Session TP8: Poster Session VII: DIII-D II; Divertor, Heating and Current Drive; Low Temperature Plasmas; Z-pinches, X-pinches, and Dense Plasma Focus

Velocimetry Analysis of 2D Turbulence Imaging Data from Beam Emission Spectroscopy

The time-resolved velocity field of density fluctuations contains pertinent dynamics on critical features of plasma turbulence, including zonal flow and geodesic acoustic mode behavior, and the $E\times B$ motion from underlying electrostatic potential fluctuations. This velocity field may be used to infer the Reynolds stress ($d/dr$), thought to drive zonal flows, and the radial turbulent particle flux. The beam emission spectroscopy (BES) system on DIII-D obtains radially and poloidally resolved images (up to 8x8) of density fluctuations at $\sim$1 cm resolution. The orthogonal dynamic programming method, developed in fluid dynamics, is used to extract the 2D flow field, $v(r,z,t)$ from the fluctuating densities. A vector-matching method determines structure motion from one frame to another. The technique will also be applied to density data from nonlinear GYRO simulations of turbulence, using an appropriate BES synthetic diagnostic. The inferred 2D density fluctuation velocity field will be compared with the $E\times B$ fluctuations also calculated from GYRO to discern their relationship and wavenumber sensitivity.   

Turbulent Ion Temperature Fluctuation Measurements on DIII-D

A novel dual-channel, high throughput, high efficiency, charge exchange spectrometer (UF-CHERS) has been developed to measure impurity ion temperature and toroidal velocity fluctuations with 1 $\mu$s time resolution. These measurements are primarily needed for identifying underlying instabilities and validation of transport simulation codes. Based on the measured photon flux levels for the entire spectral line, a photon noise floor $\sim$1\% is expected. Statistical averaging over long data records should provide turbulence measurements to 1/5 to 1/10 of the estimated photon noise floor. Correlation measurements in DIII-D plasmas demonstrate broadband ion-temperature fluctuations from 0-150 kHz, while cross-correlation with BES measurements of density fluctuations exhibits cross-power between density and ion temperature fluctuations to 250 kHz in ECH-heated low-collisionality L-mode discharges. The fluctuation level is $\tilde{T}_i/T_i \sim 2$\%, with $\langle \tilde{n}\tilde{T}_i\rangle$ correlation lengths in the range of 3-5 cm. A new detector system will provide improved signal-to-noise level and higher frequency measurements.   

Probing Electron Temperature Critical Gradients in Experiment and Simulation

In DIII-D, localized electron cyclotron heating (ECH) is used to probe the critical gradient of, and onset of stiffness in, the electron temperature $T_e$ profile. While keeping the total injected ECH power constant, the deposition profile was varied to investigate the relationship between the $T_e$ gradient and the electron power balance heat flux. A critical temperature gradient was observed, above which both the heat diffusivity and the $T_e$ fluctuations increase sharply. To compare to modeling, efforts have been made to produce the most realistic equilibrium reconstructions by using kinetic pressure constraints and motional Stark effect measurements of the local magnetic pitch. With these reconstructions, the gyrokinetic stability codes GYRO and TGLF predict that there is a critical $T_e$ gradient, similar to the experimentally observed gradient, above which electron modes exist and whose growth rates dominate over the ion growth rates.   

The Role of Zonal Flows and Predator-Prey Oscillations in Triggering the \hbox{L-H} Transition and in Internal Transport Barriers

Low frequency Zonal Flows (ZFs) have been observed to trigger the \hbox{L-H} transition near the power threshold, by either an extended predator-prey limit cycle oscillation (LCO [1]) or a short ($\sim 0.5-1.5$~ms) ZF burst executing only part of one limit cycle. Localized turbulence suppression ($k_\theta \rho_s\sim 0.5$) is initiated as the ZF shearing rate approaches the turbulence decorrelation rate. Turbulence-flow correlations (via Doppler Backscattering) show that the ZF amplitude and shear initially lag the rms fluctuation level by 90$^\circ$ during LCO, transitioning to 180$^\circ$ as the increasing ion pressure gradient and resulting equilibrium {\bf E}x{\bf B} shear secure the final transition to ELM-free \hbox{H-mode}. In a separate experiment, localized suppression of electron-scale fluctuations ($k_\theta \rho_s\sim 3$) by ZF shear is also observed in an internal thermal electron transport barrier. However, in contrast to the \hbox{L-H} transition, here the density fluctuation level is always anti-correlated (180$^\circ$ out of phase) with the ZF shearing rate. \vskip4pt\noindent [1] L. Schmitz et al., Phys. Rev. Lett. {\bf 108}, 155002 (2012).   

Particle Transport and Turbulence Dependence on Collisionality on \hbox{DIII-D} and Comparisons to GYRO and TGLF

Understanding of the physics of collisionality ($\nu^\ast$) dependence of particle transport is critically important in extrapolating existing experiments to the burning plasma regime as it governs the peaking of the plasma density profile and impurity accumulation. Recent studies of particle transport have been facilitated by the significant new measurement and modeling capabilities in \hbox{DIII-D}. High resolution profile reflectometry measurements during \hbox{L-mode} plasmas have revealed an insensitivity of the electron density peaking to $\nu^\ast$ variation for a factor of 3$-$5, in contrast to the \hbox{H-mode} scaling results. Simultaneous measurements indicate a broadening of the intermediate-$k$ turbulence as $\nu^\ast$ increases, suggesting a change in the underlying turbulence dynamics. Initial estimates for the trends in particle fluxes appear consistent with GYRO predictions. Detailed comparisons of measured perturbative particle diffusion coefficient and pinch velocity to the predictions of TGLF, which can simulate both perturbative and equilibrium transport rates now, are ongoing.   

Geodesic Acoustic Mode Structure in DIII-D

Geodesic Acoustic Modes (GAMs) are coherent flows induced by plasma turbulence that in turn affect the turbulence and turbulent transport. Recently, in a neutral beam and electron cyclotron heated L-mode plasma in the DIII-D tokamak, strong GAM oscillations have been observed in electron temperature fluctuations $\tilde{T}_e$ in addition to the often-observed GAM density fluctuations. The mode frequency is constant over a radial range ($\delta \rho \sim 0.2$), as expected of an eigenmode, with two different frequencies observed depending upon radius. Both modes exist at the location where one frequency transits to another as detected in $\tilde {T}_e$. GAM oscillations in density and $ExB$ flow peak at far edge (at $\rho\sim0.9$) and have similar profile shapes. In contrast, the GAM oscillations in $\tilde{T}_e$ peak much deeper into plasma (at $\rho\sim 0.7$). After the auxiliary heating power is turned off for $t\,\underline{>}$ 100 ms, the eigenmode feature evolves into a continuum. This observation of GAM properties may provide challenges for existing theories to understand GAMs and plasma turbulence.   

ECE Imaging of Temperature Fluctuations and Drift Waves in DIII-D Plasmas

Recent observations of 2-D turbulent structures have been performed with the ECEI instrument on DIII-D. The experiments were performed in NBI and ECH-heated plasmas, over a range of external heating power. Correlation techniques similar to those used in Correlation Electron Cyclotron Emission (CECE) systems are employed, with the advantage that the ECEI system detects a full 2-D array of plasma locations: vertical separation is provided by an optical system and horizontal separation is provided by frequency discrimination in the detection electronics. Among the results are 2-D images of poloidally-propagating drift-waves, and correlation properties of fluctuations ($<200$ kHz) in both the radial and poloidal directions. Scaling and parameter dependencies on plasma and heating conditions will be presented. In addition to the physics results, the data demonstrates the viability of the ECEI system in the presence of ECH heating, which will also be discussed.   

Initial tests of NSTX millimeter-wave polarimeter on DIII-D

Polarimetry is a powerful diagnostic technique to probe plasma equilibria and magnetic fluctuations in fusion plasmas. In high beta plasma devices such as NSTX, these measurements are important for understanding the stability, structure and anomalous transport induced by electromagnetic turbulent fluctuations. A 288 GHz polarimeter operating along the major radius has been developed and is being tested on DIII-D prior to deployment on NSTX-U. The system launches a rotating linearly polarized beam and detects phase shifts related to polarization changes due to the plasma. To improve phase resolution, quasi-optical isolation is used to minimize multi-path feedback effects. Preliminary data indicates that equilibrium results are consistent with synthetic diagnostic calculations. Typically, phase resolution of $<$1 degree is observed over a frequency range from 5 to 300 kHz, opening the possibility for the measurement of magnetic fluctuations. Measured spectra indicate the presence of various coherent modes, e.g. Neoclassical Tearing Modes, Toroidal Alfv\'en Eigenmodes. Analysis is underway to establish whether these spectral components are primarily caused by magnetic fluctuations.   

Microwave imaging reflectometry (MIR) for visualization of the 2-dimensional structure of density fluctuations on DIII-D

An imaging diagnostic capable of measuring simultaneously the poloidal and radial structure of density fluctuations is being developed for DIII-D. The success of electron-cyclotron emission imaging developed by UC Davis for DIII-D is a testament to the powerful utility of microwave imaging diagnostics for tokamaks. Since its first deployment on TEXTOR, the MIR concept has undergone several improvements in optical and electronics design. For example, the shape of the wavefront of the probing beam and the curvature of the cutoff layer strongly affect the integrity of the reflected signal. This is addressed with transmitting optical elements that are designed to control the shape of the probing beam. Advances in microwave electronics make it possible to transmit and detect multiple frequencies simultaneously, permitting fluctuation measurements at multiple radial locations. Interesting physics occurs over the entire poloidal cross-section of the plasma, on disparate spatial scales. MIR is flexible in this respect, allowing a remote user to rapidly tune the individual probing frequencies for a variety of correlation studies. Synthetic diagnostic simulations and extensive laboratory tests corroborate our confidence in a successful implementation of MIR on DIII-D.   

Characterization of Intense Bursts of mm-wave Emission Using New RF Spectrometer on the DIII-D Tokamak

Intense bursts of mm-wave emission with duration of 5-10$\,\mu$s have been observed by both Electron Cyclotron Emission (ECE) radiometer [1] and Electron Cyclotron Emission Imaging (ECEI) systems during edge localized modes. Both the ECE radiometer system and the ECEI system employ heterodyne detection methods and have overlapping intermediate frequency (IF) bands. A new RF spectrometer, spanning this IF frequency range of approximately 2-10 GHz, has been installed on the DIII-D tokamak in order to more fully characterize the frequency, intensity, and localization of these bursts. This data has been used to better understand the generation mechanism for these bursts that are believed to relate to runaway electrons maser radiation [2]. Various consequences for diagnostic development will also be addressed.\par \vskip6pt \noindent [1] Ch. \ Fuches {\em et al.,} Phys.\ Plasmas {\bf8}, 1594 (2001).\par \noindent [2] B.~Kurzan {\em et al.,} Phys.\ Rev.\ E {\bf55}, 4608 (1997).   

Evaluating pedestal gradients and scale lengths without functional fits in order to test for non-diffusive transport processes

The advent of the recent spatial resolution upgrade to the edge Thomson scattering diagnostic at \hbox{DIII-D} allows re-examination of methods for measuring electron density and temperature scale lengths. The modified hyperbolic tangent fit is widely used, however, this function is clearly inappropriate in some situations such as when density profiles are distorted by applied resonant magnetic perturbations (RMPs). In these cases, a flattening of the density profile is observed at or near the separatrix while the RMP is applied. However, no similar structure is observed in the temperature profile so far. Furthermore, the tanh fit is based on a diffusive model and recently observed differences between the tanh fit and measured profiles using newly available high spatial resolution data are revealing more subtle transport processes at the mm scale.   

A Method for Measuring Poloidal Rotation in the DIII-D Tokamak

A new method of inferring poloidal rotation by combining charge exchange measurements of impurity tangential rotation on the high- and low-field side of the magnetic axis has been developed on \hbox{DIII D} [1]. This method has the advantage of not requiring the calculation of atomic physics corrections to account for the energy dependent charge exchange cross section, and it has been used in conjunction with charge exchange measurements of impurity poloidal rotation from the vertical charge exchange views to investigate poloidal rotation in DIII-D plasmas. Measurements of poloidal rotation have been made for a large range of temperature, toroidal field, and toroidal rotation. In addition, poloidal rotation has been measured during the formation of an internal transport barrier, and the dependence of poloidal rotation on normalized collisionality has been investigated. Comparisons with the neoclassical theory of poloidal rotation will be made.\par \vskip6pt \noindent [1] C.~Chrystal, {\em et al.}, Rev.\ Sci.\ Instrum.\ {\bf 83}, 10D501 (2012)   

Calculation of Intrinsic Edge Rotation Using XGC0

XGC0 is used to interpret Mach probe measurements on DIII-D indicating a main-ion co-current rotation layer near the plasma separatrix. This intrinsic edge rotation could have multiple sources, including non-isotropic loss of ions on collisionless orbits, turbulent Reynolds stress, finite-orbit effects, neutral charge-exchange and sheath-driven potentials. The sources and sinks are explored using XGC0, a 5D full-f gyrokinetic code that self-consistently computes particle transport (electrons, main-ions and impurity ions) in the pedestal and scrape-off layer for a realistic diverted plasma geometry with divertor recycling. A small amount of anomalous ambipolar diffusion is added to the computed neoclassical transport to improve agreement between simulations and experiment. Preliminary results indicate good quantitative agreement between XGC0 and Mach probe measurements when the gyro-viscosity is on the order of the effective heat diffusion near the separatrix. The gyro-viscosity reduces the poloidal flows, leading to a non-ambipolar loss of counter-$I_p$ ions through the X-point, which is balanced by a pinch of isotropic cold ions from the SOL.   

Toroidal and Poloidal Rotation of Deuterium Ions in DIII-D Intrinsic Rotation Conditions

Comparisons of the bulk deuterium ion toroidal rotation to neoclassical theory reveal a significant discrepancy. The source of this discrepancy lies in the prediction of the main-ion poloidal rotation. In low toroidal rotation plasmas, $E_r$ is dominated by the pressure and poloidal rotation contributions; hence, an accurate determination of the poloidal flow is required in these conditions. We infer the main-ion poloidal rotation from measured main-ion toroidal rotation and the radial force balance relation. Inferred main-ion poloidal flow is significantly larger in the ion diamagnetic direction than NCLASS predictions. By experimentally performing scans of the plasma current, toroidal field and heating mix, the dependence of main-ion toroidal and poloidal rotation on these parameters can be understood. Comparisons of main-ion charge exchange recombination measurements of rotation with Mach probe data and several neoclassical rotation models will be performed.   

Comparison of some turbulent confinement models for \hbox{DIII-D}, ARIES and FNSF

We give a detailed comparison of three of the leading turbulent confinement models applied to \hbox{DIII-D} and two advanced tokamak scenarios representing ARIES and FNSF. Of particular current interest is the recently installed Lehigh MMM7.1 transport module and its predictions compared to GLF23 and TGLF. A number of recent L- and \hbox{H-mode} \hbox{DIII-D} discharges are analyzed to check agreement with experiment for the three confinement models. The models can differ significantly in their predictions of tokamak confinement and, in particular, for steady state AT scenarios, the resulting rf current drive requirements can be somewhat different. For the AT cases, we show how fast wave on axis current drive together with ECH and \hbox{L-H} can be combined to produce favorable, negative shear $q$ profiles that have acceptable predicted (less than about 200~kA) residual ohmic current in steady state. The sensitivity of the results to particular distinguishing features of the confinement (e.g. drift resistive ballooning, and trapped electron modes) is examined.   

High $\beta_N$ steady state scenario development on DIII-D

On \hbox{DIII-D}, on- and off-axis neutral beams and electron cyclotron heating have expanded access to a wide range of $q$-profiles. Plasmas have been sustained with $q_{min}=1.3-2.5$ to evaluate the suitability for high $\beta_N$, high performance steady state operation. Nearly stationary plasmas were sustained for two current profile relaxation timescales (3~s), with $q_{min}=1.5$, $\beta_N=3.5$, and performance that projects to $Q=5$ in ITER. The duration of the high $\beta_N$ phase is limited only by the available NBI energy. Low-order tearing modes are absent and the predicted ideal-wall $n=1$ kink $\beta_N$ limit is $>$4. To achieve a steady state, higher $\beta_N$ is needed to increase the bootstrap current. Higher $q_{min}$ decreases the required external current drive near the axis and can increase the stability $\beta_N$ limit. Experiments to produce $\beta_N =4-5$ and $q_{min}\geq 2$ with $B_T=1.75-2$~T were limited to $\beta_N<3.3$ by relatively low energy confinement (H$_{\rm 89}<2$) rather than tearing modes. Low H$_{\rm 89}$ is likely due to a combination of increased thermal transport at high $q_{min}$ (low poloidal flux), and depositing more power at larger radius. We will discuss upcoming experiments to achieve higher $\beta_N$ and improved confinement.   

Optimization of self-consistent \hbox{DIII-D} AT scenarios with the OMFIT framework

Integrated modeling is an established technique to obtain improved description of physical processes. OMFIT is a workflow manager designed to facilitate and automate all steps in the modeling/validation/verification process of integrated simulations--specifying the physics to be modeled, retrieving experimental data, running the simulation or analysis, and visualizing the results. In OMFIT modeling tasks are organized into modules, which can be easily combined to create arbitrarily large multi-physics simulations. A unified Graphical User Interface (GUI) oversees all of the aspects in the management of the analyses carried out by users. Most importantly, OMFIT allows integration of existing simulation tools without requiring those tools to comply to any standardized data format. This poster describes OMFIT and presents some applications of the framework, including the self-consistent optimization of \hbox{DIII-D} AT scenarios using a cost-function approach to achieve multiple simultaneous goals.   

Data-driven Model-based Combined Magnetic and \hbox{Kinetic} Control on DIII-D

In order to take into account the coupling between the different magnetic and kinetic parameters, a multi-input-multi-output (MIMO) model-based controller is introduced to regulate the rotational transform profile and $\beta_N$ in \hbox{DIII-D}. This approach is based on a linear two-time-scale model derived from experimental data. A singular value decomposition of the plasma model is carried out to decouple the system and identify the most relevant control channels. Then, a robust-control technique is used to determine a controller that minimizes the reference tracking error and rejects external disturbances with minimal control energy. Finally, the feedback controller is augmented with an anti-windup compensator, which keeps the given controller well behaved in the presence of actuator saturation. Experimental results illustrate the performance of the proposed controller, which is one of the first plasma profile controllers integrating magnetic and kinetic variables implemented in DIII-D.   

