Session GP1: Poster Session III: DIII-D I; Basic Studies and Complex Plasmas; Energetic Phenomena in Astrophysics and Lab; HEDP: Z-pinches and Incoherent Radiation

Steady-State, High-Performance Operation of DIII-D

The DIII-D AT program objective is to develop and demonstrate a steady-state scenario with performance that extrapolates to Q$>$5 in ITER. This year, we examine two aspects of AT optimization. Using the newly improved pumping, we have established a double-null, high triangularity AT reference plasma. Further optimizing this shape by varying squareness ($\zeta$) has shown that confinement appears to improve with reduced $\zeta$ and that there is an optimum $\zeta$ for MHD stability. Changes in pedestal and ELM conditions with varying $\zeta$ are being analyzed. We are also undertaking experiments to optimize the $q$ profile for AT operation. As the tools needed for maintaining a stationary high performance $q$ profile are not yet available, we concentrate on optimization under slowly varying transient conditions, using varying combinations of co- and counter-injection as well as B$_{\rm T}$ ramping to modify the current profile. The variables are $q_{min}$ ($>$2), $q_0$- $q_{min}$ (range 0-1), and $\rho_{q_{min}}$ ($>$0.5). Issues being addressed are the nature of the limiting instabilities ($n=2$ and 3 are predicted to dominate), the dependences of the $\beta$ limit and of $f_{NI}$ on $q_{min}$ and rotation.   

Development and Characteristics of Hybrid Discharges With Low Rotation in the DIII-D Tokamak

Hybrid scenarios with high plasma pressure and moderate \mbox{($\sim40\%$)} bootstrap current, offer the potential of high fusion gain and long duration plasmas in ITER. Stationary hybrid discharges lasting up to three resistive diffusion times and with fusion gain corresponding to an energy gain Q $\agt 10$ in ITER, have been achieved in the DIII-D tokamak. While past discharges were obtained at high toroidal rotation, rotation was reduced in recent experiments by injecting counter neutral beams, that have now become available. The hybrid discharge is seen to be sustained robustly with high performance and without triggering deleterious $n=1$ modes, at a rotation even at $20\%$ with pure co-injection value. Thermal confinement, though decreased from high rotation cases, remains high ($\sim10\%$ higher than that projected for conventional ITER H-mode scenario). Analysis is under way to determine if the decrease is attributable to a decrease in ExB stabilization of turbulence and/or increase in the size of tearing modes that accompany hybrid discharges.   

Real-Time Control of Plasma Rotation and Stored Energy in DIII-D

Real time control of plasma profiles throughout the plasma discharge is an important requirement for high-performance steady-state operation. To further this goal on DIII- D, significant efforts have been made to implement feedback control of plasma rotation and ion temperature using real time measurements obtained from the charge exchange recombination spectroscopy system. Plasma rotation is controlled by simultaneous neutral beam injection (NBI) in the same direction as the plasma current (co-NBI) and counter to the plasma current (counter-NBI), a capability made available for 2006. Feedback control of plasma rotation and ion temperature allows for studies of plasma transport and plasma instabilities. For example, feedback control of the plasma rotation at low or zero plasma rotation allows studies of the stabilizing effect of external feedback coils on RWMs. Details on measurement, analysis and control algorithms for plasma rotation will be presented as well as details on experiments to simultaneously feedback control the plasma rotation and stored energy.   

Anomalous Effects on the Current Evolution in \mbox{DIII-D}

We explore configurations where the current profile formation and evolution exhibit features consistent with non-neoclassical resistive effects or self-organizing mechanisms. In these discharges, evolution of the current density that determines $q$ achieves a stationary configuration where the inductively driven flux diffusion is balanced by external, non-inductively driven current and/or by anomalous flux or current diffusion processes. This stationary evolution of $q$ has been observed in both hybrid and quiescent, high-confinement (QH) modes of operation. By contrasting measurements with the neoclassical evolution, we infer the location and amount of anomalous current diffusion required to maintain these discharges. A hyper-resistive model is applied to provide at least a heuristic understanding of the current evolution observed in QDB modes. We present a combination of experimental data analysis and simulation results using the CORSICA code to demonstrate the anomaly in current profiles and their evolution.   

The DIII-D Plasma Control System as a Scientific Research Tool

The digital plasma control system (PCS) is an essential element of the \mbox{DIII-D} tokamak as a scientific research instrument, providing experimenters with real-time measurement and control of the plasma equilibrium, heating, current drive, transport, stability, and plasma-wall interactions. A wide range of sensors and actuators allow feedback control not only of global quantities such as discharge shape, plasma energy, and toroidal rotation, but also of non-axisymmetric magnetic fields and features of the internal profiles of temperature and current density. These diverse capabilities of the PCS improve the effectiveness of tokamak operation and enable unique physics experiments. We will present an overview of the PCS and the systems it controls and interacts with, and show examples of various plasma parameters controlled by the PCS and its actuators.   

Modeling for Control of the Current Profile Evolution During the Ramp-Up Phase at DIII-D

Simultaneous real-time control of the current and pressure profiles can lead to the steady-state sustainment of an internal transport barrier (ITB). It has also been suggested that global current profile control, eventually combined with pressure profile control, can be an effective mechanism for neoclassical tearing mode (NTM) control and avoidance. A key goal in control of an advanced tokamak (AT) discharge is to maintain a safety factor (q) and pressure profiles that are compatible with both MHD stability at high toroidal beta and at a high fraction of the self-generated bootstrap current. Active feedback control of the q profile evolution at DIII-D has been already demonstrated. We report progress towards enabling model-based active control of the current profile during both plasma current ramp-up and flattop phases. Initial results on modeling and simulation of the dynamic evolution of the poloidal flux profile during the ramp-up phase, open-loop control results, and an assessment of control needs are presented.   

Modeling of the Current Profile Evolution in \mbox{Hybrid} Discharges on DIII-D

\mbox{Hybrid} discharges are being studied as they offer the potential of improved performance over the ITER baseline scenario. In many discharges, a stationary state is reached in which the $q$-profile is everywhere above unity. The mechanism by which this occurs is not yet understood, but clearly involves anomalous current transport. This has been verified with simulations in which the current is evolved neoclassically. In such simulations $q_{min}$ invariably falls below unity, in conflict with the experimental observations. By introducing a heuristic current in the simulation, the $q $-profile can be maintained above unity. The spatial structure of the heuristic current required to achieve this can shed light on the physical mechanisms causing the anomaly. Current evidence suggests that neoclassical tearing modes play a role in the current redistribution, as many features are correlated with their presence. We present the results of simulations of hybrid discharges and determine the spatial structure of a heuristic current source required to sustain the $q$-profile.   

Beta Limit Due to $m/n=2/1$ Tearing Mode Onset in the DIII-D Hybrid Scenario

The ultimate performance of hybrid scenario plasmas in DIII-D ($\beta_N\approx 3$, $q_{95}\approx 4.4$, $f_{BS}\approx 0.5$, $\rm{H}_{89} > 2$) is limited by $m/n=2/1 $ tearing modes. Unlike conventional plasmas [1], the scaling of the instability onset global beta on local $\rho_i^*$ at $q=2$ is much less than linear. The weak $\rho_i^* $ scaling suggests that small island neoclassical tearing mode (NTM) threshold physics does not dominate the tearing criterion. The hybrid scenario tends to go unstable just at or below the no wall $n=1$ ideal kink beta limit of about $4\,\,\ell_i$. Experimentally $4\,\,\ell_i$ decreases with beta as $4\,\,\ell_i\approx 7.4$* $\beta_N^{-5/6}$. Thus the ``ceiling" in beta due to coupling of tearing to the ideal kink comes down as beta is increased. Scaling of the tearing unstable beta that combines both NTM threshold physics and a pole in $\Delta^\prime r$ due to coupling to the ideal kink will be presented. Also shown will be an experimental example of ``pre-emptive" electron cyclotron current drive to operate stably at the no wall ideal kink limit. \par\vskip6pt \noindent [1] T.C. Hender, et al., Nucl. Fusion \bf{44}, 798 (2004).   

Motional Stark Effect and ECE Measurements of Rotating and Interacting Magnetic Islands on DIII-D,

2D images of magnetic islands rotating at few kilohertz prior to locking and/or with balanced neutral beams were resolved in electron cyclotron emission (ECE) measurements and corroborated by fast motional Stark effect measurements. Islands of various poloidal/toroidal mode numbers $m/n$ were identified, including modes ambiguous on the basis of magnetics alone. O- and X-points were radially and toroidally localized by ECE. A real-time application to neoclassical tearing mode suppression by modulated EC current drive is proposed. The shape and internal structure of islands was also measured and transport and confinement within the island were inferred. When locked to each other, a well-developed 3/2 mode deforms the 2/1 island and gives rise to a 4/2 component, or amplifies it if it is already present. This is ascribed to the attraction between parallel filaments of current. Ergodization of the islands is also observed in the very last stage before locking.   

Structure and Scaling Properties of Zero-Mean-Frequency Zonal Flows and Geodesic Acoustic Modes in DIII-D,

Zonal flows, including the zero-mean-frequency (ZMF), spectrally broad residual flows and the oscillatory geodesic acoustic mode (GAM), are observed in the core of DIII-D plasmas. These flows have been identified using a high-sensitivity, 2D measurement of the density turbulence and its velocity-field using BES. The ZMF zonal flow peaks near zero frequency and exhibits a width of $\Delta f\sim 10$ kHz, a long poloidal wavelength but short radial correlation length (a few cm). This velocity spectrum is dominated near the edge of the plasma by the GAM, while the ZMF zonal flow dominates deeper in the plasma core. The GAM amplitude peaks near $r/a=0.9-0.95$ and is a strong function of the safety factor, $q_{95}$. Furthermore, the GAM frequency deviates increasingly from the theoretically predicted value ($c_s/R$) as the GAM is more strongly damped at lower $q$. This may suggest stronger coupling to and damping on sound waves.   

Turbulence and Flow Dynamics via Velocimetry at the L-H Transition in DIII-D,

Turbulence dynamics and the effects of sheared poloidal flows on turbulence in the pedestal region are examined at the L-H transition. The dependence of L-H transition characteristics on applied torque is investigated by varying the co- and counter-beam injection. High frequency turbulence velocity measurements are obtained via application of the dynamic programming method of time-delay estimation [1] to high- sensitivity 2D density fluctuation data from the beam emission spectroscopy diagnostic on DIII-D. The high-frequency motion of turbulent eddies in 1 and 2 dimensions is explored prior to and across the transition time. The role of zonal flows, including the Geodesic Acoustic Mode and other low-frequency zonal flows, on L-H transition behavior is investigated. Characteristic turbulence parameters are compared across the L-H transition. Nonlinear coupling between turbulence modes prior to, during, and just after the L-H transition is measured using bispectral and other analysis methods. \par\vskip3pt\noindent [1] D.J. Schlossberg, et al., Rev. Sci. Instrum. (2006).   

Analysis of High Frequency and Wavenumber Measurements From the DIII-D Phase Contrast Imaging Diagnostic,

The phase contrast imaging (PCI) diagnostic on DIII-D was upgraded for the most recent run period to improve the response at high frequency and high wavenumber, extending the response to cover the ETG-range of plasma turbulence. This allows us to compare the response of the turbulence in the ETG and ITG ranges to various changes in the plasma, including variation in bulk parameters and confinement regime transitions. Previous analysis of PCI data interpreted the results in a Fourier space localized to the measurement location. However, there is little framework in transport theory to understand the observed characteristics, such as the relative amplitude of the inward and outward propagating modes or the strong correlation between frequency and wavenumber. We now calculate the PCI response to the eigenmodes predicted from numerical modeling and reinterpret the data as a measure of the relative amplitudes of these modes.   

Localized Measurement of Short Wavelength Plasma Fluctuations With the DIII-D Phase Contrast Imaging Diagnostic,

The DIII-D phase contrast imaging (PCI) turbulence diagnostic measures density fluctuations previously line-integrated over the entire viewing chord. In 2004, a rotating mask system was installed that takes advantage of the vertical variation of radial magnetic field to make localized measurements along the PCI chord. The region of localization is proportional to wavenumber, making this technique more favorable for short wavelength modes ($k > 15/\rm{cm}$). This year the PCI has been fitted with Fiber Optic Links for data transmission, new low-noise amplifiers and high-pass filters. With these upgrades, we have improved S/N, measured wavenumber response to 35/ cm using ultrasonic calibration, and extended the frequency response to 5 MHz. Results from this new configuration will be presented with particular focus on localized measurements.   

Turbulence Dynamics During Internal Transport Barrier Formation Via Beam Emission Spectroscopy on DIII-D,

The formation of localized internal transport barriers (ITBs) is observed at the appearance of low order values of $q_{min}$ surfaces in negative central shear L-mode discharges on DIII-D. Related GYRO simulations suggest that increased zonal flows may be responsible for such ITBs [1]. Newly expanded high-sensitivity 2D beam emission spectroscopy (BES) fluctuation and flow measurements will be utilized to quantitatively examine turbulence dynamics and test this prediction of a zonal flow- driven ITB trigger mechanism. Time delay estimation via dynamic programming is applied to the fluctuation data to measure high-frequency poloidal velocity fluctuations to search for increased zonal flow activity during $q$-triggered ITB formation. In addition, newly implemented rotation control capability via co- and counter-neutral beam injection on DIII-D will be exploited to study ITB formation with varying momentum input. \par\vskip3pt\noindent [1] R.E. Waltz, et al., Phys. Plasmas \bf{10}, 052301 (2006).   

Electron Temperature Fluctuations in DIII-D \mbox{Using} the New UCLA Correlation ECE Diagnostic,

The new UCLA correlation ECE diagnostic recently installed on DIII-D allows for the study of broadband electron temperature fluctuations as well as coherent modes. Measurements of broadband electron temperature fluctuations are essential for understanding anomalous electron heat transport in advanced tokamak plasmas. The UCLA correlation ECE diagnostic has sub-centimeter radial resolution and the use of Gaussian optics results in a poloidal spot size ($2 w_0 \sim 3.5$ cm) that is comparable to anticipated turbulent correlation lengths. Remotely tunable 100 MHz wide YIG filters are utilized to select adjacent frequency bands from overlapping plasma volumes. Using established cross-correlation techniques, the amplitude and spectrum of temperature fluctuations can be extracted. The system design and data analysis will be discussed and initial measurements of broadband electron temperature fluctuations in DIII–D will be presented.   

New Diagnostic for Doppler Reflectometry and Correlation Measurements of Electron Temperature and \mbox{Density} Fluctuations in DIII-D,

Local fluctuation measurements are required to evaluate the importance of different turbulent transport channels. Doppler reflectometry at 50-65 GHz is employed to measure the density fluctuation spectrum and the ExB flow velocity in DIII-D. A parabolic mirror is used to achieve a narrow beam spot size ($W_0 \sim 2.5$ cm). The plasma flow velocity is obtained from the measured Doppler frequency shift $f_D$ of the received signal: $v_\perp = f_D \lambda_0/(2 \sin \theta)$, where $\theta$ is the antenna tilt angle (7-15 deg). An ECE correlation technique is used to extract electron temperature fluctuations (described in detail [1]). By matching the reflectometer \mbox {X/O-mode} cut-off to a particular ECE emission location, we expect that the correlation and relative phase $\phi_{T,n}$ of electron temperature and density fluctuations can be measured in quiescent plasmas (QH-mode). Quantitative comparisons of the measured $\tilde n$, $\tilde T$, and $\phi_{T,n}$ with gyrokinetic code results are now feasible. \par\vskip4pt\noindent [1] A.E. White, et al., this conference.   

