Session TP8: Poster Session VII: International Tokamak, C-Mod, ST, Plasma-Surface, ICF Diagnostics, Computer Methods, Single-Species

Improvements to the vertical control of KSTAR

The control of plasma shape in KSTAR requires, a fast vertical control system to maximize the range of elongation that can be controlled. Slow motion of the vertical center of the plasma can be controlled by the superconducting coils, but the dynamic response of these coils is too slow to provide control of fast transients. The internal vertical control coil (IVC) is the actuator best suited to control of fast transients. To date, KSTAR has relied on a simple combination of magnetic probe signals to provide a fast z position measurement. During the 2012 run, two pairs of voltage loops, each pair consisting of one loop above and one loop below the plasma at the same major radius, were instrumented to provide their difference to the plasma control system (PCS). These two signals are sensitive to the plasma vertical velocity (dz/dt). The use of these signals in the fast vertical control feedback loop during the 2013 KSTAR experimental campaign will be described and their efficacy in extending the available plasma elongation in KSTAR will be discussed.   

Radiative L-mode Edge Plasma Regimes in JET and ASDEX Upgrade

Low-Z impurities must be seeded into the edge plasmas of future high-power full-metal devices like ITER and DEMO in order to dissipate the exhaust power through radiation. Extensive studies have been performed in ASDEX Upgrade and JET to understand the effects of N-seeding on the divertor and scrape-off layer plasmas of full-metal tokamaks of different size and different geometry of the W divertor. The radiation patterns and divertor regimes change substantially in L-mode discharges for various levels of seeding and fuelling, and distinct transitions between the different radiative regimes are observed. In this contribution, conditions for these radiative regimes are described with the help of 2D simulations and the physics behind existing scaling laws for power exhaust are discussed.   

KSTAR stability and rotation control results for high normalized beta plasmas exceeding the ideal MHD no-wall stability limit

Plasma stability parameters in KSTAR have reached and exceeded the $n$~$=$~1 ideal no-wall limit computed for H-mode profiles. Normalized beta up to 2.9 has been achieved and sustained with plasma internal inductance near 0.75. The ratio $\beta_{N}$/$l_{i}$ has exceeded 3.6 (an 80{\%} increase over the prior year). Plasma stored energy has exceeded 0.5~MJ. Non-axisymmetric field spectra with dominant $n$~$=$~2 component were applied to alter the plasma rotation profile by non-resonant neoclassical toroidal viscosity (NTV). The rotation profile was significantly altered without tearing activity or mode locking. Changing the in-vessel control coil current in steps altered rotation in a controlled fashion without hysteresis. The core rotation was lowered by 50{\%} as measured by charge exchange spectroscopy, x-ray crystal spectrometer, and supported by magnetic diagnostics. H-mode energy confinement was maintained at reduced rotation while the resultant profile was peaked, as found in L-mode. Tearing mode onset conditions and mode locking criteria due to the applied $n$~$=$~1,~2 applied fields were investigated. Additionally, ELMs were mitigated using sufficient $n$~$=$~2 field strength by using midplane coils alone. Advances from the recent run campaign will be reported.   

Study of poloidal rotation induced by inboard-outboard asymmetry of NBI momentum input in KSTAR based on the gyro-center shift

The experimental measurement and theoretical analysis are performed for the origin of poloidal rotation by Neutral Beam Injection (NBI) momentum input. The theory of Gyro-Center Shift (GCS) suggests the radial electric field formation by the perpendicular component of NBI momentum input [1]. Most of induced ExB drift is balanced with the parallel rotation so that the plasma rotation is purely toroidal but there are regions where the induced radial electric field generates poloidal rotation since NB is injected into the same flux tube with different pitch angles when it propagates at inboard side and outboard side. The poloidal rotation measurements from Beam Emission Spectroscopy (BES) for the edge region and X-Ray Imaging Chrystal Spectrometer (XICS) for the core region with analysis using NUBEAM code will be presented. \\[4pt] [1] K. C. Lee, ``Analysis of Turbulence Diffusion and H-mode Transition in Conjunction with Gyrocenter Shift at the boundary of Fusion Devices,'' \textit{Plasma Phys. Cont. Fusion}, 51, 065023 (2009).   

Modelling of full toroidal plasma response to externally applied nonaxisymmetric magnetic perturbation fields in EAST

Since the increasing importance of Resonant Magnetic Perturbations (RMPs) for tokamak plasmas, it is planned to install the external coils in EAST to facilitate the related physics investigations. Predictive modelling of plasma response is under way by using MHD code MARS-F. It is in the framework of linear single-fluid, resistive MHD approximation and full toroidal geometry. It allows us to study the linear response of plasma to RMP fields with fixed plasma rotation. Full toroidal coupling enables us to study both the response of the nonresonant and resonant harmonics. A systematic study of various plasma conditions (resistivity, rotation, pressure) and coil configurations (midplane coils, off midplane coils, various toroidal mode numbers) on the plasma response to the nonaxisymmetric fields produced by the RMP coils, based on MARS-F full toroidal computations, is ongoing. Response from plasma based on different models (vacuum, ideal, resistive) will be presented.   

ELM experimental study on the EAST Tokamak

Atypical Type III ELM is observed on EAST tokamak. This type of ELM has MHD precursor and high collisionality at the edge, and also the threshold power is close to the scaling law. But the frequency of the ELM does not decrease with the injected power. Power threshold is lower with the molybdenum wall in double null (DN) on EAST. Considering the effects of the plasma surface (S) to the threshold power, Double Null has the lowest power threshold. Better energy confinement has been observed in DN compared to Single-null (SN) at same power loss. But with the same power loss, Upper Single Null (USN) with the grad-B drift pointing backwards the active X-point (favorable direction) on EAST has the lower energy confinement time than Lower Single Null (LSN). Low Hybrid Wave (LHW) can mitigate ELMs. The power deposition should be near the edge in the H-mode phase. Not only the LHW decreases the max gradient of the density in the pedestal region, but also brings the density oscillations. Low X-point configurations in Lower single null have a lower power threshold. The low X-point discharges on EAST is closer to the DN. Approaching to the DN configuration should be the reason of the lower power threshold caused by the lower X-point on EAST.   

Recent Doppler Backscattering results from EAST tokamak

A Doppler reflectometer system has recently been installed in the EAST tokamak. It includes two separated systems, one for Q-band and the other for V-band. The optical system consists of a fixed flat mirror and a steerable parabolic mirror, which enabling the measurement of perpendicular wave number in the range of 4-22 /cm, with the wave number resolution around 2 /cm, while the radial location can cover the whole minor radius for L mode and the whole pedestal for H mode on EAST. A 2D Gaussion Ray tracing code is used to calculate the scattering location, the perpendicular wave number and the resolution. In EAST last experimental campaign the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated. The Er evolution during L-H and H-L transition have also been measured. The two separated systems are also used as a poloidal coherent system together to study the GAM in EAST tokamak.   

Modeling of Steady-state Scenarios for the Fusion Nuclear Science Facility, Advanced Tokamak Approach

A Fusion National Science Facility (FNSF) would complement ITER in addressing the community identified science and technology gaps to a commercially attractive DEMO, including breeding tritium and completing the fuel cycle, qualifying nuclear materials for high fluence, developing suitable materials for the plasma-boundary interface, and demonstrating power extraction. Steady-state plasma operation is highly desirable to address the requirements for fusion nuclear technology testing [1]. The Advanced Tokamak (AT) is a strong candidate for an FNSF as a consequence of its mature physics base, capability to address the key issues with a more compact device, and the direct relevance to an attractive target power plant. Key features of AT are fully noninductive current drive, strong plasma cross section shaping, internal profiles consistent with high bootstrap fraction, and operation at high beta, typically above the free boundary limit, $\beta_N > 3$.   

All Superconducting Hybid Magnets for High Field Confinement Experiments*

The limitations on pulse duration and duty cycle that the use of copper magnets for the highest field components of advanced confinement experiments can be overcome by the adoption of ``All Superconducting Hybrid'' (ASH) magnets in their design. These consist in having MgB$_{2}$ superconducting coils, in the outer portion of the magnet, that operate at about 10 K like those adopted for the Ignitor vertical field coils, but can produce up to 10T as in the case of the hybrid magnet with a copper core under construction at Grenoble. Instead, in the case of the envisioned ASH magnets the inner core will be made of high temperature superconductors capable of operating at very high fields. The inclusion of advanced solutions for the coupled toroidal magnet and central solenoid such as that presented in Ref. [1] is envisioned. *Sponsored in part by the US DOE.\\[4pt] [1] B. Coppi and L. Lanzavecchia, Comm. Pl. Phys. Cont. Fus. 47, 11 (1987).   

Structural Analyses and Concept Developments and Physics for the Ignitor Program*

The design of the Ignitor machine features high toroidal currents ($I_{p}\simeq$ 10MA) and relatively small dimension $R_{0}\simeq$ 1.32m), with realistic values of the safety factor against the onset of macroscopic instability, involving record values of $B_{p}$ the mean poloidal field. One of the near term developments is a novel optimized distribution of the cooling channels that has been fully analyzed and is presently included in the design of the toroidal magnet plates and the associated C-clamps [1]. This solution increases the duty cycle for the machine operating at full parameters by a factor of 2. A complete structural analysis of the plasma chamber, as presently designed, has been carried out identifying the limits on the numbers and the magnitudes of the disruptions that can be tolerated on the basis of well accepted criteria. Then a program of further activities concerning the overall structural design of the Ignitor core has been formulated. *Sponsored in part by the U.S. Department of Energy.\\[4pt] [1] B. Coppi et al., 2012 IAEA International Conference Fusion Energy, Paper OV/P-02 (Vienna, 2012) to be published in Nucl. Fus. (2013).   

Control and Data Acquisition for the Spherical Tokamak MEDUSA-CR

The former spherical tokamak (ST) MEDUSA (Madison EDUcation Small Aspect.ratio tokamak, R\textless 0.14m, a\textless 0.10m, B$_{T}$\textless 0.5T, I$_{p}$\textless 40kA, 3ms pulse) [1] is being recommissioned in Costa Rica Institute of Technology. The main objectives of the MEDUSA-CR project are training and to clarify several issues in relevant physics for conventional and mainly STs, including beta studies in bean-shaped ST plasmas [2], transport, heating and current drive via Alfv\'{e}n wave, and natural divertor STs with ergodic magnetic limiter [2,3]. We present here the control and data acquisition systems for MEDUSA-CR device which are based on National Instruments (NI) software (LabView) and hardware on loan to our laboratory via NI-Costa Rica. The interface with the energy, gas fueling, and security systems are also presented. \\[4pt] [1] G.D. Garstka, PhD thesis, University of Wisconsin at Madison, 1997\\[0pt] [2] C. Ribeiro et al., Proc. 25$^{\textrm{th}}$ Symposium on Fusion Engineering, San Francisco, US, 2013\\[0pt] [3] C. Ribeiro et al., Proc. 39$^{\textrm{th}}$ EPS Conf. Contr. Fusion and Plasma Phys., Sweden, 2012   

Energy, Vacuum, Gas Fueling, and Security Systems for the Spherical Tokamak MEDUSA-CR

The former spherical tokamak (ST) MEDUSA (Madison EDUcation Small Aspect.ratio tokamak, R\textless 0.14m, a\textless 0.10m, B$_{T}$\textless 0.5T, I$_{p}$\textless 40kA, 3ms pulse) [1] is being recommissioned in Costa Rica Institute of Technology. The main objectives of the MEDUSA-CR project are training and to clarify several issues in relevant physics for conventional and mainly STs, including beta studies in bean-shaped ST plasmas [2], transport, heating and current drive via Alfv\'{e}n wave, and natural divertor STs with ergodic magnetic limiter [2,3]. We present here the energy, vacuum, gas fueling, and security systems for MEDUSA-CR device. The interface with the control and data acquisition systems based on National Instruments (NI) software (LabView) and hardware (on loan to our laboratory via NI-Costa Rica) are also presented.\\[4pt] [1] G.D. Garstka, PhD thesis, University of Wisconsin at Madison, 1997\\[0pt] [2] C. Ribeiro et al., Proc. 25$^{\textrm{th}}$ Symposium on Fusion Engineering, San Francisco, US, 2013\\[0pt] [3] C. Ribeiro et al., Proc. 39$^{\textrm{th}}$ EPS Conf. Contr. Fusion and Plasma Phys., Sweden, 2012   

Spherical tokamaks with plasma centre-post

The metal centre-post(MCP) in tokamaks is a structure which carries the total toroidal field current and also houses the Ohmic heating solenoid in conventional or low aspect ratio (Spherical)(ST) tokamaks.~ The MCP and solenoid are critical components for producing the toroidal field and for the limited Ohmic flux in STs. Constraints for a ST reactor related to these limitations lead to a minimum plasma aspect ratio of 1.4 [1] which reduces the benefit of operation at higher betas in a more compact ST reactor. Replacing the MCP is of great interest for reactor-based ST studies since the device is simplified, compactness increased, and maintenance reduced. An experiment to show the feasibility of using a plasma centre-post(PCP) is being currently under construction and involves a high level of complexity[2]. A preliminary study of a very simple PCP, which is ECR(Electron Cyclotron Resonance)-assisted and which includes an innovative fuelling system based on pellet injection, has recently been reported. This is highly suitable for an ultra-low aspect ratio tokamak(ULART) device[3]. Advances on this PCP ECR-assisted concept within a ULART and the associated fuelling system are presented here, and will include the field topology for the PCP ECR-assisted scheme, pellet ablation modeling, and a possible global equilibrium simulation.\\[4pt] [1] R.D.Stambaugh et al., Fus. Eng. Design, 41, p385, Sep98\\[0pt] [2] P.Micozzi et al., Nucl. Fusion, 50, p1, Jul10\\[0pt] [3] C.Ribeiro, Proc. 25$^{\textrm{th}}$ Symp Fus. Eng., SF, US, Jun13   

Measurements of Lower-Hybrid Waves with an Interferometer on the TST-2 Spherical Tokamak

Development of non-inductive startup scenarios is a critical issue for spherical tokamaks due to limited space at the center of the machine. Among the various methods, the TST-2 group has been focused on startups using rf waves. Recently, an electrostatically coupled combline antenna that launches lower-hybrid waves was installed for the starup experiments. Direct measurements of waves provide valuable information about wave propagation and damping. Interferometer signals are simply the line-integrated electron density, and suited for accurate quantitative studies. Design of a new interferometer to measure small scale fluctuations and a full-wave analysis of lower-hybrid waves for the relevant discharges will be presented. Comparison with the actual measurements will also be shown if available.   

