Session UP12: Poster Session VIII (Pinches, Diagnostics, Codes and Modeling, One Component, Laser-Plasma Ions, Strongly Coupled and Dusty Plasmas)

Polytropic scaling of a flow Z-pinch

The ZaP Flow Z-Pinch project investigates the use of velocity shear to mitigate MHD instabilities. The ZaP-HD experiment produces 50 cm long pinches of varying radii. The power to the experiment is split between the plasma formation and acceleration process and the pinch assembly and compression process. Once the pinch is formed, low magnetic fluctuations indicate a quiescent, long-lived pinch. The split power supply allows more control of the pinch current than previous machine iterations, with a designed range from 50 to 150 kA. Radial force balance leads to the Bennett relation which indicates that as the pinch compresses due to increasing currents, the plasma pressure and/or linear density must change. Through ion spectroscopy and digital holographic interferometry coupled with magnetic measurements of the pinch current, the components of the Bennett relation can be fully measured. A scaling relation is then assumed to follow a polytrope as the pinch pressure, initially approximately 250 kPa, increases from an initially formed state to much higher values, approaching 100 MPa. A preliminary analysis of pinch scaling is shown corroborating with other diagnostics on the machine along with extrapolations to required currents for an HEDLP machine.   

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

The ZaP-HD flow Z-pinch project investigates scaling the flow Z-pinch to High Energy Density Plasma, HEDP, conditions by using sheared flow stabilization. ZaP used a single power supply to produce 100 cm long Z-pinches that were quiescent for many radial Alfven times and axial flow-through times. The flow Z-pinch concept provides an approach to achieve HED plasmas, which are dimensionally large and persist for extended durations. The ZaP-HD device replaces the single power supply from ZaP with two separate power supplies to independently control the plasma flow and current in the Z-pinch. Equilibrium is determined by diagnostic measurements of the density with interferometry and digital holography, the plasma flow and temperature with passive spectroscopy, the magnetic field with surface magnetic probes, and plasma emission with optical imaging. The diagnostics fully characterize the plasma from its initiation in the coaxial accelerator, through the pinch, and exhaust from the assembly region. The plasma evolution is modeled with high resolution codes: Mach2, WARPX, and NIMROD. Experimental results and scaling analyses are presented.   

Magnetic field topology analysis for the ZaP-HD sheared flow stabilized Z-pinch

The ZaP-HD Experiment investigates high energy density plasmas in a sheared flow stabilized Z-pinch. The ZaP-HD device generates 5-10 mm radius Z-pinch plasmas with peak magnetic fields greater than 1 T. An array of 56 dual-winding magnetic field probes incased in boron nitride shields and surface mounted in the outer stainless steel electrode measures the azimuthal and axial field. The field gives instantaneous information about the magnitude and position of the plasma current. An analysis tool is created in order to visualize the complete 3D, time-dependent magnetic topology of the plasma column using the magnetic field value at each probe location. The information is used to investigate large scale structure and dynamics. Fourier transformations of the data provide frequency and phase information of the magnetic field fluctuations. These properties can give insight about spatial and temporal propagation of perturbations to better characterize the plasma evolution.   

Z-pinch equilibrium and instability analysis with digital holographic interferometry

The ZaP-HD Flow Z-Pinch project generates flow shear stabilized Z-pinches, providing a platform to explore how such plasmas could scale to HEDP and fusion reactor conditions. To scale up the plasma's density and temperature, it must be compressed to a smaller size making measurements more difficult. Digital holographic interferometry (DHI) employing a pulsed Nd:YAG laser and consumer DSLR camera can spatially resolve the plasma's electron density. The Fresnel reconstruction method allows expedient numerical data reconstruction.\footnote{Kreis, T. \textit{Handbook of holographic interferometry}.} Obtaining electron density radial profiles relies on applying an Abel inversion to convert measured line-integrated density, and the inversion process provides an independent measure of plasma symmetry. Entire Z-pinch equilibria (n, P, T, and B profiles) can be computed by applying physical models to the density data. Tracking the time evolution of pressure and density can reveal the presence of non-adiabatic heating mechanisms. Imaging the size scales of instabilities enables relative measures of viscosity at different positions and times. Error estimation of measured density profiles is presented along with observed asymmetric instabilities.   