Gyrokinetic Transport Database and Comparisons to the TGLF Theory-Based Transport Model,

A database with over 300 nonlinear gyrokinetic simulations has been created using the GYRO code [1,2]. Using a parameterized equilibrium model for shaped geometry, simulations show that the GYRO normalized ITG/TEM diffusivities exhibit an inverse linear dependence on elongation at fixed midplane minor radius. Kinetic electron simulations show the ExB shear quench rule is robust in shifted circle geometry. With real geometry, the quench point varies systematically with elongation and aspect ratio. Using the results, a new version of the quench rule is found that captures the variation of the quench point with these two geometric quantities. Finally, we compare the results from the TGLF driftwave model [3] with the GYRO simulations. Using the TGLF eigenmodes, we compute quasilinear fluxes using a turbulence saturation model and assess the quality of the fit to the GYRO transport database. \par\vskip4pt\noindent [1] J. Candy, R.E. Waltz, Phys. Rev. Lett. \textbf{91}, 45001 (2003).\par\noindent [2] http://fusion.gat.com/comp/parallel\par\noindent [3] G.M. Staebler, J.E. Kinsey, R.E. Waltz, Phys. Plasmas \textbf{12}, 102508 (2005).   

Divertor Impurity Sources and Core Content During ELM Controlled Regimes in DIII-D,

In low collisionality ($\nu\ast$) plasmas, ELM suppression by $n=3$ edge resonant magnetic perturbations (RMPs) in DIII-D can produce substantial changes to the divertor plasma that can lead to increases in impurity sources and core impurity content. For some conditions the divertor appears to lose the high recycling regime and become sheath limited. These changes are a consequence of the extremely low density used to achieve ITER level of pedestal $\nu\ast$ in some of these RMP experiments. In other cases, at slightly higher $\nu\ast$ or when a small level of edge MHD is present, the increase in core impurity content is not seen during RMP ELM suppression. The increase is also not seen during ELM-free QH-mode at comparable densities. In this case the Edge Harmonic Oscillation (EHO) may play a role in keeping impurities out of the core. Analysis of impurity sources and core content from low $ \nu\ast$ ELM controlled regimes (RMP and QH-mode) for low triangularity ($\delta$) LSN and higher $\delta$ more ITER-like shaped plasmas will be presented.   

Dependence of ELM Control Using RMPs in \mbox{DIII-D} on Plasma Shape and Toroidal Rotation

The size and frequency of Type-I ELMs has been controlled in \mbox{DIII-D} H-mode plasmas using n=3 resonant magnetic perturbations (RMPs) produced by an internal coil. The operating space of ELM control has been expanded to include both low and high triangularity shapes for both low and high pedestal collisionalities. The dependence of the ELM control on density, power, plasma beta and RMP amplitude in an ITER similar shape (ISS) will be compared with previous results [1-3] in a lower average triangularity (LAT) shape. New results showing the effect of toroidal rotation on the RMP assisted ELM control will also be presented and compared with available theories [4] for the screening of the RMP fields by a rotating plasma.\par \vskip6pt \noindent [1]~T.E.\ Evans, Nat.\ Phys.\ {\bf 2}, 419 (2006).\par \noindent [2]~T.E.\ Evans, Phys.\ Plasmas {\bf 13}, 056121 (2006).\par \noindent [3]~R.A.\ Moyer, Phys.\ Plasmas {\bf 12}, 056119 (2005).\par \noindent [4]~R.\ Fitzpatrick, Phys.\ Plasmas {\bf 5}, 3325 (1998)   

Edge Density Characteristics in DIII-D ELM Suppressed Operations,

Large Type-I ELMs have been suppressed using the internal MHD coils in DIII-D. In these ELM suppressed operations, the characteristics of edge density profiles and fluctuations have been investigated using high-resolution reflectometry and FIR scattering. At low collisionality, reductions in density pedestal height and gradient indicate enhanced particle transport. Increased density fluctuations measured by FIR and reflectometry are consistent with enhanced transport. At high collisionality, the SOL density profile is modified by the irregular $D_\alpha$ oscillations during the I- coil phase. Although the pedestal gradient decreases slightly the pedestal height remains unchanged. There is no significant increase of density fluctuations, however, enhanced magnetic fluctuations are observed. The results may imply strong transient Type-I ELM-induced transport is replaced by the lower impact transport associated with irregular $D_\alpha$ oscillations during the I-coil phase.   

Observation of Toroidal Asymmetries in the \mbox{DIII-D} Divertor During ELM-Suppressed I-Coil Operation,

During operation of the internal coil set (I-coils) in DIII-D to suppress ELMS, we observe toroidal asymmetries in heat flux or visible light in the divertor. Pronounced heating at tile joints is observed in some cases. We often observe a double-peaked profile in the visible light emission at one toroidal location while the IRTV at another toroidal location shows no split. In other cases infrared camera images show toroidal variation in the splitting within the view of a single camera. We compare this toroidal asymmetry to field line calculations by I. Joseph using the E3D code (this conference), in this poster and that of Joseph, and find cases of striking agreement.   

Improved Stochastic Transport Modeling for DIII-D,

The E3D two-fluid code is used to model the effect of resonant magnetic perturbations (RMPs) on DIII-D pedestal and SOL particle and thermal transport. E3D uses Monte Carlo integration to accurately compute the stochastically enhanced perpendicular transport caused by the 3D magnetic geometry. Progress has been made in adding parallel momentum balance and in efforts to use anomalous particle and thermal diffusivities inferred from 1D and 2D transport models. Qualitative agreement with observations of the RMP-induced magnetic footprint structures observed by IR-TV and Xpt-TV have been verified and may lead to direct probes of field penetration. Future improvements require self-consistent modeling of the induced non-axisymmetric currents, electric potential, and neutral transport.   

Pedestal Performance Dependence Upon Plasma Shape,

Higher moments of the plasma shape than triangularity are found to significantly affect the pedestal pressure and ELM characteristics in DIII-D. The shape dependence of the pedestal pressure was experimentally examined by holding the triangularity fixed and varying the squareness, the next higher moment of the shape, in the proposed ITER configuration. At low squareness the pedestal pressure increased by $\sim 20\%$ compared to the ITER target shape while the pressure decreased by $\sim 20\%$ at higher squareness. The ELM energy also varied with the shape and was a nearly constant fraction of the pedestal energy. Equilibrium reconstructions with variations of the pressure and current about the measured experimental conditions produced a stability map consistent with the experimental measurements of the pedestal pressure. The pedestal stability limit dependence upon global beta and shape is also examined.   

Edge Current Dynamics During the Type 1 ELM Cycle,

Initial studies have been made with the DIII-D LIBEAM system to examine the behavior of the edge current density $j(r)$ during the Type 1 ELM cycle. While the ion and electron pressure profiles have been extensively studied, the behavior of the edge current is less known. To address this need, the LIBEAM system has been developed to provide a finely spaced profile of the edge poloidal magnetic field from which one can infer $j(r)$. Conditional averaging of the signals for multiple ELMs improves the sensitivity and allows us to examine the dynamics of edge ($j$, $\nabla p$) growth and decay as a fraction of ELM spacing, or fixed absolute time after an ELM. Initial analysis shows that the current peak can relax by about a factor of two within a few milliseconds after an ELM, consistent with edge resistive decay times. The physics mechanism for the reduction of current has not yet been studied. The current appears to lag the pressure gradient evolution, both during the collapse and during the rest of the cycle, where the pressure has already returned to its pre-ELM value but the current continues to increase until the next ELM.   

Preparation of DIII-D Data to Test Edge Physics Models,

DIII-D experimental data are being assembled as inputs for models of the plasma edge. One goal of this work is to provide data to test gyrofluid and gyrokinetic transport models of the H-mode pedestal, several of which are now in development. For these purposes, well-characterized discharges with long ELM-free periods have been identified. Time-dependent profiles are available and being assembled for: electron density, electron temperature, ion temperature, toroidal rotation (carbon), and $Z_{eff}$. These profiles are fit with smoothing functions that provide good representations of both the pedestal and core profiles. These fits are used as input to the 1.5D TRANSP or ONETWO codes to compute the fast ion density and pressure and the total heating profile. The best way to obtain the particle source is still a subject of study. Density fluctuation measurements are available and can provide constraints on pedestal transport models. The data assembled here will be stored in an MDSplus database, with a format specified for the ITPA profile database.   

A Pellet Dropper for ELM Pace Making Studies on DIII-D

A pellet making device that cuts and drops deuterium or impurity pellets into the edge plasma for triggering ELMs at high repetition rates has been designed at ORNL for use on DIII-D. The device known as a pellet dropper can produce small $\sim$1 mm size cylindrical pellets and drop them at speeds of $\sim$20 m/s. It is under fabrication now and will be installed in late 2006. The design of the device and its initial performance in the laboratory will be presented. Calculations of the dropper pellet penetration in DIII-D H-mode plasmas have been performed based on the neutral gas shielding model [1]. Experimental results of pellets triggering ELMs on DIII-D will be reviewed and compared with the expected performance of the pellet dropper. \par\vskip6pt\noindent [1] P.B. Parks and P. Turnbull, Phys. Fluids, 1735 (1978).   

Gyrokinetic Microstability Analysis of the Inner Boundary of the H-mode Pedestal

A yet unresolved puzzle concerning the H-mode pedestal is which instabilities determine the width of the steep-gradient region. One theory [1] posits that the steep-gradient region extends into the core only as far as the $E\times B$ shearing rate can overwhelm the maximum linear growth rate of the instabilities. We examine this conjecture by analyzing the region just at the top of the pedestal where the gradients are shallow enough so that the gyrokinetic treatment is certainly valid. We employ the continuum codes GKS, GYRO, and GS2 --- the latter using actual numerical equilibria --- to calculate the linear growth rates and then compare them to the $E\times B$ shearing rates calculated from data. This is done for a particularly well diagnosed low-density DIII-D ELMy H-mode plasma and a high-density Alcator C-Mod enhanced-D$_\alpha$ H-mode plasma.\par \vskip6pt \noindent [1]~F.L.\ Hinton and G.M. Staebler, Phys.\ Fluids {\bf B5}, 1281 (1993).   

Inferred Edge Thermal Transport in L- and ELM-Free H-Mode in DIII-D

We have recently developed a methodology for inferring thermal transport coefficients in the plasma edge from measured density and temperature profiles which takes into account important convective, ion-electron equilibration and atomic physics effects [1]. This methodology has been applied to infer ion and electron thermal diffusivities, $\chi_{i,e}(r)$, profiles in the edge of a DIII-D shot with L-mode and ELM-free H-mode phases. The expected order of magnitude reduction in transport coefficients was found, but across the entire edge region including both the pedestal ``flattop" and ``steep-gradient" regions, rather than just in a localized ``transport barrier" coincident with the steep-gradient region. The inferred $\chi_{i,e}(r)$ are compared with various theoretical predictions.\par \vskip6pt \noindent [1]~W.M.\ Stacey and R.J.\ Groebner, Phys.\ Plasmas {\bf 13} (2006).   

Magnetic Field Patterns Calculated From Scrape-Off Layer Currents (SOLC) During ELMs in DIII-D Tokamak

Current flowing along open field lines in the SOL may play a role in the ELM trigger process through the error field it generates~[1]. A toroidally narrow ribbon-like SOLC, similar to those often found experimentally in the precursor phase of the ELM, produces a characteristic helical field structure, with appreciable intensity concentrated only over a narrow toroidal region on the outboard side and spread more evenly toroidally on the inboard side. This field pattern, calculated based in part on the spatial distribution of the SOLC observed during the ELM, has many features similar to those measured with Mirnov coil arrays. The SOLC during the ELM is non-axisymmetric, and reverses its direction as a function of its radial location. Impact of this structure on the intensity and distribution of the error field it generates is also investigated.\par \vskip6pt \noindent [1]~H.\ Takahashi, et al., EPS 2005 Paper 4.018, Tarragona, Spain.   

Cross Comparison on DIII-D of Experimental Techniques for Measuring ${\bf n_e}$ and ${\bf T_e}$ in Detached Divertor Plasmas

Spectroscopy of high-n, Balmer line transitions provides a means of measuring $n_e$ and $T_e$ in recombining plasmas [1]. The relative intensities of Rydberg series lines near the ionization limit are a sensitive diagnostic of $T_e$ for $T_e <\,$1~eV. Stark broadening of these same lines provides a measure of local $n_e$ and with less accuracy of $T_e$. Predictions from Balmer line spectroscopy are compared with those from divertor Thomson scattering to evaluate the accuracy of different theoretical models of line broadening [2,3]. In particular, the detailed dependence of line width on principal quantum number is used to distinguish which line-broadenign model best accords with experiment.\par \vskip6pt \noindent [1]~J.\ Terry, Phys.\ Plasmas {\bf 5}, 3373 (1998).\par \noindent [2]~H.\ Griem Spectral Line Broadening by Plasmas, Academic Press, New York (1974).\par \noindent [3]~E.\ Oks Stark Broadening of Hydrogen and Hydrogenlike Spectral Lines in Plasmas: The Physical Insight, Alpha Science International, Oxford, UK (2006).   

Plasma Interaction With the Outboard Main Chamber Wall in DIII-D

Plasma interaction with the outboard main chamber wall in DIII-D is studied using Langmuir probes, single point and imaging line emission diagnostics. In L-mode, plasma-wall interaction increases with the discharge density. In H-mode ELMs cause strong wall interaction that changes character with increasing discharge density. In low-density high-power H-modes ELM filaments propagate all the way to the outboard wall with little density dissipation. In high-density H-mode ELM filaments decay as they propagate through the SOL and their relative contribution to the wall fluxes versus inter-ELM periods decreases. A decreasing gap between the separatrix and the outer wall causes an increase in the plasma-wall interaction. A capability to expose material samples in far outboard SOL for in-situ erosion/deposition studies has been recently added.   

Long-Term Monitoring of DIII-D Wall Conditions Following a Boronization

A standard discharge has been developed to monitor long term changes in impurity and particle wall sources on DIII-D. This discharge is run routinely as the second discharge on most operation days. The discharge starts with a L-mode phase held at a fixed line average density of 0.27$\times$10$^{20}\,$m$^{-3}$ ($n_e/n_{GW} \sim 0.25$) followed by a slow ramp of injected beam power providing a measure of the L-H transition threshold. After a long ELM-free period ELMs begin, then the power is increased to 4~MW and a long period of stationary ELMing H-mode operation results. Over these periods the core and edge line emission is recorded with vuv spectroscopy, along with gas fueling and core particle content. These and other measurements are stored in a database and examined for long-term trends relative to major wall conditioning activities such as boronization. Preliminary results indicate that the effects of boronization do not degrade over a period of several weeks of standard operation, roughly 2800 plasma-seconds.   

Paleoclassical Model for Edge $T_e$ Pedestal

A model is proposed for the edge electron temperature profile $T_e(\rho)$ in high (H) confinement mode, diverted tokamak plasmas based on the paleoclassical model [1] for the minimum possible radial electron heat transport. In the paleoclassical model as one moves inward from the separatrix the electron heat diffusivity first decreases (until $\lambda_e\sim\pi Rq$); then it increases moving further inward into the paleoclassical collisional (Alcator-scaling) regime. The $T_e$ profile predictions from the paleoclassical model as one moves inward from the separatrix are: 1) first an increasing $T_e$ gradient with $\eta_e \equiv d\ln T_e/d\ln n_e=2$, 2) a maximum $|\nabla T_e|$ where $q$ drops to $\sim$ 5--7, 3) then a decreasing $T_e$ gradient, and 4) finally a pedestal electron pressure determined by balancing collisional paleoclassical transport against gyro-Bohm-scaled anomalous electron heat transport, $\beta_e^p \equiv n_e^p T_e^p / (B^2/2\mu_0) \propto a/Rq$, which implies $p_e^p \equiv n_e^p T_e^p \propto B_pB_t$. The relatively favorable omparisons of these paleoclassical model predictions with DIII-D experimental data on H-mode $T_e$ pedestals just before an ELM will be shown. \vspace{0.5ex}\\ \mbox{[1]} J.D. Callen, Phys.\ Plasmas {\bf 12}, 092512 (2005).   