The Physics of Local Helicity Injection Non-Solenoidal Tokamak Startup

Non-solenoidal startup via Local Helicity Injection (LHI) uses compact current injectors to produce toroidal plasma current $I_{p} $ up to 170 kA in the \textsc{Pegasus} Toroidal Experiment, driven by 4--8 kA injector current on timescales of 5--20 milliseconds. Increasing the $I_{p} $ buildup duration enables experimental demonstration of plasma position control on timescales relevant for high-current startup. LHI-driven discharges exhibit bursty MHD activity, apparently line-tied kinking of LHI-driven field lines, with the bursts correlating with rapid equilibrium changes, sharp $I_{p} $ rises, and sharp drops in the injector impedance. Preliminary NIMROD results suggest that helical LHI-driven current channels remain coherent, with $I_{p} $ increases due to reconnection between adjacent helical turns forming axisymmetric plasmoids, and corresponding sharp drops in the bias circuit impedance. The DC injector impedance is consistent with a space charge limit at low bias current and a magnetic limit at high bias current. Internal measurements show the current density profile starts strongly hollow and rapidly fills in during $I_{p} $ buildup. Simulations of LHI discharges using the Tokamak Simulation Code (TSC) will provide insight into the detailed current drive mechanism and guide experiments on \textsc{Pegasus} and NSTX-U.   

Predictive Power-balance Modeling of \textsc{Pegasus} and NSTX-U Local Helicity Injection Discharges

Local helicity injection (LHI) with outer poloidal-field (PF) induction for solenoid-free startup is being studied on \textsc{Pegasus}, reaching $I_{p} \le 0.175$ MA with 6 kA of injected current. A lumped-parameter circuit model for predicting the performance of LHI initiated plasmas is under development. The model employs energy and helicity balance, and includes applied PF ramping and the inductive effects of shape evolution. Low-$A$ formulations for both the plasma external inductance and a uniform equilibrium-field are used to estimate inductive voltages. \textsc{Pegasus} LHI plasmas are created near the outboard injectors with aspect ratio ($A)$ $\approx $ 5--6.5 and grow inward to fill the confinement region at $A\le 1.3$. Initial results match experimental $I_{p} (t)$ trajectories within 15 kA with a prescribed geometry evolution. Helicity injection is the largest driving term in the initial phase, but in the later phase is reduced to 20--45{\%} of the total drive as PF induction and decreasing plasma inductance become dominant. In contrast, attaining $\sim $1 MA non-solenoidal startup via LHI on NSTX-U will require operation in the regime where helicity injection drive exceeds inductive and geometric changes at full size. A large-area multi-injector array will increase available helicity injection by 3--4 times and allow exploration of this helicity-dominated regime at $I_{p} \sim 0.3$ MA in \textsc{Pegasus}. Comparison of model predictions with time-evolving magnetic equilibria is in progress for model validation.   

Local Helicity Injection Systems for Non-solenoidal Startup in the\textsc{ Pegasus }Toroidal Experiment

Local helicity injection is being developed in the \textsc{Pegasus} Toroidal Experiment for non-solenoidal startup in spherical tokamaks. The effective loop voltage due to helicity injection scales with the area of the injectors, requiring the development of electron current injectors with areas much larger than the 2 cm$^{2}$ plasma arc injectors used to date. Solid and gas-effused metallic electrodes were found to be unusable due to reduced injector area utilization from localized cathode spots and narrow operational regimes. An integrated array of 8 compact plasma arc sources is thus being developed for high current startup. It employs two monolithic power systems, for the plasma arc sources and the bias current extraction system. The array effectively eliminates impurity fueling from plasma-material interaction by incorporating a local scraper-limiter and conical-frustum bias electrodes to mitigate the effects of cathode spots. An energy balance model of helicity injection indicates that the resulting 20 cm$^{2}$ of total injection area should provide sufficient current drive to reach 0.3 MA. At that level, helicity injection drive exceeds that from poloidal induction, which is the relevant operational regime for large-scale spherical tokamaks. Future placement of the injector array near an expanded boundary divertor region will test simultaneous optimization of helicity drive and the Taylor relaxation current limit.   

Impurity Ion Temperature and Flow Dynamics During Local Helicity Injection on the \textsc{Pegasus} Toroidal Experiment

Anomalous energetic thermal and non-thermal minority ion distributions are observed during local helicity injection current startup. Energetic ions in significant numbers can transfer a large amount of power to plasma electrons during helicity injection, which can alter the helicity balance and consequent plasma startup via reduced resistive dissipation. Multi-spatial point spectra from a 1 m F/8.6 Czerny-Turner polychromator are recorded by an intensified high-speed camera with a time resolution of 500 $\mu $s. $T_{e} $ remains low during helicity injection, wherein the plasma experiences large magnetic fluctuations and strong reconnection activity near the injection region. Partially ionized low-Z impurities (CIII, NIII, and OIII) exist in the core plasma region, which allows core $T_{i} $ measurements. Strong impurity ion heating ($T_{i} \approx 1.2$ keV, $T_{e} \approx 0.1$ keV) correlates with $n=1$ MHD activity. High frequency magnetic fluctuations are indicated at frequencies close to the impurity ion cyclotron frequencies and may act as the source of energy for the ions. These observations motivate the deployment of a neutral particle analyzer to measure the working gas ion distributions in these plasmas. In addition, a high-throughput polychromator with 2 $\mu $s resolution is being installed to more directly correlate the observed impurity ion heating and flows with MHD and reconnection activity.   

Access to and Characterization of Ohmic H-mode Plasmas at Near-Unity Aspect Ratio

The low H-mode transition power threshold at near-unity aspect ratio allows access to H-mode in the \textsc{Pegasus} experiment with only Ohmic heating. Ohmic H-mode plasmas are achieved in both a limited and a new separatrix-limited magnetic configuration. H-mode is attained with high-field-side centerstack fueling, with densities from 1 to \textgreater\ 3x10$^{19}$ m$^{-3}$ and Greenwald fractions $\sim $ 0.2--0.7 for $I_{p} \sim 0.13$ MA. Compared to L-mode plasmas, H-modes show: a doubling of the stored energy; reduced D-$\alpha $ emission; edge current pedestal with characteristic width of $\sim $ 2 cm, with 6 cm for L-mode; reversal of the edge toroidal flow from counter-current to co-current; reduced V-sec consumption due to increased temperatures; and ELM excitation. Operation at A $\sim $1.15 results in strong particle trapping, f$_{T}\sim $ 0.7 -- 0.9, and associated neoclassical effects even at modest plasma temperatures so that P$_{OH}\sim $0.4 MW, which readily surpasses the estimated threshold power of \textless~0.1 MW. Low-field-side fueling appears to degrade access to and quality of the H-mode plasma. Characterization of H-mode access in \textsc{Pegasus} will provide unique data at near-unity A and guide detailed studies of ELM dynamics, as well as provide a critical tool for exploring the extremely high-$\beta_{T}$ regime at A $\sim $ 1.   

Initial Investigations of H-mode Edge Dynamics in the \textsc{Pegasus} Toroidal Experiment

Experiments with ultra-low aspect ratio ($A<1.2)$ H-mode plasmas in \textsc{Pegasus} enable unique measurements of Edge Localized Mode (ELM) phenomena of import to next-step fusion devices. The modest temperatures and pulse lengths in \textsc{Pegasus} allow the use of insertable probes to diagnose the edge plasma with high spatial and temporal resolution. In particular, the compatibility of the Hall probe J$_{edge}$ diagnostic with the H-mode edge to date affords the opportunity to study current profile dynamics throughout the ELM cycle. A pedestal in J$_{edge}$ is formed following the L-H transition that is transiently destroyed during ELMs. Presently, Type I and Type III ELMs are accessible. Both types generate field-aligned filaments during the ELM. A prominent current-hole J$_{edge}$ perturbation and low-$n$ MHD signature is evident during Type III ELM crash events, similar to that seen in prior peeling mode studies conducted in L-mode with strong edge current drive\footnote{ M.W. Bongard \textit{et al.}, Phys. Rev. Lett. \textbf{107}, 035003 (2011)}. In contrast, Type I ELMs are found to have a complex MHD signature comprised of multiple intermediate toroidal mode numbers ($5 [Preview Abstract]

Commissioning of Thomson Scattering on the \textsc{Pegasus} Toroidal Experiment

A new multipoint Thomson scattering diagnostic has been installed on the \textsc{Pegasus} Toroidal Experiment. It employs a frequency-doubled Nd:YAG laser ($\lambda_{0\, }=$ 532 nm) and spectrometers using volume phase holographic gratings and gated, intensified CCD cameras. Spectral, temporal and intensity calibrations of the spectrometer systems were conducted. Sources of laser energy loss were identified and reduced, beam termination was optimized to minimize reflections during collection time, and inter-shot alignment monitoring was installed. Rayleigh and Raman calibration efforts revealed significant stray light from in-vessel reflections; hence, a vacuum-compatible optical baffling system was designed, fabricated, and is being installed. Operation of the diagnostic will support characterization of helicity dissipation mechanisms and confinement scaling during local DC helicity injection startup on \textsc{Pegasus}. Additionally, H-mode temperature and density profiles will be obtained to support equilibrium reconstructions and stability studies of ELMs in the H-mode plasma edge. Initial measurements will be conducted with an 8-spatial channel array; expansion to 24 channels is in progress.   

Development of a Technique for Measuring Local Electric Field Turbulence in a Tokamak Plasma

Accessible methods for measuring $\tilde{E}(R,t)$ in large-scale magnetic confinement experiments are highly desired for validation studies of plasma turbulence models. A new technique based on neutral beam emission spectroscopy is being developed to address this need. Rapid fluctuations in the separation of spectral components of the motionally induced Stark spectrum can reflect fluctuations in the intrinsic electric field of the plasma. Polarization spectroscopy via high resolution, high-throughput spectrometers that compensate for field-of-view broadening is being developed to isolate and measure these fluctuations. Cross-power correlation analysis between the linewidth fluctuations and plasma density fluctuations will be employed to extract the expected small signals. Electric field fluctuations at mid-minor-radius, normalized to an estimated MSE field, are expected to be on the order of $\tilde{E}/E_{MSE} \approx 1\times 10^{-3}$ in the \textsc{Pegasus} Toroidal Experiment and are comparable to those expected in NSTX and in DIII-D.   

Overview of results from the Lithium Tokamak eXperiment (LTX)

LTX is a modest ST with R=0.4 m, a=0.26 m, and elongation = 1.5. Lithium is deposited on a conformal copper liner faced with stainless steel, which may be heated to 350C to provide a molten lithium PFC, or operated at lower temperatures with solid lithium coatings. In 2010 and 2011, solid coatings of lithium, applied at the start of a run day, were explored. DEGAS2 analysis indicates that recycling coefficients of 0.7 to 0.8 were achieved. In 2012 experiments used molten lithium films at temperatures up to 350C. With a static molten film of lithium, impurities segregated to the surface, and discharges were neither low recycling nor free of impurities. In 2013 an electron beam will be used to heat and stir a lithium pool in the lower portion of the liner, to eliminate surface films and maintain a clean lithium surface. An overview of results will be presented.   

Energy Confinement for Low Recycling Wall Conditions in the Lithium Tokamak Experiment

The Lithium Tokamak Experiment (LTX) is a spherical tokamak designed to study the low-recycling regime through the use of lithium-coated shells conformal to the LCFS. A lowered recycling rate is expected to flatten core $T_e$ profiles, raise edge $T_e$, strongly affect $n_e$ profiles, and enhance confinement. A Thomson scattering diagnostic uses a 20 J, 36 ns FWHM pulsed ruby laser to measure $T_e$ and $n_e$ at 11 radial points on the horizontal midplane, spaced from the magnetic axis to the outer edge at a single temporal point for each discharge. Scattered light is imaged through a spectrometer onto an intensified CCD. The diagnostic is absolutely calibrated using a precision light source and Raman scattering. Measurements of $n_e$ are compared with line integrated density measurements from a microwave interferometer. The system can make measurements at $n_e\ge2\times10^{18}\,\mathrm{m^{-3}}$. $W_{kin}$ is calculated from $T_e$ and $n_e$ profiles with CHERS measurements to constrain $T_i$. $W_{tot}$ is measured using a compensated diamagnetic loop. These measurements and a magnetic equilibrium allow determination of $\tau_E$, which is compared to scaling law predictions under various wall conditions. Dependence of $T_e$ profile shapes on wall conditions is also discussed.   