Data-Driven Modeling and Control of the Poloidal Flux Profile for Advanced Tokamak Scenarios in DIII-D

Theory-based mathematical models derived from flux averaged transport equations may result in complex expressions not suitable for real-time control implementation. At the expense of less model accuracy and controller capability, data-driven linear models constructed from system identification techniques offer a potentially practical and relatively simple alternative suitable for control design when the goal is regulation around an equilibrium point. This work considers the evolution of the poloidal flux profile in response the inductive electric field as well as to heating and current drive (H\&CD) systems. Based on the identified linear models, an optimal state feedback controller with integral action is designed to regulate the poloidal flux profile around a desired target in the presence of perturbations. Closed-loop experiments on DIII-D and simulations based on predictive codes illustrate the effectiveness of the controller.   

Impurity Flow Measurements in DIII-D using Coherence Imaging Spectroscopy

Imaging interferometers have been used to measure the \hbox{2-D} distribution of the Doppler shift of impurity emission in both the lower and upper DIII-D divertors. The interferometer design has been simplified to a single birefringent plate between two polarizers, and improved calibration techniques have been implemented, including temperature stabilization. Measurements of other impurity species such as CII have been added. An image-intensified camera in the upper divertor has enabled measurement of the flows in the crown of the plasma during lower single-null divertor operation. In general, flows are in opposite directions on the inner and outer scrape-off layers in the divertor, as expected from the magnetic geometry. Initial results from a wide view periscope of the whole plasma cross section will also be presented.   

OEDGE Assessment of Pressure and Power Balance Methods for Separatrix Identification

The OEDGE code is used to assess several methods of determining the upstream separatrix location. The inter-ELM phase of a well-diagnosed ELMing H-mode discharge is being used for this comparison. The OEDGE code utilizes 1D plasma fluid models calculated along the field lines on a 2D computational grid of a poloidal cross-section of the discharge magnetic geometry to produce a 2D model of the background plasma. Langmuir probe data at the targets are used as input to the 1D models. Additional diagnostic measurements, including Thomson scattering, reciprocating probe, divertor spectroscopy and infra-red measurements of target heat flux, may be used to further constrain the plasma background determined by OEDGE. This plasma background thus found, is then used to identify the location of the separatrix in the experimental data by comparing the upstream plasma profiles from OEDGE to the experimental measurements. The OEDGE result is then compared to the separatrix locations predicted using simple pressure balance and power balance considerations.   

Near infrared spectroscopy of the DIII-D divertor

A high speed, high resolution near infrared (NIR) spectrometer has been installed at \hbox{DIII-D} to make first-of-its-kind observations of the $0.8-2.2$~$\mu$m region in a tokamak divertor. The goals of this diagnostic are (1) to study Paschen spectra for line-averaged measurement of low temperature plasma parameters, (2) to benchmark the chemical and physically sputtered sources of neutral carbon using the lineshape of the CI, 910~nm multiplet, and (3) to quantify contamination of the 0.75$-$1.1~$\mu$m region where Thomson-shifted laser light is measured by the Thomson scattering diagnostic. Diagnostic capabilities include a 300~mm, $f/3.9$ design, 300$-$2400~Gr/mm gratings providing optical resolution of $\sim$0.65$-$0.04~nm, and readout at up to 900 frames/second. Data are presented in \hbox{L-mode} plasmas, and in \hbox{H-mode} between ELMs and during the ELM peak. Results acquired by this diagnostic will be applied to design of a proposed divertor Thomson diagnostic for \hbox{NSTX-U} and aid validation of the Thomson system on ITER.   

Effect of Divertor Shaping on Divertor Plasma Behavior on DIII-D

Recent experiments examined the dependence of divertor density ($n_{TAR}$), temperature ($T_{TAR}$), and heat flux at the outer divertor separatrix target on changes in the divertor separatrix geometry. The responses of $n_{TAR}$ and $T_{TAR}$ to changes in the parallel connection length in the scrape-off layer (SOL) ($L_{||}$) are consistent with the predictions of the Two Point Model (TPM). However, $n_{TAR}$ and $T_{TAR}$ display a more complex response to changes in the radial location of the outer divertor strike point ($R_{TAR}$) than expected based on the TPM. SOLPS transport analysis indicates that small differences in divertor geometry can change neutral trapping sufficient to explain differences between experiment and TPM predictions. The response of the core and divertor plasmas to changes in $L_{||}$ and $R_{TAR}$, under both radiating and non-radiating divertor conditions, will be shown.   

Electron Temperature Fluctuations in DIII-D SOL

We present an overview of electron temperature $T_e$ fluctuation properties in the scrape-off layer (SOL) of low (L) and high (H) confinement discharges, over L-H transitions, and during edge localized modes. $T_e$ fluctuations play an important role in the tokamak SOL, being responsible for the ``conductive'' (due to correlated fluctuations of $T_e$ and poloidal electric field $E_\theta$) part of the cross-field turbulent transport. In DIII-D, SOL $T_e$ fluctuations are measured using a harmonic technique deployed on the midplane reciprocating probe and having a bandwidth of up to 100 kHz. Relative $T_e$ fluctuation levels range from 0.1-0.2 inside the last closed flux surface (LCFS) to 0.3-0.5 in the SOL. $T_e$ fluctuations tend to be roughly in phase with the electron density $n_e$ fluctuations. ``Conductive'' and ``convective'' (due to correlated $n_e$ and $E_\theta$ fluctuations) components of the cross-field turbulent heat fluxes are comparable near the LCFS, while in the far SOL the convective component tends to be larger. Most of the $T_e$ fluctuation and heat flux spectral energy is below 50 kHz. Cross-field heat fluxes measured near the LCFS in L-mode are in reasonable agreement with the SOL power balance.   

Fueling with edge recycling to high-density in \hbox{DIII-D}

Pedestal fueling through edge recycling is examined with the interpretive OEDGE code for high-density discharges in \hbox{DIII-D}. A high current, high-density discharge is found to have a similar ionization source profile as a lower current, lower density discharge. The higher density discharge, however, has a greater density gradient indicating a pedestal particle diffusion coefficient that scales near linear with $1/I_p$. The time dependence of density profile is taken into account in the analysis of a discharge with low frequency ELMs. The time-dependent analysis indicates that the inferred neutral ionization source is inadequate to account for the increase in the density profile between ELMs, implying an inward density convection, or density pinch, near the top of the pedestal.   

Survey and Cleaning of Metal Contamination in Graphite Plasma-Facing Tiles in DIII-D

During the DIII-D FY11 and FY12 campaigns, relatively high levels of high Z metallic core plasma impurities impeded high performance plasma operation. Observations made during a vessel entry revealed potential sources of the increased metals, including: copper and Inconel splatter from a probe head damaged by runaway electrons, partial melting of a neutral beam molybdenum shield plate, and exposed metals on the Fast Wave antenna Faraday shields. Portable beta-backscattering and x-ray fluorescence diagnostics were used to map the areal density of metals deposited on the graphite plasma-facing tiles around the vessel. Tile surfaces with deposits exceeding $7\times 10^{16}$ metal atoms/cm$^2$ were sanded in place or grit blasted outside of the vessel to remove impurities. The distribution of metals before and after resurfacing and the effectiveness of the tile resurfacing techniques on subsequent plasmas will be presented.   

High Power Fast Wave Coupling and Heating in \hbox{H-mode} Plasmas on DIII-D

Up to 2.5 MW of fast wave (FW) heating power has been coupled to the core of ELMing H-mode discharges with $\beta_N \leq 2.5$ in conjunction with 3-7 MW of neutral beam injection and 2.6 MW of electron cyclotron heating. Core FW heating efficiency has been found experimentally to approach 100\% in the Advanced Inductive regime, consistent with the excellent absorption predicted by ray-tracing models in this high $\beta_e$ regime. Low antenna loading (high rf voltages) characteristic of such regimes makes increasing the FW power challenging. A study of techniques to enhance FW antenna loading has been carried out in DIII-D, with emphasis on maintenance of good confinement. The loading is in absolute agreement with modeling when edge density profiles measured with reflectometry are used in the model. Recent work extending the range of H-mode regimes to which FW heating has been applied and on increasing the FW power coupled to those regimes is described.   

Dependence of ICRF Antenna Loading on ELM Frequency, Type and Size

The ITER ICRF antenna is required to couple 20 MW to the plasma in the presence of edge localized modes (ELMs), which can cause very fast changes in the ICRF antenna loading [1]. Three fast wave antennas are in use on the DIII-D tokamak, one with a frequency of 60 MHz and two operating at 90 MHz. The impact of edge profile modifications due to ELMs on fast wave antenna loading was investigated during low power operation into ELMy H-mode discharges. A variety of diagnostics are used to study the edge plasma and antennas, including: fast ion gauges, filterscopes, and an edge reflectometer. Antenna loading as a function of ELM frequency, size and type will be reported.\par \vskip5pt \noindent [1] D.W.\ Swain and R.~Goulding, ``ITER Ion Cyclotron System: Overview and Plans," Fusion Eng.\ and Design {\bf 82}, 603 (2007).   

Study of Fast Wave Coupling Through the \hbox{DIII-D} Edge Plasma Using the AORSA Full Wave Code

The AORSA full wave code has recently been extended to include the edge plasma in its solution domain and has been applied to the calculation of 30 MHz high harmonic fast wave power coupling and propagation in NSTX experiment [1]. A similar analysis is being carried out for the Fast Wave system used for central electron heating and current drive on DIII-D. Two of the three 4-strap arrays in DIII-D are identical and typically operate close to 90 MHz; the third differs in its geometry and runs at 60 MHz. Power coupling through and propagation within the edge plasma are being analyzed as a function of the plasma outer gap, edge density profile, and array operating frequency and spectra. Although the uniform grid-meshing scheme imposes limits on the fine structure resolution of the antenna geometry, the consequences of shape mismatching between the current strap surfaces and the plasma's last closed flux surface can be evaluated.\par \vskip6pt \noindent [1] D.L.\ Green, {\em et al.}, Phys.\ Rev.\ Lett.\ {\bf 107}, 145001 (2011).   

Operational Performance of the ECH System on \hbox{DIII-D}

The measurement of the rf power in the ECH system on DIII-D is showing the history of the performance for the six 110 GHz, 1 MW class gyrotrons. Four of the six systems show a general trend to higher values for the power injected in the tokamak after improvement of the transmission line, while for the other two systems the lower injected power is explained by operation at lower input power for reliability. The power calibration is based on the measured linearity of the injected power with the gyrotron cavity loading for all 6 systems. Total collector loading was measured versus the beam voltage. The measured transmission loss for 4 of the transmission lines is less than 1.1 dB, close to the theoretical value. The HE$_{11}$ mode content is over 85\% for all the lines. An average gain of 0.035 in the total transmission coefficient in the lines is due to a reduced number of miter bends in the system, reduced waveguide run, and improved angular alignment of the rf beam at the waveguide input. Measurements using a 4-port monitor and a dummy load have shown that the maximum power transmitted to a load corresponds to a maximum in the HE$_{11}$ mode.   

Upgraded Waveguide Components for New 1.2 and \hbox{1.5~MW} Gyrotrons on the DIII-D Tokamak

The present gyrotron system on the DIII-D tokamak comprises 110~GHz gyrotrons in the 1 MW class with designed pulse lengths of 10~s. The system is being upgraded with two types of depressed collector gyrotrons producing 1.2~MW at 110~GHz and 1.5~MW at 117.5~GHz, for which waveguide components having higher power ratings will be required. New power monitors and polarizers have been designed and fabricated, which are capable of operating for 10 s pulses at the higher power levels. This presentation reports an analysis of the component heat loading to obtain a thermal equilibrium. Using this equilibrium, a stress strain analysis was performed to calculate life expectancies. The calculations take into account the temperature dependence of the heat transfer coefficient in the component coolant channels. Although the high heat load components required upgrading, the waveguide lines themselves have adequate margins for the expected power and pulse length. A summary of the thermal capabilities of other components will also be presented.   

Upgrades for the ECH System on DIII-D

The gyrotron system for electron cyclotron heating on the DIII-D tokamak is being upgraded with the addition of higher efficiency gyrotrons having collector potential depression. Two new gyrotrons, operating at the present frequency of 110 GHz and generating 1.2~MW per unit, have been manufactured and are being installed and tested. The subsequent group of gyrotrons have been designed to generate 1.5~MW for 10~s pulses at 117.5~GHz. The first of these tubes is presently being manufactured at Communications and Power Industries. By the end of 2013, the system will comprise eight high power gyrotrons and, pending the successful performance of the 1.5~MW tube, an upgrade to a 15~MW system will begin. High voltage power supplies, transmission lines, launchers and associated control and data acquisition systems are included in the upgrades as is enhanced ability to steer the rf beams under a variety of pre-programed and reactive scenarios.   

Cross-Field Thermal Diffusion in Limiter and Divertor SOL, in the Conduction-Limited Regime

Earlier work based on a 2-D thermal diffusion code showed that scrapers in the SOL of divertor plasmas should receive much greater heat flux near their tips than projected by standard analyses. Indeed \textit{dP/dr} diverges as ($r-r_{tip})^{-1/2}$ because the cold region defined by the scraper surface concentrates heat flow. The same high heat flux is deduced for limiters at the plasma contact point, doubling from the projected value at a distance $\delta r\sim {\lambda _{SOL} } \mathord{\left/ {\vphantom {{\lambda _{SOL} } 8}} \right. \kern-\nulldelimiterspace} 8$ from the limiter tip, perhaps consistent with recent IR measurements on JET. Since \textit{dl/dr} along a parabolic limiter has a complementary divergence, the peak in the heat flux can be mitigated with such shaping. The same thermal diffusion code has been used to analyze the Eich/Wagner model for divertor target heat flux, in which an exponential heat flux profile is posited at the X-point, and is then convolved with a Gaussian to represent thermal diffusion along the divertor leg. This model is formally only applicable for convective parallel heat flux, with radially-independent velocity, coupled with radially-independent cross-field diffusion. These assumptions are likely incorrect, but in both experiments and these computations, the Eich/Wagner model fits results well. The method is shown to be very effective in deconvolving the original exponential term from data in conditions strongly violating the model assumptions. Its usefulness for determining diffusivity along the divertor leg will be explored. This work supported by DOE Contract {\#}DE-AC02-09CH11466.   

Axisymmetric Modeling of a Tokamak Edge with the Continuum Gyrokinetic Code COGENT

COGENT is a continuum gyrokinetic code being developed by the Edge Simulation Laboratory for edge plasmas. The code is distinguished by the use of a fourth-order finite-volume (conservative) discretization combined with arbitrary mapped multiblock grid technology (nearly field-aligned on blocks) to handle the complexity of divertor geometry without loss of accuracy. COGENT is written in parallel velocity - magnetic moment coordinates, and includes a number of options for collision models. In the present work we make use of a closed-flux-surface version of the code to investigate the influence of a strong self-consistent radial electric field (characteristic of a tokamak edge under H-mode conditions) on the neoclassical transport and decay of geodesic acoustic modes. In addition, we present initial simulations performed with a divertor version of the code exploring the effects of ion orbit losses on macroscopic flows in the edge and scrape-off layer.   

On the evaluation of a neoclassical radial electric field in the edge of a tokamak

The use of the standard approaches for evaluating a neoclassical radial electric field E$_{r}$, i.e., the Ampere (or gyro-Poisson) equation, requires accurate calculation of the difference between the electron and ion particle fluxes (or densities). In the core of a tokamak, the nontrivial difference appears only in high-order corrections to a local Maxwellian distribution due to the intrinsic ambipolarity of particle transport. The evaluation of such high-order corrections may be inconsistent with the accuracy of the standard first-order drift kinetic equation (DKE), thus imposing limitations on the applicability of the standard approaches [e.g., F. I. Parra and P. J. Catto, Phys. Plasmas \textbf{17}, 056106 (2010)]. However, in the edge of a tokamak, charge-exchange collisions with neutrals and prompt ion orbit losses can drive non-ambipolar particle fluxes for which a nontrivial (E$_{r}$-dependent) difference between the electron and ion fluxes appears already in a low order and can be accurately predicted by the first-order DKE. The parameter regimes where the radial electric field dynamics in the tokamak edge region is dominated by the nonambipolar processes, thus allowing for the use of the standard approaches, are discussed.   

Liquid Lithium Limiter for Carbon Wall Conditioning on RFX-mod

The \emph{Li}quid \emph{Li}thium \emph{Li}miter (\emph{Li3}) with capillary porous system originally developed for the FTU tokamak, has been tested for the first time on the Reversed Field Pinch RFX-mod, a machine equipped with a first wall completely covered by graphite tiles. The operation in limiter configuration was restricted by a defect on the limiter, which coupled with the plasma wall interaction with a relatively limited power (2-3 $MW/m^2$) caused a damage to the device. The \emph{Li3} has then been operated as an evaporator, being the Lithium depositions preceded by prolonged glow discharges in Helium to remove the Hydrogen trapped into the graphite. The enhanced retention capability and the lowered recycling factor of the first wall obtained with this treatments in respect to standard operational conditions, allowed a good degree of control on the density of the RFP discharges and to reach high density regimes at high current ($n/n_{Greenwald}$ $\sim$ 0.8 at plasma current $\sim$ 1.6 $MA$).   

ELM Triggering with the New PPPL Lithium Granular Injector

A Li granular injector based on a high-speed rotating impeller has been developed at PPPL. The injector is capable of injecting spherical particles with diameters up to 1.3 mm and velocities of up to 100 m/s and has several possible applications. Primarily, the injector was developed as a tool to induce ELMs for ELM pacing experiments in plasmas operating in the H-mode. It can also operate as a real-time wall conditioning tool or as a method to resupply Li during a discharge to devices where Li is applied to the PFC's prior to a discharge. The injector is also capable of horizontally injecting small dust particles of any variety for plasma-dust transport studies. The first injector has recently been successfully installed on the EAST tokamak in Hefei, China where ELMs were induced with near 100\% efficiency when 0.7mm spheres were injected at $\sim$ 40m/s into the midplane SOL. The injector will be described and supporting data for ELM triggering will be presented.   