Divertor Target Plate Measurements With the New DIII-D Langmuir Probe Array

A new Langmuir probe array with higher power handling capability was installed in the DIII-D lower divertor. The new pyrolytic graphite probes have 100X larger mass than the previous design that increases the total energy capacity for longer plasma pulses, a fixed 12.5~degree surface angle for more uniform heat flux, and a symmetric ``rooftop" design to allow operation with reversed toroidal field. The 4~mm wide probes are located 1.5~cm apart near the pumping baffle entrance for improved spatial resolution and 3~cm apart on the shelf above the pump. Target plate profiles, of $n_e$, $T_e$, and $I_{sat}$ at high spatial resolution during radiative divertor and ELM suppression experiments, which use strong divertor pumping, will be shown.   

Analysis of Dust Particles Observed by Laser Scattering from 2004-2006 at DIII-D

Dust particles are observed by Rayleigh/Mie scattering of ND:YAG lasers during plasma operations at DIII-D. Typically, about 1 particle is observed in the SOL during each shot and this rate has a strong dependence on operating parameters. The mean density (4000~m$^{-3}$) is small compared to observed carbon density in the plasma and is unlikely to be a significant source of contamination but understanding dust is important because it can be a significant safety issue for future burning plasma machines. Studies of these particles show large differences in the dust densities for different plasma configurations. There is a significant increase in dust density with H-mode discharges relative to L-mode discharges. Plasma configuration is also very important for dust production and upper single-null plasmas have double the dust density in the SOL compared to lower single-null plasmas with similar confinement. Dust collection on the vessel floor has also been observed with an increase in the dust rate for LSN discharges after a period of USN discharges.   

An Upgraded Porous Plug Injection System for Studies of Hydrocarbon Dissociation and Transport in DIII-D

The objective of the Porous Plug Injector (PPI) is to obtain calibrated spectroscopic measurements of dissociation fragments resulting from hydrocarbon influx in a tokamak divertor. This is done by admitting methane through a porous surface, $>$90\% graphite, such that the injected molecules ``see" a local carbon surface, similar to the re-deposition environment seen by molecules emitted by chemical sputtering. Following its initial use in DIII-D DiMES, the PPI has been significantly upgraded to include a finer porous graphite cap, a small orifice flow restrictor for precise gas control, and a built-in Langmuir probe to make direct measurement of possible perturbation to the local plasma as a consequence of gas injection. Design of these and other improvements are presented.   

OEDGE Modeling of Carbon Spectroscopy and Carbon Core Leakage for Puffing $^{\bf 13}$CH$_{\bf 4}$ into the Main SOL of DIII-D

Methane puffing experiments were conducted on DIII-D in both L- and H-mode conditions. The puffing was toroidally symmetric into the crown of a series of well-characterized LSN discharges in which the plasma conditions, carbon emissions and core carbon increment were measured. The hydrocarbon species are modeled in OEDGE using an improved methane kinetics and breakup model, with the latest Janev-Reiter reaction data. The resulting carbon is followed until it deposits on surfaces. Code calculated emissions from CI, CII and CIII, as well as the carbon increment in the confined plasma, for varying scenarios of parallel and radial flow, are compared to the measured values. The flows required to reproduce the experimental measurements are estimated and the expected sensitivity of the measurements to underlying flows is examined.   

Magnetohydrodynamic Simulation of Exploding Wire in Water.

We present one-dimensional time-dependent magnetohydrodynamic (MHD) simulation of exploding wires in underwater discharge experiment. The simulation employs Saha equilibrium equation-of-state (EOS) data while the electrical conductivity is obtained from a nonideal model that takes into account the Coulomb coupling effect in warm dense plasma [1]. It is assumed that the wire is initially in warm plasma state with a uniform density distribution. The MHD behaviors of aluminum and copper plasmas and the shock propagations in the surrounding water are calculated and compared with the observations made by a frame camera. Comparisons of the temporal behaviors of plasma boundaries and water shock propagations show reasonable agreements considering the simple EOS model used in the simulation. In addition, the calculations seem to effectively reproduce the measured electrical conductivity behaviors in the early stages of discharge. [1] Deok-Kyu Kim and Inho Kim, Phys. Rev. E 68, 056410 (2003).   

Laser driven shock waves in a large magnetized plasma

We will present the first experiments on the interaction of an energetic, rapidly expanding laser-produced plasma with an ambient magnetized plasma that supports Alfven waves. The experiments are performed with a high-power laser coupled to the Large Plasma Device (LAPD). Focused laser intensities in excess of 10$^{14}$ W/cm$^2$ produce an ablating plasma-plume with expansion velocities of several 100 km/s. Prior to the laser pulse an ambient plasma with a size of 18 m lengths and 50 cm diameter at 4x10$^{12}$ cm$^{-3}$ and Te=5 eV is created in an axial magnetic field of 400 G. The interaction of the two plasmas will lead to the formation of shock waves with M$_A$ and $\beta$ above unity and a density sufficiently small to approach the collisionless regime. We will show measurements of the magnetohydrodynamic turbulence that the shock creates as well as its effect on the particle velocity distribution.   

Kinetic simulations of laser-plasma interaction in realistic plasma profiles

Kinetic (particle-in-cell) simulations of laser-plasma interaction have been performed using density and velocity profiles obtained from radiation-hydro calculations. Because the size of the system, and the density range, are unusually large for PIC simulations, it is possible to see the competition of many processes across the profile. For one, light from Raman scatter propagates down the gradient to its quarter-critical density and there undergoes a second Raman decay. This depletes the light available experimentally to diagnose the initial Raman process. Roles of ion response in evolution of Raman scatter are illustrated via synthetic streak diagnostics and animations.   

Expansion of a laser created plasma in a cusp magnetic field

The use of a magnetic field with a laser-created plume is especially interesting, as the magnetic field can be used to help better control the dynamic properties of these transient and energetic plasmas[1]. Previous experiments were carried out striking a carbon target with a 1.5 J, infrared Nd:YAG laser pulse of 10 ns, immersed in a background magnetized plasma (1 kG) [2]. In this work we created a magnetic cusp starting at the target and follow the plasma dynamics by fast photography with an intensified camera (ICCD) placed orthogonal to the plasma expansion direction. Spectroscopic analysis of the atom and ion emission lines were performed and different velocities distributions were obtained by time of flight (TOF), for different configurations of the magnetic field (straight or cusp) and background plasma parameters. We will present the perturbation caused by the plasma plume into the background magnetized plasma, measured with magnetic probes. [1]. P.B. Parks, Phys. Plasmas \textbf{12}, 102510~(2005). [2]. M. VanZeeland and W. Gekelman, Phys. Plasmas \textbf{11}, 320 (2004).   

Using laser-produced energetic electrons to model ionospheric phenomena

Bursts of energetic electrons are commonly observed throughout the earth’s auroral zone and are associated with a host of physical phenomena including VLF saucers, Alfven waves, magnetic reconnection, and (possibly) electron phase-space holes. In this laboratory experiment, a burst of field-aligned electrons is produced by the expansion of a dense, laser-produced plasma (lpp) into a uniform, low-density background plasma. The experiment is conducted UCLA’s Basic Plasma Science Facility. The background plasma is He, Ne, or Ar, with plasma parameters: $\omega_{ce}/\omega_{pe}$=0.1-0.3, $T_{e}$/$T_{i}\approx$6, plasma radius=66$r_{ci}$, L=17m. A solid target is struck with a NdYAG laser (1.5J, 7ns pulse) focused to a spot size of less than one millimeter. The ions in the lpp (with energies of several keV) are initially unmagnetized. The electrons, however, remain magnetized and a fraction of them jet away from the point source of the laser impact in a field-aligned burst. We present data on the generation of lower hybrid waves by the energetic electrons and the subsequent trapping of these waves within density depletions and show preliminary data on ion heating within the cavities. We also report on the current status of measuring Debye-scale (15$\mu$m) electron holes (ion blobs) within the beam using novel miniaturized probes.   

Magnetic Turbulence in colliding laser produced plasmas

We describe a series of experiments, which involve the collision of two dense (initially, $\delta n_{lpp} /n_0 >>1$) laser-produced plasmas (llp) within an ambient, highly magnetized (R$_{ci} <$Dia-plasma) capable of supporting Alfv\'{e}n waves. Colliding plasmas can be used to study generation of magnetic turbulence and spontaneous generation of magnetic fields. The plasma column (He, Ne, 1-4 X10$^{12}$ cm$^{3})$ is 18 m long and 60 cm in diameter, 15 ms duration and pulsed at 1 Hz. Two carbon targets are struck by 1.5 J (10 ns,10 $\mu $, 1 Hz) laser beams. The lpp's form diamagnetic bubbles in which a large percentage of the background magnetic field (600G $<$B$_{0 }<$1 kG) has been expelled [1], jet across the magnetic field and collide. Fast camera (3 ns) photographs show the bubble surfaces become corrugated after the collision. Small magnetic field probes are used to study the magnetic turbulence. One probe is fixed and the second moved in a small volume close to the targets. An ensemble each location within the volume is used to determine correlations and cross-spectral functions of the magnetic turbulence. The current systems of the waves are fully three-dimensional and are reported in the adjacent poster by Collette et al. [1] M. Van Zeeland, W. Gekelman, Laser Plasma Diamagnetism in the presence of an ambient magnetized plasma, Phys. Plasmas, \textbf{11}, 320 (2004)   

Current Systems Generated by Colliding Laser-Produced Plasmas

There are many situations, either in space (e.g. CMEs, supernovae), or man-made, in which a dense plasma expands into a magnetized background plasma capable of supporting Alfv\'{e}n waves. The generation of shear Alfv\'{e}n waves by an expanding laser-produced plasma (lpp) has been observed previously at the LArge Plasma Device at UCLA [1]. In a recent experiment at LAPD, the collision of two dense ($\delta $n/n$_{0} \quad >>$ 1, where n$_{0} \quad \sim $ 1-4 x 10$^{12}$/cm$^{3})$ lpps within an ambient, highly magnetized ($R_{ci} \quad \sim $ 4mm $<< \quad \Delta \quad \sim $ 1m, where $\Delta $ is the machine diameter) is studied. A 150MW laser is pulsed with the background plasma at 1Hz, in a reproducible experiment to obtain a large volumetric dataset. We have directly observed a system of complex, fully 3-dimensional time-dependent current systems associated with the shear Alfv\'{e}n waves generated by the two lpps. The magnetic helicity associated with these current systems will be presented and compared to their topology, along with a process which strongly resembles magnetic reconnection at a point where the two current channels pull apart. Data will be presented as a 3-D movie. \newline \newline [1] M. Van Zeeland \textit{et al}, Phys. Rev. Lett., \textbf{87}(10):100501-1-100501-4, 2001.   

Dust particle size measurement by the multi-channel laser light scattering method

The measurement of the spatial distribution of dust particle size was performed by the multi-channel laser light scattering method. To self-consistently determine the time evolution of the particle size, in-situ polarization-sensitive laser light scattering was used using a 30 mW He-Ne laser. Polarization light intensities (incident and scattered light intensities with the same polarization) were measured at 71\r{ }. Before applying the method to the dusty plasmas, the measurement accuracy was confirmed using a distilled water solution of the size-known particles. In addition, the size-known particles were injected into the argon plasma, and the particles trapped inside the plasma were used for the accurate measurement of the light scattering angle. The measured size of the dust particles in an argon diluted silane capacitively-coupled plasma at 160 mTorr, 150 W, (11.4-11.8) s after the plasma on was (80-110) nm. In comparison, the scanning electron microscope photographs of the fallout particles showed (90-100) nm spherical particles under the similar experimental condition. The time evolution of the spatially distributed particle size at various plasma conditions was studied by using a 2-dimensional 16 channel photomultiplier tube as a detector of scattered laser light.   

Measurement of the ion drag force on falling dust particles in a low-pressure plasma

The ion drag force on falling dust particles in a low-pressure discharge was measured experimentally. The plasma was produced in a multidipole hot-filament discharge using argon gas at pressures below 1 mTorr. The plasma density, electron temperature and space potential were measured using a planar Langmuir probe. Typically, the electron density was in the range of 10$^{15}$ -10$^{16}$ m$^{-3}$, and the electron temperature in the 2 -- 5 eV range. A weak electric field, present along the axis of the plasma, induced a drift motion of the argon ions to velocities several times the ion thermal velocity. Glass micro-balloons (mean diameter 40 or 59 micron), dropped into the plasma using a dust shaker, were deflected by the horizontally-directed drag force produced by the drifting ions. The ion drag force was deduced from measurements of the deflection angles of the particle trajectories which were observed by laser light illumination and recorded by a CCD camera. The measurements will be compared to theoretical models of the ion drag force.   

Charge of dust on surfaces in plasma

Experimental investigations are reported on the charging of dust particles resting on conducting and insulating surfaces beneath plasma. The surfaces are agitated so that the particles drop through a small hole into a Faraday cup where the charge on each particle is measured. Both conducting (Ni) and insulating dust (SiO$_2$, Al$_2$O$_3$ and JSC-1 lunar simulant) are investigated. The conducting surface is given a bias voltage above or below the floating potential to create electron flux dominant or ion flux dominant conditions, respectively, to the surface. The dust charges more positively in ion flux dominant conditions and more negatively in electron flux dominant conditions. The insulating dust retains the charge for a long period of time ($\sim $1 hour) after the plasma is turned off. Agitation of the dust, causing it to roll around on the surface, causes the charge to decay more quickly. The conducting dust has a lower level of charge than the insulating dust because the charge is conducted from the dust particle into the conducting surface. Conducting and insulating dust having fallen from an insulating surface have approximately the same level of charge.   

Smoky Plasma

The mesosphere contains nanometer-sized smoke particles that have formed in the vapor trails of meteors and that are thought to be the condensation nuclei for noctilucent clouds. Laboratory dusty plasmas often have the dust particles in a layer at the lower sheath boundary. We examine the possibility of creating in a double-plasma device a smoky plasma in which the particles would be sufficiently small to fill the plasma nearly uniformly while being sufficiently large to exhibit multiple charge states that would distinguish the smoky plasma from one containing heavy negative ions. For example, nanometer sized atomic clusters of Ag (4 nm radius, 10,000 atoms) can be generated in an oven with an inert gas that carries the particles into the plasma chamber. These particles will become charged negatively with about 8 electrons and will then be electrostatically contained by the presheath electric field The confining electric force will also be greater than the ion drag force that could otherwise create a void in the smoke particle density distribution. This plasma would make possible, for example, experiments on the coupling of electrostatic waves to fluid turbulence by the neutral drag force. An acoustic wave propagating in smoky plasma will exert different drag forces on electrons, ions, and smoke particles thus creating a charge-separation electric field that can be measured by potential probes. This coupling may be the origin of electrostatic fluctuations seen by rocket-borne electric field probes in the mesosphere. Supported by the NSF/DOE Plasma Science Initiative.   

Suggestion for a two-dimensional cryogenic complex plasma

We propose and discuss theoretically a novel type of 2D complex (dusty) plasma formed by positioning charged dust grains on the surface of liquid helium (He).\footnote{Rosenberg, M. and Kalman, G. J., \textit{Europhys. Lett}., submitted, 2006.} Liquid He provides a nearly ideal flat substrate, has very low polarizability and conductivity, and has been used previously to study 2D systems of electrons, ions, and charged clusters. The 2D cryogenic complex plasma system has several possible advantages compared with traditional 2D complex plasmas: (1) a more controlled environment; (2) the grains interact via an unscreened Coulomb interaction; (3) in addition to micron-size particles, nanoparticles might be used; (4) effects related to intrinsic magnetic dipole moments, as well as phenomena involving magnetized dust, may be more amenable to study; (5) binary mixtures of different charges and masses could be studied. At the same time, there are issues that invite further investigation: (a) the type of grain to use; (b) the possible choice of other, denser cryogenic liquids; (c) optimal methods for charging and discharging; (d) means of confinement; (e) possible diagnostic methods; (f) possible He transmitted damping mechanisms.   