Equilibrium Reconstructions and Improved Loop Voltage Control of LTX Plasmas

The Lithium Tokamak eXperiment (LTX) is a spherical tokamak with a close fitting low-recycling wall composed of thin evaporated lithium layers on stainless steel-lined copper shells. The combination of high electrical conductivity of the copper shells and transient coil and plasma currents results in long-lived eddy currents with large spatial extent in the close fitting wall which, in turn, affects the start-up and operations of tokamak plasmas. The ohmic heating power supply control system was configured to allow better control of the loop voltage while reducing the induced eddy currents. Upgrades to the magnetic diagnostic system include repairs to existing diagnostics and the addition of poloidal arrays of sensors and saddle coils to more effectively perform equilibrium reconstructions of the plasma discharge. Details of the signal processing will be presented and 2D and 3D reconstructions will be compared.   

Latest Results from the LTX High-Speed Digital Holography System

During the last year research efforts for the LTX Digital Holography system have been concentrated on reducing noise and diagnosing the flow pattern of the LTX Supersonic Gas Injector. A high-speed CO$_{\mathrm{2}}$ laser digital holography system (500 frames per second (FPS) at 256 x 256 pixels, 1500 FPS at 128 x 128 pixels, etc., to a maximum of 43,000 FPS at 64 x 4 pixels) has been built for high-resolution imaging of electron density on the Lithium Tokamak Experiment (LTX). The laser operates at 9.1 microns by using an Oxygen-18 isotope, and has a power output up to 20 W. A FLIR SC4000 IR camera is used to capture the digital holograms. An acousto-optic modulator (AOM) is used to ``shutter'' the laser so that effective camera integration times down to less than one microsecond are possible. The system will be used for imaging measurements on LTX during molecular cluster injection (MCI), supersonic gas injection (SGI), and injection from edge gas puffers. Results of noise reduction efforts along with ultra low noise flow-pattern images from the SGI will be presented.   

A Microwave FMCW Reflectometer for Electron Density Measurements on LTX

An FMCW (frequency-modulated continuous-wave) reflectometer is being installed on the Lithium Tokamak Experiment (LTX) for electron density profile and fluctuation measurements. The system has two channels covering 13.5--33 GHz for (O-mode) electron density measurements in the range of $0.2$$-$$1.3$$\times$$10^{13}$ cm$^{-3}$. The diagnostic can operate at ultrafast full-band sweep intervals ($\Delta t\ge 4$ $\mu$s), which allows the system to function as both a profile and fluctuation monitor. The reflectometer utilizes a mid-plane port on LTX and views the plasma through a 4.8$''$ gap between the upper and lower in-vessel shells. A pair of bi-static conical horns are attached to the ends of 18$''$ long circular waveguide sections and mounted on a rotatable flange. This sub-assembly is attached to a jacking stage such that the horns can be positioned arbitrarily close to the plasma edge, or retracted outside the main chamber. A rotary joint allows the polarization of the launch and receive waves to be independently selected. Further details of the design and capabilities of the diagnostic, along with preliminary data, will be presented at the meeting.   

Improved doppler spectroscopy measurements on LTX

Lithium wall coatings in the Lithium Tokamak Experiment (LTX) reduce recycling, thus reducing collisions with edge neutrals that cause drag on rotation and energy losses. In order to measure the predicted improvements in plasma momentum and ion energy confinement, significant hardware upgrades to the passive doppler spectroscopy diagnostics on LTX have been performed. New collection optics for the poloidal and toroidal views have more sightlines and greater spatial coverage for measurements of line-integrated density, temperature, and rotation velocity. In addition to the existing Kaiser Holospec spectrometer, a novel high-throughput, high-resolution variable wavelength spectrometer has been installed and can be operated in parallel to measure emission in the remaining sightlines. It has similar capabilities to the Kaiser spectrometer, but can be adjusted between shots to measure any lines in the visible range with high accuracy. Measurements of the various impurity lines and charge states in the plasma with these improved capabilities will be used to study the dependence of plasma momentum and ion energy confinement on the wide variety of wall conditions achievable in LTX.   

Study of plasma-facing components in the Lithium Tokamak Experiment with the Materials Analysis and Particle Probe

The Lithium Tokamak Experiment (LTX) is a spherical torus designed to accommodate solid or liquid lithium as the primary plasma-facing component (PFC). We present initial results from the implementation on LTX of the Materials Analysis and Particle Probe (MAPP) diagnostic, a collaboration among PPPL, Purdue University, and the University of Illinois. MAPP is a compact \emph{in vacuo} surface science diagnostic, and its operation on LTX will provide the first ever \emph{in situ} surface measurements of a tokamak first wall environment. With MAPP's analysis techniques, we will study the evolution of the surface chemistry of LTX's first wall as a function of varied temperature and lithium coating. During its 2013 run campaign, LTX will use an electron beam to evaporate lithium onto the first wall from an in-vessel reservoir. We will use two quartz crystal microbalances to estimate thickness of lithium coatings thus applied to the MAPP probe. We have recently installed a set of triple Langmuir probes on LTX, and they will be used to relate LTX edge plasma parameters to MAPP results. We will combine data from MAPP and the triple probes to estimate the local edge recycling coefficient based on desorption of retained hydrogen.   

Critical need for MFE: the Alcator DX advanced divertor test facility

Three critical challenges must be met before a steady-state, power-producing fusion reactor can be realized: how to (1) safely handle extreme plasma exhaust power, (2) completely suppress material erosion at divertor targets and (3) do this while maintaining a burning plasma core. Advanced divertors such as ``Super X'' and ``X-point target'' may allow a fully detached, low temperature plasma to be produced in the divertor while maintaining a hot boundary layer around a clean plasma core -- a potential game-changer for magnetic fusion. No facility currently exists to test these ideas at the required parallel heat flux densities. Alcator DX will be a national facility, employing the high magnetic field technology of Alcator combined with high-power ICRH and LHCD to test advanced divertor concepts at FNSF/DEMO power exhaust densities and plasma pressures. Its extended vacuum vessel contains divertor cassettes with poloidal field coils for conventional, snowflake, super-X and X-point target geometries. Divertor and core plasma performance will be explored in regimes inaccessible in conventional devices. Reactor relevant ICRF and LH drivers will be developed, utilizing high-field side launch platforms for low PMI. Alcator DX will inform the conceptual development and accelerate the readiness-for-deployment of next-step fusion facilities.   

Access to and performance of I-mode plasmas on Alcator C-Mod

The I-mode regime of operation features an edge thermal transport barrier, without a particle barrier. Stationary conditions are thus achieved without impurity accumulation, and usually without ELMs. In contrast to the EDA H-mode regime on Alcator C-Mod, it is readily accessed at low q$_{95}$ and low collisionality, both relevant for ITER. Analysis of a dataset of 400 discharges at q$_{95}$ $\sim$ 3 shows normalized energy confinement in I-modes reaches or exceeds that in most H-modes, up to H$_{98} =$ 1.2. Confinement and pedestal temperature increase with input power. In some cases I-mode is maintained up to the maximum available power (5 MW ICRF) while in others a transition to H-mode limits the performance. Understanding and extending the conditions for entering and staying in I-mode is thus critical for extrapolation of the regime. Experiments have extended the regime both to lower densities and to higher densities and powers through gas puffing into established I-modes. Results from an expanded database of C-Mod discharges will be presented, along with details of I-mode profiles and fluctuations, including GAMs and a weakly coherent mode, which are providing insights into the physics of the regime.   

A Study of Electron Modes in Off-axis Heated Alcator C-Mod Plasmas

Understanding the underlying physics and stability of the peaked density internal transport barriers (ITB) that have been observed during off-axis ICRF heating of Alcator C-Mod plasmas is the goal of recent gyro-kinetic simulations. Two scenarios are examined: an ITB plasma formed with maximal (4.5 MW) off-axis heating power; also the use of off-axis heating in an I-mode plasma as a target in the hopes of establishing an ITB. In the former, it is expected that evidence of trapped electron mode instabilities could be found if a sufficiently high electron temperature is achieved in the core. Linear simulations show unstable modes are present across the plasma core from r/a$=$0.2 and greater. In the latter case, despite establishing similar conditions to those in which ITBS were formed, none developed in the I-mode plasmas. Linear gyrokinetic analyses show no unstable ion modes at r/a \textless 0.55 in these I-mode plasmas, with both ITG and ETG modes present beyond r/a$=$0.65. The details of the experimental results will be presented. Linear and non-linear simulations of both of these cases will attempt to explore the underlying role of electron and ion gradient driven instabilities to explain the observations.   

Non-local Heat Transport in Alcator C-Mod Ohmic L-Mode Plasmas

Non-local heat transport experiments are performed in Alcator C-Mod Ohmic L mode plasmas by inducing edge cooling with laser blow-off impurity injection. The non-local effect is observed in low collisionality linear Ohmic confinement (LOC) regime plasmas. Transport analysis shows this phenomenon can be explained by a fast drop of the core diffusivity, or the sudden appearance of a heat pinch. Experiments with repetitive cold pulses experiments show that in LOC plasmas the electron thermal transport is not purely diffusive. In high density saturated Ohmic confinement (SOC) regime plasmas, the thermal transport becomes local. Measurements from a high resolution imaging x-ray spectrometer show that the ion temperature has a similar behavior (with a time delay) as the electron temperature in response to edge cooling, and the transition density of non-locality correlates with the density at which rotation reverses. This correlation may indicate the possible correction between thermal and momentum transport, which is also linked to the TEM to ITG mode dominance transition.. Linear gyrokinetic simulations suggest the turbulence outside r/a$=$0.75 changes from TEM dominance in LOC plasmas to ITG mode dominance in SOC plasmas.   

Impact of Poloidal Density Variation on Radial Impurity Transport

Results are presented from a first of its kind experiment to test the impact of poloidal asymmetries of impurity density on their flux-surfaced averaged radial transport in tokamak plasmas. Laser blow-off of molybdenum was introduced in ICRH heated, Alcator C-Mod L-mode plasmas where changes to the hydrogen minority resonance layer were used to modify the in/out asymmetry between -0.2 \textless~n$_{\mathrm{z,cos}}$/$\langle$n$_{\mathrm{z}}\rangle$ \textless~0.2 at mid-radius. Changes in the impurity confinement time by a factor of two, measured using radiated power and vacuum ultraviolet spectroscopy diagnostics, are correlated to changes in the asymmetry. Radial profiles of Mo$^{32+}$ for r/a \textless~0.8 are measured using x-ray imaging crystal spectroscopy, and are used to constrain STRAHL simulations which provide details on changes in the gradient scale length of the impurity density.   

Uncertainty quantification of experimentally-derived quantities

Uncertainty quantification (UQ) is an essential step in both the interpretation of experimental data and the validation of simulations. Use of rigorous UQ techniques can reduce the number of code runs needed while at the same time increasing confidence in the uncertainties computed. To this end, work is underway on UQ of experimental impurity transport coefficients in Alcator C-Mod plasmas. The transport coefficients are obtained by fitting the output from the impurity transport code STRAHL to the signal from an x-ray imaging crystal spectrometer. In previous work, naive Monte Carlo sampling was implemented by randomly perturbing the observed $n_e$, $T_e$ profiles within error bars then fitting each perturbed dataset with a spline. In the new approach, the profiles are smoothed using Gaussian process (GP) regression. The GP posterior can then be sampled using a variety of techniques. Further harnessing of UQ techniques should enable improved error estimates for experimentally-derived quantities and simulation validation.   

Search for gyrokinetic dependencies in helium transport at Alcator C-Mod

Helium-3 and helium-4 impurity transport measurements and density profile measurements have been obtained on Alcator C-Mod in a variety of discharge conditions, using the core Charge Exchange Recombination Spectroscopy (CXRS) diagnostic. The helium concentrations range from trace ($ < 2 $ \%) to large minority ($\sim 20 $ \%). L-mode, H-mode, and I-mode results are included, with Ohmic heated, ICRF heated, and LH heated plasmas. Helium profiles are observed to vary with plasma current, and also change in time during ICRF shots. Linear and nonlinear gyrokinetic simulations are performed for some shots using the GENE code. Sensitivity scans are done for magnetic shear, impurity density, and other plasma parameters and transport scalings are compared with experimental results. Simulated transport flux is compared with experimentally derived D and v parameters.   

Effects of Sawtoothing on Peaking of Impurity Profiles in Alcator C-Mod

Sawtoothing has a role in limiting impurity accumulation and might be used to control impurity concentration. In this poster, the effect of sawtoothing on the impurity peaking factor is examined for Alcator C-Mod. Sawtooth parameters are obtained by coherent averaging of \textit{Te} profile data from the University of Texas's 32-channel electron cyclotron emission (ECE) radiometer; impurity peaking factors for B and He are supplied by active core CXRS. Initial results suggest core impurity peaking factors for B are not strongly dependent on sawtooth period. Sawteeth in soft x-ray emission (SXR) are compared with sawtooth parameters derived from ECE data to investigate the response of heavy impurities to sawtoothing. In early results, discharges have either inverse or normal sawtoothing observed in core SXR. Inverted SXR sawteeth may arise from transport but are more likely due to the atomic physics of the emission. Simulations to verify these assertions for light and heavy impurities are in progress.   