Magnetohydrodynamic Simulation of the Chordal Wire-Array Plasma Flow Switch

The coaxial plasma flow switch (PFS) using a chordal wire array armature was first studied experimentally and computationally in the 1980's. That work revealed significant current interruption (dI/dt $\sim$ 5 MA/$\mu $s) as well as continuum x-ray emission representative of 30-45 keV bremsstrahlung. The work concluded that the voltage spike associated with the current interruption accelerated highly magnetized ions downstream at high velocity, and that energy exchange between the ions and electrons and their subsequent acceleration at the downstream boundary of the apparatus were responsible for the x-ray production. This work revisits the PFS operation up to and just beyond the point of armature lift-off from the coaxial section, where the magnetohydrodynamic model is valid and relevant. The early-time energy deposition in the wires from the pulse discharge is modeled in high-resolution 1-D and is used to set the initial conditions for the full-scale 3-D calculation. The wire array is assumed to have expanded from the initial r$=$0.01 cm uniformly and only in the axial direction, while the areal mass density retains its intended variation with radius. 3-D calculations are used to examine the armature, including magnetic field diffusion, as it is propelled along the coaxial geometry. These calculations will be used to set the initial conditions for follow-on particle or particle-fluid hybrid calculations of the propagation of ions and electrons to downstream obstacles and to calculate the x-ray production from the interactions of the flowing plasma with the obstacles.   

Faraday rotation measurements of magnetic field pile-up in a reverse shock formed by the stagnation of a supersonic magnetized plasma jet with a conducting obstacle

We present measurements of the magnetic field distribution formed by the stagnation of a magnetized plasma with a conducting obstacle. This jet is formed by plasma flows produced using radial foil or wire array z-pinch configurations driven by 1.4MA, 250ns current pulse on the MAGPIE generator at Imperial College. The jets typically have internal Mach numbers of 3-20, Reynolds numbers of \textgreater 10$^{5}$ and densities of $\sim$ 10$^{18}$-10$^{19}$cm$^{-3}$. The structure of the reverse shock was investigated using laser interferometry and Thompson scattering diagnostics, which provide spatially resolved measurements of the flow velocity and plasma temperature in the shock front. Faraday rotation measurements, carried out using a 1053 nm probe, were combined with interferometric measurements of electron density distribution in order to measure the distribution of magnetic field in the plasma. These measurements show that the magnetic field accumulated in the post-shock region plays a dynamically significant role, balancing the ram pressure of the plasma flow.   

On the Observation of Jitter Radiation in Solid-Density Laser-Plasma Laboratory Experiments

Plasmas created by high-intensity lasers are often subject to the formation of kinetic-streaming instabilities, such as the Weibel instability, which lead to the spontaneous generation of high-amplitude, tangled magnetic fields. These fields typically exist on small spatial scales, i.e., ``sub-Larmor scales''. Radiation from charged particles moving through small-scale electromagnetic (EM) turbulence, known as jitter radiation, has spectral characteristics distinct from both synchrotron and cyclotron radiation, and it carries valuable information on the statistical properties of the EM field structure and evolution. Consequently, jitter radiation from laser-produced plasmas may offer insight into the underlying electromagnetic turbulence. Here we investigate the prospects for, and demonstrate the feasibility of, such direct radiative diagnostics for mildly relativistic, solid-density laser plasmas produced in lab experiments.   