Ion-Acoustic Surface Waves in a Complex Plasma with Lorentzian Distribution

The electrostatic modes of dusty ion-acoustic surface waves propagating on the interface between a vacuum and a complex (dusty) plasma are kinetically investigated by using the dispersion relation based on the Vlasov-Maxwell equations. The complex plasma consists of the electron and ion plasmas with Lorentzian distributions and dust particles in the form of $f_{d}$ = $n_{d}$\textit{$\delta $}($r-r_{d})$ where $r_{d}$ indicates the location of dust particles. In the long wavelength limit, the ion-acoustic surface wave in a complex Lorentzian plasma is found to be slower as the non-thermality of electron and ion plasmas increases when the phase velocity is in the range of $v_{i}<<\omega $/$k_{x}< [Preview Abstract]

Ground State and Collective Modes of Magnetic Dipoles Fixed on Two-Dimensional Lattice Sites

In complex (dusty) plasmas the grains may be endowed with intrinsic dipole moments. We present here our results of theoretical calculations\footnote{J D Feldmann, G J Kalman and M Rosenberg, J. Phys. A: Math. Gen. 39 (2006) 4549-4553} accompanied by and Molecular Dynamics simulation findings on the ground state configuration and on the collective modes mode spectrum of a system of magnetic dipoles, interacting via the magnetic dipole pair-dipole potential, fixed on two-dimensional (2D) lattice sites. In particular, we We study a family of lattices that can be characterized by two parameters: (parallelogram)---the aspect ratio, c/a, and the rhombic angle, phi. The The new collective modes of in the system associated with the dipole-dipole interaction are the angular oscillations (or wobbling) of the direction of the dipoles about their equilibrium configurations. We identify in-plane and out-of-plane modes and display their dispersions. Orders of magnitudes of the parameters of the system relevant to possible future experiments will be discussed.   

Dustwheel: a new experiment for magnetized dusty plasma

We report on the construction and features of a new magnetized dusty plasma experiment. A set of 24 water-cooled magnets produces a steady-state magnetic field {\bf B} $\le$ 0.7 T. The magnets have a bore of 30 cm diameter and the magnetic field is homogeneous over a length of L = 1 m. The magnets are suspended in a wheel-shaped cage, which allows tilting the entire experiment to any position between horizontal and vertical. In this way, the angle between the magnetic field direction and gravity can be chosen at will. This allows exciting dust flows by a variation of the residual component of gravity. The dusty plasma is generated by means of a high frequency discharge at 13.56 MHz. The objective of the experiments is the study of wave phenomena in magnetized dusty plasmas. In particular, we are interested in the influence of dust on the destabilization and propagation of drift waves. Drift waves are plasma surface modes which are excited by the radial pressure gradient in magnetized plasma columns. In the presence of dust the reduction of free electron density and the ion drag by the Coulomb collisions with dust will affect the drift waves. The planned investigations will be focussed on the range of existence of different modes, the threshold conditions for the excitation of drift waves and drift-wave dispersion. The investigations will subsequently be expanded to drift-wave turbulence. Supported by Deutsche Forschungsgemeinschaft SFB-TR24/A2.   

Digital in-line holography of coulomb balls

Recently, the field of dusty plasmas has a growing interest in three dimensional dust clouds. To observe dynamic processes of micrometer sized particles in a 3D volume with a sufficient time resolution, it is necessary to develop novel diagnostic methods. Beside other methods, digital holography appears to be a promising technology for instantaneous 3D measurement of particle fields. Holograms are directly recorded with a CCD sensor and numerically reconstructed using the Fresnel integral. This does not only eliminate wet chemical processing and mechanical scanning, but also enables the use of complex amplitude information which is inaccessible by optical reconstruction. Thereby, the small depth resolution of digital holography, which is caused by the low pixel resolution of solid state sensors, amends vastly. Further, this technique even allows the observation of dynamical processes. This contribution describes the development and performance test of a digital in-line holography setup and discusses the applicability to coulomb balls.   

Dynamics of stable and unstable externally driven dust clouds

An individual dust cloud, formed in the anodic glow discharge immersed in a radio-frequency-produced plasma, is shown to support dust-acoustic waves (DAW) if the number of dust grains is sufficiently large. Below this threshold, the dust cloud sloshes and compresses synchronously when the anode voltage is modulated (1-50 Hz). Above this threshold, the dust acoustic waves become resonant when the modulation amplitude is large and the modulation frequency is near the spontaneous dust acoustic frequency. We report on the three-dimensional structure and dynamics of both the cloud and the dust acoustic wave.   

Self-excited dust-acoustic waves in dusty plasmas under microgravity conditions

We have performed dusty plasma experiments under microgravity in the rf-driven IMPF-K plasma chamber. Under these conditions, a particle-free region (``void'') appears in the center of the discharge. In dependence on the neutral gas pressure and the particle density in the cloud, self-excited compressional dust density waves are released from the void's sharp boundary and propagate radially outwards. On their way towards the cloud edge, the wave is refracted and propagates under a distinct angle along the medium boundary with almost constant intensity. The instability has been characterized by its frequency and wave number over a range of discharge parameters. The observation is compared with theoretical dispersion relations for the ordinary dust-acoustic wave and the dust-acoustic surface wave. Conclusions on the driving mechanism can then be drawn on base of the refractive behavior and the background plasma parameters which have been gained by Langmuir probe measurements.   

Experiments on Structural Properties of Yukawa balls

Recently, it was shown that it is possible to confine spherical dust clouds in a plasma~[1,2]. It was found that these dust clouds have a crystalline structure which differs noteably from the well known fcc, bcc and hcp order in extended crystalline systems. These objects are called 'Yukawa Balls' because of the screened Coulomb interaction between the particles. The experiments show that the particles arrange in nested shells with hcp order on individual shells. This seems to be a unique feature of few-particle systems with strong coupling as it is also reported for trapped laser-cooled ions [3]. Interestingly, the structure in the center of ions clouds changes to bulk order as the ion number and hence the cloud size grows. Here, we present results from experiments on small and large Yukawa balls to discuss whether this can be observed for Yukawa balls as well. Additionally, first experiments are reported which investigate structural changes due to a elliptical deformation of the dust cloud. \newline [1] O. Arp \textit{et al.}, Phys. Rev. Lett. \textbf{93}, 165004 (2004). \newline [2] O. Arp et al., Phys. of Plasmas 12, 122102 (2005). \newline [3] D.H.E. Dubin and T.M. O'Neill, Rev. Mod. Phys. 71, 87 (1999).   

Thermodynamic properties of screened Coulomb balls

Complex plasmas in parabolic traps [1,2], especially Coulomb balls, can easily reach a strongly coupled state which is of great current interest in many fields, including trapped ions, ultracold plasmas and condensed matter. The advantage of the dust crystals is the direct experimental access to the individual particle positions, allowing for precision comparisons with theoretical models and numerical simulations. In this work the dependence of melting points of mesoscopic spherical crystals on the screening and particle number is analyzed. We present analytical results which are compared with simulation and experimental data [3,4,5]. It is shown that the influence of the screening on structural properties of these mesoscopic systems exhibts also a strong impact on the melting behavior. This analysis is based on Metropolis thermodynamic Monte Carlo simulations to obtain first principle thermodynamic properties of the strongly correlated Coulomb clusters. Finally, our results allow to propose a new non-invasive diagnostic to determine the dust temperature. [1] O. Arp, A. Piel and A. Melzer, Phys. Rev. Lett. 93, 165004 (2004).[2] P. Ludwig, S. Kosse and M. Bonitz, Phys. Rev. E 71, 046403 (2005).[3] M. Bonitz, D. Block, O. Arp, V. Golunychiy, H. Baumgartner, P. Ludwig, A. Piel and A. Filinov, Phys. Rev. Lett. 96, 075001 (2006). [4] O.S. Vaulina, S.A. Khrapak and G.E. Morfill, Phys. Rev. E 66, 016404 (2002). [5] J.P. Schiffer, Phys. Rev. Lett. 88, 205003 (2002)   

Mach Cones in Three-Dimensional Yukawa Crystals

Mach cones have been observed in two-dimensional dusty plasma experiments (D. Samsonov et al., Phys. Rev. Lett., 83, 3649, 1999) and molecular dynamics (MD) simulations assuming that the dust particles interact via a Yukawa potential (Z. W. Ma and A. Bhattacharjee, Phys. Plasmas, 9, 3349, 2002). We present new simulation results of Mach cones in three-dimensional Yukawa crystals excited by external laser forcing. As is well known, these crystals can be of the bcc and fcc type, and experiments have produced crystals with both types coexisting. Under a variety of conditions, our simulations show stable three-dimensional Mach cones with a tent structure. While the two-dimensional projection of these cones resemble the multiple cone structure of two-dimensional cones, they need larger dust charge and higher-amplitude forcing for their excitation. We present results on the effect of melting on these Mach cones, and their structures in the near-field and far-field regions.   

Three-dimensional Bernstein-Greene-Kruskal modes in a multi-species plasma: Void solutions in a dusty plasma?

A recent theory on three-dimensional (3D) Bernstein-Greene-Kruskal (BGK) mode [Ng and Bhattacharjee, Phys. Rev. Lett., {\bf 95}, 245004 (2005)] is generalized to the case of a multi-species plasma. One particular class of exact Vlasov solutions is sought and some are constructed explicitly with electrons and ions following Boltzmann distributions and the distribution function of the other species depending on energy and angular momentum. Some of these solutions are shown to have a depleted (void) or enhanced (clump) density of the third species, or even more complex structures such as a void shell. Comparison with dust voids observed in dusty plasma experiments will be made, as well as a recent theory of the dynamical formation of dust voids [Avinash, Bhattacharjee and Hu, Phys. Rev. Lett., {\bf 90}, 075001 (2003)], which has been generalized to 3D [Ng et. al., Phys. Plasmas, submitted].   

On constructing thermodynamic state variables for a dusty plasma from 3-D velocity measurements

Over the past two decades, dusty plasmas have become a widely studied system because they offer the opportunity to study many plasma phenomena at the kinetic (particle) scale. In particular, laboratory experiments on dusty plasmas can be “constructed” to exist in a variety of configurations – from a highly ordered crystal-like state to a disordered gas-like state. Through the use of stereoscopic particle image velocimetry (stereo-PIV) techniques, it is now possible to examine the velocity space distributions and its moments to obtain information on the thermodynamic properties of dusty plasmas. This presentation reports on the application of this velocity space analysis method to construct “state variables” for a dusty plasma – specifically, pressure, temperature, and a new quantity, flow. In this study, several physical systems that have been measured using PIV techniques are reanalyzed using this approach. In particular, stereo-PIV measurements of particle cloud formation, streams, and dust acoustic waves will be considered.   

Collective Modes in 2D Yukawa Solids and Liquids

We report comparative studies on collective excitations in 2D strongly-coupled complex plasmas, interacting through a Yukawa potential, encompassing both the solid and the strongly coupled liquid states. Dispersion and polarization of the collective modes in the solid state are calculated through lattice-summations$^{1}$, while in the liquid state through Molecular Dynamics simulations$^{2}$ in conjunction with theoretical Quasilocalized Charge Approximation$^{3}$ (QLCA) analysis. The latter closely emulates the dispersion resulting from angular averaging in the lattice. In general, however, the lattice dispersion is substantially different from that of liquid. MD simulations show the dramatic transformation of the anisotropic phonon dispersion of the crystal lattice near the solid-liquid transition into the isotropic liquid dispersion. The transition boundary is identified through independent equilibrium analysis$^{4}$. $^{1}$T. Sullivan et.al., \textit{JPA }\textbf{39} 4607 (2006);$^{2}$G.J. Kalman et.al., \textit{PRL} \textbf{92} 065001 (2004);$^{3}$K.I. Golden, G.J. Kalman, \textit{Phys. Plasmas} \textbf{7} 14, (2000);$^{4}$P. Hartmann et.al., \textit{Proc. of 11}$^{th}$\textit{ WS on Dusty Plasma }(2006), to be publ.   

Beam-Plasma Interaction in a 2D Complex Plasma

In a complex (dusty) plasma, penetrating ion or electron beams may lead to beam-plasma instabilities. The instability displays interesting new properties when either the plasma or the beam, or both, are strongly interacting$^{1}$. Foremost amongst them is the possible generation of \textit{transverse} instabilities. We consider the case when a 2D plasma is in the crystalline phase, forming a lattice, and the beam is moving in the lattice plane. Both the grains and the beam particles interact through a realistic Yukawa potential. The beam particles are assumed to be weakly coupled to each other and to the lattice$^{2}$. Using the full phonon spectrum for a 2D hexagonal Yukawa lattice$^{3}$, we determine and compare the transverse and longitudinal growth rates. The behavior of the growth rates depends on the direction of the beam and on the relationship between the beam speed $v$ and the longitudinal and transverse sound speeds $s_L$, $s_T$. For beam speeds between the longitudinal and transverse sound speeds, the transverse instability could be more important, because it appears at lower $k$ values. \newline $^{1}$ G. J. Kalman and M. Rosenberg, J. Phys. A: Math. Gen. \textbf{36} 5963 (2003) $^{2}$ M. Rosenberg, G. J. Kalman, S. Kyrkos and Z. Donko, J. Phys. A: Math. Gen. \textbf{39 }4613 (2006) $^{3}$ T. Sullivan, G. J. Kalman, S. Kyrkos, P. Bakshi, M. Rosenberg and Z. Donko, J. Phys. A: Math. Gen. \textbf{39} 4607 (2006)   

Measurements of the kinetic dust temperature of a weakly-coupled complex (dusty) plasma

A complex (dusty) plasma (CDP) is a four-component system composed of ions, electrons, neutral particles and charged microparticles. The presence of the microparticles gives rise to new plasma phenomena and allows the study of fundamental aspects of plasma physics on the kinetic level. Using stereoscopic particle image velocimetry (stereo-PIV), one can measure the velocity of the microparticles in three dimensions and extract a three-dimensional distribution of velocities. From this velocity distribution, one can extract a kinetic temperature for the microparticle component of a CDP. This presentation describes ongoing experimental measurements of the velocity space distribution of the dust component and ongoing numerical studies on the application of stereo-PIV to the measurement of thermal properties of CDPs. It will be shown that the temperature of the dust component is anisotropic and significantly larger than the other plasma components (electrons, ions and background neutrals), a result which is consistent with previous measurements of the kinetic temperature using the plasma crystal.   

Laser manipulation experiments in dc glow discharge dusty plasmas

The use of lasers to actively manipulate charged microparticles in a plasma has been a valuable tool in the dusty plasma community. However, almost all of these studies have been performed in the strongly-coupled regime in rf discharge dusty plasmas. This presentation reports on the application of laser manipulation techniques to a dc discharge dusty plasma. In these studies, a Nd:YAG laser with a maximum output power of 600 mW is used as the manipulation laser. The manipulation laser uses a voltage-controlled, two-mirror system that allows the laser to be scanned throughout the volume of the dusty plasma. This presentation will highlight three main results. First, it will demonstrate the active manipulation of microparticles in the plasma. Second, it will use the manipulation of the particles at the particle cloud – plasma interface to characterize the potential structure at that boundary. Third, it will illustrate the effect of scanning the laser through the particle cloud and observing driven flows.   