Resolving the non-equilibrium MSE spectrum and its application at Alcator C-Mod

The high-resolution MSE spectrum has been thoroughly measured in the Alcator C-Mod tokamak using high-throughput polarization-corrected optics and an f/1.8 holographic imaging spectrograph. The measurements have been done with a 50~keV/u diagnostic neutral beam in Alcator C-Mod high-density Ohmic plasmas covering the parameter range: n$_{\mathrm{e}}$~$=$~\textbraceleft 0.6-1.3\textbraceright x10$^{20}$~m$^{-3}$, B$_{\mathrm{T}}$~$=$~4-6~T at T$_{\mathrm{e}}$~$=$0.4-2.5~keV. Multiple Stark components were resolved with application of the advanced constrained multi-line spectral fit. Measured ratios of Stark spectral components $\sigma_{1}$/$\sigma_{\mathrm{0}}$, $\pi _{4}$/$\pi_{3}$ and $\Sigma \sigma $/$\Sigma \pi $ were compared to an $n$-resolved statistical and a \textit{nkm}-resolved collisional radiative model (Marchuk, PPCF, 54, 2012). Results showed a clear deviation from $n$-resolved model and good agreement with \textit{nkm}-resolved model. The comparison results were extrapolated toward 100~keV/u and 500~keV/u ITER beams (Bespamyatnov, NF, submitted). Here we present an extended analysis of the MSE spectral measurement and spectral fitting and attempt to employ the extracted line ratios for the measurement of the magnetic field line pitch angle. Feasibility of using spectral MSE as a supporting or complimentary diagnostic to the MSE polarimetry will also be assessed.   

Using signals at the 2$^{\mathrm{nd}}$, 4$^{\mathrm{th}}$ and combined PEM Harmonics in MSE Polarimeters

The use of signal intensities at various harmonic frequencies of paired photo-elastic modulators (PEMs) in polarimeters to measure the polarization angle of linearly polarized light is explored. The Alcator C-Mod Motional Stark Effect diagnostic has been calibrated at the fourth PEM harmonics to an accuracy of about 0.05$^{\circ}$, which is within a factor $\sim$ 2 of the calibration accuracy at the traditionally used second harmonic, but requires additional calibration terms. A new mode of operation is derived analytically and verified experimentally; if the PEM retardance is chosen to maximize the \underline {combined} signal strength at the 2nd and 4th harmonic, and if the polarization angle is deduced from the ratio of the summed signal amplitudes at the second and fourth harmonic, then the magnitude of the cos (4$\theta )$ correction term is smaller than operating at the customary PEM retardance and using the second harmonic. In addition, the system is less sensitive to small drifts in the PEM retardance. This new regime of operation provides a 40{\%} improvement in photon statistics without compromising the polarimeter calibration or sensitivity to PEM retardance drift.   

Recent Results from the C-Mod Polarimeter

The C-Mod 3 chord FIR polarimeter, with a 2 MHz bandwidth, is capable of responding to both fast changes in the plasma equilibrium and high frequency fluctuations. It operates under ITER-like plasma conditions and magnetic fields, and uses an optical layout and FIR sources very similar to those proposed for the ITER polarimeter. Results from the polarimeter as a function of plasma density and current will be discussed, as well as the effects of lower hybrid power levels, phasing and plasma density on the current drive efficiency. The possible identification of some broadband fluctuations as primarily magnetic in nature, and gyrokinetic simulation results from the modeling of these fluctuations will also be presented. Estimates of the localization of this mode will be described. The polarimeter response to low frequency MHD modes will be compared with results from the Fast Two-Color-Interferometer.   

Comparison of Edge Turbulence Velocity Analysis Techniques using Gas Puff Imaging Data on Alcator C-Mod

Two methods for analyzing turbulence velocities have been used in the past on Gas Puff Imaging (GPI) data. One is based on time-delay cross-correlation of successive images that is used to track motion of discrete structures. The other uses Fourier analysis to obtain frequency and poloidal wavenumber spectra, from which phase velocities are derived. Recent experiments were completed with imaging at the outboard midplane and analysis using the cross-correlation technique, yielding magnitudes of poloidal velocities in the 0.1-1.4 km/s range [Zweben et al., in press PoP (2013)]. The Fourier analysis yielded values up to an order of magnitude larger for the same data. To understand the reasons for these differences, we have created and analyzed synthetic data. Comparisons between the two analysis techniques applied to both the actual experimental and synthetic data will be presented.   

Destabilization of internal kink by LHCD suprathermal electron pressure

New observations of the formation of short-lived modes have recently been carried out on Alcator C-Mod. A (1,1) internal kink appears to be destabilized by the fast-electron pressure carried by the suprathermal electrons driven by Lower Hybrid Current Drive (LHCD). Surprisingly, the (1,1) fishbone-like activity can coexist with sawteeth, suggesting that the two modes have independent driving mechanisms. The frequency of the mode is comparable to the core toroidal rotation and that of the precursors. The electron energies responsible for driving the mode have been measured for the first time using the downshift of electron gyrofrequency due to relativistic effects. This is a new explanation for the so-called electron-fishbones. This work was performed under US DoE contracts DE-FC02-99ER54512, DE-AC02-09CH11466 and DE-FG03-96ER-54373.   

Neutral Excitation, Ionization and SOL Power Loss of Lower Hybrid Waves in the Alcator C-Mod Tokamak

The efficiency of Lower Hybrid Current Drive (LHCD) in Alcator C-Mod discharges diminishes precipitously in high density (line-averaged $\bar{n}_e > 10^{20} [m^{-3}]$), diverted plasmas as seen by the lack of indicative hard X-ray (HXR) bremsstrahlung and reduction in loop voltage. VUV, Visible and infrared light, as well as measurements of ne,Te in the SOL show significant change in the high density regime with the application of Lower Hybrid power. Poloidal dependence of LHCD-induced hydrogen Lyman-alpha emission in high density plasmas was investigated using a filtered poloidally-viewing pinhole camera. Due to limitations in the camera radial resolution, \emph{a priori} assumptions of the emission region were used to extract global emission values. Estimations are made of the electron cooling rate utilizing the Lyman-alpha emission and S/XB coefficients and are correlated to various experimental parameters for the dependency of power loss. The measurements indicate that Lyman-alpha power is enhanced globally by LHCD. Work supported by USDOE awards DE-FC02-99ER54512 and DE-AC02-76CH03073.   

Development of active stub tuning networks in the Lower Hybrid Range of Frequencies

Active stub tuning with a fast ferrite tuner (FFT) allows for the RF matching network to respond dynamically to changes in the plasma impedance such as during edge density fluctuations or the L-H transition, and has greatly increased the effectiveness of fusion ion cyclotron range of frequency systems. A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system at 4.6 GHz. Construction and test-stand performance of the high power stub tuner will be discussed. Cross-coupling of reflected power between columns must be considered when evaluating the performance of a matching network for a LHCD phased array launcher. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution for the stub lengths is advanced in an iterative manner to simulate the time-dependent behavior of the real system under conditions of time-varying plasma load conditions. System performance is presented for a range of edge density conditions from under-dense to over-dense and a range of launched $n_{||}$.   

Modification of Launched ICRF Wave Number Spectrum

A principle challenge of ICRF heating in tokamaks with high-Z walls is the minimization of impurity contamination associated with ICRF operation. A source mechanism introduces impurities by sputtering of PFCs by energetic ions. Additionally, a transport mechanism alters core impurity concentration through spatial variation of plasma potentials in front of the antenna, establishing local E x B drifts that affect edge transport. Here, RF sheath formation driven by ICRF E-parallel, is implicated as the root cause. Recent results on Alcator C-Mod using a field-aligned (FA) antenna suggest that the launched wave spectrum may differ significantly from the predicted wave spectrum for certain toroidal phasings. A novel method for modifying the launched ICRF wave number spectrum and improving the cancellation of E-parallel involves directly imposing a potential bias on different regions of the antenna structure. 3D FEM simulations using a cold plasma tensor were implemented using DC and RF biasing on local limiters, tiles, and antenna septa. The effect of biasing antenna structures was investigated to determine the feasibility of improving E-parallel cancellation for different toroidal antenna phasings.   

Turbulence and Zonal-Flow evolution at the L-to-H transition in Alcator C-Mod

Transitions of tokamak confinement regimes from low- to high-confinement are studied on Alcator C-Mod using gas-puff-imaging (GPI) with a focus on the interaction between the edge drift-turbulence and the local shear flow. Results will be presented on the evolution of the energy transfer rate and turbulence spreading, with the transfer rate reaching the estimated value of drift turbulence growth rate at the time the turbulent kinetic energy starts to drop, leading to an expected loss of turbulence power comparable to the observed changes. The above behavior is demonstrated across a series of experiments. Thus a lossless kinetic energy conversion mechanism is shown to both drive a zonal flow and be responsible for the initial reduction of turbulence fluctuation power, consequently mediating the transition into H-mode. The history of the energy transfer is compared with the evolution of the pressure pedestal for the purpose of developing microscopic models of thresholds between L-mode, limit-cycle-oscillating regimes and H-mode.   

Comparing density, electron temperature, and magnetic fluctuations with gyrokinetic simulations using new synthetic diagnostics

Three new synthetic turbulence diagnostics are implemented in GS2 and compared with measurements: phase contrast imaging, polarimetry, and electron-cyclotron (ECE) emission. Our new synthetic diagnostic framework is based on transforming to a real-space annulus in Cartesian coordinates. This allows straightforward convolution with diagnostic point-spread functions, or integration over viewing chords. Wavenumber spectra and fluctuation amplitudes, as well as transport fluxes, are compared with measurements. Both phase contrast imaging and newly observed ECE electron temperature fluctuations, closely follow the electron temperature in an internal transport barrier during on-axis heating pulses, consistent with the role of TEM turbulence [D. R. Ernst et al., APS Inv. (2012), IAEA/TH/1-3 (2006)]. New C-Mod polarimetry measurements, showing strong broadband core magnetic fluctuations, will also be examined against gyrokinetic simulations. The new framework is readily extended to other fluctuation measurements such as two-color interferometry, beam emission spectroscopy, Doppler back-scattering, ECE imaging, and microwave imaging reflectometry.   

Kinetic modelling of energy deposition and erosion of EAST divertor targets during ELMs

Edge localized modes (ELMs) produce a periodic expulsion of plasma particles and energy from the edge region into the scrape-off layer (SOL) towards the divertor target and other plasma facing components (PFCs) in fusion devices. High energy particles due to ELMs burst lead to the excessive heat load on the divertor plate, which results in material erosion, melting and vaporization. This has a serious impact on the lifetime of PFCs and further the operation of fusion reactor. Therefore, the modelling and understanding of the effects of the ELMs on energy deposition and erosion of divertor target is one of the most crucial issues concerning the design and performance of PFCs. In this study, a one-dimension-in-space and three-dimension-in-velocity (1d3v) Particle-In-Cell Monte Carlo collision (PIC-MCC) code SDPIC (SOL {\&} Divertor PIC simulation) has been developed to investigate energy deposition and erosion of divertor target of EAST. Time evolution of energy flux and erosion of the divertor target has been studied. The spatial distribution of potential and plasma density during ELMs burst has been investigated. In addition, heat conduction equation has been implemented into SDPIC to study variation of surface temperature during ELMs.   

The effects of divertor parameters on the plasma penetration depth of the castellated tile gaps: a kinetic simulation

Castellated tiles construction is thought to be the best solution to ensure the thermo-mechanical durability and integrity of materials under high heat flux loads. However, issues such as material migration into gaps and the subsequent fuel retention, are of crucial important for the fusion devices with castellation structure. Therefore, concerns over the fuel accumulation and impurity deposition in the gaps calls for dedicated studies. To know how the fuel retained inside the gap, the plasma sheath around the gaps should be understood first. Since PIC model possesses the merits of kinetic methods, it has been applied extensively to edge plasma studies. In this work, a 2D PIC model is applied to study plasma around the divertor gaps with the focus on the H$+$ penetration depth inside the gaps. By varying the magnetic field and plasma temperature, the relationship between penetration depth and cyclotron radius of the ions is obtained, we find the H$+$ cyclotron radius has a significant effect on the penetration depth. Besides, the effects of gap width, plasma parameters and magnetic field are analyzed and discussed. Finally, effect of penetration depth on the fuel retention inside the tile is illustrated, which shows it can increase retention dramatically.   

Broad Distribution of Traps in Metallic Plasma Facing Components

In the vacuum vessel of the ITER device, plasma facing components will be exposed to various plasma conditions that will alter both surface and bulk of material [1], affecting dynamic plasma wall interactions (PWI). In most studies of retention processes involved in PWI, hydrogen outgasing is not well reproduced in the context of long-pulse plasma operation regimes, due to small number of hydrogen traps considered. In this work, we model hydrogen outgasing from metallic wall with a large number of traps. We identify two regimes of ougasing from the material bulk depending on trapping energy and two regimes of hydrogen desorption from material surface, which depend on hydrogen concentration. Modeling of thermodesorption experiments performed on W samples implanted with D at high fluences reveals broad distributions of traps. Effects of such broad trap distributions on hydrogen outgasing are discussed. Work is performed under the auspices of USDOE Grant No. DE-FG02-04ER54739 and the PSI Science Center Grant DE-SC0001999 at UCSD. \\[4pt] [1] R.A. Causey, J. Nucl. Mater. 300 (2002) 91   

Initial Operation of the PhIX High Intensity Plasma Source

The Physics Integration eXperiment (PhIX) is a linear high-intensity rf plasma source that combines a high-density helicon plasma generator with an electron heating section. It is being used to study the physics of heating over-dense plasmas in a linear configuration, as well as exploring source interactions with a downstream target. The helicon plasma is produced by coupling 13.56 MHz rf power at levels up to 100 kW. Microwaves at 18 GHz are coupled to the electrons in the over-dense helicon plasma via whistler waves and Electron Bernstein Waves (EBW). An energy analyzer embedded in the target substrate is being used to determine the ion energy and ion flux at the target, while a microwave interferometer and Langmuir probes are used to determine plasma parameters near the source and near the target. High plasma densities have been produced in He (\textgreater\ 5x10$^{19}$/m$^{3})$ and H (\textgreater\ 1.5x10$^{19}$/m$^{3})$, and operation in magnetic field strengths up to 0.5T has been demonstrated. Details of the experimental results will be presented, as well as future plans for studying plasma surface interactions and rf antenna plasma interactions.   