Optical Spectroscopy of a Mega-Ampere Dense Plasma Focus

An optical streaked spectroscopy system was developed to evaluate the spectral emission of the run-down, run-in and pinch phase on the Gemini Dense Plasma Focus (DPF). Time-resolved emission spectra were captured for hydrogen, deuterium, argon, and krypton gas from these phases. The emission was focused onto a fiber, and fed to a spectrometer that was coupled to a streak camera. Spectra of hydrogen, deuterium, argon, and krypton gas were modeled using Spec3D. Plasma parameters including electron density and temperature, from LSP simulations of the DPF discharge, were loaded into the Spec3D simulation to evaluate the emission spectra. Spectra collected from DPF on the streaked spectrometer system were then compared to the Spec3D simulations, and used to verify known optical emission lines for the various gases and to identify possible contaminants. This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946, and by Los Alamos National Laboratory, under Contract no. DE-AC52-06NA25396 with the U.S. Department of Energy. DOE/NV/25946--2519   

Plasma dynamics of laser produced plasma plumes propagating in an axial magnetic field

We have performed experimental studies of the effect of static axial magnetic fields on the plasma dynamics of laser produced carbon and titanium plasmas. The laser plasmas are produced in vacuum, with a Nd:YAG laser, 3.5 ns, 340 mJ at 1.06 4 $\mu$m, operating at 10 Hz, and propagate in static magnetic fields of maximum value $\sim$0.2 T. Laser plasma features are characterized using 50 ns time resolved plasma imaging, time and space resolved visible spectroscopy and Faraday cup measurements. The presence of the magnetic field is found to affect plasma dynamics, plasma emission and plasma ions energy spectrum. Based on these measurements, a detailed analysis of the confinement effects of the magnetic field on the laser plasma will be presented.   

Dynamics of magnetic fields in high-energy-density plasmas for fusion and astrophysics

An overview of our recent experimental and theoretical work on the dynamics of magnetic fields in high-energy-density plasmas will be presented. This includes: (1) precision mapping of the self-generated magnetic fields in the coronal plasma and the Nernst effect on their evolution [1], (2) characterizing the strong magnetic field generated by a laser-driven capacitor-coil target using ultrafast proton radiography [2], and (3) creating MHD turbulence in Rayleigh-Taylor unstable plasmas. The experimental results are compared with resistive MHD simulations providing a stringent test for their predictions. Applications in relevance to ignition target designs in inertial confinement fusion, material strength studies in high-energy-density physics, and astrophysical systems such as plasma dynamos and magnetic reconnection will be discussed. Future experiments proposed on the National Ignition Facility will be described.\\[4pt] [1] L. Gao et al., Phys. Rev. Lett. 114, 215003 (2015).\\[0pt] [2] L. Gao et al., submitted.   

A short-pulse mode for the SPHINX LTD Z-pinch driver

The SPHINX machine is a 6MA, 1 $\mu$s, LTD Z-pinch driver at CEA Gramat (France) and primarily used for studying radiation effects. Different power amplification concepts were examined in order to reduce the current rise time without modifying the generator discharge scheme, including the Dynamic Load Current Multiplier (DLCM) proposed by Chuvatin [1]. A DLCM device, capable of shaping the current pulse without reducing the rise time, was developed at CEA. This device proved valuable for isentropic compression experiments in cylindrical geometry [2]. Recently, we achieved a short pulse operation mode by inserting a vacuum closing switch between the DLCM and the load. The current rise time was reduced to $\sim $300 ns. We explored the use of a reduced-height wire array for the Dynamic Flux Extruder in order to improve the wire array compression rate and increase the efficiency of the current transfer to the load. These developments are presented. Potential benefits of these developments for future Z pinch experiments are discussed.\\[4pt] [1] A.S. Chuvatin, ``Dynamic Current Multiplier''; 14$^{th}$ Symposium on High Current Electronics, Tomsk, Russia, pp 232-235 (2006).\\[0pt] [2] T. d'Almeida \textit{et al,} \textit{Phys. Plasmas}, \textbf{20}, 092512-1 092512-16 (2013).   

Shock dynamics in counter-streaming plasma flows

The collision between two counter-streaming plasma flows is studied on the MAGPIE generator by introducing a 1.4MA, 250ns electrical current into two oppositely-facing radial foils. The interaction between the flows leads to the formation of different shock features, particularly a bow shock on the axis of the system. We present results of bow shock dynamics with different foil thicknesses and materials, together with an analysis of the effects of radiative cooling in the shock.   