Thermoelectric-Driven Liquid-Metal Plasma-Facing Structures (TELS)

CPMI is embarking on the development of a new, innovative liquid divertor PFC that can withstand heat fluxes above 15 MWm$^{-2}$. It will be based on the lithium-metal infused trenches (LIMIT) concept which has been demonstrated at Illinois and HT-7. TELS will extend the work that has been done at CPMI is four ways: 1. Develop, refine and test new geometries for thermoelectrically driven structures 2. Expansion of the Illinois pulsed and continuous systems so that pulsed plasma heat loads impinge on a surface that already has a continuous heat load on it 3. Increase the magnetic field so that a broader range of ``fusion type environments'' can be studied 4. Include other PFC materials such as tin and tin-lithium eutectics. The importance of testing with a pulsed plasma heat load is clear since magnetic fusion devices surfaces are subject to ELMs, disruptions, start-up and a variety of other plasma incursions and a PFC needs to show that it is robust under these extreme conditions. Plans for building TELS using the flowing lithium experiment (SLiDE), LiMIT and a pulsed-plasma theta pinch (DEVeX) will be presented. Thus heat removal systems can be systematically investigated and prototypes designed for installation on major fusion experiments around the world.   

Inter-machine scalings of plasma filament electromagnetic features

Electromagnetic features of turbulent filaments, emerging from turbulent background, have been studied in four different magnetic configurations: the stellarator TJ-II, the Reversed Field Pinch RFX-mod, a device that can be operated also as a ohmic tokamak, and the Simple Magnetized Torus TORPEX. In all cases, direct measurements of both field-aligned current density and vorticity were performed inside the filament. The inter-machine comparison revealed a clear dependence of the filament vorticity upon the local time-averaged ExB flow shear. Furthermore, a wide range of local beta was explored allowing concluding that this parameter plays a fundamental role in the appearance of filament electromagnetic features.   

Synchronous observation of filament properties using a fast camera and a hybrid probe in Heliotron J

A perpendicular-view gas puff imaging system and a hybrid probe (combination of a Langmuir probe array and three magnetic probe coils) have been installed in Heliotron J for synchronous measurement of the filament behaviors near the last closed flux surface (LCFS). To get the information of fast camera observation position, we have compared the poloidal velocity of turbulence and intermittent ``blob'' filaments from the data of radial-scanned ion saturation current (Is) data and fast camera data, respectively.   

Imaging-based measurements of plasma turbulence in a linear device

We present the status of our ongoing study of imaging-based plasma turbulence measurements in the Controlled Shear Decorrelation Experiment (CSDX) at the University of California, San Diego. CSDX is a well-characterized linear machine producing dense plasmas relevant to the tokamak edge ($T_e \sim 3$ eV, $n_e \sim 10^{13}$/cc). Electrostatic fluctuations are measured with Langmuir probe arrays in concert with fast imaging over a range of plasma parameters. Drift-wave-like modes are observed with frequencies of $3-30$ kHz ($\omega L_n/c_s \sim 0.2-2$) and wavenumbers of $0.3-6$ cm$^{-1}$ ($k\rho_s \sim 0.1-10$). Time-resolved velocity fields are obtained through pattern-matching velocimetry, allowing access to flow/turbulence interaction dynamics across the plasma radius. Current work includes measurements of mode structure, velocity profiles, Reynolds stress profiles, and ion-neutral coupling.   

Space resolved XUV/VUV spectroscopy of low temperature plasmas

Transmission grating based survey imaging spectrometers in the XUV/VUV have been developed for diagnosing tokamak divertor and edge. The XUV and the VUV spectrometers had moderate spectral resolution ($\lambda/\delta \lambda \sim 30$), but wide spectral range ($30-800$ \AA\ for the XUV, $100-2000$\AA\ for the VUV). The XUV(VUV) spectrometer covered an angular view of $\pm3.5^0$($\pm1.6^0$) with a $0.45^0$($0.2^0$) resolution. These spectrometers were tested on two different low temperature plasma experiments - (a) A Penning ionization discharge (PID) at JHU ($n_e \sim 10^{19}/\textrm{m}^3, T_e \sim 1$eV) and (b) The PISCES-A linear plasma experiment at UCSD ($n_e \sim 10^{18}/\textrm{m}^3, T_e < 6$eV). Distinct radial emission profiles were measured in both the experiments from a variety of ions: H, He, C, Ne and Al. For most ions, higher charge states were observed on the PID than on PISCES-A. For example upto Al$^{+3}$ and Ne$^{+3}$ were observed on the PID and only upto 2 times ionized Al and Ne were observed on PISCES-A. This may be attributed to the presence of non-Maxwellian electrons in the PID. However, the PISCES-A experiment observed up to C$^{+2}$, while only C$^{+1}$ was observed on the PID. This poster will present the experimental spectra and the related modeling.   

Microscopic Dynamics of Plasma Blob

Recently it has been observed that filamentary coherent structures are formed intermittently and propagate from the edge of core plasma to the first wall in scrape-off layer (SOL) of magnetic confinement fusion devices. Such structures are called ``blobs'' and are believed to transport plasma particles and heat flux into the far SOL across magnetic field lines. Many authors have studied dynamics of blobs on the basis of two-dimensional reduced fluid models. In such kind of macroscopic model, however, kinetic effects, such as sheath formation between plasma and divertor plate and velocity difference between ions and electrons, are treated under some assumptions and parameterization. Thus, in this study, we investigate microscopic dynamics of blobs with a three dimensional electrostatic plasma particle simulation. In the simulation, we have found the spontaneous electric current system in a blob. Further, the current shear is formed. In this paper, the investigation into the effect of this current shear will be reported.   

Modeling of fluctuating edge plasmas and ELMs with macro-blob approach

The Macro-Blob (MB) approach has been developed [1] to simulate the intermittent non-diffusive transport (via blobs and ELMs) in edge plasmas within the framework of 2-D transport code UEDGE (the corresponding version is UEDGE-MB). UEDGE-MB has shown [2] its capability of simulating both long-scale evolution of background plasma and fast spatiotemporal dynamics of blobs, resulting in dynamic plasma equilibrium under periodic sequence of MBs. We present new results of UEDGE-MB modeling, in which intermittent transport is represented as a random sequence of many macro-blobs (RSMB) based on experimental distributions of ordinary blobs over sizes and velocity for various regimes on DIII-D tokamak. Characteristics of edge plasma in RSMBs, both fluctuating and averaged over time interval series, will be analyzed and compared with experimental data. Intermittent particle/energy fluxes, plasma radiation, and plasma-wall interactions will be quantified. We also discuss the extended UEDGE-MB model to simulate type-I ELMs. Initial results on ELM cycle modeling will be presented and analysis of plasma particle/energy transport, radiation, impurity generation rates, and power loadings on material surfaces will be given.\\[4pt] [1] A. Pigarov et al., PoP 18, 092503 (2011)\\[0pt] [2] A. Pigarov et al., PoP 19 (2012).   

Tokamak SOL fluid simulations with self-consistent boundary conditions at the magnetic presheath edge

Fluid codes simulating the dynamics of magnetized plasmas with field lines terminating on the device vessel require boundary conditions that are consistent with the plasma-wall transition region. When the magnetic field strikes the wall at an oblique angle, this transition region corresponds to the magnetic presheath entrance (MPE). A complete analytical set of boundary conditions at the MPE is provided here for the density, temperature, potential, vorticity, and parallel ion and electron velocities, that is fully consistent with kinetic simulations of the plasma-wall transition. These boundary conditions are implemented in a 3D global fluid code simulating the tokamak SOL turbulence in a limiter configuration. This allows investigating for the first time the effect of the sheath on the tokamak SOL dynamics, in particular on the steady state profiles, plasma circulation, and blob propagation, and the effect of ExB on the intrinsic plasma rotation.   

Effect of Boussinesq approximation on SOL turbulence computations

Fluid models of edge-scrape-off layer turbulence typically involve the drift approximation, thereby reducing the momentum conservation perpendicular to the magnetic field to a single vorticity equation. A further assumption of scale-separation between fluctuation and background density length-scales is used to linearize the vorticity expression for ease of computation. This assumption is referred to as the Boussinesq approximation. However, in practice there is no significant scale separation between background density variation and fluctutaion scales in edge-SOL turbulence. Considering the case of an isolated blob in the SOL region, we report the effect of the Boussinesq approximation in the evolution of a density perturbation into a convecting dipole, both in linear analysis and in nonlinear simulations using the code TOKAM-2D. We will also compare the turbulent density and potential fields that are obtained using TOKAM-2D, and examine the validity and effect of the Boussinesq approximation with respect to the fluctuation scales.   

Analysis of plasma particle and energy fluxes to material surfaces from tokamak edge turbulence simulations

Recent BOUT simulations of edge plasma turbulence in L-mode regime in the boundary region of DIII-D tokamak have demonstrated reasonable match with key edge diagnostics [1]. Order-of-magnitude level agreement has been found in the characteristic amplitude, wavenumber, and frequency of turbulent fluctuations, as compared with experimental data from reciprocating edge Langmuir probe and Beam Emission Spectroscopy systems. Owing to this encouraging agreement, output data from these simulations are analyzed to get insights on physical mechanisms and properties of plasma particle and energy fluxes to material surfaces. Of particular interest is plasma turbulence propagating into, or generated in, the far scrape-off layer region where plasma interacts with material walls. Results of statistical analyses of simulated turbulence plasma transport will be presented and physical implications will be discussed. \\[4pt] [1] B.I. Cohen et al., APS-DPP 2012   

Multi-species Kinetic-Kinetic Plasma-Neutral Transport Simulations with XGC-DEGAS2

The role of neutral fueling in the buildup of the H-mode pedestal is of great interest due to the predicted sensitivity of ITER's performance to pedestal parameters. The effects of kinetic neutral and plasma phenomena on the pedestal buildup are being examined via the coupled DEGAS2 Monte Carlo neutral and XGC0 neoclassical particle transport codes. The coupled DEGAS2 neutral and XGC1 turbulence codes will be discussed at this conference by J. Lang, et al. Example pedestal fueling simulations utilizing XGC0-DEGAS2's time-dependent, consistent recycling capability have been described previously.\footnote{D. P. Stotler et al., in Proceedings of the 20th PSI Conference, Germany, 2012} However, only deuterium ions and atoms were considered in that paper. Here, we will report on a generalized plasma-on-neutral collision routine in XGC0-DEGAS2 that handles multiple ion species and extends the list of modeled atomic collision processes. First, we will utilize XGC0's existing kinetic electron capability in processing electron impact ionization. Second, neutral impurities will be added as the next step towards a complete impurity model in XGC0. Third, deuterium molecules and molecular collision processes will be incorporated. Relevant edge physics results will also be discussed.   

Effect of neutral recycling on pedestal turbulence and transport

Neutral ionization and charge exchange are one of the dominant parts of the pedestal physics. However, study of their kinetic effects on the gyrokinetic micro-turbulence has been difficult due to the multi-physics complexity and the lack of high performance computing resource. In this work, we turned on the neutral Monte-Carlo routine in the comprehensive full-f gyrokinetic code XGC1 in diverted magnetic field geometry. Neoclassical and electrostatic turbulence physics are considered together. Simulation shows that the neutral recycling plays an important role in the pedestal formation, and the turbulence and transport. Neutrals provide not only a particle source through ionization, but also a heat sink through charge exchange. The physics involved is nonlocal. As the neutral recycling is reduced, the edge ITG turbulence intensity decreases, consistently with the observations from L-H transition and Li coating. In addition, the nonlocal turbulence affects the penetration of cold particles from the edge to the core region.   

Dynamic divertor by plasmoid ejection

We developed the new concept of divertor called the dynamic divertor which insulates the divertor plate from the main plasma using repetitive plasmoid ejection from the main plasma to the divertor coil. In this concept, current drive or heating causes the main plasma to expand and to form a plasmoid with helium ashes. Second, the expanding core plasma finally pinches off the small plasmoid and then, the plasmoid isolated from the main plasma is cooled down by argon gas puffing and finally is connected with the divertor plate. In this series of motions, divertor plate is not connected to the main plasma, indicating significant reduction of heat flux into the divertor plate. We demonstrated for the first time one cycle of the dynamic divertor action in TS-4 ST experiment. The plasmoid was formed at bad curvature at the main plasma and was translated to the divertor coil in agreement with the corresponding 2D MHD simulation results. The 2D MHD simulation also demonstrated the repetitive ejection of plasmoid from the main plasma by controlling the divertor coil currents. The remaining problem is to annihilate the common flux between the core plasma and the divertor coil and to demonstrate the repetitive plasmoid ejection simply by heating and current drive of the main plasma.   

Lower Hybrid Heating and Current Drive

Lower hybrid current drive (LHCD) is the most robust and efficient method of driving the tokamak current with external radio frequency waves in steady-state tokamak operation. The electron distribution functions in the LHCD experiments contain substantial parallel thermal fluxes with radial gradients that are greater than those in the current and temperature profiles. We re-examine the growth rates of the electron temperature gradient (ETG) modes in these plasmas based on an analytic model for electron distribution function with three temperatures $T_\perp$, $T_{\|F}$, and $T_{\|B}$. The stability and turbulent transport is also analyzed using the electron distribution functions computed with a combined ray tracing/Fokker Planck code (DELPHINE C3P/LUKE). Electron Landau damping is reduced compared to its value in a Maxwell distribution. These potential instability drives are controlled by the magnetic sheared induced electron Landau damping that becomes strong as the fluctuations propagate into regions of large parallel wavenumber away from the mode rational surfaces. The feedback of the ETG turbulence on the propagation of the penetration of RF fields that shape the electron distribution function feeding the ETG growth rate make the problem a complex dynamical system.   

Burn Control for the IGNITOR Experiment by External ICRH Heating

The non-linear thermal balance equation for thermal equilibrium and stability, is analytically and numerically investigated by including the ICRH external wave heating term in order to control the thermonuclear instability in IGNITOR experiment facility. The expressions for ion and electron thermal coefficients, introduced in the thermal balance equation, are obtained by solving the nonlinear transport equations estimated in the several collisional transport regimes (in particular the banana collisional transport regimes). The scaling law of the thermal coefficients with respect to temperature is obtained by fitting the, magnetic surface, averaged profiles of these coefficients against temperature. The ICRH heating in the IGNITOR experiment, among other applications, is expected to stabilize the power of the thermonuclear burning by automatic regulation of the RF coupled power. Here a scenario is considered where IGNITOR is led to operate in a slightly sub-critical regime by adding a small fraction of He3 to the nominal 50-50 Deuterium-Tritium mixture. The difference between power lost and alpha heating is compensated by additional ICRH heating, which should be able to increase the global plasma temperature via collisions between He3 minority and the background D-T ions.   

Plasma Start-up Experiments Using the Lower Hybrid Wave Excited by a Dielectric Loaded Waveguide Array Antenna on the TST-2 Spherical Tokamak

Plasma current start-up experiments were performed on the TST-2 spherical tokamak (R= 0.38 m, a = 0.25 m, B$_{t}$ = 0.3 T, I$_{p}$ = 0.1 MA) using the lower hybrid wave (LHW) at f = 200 MHz. A waveguide array antenna consisting of four dielectric (alumina, $\varepsilon _{r}$ = 10.0) loaded waveguides was used. The coupling characteristics of this antenna were investigated by low power experiments (P$_{FWD} <$ 5 kW). The measured characteristics were qualitatively consistent with those predicted by calculations using a finite element method solver package (COMSOL). The experimentally observed reflection coefficient is large (greater than 36 {\%} averaged over four waveguides), and there are large differences in reflectivities in neighboring waveguides. It was necessary to take into account of the private limiter surrounding the antenna in order to reproduce these features. Non-inductive plasma current start-up to 6 kA has been demonstrated using 20 kW of LHW power. In this experiment, the reflection coefficient was very high because the initial plasma density was much lower than the predicted optimum plasma density.   

ECCD-induced tearing mode stabilization via active control in coupled NIMROD/GENRAY HPC simulations

Actively controlled electron cyclotron current drive (ECCD) applied within magnetic islands formed by neoclassical tearing modes (NTMs) has been shown to control or suppress these modes. In conjunction with ongoing experimental efforts, the development and verification of integrated numerical models of this mode stabilization process is of paramount importance in determining optimal NTM stabilization strategies for ITER. In the advanced model developed by the SWIM Project, the equations/closures of extended (not reduced) MHD contain new terms arising from 3D (not toroidal or bounce-averaged) RF-induced quasilinear diffusion. The quasilinear operator formulation models the equilibration of driven current within the island using the same extended MHD dynamics which govern the physics of island formation, yielding a more accurate and self-consistent picture of 3D island response to RF drive. Results of computations which model ECRF deposition using ray tracing, assemble the 3D quasilinear operator from ray/profile data, and calculate the resultant forces within the extended MHD code will be presented. We also discuss the efficacy of various numerical active feedback control systems, which gather data from synthetic diagnostics to dynamically trigger and spatially align RF fields.   

Experimental Study of RF Sheaths due to Shear Alfv\'{e}n Waves in the LAPD

Ion cyclotron resonance frequency (ICRF) heating is an important tool in current fusion experiments and will be an essential part of the heating power in ITER. A current limitation of ICRF heating is impurity generation through the formation of radiofrequency (RF) sheaths, both near-field (at the antenna) and far-field (e.g. in the divertor region). Far-field sheaths are thought to be generated through the direct launch of or mode conversion to shear Alfv\'{e}n waves. Shear Alfv\'{e}n waves have an electric field component parallel to the background magnetic field near the wall that drives an RF sheath.\footnote{D. A. D'Ippolito and J. R. Myra, \emph{Phys. Plasmas} \textbf{19}, 034504 (2012)} In this study we directly launch the shear Alfv\'{e}n wave and measure the plasma potential oscillations and DC potential in the bulk plasma of the LAPD using emissive and Langmuir probes. Measured changes in the DC plasma potential can serve as an indirect measurement of the formation of an RF sheath because of rectification. These measurements will be useful in guiding future experiments to measure the plasma potential profile inside RF sheaths as part of an ongoing campaign.   