Surface chemistry of Li coatings on polycrystalline W and ATJ graphite irradiated by D and He ions

Because Li has been shown to improve the performance of fusion plasmas, experiments have been performed to investigate the fundamental chemical behavior of these coatings. Earlier experiments on graphite showed that O atoms are the preferential bonding sites for deuterium. This implies that the O, not Li, retains the deuterium but from where does the O originate. ATJ graphite was irradiated with D to remove surface O, next a layer of Li was deposited of various thicknesses, and then some of the coatings were irradiated with D or synergistic D/He. Data from the samples without the post Li irradiation show that the surface O is likely from the Li evaporator or the ambient. Additional experiments investigated lithium coatings on tungsten at medium ($\sim$10$^{14} cm^{-2}s^{-1}$) and high ($\sim$10$^{20} cm^{-2}s^{-1}$) fluxes of D, He, and synergistic D/He ions at low energies ($\le$ 1 keV). The medium flux D results show a similar retention behavior in lithiated W, however with He ions mixed in, the D retention is reduced for both graphite and W substrates. High flux results will also be presented from experiments at DIFFER.   

Dust generation at interaction of plasma jet with surfaces

Coatings of W and C with widths of a few microns will be exposed to plasma jet for studying the erosion of the surface and detachment of micron size dust particles. A coaxial plasma gun has been built inside a vacuum chamber for producing supersonic plasma jets. Its design is based on a 50 kJ coaxial plasma gun which has been successfully used for accelerating hypervelocity dust. Initial shots were carried out for a capacitor bank with C=12 $\mu$F and charged up to 2 kV. Currents of tens of amps were measured with a Rogowsky coil and plasma flow speeds of 4 km/s were inferred from high-speed images of jet propagation. An upgrade consisting in adding capacitors in parallel will be performed in order to increase the energy up to 2 kJ. A coil will be installed at the gun muzzle to compress the plasma flow and increase the energy density of the jet on the sample surface. A CCD camera with a maximum recording speed of 100 k fps and a maximum resolution of 1024x1024 pixels was set for image acquisition of the plasma and dust. A laser system used to illuminate the ejected dust from the surface includes a laser diode emitting at 650 nm with a beam power of 25 mW.   

Secondary Electron Emission Yield in the Limit of Low Electron Energy

Secondary electron emission (SEE) from solids plays an important role in many areas of science and technology. In recent years, there has been renewed interest in the experimental and theoretical studies of SEE. Several recent studies proposed that the reflectivity of very low energy electrons from solid surface approaches unity in the limit of zero electron energy, see e.g. discussion in Ref. [1]. If this were indeed the case, this effect would have profound implications on the formation of electron clouds in particle accelerators [2], plasma measurements with electrostatic Langmuir probes, and operation of Hall plasma thrusters for spacecraft propulsion. It appears that, the proposed high electron reflectivity at low electron energies contradicts to numerous previous experimental studies of the secondary electron emission. We address possible causes of these contradictions.\\[4pt] [1] J. Cazaux, J. Appl. Phys. 111, 064903 (2012).\\[0pt] [2] R. Cimino, et al, Phys. Rev. Lett. 93 014801 (2004)   

Characterization of a 20 kW helicon source for fusion relevant plasma-surface interactions using microwave and electrostic diagnostics

The MAGnetized Plasma Interaction Experiment (MAGPIE) is a non-uniform axial magnetic field helicon source built to study fusion relevant plasma-surface interactions. In this work we describe its operation with a new 20 kW pulsed RF source in H$_{2}$ and H$_{\mathrm{e}}$ under various discharge configurations. Diagnostics such as RF double probes and a 140 GHz heterodyne Michelson microwave interferometer are used to characterize the performance of the device over a wide range of operational regimes. During initial characterization we have measured plasma densities in excess of 1x10$^{19}$ m$^{-3}$ in H$_{2}$ at 12 kW of RF power. Finally, we report on recent work conducted in MAGPIE in close collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO) and the Plasma Research Laboratory (PRL) where biased material samples are subjected to H$_{2}$ plasma. These samples are then analyzed ex-suti using a variety of material characterization techniques. Materials being investigated include graphite, diamond and tungsten.   

Combining a plasma gun and theta-pinch to study plasma-facing components under fusion-relevant conditions

A recent paper [D.N.Ruzic et al. Nuclear Fusion 51, 102002, 2011] on lithium-metal infused trenches shows that liquid lithium in metal trench flows by thermoelectric magneto-hydrodynamic force in such a way that it removes the heat flux and refreshes the surface with pure lithium. To further investigate behaviors of the liquid flow in a similar condition to extreme events in a tokamak, development of a high energy pulsed plasma simulator has been proposed. The new plasma simulator consists of mainly two components: a plasma gun to provide preionized high density plasma and directional momentum, and a theta pinch to magnetically heat up the plasma. The use of the theta pinch efficiently increases the ion temperature and creates similar conditions to an ELM in a tokamak. A gas-puff system has been developed to reduce neutral atom collisions with the hot plasma. Diagnostics include a triple Langmuir probe, an optical time-of-flight, a gridded ion energy analyzer and calorimetry. These quantitatively estimate plasma parameters to understand the effect of combining the plasma gun with the theta-pinch.   

Laser-based diagnostics for characterizing materials exposed to a plasma environment

To address the needs of fusion reactors, diagnostic techniques for plasma-material interactions (PMI) are being developed at ORNL. Laser-based diagnostic techniques (LBDT) will be used to both characterize the plasma environment and probe the material surface during plasma exposure. A Nd:YAG laser is needed for LBDT. Initial setup and diagnostic testing of the beam will be performed before installing it onto the ORNL device, PHISX (Prototype High Intensity Source Experiment). Installation of the Nd:YAG laser on PHISX, will enable Thomson Scattering (TS) measurements as well as Laser Induced Ablation/Breakdown/Desorption Spectroscopy (LIAS/LIBS/LIDS) to be performed \textit{in-situ} on material targets. The material targets can be further characterized \textit{ex-situ} by surface techniques available at ORNL; \textit{ex-situ} results will be compared to the \textit{in-situ} characterizations. This poster will show the initial setup and plans for LBDT on PHISX at ORNL.   

Effects of Electron Emission on Plasma-Surface Interaction

Most models of sheaths~facing emitting surfaces~invoke assumptions that~the sheath is time-independent,~the~wall potential is negative, ions enter~the~sheath at Bohm velocity, the presheath is weakly affected, and one wall is considered [1]. We present theory and PIC simulations showing that these assumptions can break down in practice. When emission is strong, the sheath potential can become positive, repelling ions from the wall [2,3]. Emitted electrons~entering the plasma~can drastically affect the presheath structure too. If their mean-free-path is large, emitted electrons can transit~the~plasma and~impact~the opposite wall; hence~wall charging becomes a complex global problem [4]. Secondary emission can trigger sheath instabilities~preventing plasma-wall systems from reaching steady state [5,6]. Implications are discussed for tokamaks, Hall thrusters, dusty plasmas, hot cathodes, RF discharges and spacecraft. [1] G.D. Hobbs and J.A. Wesson, Plasma Phys. 9, 85 (1967). [2] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. 108, 255001 (2012). [3] M.D. Campanell, submitted to Phys. Rev. E (2013). [4] M.D. Campanell and H. Wang, submitted to Appl. Phys. Lett. (2013). [5] M.D. Campanell, A.V. Khrabrov and I. D. Kaganovich, Phys. Rev. Lett. ~108, 235001 (2012). [6] M.D. Campanell et al., Phys. Plasmas 19, 123513 (2012).   

Recycling at tungsten wall and its impact on boundary plasma profile

High-Z refractory metals like tungsten, hydrogen absorbing metal like lithium, and carbon tiles represent three distinct choices in recycling properties. Tungsten divertor/first wall is considered a high recycling boundary, in contrast to the hydrogen absorbing lithium, by the usual definition of returned neutral versus incoming ion flux. Unlike carbon tile which is also high recycling, tungsten wall does not take in the ion heat flux as well in that most ions are reflected back to the plasma, keeping most of their kinetic energy. The inability of plasma ions to efficiently exhaust in particle and energy implies a high temperature boundary plasma. This leads to the peculiar scenario that tungsten wall tends to maintain high edge density like the carbon tiles, but retain a high ion temperature like the lithium surface. This is of course an unstable scenario, which must be resolved by either modifying the recycling at the surface, or exhausting the energy flux by other channels. Our calculations will (1) show the characteristics of particle and energy recycling at the tungsten surface, and (2) illustrate the effect of tungsten wall recycling on boundary plasmas, and the mitigation strategies. The issue of helium ions is a particular focus. {\em Work supported by OFES}   

Investigation of Vapor Cloud Effects on Fusion Plasma-Dust Interactions for High-Z Materials

Recent experimental and computational studies have shown that dust grains can have a significant impact on fusion tokamak operations. Formed by intermittent heat loads to plasma facing components (PFCs) and various other edge plasma-surface interactions, massive dust grains can penetrate deeply into the plasma where further heating generates plasma impurities by ablation. In extreme cases, dust ejection from PFCs has been experimentally shown to cause termination of the plasma discharge. Currently, the models used to simulate fusion plasma-dust interactions ignore the vapor cloud around each grain and its effects such as gas dynamic shielding as well as electrostatic shielding and radiative cooling due to the formation of a partially ionized secondary plasma. Previously, by studying these effects for the low-Z materials Li, Be, and C, it was found that the current models are applicable only for sufficiently small dust grains which produce vapor clouds of negligible density. In this work, we present and compare the same limits of applicability for grains composed of the high-Z materials Fe, Mo, and W. We also discuss the appearance of a bifurcation in the maximum allowable dust grain radius for Mo and W due to thermionic electron emission at high plasma density and temperature.   

Transport of Aluminum impurities in Helium Plasma

Impurity radiation losses at the edge of fusion devices are crucial for establishing detached divertor regimes in ITER and future tokamak reactors, despite the problem they cause in reducing plasma efficiency. Complex parallel and cross-field impurity transport suggest a rather fluid description when treating edge dynamics, leading somehow to marginal simulation results of the impurity transport problem. A kinetic description accounting for impurity/plasma collisions should be used instead, generating more details on the collision dynamics, while the relatively high mass difference between colliding particles leads to major simplifications in the physics of the problem. Modeling of Aluminum injection and entrainment into steady-state Helium plasma is presented. Multiple ionization and radial losses are included and numerical results are then compared to experimental data obtained from PISCES machine.   

Development of the diagnostic laser for deep UV probing of the dense Z-pinch

Lasers are powerful tools for investigation of dense plasmas. UV laser diagnostics at the wavelength of 266 nm were recently developed for Z-pinch plasma. The absorption and refraction in plasma are significantly smaller in the UV range that allows investigation of the fine structure of the stagnated dense Z-pinch. Further development of deep UV diagnostics needs in a laser with high energy, short pulse duration, and smooth beam profile. A Nd:glass laser meets these requirements if operates at the fourth, fifth and sixth harmonics. We have developed a Nd:glass laser for operation at 263 and 211nm with pulse durations of 150ps and 2ns, output aperture of 45mm, and energy \textgreater~1J at 263nm and \textgreater~0.2J at 211nm. The laser will allow the development of new deep UV diagnostics for dense Z-pinches: multiframe interferometry, tomography, streaked shadowgraphy, and high-resolution Faraday rotation diagnostics.   

Nike Facility Diagnostics and Data Acquisition System

The Nike laser-target facility is a 56-beam krypton fluoride system that can deliver 2 to 3 kJ of laser energy at 248 nm onto targets inside a two meter diameter vacuum chamber. Nike is used to study physics and technology issues related to laser direct-drive ICF fusion, including hydrodynamic and laser-plasma instabilities, material behavior at extreme pressures, and optical and x-ray diagnostics for laser-heated targets. A suite of laser and target diagnostics are fielded on the Nike facility, including high-speed, high-resolution x-ray and visible imaging cameras, spectrometers and photo-detectors. A centrally-controlled, distributed computerized data acquisition system provides robust data management and near real-time analysis feedback capability during target shots. Work supported by DOE/NNSA.   

A Magnetic Particle-Time-Of-Flight (MagPTOF) diagnostic for simultaneous measurements of shock- and compression bang-times at the NIF

A magnetic particle-time-of-flight (MagPTOF) diagnostic has been designed for simultaneous measurements of shock- and compression-bang time at the National Ignition Facility (NIF). This type of measurement combined with the measured shock-burn weighted $\rho$R will significantly constrain the modeling of the implosion dynamics. The MagPTOF design is an upgrade to the existing particle time-of-flight (pTOF) diagnostic, which has recorded accurate bang times in cryogenic DT implosions, DT exploding pushers and D$^3$He implosions with accuracy better than 70 ps. The inclusion of a deflecting magnet will increase proton signal-to-background by a factor of 1000, allowing for simultaneous measurements of shock- and compression-bang times in D$^3$He-filled surrogate implosions using D$^3$He protons and DD-neutrons, respectively. This work was supported in part by the U.S. DOE, NNSA, LLNL and LLE.   