Particle transport in a dc glow discharge dusty plasma with two particle size distributions

Recent experiments performed on the Auburn Complex Plasma Experiment (A-COMPLEX) have shown a tendency for the dust cloud, suspended in an argon dc glow discharge plasma, to self-organize into an ordered structure with multiple internal boundaries. At these internal boundaries, there is a clear change in the microparticle transport. Initial investigations suggest that the mechanism behind the dust cloud self-ordering is the mass distribution of the silica particles that comprise the cloud. To test this, a new experiment involving two dust particle distributions, 10 micron and 40 micron diameter silica particles, is performed in the A-COMPLEX device to create distinct microparticle populations within the suspended dust cloud. Two-dimensional Particle Image Velocimetry (PIV) techniques will be used to characterize the particle transport. Measurements will be presented on the internal transport of the microparticle in this system.   

DEMOCRITUS code: A kinetic approach to the simulation of complex plasmas

The DEMOCRITUS code is a particle-based code for plasma-material interaction simulation. The code makes use of particle in cell (PIC) methods to simulate each plasma species, the material, and their interaction. In this study, we concentrate on a dust particle immersed in a plasma. We start with the simplest case, in which the dust particle is not allowed to emit. From here, we expand the DEMOCRITUS code to include thermionic and photo emission algorithms and obtain our data. Next we expand the physics processes present to include the presence of magnetic fields and collisional processes with a neutral gas. Finally we describe new improvements of the code including a new mover that allows for particle subcycling and a new grid adaptation approach.   

Simulation of Dust Charging and Shielding in the Presence of a Magnetic Field

We explore the charging of a dust particle immersed in a plasma in the presence of a magnetic field. The dust particle charges due to the flowing electrons and ions within the plasma and is allowed to emit electrons via thermionic emission and photoemission. Several parameters are obtained and compared with basic simulations without the magnetic field using the 2-D, 3-V DEMOCRITUS code developed at LANL. Next we look at the effect of this dust particle charging in the presence of another dust particle. Delzanno, et al. [1] showed that for a thermionically emitting particle immersed in a plasma, an attractive potential well can form. This leads to the attraction of particles with like charges, such as another dust grain. We explore the attractive forces between two particles as a function of their separation. If the attractive potential well is deep enough, the two particles will combine, thereby creating macro-particles. We study this in an astrophysical sense, looking at this phenomenon as a possible source of galactic formation. \newline \newline [1] G.L. Delzanno, G. Lapenta, and M. Rosenberg, Phys. Rev. Lett. 92 (3), 035002 (2004)   

Laboratory Astrophysics: effects on radiative shock of lateral radiative losses.

Radiative shock waves are observed around astronomical objects in a wide variety of environments, for example they herald the birth of stars and sometimes their death. They can also be created in the laboratory using energetic lasers, with control on the experimental conditions. We show here the first experiment on radiative shock performed at the PALS laser facility with a Xenon filled cell. It is also the first time that the slow-down of the radiative precursor is clearly seen in an experiment. During the first 40 ns of the experiment, we have traced the radiative precursor velocity, which shows a strong decrease at that stage. Three-dimensional numerical simulations, including state of art opacities, indicate that the slowing down of the precursor is consistent with a radiative loss induced by a reflection coefficient of about 40 {\%} at the walls of the cell. *We would like to thank Hamamatsu, LASERLAB and JETSET for their support in doing the experiment at PALS.   

Study of Perturbations on High Mach Number Blast Waves in Various Gasses

We have performed a series of experiments examining the properties of high Mach number blast waves. Experiments were conducted on the Z-Beamlet$^{1}$ laser at Sandia National Laboratories. We created blast waves in the laboratory by using 10 J- 1000 J laser pulses to illuminate millimeter scale solid targets immersed in gas. Our experiments studied the validity of theories forwarded by Vishniac and Ryu$^{2-4}$ to explain the dynamics of perturbations on astrophysical blast waves. These experiments consisted of an examination of the evolution of perturbations of known primary mode number induced on the surface of blast waves by means of regularly spaced wire arrays. The temporal evolution of the amplitude of the induced perturbations relative to the mean radius of the blast wave was fit to a power law in time. Measurements were taken for a number of different mode numbers and background gasses and the results show qualitative agreement with previously published theories for the hydrodynamics of thin shell blast wave. The results for perturbations on nitrogen gas have been recently published$^{5}$. .$^{1 }$P. K. Rambo, I. C. Smith, J. L. Porter, et al., Applied Optics \textbf{44}, 2421 (2005). $^{2 }$D. Ryu and E. T. Vishniac, Astrophysical Journal \textbf{313}, 820 (1987). $^{3 }$D. Ryu and E. T. Vishniac, Astrophysical Journal \textbf{368}, 411 (1991). $^{4 }$E. T. Vishniac, Astrophysical Journal \textbf{274}, 152 (1983). $^{5 }$A. D. Edens, T. Ditmire, J. F. Hansen, et al., Physical Review Letters \textbf{95} (2005).   

3D Rayleigh-Taylor Instability in Decelerating Interface Experiments

Our goal is to experimentally confirm or disprove the hypothesis that the Rayleigh-Taylor instability could be responsible for the observed transport of heavy elements from the core of SN1897A into its outer layers. Our experiments bridge the gap between simulations and observations by using intense lasers to create an extremely large amount of energy in a small volume. Experiments performed at the Omega Laser facility use $\sim $ 5kJ of laser energy to create a blast wave similar to those in supernovae. The blast wave crosses a perturbed interface with a density drop and produces RT growth. By performing experiments with more complex initial conditions, we hope to observe the effect their complexity has on Rayleigh-Taylor instability. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliance through DOEGrant DE-FG52-03NA00064, and DE FG53 2005 NA26014.   

Preliminary Hyades Modeling of Multi-Interface Diverging Experiments for NIF

In experiments using the Omega Laser at the University of Rochester, researchers studying supernova dynamics have been able to observe the growth of Rayleigh-Taylor instabilities in a high energy density system, using a planar target with a single, multi-mode sinusoidal interface. These experiments will be expanded to incorporate multiple layers for use at the NIF laser. We report attempts to perform scaling simulations and model the interface dynamics of a multilayered diverging Rayleigh-Taylor experiment for NIF using a combination of 1D and 2D Hyades, a Lagrangian 3-temperature, 1-fluid hydrodynamic simulation code used within the high energy density physics community. The 2D simulations use a two-dimensional mesh defined in R-Z coordinate space with a rotation axis of symmetry at R=0. The simulation work will be used to help develop parameters for experimental design.   

Three-dimensional Modeling of Laboratory Jets Interacting with Obstructions

We have fielded hohlraum-driven high-Mach-number jet experiments on the Omega laser at the U of Rochester that scale to astrophysical jets. We have obtained high-resolution x-ray images of jets deflected from a high-density sphere that simulate astrophysical jets interacting with stellar clouds at different impact parameters. We will present 2- and 3-d simulations of these jets with the continuous-adaptive-mesh-refinement radiation-hydrodynamics code RAGE. 3d calculations are not only required because of the 3-d initial configuration of the jet-ball interactions, but also because of the breakup of the jet after the deflection. We have developed the technique of running high resolution 2d simulations to obtain the correct impulse delivered to the jet; then rotating the jet into 3d, adding the 3d configuration of the ball; running relatively low resolution for some time and later higher resolution to capture details of the breakup after the jet deflection. We will also show simulations of jet and shock interactions with multiple balls to be fielded in the future under the NLUF program.   

Study of supersonic jet formation and propagation in low current x-pinches

A comprehensive study of the supersonic jets created from x-pinches has been performed. A compact pulsar was used to drive 4 and 6 wire x-pinches with an $\sim $80kA current having a risetime of 40ns. The wire materials studied include Al, Mo, and W. The electrode separation was 10mm. The propagation of the jets is studied at the top of the electrode. XUV framing images show that the jets move with a velocity of 6 x 10$^{5}$ m/sec. Additionally, the density of the jets was obtained with laser interferometry and was in the range of 10$^{17}$-10$^{18}$ cm$^{-3}$. These results indicate that jets of astrophysical interest can be produced with compact, low current, x-pinches.   

A Simplified Theory of Plasma Jets

A simplified model describing a plasma jet is presented. In this model, the plasma jet is assumed to obey the ideal MHD equation in the Woltjer-Taylor equilibrium state locally. The ratio of the self magnetic energy to the self-magnetic helicity is assumed to be slowly varying. Under these assumptions, a complete set of equations governing the plasma jet is derived. Techniques for solving these equations are discussed.   

Plasma Wakefields Driven by Photons, Electrons and Neutrinos's

Employing the relativistic kinetic equations for photons, electrons and neutrinos we investigate the formation of plasma wakes due to both short and long pulses driven for photons, the latter are known as self-modulated \underline {wakefields.} We also investigate the effects of energy spread of the driver and show that modulational type instabilities exist for all drivers. Quasi -- Linear equations are obtained for photons and neutrinos and we demonstrate significant energy transfer to the plasma particles. Applications to laboratory plasma accelerators and astrophysical accelerators will be discussed.   

Theoretical aspects of jitter radiation from Weibel turbulence in Hercules experiment

Weibel instability development and structure will be studied in Hercules experiment. A primary beam will induce current filamentation whereas the secondary beam is used to probe the generated magnetic field structure. In particuar, jitter radiation, which will be emitted by the electrons of the secondary beam, can be use for accurate diagnostics. For this purpose, we further develop the theory of jitter radiation from small-scale magnetic fields generated by the Weibel instability and demonstrate that the spectra vary considerably with the viewing angle. Furthermore, we quantify how the low-energy photon index, alpha, ranges changes with the apparent viewing angle for various models of magnetic field spatial distribution. We discuss astrophysical applications of this study.   

Modeling the spectral evolution of prompt GRBs and X-ray flares

We use the detailed theory of jitter radiation from relativistic shocks containing small-scale magnetic fields and relativistic shock kinematics to build a numerical model of spectral variability of GRB emission. It is, then, applied to the conditions of the internal shocks in order to model the prompt phase and X-ray flares. We demonstrate that the model lightcurves and spectra agree well with observation data. We discuss how one can deduce certain parameters of the plasma of the shock and the ejected material.   

Efficiency of proton-driven Weibel instability at thermalizing initially two-temperature astrophysical plasmas

Whether an efficient collisonless temperature equilibration mechanism exists for a two-temperature ion-electron plasma, with $T_i>T_e$, is important for understanding astrophysical phenomena such as radiatively inefficient accretion flows and supernova remnants. Here we study whether Weibel instability driven by a proton temperature anisotropy can be such a mechanism. Analysis and PIC simulations find that in an unmagnetized plasma, although the instability grows at a rate much larger than the ion-electron collision rate, the saturated magnetic field is low and inefficient to couple the ions and electrons. It is speculated that in a magnetized plasma the instability can provide a more efficient coupling.   

Acceleration of High Energy Cosmic Rays in the Nonlinear Shock Precursor

The problem of understanding acceleration of very energetic cosmic rays to energies above the 'knee' in the spectrum at 10$^{15}$-10$^{16}$eV remains one of the great challenges in modern physics. Recently, we have proposed a new approach to understanding high energy acceleration, based on exploiting scattering of cosmic rays by inhomogenities in the compressive nonlinear shock precursor, rather than by scattering across the main shock, as is conventionally assumed. We extend that theory by proposing a mechanism for the generation of mesoscale magnetic fields (kr$_{g}<$1, where r$_{g}$ is the cosmic ray gyroradius). The mechanism is the decay or modulational instability of resonantly generated Alfven waves scattering off ambient density perturbations in the precursors. Such perturbations can be produced by Drury instability. This mechanism leads to the generation of longer wavelength Alfven waves, thus enabling the confinement of higher energy particles. A simplified version of the theory, cast in the form of a Fokker-Planck equation for the Alfven population, will also be presented. This process also limits field generation on r$_{g}$ scales.   

Micro and macro-instabilities driven by cosmic rays accelerated in front of strong shocks

Cosmic rays (CR) are thought to be accelerated in strong shocks such as supernova remnant shocks. Their acceleration, however, significantly modifies the shock environment and thus influences the acceleration process itself. The paradigm of enhanced CR acceleration by scattering them off the self-generated Alfven waves seems to explain the supernova remnant origin of galactic CRs up to the energies $\sim 10^{15}$ eV. At the same time recent improved observations and analyses suggest a possibility of acceleration of CRs to even higher energies ($10^{17}-10^{18}$eV) in the same sources. This requires a thorough reconsideration of the backreaction of accelerated particles on the shock structure and on the MHD turbulence that helps to confine and accelerate particles. We consider the Alfven wave instability driven by the CR cloud ahead of the shock in a regime of strong rms magnetic field (exceeding an ambient field) which is more appropriate for an enhanced acceleration. The second part of this study deals with the structural stability and bifurcation of the shock which also strongly influences the acceleration rate by altering the flow velocity gradient and the shock compression.   

Quantum Plasmas in Neutron Stars

Observations of X-rays from 1E1207.4-5209 (an isolated hot neutron star/pulsar in the supernova remnant KS1209-52) reveal that its X-ray spectrum contains absorption features that display quantization in their energy. We show that such features can be explained by quantizing the Larmor orbits in the extremely strong magnetic field of the neutron star. However, the magnetic field strength necessary to explain the quantized absorption features \underline {(8x10}$^{10}$ \underline {G)} is a factor 40 too small to explain the known rotational spin-down rate of the pulsar \underline {(2-3x10}$^{12}$ G). We provide a solution to this dilemma by showing that X-ray emission arises from confined torodial plasma surrounding the equator of the neutron star/pulsar (a ``pulsarsphere''). This emission is then situated well above the surface of the neutron star and hence reveals a much lower apparent magnetic field strength.   

Recent Progress on Laser-Driven 10.3 keV X-Ray Sources

Underdense, laser-driven targets have been shown to be efficient converters of laser light into X rays. We report on recent experiments at the OMEGA laser carried out in order to optimize the X-ray yield from Ge-doped SiO$_{2}$ aerogel targets. We have varied the aerogel-target density from 3.6 to 6.5 mg/cm$^{3}$ keeping the fraction of Ge atoms fixed at 20{\%}. Laser intensity on target varied from 2$\times $10$^{15}$ -- 7$\times $10$^{16}$ W/cm$^{2}$ and laser-pulse lengths from 1 to 6 ns; the total energy delivered to the targets is nearly 20 kJ. The X-ray output from the targets has been measured with absolutely calibrated crystal spectrometers and photo-conductive devices (PCDs) and X-ray diodes, which also provide a temporal history of the X-ray output. Target performance in the 10.3 keV X-ray band is presented as a function of these parameters. We find $\approx $1{\%} of the laser energy is converted into X rays in the 9--15 keV band. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405ENG-48. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract DE-AC04-94AL8500.   

Titanium lined hohlraums as multi-keV x-ray converters

Developments of bright multi-keV K-shell emission sources [1-3] are necessary for ICF studies such as radiography of dense materials. Our recent works with prepulsed foils of titanium (He$_{\alpha }$ at 4.7~keV), copper (He$_{\alpha }$ at 8.3~keV) and germanium (He$_{\alpha }$ at 10.3~keV) showed high multi-keV x-ray conversion efficiencies up to 8.0{\%}, 1.0{\%} and 2.5{\%} (respectively) [1,2]. In comparison with thick foils, the preexploded foils conversion efficiencies are increased by a factor of more than 2. Hohlraums with a titanium liner have been used on the OMEGA laser facility in Rochester to quantify the multi-keV x-ray conversion. For the first time, a laser pulse with a picket prior to the main bulk of laser power has been employed with a Ti-lined hohlraum. X-ray produced with this laser pulse with picket is compared to the case with a square 1 ns pulse shape. X-ray power was measured by the broadband spectrometer DMX (filtered diodes) and the absolutely calibrated crystal spectrometer HENWAY. Multi-keV emission is diagnosed by a full set of diagnostics giving conversion efficiencies, time dependant x-ray power and imaging, time integrated imaging and high resolution spectra of titanium. [1] F. Girard et al., Phys. Plasmas, \textbf{12}, 092705 (2005) [2] D. Babonneau et al., submitted in Phys. Rev. Lett. [3] K. B. Fournier et al., Phys. Rev. Lett., \textbf{92}, 165005 (2004)   

Electron kinetic simulations of nanosecond pulse interaction with foam targets for X-ray production

SiO$_{2}$ aerogel targets doped with Ge or Ti have been used as sources of multi-keV X-rays for backlighting [1,2], and a supersonic ionization wave was observed to propagate in the underdense plasma. To complement previous LASNEX 2D fluid simulations [1,2], and to better understand the heat transport, we performed planar 1D simulations with our electron kinetic code ``FPI'' [3], including atomic physics (ionization and excitation of Si). Both axial and radial transport were addressed, albeit in turn. Non Maxwellian energy distribution functions, due to strong laser heating and to nonlocal electron heat flow were seen. K.B.F.'s work performed under the auspices of the US Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. \newline [1] C. Constantin et al., Phys. Plasmas \textbf{12}, 063104 (2006). \newline [2] K.B. Constantin et al. Proc. of SPIE \textbf{5918}, 59180N. \newline [3] S. \'{E}thier and J.P. Matte, Phys. Plasmas \textbf{8}, 1650 (2001).   