Test results of 3.7 GHz 500kW CW klystron for SST1 LHCD system

A 3.7 GHz, LHCD system aims to driving non inductive plasma current for SST1 machine. Its capability has been enhanced up to 2 MW by adding two additional klystrons, each rated for 500kW, CW power. The additional klystrons are installed and commissioned at site, for rated power, for more than 1000 seconds, before connecting them to main LHCD system. The auxiliary systems, like supporting power supply system (magnet, filament, ion pump, etc.), active heat management system, slow and fast interlock system, transmission line pressurization system, low power rf drive system, etc. are inter-connected with klystron system through VME based data acquisition and control system for remote CW operation of klystron at rated power. The calorimetric measurements, employing Pt-100 sensors, suggests that the maximum rf power ($\sim $500kW CW) extracted from klystron is dissipated on water cooled dummy loads. The unspent DC power ($\sim $800 kW CW) is dissipated in collector which is heavily cooled with water flowing at $\sim $1300 litres/min (lpm). The power loss in the klystron body remained within 20 kW. The cavity temperature, measured using J-type thermocouple, remained below 150 $^{\circ}$C. The output rf power, sampled through directional couplers and measured by rf detectors shows good agreement with calorimetric measurements. A detailed description of the klystron test set up and the test results obtained during its commissioning is presented in this paper.   

Effect of Electron Seeding on Experimentally Measured Multipactor Discharge Threshold

Multipactor is a vacuum phenomenon in which electrons, moving in resonance with an externally applied electric field, impact material surfaces. If the number of secondary electrons created per primary electron impact averages more than unity, the resonant interaction can lead to an electron avalanche. Multipactor is a generally undesirable phenomenon, as it can cause local heating, absorb power, or cause detuning of RF circuits. In order to increase the probability of multipactor initiation, test facilities often employ various seeding sources such as radioactive sources (Cesium 137, Strontium 90), electron guns, or photon sources. Even with these sources, the voltage for multipactor initiation is not certain as parameters such as material type, RF pulse length, and device wall thickness can all affect seed electron flux and energy in critical gap regions, and hence the measured voltage threshold. This study investigates the effects of seed electron source type (e.g., photons versus beta particles), material type, gap size, and RF pulse length variation on multipactor threshold. In addition to the experimental work, GEANT4 simulations will be used to estimate the production rate of low energy electrons ($<$ 5 keV) by high energy electrons and photons. A comparison of the experimental fluxes to the typical energetic photon and particle fluxes experienced by spacecraft in various orbits will also be made. Initial results indicate that for a simple, parallel plate device made of aluminum, there is no threshold variation (with seed electrons versus with no seed electrons) under continuous-wave RF exposure.   

Guided radar system for arc detection: new results at DIIID

A guided radar arc detection and localization system has been designed, fabricated, installed in the feed line to one of the resonant loops on the 285/300 FW antenna, and successfully tested during vacuum conditioning. The system injects a train of binary phase-modulated pulses at a carrier frequency of 25 MHz up-shifted to around 450MHz into the main high power transmission line connected to the antenna through a septate coupler and a circulator. The pulses are reflected by arcs, and the time delay provides the distance to the arc. The reflected signals are analyzed in real time, with a time response sufficient to provide active arc detection as well as localization. RF pulses have been injected into the antenna at a power level of up to 650kW. The arc location was varied by either puffing gas into the vacuum vessel, in which case arcs always occurred in the antenna, or injecting RF without a gas puff, in which case the arcs almost always occurred in the transmission line feeding the antenna. The localization obtained during these initial tests had a relatively low resolution of about 2 m, but arcs occurring inside or outside the antenna could clearly be differentiated and corresponded with the expected location. The septate coupler proved fully compatible with the antenna feed and matching network and improved performance significantly in comparison to the use of directional couplers.   

Nonlinear kinetic simulations of ion cyclotron emission from fusion products in large tokamak plasmas

Ion cyclotron emission (ICE) was the only collective radiative instability, driven by fusion-born ions, observed from deuterium-tritium plasmas in both JET and TFTR (R O Dendy et al., Nucl. Fusion \textbf{35}, 1733 (1995)). Suprathermal emission, peaked at sequential ion cyclotron harmonics at the outer mid-plane edge, was detected using heating antennas as receivers on JET and using probes in TFTR. The intensity of ICE spectral peaks scaled linearly with fusion reactivity. The underlying emission mechanism appears to be the magnetoacoustic cyclotron instability (MCI), which involves resonance between: the fast Alfv\'{e}n wave; cyclotron harmonic waves supported by the energetic ions and by the background thermal plasma; and a set of centrally born fusion products, lying on barely trapped orbits, which undergo large drift excursions. Analytical studies show that the linear growth rate of the MCI corresponds well with certain observational features of ICE, including ones where a nonlinear treatment might be thought essential. To help explain this, we have carried out direct numerical simulations using a particle-in-cell (PIC) code. We focus on the results of extending MCI theory from the linear into the nonlinear regime for large tokamak parameters.   

Advanced electric field computation for RF sheaths prediction with TOPICA

The design of an Ion Cyclotron (IC) launcher is not only driven by its coupling properties, but also by its capability of maintaining low parallel electric fields in front of it, in order to provide good power transfer to plasma and to reduce the impurities production. However, due to the impossibility to verify the antenna performances before the starting of the operations, advanced numerical simulation tools are the only alternative to carry out a proper antenna design. With this in mind, it should be clear that the adoption of a code, such as TOPICA [1], able to precisely take into account a realistic antenna geometry and an accurate plasma description, is extremely important to achieve these goals. Because of the recently introduced features that allow to compute the electric field distribution everywhere inside the antenna enclosure and in the plasma column, the TOPICA code appears to be the only alternative to understand which elements may have a not negligible impact on the antenna design and then to suggest further optimizations in order to mitigate RF potentials. The present work documents the evaluation of the electric field map from actual antennas, like the Tore Supra Q5 and the JET A2 launchers, and the foreseen ITER IC antenna. \\[4pt] [1] D. Milanesio et al., Nucl. Fusion 49, 115019 (2009).   

Fully kinetic simulation of radio frequency wave in fusion plasmas

We are looking into a new nonlinear kinetic simulation model to study the radio frequency heating and current drive of fusion plasmas using toroidal code GTC. In this model ions are considered as fully kinetic (FK) particles using Vlasov equation and the electrons are treated as drift kinetic (DK) particles using drift kinetic equation. This scheme is particularly suitable for plasma heating and current drive with wave frequencies lower than the electron cyclotron frequency, ranging from fast wave and ion cyclotron wave to lower hybrid wave. This model also can handle physics with realistic electron-to-ion mass ratio and nonlinear dynamics in the full torus simulation. The implementation of fully kinetic ions has been verified in the GTC simulation of ion plasma waves. The real frequency measured in GTC agrees very well with theoretical value for various ion temperatures. The ion Debye shielding potential from simulation also agrees with theoretical solution.   

Particle-in-cell simulations of lower hybrid waves in plasmas

In this work, simulations based on the GeFi framework [Lin et al., 2005] are conducted to study the nonlinear wave-plasma interactions. The obtained dispersion relation is in excellent agreement with the linear theory prediction. When the wave power is sufficiently high, the parametric decay instability is observed.   

Wave-particle interactions in toroidally confined fusion plasmas

Radio frequency (RF) waves are routinely used for heating and controlling the current profile in fusion plasmas. RF waves modify the particle distribution functions away from an equilibrium distribution through wave?particle interactions, while collisions try to restore the distribution function to its equilibrium state. In high temperature plasmas, RF waves modify the particle distribution function over times much shorter than collisional times. In this long mean free path limit, particles do not undergo Brownian/Markovian diffusion. There persist long time correlations which require special attention. We have developed a kinetic theory for RF wave-particle interactions in the long mean-free path limit [1]. In order to be fully consistent, the distribution function has to be evolved concurrently with the particle motion. The ensuing diffusion tensor depends on time and the action variables describing particle motion in tokamaks. This leads to results that are different from the usual quasilinear theories of wave-particle interactions. The consequences of our diffusion tensor will be illustrated through the evaluation of averaged quantities, like current and temperature, for ITER and DEMO plasmas.\\[4pt] [1] Y. Kominis, A.K. Ram, and K. Hizanidis, Phys. Rev. Lett. 104, 235001 (2010).   

PiC simulations of the anomalous Doppler resonance for a scaled laboratory experiment

The anomalous Doppler resonance occurs due to coupling between a negative harmonic of the electron cyclotron frequency and an electromagnetic wave, as such this regime is only applicable in slow-wave media like a plasma or dielectric loaded waveguide. In nuclear fusion devices the generation of fast electrons by Lower Hybrid Current Drive or in extreme cases Dreicer acceleration, can lead to the criterion for the anomalous Doppler resonance being fulfilled. The anomalous Doppler resonance is also relevant in the nature of pulsar radio emission. Simulations have been developed to study non-thermal electrons drifting at relativistic velocities along a magnetic field with a background plasma acting as the slow-wave media. The simulations will be used to inform the design of a scaled laboratory experiment at Strathclyde, the results of which will be used to compare with the prediction of the numerical simulations and analytical theory. Once benchmarked by the experiment simulations will investigate regimes relevant to tokamak and astrophysical plasmas.   

Adding kinetic effects to finite-difference frequency-domain simulation of ion-cyclotron heating

The kinetic plasma current required for full-wave simulation of radio-frequency (RF) heating of Tokamak plasmas is typically included in frequency-domain simulation by application of the pseudo-spectral method and the Stix hot plasma dielectric tensor. Since the Stix dielectric is derived using a Fourier basis set, the choice of basis when applying the pseudo-spectral method is constrained to also use the Fourier basis. This presents problems when simulating bounded domains with complex geometries (i.e., the antenna and vacuum vessel structures), and is further limited to a uniform spatial resolution. Short of re-deriving the Stix dielectric for a more suitable spectral basis (e.g., the Chebychev basis), here we investigate including a kinetic plasma current through direct integration of the RF force on a discrete set of particle trajectories under a given estimate of the RF electric wave field. With an initial estimate based on a cold-plasma, an iteration where the particle kinetic plasma current updates the finite-difference solution may converge to the correct kinetic solution. Here we present initial work investigating the proposed method for an electron Langmuir wave, with comparisons to the pseudo-spectral solution.   

Application of the delta-f method to ICRF heating

Inclusion of ion kinetic effects (which resolve resonances and determine ion absorption) into the computation of plasma response to ICRF wave fields has often involved simplifications, such as small-but-finite Larmor radius expansions and neglect of drift excursions (finite banana width). The accuracy of such approximations is not always clear. Time-domain particle-based techniques don't suffer such restrictions, but do suffer from sampling noise. The utility of an optimized (for noise reduction) delta-f particle-based computation of self-consistent plasma currents is examined for a prototypical mode-conversion problem--electromagnetic to plasma wave conversion at the critical layer (where the em wave frequency equals the plasma frequency). A prescription for incorporation of the method into full-wave ICRF propagation solvers is discussed.   

Finite Orbit Width versions of the CQL3D code: Hybrid-FOW and Full-FOW

Finite-Orbit-Width (FOW) effects are being added into the CQL3D bounce-averaged Fokker-Planck code [1] using two main options. In the Hybrid-FOW option, partial FOW capabilities are implemented which add FOW features into the particle source (NB) operator, RF quasilinear operator, diagnostics, and guiding center orbit losses with gyro-radius correction. Collisions remain Zero-Orbit-Width (ZOW). The Hybrid-FOW version provides a greatly improved agreement with signals measured by the NSTX Fast Ion Diagnostic [2]. The advantage of the Hybrid-FOW version is that run time increases by only a factor of two compared to ZOW runs. The Full-FOW option further adds neoclassical radial transport features into the FP coding. The collisional coefficients are averaged along guiding center orbits, with a proper transformation matrix from local coordinates to the midplane coordinates, where the FP equation is solved. All radial terms are included. The computations are parallelized in velocity-grid index, typically using 128 CPU cores. We emphasize that this theory includes nonthermal and full-orbit, not first order correction, neoclassical theory. \\[4pt] [1] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' www.compxco.com/cql3d \\[0pt] [2] R.W. Harvey, Yu. Petrov, D. Liu, W. Heidbrink, P. Bonoli, this mtg (2012)   

Far field radio-frequency sheath modeling

Recent probe data on Alcator C-Mod suggests that fast wave propagation to regions far from the antenna can produce large rf sheaths ($\sim $ 100 V). This data provides a good test case for 1D and 2D models of far field sheath generation. The 1D model [D. D'Ippolito et al., Phys. Plasmas 15, 102501 (2008)] shows that coupling between the fast and slow waves at the limiter can drive sheath potentials when rapid spatial variation (associated with the limiter geometry) is assumed. Using the rfSOL code [H. Kohno et al., Phys. Plasmas 19, 012508 (2012)] a new 2D simulation shows that rapid spatial variation in the magnetic field direction relative to the limiter can produce slow waves and rf sheath potentials varying rapidly along the sheath. While neither of these models treats the actual experimental geometry, they provide support for the idea that the observed potentials are driven by fast wave -- slow wave coupling at the limiter. Understanding this physics is important for minimizing rf-enhanced impurities in rf experiments.   

Self-consistent modeling of the tokamak RF antennas, edge plasma, and sheath voltages

We model the 24-strap ITER RF antenna with a time-domain electromagnetic simulation package [1] that faithfully represents the 3D complexity of the launcher geometry. The simulations include a cold-plasma fluid model of the edge plasma [2], with an RF sheath sub-grid model which allows for realistic behavior of plasma in contact with metallic structures, such as Faraday shields [3]. Interestingly, localized short wavelength modes, likely slow waves, have been observed in the vicinity of the launcher, and are very sensitive to density. We investigate the effect on these waves for varying density, density profile, and magnetic shear. We further investigate the contribution to high sheath potentials such waves might have. We also present status and additional highlights of the continuing evolution of the overall model. This includes studies to benchmark the nonlinear sheath width and loss parameters, and more diagnostics aimed towards better characterizing energy balance. It also includes application of the analysis on larger problem domain size, with scaling-study results. Finally, we review recent work to improve the model for warm plasma, and nonlinear effects. Work supported by US. DOE Grants DE-FG02-09ER55006 and DE-FC02-08ER54953.\\[4pt] [1] Nieter, C. and Cary, J. R., JCP 196 (2004) 448-473.\\[0pt] [2] Smithe, D., Physics of Plasmas 14, 056104 (2007).\\[0pt] [3] Myra and D'Ippolito, PRL 101, 195004 (2008).   

Scattering effects on lower hybrid wave propagation

The effects of edge plasma density fluctuations on the scattering of lower hybrid (LH) waves are studied. Scattering can improve the penetration of LH waves into the plasma core due to the $k_\parallel$ upshift that occurs through the poloidal field (because the rotation of $k_\perp$ induces a finite poloidal mode number). Scattering can also inhibit wave penetration depending on the density fluctuation levels, resulting in enhanced collisional absorption of the waves in the SOL at high density. These two effects might contribute, respectively, to resolving the ``spectral gap'' problem [Bonoli P. T. and R. C. Englade, Phys. Fluids 9 (1986) 2937] and the ``density limit'' in the efficiency of LHCD [Wallace G. et al., Phys. Plasmas 17 (2010) 082508]. The scattering model used is based on the work of Bonoli and Ott [Phys. Fluids 25 (1982) 361] that introduces an electromagnetic wave kinetic equation solved by a Monte Carlo technique. This equation has been implemented in the ray tracing code GENRAY, which explicitly includes the SOL region. A detailed analysis of this scattering model will be presented in comparison with the experimental observations of LHCD for Alcator C-Mod tokamak.   

A careful comparison between ray tracing and full wave approaches to lower hybrid current drive

Lower hybrid (LH) waves in fusion plasmas have perpendicular wavelengths of $\approx$ 0.5 mm and parallel wavelengths of $\approx$ 1cm. Historically, the propogation and power deposition of these waves has been modeled by coupled geometric optics (ray tracing) and Fokker-Planck codes. In the past few years several authors have sought to address the effects of physical optics on LH propogation and power and current deposition [Wright, J. et al {\em Phys. Plasmas\/} {\bf 17} 056119 (2009) ;Meneghini, O. et al {\em Phys. Plasmas\/} ]. In these works, differences between ray tracing and full wave or beam tracing are attributed to physical optics effects such as diffraction and focusing or the treatment of ray caustics and reflections. We show that some observed differences are due to the differences in the correspondance between initial or boundary conditions between the two approaches or differences in dielectric models. Focusing at caustics is identified as the primary cause of remaining differences in the case of linear damping. When evolution of the electron distribution function is taken into account we see more significant differences that we show are due to effects of interference in quasilinear diffusion that are captured in the fullwave approach.   

Initial predictive studies of HHFW heating in the NSTX-U device

Plasma heating and non-inductive current drive via applied fast waves at high harmonics of the ion cyclotron frequency have been used successfully on NSTX, though rf power losses outside of the last closed flux surface and interactions of the HHFWs with co-injected NBI, which are relevant in ITER and other devices, are not fully understood. NSTX-U will operate with toroidal magnetic fields up to 1 T, nearly twice the value used in the experiments on NSTX. While the dominant rf heating mechanisms in NSTX were found to be TTMP damping on electrons and fast ion damping at high cyclotron harmonics, at the mid-harmonic range expected on NSTX-U the power partitioning may change. Initial simulations with the AORSA, TORIC, METS and GENRAY codes indicate that significantly more thermal D damping may occur in NSTX-U. Detailed predictions of the fields and power deposition obtained from these codes will be discussed.   