A linear electrostatic accelerator for education and advanced diagnostics development for OMEGA and the NIF

The MIT Linear Electrostatic Accelerator generates D-D and D-3He fusion products, which are used for development of nuclear diagnostics for OMEGA and the NIF. Fusion reaction rates of about 10$^{6}$ s$^{-1}$ are routinely achieved, and fluence and energy of the fusion products have been accurately characterized. Diagnostics developed and calibrated at this facility include CR-39 based charged-particle spectrometers, neutron detectors, and the particle Time-Of-Flight (pTOF) CVD-diamond-based bang time detector. The accelerator is also a vital tool in the education of graduate and undergraduate students at MIT. This work was supported in part by SNL, DOE, LLE and LLNL.   

Single order X-ray diffraction with binary sinusoidal transmission grating

All existing x-ray dispersive devices including crystals, multilayers and diffraction gratings generate spectra in multiple orders, whereas soft x-ray spectroscopy applications usually require only the first order spectrum. The other diffraction orders can overlap and contaminate the first order spectrum of interest. In this letter we describe how an axis-symmetrically-distributed sinusoidal-shaped aperture with binary transmittance values can be used to disperse x-rays and with a superior diffraction pattern where, along its symmetry axis, all higher-order diffractions can be effectively suppressed. Hence this sophisticated dispersive element generates pure soft x-ray spectra in the first diffraction order, free from interference from higher diffraction orders.   

Absolute Calibration of the OMEGA Streaked Optical Pyrometer

High-energy-density-physics (HEDP) experiments often rely on temperature measurements using optical pyrometry. Laser-driven experiments have time scales of picoseconds, requiring the use of a streak camera as a detector. This complicates the already formidable task of absolute calibration. We report on multiple calibration runs that used a NIST-traceable tungsten-filament lamp to calibrate the optical response of the streaked optical pyrometer on OMEGA. This entailed constructing a spectral-response function from measurements and estimates of the transmissions and responses of all components in the system as well as measurements using narrowband (30-nm) optical filters. The latter is used to normalize the estimated response. The resulting response function predicts the wideband ($\sim $300-nm) response of the system to high precision. The performance of a spectral calibration device is also presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Extending crystal options in x-ray polarization splitting

Anisotropy in a plasma, as may be produced by some anisotropic heating mechanism like an electron beam possibly accelerated by a laser, can sometimes be inferred from the polarization of the plasma's x-rays. The polarization is the difference between two linearly polarized spectra. These are usually obtained with two diffracting crystals in two different locations, hence not necessarily from the same plasma. Interweaving the two crystals [1], as is possible when crystals have threefold symmetry, ensures that the two polarized spectra come from the same radiation source. This paper discusses how crystals of the right type could be used for polarization splitting even though they may not have been cut expressly for the purpose. With the proper mounting common high quality but low-cost crystals such as Si (111) can be used for polarization splitting, and even quartz crystals intended for polarization-spitting could be used with unanticipated photon energies in an asymmetric orientation.\\[4pt] [1] E O Baronova and M M Stepanenko, ``A novel x-ray polarimeter based on hexagonal crystal, for application to thermonuclear fusion experiments,'' Plasma Phys. Control. Fusion 45 1113 (2003)   

Magnetic Field Measurement in Magnetized Laser Plasmas Using Zeeman Broadening Diagnostics

The Zeeman effect has been used to measure the magnetic field in high energy density plasmas. The measurements are difficult when the field orientation is fluctuating in the plasma volume or when the line broadening due to the high plasma density and temperature surpasses the Zeeman splitting. Based on an idea proposed by Tessarin \textit{et al}. (2011), we implemented a solution to this problem to the field measurement in magnetized laser plasmas. High resolution spectra were obtained at the Nevada Terawatt Facility for plasmas created by 20 J, 400 fs Leopard laser pulses in the azimuthal magnetic field produced by the 0.6 MA Zebra pulsed power generator. The components of the Al III 3s $^{\mathrm{2}}$S$_{\mathrm{1/2}}$ -- 3p $^{\mathrm{2}}$P$_{\mathrm{1/2,3/2}}$ were recorded with space resolution along the direction normal to the target, which coincided with the magnetic field radius. In several shots, the spectra were time gated for 10 ns at different values of the magnetic field. In these measurements the Zeeman splitting was not resolved, but the magnetic field strength can be measured from the difference between the widths of the line profiles.   

Laser-Plasma Density and Temperature Measurements with Triple Langmuir Probes

Experiments to investigate shocks produced by the explosive expansion of a laser-plasma plume against a gas background were performed on the Titan laser (LLNL). Knowledge of density and temperature is essential for understanding the underlying processes. Triple Langmuir probes (TLP) were used for measuring these quantities as function of time at a given location in the plasma. In the experiment, laser ablation plasma from a carbon rod expanded in hydrogen, helium, or argon ambient gas. Density and temperature jumps in the TLP measurements can be correlated with shocks detected by interferometry and proton deflectometry.   

New Features in Nuclear Diagnostic Modeling Using HYDRA

New methods in HYDRA have been developed to allow more accurate and flexible modeling of nuclear reactions with a focus on measurements at the National Ignition Facility. Two developments are highlighted: radiochemistry and compound nuclei. Low probability nuclear reactions in an ICF capsule are best simulated using radiochemistry techniques. HYDRA now has both an inline and a post-processing capability, which uses the new code KUDU. Calculation of the 4.4 MeV $^{12}$C(n,$\gamma$n$^{\prime}$) $\gamma$ is shown to be greatly improved relative to an analog Monte Carlo calculation. This $\gamma$ measured along with the T(D,$\gamma$n) $\gamma$ in an ICF implosion provides a measurement of mix, areal density, and timing. HYDRA now also provides a facility to define the properties of a compound nucleus in a thermonuclear reaction. By using this new capability and recently measured $\gamma$ and neutron spectra to inform the $^{5}$He state, the simulation of T(D,n$\gamma$) and TT fusion reactions that share the intermediate $^{5}$He state has been significantly improved.   

On FAST3D simulations of directly-driven inertial-fusion targets with high-Z layers for reducing laser imprint and surface non-uniformity growth

Modifications to the FAST3D code have been made to enhance its ability to simulate the dynamics of plastic ICF targets with high-Z overcoats. This class of problems is challenging computationally due in part to plasma conditions that are not in a state of local thermodynamic equilibrium and to the presence of mixed computational cells containing more than one material. Recently, new opacity tables for gold, palladium and plastic have been generated with an improved version of the STA code [A. Bar-Shalom \textit{et al}., Phys Rev A, \textbf{40} (1989)]. These improved tables provide smoother, higher-fidelity opacity data over a wider range of temperature and density states than before, and contribute to a more accurate treatment of radiative transfer processes in FAST3D simulations. Furthermore, a new, more efficient subroutine known as ``MMEOS'' has been installed in the FAST3D code for determining pressure and temperature equilibrium conditions within cells containing multiple materials [M. Gittings \textit{et al}., Comput. Sci. Disc. \textbf{1}, 015005 (2008)]. We will discuss these topics, and present new simulation results for high-Z planar-target experiments performed recently on the NIKE Laser Facility.   

Comparisons of electronic transport properties computed via classical and quantum molecular dynamics

We have applied the ddcMD molecular dynamics (MD) code to the computation of the electrical conductivity and thermal conductivity of hydrogen plasmas at several points in phase space. Quantum mechanical effects on the electronic degrees of freedom are incorporated through the use of temperature-dependent statistical potentials. In order to examine the validity of this approach, we make comparisons with results from quantum MD simulations. We find that, while the electrical conductivities computed via classical MD are in reasonably good agreement with the quantum MD calculations, the thermal conductivity computed via classical MD is lower than the quantum MD result by a factor of 2-3. The Lorenz number determined from the classical MD is a factor of 2-3 lower than the Spitzer prediction. Similar discrepancies with Spitzer were also observed by Bernu and Hansen. LLNL-ABS-640881 This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC.   

Double P1 approximation to electron distribution function for purposes of computing non-local electron transport

In Spitzer Harm theory of thermal conductivity, a zero net electron current condition is imposed. For direct drive implosions, this represents a balance of the high-energy inflow current of laser driven electrons by the outflow of low energy electrons acting to reestablish charge neutrality. Previous formalisms\footnote{Schurtz et. al. Phys. Plasmas \textbf{7}, 4238 (2000).}$^,$\footnote{Manheimer et. al. Phys. Plasmas \textbf{15}, 083103 (2008).} have made use of a P1 expansion of the Fokker-Planck equation to create a diffusion model for electron thermal conduction. This work aims to formulate a double P1 expansion of the Fokker-Planck equation in order to take advantage of the strong correlation between electron energy and direction. Preliminary results of this model will be presented. This work was supported by the University of Rochester Laboratory for Laser Energetics.   

Numerically robust and efficient nonlocal electron transport in 2D DRACO simulations

An improved implicit algorithm\footnote{Private communications with M. Marinak and G. Zimmerman, LLNL.} based on Schurtz, Nicolai and Busquet (SNB) algorithm\footnote{Schurtz, Nicolai and Busquet, ``A nonlocal electron conduction model for multidimensional radiation hydrodynamics codes,'' Phys. Plasmas 7, 4238(2000).} for nonlocal electron transport is presented. Validation with direct drive shock timing experiments\footnote{T. Boehly, et. al., ``Multiple spherically converging shock waves in liquid deuterium,'' Phys. Plasmas 18, 092706(2011).} and verification with the Goncharov nonlocal model\footnote{V. Goncharov, et. al., ``Early stage of implosion in inertial confinement fusion: Shock timing and perturbation evolution,'' Phys. Plasmas 13, 012702(2006).} in 1D LILAC simulations demonstrate the viability of this efficient algorithm for producing 2D lagrangian radiation hydrodynamics direct drive simulations. Additionally, simulations provide strong incentive to further modify key parameters within the SNB theory, namely the ``mean free path.'' An example 2D polar drive simulation to study 2D effects of the nonlocal flux as well as mean free path modifications will also be presented. This research was supported by the University of Rochester Laboratory for Laser Energetics.   

Simulation of Ge Dopant Emission in Indirect-Drive ICF Implosion Experiments

We present results from simulations performed to study the radiative properties of dopants used in inertial confinement fusion indirect-drive capsule implosion experiments on NIF. In Rev5 NIF ignition capsules, a Ge dopant is added to an inner region of the CH ablator to absorb hohlraum x-ray preheat. Spectrally resolved emission from ablator dopants can be used to study the degree of mixing of ablator material into the ignition hot spot. Here, we study the atomic processes that affect the radiative characteristics of these elements using a set of simulation tools to first estimate the evolution of plasma conditions in the compressed target, and then to compute the atomic kinetics of the dopant and the resultant radiative emission. Using estimates of temperature and density profiles predicted by radiation-hydrodynamics simulations, we set up simple 2-D plasma grids where we allow dopant material to be embedded in the fuel, and perform multi-dimensional collisional-radiative simulations using SPECT3D to compute non-LTE atomic level populations and spectral signatures from the dopant. Recently improved Stark-broadened line shape modeling for Ge K-shell lines has been included. The goal is to study the radiative and atomic processes that affect the emergent spectra, including the effects of inner-shell photoabsorption and K$\alpha $ reemission from the dopant.   

Implicit XMHD Modeling of Fast Z-Pinches

The numerical modeling of fast Z-Pinches as applied to magnetically driven inertial confinement fusion concepts is typically performed under the resistive- magnetohydrodynamic (MHD) model. We derive the limitations of this model as currently applied to modeling such targets and present numerical test problems that demonstrate the physical error introduced through the approximations inherent in resistive-MHD. We then compare the resistive-MHD model to simulations utilizing new implicit algorithms for the efficient solution of the extended-magnetohydrodynamic (XMHD) system of equations. Herein we define XMHD as a quasi-neutral electro-magnetic two-fluid model. We present specific examples where the XMHD system of equations is required for modeling magnetically driven ICF targets if large physical errors are to be avoided in the numerical solution of the system. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Incorporation of a Truncated Azimuthal Mode Expansion into the OSIRIS PIC Simulation

Here we incorporated a new algorithm into the OSIRIS simulation framework. The electromagnetic fields are solved using a truncated Fourier expansion along the poloidal direction in cylindrical coordinates. This provides a three-dimensional simulation of a physical system with a low-modal azimuthal symmetry while maintaining a computational load similar to two-dimensional simulations. Many problems involving laser or beam-driven plasma wakefield accelerators fit this description. Preliminary results on LWFAs and PWFAs using this computational method will be provided.   

A base-line model for direct-drive ICF implosions in the xRAGE code

xRAGE is a radiation-hydrodynamics code using a Godunov solver on an Eulerian mesh with an adaptive mesh refinement (AMR) algorithm, and a radiation diffusion algorithm. It has been used to study fluid flow in highly distorted systems, where arbitrary Langrian Eulerian (ALE) methods are not the method of choice, which can include ICF. A version of the code, called CASSIO, uses an implicit Monte Carlo (IMC) method for radiation transport. However, specific physics packages relevant to ICF have not been available in the past, and which include laser propagation, three-temperature plasma physics and non-LTE opacity calculations. As these physics packages become available and undergo testing, a suite of validation problems is being developed to test the code under conditions relevant to ICF. The direct-drive ICF capsules fielded for the High-Z project [2] will be used as the initial suite of validation problems. This presentation will discuss the capsule experiments and the physics used in the modeling, as well as a brief overview of the software framework used to standardize the verification and validation process.\\[4pt] [1] M. Gittings, R. Weaver, M. Clover, et al., Comp. Sci. and Disc., 1 015005 (2008).\\[0pt] [2] E. S. Dodd, J. F. Benage, G. A. Kyrala, et al., Phys. Plasmas, 19 042703 (2012).   