ZaP Flow Z-pinch EUV Light Source for Lithography

The density of features on semiconductor integrated chips (ICs) can increase as the wavelength of the light used for lithography decreases. Present lithography operates at 193 nanometer (nm) wavelength to produce ICs with features at the 90 nm node. By 2015, the semiconductor industry's goal is to operate lithography at the 44 nm node. To accomplish this, an extreme ultraviolet (EUV) light source operating at 13.5 nm wavelength is required, at a power of at least 115 Watts. Using a xenon gas, the ZaP experiment is expected to produce plasma that will emit EUV radiation at the 13.5 nm wavelength. The ZaP Flow Z-Pinch Experiment is presently studying the effect of sheared flow on gross plasma stability. In the experiment, hydrogen gas has been used to produce plasma with quiescent periods in the magnetic mode activity which are 2000 times longer than other plasma concepts for creating EUV light, with 300 times the volume. Similar results have been found with xenon gas. Presently, an EUV detector is being designed using an AXUV100, Silicon/Zirconium filtered photodiode with an 11-18 nm band pass to detect any EUV emissions within that spectrum and the total power of the emissions. The design allows for other photodiodes with narrower band passes to be installed. The design of the detector and initial results will be presented.   

Comparison of Blast Wave Simulations Using Spatially Uniform and Experimentally Measured X-ray Sources

The dynamics of energy loss through diagnostic and/or laser-entrance holes with or without shine shields is of interest to a class of inertial confinement fusion experiments envisioned for the National Ignition and ZR Facilities. We discuss the energy source in 2-D radiation-hydrodynamic simulations using Lasnex [G. Zimmermann et al.] for blast wave experiments recently fielded at the Z facility driven by a dynamic hohlraum. In the past the simulations used a time and spectrum-dependent 1-D source, which created a blast wave in an adjacent region of SiO$_{2}$ aerogel foam. We have now included a spatial dependence in the source and constrained this dependence and the spectrum to closely resemble recent data [Nash et al., 2001]. We show a comparison of results from both simulations to identify sensitivities to the characteristics of the source function.   

X-ray imaging diagnostics for axially-located experiments using the Z / ZR dynamic hohlraum

Radiation transport is among a class of inertial confinement fusion problems being examined using the Z dynamic hohlraum (DH) and the future ZR DH. We describe experiments wherein a target is mounted above the DH to permit radiographic access. X-ray diodes and bolometers characterize the $\sim$100 kJ quasi- Planckian radiation (T$_{rad} \sim$ 150-220 eV) emitted on-axis from the DH. High-resolution, soft x-ray imaging of hohlraum wall ablation and motion is sometimes complemented by a blast wave (BW) calorimeter. As an example, we describe an experimental configuration fielded at Z that imaged BWs emerging from copper wall hohlraums using the 6.15 keV monochromatic imaging system [Sinars et al., 2004]. This experiment diagnosed energy loss through gaps or holes similar to diagnostic and/or laser entrance holes in hohlraums. Future experiments require higher energy backlighters (9-25 keV) to study, e.g., how high-albedo hohlraum material ablates and fills holes. We also discuss possible x-ray diagnostics for experiments that use the ZR DH as a radiation source.   

Properties of the Blowoff Plasma Emitted from the Ends of a Cylindrical Dynamic Hohlraum

A Dynamic Hohlraum\footnote{T. W. L. Sanford \textit{et al.,} Phys. Plasmas \textbf{13,} 012701 (2006); J. P. Apruzese \textit{et al.,} Phys. Plasmas \textbf{12}, 012705 (2005).} is created when arrays of tungsten wires imploded by Sandia National Laboratories' Z generator impact and heat an on-axis cylindrical foam target. The resulting radiation, confined by the tungsten wire plasma, is nearly Planckian and is characterized by temperatures of $\sim $ 200-250 eV. The internal radiation field can be used to implode a capsule containing fusionable material. Radiation emitted from the ends of the cylindrical target can be employed for radiation flow and material interaction studies. This external radiation is accompanied by an expanding blowoff plasma. To diagnose this blowoff plasma, Al and/or Mg bearing tracer layers have been placed at the ends of some of the Dynamic Hohlraum targets. Deep absorption lines of the K-shell stages of Al and Mg are seen in the spectra of these tracers. In the present work, these spectra are analyzed to obtain the properties of the blowoff plasma, the radiation field backlighting it, and to compare it with the hohlraum interior.   

Performance Metrics of the Z Pinch Dynamic Hohlraum

The z-pinch dynamic hohlraum is used as a high-power x-ray source for a variety of HEDP applications including radiation physics, opacity measurements, and inertial confinement fusion (ICF). In each of these applications, the usefulness of the source depends on the reproducibility of the pulsed power performance and the resulting x-ray energy emission and pulse shape. A statistical analysis of a number of different performance metrics has been completed for $> 10$ experiments with nearly identical z-pinch target geometry and diagnostic viewing access. It is found that the 1-sigma reproducibility of the x-ray energy emission and pulse-shape is $< 13\%$ and $< 4\%$ respectively. A discussion of this analysis and the impact on the various HEDP applications is included.   

X-ray Power and Energy output of Z-Machine Dynamic Hohlraums

Los Alamos performs radiation flow experiments at the Z-machine in order to verify their modelling codes. Critical input to these codes is the actual radiation power profile which flows into the experiment. Our standard diagnostic suite consists of X-ray Diodes (XRD), silicon photodiodes, and nickel thin film bolometers. Custom written computer software examines the raw data to determine the data quality, folds in detector spectral response, calculates a multi-detector spectral unfold, and yields an equivalent Planckian temperature profile. Sets of diagnostics view the dynamic hohlraum from the side, top axial anode side, and bottom axial cathode side. Results to date yield some interesting conclusions: Correlation between the various diagnostic views seems tenuous at best. Identical nickel foil bolometers usually agree within 10{\%}. At low bolometer-foil temperature increases the bolometers agree with integrated XRD power unfolds but diverge at higher temperature increases. For identically filtered X-ray diodes the integrated response of photocathodes may vary an factor of two. XRD's usually unfold to yield a Planckian-like spectrum. Top axial measurements consistently yield higher temperatures than bottom axial diagnostics. In our presentation we will compare the diagnostic techniques, analysis, and results to establish drive conditions for our experiments.   

Faster, 80ns, Current Scaling Experiments Yield Higher Radiated X-Ray Power and Approach Quadratic Dependence

We report the results of a new series of current scaling experiments with the Z accelerator. The novelty of this work is the shorter implosion times of 80 ns as compared with the 95 ns of the previously reported work. In the present study we utilized lighter weight tungsten wire arrays, which had 300 wires, 20-mm diameter and 10-mm height. We measured the z-pinch radiated x-ray power and energy as a function of the peak current. Two different currents were driven through the loads with the maximum attainable difference in peak values. The x-ray power measured at full 90-kV charge was the highest ever observed with a single 20-mm diameter, 10-mm high tungsten wire array and was of the order of 185 TW. The superior performance of the faster implosions can be attributed to shorter ablation times and to less mass left behind at the initial array radius. The present results are compared with the predictions of heuristic models and enhanced resistivity models.   

3D Resistive MHD Modeling of Wire Array Z-Pinches

We present 3D resistive MHD calculations of the implosion of high wire number arrays on the Z generator at Sandia National Laboratories. We demonstrate full circumference calculations of the implosion of 300 wire tungsten arrays. To access the high resolutions required to adequately resolve discrete initial wires in these arrays we present details of a highly parallel finite volume MHD code for the solution of resistive MHD across a locally refineable Cartesian grid. Furthermore, using a R-Phi-Z wedge shaped geometry we perform calculations of the implosion of nested wire arrays stagnating onto foam liners, to examine possible mechanisms for the origin of an observed axial asymmetry seen in the X-ray radiation obtained from Dynamic Hohlraum experiments.   

Physics issues relevant during stagnation phase of the Z pinch

Wire-array Z pinches are the most powerful laboratory x-ray sources. However, the physics driving the stagnation phase, during which the pinch collapses on axis and subsequently radiates, remains somewhat mysterious. In particular the stagnated pinch resists undergoing radiative collapse, and the radiated energy is several times the kinetic energy of the imploding plasma. In recent years, a number of theories, as well as direct 3D numerical simulation, have been developed to address these phenomena. This work will highlight some of the physics issues relevant during this complicated phase of the Z pinch. Possibilities for future work will be discussed, and, with a little luck, performed.   

K-Shell Emission from Z-pinches: Z to ZR

K-shell z-pinch sources studied at the Z Aaccelerator over the last few years have produced significant K-shell x-ray output from argon, titanium, stainless steel, and copper. K-shell scaling theories (1), which identify initial load conditions necessary to get enough mass to implode at a high enough velocity to achieve plasma temperatures and densities required for K-shell emission, were benchmarked against K-shell emission data from $<$8 MA facilities and photon energies up to 3 keV. The experimental results from the sources at Z have demonstrated the heuristic validity of the models and provided data to fine tune the models for higher photon energies and currents. In this paper, the results of K-shell scaling experiments from Z will be presented, as well as the expected scaling of these sources to ZR (26 MA). Plasma conditions from the Z experiments will be discussed within the context of efficient K-shell production, to identify the appropriate plasma conditions necessary for efficient production at ZR. Calculated results for expected ZR outputs will be presented. (1) K.G. Whitney et. al., J. App. Phys., 67 1725 (1990). 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. Naval Research Lab work was supported by DTRA.   

X-ray Spectroscopy of Plasmas on the Z Accelerator

Tests on the Z-accelerator have been performed to optimize the K-shell emissions from aluminum, stainless steel and copper wire arrays. This process entails changing the array's initial diameter, wire number, or wire thickness, and is influenced by considerations of final implosion velocity and initial mass. Measurements of the plasma temperature and density provide important benchmarks for modeling and theoretical studies, but they can also provide important feedback to experimentalists. If H-like lines dominate the spectrum the plasma is clearly overheated, and mass can be added to the plasma without a significant temperature penalty. We will show examples of K-shell spectra (and where available, L-shell spectra) generated by the Z-machine, and how these spectra reflect array-design considerations. In addition we will show recent data from a stainless steel array where both time- and space-resolved spectra were captured, and examples of parameters that can be extracted: plasma size, temperature and density. All together, these measurements can provide insight into the dynamic process responsible for these final states.   

2D Radiation MHD Modeling of Ar Gas Puff Experiments on Z, ZR, Saturn, and DQ

Large diameter nozzle argon gas puff experiments present us with the opportunity to demonstrate the progress that has been made in 2D state-of-the-art modeling of K-shell emitting gas puff plasmas. In this modeling the Mach2 2D MHD code is extended by incorporating into it a computationally efficient and reasonably accurate model for the non-LTE equation of state and radiation transport present in K-shell emitting multi-dimensional plasmas. This extension is called the tabular collisional radiative equilibrium model. This fully 2D radiation -- MHD capability is used to investigate how the initial, multi-dimensionally structured, gas-puff density distribution affects the evolution of the pinch and produces substantial deviations from 1D behavior. The density and temperature profiles of the pinch, the current profile, the implosion time, and the radiation characteristics are all substantially affected. The 2D calculations can be employed to project the K-shell yield behavior of different nozzle designs on higher current machines. Here we project the K-shell behavior of L3's 4-3-2-1 nozzle and their 6-5-3-2 with-central-jet nozzle$^{ }$to the Saturn and ZR pulse power generators.   

Neutral Gas Density Measurements to Predict Z-Pinch Implosion Dynamics

Neon z-pinch implosions were investigated using the Hawk pulsed power generator [1]. The gas pressure in the inner and outer shells of an 8-cm-diameter shell-on-shell nozzle was varied to change the initial gas distribution and determine its effect on the z-pinch dynamics and K-shell x-ray emission. A holographic interferometer was used to record the z-pinch density distribution at various times during the implosion [2]. In this work, the neutral gas distribution from the nozzle is measured using a high-sensitivity, multichord interferometer. The interferograms and initial gas density are correlated with radiation traces and plasma-radius histories from the measured load inductance. Snow-plow modeling based on the measured gas distributions is compared with the measurements. [1] D.P. Murphy et al., Proc. Beams 2004, St. Petersburg, Russia, p. 7048. [2] D.G. Phipps et al., ICOPS 2006, Book of Abstracts, p. 314.   

X-ray blast windows with mechanically strengthened lithium

Z-pinch- and laser-produced X-ray sources not only emit the few-keV X-rays that are intended, but also X-rays that are much softer than desired, plasma, and hot gas. An X-ray filter along the line of sight easily suppresses the softer X-rays, but when the pulse is powerful enough it evaporates, to become a secondary source of material that must be stopped by an X-ray transmitting blast window. For the X-ray filter the best material is lithium metal: Li has the highest mass per unit area and largest heat capacity for a given X-ray transmission. Li would be a good candidate material for the blast window too if it were mechanically stronger. We show initial results on the strengthening of lithium by mixing it with lithium hydride powder, the only ceramic that would not decrease Li's X-ray transmission. Cooling the resulting Li-LiH cermet to 77 K increases the strength more than an order of magnitude (compared to room-temperature Li): cryogenically cooled Li-LiH is then just as strong, or perhaps stronger, than beryllium.   

Integrated LiH Debris Shields for Warm PRS Loads

Plasma radiation sources can profitably employ thick LiH debris shields integrated into the return current surface. Shield material selection demands the highest specific enthalpy ${\cal H}_m$ to vaporization or decomposition in an attenuation length $\lambda(E_{prs})$ at the k-shell transmission energy of interest. As a figure of merit then, ${\cal E} \equiv {\cal H}_m \rho/ {\cal M} ~ \lambda(E_{prs}) [{\frac{kJ}{cm^2}}]$ can order candidate materials. LiH is the best possible material for this purpose, offering ${\cal E}$ values of $\approx$ 130 for the 13 keV k-shell lines of Kr. In a level comparison, Be offers only 47 and pure Li, 77.5. For the Ti k-shell lines, a similar ordering is: LiH, 22.44; Be, 3.08; Li, 10.40. Early studies\footnote{R.E.Terry, NRL Memo Report 6720--96-7868.} of these designs showed promise even for the more opaque Ar and Ti k-shell radiation. Here we examine the much easier problem for a Kr k-shell PRS and consider the influence of l-shell and m-shell radiation on the hydrodynamic flows developed in the LiH and its decay products. The radiation loading of the LiH shield is computed self consistently within the more generous trade space of stagnation energy, machine current, implosion time, and delivered mass for drivers capable of useful Kr k-shell yield.   