Integrated Plasma Simulation of Ion Cyclotron and Lower Hybrid Range of Frequencies Actuators in Tokamaks

Recent upgrades to the ion cyclotron RF (ICRF) and lower hybrid RF (LHRF) components of the Integrated Plasma Simulator [1] have made it possible to simulate LH current drive in the presence of ICRF minority heating and mode conversion electron heating. The background plasma is evolved in these simulations using the TSC transport code [2]. The driven LH current density profiles are computed using advanced ray tracing (GENRAY) and Fokker Planck (CQL3D) [3] components and predictions from GENRAY/CQL3D are compared with a ``reduced'' model for LHCD (the LSC [4] code). The ICRF TORIC solver is used for minority heating with a simplified (bi-Maxwellian) model for the non-thermal ion tail. Simulation results will be presented for LHCD in the presence of ICRF heating in Alcator C-Mod. \\[4pt] [1] D. Batchelor \textit{et al}, Journal of Physics: Conf. Series \textbf{125}, 012039 (2008).\\[0pt] [2] S. C. Jardin \textit{et al}, J. Comp. Phys. \textbf{66}, 481 (1986).\\[0pt] [3] R. W. Harvey and M. G. McCoy, Proc. of the IAEA Tech. Comm. Meeting on Simulation and Modeling of Therm. Plasmas, Montreal, Canada (1992).\\[0pt] [4] D. Ignat \textit{et al, }Nucl. Fus. \textbf{34}, 837 (1994).\\[0pt] [5] M. Brambilla, Plasma Phys. and Cont. Fusion \textbf{41},1 (1999).   

SOLPS modeling of the ORNL helicon and PhIX experiments

The ORNL helicon experiment has produced large cross section plasmas (12cm) at high densities (up to 4x10$^{19}$/m$^{3}$ in D and 6x10$^{19}$/m$^{3}$ in He) at high powers (up to 90kW). The Physics Integration eXperiment (PhIX) will investigate adding electron heating with Whistler waves (18GHz) and EBW (18GHz) to the helicon source plasma in order to increase T$_{e}$. Interpretative analyses of the helicon discharges in D and He with the SOLPS transport code show that 2D heating profiles based on resonant power absorption calculations reproduce the main features of the measured density and temperature distributions. The PhIX plasma column, including the helicon and RF heated mirror cell, has diameter $\approx $ 12-15 cm and length $\approx $ 3 m. Predictive SOLPS simulations of PhIX, with additional EBW and Whistler power absorption profiles from the GENRAY-C code, indicate a doubling of T$_{e}$ in the mirror cell. With plasma fueling by an upstream gas puff, calculations indicate $\approx $ 2:1 power split between the downstream and upstream targets.   

Design of an ICRH antenna for RF-plasma interaction studies

The interaction between an ion cyclotron resonant heating antenna and the near-field plasma can lead to rectified (high voltage) sheath formation and subsequent material erosion. This issue will be studied by using a simple loop antenna operated on the Physics Integration eXperiment (PhIX) at ORNL, which is a linear plasma device that uses an ECH heated helicon plasma source to create a high-density plasma suitable for use in a plasma-material interaction test stand. The antenna consists of a single strap with a single-tier Faraday shield. The antenna is $\sim $one-quarter wavelength long at 50 MHz and grounded at one end, which will allow for strap voltages of $>$20 kV to be located near the plasma. The PhIX edge plasma near the antenna is similar to typical edge conditions, with n$_{e}\sim $1-2x10$^{18}$/m$^{3}$ and T$_{e}$=5-10 eV, with a magnetic field of 0.1-0.2 Tesla. Several diagnostics will be used to characterize the near-field interaction, including Langmuir and capacitive probes, energy analyzers, Stark effect spectroscopy, and local/remote material erosion measurements. Details of the antenna design and initial characterization will be presented.   

The PhIX High Intensity Plasma Source

The Physics Integration eXperiment (PhIX) is a linear high-intensity rf plasma source presently being constructed at ORNL that combines a high density helicon plasma generator with an electron heating section. It will be used to explore the physics related to heating an overdense, streaming plasma in a linear geometry by whistler waves and Electron Bernstein Waves (EBW), including optimization of heating efficiency and maximization of particle flux. Interactions between the plasma production and heating regions, and the source and a downstream target, will also be investigated. Experiments using the device will provide data for the design of an rf powered high particle flux ($\sim 10^{24}/m{^2}- s$), high heat flux($\sim 10 MW /m^{2}$) steady-state linear plasma-materials test station (PMTS). In preparatory experiments, the helicon device has operated at power levels up to 90 kW, producing high plasma densities in He ($6 \times 10^{19} m^{-3}$) and D ($> 4 \times 10^{19} m^{-3}$), and has also operated at high magnetic field strength up to 0.5 T. Separate ECH experiments have demonstrated both whistler and EBW coupling at 6 GHz to an overdense plasma. A review of these experiments will be presented, as well as an overview of PhIX and its status.   

Cherenkov radiation of shear Alfv\'{e}n waves in plasmas with two ion species

In magnetized plasmas with two ion species there exists a unique frequency, the ion-ion hybrid frequency, which significantly alters the dispersion relation for Alfv\'{e}n waves. Multiple ion species plasmas are encountered both in fusion devices and in planetary magnetospheres. Further, in these environments, various mechanisms exist that give rise to fast, energetic particle populations. It is of interest to examine the excitation of Alfv\'{e}n waves by these fast particles. Results from a theoretical study of Cherenkov radiation by charged particle bursts in a two ion-species plasma are reported. Due to the presence of two ion-species, the Alfv\'{e}n waves propagate within two different frequency bands separated by a gap. The radiation pattern in the lower frequency band is found to exhibit essentially the same properties reported in a previous study [B. Van Compernolle et al., Phys. Plasmas 15, 082101 (2008)] of a single species plasma. However, the upper frequency band differs from the lower one in that it always allows for the Cherenkov radiation condition to be met. The methodology is extended to examine the Alfv\'{e}nic wake of point-charges. The wake is illustrated for conditions applicable to a fusion-born alpha particle in ITER.   

Glow Plasma Discharges inside Externally Excited Porous Spherical Cavity Resonators

A porous spherical cavity resonator (PSCR) provides amplification of externally incident electric fields a resonant frequencies corresponding to discrete modes. The PSCR has a mesh surface with a large number of polygon (hexagon and pentagon) holes. The size of the holes is adjusted to maximize the Q of the resonator for production of maximum internal electrical fields. Amplification factors for a PSCR are about 1000. The high resonator Q requires precise tuning of the incident wave frequency to a resonant frequency. The PSCR is placed in a low-pressure (1 T) gas chamber and excited by an external microwave horn for a chosen spherical cavity resonator mode. At the resonant frequency, a glow discharge occurs inside the cavity producing a plasma cloud in the shape of electric field modes. Varying the neutral gas pressure inside the chamber (1) yields variations in the glow discharge light intensity and (2) affects the shapes of the plasma cloud. If the plasma frequency in the electron cloud approaches the incident wave frequency, self-action produces localized regions of dense plasmas. The PSCR apparatus can be used to study cavity resonator modes in the low pressure environment and electromagnetic wave interactions in high pressure plasmas.   

A Low-Voltage Heated-Cathode Discharge Device for Nonlocal Control of Plasma Properties

In this research a low-voltage gas discharge device with heated cathode has been used for demonstration of controlling plasma properties by means of regulation of nonlocal energetic electrons. The discharge is formed between a heated cathode and an anode. A special molybdenum diaphragm, the control electrode, is placed between cathode and anode. Experiments and modeling of the device suggest the presence of two dramatically different modes, which are dependent on the diaphragm voltage. The transition between modes leads to a significant variation in plasma properties. It is experimentally shown that increasing the gas pressure (which leads to transition from plasma with nonlocal electron energy distribution (EDF) to plasma with local EDF) will eventually terminate this effect and for higher pressure there is only one mode in the discharge. Modeling for different radii of the diaphragm opening allows demonstrate modification of the effect.   

Application of Oxide cathode discharge source in a linear plasma device

A plasma source using barium oxide indirectly heated cathode is attached to one end of a linear plasma device. The homemade source provides a uniform, quiet and reproducible plasma column with a diameter 10 cm in the 2 meters long linear chamber. The peak density is around 1019 m-3 with a neutral pressure 5E-2 Pa. Besides the normal pulse running mode, a quasi-steady state running mode is also tested, in which a low density plasma is maintained for days long. We also found that the electron emissivity of the cathode is doubled when scandium oxide is added into the cathode as an impurity. Initial result of the low frequency drift wave turbulence and related transport are presented.   

Initial Operation of the Miniaturized Inductively Heated Plasma Generator IPG6

In close collaboration between the Center for Astrophysics, Space Physics and Engineering Research (CASPER) at Baylor University, Texas, and the Institute of Space Systems (IRS) at the University of Stuttgart, Germany, two plasma wind tunnel facilities of similar type have been established using the inductively heated plasma source IPG6 which is based on proven IRS designs. The facility at Baylor University (IPG6-B) works at a frequency of 13.56 MHz and a maximum power of 15 kW. A vacuum pump of 160m$^{3}$/h in combination with a butterfly valve allows pressure control in a wide range. First experiments have been conducted with Air, O$_{2}$ and N$_{2}$ as working gases and volumetric flow rates of up to 14 L/min at pressures of a few 100 Pa, although pressures below 1 Pa are achievable at lower flow rates. The maximum tested electric power so far was 8 kW. Plasma powers and total pressures in the plasma jet have been obtained. In the near future the set up of additional diagnostics, the use of other gases (i.e. H$_{2}$, He), and the integration of a dust particle accelerator are planned. The intended fields of research are basic investigation in thermo-chemistry and plasma radiation, space plasma environments and high heat fluxes e.g. in fusion devices or during atmospheric entry of spacecraft.   

Excitation of Ion Acoustic Waves by Electron Beams

The interaction of electron beams with plasmas is of considerable importance particularly for hybrid DC/RF coupled plasma sources used in plasma processing [1]. An electron beam is formed by emission from one surface, is accelerated through a dc bias electric field and enters the bulk plasma. Emitted electrons excite electron plasma (Langmuir) waves through the two-stream instability. Due to the high localized plasmon pressure, ion acoustic waves are excited parametrically. The plasma waves saturate by non-linear wave trapping. Eventually coupling between electron plasma waves and ion acoustic waves deteriorates the Langmuir waves, which leads to a bursting behavior. The two-stream instability and the consequent ion fluctuations are studied over a wide range of system parameters using the particle-in-cell codes EDIPIC and LSP. The influenceof these instabilities on collisionless electron heating are presented for a hybrid RF-DC plasma source.\\[4pt] [1] Lin Xu, et al, Appl. Phys. Lett., 93, 261502 (2008).   

Geometrical Optics of Dense Aerosols

Dense aerosols might be focused in such a way that a planar sheet of material is created, which could be ionized to form a free-standing sheet of moderately high-density plasma. Such a sheet of plasma might be useful for its optical properties, possibly as a lens or as an amplifier, since free access is available normal to the plane. We simulate dense aerosol formation under such aerodynamic focusing. The simulations include momentum coupling between the carrier gas and the particles' virtual flow field. The linear focusing problem elucidates the presence of aberrations in aerodynamic lenses designed for dense aerosols.   

Laboratory modeling of hypersonic flight conditions

One of the key issues for vehicles in hypersonic flight and during atmospheric reentry is radio blackout due to weakly-ionized air plasma formation. When a spacecraft enters Earth's atmosphere or a vehicle travels through the atmosphere at hypersonic velocities, a shock wave is formed in front of the vehicle. The shock wave converts much of the vehicle's kinetic energy into heat and as a result the air molecules are dissociated and ionized. This plasma layer prevents normal telemetry transmission. This work considers a new approach to model the conditions of hypersonic flight in laboratory environment. The approach utilizes hypersonic plasma jet created by vacuum arc that hits immovable object intended to model a hypersonic vehicle. Heating of the object by the arc causes immediate re-evaporation of the jet's metal ions being deposited on the object's surface. This mimics absence of attachment of the air molecules to the vehicle in hypersonic flight. The plasma parameters and object temperatures are measured using electrostatic Langmuir probes and thermocouples respectively. The results of these experiments can be also used as calibration tool for tuning and debugging of numerical codes intended to predict and mitigate the blackout problem.   

Python framework for kinetic modeling of electronically excited reaction pathways

The use of plasma energy to enhance and control the chemical reactions during combustion, a technology referred to as ``plasma assisted combustion'' (PAC), can result in a variety of beneficial effects: e.g. stable lean operation, pollution reduction, and wider range of p-T operating conditions. While experimental evidence abounds, theoretical understanding of PAC is at best incomplete, and numerical tools still lack in reliable predictive capabilities. In the context of a joint experimental-numerical effort at Michigan State University, we present here an open-source modular Python framework dedicated to the dynamic optimization of non-equilibrium PAC systems. Multiple sources of experimental reaction data, e.g. reaction rates, cross-sections and oscillator strengths, are used in order to quantify the effect of data uncertainty and limiting assumptions. A collisional-radiative model (CRM) is implemented to organize reactions by importance and as a potential means of measuring a non-Maxwellian electron energy distribution function (EEDF), when coupled to optical emission spectroscopy data. Finally, we explore scaling laws in PAC parameter space using a kinetic global model (KGM) accelerated with CRM optimized reaction sequences and sparse stiff integrators.   

Development of a Compact Atmospheric Pressure Plasma Source

Open plasma sources working at atmospheric pressure have a variety of uses, including applications in both the medical [1] and industrial realms [2]. We will be reporting on the development of a compact RF-driven plasma source. Operation of the system will utilize common mono- and diatomic atmospheric gases [3]. Further diagnostics, including UV-VIS emission spectra and in-situ probing, will be performed and presented. \\[4pt] [1] Plasma Medicine: Applications of Low-Temperature Gas Plasmas in Medicine and Biology, Ed. M. Laroussi, M. G. Kong, G. Morfill, and W. Stolz, Cambridge Press, 2012.\\[0pt] [2] A. Fridman, Plasma Chemistry, Cambridge Press, 2008.\\[0pt] [3] M. Capitelly, C.M. Ferreira, B.F. Gordiets, and A.I. Osipov, Plasma Kinetics in Atmospheric Gases, Springer Series on Atomic, Optical, and Plasma Physics, 2000.   

Atmospheric-pressure ionic plasmas in afterglow of atmospheric-pressure discharges in room-temperature air

Ionic plasmas, which have been studied in collosionless cases so far, are theoretically analyzed and observed in experiments in atmospheric pressure gases at room temperature. Although their densities are fairly low, collective motions of positive and negative ions with charge neutrality lead to several unique properties such as broad ion mass spectra, frequency-dependent dielectric constant, and very long life time around a few seconds. One possible situation in which atmospheric-pressure ionic plasmas is observed is in an afterglow phase of atmospheric-pressure discharges with weakly-ionized plasmas. During the discharges, electrons and positive ions are present as majority species of charged particles, and some of the electrons attach to neutral atoms and/or molecules to form negative ions. Via adjustment of charge density balance, such ionic plasmas are composed of positive and negative ions. From analysis on their dielectric properties in the fluid model, the dielectric constants are predicted to be similar to those in the Drude type, which are confirmed in a few specific experiments.   

Cold Atmospheric Plasma as an alternative therapy for cancer therapies

CAP (cold atmospheric plasma) is a technology, which is based on quasi-neutral ionized gas (plasma at low temperatures), which is being evaluated as an alternative or addition to existing cancer therapies. A recent study shows that CAP treatment can cause a significant reduction in tumor size in vivo. Thus the purpose of this study is to begin to identify the mechanism by which cancer cells are killed by CAP, i.e. to identify the mechanism of CAP action. CAP induced a robust $\sim $2-fold G2/M increase in two different types of cancer cells with different degrees of tumorigenicity. We hypothesize that the increased sensitivity of cancer cells to CAP treatment is caused by differences in the distribution of cancer cells and normal cells within the cell cycle. The expression of $\gamma $H2A.X (pSer139), an oxidative stress reporter indicating S-phase damage, is enhanced specifically within CAP treated cells in the S phase of the cell cycle together with significant decrease in EdU-signal of DNA-replicating cells. Our data suggest that more tumorigenic cancer cells are better susceptible to CAP treatment.   

Characteristics of dielectric barrier discharge plasmas in atmospheric humid air

Atmospheric pressure plasmas have a great advantage for industrial applications such as surface modifications, sterilization and film preparation. In particular, reactive plasmas including OH radicals can be generated in humid air. On the other hand, it is known that dielectric barrier discharge (DBD) plasmas in air are strongly affected by humidity. In this study, a twisted pair sample is used as a DBD electrode. The twisted pair consists of two enameled wires, and it is installed in a climate chamber to control ambient temperature and humidity. Repetitive impulse voltage pulses were applied to the twisted pair to produce DBD plasmas. Light emission, electromagnetic wave and current pulses were used to detect discharge activities. The discharge inception voltage (DIV) is basically determined by Paschen curve in air, however, the DIV was decreased by increasing the humidity. In addition, it was found that there were largely scattered data of DIV at the low humidity condition. After the pre-discharges, the DIV reached to the steady state value. On the other hand, there was no scattering of the observed DIV at the high humidity condition. Measurements of surface potential of the sample after the discharge show these behaviors could be explained by surface charge accumulation on the enameled wire. It is noted that there was no fluctuation in the DIV data in the case of unipolar voltage pulse.   

Time Evolution of Electron Density in Atmospheric Discharges

The electron density of atmospheric air discharges was found via the Stark broadening of H-beta emissions, with the time evolution of spectra captured by several methods to be compared: a spectrometer with a streak camera, a spectrometer with a gated ICCD camera, and photodiodes with filters. Discharges on the order of 10 microseconds in duration, a joule in energy and a few millimeters in length, were created across a triggered spark gap. A second spark gap allowed shunting away of current to yield a roughly rectangular current pulse through the discharge, permitting the examination of plasma afterglow. Additionally, evolution of discharges exposed to low level ionization radiation and an ion generator are examined to determine how background ion level impacts the observable discharge dynamics, with potential applications in remote detection of radiation.   