Spectroscopic and x-ray scattering models in SPECT3D

Spectrally resolved x-ray scattering has become a very effective method for diagnosing electron temperatures, densities, and average ionization in warm dense matter. We present a newly implemented capability to simulate scattering signatures from realistic experimental configurations, which include the influence of plasma non-uniformities and collecting scattered x-rays from a range of angles. The method is based on a formalism developed by G. Gregori. The x-ray scattering modeling has been added to the multi-dimensional collisional-radiative spectral and imaging package SPECT3D. The ability to simulate the emissivity and attenuation of scattered photons within a multi-dimensional multi-volume-element plasma with non-uniform temperature and density distributions adds a major new capability to existing model. We will discuss details of the modeling and show results relevant to ongoing experimental investigations at Sandia National Laboratories.   

Latest Developments to the FLASH Laser Energy Deposition Package

We describe recent improvements to the FLASH laser energy deposition package. FLASH is an open source, compressible, spatially-adaptive, radiation hydro/MHD code based on an Eulerian AMR grid. Laser energy deposition is modeled using geometric optics ray-tracing algorithms and the inverse-Bremsstrahlung process. A large variety of options exist for users, which allows for a flexible setup of the laser. Several domain geometries are possible (1D, 2D cartesian and cylindrical, 3D in 2D ray-tracing, 3D cartesian) and several beam cross-sections are available (ray placements on square, radial or statistical grids). The original treatment is based on the Kaiser algorithm, which represents the electron number density as a cell-by-cell, piece-wise linear continuous function. We have added a second option that uses cubic interpolation of the electron number density, resulting in a smoother distribution of the energy deposition. We have also improved the computational performance of the package through threading and asynchronous communication when rays cross a block boundary. We present the results of performance and verification tests of the improved package.   

Predictive radiation-MHD simulations with FLASH: Magnetic field generation and turbulent amplification experiments with the Omega EP laser

The process of generation and amplification of Biermann battery magnetic fields is closely linked to the development of turbulence. In an astrophysical environment, a small seed field can be formed in asymmetric supernova remnant blast waves due to misaligned pressure and density gradients. Inhomogeneities in the density distribution can cause the flow to become turbulent and the B-field can be amplified via dynamo action. In this context, the COSMOLAB team will perform experiments using the Omega EP laser at LLE, that represent a scaled-down model of the astrophysical process in a controlled environment. The experiments involve the illumination of a slab-like target, which produces a plasma flow and a Biermann battery field. The flow then propagates through a grid that creates turbulence and amplifies the field. In this study we describe 2D and 3D radiative MHD simulations of the experimental setup, carried out using the FLASH code on Mira (BG/Q) at ALCF. The objective of these simulations is to explore the morphology and strength of the B-fields generated by ablation of target material by the laser, and their amplification due to the grid.   

Validity and Numerical Implementation of the 13 Moment Multi-Fluid Plasma Model

Fluid-based plasma models have typically been applied to parameter regimes where a local thermal equilibrium is assumed. While this parameter regime is valid for most fusion and confinement applications, it begins to fail as plasmas near the collisionless regime and kinetic effects dominate the physics. To avoid costly kinetic calculations, the validity of the fluid regime is expanded using an anisotropic 13 moment fluid model derived from the Pearson type-IV probability distribution function. This model evolves the heat flux vector in addition to the density, momentum, and energy, and the plasma species are coupled through Lorentz forces, Maxwell's equations, and collision operators. For this study, Maxwell's equations utilize a parabolic cleaning method to locally remove divergence errors in the electric and magnetic fields arising from the numerical scheme. The full multi-fluid plasma model is tested against the generalized Brio-Wu electromagnetic shock problem for various scalings of the Debye length and Larmor radius. The physical model is implemented using a hybrid CENO finite volume method for unstructured meshes developed within the University of Washington's WARPM (Washington Approximate Riemann Plasma) framework for use on heterogeneous GPU clusters.   

Advanced kinetic plasma model implementation for new large-scale investigations

A kinetic plasma model for of one or more particle species described by the Vlasov equation and coupled to fully dynamic electromagnetic forces is presented. The model is implemented as evolving continuous PDF (probability density function) in particle phase space (position-velocity) as opposed to particle-in-cell (PIC) methods which discretely sample the PDF. A new boundary condition for the truncated velocity-space edge, motivated by physical properties of the PDF tail, is introduced. The hyperbolic model is evolved using the discontinuous Galerkin numerical method, conserving system mass, momentum, and energy -- an advantage compared to PIC. Simulations of two- to six-dimensional phase space are computationally expensive. To maximize performance and scaling to large simulations, a new framework, WARPM, has been developed for many-core (e.g.~GPU) computing architectures. WARPM supports both multi-fluid and continuum kinetic plasma models as coupled hyperbolic systems with nearest neighbor predictable communication. Exemplary physics results and computational performance are presented.   

Blended Finite Element Method for Multi-Fluid Plasma Modeling

Taking moments of the Boltzmann equation provides equations that govern the evolution of the moments of the distribution function. Generalizing the moment approach to include an arbitrary number of species yields the multi-fluid plasma model. The multi-fuid plasma model is implemented using a blended finite element method (BFEM) through the simultaneous use of continuous and discontinuous Galerkin (CG and DG) spatial discretizations to represent the plasma fluids and electromagnetic fields. The electron fluid and electromagnetic fields are modeled using a nodal CG method while the ion fluids are modeled using modal DG. The approach uses the shock capturing capability of the DG method where appropriate and the computationally efficient CG method for variables that are unlikely to form discontinuities. The variables using the CG representation are associated with the fastest dynamics. To reduce the time step restrictions the CG formulation is integrated in time using an implicit method while the DG time integration is accomplished using a 2nd, 3rd or 4th order Runge-Kutta scheme. The BFEM is implemented into the WARPX (Washington Approximate Riemann Plasma) code framework and is used to study species separation and fuel mixing during ICF implosions of DT capsules.   

Vlasov-Fokker-Planck modeling of High Energy Density Plasmas

Vlasov-Fokker-Planck simulations can be applied to a wide variety of problems in High-Energy-Density Plasmas. They can be used with an explicit solver to study the physics of waves in plasma media, including Landau Damping, echoes, instabilities etc., just like standard Vlasov codes. Moreover, they allow us to study the effect of collisions on these kinetic phenomena. On the other had, using an implicit solver, they enable kinetic simulations of realistic temporal and spatial scales. Recent simulations with the VFP code OSHUN [1] will be presented for all of the aforementioned problems. The algorithmic improvements that have facilitated these studies will be also be discussed.\\[4pt] [1] M. Tzoufras, A.R. Bell, P.A. Norreys, F.S. Tsung, JCP 230 (17), 6475-6494 (2011); M. Tzoufras, A. Tableman, F.S. Tsung, W.B. Mori, A.R. Bell, Phys. Plasmas 20, 056303 (2013)   

Vlasov-Fokker-Planck modeling of plasma near hohlraum walls heated with nanosecond laser pulses calculated using the ray tracing equations

Here, we present 2D numerical modeling of near critical density plasma using a fully implicit Vlasov-Fokker-Planck code, IMPACTA, which includes self-consistent magnetic fields as well as anisotropic electron pressure terms in the expansion of the distribution function, as well as an implementation of the Boris CYLRAD algorithm through a ray tracing add-on package. This allows to model inverse brehmsstrahlung heating as a laser travels through a plasma by solving the ray tracing equations. Generated magnetic fields (eg. the Biermann battery effect) as well as field advection through heat fluxes from the laser heating is shown. Additionally, perturbations in the plasma density profile arise as a result of the high pressures and flows in the plasma. These perturbations in the plasma density affect the path of the laser traveling through the plasma and modify the heating profile accordingly. The interplay between these effects is discussed in this study.   

A numerical investigation of Landau damping by bounce-resonant particles with initial-condition effects

There is very little damping by particles which are velocity-resonant with an electrostatic wave in a nonneutral plasma because they remain in resonance only until they bounce off the end. The time spent in turning around dephases the particle from the wave. Bounce-resonant particles, on the other hand, which have a higher midplane velocity which compensates for the turn-around time, remain coherent with the wave in an average sense and cause damping of the wave.\footnote{M. E. Koepke, Bull. Am. Phys. Soc., 49, 40 (2004).} We have built a Particle-In-Cell (PIC) simulation that models a damped wave in a nonneutral plasma. In this simulation we can cut off the distribution function at an arbitrary velocity. As the cut-off velocity is passed through the resonant velocity, the change in plasma behavior demonstrates the effect of that group of plasma particles on the damping of the wave. Certain particles change from damping to anti-damping as they change their phase relative to the wave in the resonant region. The frequency of the wave changes by about 2\% as the cut-off velocity passes through the resonance, much larger than expected from the change in the charge in the plasma. These and similar effects will be discussed.   

Theory and Simulation of Axisymmetric Radial Bernstein Modes in a Finite-Length Plasma

We have developed an r-z particle-in-cell code to investigate the behavior of nearly-self-shielding axisymmetric radial Bernstein modes in a finite length plasma in a cylindrical Malmberg-Penning trap. This code allows us to examine both the structure and the frequency shift caused by the finite length as compared with an infinite length cylindrical plasma. The code also allows us to evaluate the possibility of exciting and detecting these modes in an experiment. The simulation results will be compared to a finite length kinetic theory.   

Empirical model for low-frequency asymmetry-induced transport

We are currently developing an empirical model of asymmetry-induced transport in our non-neutral plasma trap with an eye toward providing guidance for further theoretical development. Our previous efforts\footnote{D.~L. Eggleston, Phys. Plasmas {\bf 17}, 042304 (2010).} have focused on radii where the asymmetry frequency $f$ matches the local $\bf{E}\times\bf{B}$ plasma rotation frequency $f_R$. We now study the radial particle flux $\Gamma$ produced by frequencies below $f_R$. The flux produced by these frequencies is typically largest at the outer edge of the plasma, $r/R\ge 0.75$, where $R$ is the wall radius. The data support an empirical model $\Gamma(r)\propto\exp{[-(f_0-f)/f_*]}$. Both of the parameters $f_0$ and $f_*$ are proportional to $\phi_{cw}/B$, where $\phi_{cw}$ is the bias of our central wire electrode and $B$ is the axial magnetic field. This scaling suggests a relation with $f_R$ or its derivatives. If we assume the former, then $f_0\approx 1.5f_R$ and $f_*\approx f_R/3$. This model is consistent with empirical constraints obtained\footnote{Eggleston 042304} near the $f=f_R$ points. The physical basis for this model, however, remains to be found.   

A Novel Damping Mechanism for Diocotron Modes

Recent experiments with pure electron plasmas in a Malmberg-Penning trap have observed the algebraic damping of $m=2$ diocotron modes.\footnote{A.A. Kabantsev, T.M. O'Neil and C.F. Driscoll, NNP 2012 Workshop, Greifswald; AIP Conf. Proc. \textbf{1521}, 35-42 (2013).} Transport due to small field asymmetries produces a low density halo of electrons moving radially outward from the central plasma core, and the mode damping begins when the halo reaches the resonant radius of the mode [i.e., where $\omega=m\omega_{E\times B}(r)$]. Here we propose a theoretical explanation for the damping using conservation of canonical angular momentum. As the wave sweeps halo electrons across the resonant layer, it imparts canonical angular momentum to the electrons, and in response the wave angular momentum decreases, that is, the wave damps. An alternative more mechanistic picture of the process is that the electrons in the resonant layer form a quadrupole density distribution, and the electric field from this quadrupole produces $\mathbf{E}\times\mathbf{B}$ drift motion that symmetrizes the surface of the plasma core, that is, damps the mode.   

Measurements of Plasma Wave Decay to Longer Wavelengths

We measure the decay of plasma waves to longer wavelengths, for both ``standard'' Langmuir waves with $\mathrm{v}_{\mathrm{phase}} \gg \overline{\mathrm{v}}$, and for the lower phase velocity ``EAW'' modes with $\mathrm{v}_{\mathrm{phase}} \sim \overline{\mathrm{v}}$. These are $\theta$-symmetric standing modes on pure ion or pure electron plasma columns with discrete wavenumbers $k_z = m_z ( \pi / L_p )$. A large amplitude $m_z \!=\!2$ Langmuir wave causes phase-locked exponential growth of the $m_z \!=\!1$ wave when they are near resonant, at growth rates $\Gamma_e \propto \delta n_2 /n_0$ consistent with cold fluid theory. For larger detuning $\Delta \omega\equiv 2\omega_1 -\omega_2$, mode amplitude $A_1$ is observed to ``bounce'' at rate $\Delta\omega$, with amplitude excursions $\Delta A_1\propto\delta n_2/n_0$ also consistent with cold fluid theory; but $A_1$ often exhibits a slower overall {\it growth}, as yet unexplained by theory. In contrast, a large amplitude $m_z\! =\!2$ EAW mode generally causes either strong phase-locked $m_z \!=\!1$ growth or no growth at all, apparently because the EAW ``frequency fungibility'' enables $\Delta\omega =0$, and EAW mode damping is strong until the velocity distribution $F (\mathrm{v}_{\mathrm{phase}} )$ is ``flattened.''   