The Impact of Non-LTE Multiplet Dynamics on the Modeling of Tungsten Ionization Dynamics

Modeling the heating and ionization dynamics of high atomic number plasmas generally requires the use of approximations to the M- and N-shell atomic structure of these plasmas. The average atom model is the most commonly employed such approximation. It effectively assigns to each ionization stage the same atomic structure, thereby ignoring large differences in state structure that exist throughout the different M- and N-shell ionization stages. An alternative approximation is to treat each ionization stage separately, but to lump the states within each nl multiplet\footnote{K. G. Whitney and M. C. Coulter, IEEE Trans. on Plasma Sci., \bf{16}, 552 (1988).}. Historically, this approximation has been applied assuming the multiplet substates are in LTE with respect to one another. In this presentation, we calculate the non-LTE behavior of the n=4 states of nickel-like tungsten, whose emissions are seen prominently in exploding wire experiments\footnote{P. G. Burkhalter, et. al., Phys. Rev. A, \bf{15}, 700 (1977).} The non-LTE behavior's effect in modifying the Einstein decay and deexcitation rates of these lumped states will be discussed.   

Modeling and Spectroscopic Analysis of non-LTE Krypton Plasmas

We have developed a detailed multilevel atomic model for K-, L- and M-shell krypton, and investigated its impact on the radiation hydrodynamics on a krypton gas puff driven by the redesigned Sandia National Laboratory ZR accelerator. The atomic model employs an extensive atomic level structure, which is necessary to accurately model the pinch dynamics and the spectroscopic details of the emitted radiation. The atomic data was obtained using the state-of-the-art Flexible Atomic Code, and all relevant radiative atomic processes were included in generating the model. The enormous number of fine-structure levels were judiciously lumped to create a database that is detailed but manageable. We have analyzed the behavior in the krypton K- through M-shell ionization stages using temperature and density conditions that have been predicted in 1-D and 2-D MHD calculations of implosions on ZR   

A Neutron Imaging Diagnostic for Z and ZR

The Sandia pulsed power machine Z produces both gammas and neutrons during Deuterium gas puff shots. Currently, the time history of the gamma and neutron emission is measured during these experiments. Significant interest exists in imaging these neutrons. A prototype pinhole neutron imaging system has been fielded on Z with some success. Plans exist to field an imaging system on the upgraded ZR. Both the design and some results from Z will be presented. This work was performed by the University of California LLNL under the auspices of the DOE under contract W-7405-ENG-48.   

Measurements of D-D neutrons in Z-pinches

Fusion neutron measurements have been carried out on a small Z-pinch device (Z-150, CTU in Prague, 100 kA, 800 ns) as well as on a large Z-pinch generator (S-300, Kurchatov Institute in Moscow, 3 MA, 100 ns). The main goal of this research was to give an insight into the acceleration of fast ions and electrons, i.e. the most energetic processes in Z-pinches. For that purpose we used various deuterated Z-pinch loads and we studied the production of D-D neutrons together with the emission of soft and hard X-rays. The emphasis was put on the finding of (i) the evolution of Z-pinch plasmas, (ii) the time and duration of neutron production, (iii) the time and duration of X-ray generation, (iv) the region of X-ray emission, (v) neutron yields, (vii) X-ray energies, (viii) the neutron energy distribution function, (ix) the anisotropy of neutron emission, (x) the plasma current and voltage. Having obtained these pieces of information, the acceleration mechanism could be discussed.   

Improvement of Methods Used for Reconstruction of Time Resolved Neutron Energy Spectra in D-D Fusion Reactions

Several methods (Monte Carlo, maximum entropy, genetic algorithm, etc.) for the reconstruction of the time resolved neutron energy spectra have been developed in the last few decades. The energy spectra are reconstructed from time-resolved neutron signals which are recorded by several detectors in one direction at different distances. The basic formulation of the problem as well as test results indicate that the methods could give better results when detectors in the opposite direction are also included. If we want to employ both directions of neutron detection, we must know the relation between the neutrons which are emitted in one direction and the opposite direction. Considering these facts, one of the methods (namely the Monte Carlo reconstruction method) was improved. Our improvement includes the anisotropy in neutron yields and neutron energies. The transformation is based on the scattering theory and it is applied specifically to the D-D fusion reaction. This improved Monte Carlo method has been used to process data from experiments on the PF 1000 plasma focus (IPPLM, Warsaw) and on the S-300 Z-pinch (Kurchatov Institute, Moscow).   

Electron Beams in Wire Array Z-pinches

The intensity and the radial profile of electron beams from wire-array z-pinches have been measured on the MAGPIE generator (1MA, 240ns) at Imperial College, London. A Faraday-cup was used to measure the electron beam current from both aluminium and copper wire-arrays with spatial and temporal resolution. Time integrated hard x-ray spectroscopy using spherically-bent crystals produced axially resolved images of the copper K$_{\alpha }$ line, a signature of the presence of electron beams. This research was sponsored by Sandia National Laboratories Albuquerque, the SSAA program of NNSA under DOE Cooperative Agreement DE-FC03-02NA00057.   

Plasma dynamics in an inverse wire array z-pinch

We describe experiments on the MAGPIE facility (1MA, 250ns) with inverse wire array z-pinches, in which the wires act as a return current cage placed around a central current conductor. In this configuration the plasma ablated from the wires is pushed by the JxB force in the radially outward direction and expands into the region free of the magnetic field. The parameters of the coronal plasma were measured using laser interferometry, X-ray and XUV imaging. The experimental set-up also allows addition of axial and radial magnetic fields, and we study the effects of these fields on the plasma dynamics and the ablation rate. The complete ablation of the wires triggers radial ``explosion'' of the plasma which proceeds through formation of gaps in the wire cores, similar to the dynamics of standard wire arrays. The opening of the current path in the final stages could potentially be used as a plasma opening switch.   

Experimental study of wire array plasma dynamics by laser probing

Dynamics of the ablation stage and the beginning of the implosion stage in low wire number cylindrical, nested, and linear arrays were investigated in the 1-MA generator. Plasma bubbles arise in breaks on the wires in the beginning of implosion. The leading edges of the bubbles carry material with the speed $>$250 km/s. In linear wire arrays plasma cascades to the center of the array from wire to wire. In nested arrays, the implosion begins in the external cylinder. The plasma bubbles then hit the plasma columns in the internal cylinder and collapse to the center. Configuration of the magnetic field in the linear array was changed by variation of wire spacing. The regimes of ablation and implosion in the wire arrays with different mutual magnetic fields are compared. Influence of magnetic field reconnection in the central region on ion and electron acceleration is also discussed, and experimental evidence of conversion of magnetic energy to plasma particles is demonstrated.   

Experimental study of the dynamics of energetic electron beams in planar and cylindrical wire arrays and x-pinches by means of hard x-ray spectroscopy and spectropolarimetry on the 1 MA z-pinch generator at UNR

Experiments with planar and cylindrical wire arrays and x-pinches were performed at the 1MA, 100 ns rise time Zebra generator. A time-resolved hard x-ray polarimeter ($>$30kev) based on Compton scattering effect has been used together with fast hard and soft x-ray detectors, a time-gated pinhole camera, and a hard x-ray spectrometer. The different types of electron beams were investigated using silicon diode signals (filters with cut off energies $>$2kev,$>$9kev,$>$30kev ) by comparing with hard x-ray polarimeter signals. The experimental observation of cold K$\alpha $ lines were also used for investigation of high energy electron beams. Estimated degrees of polarization of x-ray radiation varies from 38 {\%} to 70{\%} for different types of plasma electron beams. Electron beams are more collimated in a cathode to anode direction for cylindrical wire arrays than for planar wire arrays and X-pinches. Work supported by the DOE/ NNSA under UNR grant DE-FC52-01NV14050.   

Zero-dimensional implosion models for the Nested and Planar Wire Arrays

In order to maximize power of the plasma radiation sources produced by the implosions of wire arrays the array implosion time should perfectly match the shape and duration of the current pulse. The most effective and reliable tool used so far for optimization of cylindrical array loads is so-called 0D model, which neglects plasma ablation from wires. To apply this model to more complex wire array configurations, such as cylindrical and planar arrays, one has to couple self-consistently the equation of wire motion in the global magnetic field with an algorithm of calculation of the inductive division of current between the wires. The resultant generalized 0D model allows fast calculations of the array implosion time and the amount of the energy coupled with plasma due to the motional impedance, and can be used for the optimization of the nested and planar wire array loads, including the control of Z-pinch radiation performance. This model also reveals some interesting features of the implosion dynamics of nested and planar arrays due to the effect of the current transfer. Detailed study of these features in conjunction with 3D MHD modeling is very important for the improvement of Z-pinch stability.   

Saturation of Electromagnetic Flute Mode Instability in Z-pinch Plasma of a Precursor

Linear analysis of the electromagnetic flute mode instability in the high beta current carrying plasma of the precursor, has demonstrated good agreement between theory and experimental data obtained during wire array implosion experiments on the Zebra pulsed power generator in terms of excited wavelengths and characteristic growth rates. In order to solve numerically nonlinear equations which describe saturation of the instability, we derived expressions for the possible equilibrium profiles for density and magnetic field and developed a 2D numerical code based on the pseudo-spectral method for spatial representation and the two-step predictor corrector method for time advance. In the linear stage numerical results are in good agreement with linear theory. In the nonlinear stage numerical solutions show saturation of the instability, appearance of large scale structures as well as emergence of shorter wave lengths in the excited wave spectrum. The role of large scale structures in anomalous plasma transport will be also discussed.   

Modeling of EUV Radiation from low-Z plasmas with applications to pulsed power experiments

Investigation of MITL plasma formation is important for finding ways to increase power and energy transmission to the load of a pulse power generator. The MITL plasma consists of low-Z elements such as, for example, carbon and oxygen. An appropriate method of diagnosing these ions is EUV spectroscopy. Recently, UNR has experimentally simulated plasma conditions of a SNL-Z MITL using a compact laser-plasma x-ray/EUV facility. EUV spectra from experiments with polyethelene and mylar slabs were collected by a spectrograph with a sliced multilayer grating which can cover a broad spectral region of 130-280 {\AA}. In the present work, we report the further improvement of previously developed non-LTE carbon and oxygen models. In particular, the detailed comparison of our atomic data for C and O ions with results from other codes and with NIST atomic data was performed. Then, more ionization stages and configurations were added to the models to improve the comparison with the EUV spectra. As a result, the new temperature- and density-sensitive, diagnostically important spectral lines were identified and suggested to use for MITL diagnostics. Work supported by the DOE/ NNSA under UNR grant DE-FC52-01NV14050 and by Sandia National Laboratories under DOE contract DE-AC04-94AL85000.   

Modeling of L- and K-shell x-ray radiation from stainless steel wire arrays and X-pinch implosions on the 1MA pulsed power generator at UNR

Modeling of x-ray spectra from the implosion of stainless steel wire loads is challenging because of the overlapping contributions of L-shell spectra from the Fe and Ni ions. Nevertheless, it is a useful diagnostic because of the broad region of electron temperatures (0.2-2keV) corresponding to L- and K-shell radiation. L- and K-shell x-ray spectra have been collected from stainless steel single-wire, X-pinch, conical, and planar array experiments on the 1MA Zebra generator and have been analyzed in detail and compared. Non-LTE Fe, Ni, and Cr kinetic models have been developed to account for the K- and L-shell radiation from these ions. Two atomic data sets were employed and will be compared for the Fe model. The resulting plasma parameters from the axially-resolved L-shell and spatially-integrated K-shell spectra and their dependence on the load type will be discussed. Work supported by the DOE/NNSA under UNR grant DE-FC52-01NV14050, by Sandia National Laboratories under DOE contract DE-AC04-94AL85000, and in part by fellowship support from the National Physical Science Consortium with Sandia National Laboratories.   

Spectroscopic Modeling of Mo Planar Wire Arrays produced on the 1 MA Zebra generator at UNR

Planar wire arrays produce the record-high peak powers and yields in sub-keV and keV among all other configurations of the X- and Z-pinch loads tested on 1 MA UNR Zebra [1]. In particular, Mo planar wire arrays implosions showed the maximum radiation yield of all wire materials tested on this device so far ($\sim $ 18 kJ) and the highest L-shell plasma electron temperature Te$\ge $800 eV. To estimate and analyze evolution of plasma parameters in space and time, spatially-resolved, time integrated and time-gated, spatially integrated L-shell Mo spectra were collected and analyzed. An earlier developed and successfully used non-LTE kinetic Mo model [2] was applied in the present work. Results are compared with the previous results on UNR Zebra Mo X-pinches and SNL-Z Mo nested wire arrays [3]. References: [1]. V.L. Kantsyrev, A. S. Safronova, D. A. Fedin \textit{et al}, IEEE TPS, v. 34, 194 (2006). [2]. A.S. Shlyaptseva, S.B. Hansen, V.L. Kantsyrev \textit{et al}, PRE, v. 67, 026409 (2003). [3]. P. D. LePell, S.B. Hansen, A.S. Shlyaptseva \textit{et al}, PoP, v. 12, 12032701 (2005).   

Study of planar wire arrays with various distribution of mass along the array plane on the 1 MA Zebra generator

Recent studies of planar array implosions have shown remarkable x-ray yield and high-powered pulses. The study of the dependence of radiation properties on the distribution of mass along the array plane were performed using spatially-resolved, time-integrated x-ray/EUV spectroscopy, time-gated x-ray and laser probing imaging, fast x-ray/EUV diodes, and a Ni bolometer. The following planar mass distributions were measured and will be discussed: the linear mass of the exterior wires was four times more than the interior wires with gradual linear mass transition, the linear mass of the interior wires was four times more than the exterior wires with gradual linear mass transition, constant linear mass distribution across an even number of wires such that the implosion took place on inter-wire gap, and constant linear mass distribution across an odd number of wires such that the implosion took place on the central wire. The results of a periodic variation of the linear mass will also be reported.   

Copper Wire Arrays at the 1 MA Zebra Facility

Experiments to study the implosion dynamics and radiation characteristics of copper z-pinches have been fielded at the 1 MA Zebra facility. At Zebra, load diameters ranged from 8mm to 16mm, with 6 to 14 wires, and load masses of 80 to 200 $\mu $g. All arrays utilized 10 micron diameter wire. The impact of initial load mass and initial load diameter on the precursor and the stagnated plasma has been evaluated through spectroscopy, shadowgraphy, and fluence measurements. Plasma parameters have been extracted from modeling of the time-integrated L-shell spectra to study temperature and density variations as a function of spatial position and initial load configuration. Comparisons will be made with harder x-ray spectra and pinhole images to identify regions of highest temperature and density. Shadowgraphy has been fielded to study the formation of the precursor and the main implosion; significant structure is observed. Substantial radiation is observed from the precursor, with variations in the precursor associated with initial load configuration. *Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

Optical streak camera images of wire-array z-pinches on the 1-MA COBRA pulsed power generator

Initial optical streak camera imaging experiments of wire array z-pinches on the 1 MA COBRA pulsed power generator are presented. The imaging system makes use of a Hamamatsu C7700 streak unit, which is coupled to a V7669-06 image intensifier with an MCP, and a C4742-98 CCD camera. A long focal length optical system is employed to relay the z-pinch produced light from the experiment chamber to the input slit of the streak camera -- a total transmission distance of approximately 14 m. The optical streak camera images produced, along with data from other supporting diagnostics, are presented for z-pinch implosions of various wire array geometries and materials.   

Doppler shift and broadening of H-like and He-like Al lines in imploded arrays on MAGPIE and COBRA pulsers experiments

Strong broadening and clearly visible ``red'' and ``blue'' shifts of K-shell spectral lines as a function position along the array axis have been observed in the time-integrated spectra of 8-32 wire Al arrays in experiments on the MAGPIE (1.2 MA, 240 ns risetime) and COBRA (1.1 MA, 100 ns risetime) pulsed-power generators. Spectra were recorded using focusing spectrographs with spherically bent mica crystals (FSSR) in configurations in which the film was positioned out of Rowland circle and the influence of the spatial position on a line position in the spectral direction on the film is not negligible. To separate the spectral and spatial shift of the spectral lines, two wide-band spectrographs with 3 spherically bent crystals each (WB-FSSR) with radius of curvature 182 mm have been used in a mirror-symmetric configuration. The measured Doppler shift of the spectral lines varied along the pinch axis and reached $\pm $0.015 {\AA} corresponding to a radial plasma velocity of $\pm $5x10$^{7}$ cm/s. This research was sponsored by the NNSA under DOE Cooperative Agreement DE-F03-02NA00057.   