Spectroscopy Measurements of Air Breakdown Initiated with a Focused 110 GHz, MW-Level Quasioptical Gyrotron Beam

We present spectroscopic measurements of air breakdown that is created with a focused 110 GHz gyrotron beam at fluxes exceeding 1 MW/cm$^{2}$. Excitation, rotational, and vibrational temperature measurements are made over the pressure range of 1-100 Torr. The gyrotron power is varied to measure the behavior of these plasma temperatures as the field is raised above the threshold field required for breakdown. Rotational temperature measurements of the plasma show minimal gas heating, with gas temperatures in the range of 300-500 K. The vibrational and excitation temperatures were measured to be on the same order and vary between 4200-6200 K and 0.4-0.65 eV, respectively. In order to calculate the excitation temperature it was necessary to include the deviation from Local Thermodynamic Equilibrium (LTE) in the analysis because the electron density of the discharge is not high enough to satisfy the conditions for LTE. The excitation temperature is observed to vary little with applied field, while the vibrational and rotational temperatures increase as the field is increased above threshold. The temperature measurements indicate that the plasma is in a state of thermal non-equilibrium, with heavy ions and neutrals heated by collisions with much hotter electrons.   

Focused Laser Initiated RF Sustained High Pressure Air Plasmas

Measurements and analysis of air breakdown processes and plasma production were done by focusing 193 nm, 300 mJ, 15 MW high power laser radiation inside a helical RF coil. We observe quantum resonant multi-photon and collisional cascade laser ionization processes that produce high density (ne$\sim $5*10$^{15}$/cm$^{3})$ cylindrical seed plasmas. We installed an improved capacitive system that better matches the antenna impedance before plasma is produced, which increases the breakdown pressure from 20 to 60 torr with 5 kW incident RF power only. The focused laser and associated shock wave produces a plasma seed for sustaining by the RF (1-10 kW, 0.5-1.8 s) pulse. We find that triggering 20 ns multi-laser pulses at 20 Hz during one RF pulse increases the breakdown pressure from 70 to 85 torr single laser pulse. Measurements of the helical RF antenna plasma-loaded impedance are obtained by measuring the complex reflection coefficient with and without the laser pulse. Additional diagnostics are obtained with a 105 GHz interferometer to measure plasma density, collision frequency and electron temperature. Spectroscopic measurements of the plasma and comparison with the SPECAIR code are made to determine rotational and vibrational neutral gas temperatures. The results demonstrate that the laser formed seed plasma allows RF sustainment at higher initial air pressures than with RF only initiation.   

Electron and Negative Ion Production Rates in Air Plasmas with Ionizing Radiation

Electron and negative ion production rates during atmospheric discharges in the presence of ionizing radiation are investigated. Ionizing radiation creates free electrons and negative ions with number densities that may be orders of magnitude higher than background conditions. These high densities not only facilitate air breakdown between high-voltage electrodes or at the focal point of high-power electromagnetic beams, but also change the breakdown evolution and the neutrals recombination history after the power source is turned off. Time dependencies of breakdown and recombination rates on radiation levels and aerosol concentrations, are modeled and compared to measurements of breakdown between electrodes. This research is part of a wider effort to investigate the feasibility of a remote detection scheme for radioactive materials, utilizing sub-THz beams to produce air breakdown [G. S. Nusinovich, et al. J. Appl. Phys. \textbf{109}, 083303 (2011)].   

Electron ranaway and ion-ion plasma formation in afterglow low-pressure plasma of oxygen-containing gas mixtures

Experimental investigation of temporal evolution of charged plasma species in afterglow plasma of oxygen-containing mixtures have been investigated. The probe VAC and the time dependence of the saturation positive and negative particles currents to a probe in a fixed bias voltage were performed. The decay of afterglow low-pressure electronegative gas plasmas take place in two distinct stages (the electron-ion stage, and the ion-ion stage) as it was shown in [1] for pure oxygen. In the first stage, the negative ions are locked within a discharge volume and plasma is depleted of electrons and positive ions. The electron density decay is faster, than exponential, and practically all electrons leave plasma volume during finite time followed by the ion--ion (electron-free) plasma formation. The decay of the ion-ion plasma depends on the presence of detachment. With a large content of electronegative gas (oxygen) in a mixture, when there is a ``detachment particles,'' a small fraction of the electrons appearing as a result of the detachment continue to hold all negative ions in the discharge volume. In this case, the densities of all charged plasma components decay according to the same exponential law with a characteristic detachment time. At a low oxygen content in the gas mixture there is no detachment and plasma decays by an ion--ion ambipolar diffusion mechanism.\\[4pt][1]. S.A.Gutsev, A.A.Kudryavtsev, V.A.Romanenko. Tech.Phys. 40, 1131, (1995).   

2D ion velocity distribution function measurements by laser-induced fluorescence above a radio-frequency biased silicon wafer

Ion dynamics have been measured in the sheath above a 30 cm diameter, 2.2 MHz-biased silicon wafer in a plasma processing etch tool using laser-induced fluorescence (LIF). The velocity distribution function of argon ions was measured at thousands of positions above and radially along the edge of the wafer by sending a planar laser sheet from a pulsed, tunable dye laser into the tool. The RF sheath is clearly resolved. The laser sheet entered the machine both parallel and perpendicular to the wafer in order to measure the distribution function for both parallel and perpendicular velocities/energies (0.4 eV $< E_{max} <$600 eV). The resulting fluorescence was recorded using a fast CCD camera, which provided spatial (0.4 mm) and temporal (30 ns) resolution. Data was taken at eight different phases of the 2.2 MHz cycle. The distribution functions were found to be spatially non-uniform near the edge of the wafer and the distribution of energies extremely phase-dependent. Several cm above the wafer the distribution is Maxwellian and independent of phase. Results are compared with simulations; for example, the experimental time-averaged ion energy distribution function compares favorably with a computer model carefully constructed to emulate the device.   

Overview of Inertial Electrostatic Confinement Plasma Physics Research at the University of Wisconsin

In inertial-electrostatic confinement (IEC) fusion devices, a voltage difference between nearly transparent electrodes accelerates ions to fusion-relevant velocities, typically in spherical geometry. University of Wisconsin IEC research has produced $\sim10^8$ steady-state and $\sim10^{10}$ pulsed DD neutrons per second, plus $\sim10^8$ D$^3$He protons per second~[1]. The neutrons have been used to detect highly enriched uranium (HEU) and C-4 explosives; the protons have produced radioisotopes for positron emission tomography at proof-of-principle levels~[1]. A new 300 kV, 200 mA power supply will begin operation in 2012, which should increase fusion reaction rates. Presently, the investigation of IEC plasma physics issues at the University of Wisconsin comprises: (1) theoretical analysis of ion and neutral flow through atomic or molecular gases; (2) negative-ion production; (3) fusion of DD, D$^3$He, and $^3$He$^3$He; (4) converging ion beams; and (5) ion-surface interactions. Diagnostic development includes: (a) charged fusion product Doppler-shift and time-of-flight; (b) movable Faraday cup; and (c) double Langmuir probe.\\[4pt] [1] G.L. Kulcinski, et al., {\it Fusion Science and Technology} {\bf 56}, 493, (2009).   

Modeling Two-Charge State Helium Plasmas

A computational model for the flow of energetic helium ions and atoms through a background neutral helium gas is being developed. The essence of the method is to consider atomic reactions as creating a new source of ions or neutrals if the energy or charge state of the resulting particle is changed. A set of conservation equations in a two-dimensional (position -- energy) phase space is formulated. Atomic reactions that lead to ions being born with zero kinetic energy are modeled with a 1-D Volterra integral equation [1] that can quickly be solved numerically by finite differences. Atomic reactions leading to ions being born with finite kinetic energy are formulated as source terms in the position-energy phase space. The conservation equations are solved iteratively using the solution to the Volterra equation as a starting point. The current work focuses on multiple-pass, 1-D ion flow through neutral gas in a nearly transparent anode and cathode pair in planar, cylindrical, and spherical geometry for application to $^{3}$He-$^{3}$He and D-$^{3}$He inertial electrostatic experiments.\\[4pt] [1] G.A. Emmert and J.F. Santarius, ``Atomic and Molecular Effects on Spherically Convergent Ion Flow I: Single Atomic Species,'' Phys. Plasmas 17, 013502 (2010)   

Experimentally Study of Micro-Cathode Arc Thruster ($\mu $CAT)

A micro-Newton level cathode arc thruster ($\mu $CAT) with magnetically enhanced system has been proposed to address the long-time operation of micro-thruster for the nano-satellite propulsion. One important parameter governing the thrust force is velocity of the ions. In this work, we present the methodology of the Ti ion velocities measurement produced by $\mu $CAT and especially address the influence of magnetic field on the ion motion. The ion velocities are studied by means of time-of-flight (TOF) method equipped with enhanced ion detection system (EIDS). The EIDS method consists of perturbations (spikes) on arc discharge current waveform to generate denser plasma bunches and following detection of moments of time when perturbations arrives at the detectors. The novel double probes ion detection system could overcome the problem of noise generation simultaneously with the arc current perturbation associated with utilization of conventional single probe detector. When plasma bunch crosses each of the double probes, the spike on the probe current is detected following variation of plasma density. By measuring the delay times between the neighbor probes the average ion velocity can be determined. The Ni ion velocities are measured to compare with Ti ion velocities.   

Enhanced Output of the High Power Helicon with Addition of a Downstream Accelerating Antenna

The high power helicon (HPH) is a compact plasma source that can generate downstream densities of 10$^{17}$-10$^{18}$ m$^{-3}$ and directed ion energies of 50-70 eV that continue to increase tens of centimeters downstream of the source. Previous results indicated a diamagnetic perturbation more than 15 gauss in strength that propagates tens of centimeters downstream and cancels out the base magnetic field on axis. This was correlated with the helicon wave being driven by the antenna and indicated an azimuthal current density which peaked at 20 kA m$^{-2}$. In order to increase the energy coupled into the plasma and drive a larger diamagnetic perturbation a further distance downstream a second, larger radius antenna was added roughly one wavelength downstream co-axially with the first antenna and driven in phase with the first. This resulted in improved collimation of the plasma beam over a meter downstream, increased diamagnetic perturbation, and an increase in the ion energies downstream of more than 20 eV. Ion energy distributions and plasma density measurements will be presented, along with magnetic measurements that suggest enhanced performance of the plasma output.   

Progress in Development of Low Pressure High Density Plasmas on a Small Helicon Plasma Experiment (HPX)

At the Coast Guard Academy Plasma Lab (CGAPL), a small Helicon Plasma Experiment (HPX) is being developed to utilize the reputed high densities (10$^{13}$ cm$^{-3}$ and higher) at low pressure (.01 T) [1], for eventual high temperature and density diagnostic development in future laboratory investigations. HPX is designed to create repeatedly stable plasmas induced by an RF frequency in the 10 to 70 MHz range and employs an electromagnet to provide the external energy in the plasma's magnetic field to transition from the H-Mode to the Helicon Mode. An acceleration~coil, currently under construction, will place the plasma in the vacuum chamber for optical and particle probing. With the initial construction phase complete and first plasmas attained, HPX is constructing triple and mach particle probes, magnetic probes, and a single point 300 W Thompson Scattering system backed by a 32-channel DAQ system capable 12 bits of sampling precision at 2 MS/s for plasma property investigations. Progress on the development of the RF coupling system, magnetic coils, and qualitative observations from the optical and electric diagnostics are to be reported. \\[4pt] [1] K. Toki, \textit{et al}., \textit{Thin Solid Films} \textbf{506-507} (2005).   

Examination of Ion Beam Acceleration in A High Power-Low Pressure and Gas Flow Rates Argon Plasma Created in the MadHeX Helicon Source

The modified MadHeX experimental system consists of a Pyrex tube connected to a stainless steel chamber with an axial magnetic nozzle field, variable up to 1 kG at the source region that has been upgraded to minimize neutral reflux and reduce neutral concentrations in the chamber. A half-turn double-helix antenna is used to excite helicon waves in the source. An ion beam of energy, E = 160 eV at 500 W RF power, has been observed in a low flowing argon plasma formed in the expanding region with a 340 G magnetic field. The role of plasma positive ``self-bias'' and the effects of boundary conditions are discussed. The measured density decrease factor of 18 at 100 W RF power across the expansion region yields a higher ion acceleration and agrees with a conservation-of-flux calculation. The effect of lower flow rates and pressures, higher RF powers and magnetic field strength dependence on the ion beam acceleration, plasma potential, electron density and temperature are further explored. The axial ion velocity distribution function and temperatures at higher powers are observed by argon 668 nm laser induced fluorescence with density measurements by interferometry. The electron energy distribution and its possible non-Maxwellian tail are examined using optical emission spectroscopy (ADAS and Vlcek models).   

Ion Extraction from a Helicon Plasma Source for an Inertial Electrostatic Confinement Fusion Device

HELIOS, an inertial electrostatic confinement (IEC) fusion device designed for advanced fuel studies [1], uses an external helicon plasma source, from which ions are extracted and subsequently accelerated radially into an electrostatic potential well set up by a semi-transparent cathode grid inside a spherical chamber. A campaign is underway to raise fusion rates to allow for diagnostic studies of IEC physics with helium-3 fuel, in order to benchmark the single-atomic-species formalism of VICTER, a Volterra integral-equation code on spherically convergent ion flow [2]. The helicon plasma has been characterized through double probe measurements of plasma density and electron temperature for various rf antenna and magnetic field geometries. Measurements of the extracted ion current using a witness plate and a Faraday cup are also presented.\\[4pt] [1] G.R. Piefer et al., Fusion Sci. Technol. 47, 1255 (2005).\\[0pt] [2] G.A. Emmert and J.F. Santarius, Phys. Plasmas 17, 013502 (2010).   

A Comparison of Laser Induced Florescence and Continuous Wave Ring Down Spectroscopy Measurements of Argon Ion and Neutral VDFs in a Helicon Plasma

In this work, we compare two spectroscopic methods for measuring the velocity distribution functions (VDFs) of argon ions and neutrals in a helicon plasma: laser induced florescence (LIF) and continuous wave cavity ring down spectroscopy (CW-CRDS). An established and powerful technique, LIF suffers from the requirement that the initial state of the LIF sequence have a substantial density. In most cases, this requirement limits LIF to ions and atoms with large metastable state densities for the given plasma conditions. CW-CRDS is considerably more sensitive than LIF and can potentially be applied to much lower density populations of ion and atom states. However, CRDS is a line integrated technique that lacks the spatial resolution of LIF. CRDS is a proven, ultra-sensitive, cavity enhanced absorption spectroscopy technique and when combined with a CW diode laser that has a sufficiently narrow linewidth, the Doppler broadened absorption line, i.e., the VDFs, can be measured. We present CW-CRDS and LIF measurements of the VDFs in an argon plasma using the 668.614 nm (in vacuum) line of Ar II and the 667.9125 nm (in vacuum) line of Ar I.   

A comparison of ion velocity measurements from retarding field energy analyzer (RFEA) and laser induced fluorescence (LIF) in expanding helicon plasma

We present a comparison of ion velocity measurements from RFEA and LIF where we specially focus on the ability to diagnose flow and ion beams due to current free double layers in expanding helicon plasma. An RFEA in a plasma will be surrounded by a sheath so the velocities measured by the RFEA will be the velocities of the ions after they have been accelerated by the sheath and any potential drops inside the probe. Different methods exist to relate the velocities measured by the RFEA to the velocities in the plasma. Most of them include some simple assumptions about the acceleration in the sheath. We will compare measurements from two different RFEAs, one with a grounded front grid and one with a floating front grid, with LIF measurements to assess the validity of these simple models.   

Multiple Current-Free Double Layer Formation in an Expanding Plasma

Over the past decade, experimental and theoretical studies have demonstrated the formation of current-free, stable, electrostatic double layers in plasmas with a strong density gradient resulting from a divergent magnetic field. In this work, we present evidence for the formation of multiple double layers within a single divergent magnetic field structure. Downstream of the divergent magnetic field, multiple accelerated ion populations are observed through laser induced fluorescence measurements of the ion velocity distribution function. The formation of the multiple double layer structure is a strong function of the neutral gas pressure in the experiment. The relation of the accelerated ion populations observed in these laboratory experiments to observations in the auroral zone and plasma sheet boundary layer is described.   

Direct Measurements of the Ionization Fraction in Krypton Helicon Plasmas

Helicons are efficient plasma sources, capable of producing plasma densities of 10$^{19}$ m$^{-3}$ with only 1 kW of input RF power. But the ionization fraction in the core of a helicon is usually not well known, because the neutral density is typically inferred from indirect spectroscopic measurements or from edge pressure gauge measurements. We have developed a two photon absorption laser induced fluorescence (TALIF) diagnostic capable of directly measuring the local neutral density. We use TALIF in conjunction with a Langmuir probe to measure the neutral and ion density profiles as a function of driving frequency and magnetic field. We find that when the frequency of the driving wave is greater than the lower hybrid frequency ($f_{LH})$, the core ionization fraction is small (0.1 {\%}) and the plasma density low (10$^{17}$ m$^{-3})$. As the axial magnetic field is increased, or, equivalently, the driving frequency decreased, a transition is observed at RF = $f_{LH}$. The plasma density increases by a factor of 10 or more, the plasma density profile becomes strongly peaked, the neutral density profile becomes strongly hollow, and the ionization fraction in the core reaches $\sim $100{\%}. The dramatic neutral depletion in the core is thought to be due to a combination of increased ionization and neutral pumping. The role of the latter is quantified by a comparison of flowing and static discharges.   

The Development of a Solid Fuel Plasma Source for the High Power Helicon

The high power helicon (HPH) is a compact plasma source that generates downstream densities of 10$^{18}$ m$^{-3}$ and directed ion energies greater than 50 eV. To date most of the work on helicons have involved gas fed systems. The problem with gas propellant is that it's expensive to store and the slow propagation through control valves makes it poorly suited for pulsed systems. In order to address both issues a solid propellant helicon using the same technology as pulsed plasma thrusters (PPTs) is being developed. A current pulse ablates a layer of Teflon, creating a plume of roughly 10{\%} plasma and 90{\%} neutrals. PPT electron densities on the order of 10$^{19}$ m$^{-3}$ were measured 20 cm downstream of the Teflon surface when fired at 43 J. The thruster is estimated to ablate 100 $\mu $g of propellant per pulse. PPTs are known for their compact nature but the presence of a large neutral cloud reduces the overall ISP and efficiency of the system. The HPH system provides greater than 90{\%} ionization of all ablated material and yields an extremely high ISP thruster with high power and neutral efficiency. Initial testing of the solid fuel HPH has shown comparable results to similar configurations with gas propellant while opening up the opportunity to have very discrete pulses without long ramp-up times.   

