear state of a plasma is approximated by only the most unstable eigenmode and a single stable mode. This approach is applied here to study resistive tearing-mode-driven turbulence, and the significance of the stable mode contribution is evaluated using the threshold parameter rubric as defined in Terry et al. 2006. The potential relevance of this approach to collisionless tearing modes is considered. This work is funded by the Michigan Space Grant Consortium, NASA grant \#NNX15AJ20H.   

Writhe of a Laboratory Arched Magnetized Plasma evolving in a Sheared Magnetic Field

Solar atmosphere is abundant in arched magnetized plasma structures (i.e. solar prominences, coronal loops). We study laboratory analogues of solar arched plasma structures to gain a better insight into fundamental processes governing its spatiotemporal evolution. The arched plasma is produced using a hot-cathode lanthanum hexaboride (LaB$_6$) source and it evolves in an ambient magnetized plasma produced by another LaB$_6$ source [1, 2]. Typical plasma parameters are: $\beta \approx$ 10$^{-3}$, Lundquist number $\approx$ 10$^2$ - 10$^5$, B $\approx$ 1000 Gauss at footpoints, plasma radius/ion gyroradius $\approx$ 20, B = 0-50 G in the ambient plasma, and 0.5 Hz repetition rate. We present recent results on measurements of plasma density, electron temperature, and three-dimensional magnetic field. Results demonstrate formation of S and reverse-S shaped current-filaments depending on the direction of the ambient magnetic field. Role of magnetic-shear and relative strengths of magnetic-fields of the arched and ambient magnetic fields will be discussed. These results indicate that occurrence of kink-instability is not a necessary condition for producing S or reverse-S shaped filaments on the Sun.   

Intermittent turbulence in Multi-Ion Plasmas in the LAPD

Intermittent turbulence and associated density-enhancement events ("blobs") are observed in the edge of a wide range of magnetic confinement devices. In the edge of tokamak plasmas, convective transport associated with blob propagation can dominate particle transport. Most studies of intermittency and blob transport have been performed in single ion species plasmas even though fusion plasmas will need to be mixed ion (DT). We are carrying out a study of blobs in controlled mixtures of hydrogen and deuterium in the Large Plasma Device (LAPD). We will present data and analysis of the properties of blobs (size, velocity, amplitude, transport) as the D-H mix is varied.   

Inference of Neutral Depletion in the Large Plasma Device

Quantification of neutral particle populations within a plasma is of strong importance because they can influence particle flux and energy confinement through charge exchange and impact ionization, leading to fast ion losses and decreased energy confinement. The direct measurement of neutral particle density however has proven elusive, often necessitating complex and costly experimental measurement techniques such as laser induced fluorescence and charge exchange spectroscopy. An alternative line-ratio spectroscopy-based measurement technique has been studied both experimentally and theoretically to determine the neutral density content in Helium plasma experiments on the Large Plasma Device. Considerations taken when modeling plasma emission involve non-Maxwellian collisional radiative modeling, predicting the opacity of line-integrated measurements through Beer's law, and modeling plasma ionization rates and transport to determine neutral density evolution with time.