Factors influencing the wire core and coronal plasma expansion in arrays with small numbers of wires

The initial explosion phase of individual wires in a wire array may be a critical factor in the later development of the load impedance of the entire array. Factors influencing wire core and corona plasma expansion and long scale-length instability development in arrays with small numbers of wires (2-16) have been studied on the COBRA (1 MA current 100 ns risetime) and XP (500 kA current 45 ns risetime) generators at Cornell University using two frame point-projection x-ray backlighter imaging and three frame laser schlieren imaging and interferometry. The arrays were studied both as the main load of the generators and in the return current circuit. The dependence of the wire core and corona plasma development and expansion on the wire diameter, numbers of wires and current through the individual wires has been studied for Al and W wire arrays. As an example of our results we observe a nonlinear expansion of W wire cores with time.   

Wire Array Dynamics with Varied Current Risetime on COBRA

The 1 MA COBRA accelerator is used to drive wire array loads with current risetimes from 95 to 220 ns. The dynamics of 8 x 12 micron Al arrays show a clearly different character when driven by the fast and slow current pulses. The fast ($\sim $100 ns, 1 MA) drive produces a hot pinch, high total x-ray energy in a long-duration, slowly rising x-ray pulse, multiple hot spots in the pinch, a relatively small load voltage that stays up after x-ray peak, and a large inferred current radius at x-ray peak. In contrast, the slow ($\sim $200 ns, 900 kA drive) produces a cooler pinch with lower total x-ray energy in a short, fast-rising x-ray pulse, absence of hot spots but clear evidence of high-energy ``electron beam,'' large load voltage that drops fast after x-ray peak, and small inferred current radius at x-ray peak. The diagnostic evidence of these characteristics especially x-ray imaging and spectral information, will be presented and the dynamics underlying these phenomena will be discussed.   

Positive polarity effect on 8-wire cylindrical z-pinches

The Cornell University COBRA pulser is a nominal 1MA, 1MV machine, capableof driving up to 8 wire cylindrical Z-pinches. COBRA operates normally in negative polarity. A bolt-on convolute at the load was designed to allow both positive and negative polarity drives for the array. In positive polarity the electric field lines originate on the array wires while in negative polarity the field lines terminate on the wires Data were collected on 8-wire 12.5$\mu $m Aluminum Z-pinches in both polarities. Time dependent x-ray signals were measured using diodes and diamond detectors. A 6 ns pulsed laser was used as a backlighter to obtain images that were time-stepped through the evolution of the pinch. Pinches in positive polarity showed an increase in the power and total energy of the soft X-rays, and a decrease in the power and total energy of the emitted hard X-rays as compared to negative polarity pinches. Initial observations indicated that the pinch column in positive polarity began to form earlier than in negative polarity, and the structure of the imploding material was shown to be drastically different for the two polarity configurations. This research was supported by DOE grant DE-FG03-98ER54496, by Sandia National Laboratories contract AO258, and by the NNSA Stockpile Stewardship Academic Alliances program under DOE Cooperative Agreement DE-FC03-02NA00057.   

Positive Polarity X-Pinch Operation

We present here the results of a series of experiments comparing the operation of an x-pinch in positive and negative polarity. Of particular interest are the x-ray yield, timing, radiating spot size, and the number of radiating spots in a given x-pinch. Conventionally, an x-pinch is operated in negative polarity resulting in electric field lines that terminate on the wires. This choice of polarity for the generator is due to the fact that breakdown in these machines is easier to prevent when they are operated in negative polarity. Previous experiments involving single wire explosions at low current (1-5 kA) have shown that more energy can be deposited in the wire cores when they are driven in positive as opposed to negative polarity. These results raised the question of what happens when pinch-type experiments are driven in positive polarity. Recently, it has become possible to switch the polarity of the 450 kA XP pulser at Cornell to drive x-pinches in either positive or negative polarity. After switching the polarity of the machine and matching current shapes between positive and negative shots, a direct comparison between opposing polarities can be made. This research was supported by DOE grant DE-FG03-98ER54496, by Sandia National Laboratories contract AO258, and by the NNSA Stockpile Stewardship Academic Alliances program under DOE Cooperative Agreement DE-FC03-02NA00057.   

Monochromatic X-Ray Imaging by X pinches on the COBRA Accelerator

The COBRA accelerator is a pulsed power generator capable of producing up to 1 MA current pulses with a typical rise time of 100 ns and pulse width of about 200 ns. Time-gated x-ray imaging is an important diagnostic for the study of imploding dense z-pinches. These plasmas must be studied on the nanosecond time scale, and they generally radiate strongly in the soft x-ray range. As such, they require short, intense x-ray sources to illuminate them. Monochromatic x-ray imaging using an X pinch is a way that has been used successfully in the past [1,2] to image single exploding wires and various static objects. Experiments have been performed to investigate x-ray source parameters for various materials and configurations. Of these materials aluminum (6.6343{\AA}) and zinc (9.815{\AA}) have the most potential for use as a monochromatic source on COBRA. Data presented includes x-ray spectra ($\sim $1-10 {\AA}) along with source size estimates and results from initial imaging experiments. This research was supported by the Stewardship Sciences Academic Alliances program of the National Nuclear Security Administration under DOE Cooperative agreement DE-FC03-02NA00057. \newline \newline [1] S. A. Pikuz,\textit{ et al.,} Rev. Sci. Instrum., \textbf{68}, 740 (1997). \newline [2] D. B. Sinars, \textit{et al., }Phys. Plasmas \textbf{13}, 042704 (2006).   

X-Pinch Characteristics on A Microsecond Capacitive Discharge

Two wire x-pinches of tungsten, molybdenum, nichrome, aluminum, titanium, etc., with diameters ranging from 10 -- 25 microns have been imploded with a 280 kA, 1 microsecond capacitor bank. The x-pinch plasma has been characterized with diagnostics which include soft x-ray PIN diodes, pinhole cameras, a flat crystal spectrograph and a Nd:YAG laser based interferometer. Time integrated x-ray data obtained with pinhole cameras and the spectrograph have enabled the estimation of the x-pinch source size and plasma temperature, respectively. X-rays from the pinch have been used to demonstrate point projection radiography of a simple biological system and a fine wire mesh. Plasma density measurements will also be reported from the Mach-Zender interferometer.   

Plasma dynamics and radiation properties of z-pinch formed within a conducting coil.

We present experimental observations on a z-pinch plasma formed within a conducting coil. The experiment aims to produce a single component x-ray emitting titanium plasma. The z-pinch is formed inside a thin wall titanium coil, which act as the load of a small pulsed power generator, with a nominal 150 kA, 120 ns pulse. The coil pitch is chosen so that the inductive impedance is high enough for the current path along the pinch axis to be preferred. A laser produced titanium plasma is injected from the cathode side, and pre-heated by an auxiliary hollow cathode discharge. The diagnostics include voltage and current, XRD, filtered and time resolved multi pin-hole images of the emitted x-ray plasma radiation, and time resolved XUV spectroscopy. It has been found that an axial titanium plasma column forms during the time scale of the current pulse, well detached from the inner wall of the coil. These observations will be complemented with measurements of the time evolution and spectral content of the titanium plasma radiation.   

Wire array initiation at 1MA z-pinch ZEBRA

The initial stage of W and Al wire array explosions on the 1MA z-pinch installation ZEBRA was investigated. Data from breakdown light emission, B-dot and axial/peripheral V-dots were obtained. Observation of the resistive voltage and the moment of corona generation allow calculation of the condition of the wire core after breakdown. With a current prepulse of 300ns duration, we observed plasma formation at 200ns before the main current. At this moment a first light emission and V-dot signals was observed. During the next 5-10 ns fast plasma components arrive at the center of the wire array and shunt the axial V-dot. After shunting, the axial V-dot shows signals correlated to axial current. Analyzing mass-velocity ratios, we determined that for the Al wire array hydrogen plasma arrived first at the center with velocity 666km/s, followed by carbon plasma with velocity 200km/s and later, the aluminum plasma with velocity 130km/s. Hence, before the main current starts the internal volume of the array is filled by low-density plasma. The axial V-dot signal indicates that the main current initially flows though the entire wire array cross-section, but it is rapidly rejected to the periphery. During the compression stage we observed that the current again flows through the center of the wire array.   

Control of instabilities and thermonuclear fusion in Staged Z-pinch

A Staged Z-pinch, configured for discharge parameters characteristic of multi-megajoule facilities, is studied using the 2 and 1/2 D, radiation-MHD code, MACH2. In this configuration a cylindrical, xenon plasma shell implodes radially onto a co-axial,deuterium-tritium plasma target. During implosion shock fronts are formed in both plasma. The shock waves in the DT plasma preheats the plasma up to several hundred eV before adiabatic compression takes over. In the outer region of the liner plasma,a shock front forms causing Xe mass to accumulate at the outer surface of the DT region. This causes the formation of a conduction channel that the discharge current transfers into. The outer surface of Xe liner then becomes Rayleigh-Taylor (RT)unstable while the shock front that compresses the DT target remains stable. The compression ratio of about 25 can achieve the parameters at the peak compression that can produce a thermonuclear yield from fusion neutrons more then breakeven and beyond. The interesting feature is the inner pinch remains stable even with 1\% perturbation level and only become unstable when it explodes.   

Reconstruction of an Intense Auroral Z-Pinch from Instabilities Recorded in Antiquity

The GPS locations and survey field-of view data representing some three million rock carvings at petroglyph sites world-wide has been assembled. In addition to previous sites [1], logging has been completed at two major sites in Mongolia; some three-dozen sites in the Flinders Range, South Australia; and in central Chile. The data allows a visual reconstruction of a sub-gigaampere auroral Z-pinch column whose plasma flow was bent inward towards the south polar axis, subsequently flowing around the Earth. Analysis is by means of two and three dimensional satellite and aerial orthophotography with side looking radar sets allowing near ground level to vertical inspection of all-terrain views from each site. It is found that petroglyphs, shown to be depictions of synchrotron light from MHD instabilities, possess polar south preferred orientations and right-ascension-declination information. \newline \newline [1] A. L. Peratt, ``Characteristics for the occurrence of a high-current Z-pinch aurora as recorded in Antiquity,'' IEEE Trans. Plasma Sci. V.31, 2003.   

Plasma jets interaction and merging simulation

The 3D particle LSP code is used for comprehensive analysis of plasma jets interaction dynamics. Merging of plasma jets and neutral gas jets are studied and compared for a wide range of parameters (density $10^{12}-10^{18}cm^{-3}$, temperature 1-10 eV, jet velocity 10-1000 km/s, collision angle $30^{\circ}-120^{\circ})$. Different merging regimes of plasma jets were found. Collision angle and density of jets are crucial parameters for plasma jets interaction. Well focused merged plasma jets are observed for relatively low densities and velocities of jets. High turbulent plasma flows are observed for higher densities. Higher jet velocity leads to asymmetrical twisting flow. Neutral gas jets merging simulations show different behavior as against plasma jets. Neutral gas jets tends to scatter easily as they merge and even show a stronger scattering with a higher jet density. Effects of compressibility, viscosity, asymmetry in merging jets are being considered. The plasma liner formation and dynamics, including instabilities, are studied. The required parameters to form a plasma liner, and the spatial and temporal precision required for a liner implosion on target plasmas are investigated.   

Magnetized-Target Fusion Based on Ultrahigh Speed Plasma Flows

One approach [1] to preparing the initial plasma for Magnetized Target Fusion (MTF) employs an ultrahigh speed plasma flow from a coaxial-gun arrangement known at the Plasma Flow Switch (PFS). PFS experiments [2] with aluminum demonstrated plasma flows at speeds in excess of 2000 km/s that stagnated to create fully-stripped plasma with electron temperatures of 30 keV. If D-D or D-T gas is substituted in the PFS, kilovolt-level \underline {initial} temperatures of the target plasma within an imploding liner would result. Experiments to raise the plasma temperature to fusion-levels ($>$ 10 keV) by liner compression may be within range of existing high-energy capacitor banks (e.g., Atlas or Shiva Star). Generation of the initial plasma and the liner implosion with the same bank offers an experimentally convenient approach, but requires careful timing based on initial analytical estimates and detailed numerical simulations. \newline \newline [1] P.J. Turchi, ``Ultrahigh-speed plasma sources and liner compression of high temperature plasma,'' APS Plasma Physics Meeting, 24-28 Oct 2005, Denver, CO. \newline [2] P.J. Turchi, et al, J. Appl.Phys. Vol. 69 (4), P. 1999, (Feb. 1991).   

Plasma-Jet Magnetized-Target Fusion Burn Dynamics

In magnetized-target fusion (MTF), an imploding, conducting liner compresses a magnetized plasmoid, such as a spheromak or field-reversed configuration (FRC). The increasing magnetic field of the target reduces thermal conduction and the liner's inertia provides transient plasma stability and confinement. This poster explores the burn dynamics of using plasma jets to form the liner [1]. The investigation uses the University of Wisconsin’s 1 D Lagrangian radiation hydrodynamics code, BUCKY, which solves single-fluid equations of motion with pressure contributions from electrons, ions, radiation, and fast charged particles, using either ideal-gas or table-lookup equations of state. BUCKY includes ion-electron interactions, PdV work, and fast-ion energy deposition. For this research, the code has been extended to include the magnetic field evolution as the plasmoid compresses plus the dependence of the thermal conductivity and fusion product energy deposition on the magnetic field.\hfil\break [1] Y.C. F. Thio, et al., ``Magnetized Target Fusion in a Spheroidal Geometry with Standoff Drivers,'' in Current Trends in International Fusion Research, E. Panarella, ed. (National Research Council of Canada, Ottawa, Canada, 1999), p. 113.\hfil\break * Research funded by the DOE Office of Fusion Energy Sciences, grant DE-FG02-04ER54751.   

Progress on Liner Implosions for compression of FRC's

Magnetized Target Fusion uses magnetic fields to reduce thermal conduction and required power density. Metal shell (liner) implosions driven by high current axial discharges, have been achieved with size, symmetry, and velocity suitable for compression of the Field Reversed Configuration (FRC) type of magnetized plasma. Using deformable liner -- electrode contacts enables axial access to inject FRC's. Radiography indicates 16 times radial compression of the inner surface of a 0.11 cm thick, 5 cm radius Al liner, free of instability growth. Combined 2D-MHD simulations of FRC formation with imploding liner compression indicate capture of the injected FRC by the imploding liner with suitable relative timing of the FRC formation and liner implosion discharges. Sponsored by DOE-OFES.   

Modeling Liner Compression Of FRC's: Obstacles and Advances

Compression of a field-reversed configuration (FRC) by an imploding solid liner is a possible path to magnetized target fusion. It is critical to the success of such experiments to perform full-up multidimensional computational simulations of them. However, there are numerous difficulties in performing those simulations. We have made significant progress on these issues. First, we have performed fully integrated, simultaneous simulations of liner implosion and FRC formation on the same grid. These simulations address the generation of rotation in the FRC as well as perturbations of the liner. Second, we have developed a mixed-order numerical treatment of the anisotropic heat conduction that has proven both more robust and more accurate. This improvement has enabled us to run more simulations for design purposes. Finally, we have begun to perform 3-d simulations of the final stages of compression, beginning from the self-consistent state of the 2-d axisymmetric simulation, perturbed in a mass, energy, momentum, and flux conserving way.