Pre-Stage Magnetic Coil to Enhance Helicon Mode Excitation on a Small Helicon Plasma Experiment (HPX)

Small helicon plasmas have been employed in various capacities from industry to spacecraft propulsion. At the Coast Guard Academy Plasma Lab (CGAPL), a small Helicon Plasma Experiment (HPX) is being developed to utilize the reputed high density (10$^{13}$ cm$^{-3}$ and higher) at low pressure (.01 T) [1] Helicon Mode Plasmas. HPX will become a high temperature and density diagnostic development test-bed for future laboratory investigations in addition to becoming a tool for future spacecraft propulsion devices. HPX Plasmas are created by imparting directed energy into a Pyrex tube preloaded with Ar gas with fill pressures on the order of 10$^{4}$ mTorr utalizing a power supply and matching box can deliver up 250 W of power in a 20 MHz to 100 MHz frequency range. It has been demonstrated [1] that a uniform magnetic field in lower energy level plasmas can facilitate a decrease in inertial effects, which promotes energy conservation within the plasma and provids the necessary external energy in the plasma's magnetic field to reach the Helicon Mode. HPX employes an electromagnet to establish this uniform field. An acceleration coil, currently under construction, will be used to increase the plasma velocity to facilitate partcle and optical probing within the vacuum chamber for experimental analysis. Initial accuracy and calibration measurements of the relative magnetic fields created by both electromagnets will be reported.\\[0pt][1] K. Toki, et al., Thin Solid Films 506-507 (2005).   

Supersonic ExB Rotation in the Highly Ionized, Low Temperature Plasma of a Hall Thruster

We study how pressure gradients and centrifugal forces from electron rotation affect the potential distribution near the thruster channel exit of a cylindrical Hall thruster. The region of interest is a low temperature plasma with magnetized electrons and flowing, unmagnetized ions. We use langmuir probe measurements to quantify the importance of pressure forces and centrifugal forces on determining the equilibrium potential distribution.   

Modeling the expansion of a contactor plasma

Plasma contactor technology is widely used on board spacecraft to keep spacecraft charging levels under control. On the International Space Station, for instance, it is used to prevent high current discharges between differently charged surfaces. It consists of emitting a neutral plasma to create a plasma reservoir near the spacecraft in order to balance the currents collected by the spacecraft from the magnetospheric environment. One approach to modeling the contactor plasma plume applies a self-similar solution in order to gain insight into the plume dynamics without requiring expensive numerical simulations [1, 2]. Typically, hydrodynamic fluid equations are used to model the plasma behavior. We present a comparison of different self-similar plume models existing in the literature [1, 2] and compare these with our Particle-In-Cell simulations in the near-field to assess their validity. We will consider both the unmagnetized and the magnetized limit. \\[4pt] [1] F. F. Gabdullin, A. G. Korsun, E. M. Tverdokhlebova, IEEE Trans. Plasma Science 36(5) 2207 (2008). \newline [2] M. Merino, E. Ahedo, C. Bombardelli, H. Urrutxua, J. Pelaez, ``Hypersonic plasma plume expansion in space,'' 32nd International Electric Propulsion Conference, IEPC-2011-086, Wiesbaden, Germany, 2011.   

Laser ablation with applied magnetic field for electric propulsion

Using ultrafast lasers with tera-watt-level power allows efficient ablation and ionization of solid-density materials [1], creating dense and hot ($\sim $100eV) plasma. We propose ablating small droplets in the magnetic nozzle configurations similar to mini-helicon plasma source [2]. Such approach may improve the momentum coupling compared to ablation of solid surfaces and facilitate plasma detachment. Results of 2D modeling of solid wire ablation in the applied magnetic field are presented and discussed. \\[4pt] [1] O. Batishchev et al, Ultrafast Laser Ablation for Space Propulsion, AIAA technical paper 2008-5294, -16p, 44th JPC, Hartford, 2008.\\[0pt] [2] O. Batishchev and J.L. Cambier, Experimental Study of the Mini-Helicon Thruster, Air Force Research Laboratory Report, AFRL-RZ-ED-TR-2009-0020, 2009.   

Emission spectra of electric propulsion thrusters

Accurate measurement of emission spectra of electric propulsion devices provides fast non-invasive way of operational regimes characterization and possible optimization. First, we present comparative spectra of three different thrusters [1] running on xenon propellant. Next, we analyze spectra of a versatile thruster running on noble and diatomic gases, to emphasize complex qualitative composition of the plasma exhaust. \\[4pt] [1] O. Batishchev and T. Matlock, Spectroscopic Characterization of the Electric Propulsion Thrusters, Final Report for the AFOSR grant FA9550-08-1-0223, -260p, 2010.   

Sensitivity of RF-Driven Plasma Filaments to Trace Gases in Neon

Filamentary structures have been observed in many types of plasma discharges in both natural and industrial systems (e.g. upper-atmospheric discharge phenomena and dielectric barrier discharges). Various aspects of their physics remain unclear. A common example can be found within a toy plasma globe (or plasma ball), wherein a primarily Neon gas mixture in a spherical glass vessel near atmospheric pressure ($\sim $ 740 Torr) clearly and aesthetically displays filamentation. Recent work has provided the first characterization of these plasma globe filaments [Campanell \textit{et al}, Physics of Plasmas 2010], where it was noticed that discharges of pure gases tend \textit{not} to produce filaments. We have extended this initial work to quantify the dependence on trace gases and absolute pressure on filament properties (e.g. average number, thickness). These initial results using a custom globe apparatus are here presented along with some preliminary discussion of the effects possible with a programmable high voltage supply. Ultimately, high-speed photography and in-situ probes will be used to characterize filament dynamics, allowing for a more detailed comparison with theory and simulations.   

Synthesis of single-walled carbon nanotubes and graphene composite in arc for ultracapacitors

Arc discharge supported by the erosion of graphite anode is considered as one of the most practical and efficient methods to synthesize various carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene with minimal defects and large yield due to the relatively high synthesis temperature and eco-friendly growth mechanism. By introducing a non-uniform magnetic field during synthesis process, large-scale graphene and high-purity SWCNT can be obtained in one step. In addition, the yield of graphene can be controlled by external parameters, such as the type and pressure of buffer gas, the temperature of substrate, and so on. Possessing the properties of highly accessible surface area and good electrical conductivity, the composite of graphene and SWCNT are promising nanomaterials for the electrodes of ultracapacitor, which can store electric energy with high level of capacitance. In this work, we fabricated electrodes of ultracapacitor based on nanostructures composite by wire-wound rod coating method, characterized them by SEM, EDX and Raman spectroscopy, and tested the performance by a potentiostat/galvanostat.   

Measuring plasma potential with an impedance probe in low density plasma

A recent rf technique for determining plasma potential, \textit{$\phi $}$_{p}$ , using an impedance probe was shown to be independent of probe geometry, magnetic field, and orientation. However, a problem which arises in low density plasma concerns a magnitude mismatch between typical network analyzer input impedance ($Z_{0 }$= 50 $\Omega )$ and the large value of ac resistance ($R_{ac})$ which is inversely proportional to $n_{e}$. The method relies on finding a minimum in \textit{Re(Z}$_{ac}$)\footnote{\textit{Phys. Plasmas} \textbf{17}, 113503 (2010).}$^,$\footnote{\textit{NRL Memorandum Report 6750-12-9413} (2012).} which is difficult if $R_{ac}$ is much larger than $Z_{0}$. For low density space plasmas (10$^{4}$ -10$^{5}$ cm$^{-3})$ values of $R_{ac}$ range to k$\Omega $ levels. We have developed numerical simulations based on solving the Boltzmann equation in spherical geometry for a given sheath size. These simulations include a presheath and predict values for $Z_{ac}$ which are then used to estimate the error as a function of input impedance based on the error associated with a 50 $\Omega $ load.   

Imaging x-ray fluorescence relevant to hydrodynamic mixing experiments at the National Ignition Facility

The National Ignition Facility (NIF) is capable of providing enough energy to explore areas of physics that are not possible on any previous laser system. This includes large-volume, geometrically complex hydrodynamic and radiation hydrodynamic experiments in which traditional, line-integrated radiographic techniques limit the quality of the results. As an example, we are involved in divergent hydrodynamic experiments at the NIF, motivated by supernova hydrodynamics, that cannot be diagnosed in detail with transmission radiography. X-ray scattering has been considered for this purpose and appears feasible [1]. Here we consider fluorescence imaging, a better candidate as the cross section of photoabsorption in the several-keV range is roughly 100 times larger than that of scattering. A single layer of the target will be uniformly doped with a fluorescent tracer, which will be pumped by a sheet of x-rays. The fluorescent intensity will be measured to create a density map of the doped material as it mixes with other layers. Developing this diagnostic will create a powerful tool to characterize hydrodynamic experiments with complex geometries.\\[4pt] [1] Huntington et al. High Energy Density Physics 6, 194 (2010).   

High Energy Density Z-Pinch Plasmas using Flow Stabilization: ZaP-HD

The ZaP Flow Z-pinch experiment at the University of Washington investigates the effect of sheared flows on MHD stability. Z-pinch plasmas scale to high energy density by reducing the pinch radius through any combination of lower linear density and higher pinch current. The ZaP experiment generates 100 cm long plasma pinch columns approximately 1 cm in radius. Experimental results show a period of low fluctuation levels when a sheared plasma flow is present. The quiescent period lasts for approximately 40 $\mu$s, approximately 2000 classical growth times. The length of the quiescent period is over four flow-through times for the 100 cm pinch. The experiment has focused on developing diagnostics that measure radial profiles of the plasma equilibrium (including velocity) and that measure the three-dimensional magnetic topology to determine plasma stability. The experiment could be modified to significantly increase the plasma energy density by decreasing the linear density of the pinch and by increasing the pinch current. The modified experiment, ZaP-HD, will produce plasma columns with smaller radii and, therefore, higher energy density. Experimental plans and scaling analyses will be presented.   

Insulator Modifications to Extend Plasma Lifetime on the ZaP Flow Z-Pinch

The ZaP Flow Z-Pinch at the University of Washington is a basic plasma physics experiment that utilizes sheared axial flows to maintain gross plasma stability. The experiment uses an annular acceleration region followed by a cylindrical assembly region in which the plasma column is formed and maintained. Upon Z-pinch formation, flowing plasma from the accelerator maintains the plasma supply in the Z-pinch in a quasi-steady state fashion. Past run campaigns have used changes in the injector parameters or to the annular area to alter the characteristics of the bulk plasma. Previous results show that the lifetime of the plasma is limited by the current from the power supply and by the plasma source from the accelerator. The supplied power has previously been increased to extend the current waveform. The insulated volume will be increased by 300{\%} to extend the plasma supply from the accelerator, the stability period of the Z-pinch, and thus the plasma lifetime. Preliminary results will be discussed relating velocity shear measurements and interferometric accelerator densities. The increased source duration will be compared with the quasi-steady state duration of the pinch to show an increased control of the plasma lifetime through prolonged flow shear.   

Design of a digital holographic interferometer for the ZaP Flow Z-Pinch

The ZaP Flow Z-Pinch experiment investigates how flow shear stabilizes MHD modes. An upgrade to a high energy-density plasma experiment would allow exploration of flow shear's effectiveness in this operating regime. The experiment's upgrade would include the addition of a digital holographic interferometer to measure electron density with fine spatial resolution. The design uses a pulsed Korad ruby laser with a consumer digital camera to generate and record holograms, which are then numerically reconstructed to obtain the phase shift caused by the interaction of the laser beam with the plasma. The numerical reconstruction provides a two-dimensional map of chord-integrated electron density without employing labor-intensive physical reconstruction techniques. The interferometer's accuracy has been validated with comparisons to measurements from an existing four-chord HeNe interferometer. The new diagnostic will allow the ZaP team to search for structures such as plasma shocks that were not previously resolvable. It would also be able to resolve the density profile of the smaller, high-energy pinch.   

Investigation of K-shell radiation from two-component wire arrays

Two-component plasma was studied in star and planar wire-array Z pinches. Arrays consisted of Al wires as the first component in all shots and Ti, Cu, Ni, Mo, and Au wires as the second component. Cascading implosion in star arrays provides the mixing of wire materials in one ray during implosion. The implosion dynamic was not affected by variation of materials in wire arrays that allows observation of features of the two-component plasma. Compared to pure Al plasmas, decreased Al K-shell radiation and increased soft x-ray radiation were observed in Al-Au and Al-W plasma. Mixt plasma with 80-90{\%} of Al ions displayed radiative properties similar to pure W or Au Z-pinch plasma. Al K-shell x-ray spectra simulations with the PrismSpect code showed a fall of the electron temperature from 400 eV in Al plasma to 250-300 eV in the Al-W and Al-Au mix. There was no corresponding cooling effect when the second component was Ti, Cu, and Ni. Spectra of the Z-pinch plasmas were compared with the spectra from laser produced Al-Au plasma experiments carried out at the Leopard laser. Work was supported by the DOE/NNSA under UNR grant DE-FC52-06NA27616.   

Modeling K- and L-shell Spectra from Cu Wire Array Implosions on ZR

We will examine K- and L-shell data obtained from the copper nested wire-array SNL shot Z1975, and compare it with data obtained from a simulation using the 1-D DZAPP radiation-hydrodynamics code. In addition to Cu, lines of Ni, Fe and Cr were observed in the experimental spectra, and we performed the calculations with an appropriate mixture of these elements. In the present analysis, we find support for an alternative K-alpha model which competes with the better known e-beam generation mechanism, wherein K-shell photons from hot plasma on or near the axis are absorbed in a dense, cool annular envelope via inner-shell photoionization. The resulting electronic relaxation of the absorbing ions produces the K-alpha radiation. By generating radially resolved synthetic spectra from self-consistent calculations of K-shell vacancy formation, and characterizing the energies of the resulting K-alpha radiation, diagnostics are obtained which can help differentiate between beam generated and photon driven K-alpha radiation.   

2D-RMHD Modeling of the Dynamics of a Ne Gas Puff Z Pinch

Detailed spatially resolved spectroscopic analysis of a neon gas puff Z pinch on the Weizmann 1MA generator [1,2] indicates that the radius of the K-shell regions grows to a maximum and then decreases during the radiation pulse -- the opposite of that calculated by 1D-RMHD models. Here we compare Mach2 2D-RMHD [r-z, high resolution, moving grid, non-LTE atomic populations, 3D ray trace radiation transport] simulation results to the size of the K-shell emission region as inferred from the spectroscopic analysis. In addition 2D, 3-ns time gated visible light images recorded during the neon experiments give us the opportunity to compare with the evolution of the outer pinch radius, r(z,t), as calculated by the 2D-RMHD model. Comparisons with spectroscopically inferred results and simulation results will also be made for electron and ion temperatures as well as internal energy to study the weak ion and electron temperature equilibration observed in the data.\\[4pt] [1] E. Kroupp, et al., PRL, 98, 115001 (2007).\\[0pt] [2] D. Osin, Ph.D. Thesis (2008).   

Simulation of K-$\alpha$ Emission from Highly Charged Cu ions for Pinches on ZR

Recent spectral data of Cu shots Z1975 and Z2122 from Sandia's ZR machine are believed to show strong K-$\alpha $ emissions. As these K-$\alpha $ lines provide good diagnostics, a detailed spectral model will be developed to investigate these line emissions for analyzing the data. In a Z pinch plasma, K-$\alpha $ emission can occur due to e-beams, hot electrons at the tail of a Maxwellian and also pumping from hot photons emitted near the axis. K-$\alpha $ emission that originates from collisional processes involving hot electrons in the final phase of the pinching plasmas are associated with radiationless electron capture, inner-shell electron collisional excitation and ionization. K-$\alpha $ lines from various ionization stages of various materials such as Fe, Cr, Ni, and Mn were also observed in the ZR data. Contributions from ions with strong K-$\alpha $ transitions will be included for this study which is a preliminary attempt to investigate Cu K-$\alpha $ lines due to hot electrons and photons. Photo-pumped K-$\alpha $ emission from an outer shell is spatially distinguishable from that produced by e-beam on axis.   

Simultaneous time-gated measurements of K- and L-shell radiation from brass wire array implosions on Zebra

New experiments have simultaneously measured both the copper and zinc K- and L-shell radiation with two time-gated spectrometers on the 1 MA Zebra generator at the University of Nevada, Reno. This work extends the previous brass wire implosions which only used one time-gated spectrometer [Ouart \textit{et al.}, IEEE Trans. Plasma Sci. \textbf{38}, 631 (2010) and Ouart \textit{et al.}, HEDP \textbf{8}, 247 (2012)]. The diagnostic suite also includes time-integrated spatially resolved spectrometers, time-integrated and time-gated pinhole imaging, various x-ray diodes, Ni bolometers, a Faraday cup, and laser shadowgraphy. The L-shell radiation comes from ionization stages around the Ne-like charge state that is largely populated by a thermal electron energy distribution function, while the K-shell radiation is subsequently produced by electron beams removing an inner-shell electron. A multi-zone non-LTE copper and zinc pinch model will be used to model the radiation from experiments. Diagnostic analysis will be presented using contours of line ratios and powers.   

Shell Interaction Physics of Multi-Material Gas Puff Z-Pinches for the Neutron Production on the Refurbished Z

Deuterium (D) double-shell gas-puff Z-pinch loads driven by the SNL Z accelerator has proven to be a proficient source of thermal fusion neutrons. RMHD simulations studies of the Z-pinches with the outer shell D replaced by a dense high-Z better radiating element have predicated a substantial increase in thermal fusion neutron yield. The neutron yield depends on not only the development of multidimensional structures and nonuniform gradients due to the RT instabilities but also on the complex interaction physics of the shells wherein the break-through \& penetration of the outer shell material and subsequent push-out of the interior D matter adversely affects the yield. Our investigation focuses on the understanding the interaction physics \& dynamics of the outer argon and inner D shells toward the optimization of the neutron yield using the multi-material version of the Mach2+DDTCRE 2D RMHD code. We will establish various performance metrics, in particular the neutron yields, of the Z-pinch loads as a function of mass ratio and/or radius for different load elements toward the mapping of the optimal operation parameter regime \& design of multimaterial gas-puff loads as a pulsed neutron source. A comparison of the results with 1D predictions and with pure D loads is made as well.   