Weak Nonlinearity Effects in TG and EAW Modes

We have studied the nonlinear coupling of Trivelpiece-Gould modes as well as EAW modes, in a cylindrically symmetric plasma with average density $n_0$ and periodic boundary conditions at the axial ends of plasma. For Trivelpiece-Gould modes, the cold fluid formalism gives the slow time evolution of mode amplitudes due to nonlinear couplings. For EAW modes, the Vlasov-Poisson formalism is required. We analyze the coupling between mode $m_z=2$ with frequency $\omega_2$ and mode $m_z=1$ with frequency $\omega_1$, with initial density perturbations $n_2(0)\gg n_1(0)$. For small detuning $\Delta\omega\equiv2\omega_1-\omega_2\ll\omega_1 n_2(0)/n_0$, mode amplitude $n_1$ grows exponentially in time due to resonant parametric interaction with mode $m_z=2$, at a rate $\Gamma$ which is linearly propotional to $n_2(0)$. For $\Delta\omega\gg\omega_1 n_2(0)/n_0$, mode amplitude $n_1$ oscillates about its initial value, with frequency $\Delta\omega$ and amplitude $ n^{(2)}_1\propto n_2(0) n_1(0)$. In both cases the theory is consistent with experiments.   

Structure of Two-Dimensional Plasma Crystals in Anharmonic Penning Traps

In several recent experiments charged particles have been trapped and cooled in two-dimensional (2D) crystalline configurations using a Penning trap. Usually in such traps the applied trap potential is harmonic (i.e. depending quadratically on position), and consequently the 2D crystal structure is nonuniform and riddled with defects. This poster derives a closed-form analytic expression for the density per unit area of the 2D crystal when an arbitrary anharmonic trap potential is employed, expressed as a multipole expansion. This expression is used to find the optimum potential, with a given number of multipoles, for trapping a plasma crystal with the most uniform possible density per unit area. Image charge effects are included to lowest order in (plasma size)/(electrode radius). Minimum energy states in such an optimized trap potential (including only quadrupole and octopole terms) are evaluated numerically and the resulting crystals are shown to be defect-free over the central region where the density is most nearly uniform. The poster also explores using an $l=3$ rotating wall trap potential in order to produce near-perfect crystals with triangular boundaries and no defects.\footnote{D.H.E. Dubin, Phys Rev A 88, 013403 (2013).}   

Cyclotron Mode Frequency Shifts in Multi-Species Ion Plasmas

We observe cyclotron modes varying as $\cos \left( {m_{\theta } \theta -\omega_{m} t} \right)$ on laser-diagnosed multi-species ion plasma columns. These mode frequencies are shifted away from the ``bare'' cyclotron frequency $\Omega_{s} =\left( {q_{s} /M_{s} } \right)B$ for each species s, due to electric fields and plasma rotation. For radially uniform ion species, we observe resonances at frequencies $\omega_{s} =\Omega_{s} +\left[ {m_{\theta } -2+\delta_{s} \left( {1-R_{m} } \right)} \right]\omega _{ExB} $, in close agreement with theory [1]. Here, $\omega_{ExB} $ is the measured ExB rotation frequency, $\delta_{s} $ is the relative fraction of the resonant species, and $R_{m} $ is a small image charge correction. These frequency shifts are measured on the $m_{\theta } =1$ center-of-mass mode, on the $m_{\theta } =2$ ``elliptical'' mode, and on the novel $m_{\theta } =0$ radial ``breathing'' mode. Absent laser diagnostics, these mode frequencies could be used to estimate the plasma composition $\delta_{s} $ and rotation $\omega_{ExB} $. When the plasma is laser cooled to $10^{-5}10^{-1}$ eV, the cyclotron resonances develop a multiple peak structure with temperature-dependent frequency spacing, and a connection to radially standing Bernstein modes [1] will be pursued. \\[4pt] [1] D.H.E. Dubin, Phys. Plasmas \textbf{20}, 042120 (2013).   

Temperature Control in Radiatively Cooled Plasmas through Autoresonant Drive of TG-waves

We demonstrate accurate temperature control of pure electron plasmas, using driven wave heating ``autoresonantly'' in balance with cyclotron cooling. The $m_{\theta } =0$ Trivelpiece-Gould wave frequencies are temperature-dependent, as$f_{TG} (T)=f_{TG} (0)\ast [1+\varepsilon T]$; and they exhibit a narrow Lorentzian absorption response $R(f)$ with width $\gamma \sim 10^{-3}f_{TG} $. A continuous drive amplitude $A_{dr} $ then produces plasma heating power $P_{h} \propto A_{dr}^{2} R(f_{dr} )$, which can exactly balance the cyclotron cooling power$P_{c} \propto T \mathord{\left/ {\vphantom {T {\tau_{c} }}} \right. \kern-\nulldelimiterspace} {\tau_{c} }$. This balance point is autoresonantly \textit{stable} when $f_{dr} \approx f_{TG} (T)-\gamma $: if $T$ increases, then $f_{TG} (T)$ also increases and $f_{dr} $ gets further from resonance, so the heating power decreases and $T$ decreases back to the balance point. (The second power-balance point at $f_{dr} \approx f_{TG} (T)+\gamma $ is \textit{unstable}.) In practice, we use a $m_{z} =3$ TG wave having frequency range $5.2 [Preview Abstract]

Recent Experiments with Toroidal Pure-Electron Plasma -- Trivelpiece-Gould Modes, Diocotron Mode Damping, and Asymmetry Modes in the Lawrence Non-neutral Torus II

Electron plasma is confined using a purely toroidal magnetic field ($R_{o}=$ 18 cm, $B$ \textless\ 550 G) for times ($\sim$ 1 s) that are much longer than any of the dynamical timescales of the system. The Lawrence Non-Neutral Torus II (LNT II) can be operated as a partial torus in which plasma is confined in C-shaped toroidal sectors or as a fully toroidal, closed field trap. We present results of recent wave excitation and damping studies in LNT II, including the first observations of Trivelpiece-Gould waves in toroidal electron plasma, measurements of the dependence of the $m=$1 diocotron mode damping rate on magnetic field, plasma position, and neutral pressure, and first observations of asymmetry modes in a toroidal electron plasma. Efforts are also underway to enhance the magnetic field strength and augment the diagnostic capabilities of the apparatus. This work is supported by the National Science Foundation -- Award {\#}1202540.   

Antihydrogen trapping assisted by sympathetically cooled positrons

Antihydrogen formed by carefully merging cold plasmas of positrons and antiprotons has recently been trapped in magnetic traps. The efficiency of trapping is strongly influenced by the temperature of the nascent antihydrogen, which, to be trapped, much have a kinetic energy less than the trap depth of $\sim$0.5~K$\cdot k_B$. In the conditions in the ALPHA experiment the antihydrogen temperature seems dominated by the temperature of the positron plasma used for the synthesis. Cold positrons are therefore of paramount interest in these experiments. In this article we investigate an alternative route to make ultra-cold positrons for enhanced antihydrogen trapping. We propose to sympathetically cool the positrons by merging them with laser-cooled positive ions. We investigate the effectiveness of such cooling in conditions similar to those in ALPHA, and discuss how the formation of and the nascent antihydrogen may be influenced by the presence of positive ions. We argue that this scheme is a viable alternative to schemes such as evaporative and adiabatic cooling, and may overcome limitations faced by these schemes.   

Aperture-Based Antihydrogen Gravity Experiment: Parallel Plate Geometry

A Monte Carlo simulation is presented of an experiment that would indicate whether antihydrogen falls up or down in earth's gravitational field. The work is the third iteration of ongoing research to reduce the experimental run time that would be necessary for an aperture-based experiment at the CERN Antiproton Decelerator facility. The configuration consists of two circular, parallel plates separated by a small vertical distance with an axis of symmetry directed away from the center of earth. There are one or more pairs of circular barriers that protrude from the upper and lower plates, thereby forming an aperture. The probability that an antiatom will annihilate within a ``shadow region'' on the upper or lower plate is determined for a point, line and spheroidal source of antihydrogen. Such annihilations would indicate the direction of the acceleration of antihydrogen due to gravity.   

Plasma Physics Experiments in Support of Antihydrogen Trapping in ALPHA

Spectroscopic studies of antihydrogen in ALPHA depend on the reliable production of antihydrogen atoms in quantities large enough to achieve the necessary statistics for precision studies. The efficient production of anti-hydrogen requires the simultaneous trapping of antiproton and positron populations under excellent vacuum conditions with high densities and low temperatures. Presently, we report on recent experiments designed to diagnose and control these three critical elements: (1) A new vacuum diagnostic has been developed in which excited electron plasmas ionize the background gas; the resulting plasma can then be diagnosed to identify the original composition of the gas. (2) Experiments have been performed to characterize plasma compression using electrostatic rotating wall boundary conditions, so that this technique can be optimized for the production of high density plasmas. And (3), a new Lithium ion source has been installed in order to mimic the behavior of antiprotons. This can be used to develop and optimize techniques for the cooling of multi-species plasmas.   

Cryogenic Positron-beam System for Atomic Physics

Trapped positron plasmas are routinely used to generate beams that can be used for a wide variety of experiments, such as studies of positron annihilation on molecules.\footnote{Gribakin, Young, and Surko, {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010).} While current beam generation techniques are sufficient, for example, for the measurement of positron-molecule binding energies, more detailed studies are limited by beam energy resolution. Described here is a new method of positron beam formation using a buffer gas cryogenically cooled to 50 K. Simulations of the beam formation process\footnote{Natisin, {\it et. al.}, AIP Conf. Proc. {\bf 1521}, 154 (2013).} are discussed and used to predict an energy resolution of $\approx 9$ meV FWHM; a factor of 5 improvement over current techniques. Various possible physical measurements using this technique are discussed, including the ability to resolve individual vibrational mode features in the molecular annihilation spectra.\footnote{Jones, {\it et. al.}, {\it Phys. Rev. Lett.} {\bf 110}, 223201 (2013).}   

Trapping and Storage of Non-neutral Plasmas in an Off-axis Penning-Malmberg Trap

There are many potential applications of high-capacity and/or portable antimatter traps, including multiplexing the output of high-flux positron beams, study of electron-positron plasmas, and eventually the construction of an annihilation gamma-ray laser at 0.51 MeV. We describe the details of a new experiment to test and validate several aspects of the multicell Penning-Malmberg (PM) trap for non-neutral plasma storage.\footnote{Danielson, Weber, Surko, {\it Phys. Plasmas} {\bf 13}, 123502 (2006).}$^,$\footnote{Danielson, Hurst, Surko, AIP Conf. Proc. {\bf 1521}, 101 (2013).} Progress has been made in several key areas, including the successful trapping of plasmas in off-axis cells and confinement without particle loss for hours. Details of the trapping process will be presented, as will the first studies of plasma lifetime in the off-axis cells. The effect on confinement due to using electrodes with different diameters is also investigated. Future plans to study the confinement and stability of plasmas with kilovolt levels of space charge will also be discussed.   

Design and Construction of a 21-cell Multicell Trap for Positron Storage

There are many potential applications of high-capacity and/or portable antimatter traps. We describe the construction (in progress) of a novel multicell Penning-Malmberg (PM) trap designed to store up to $10^{12}$ positrons.\footnote{Danielson, Weber, Surko, {\it Phys. Plasmas} {\bf 13}, 123502 (2006).}$^,$\footnote{Danielson, Hurst, Surko, AIP Conf. Proc. {\bf 1521}, 101 (2013).} The device consists of 21 PM cells (in three banks of 7 cells) within a UHV vacuum system and a 140~mm diameter warm-bore, 5 tesla, magnet. Each cell will use kV confinement potentials and have an azimuthally segmented electrode for diagnostics and plasma manipulation, such as the application of rotating electric fields. An independent, large-diameter master cell will be used to move plasmas, received from a buffer-gas positron accumulator, across the magnetic field to the off-axis cells using autoresonant diocotron-mode excitation.\footnote{Fajans, Gilson, Friedland, {\it Phys. Rev. Lett.} {\bf 82}, 4444 (1999).} Details of the current design will be presented, as well as scenarios for effective extraction and use of the trapped particles.   

Positron accumulation for the fueling of matter-antimatter pair plasmas

Pair plasmas have been a topic of theoretical and astrophysical interest for decades. APEX (A Positron-Electron Experiment) aims to create them in the laboratory by building upon recent advancements in several areas: devices that can magnetically confine both quasi-neutral and non-neutral plasmas, the strength of positron beams, and the manipulation of non-neutral plasmas. This last is the focus of the Positron Accumulation Experiment (PAX). PAX will provide a necessary bridge between the source of APEX's positrons (Neutron-Induced Positron Source Munich) and the magnetic confinement device for the pair plasma. PAX will cool and store incoming positrons over a 10-15 minute period, thereby accumulating them at sufficiently low energies and in sufficiently high numbers that the resulting plasma will extend at least 10 Debye lengths. The current status of PAX's construction and tests will be presented.   

Fokker-Planck simulation of positron and antiproton equilibration in antihydrogen production

Antihydrogen atoms are produced in the ALPHA experiment by autoresonantly exciting the axial oscillation of antiprotons confined adjacent to a positron plasma in a nested Penning--Malmberg trap. After gaining enough energy, the antiprotons overcome the electrostatic barrier and enter the positron plasma. Antihydrogen is then formed by a three--body combination process. This poster presents the first results of simulating the temperature re-equilibration between injected antiprotons and positrons, where the initial velocity distribution for the antiprotons is obtained from our Vlasov solver that models the injection process[1]. The frictional and diffusive coefficients in the Fokker-Planck equation are obtained through a Monte-Carlo simulation of collisions between and within the antiproton and positron distributions in the intermediate magnetized limit. \\[4pt] [1] C. Amole et. al, Phys Plasma 20, 043510 (2013)