Resolving Microstructures in Z Pinches with Intensity Interferometry

Nearly 60 years ago, Hanbury Brown and Twiss\footnote{R. Hanbury Brown and R. Twiss, Nature \underline {178}, 1046 (1956).} succeeded in measuring the 30 nanoradian angular diameter of Sirius using a new type of interferometry that exploited the interference of different photons emitted from opposite sides of the stellar disk. Its basis was the measurement of intensity correlations as a function of detector spacing, with no beam splitting or direct collection of phase information needed. Applied to Z pinches, X pinches, or laser-produced plasmas, this method could potentially yield spatial resolution well under one micron, using photon energies ranging from visible to x-ray. We consider the advantages, disadvantages, and possible complications in applying intensity interferometry to the pinch environment. Preliminary experimental designs are considered.   

Comparison of Deuterium-Deuterium-Deuterium and Neon-Deuterium-Deuterium Triple Shell Gas-Puff Z-pinch on the Level of 3 MA

The experiments of a triple shell gas-puff Z-pinch were carried out on the GIT-12 generator at IHCE in Tomsk during the April-May-June campaign in 2012. We diagnosed 17 Z-pinch shots where the triple D$_2$-D$_2$-D$_2$ (with the linear mass in the range of 50 - 255 $\mu$g/cm) and Ne-D$_2$-D$_2$ (with the linear mass in the range of 110 - 285 $\mu$g/cm) gas-puffs with diameter of 160 mm / 80 mm / 30 mm were mostly used as loads. This contribution is focused on the comparison of the results obtained by X-ray and neutron diagnostics, especially to the difference in reconstructed neutron energy spectra and obtained neutron yields (with the maximum of $3.3 \times 10^{11}$ neutrons/shot on a current level of 2.5 MA). The time correlations with other diagnostics such as electrical characteristics, a visible streak camera and MCP frames are also presented.   

Optimization of an Elliptical Spectrometer for the Study of X-Pinch Physics

Previous work has established the design of an elliptical spectrometer on the XP accelerator platform at Cornell University with the aim of studying the development of the plasma structures surrounding the core of an x-pinch. The spectrometer is aligned to view the absorption of hydrogen and helium-like resonance lines of magnesium in a sample x-pinch from a burst of continuum radiation generated by a second source pinch. In an attempt to minimize background signals reaching the detector film, a limiting aperture to the film cassette along with an aluminum band-pass filter have been included in the design. While effective at attenuating background noise, these two efforts have been proven to be incapable of reducing noise levels to an acceptable minimum. We hypothesize that the primary source of this background is due to x-rays scattering off of the nearby structural components of the spectrometer. The work presented here details our efforts to identify and further reduce the sources of the scattered background radiation signals in the collected experimental data.   

X-Pinch Experiments on UM Linear Transformer Drivers

X-pinches have been tested on the MAIZE Linear Transformer Driver (LTD) at the University of Michigan. The MAIZE LTD can supply 1 MA, 100 kV pulses with 100 ns risetime into a matched load. The x-pinch consists of a single wire separated by conical electrodes between two current return plates. The LTD was charged to +/-70 kV resulting in approximately 0.4-0.5 MA passing through a single wire. Initial tests with Mo and Al wires show several x-ray bursts over the length of the current pulse. The x-pinch is expected to be placed in parallel with a plasma load foil of 400 nm Al. Magnetic pressure causes the foil to accelerate, which drives the magneto-Rayleigh-Taylor (MRT) instability. Laser shadowgraphy has been used to image the foil and determine the growth rate, and the x-pinch is planned as a backlighter with the goal of x-ray probing the foil plasma at higher densities. A smaller 100-150 kA LTD x-pinch driver is also being developed.   

Early stage of a hybrid X-pinch plasma formation

A hybrid X pinch (HXP) configuration consisting of solid conical electrodes connected by a wire has been successfully tested in different conditions on different pulsers. But the physics of the process of single hot spot formation in HXPs is not yet understood. In order to understand the physical processes that occur in the HXPs we are carrying out an in-depth investigation of the physics of HXPs. To begin to understand this process, experiments with lower level load current have been performed. Pulsers with 10 and 4 kA current were used to study the prepulse influence on the HXP formation. Early stages of HXP formation have been studied also on the BIN pulser (250 kA, 100 ns) with 70 kA current through the HXP placed in the BIN return current circuit. Point-projection x-ray radiography and laser shadow and shlieren imaging together with interferometry were used in the experiments.   

Study of exploding Al wire plasmas using X-ray absorption spectroscopy

X-ray absorption spectroscopy is a powerful diagnostic technique useful for determining the charge state, temperature and density of plasmas under a wide range of conditions and situations. Our particular interest was the study of the core-corona system generated in electrically exploded wires and wire array Z-pinches. Two wide-bandwidth spectrographs with flat and concave cylindrically bent KAP crystals, and high-resolution spectrographs with spherically bent quartz crystals have been used on the XP and COBRA pulsers at Cornell University. The hybrid X-pinch was used as the continuum x-ray source in the photon energy range of interest for absorption spectroscopy with exploding Al wire experiments. This source is capable of producing broadband continuum x-ray pulses with micron source size and 100 ps duration. Absorption spectra of single exploded Al wires and 2 - 4 wire arrays were recorded with high spatial resolution. The parameters of the dense wire core plasmas and the ablating plasma streams were estimated under different experimental conditions. New spectral features in absorption spectra were observed.   

Study of Implosion in Wire Arrays with UV interferometry and Faraday Rotation Diagnostics

The implosion stage in wire arrays was studied with UV interferometry and Faraday rotation diagnostics at the wavelength of 266 nm implemented at the 1 MA Zebra pulsed power generator at UNR. Al cylindrical, star, and planar wire arrays were investigated. UV interferometry allows direct study of electron plasma density $>$ 10$^{20}$ cm$^{-3}$. Measurement of higher density is limited by spatial resolution, plasma motion, and plasma opacity at 266 nm. The density of the non-imploded plasma was measured at different times during the implosion stage. The first results from the UV Faraday rotation diagnostics are presented. Comparison of Faraday images with shadowgrams and interferograms allow measurement of current in the imploding plasma and non-imploded material in wire arrays.   

Analysis of Precursor Properties of mixed Al/Alumel Cylindrical Wire Arrays*

Previous studies of mid-Z (Cu and Ni) cylindrical wire arrays (CWAs) on Zebra have found precursors with high electron temperatures of $>$300 eV. However, past experiments with Al CWAs did not find the same high temperature precursors. New precursor experiments using mixed Al/Alumel (Ni 95{\%}, Si 2{\%}, and Al 2{\%}) cylindrical wire arrays have been performed to understand how the properties of L-shell Ni precursor will change and whether Al precursor will be observed. Time gated spectra and pinholes are used to determine precursor plasma conditions for comparison with previous Alumel precursor experiments. A full diagnostic set which included more than ten different beam-lines was implemented. Future work in this direction is discussed. \\[4pt] *This work was supported by NNSA under DOE Cooperative Agreements DE-FC52-06NA27588, and in part by DE-FC52-06NA27586, and DE-FC52-06NA27616. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.   

Observations of Opacity from K-shell Z-pinch Plasmas at the Z Accelerator

K-shell x-ray sources have been studied extensively at the Z Accelerator both pre- and post-facility refurbishment. Radiation and spectra from sources such as Al (5\% Mg), stainless steel, and Cu (4\% Ni) have been analyzed to understand the dominant mechanisms for the K-shell emission. Recent work (J.P. Apruzese et al., ICOPS 2012) has shown that for some experiments, the K-shell emission is dominated by initial mass and plasma density, and in other cases the emission is driven by electron temperature. In this work, the K-shell emission from the primary materials (Al, Fe, Cu) and the dopant materials (Mg, Cr, Ni) are compared to evaluate opacity effects for these z-pinch plasmas. Experimental data from pre-refurbished Z illustrating that opacity limits the Al K-shell output, but does not significantly limit the Cu K-shell output will be presented, along with observations from the same sources on post-refurbished Z. Comparisons will also be made with the dominant plasma emission properties (mass, temperature, density) to understand the correlation with opacity.   

Modeling of the Magnetic Field Entrained in Precursor Plasma of a Fast Cylindrical Shell Implosion on Z

Recent experiments on the Z machine at Sandia National Laboratories have demonstrated the measurement of magnetic fields inside an imploding cylindrical liner. The aspect ratio six beryllium liner had a two micron thick aluminum radiographic tracer layer on its inner surface and was driven with approximately 20MA of current over a 100ns rise time. B-dot probes were placed at varying radial positions inside the liner and a time-dependent magnetic field was measured. We compare the results of these experiments to simulations performed with the multi-physics ALEGRA code. These simulations suggest that the measured magnetic field is due to flux frozen into the release from the liner's inner surface. In short pulse mode, the surface magnetic pressure drives a shock into the liner. This shock has a magnetic component which is then frozen into the release wave formed when the shock reaches the liner's free inner surface. Simulations suggest this magnetized low density release then flows past the B-dot probe and is the source of the measured magnetic field. We demonstrate how these experimental measurements could be utilized to infer the amount of magnetic field at the shock front just before it releases into the vacuum. 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.   

Seeded Magneto Rayleigh Taylor Experiments on a 1-MA LTD

Recent work on the 1-MA Michigan Linear Transformer Driver, MAIZE, has focused on the Magneto Rayleigh-Taylor (MRT) instability and validation of analytic theory, developed at UM. MAIZE is a nominal 1-MA, 100 ns, 100 kV driver, capable of driving 0.1 $\Omega $ matched loads. We present here the continuing results of diagnostic development on experiments on planar and pseudo-planar foils. Some of the results will include various techniques used to seed the MRT instability on the foil. This work was conducted on 400-nm thick, 1-cm wide aluminum foils placed between two planar current return plates. The driver charge was limited to $\pm $70 kV, giving $\sim $700 kA with a risetime of $\sim $180 ns. Experiments were performed employing two methods to seed the MRT instability on either the foil. We have developed a laser-ablation mass perturbation technique using a 150 fs Ti:sapphire laser. We have also developed an initial displacement perturbation, in which the foil is pushed into a non-planar, rippled position by retractable knife-edges. The progress of these experiments is presented here. Analysis of MRT was derived from laser shadowgraphic images.   

Faraday and Kerr Effects Diagnostics for Underwater Exploding Wires

Two-channel laser polarimeter was used to measure magnetic and electric fields in vicinity of underwater exploding wire. Nd:YAG Q-switch laser with 532nm wavelength, 100mJ energy and 5ns pulse width was used for probing. Single wire, parallel wires and X and V- shaped wires was used in experiments. Electric and magnetic field induced birefringes in the water results in changing of polarization stage of probing beam after propagation through this anisotropic medium. Magnetic field results in circular anisotropy of the water, while electric field creates linear anisotropy. Magnetic field results in rotation of polarization plan of linear-polarized probing beam. Electric field effect is more complicated- polarization plan of the laser beam subjected to pulsation and changing of ellipticity. Effect of electric field depends on initial probing geometry- angle between electrical field vector E and polarization plane of probing wave. In our exploding wire experiments we found influence of both Faraday and Kerr effects. It was demonstrated existence of Kerr effect inside bubbles at high voltage electrode. Effect of magnetic fields interaction for multi-wire loads was observed.   

Viscous and Induced Current Heating in Plasma Focus Plasmoids

Recently, Abolhasani et al, proposed that the high ion energies observed in plasmoids formed in the plasma focus could be explained by viscous heating. We here elaborate this proposal, demonstrating that during plasmoid formation, ion motion along magnetic field lines can be rapidly converted, at least in part, to thermal energy through viscous diffusion. This effect is strongly enhanced by higher-z ions. We compare the theoretical predictions with the recent observation by Lerner et al, of trapped ion energies of 160 keV. In addition, we propose a second source of heating. The mildly relativistic electron beam emitted by the plasmoid, generates an induced current within the plasmoid comparable to the beam current and confined to approximately the same region. The induced current electrons, with drift velocity v$_{de}<<$v$_{b}$ of the beam electrons, are thus far more effective than the beam itself in Ohmically heating the plasmoid. We show that both these mechanisms are capable of generating ion energies of tens to hundreds of keV for a wide variety of plasmoid conditions. Finally we briefly consider a third possible heating mechanism, through ion-acoustic waves generated by the strongly sheared current and plasma flows in and near the emitted electron beam.   

Interferometry Results from Single Jet and Two Jet Merging Experiments on PLX

The Plasma Liner Experiment (PLX) is exploring single jet propagation and two jet merging of supersonic plasma jets in support of forming HED and potentially MIF-relevant imploding spherical ``plasma liners'' that can reach peak pressures $\sim $ 0.1-1 Mbar at stagnation. A novel 8 chord interferometer using a 561 nm diode-pumped solid state laser is being used to make time-resolved density profile measurements of the plasma jets. The interferometer phase shift is sensitive to electron, ion, and neutral atoms and thus is dependent on both plasma ionization fraction, $f$, and total atomic density. For argon jets both positive and negative phase shifts have been observed, where the sign of the phase shift bounds the value $f$ in the jet. Interferometry measurements coupled with spectroscopy and synthetic diagnostic data have allowed us to infer key physics such as plasma density range (10$^{16}$ -- 10$^{17}$ cm$^{-3})$, jet propagation velocity ($\sim $50 km/s), and radial and axial expansion. This poster will cover results from both single jet propagation and two jet merging experiments.   

On attaining eV energy resolution in an ionization energy shift at $\sim$ 60 keV

The K-line energy of high atomic number atoms depends to some degree on the atom's ionization stage, hence a high-resolution measurement of the K-line energy could, under the right circumstances, become a way to diagnose the plasma's ionization. Recently, we could measure a blue-shift close to $\sim$10 eV in the K$\alpha_2$ line of iridium ionized in the Plasma-Filled Rod Pinch. with a technique based on the line's transmission through a filter with a near-coincident K-edge. The presentation disxusses the measurements and analyses done to improve the systematic energy resolution to below 3 eV, with additional measurements and analysis of the K-ege filter with radiation from cold iridium excited with a PIXE source.   

Measurements of Ion Beam Production and Neutron Yields in the LLNL High Gradient Z-Pinch Experiment

Dense plasma focus (DPF) z-pinch plasmas are known to produce abundant neutrons and particle beams, but the mechanisms behind the high gradient fields in DPFs are not well understood. We have a 4 MeV deuteron beam that can be used to probe the electric field gradients produced by the DPF experiment at LLNL. This information can be used in conjunction with fully kinetic simulations of DPF plasmas to further our understanding of the mechanisms that produce these beams. This knowledge allows us to optimize the gradients in the DPF for next generation compact accelerators. The beam and neutron output from the LLNL DPF have been characterized. We present measurements of beam and neutron production for a variety of pinch currents. Acceleration gradients greater than 0.5 MV/cm have been achieved, a record for sub-kJ DPFs. Our upgraded gun design allows a probe beam to pass through the plasma, allowing for the first-ever measurements of DPF gradients. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (11-ERD-063) at LLNL.   

Fully Kinetic Simulations of Dense Plasma Focus Z-Pinch

Dense plasma focus (DPF) z pinch devices are sources of copious high energy electrons and ions, x-rays, and neutrons. The mechanisms through which these physically simple devices generate such high energy beams in a relatively short distance are not fully understood. We now have, for the first time, demonstrated a capability to model these plasmas fully kinetically, allowing us to simulate the pinch process at the particle scale. We present here the results of the initial kinetic simulations, which reproduce experimental neutron yields and high energy (MeV) beams for the first time. We present a comparison between fully kinetic, hybrid (kinetic ions/fluid electrons), and fluid simulations. Only fully kinetic simulations predict MeV-energy ions and experimental neutron yields. A frequency analysis of the electric field in the fully kinetic simulation shows plasma fluctuations near the lower hybrid frequency. This suggests the presence of lower hybrid drift instability, a possible contributor to anomalous resistivity in the plasma. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (11-ERD-063) at LLNL.   

Neutron Signatures of Non-Thermal Ion Distributions in Z-Pinch Driven ICF Plasmas

In preparation for upcoming ICF experiments on the 26 MA Z machine (e.g., D2 gas puff, MagLIF [1]), we are studying the neutron energy spectra produced by magnetically-driven loads beyond the archetypal single temperature, uniform plasma. Z-pinch sources frequently exhibit evidence of unusual neutron spectra [2], which can be attributed to three-dimensional turbulent motion, high-energy beams, and other phenomena leading to non-Maxwellian ion distributions. Understanding the nature of our plasma neutron sources is critical for understanding how they scale with increasing current. We will show Monte Carlo and analytic calculations for plausible scenarios and discuss the corresponding signatures for the existing set of time-of-flight diagnostics on Z.\\[4pt] [1] S. A. Slutz et al. Phys. Plasmas 17, 056303 (2010)\\[0pt] [2] V.V. Vikhrev and V.D. Korolev, Plasma Dynamics, Vol. 33, No. 5 (2007)   

Experimental study of z-pinch driven radiative shocks in low density gases

Results of experiments performed on MAGPIE pulsed power facility (1.4MA, 250ns) will be presented. Shocks with velocities of 50-70km/s are driven in Ar, Xe and He gases at density $\sim $10$^{-5}$g/cc using radial foil z-pinch configuration [1]. Measurements of the structure of the shocks obtained with laser probing will be presented and observations of the development of instabilities will be discussed. It was found that the structure of the shocks and the development of instabilities strongly depend on the rate of radiative cooling, increasing for gases with higher atomic numbers.\\[4pt] [1] F. Suzuki-Vidal et al., PoP 19, 022708 (2012)