GPU-Accelerated PIC/MCC Simulation of Laser-Plasma Interaction Using BUMBLEBEE

The research of laser-plasma interaction in its wide applications relies on the use of advanced numerical simulation tools to achieve high performance operation while reducing computational time and cost. BUMBLEBEE has been developed to be a fast simulation tool used in the research of laser-plasma interactions. BUMBLEBEE uses a 1D3V electromagnetic PIC/MCC algorithm that is accelerated by using high performance Graphics Processing Unit (GPU) hardware. BUMBLEBEE includes a friendly user-interface module and four physics simulators. The user-interface provides a powerful solid-modeling front end and graphical and computational post processing functionality. The solver of BUMBLEBEE has four modules for now, which are used to simulate the field ionization, electron collisional ionization, binary coulomb collision and laser-plasma interaction processes. The ionization characteristics of laser-neutral interaction and the generation of high-energy electrons have been analyzed by using BUMBLEBEE for validation.   

A Smoother Ray-Traced Power Deposition Method

We have developed a new approach to improve the smoothness of the power deposition produced by ray-tracing laser light in plasmas. The fundamental approach is to connect the traced-rays together into either sheets (in 2D) or volume-enclosing chunks (in 3D). The connected rays then sweep out areas or volumes on the underlying mesh onto which power is deposited. The resulting absorbed power distribution continuously and smoothly covers the region illuminated by the laser. This approach allows significantly less rays to be used in the ray-tracing, and reduces the message passing in the parallelized implementation. The number of rays is also independent of grid resolution. Previously we have shown results from the 2D connection method; here we show the 3D connection method and discuss its implementation in our massively parallel radiation hydrodynamics code FASTRAD3D.   

Precise Opacities for Astrophysics (Fe and Ni) and ICF modeling

Opacities of FeIII - FeXV at Te$=$15-20 eV and densities 1.e16-1.e23 cm$^{-3}$ have been computed with an improved version of the HULLAC code [1, 2]. More than 10$^{9}$ transitions have been computed, with different ways to account for configuration interactions (CI). Spectra with CI limited to each non-relativistic configuration (CIinNRC) are compared to more extended full Relativistic CI (RCI). The effect of increasing the size of the CI basis is investigated. These comparisons enable optimizing the method for each temperature/density regime. With powerful computers, HULLAC -generated opacity databases could then be envisioned, bypassing the need for statistical approximations.\\[4pt] [1] D. Gilles, M. Busquet, M. Klapisch, F. Gilleron, J.C. Pain, Open M-shell Fe and Ni LTE opacity calculations with the code HULLAC-v9, High Ener. Dens. Phys., 16 (2015) 1-11.\\[0pt] [2] M. Klapisch, M. Busquet, A. Bar-Shalom, A New And Improved Version Of HULLAC, AIP Conference Proceedings, 926 (2007) 206-215.   

Streaked optical pyrometer for measuring surface temperature of ion heated plasma

The evolution of the interface between a light and heavy material isochorically heated to warm dense matter conditions is important to the understanding of electrostatic effects on the usual hydrodynamic understanding of fluid mixing. In recent experiments at the Trident laser facility in Los Alamos National Laboratory, the target, containing a high Z and a low Z material, is heated to several eV by laser accelerated aluminum ions. We fielded a streaked optical pyrometer to measure surface temperature. The pyrometer images the back surface of a heated target on a sub-nanosecond timescale with 400nm light from the plasma. This poster presents the details of the experimental setup and pyrometer design, as well as initial results of ion heating of aluminum targets. The interface between heated diamond and gold is also observed.   

Study of high resolution x-ray spectrometer concepts for NIF experiments

Options have been investigated for DIM-insertable (Diagnostic Instrument Manipulator) high resolution (E/$\Delta $E $\sim$ 3000 - 5000) Bragg crystal x-ray spectrometers for experiments on the NIF. Of interest are time integrated Cu K- and Ta L-edge absorption spectra and time resolved Kr He-$\beta $ emission from compressed symcaps for inference of electron temperature from dielectronic satellites and electron density from Stark broadening. Cylindrical and conical von Hamos, Johann, and advanced high throughput designs have been studied. Predicted x-ray intensities, spectrometer throughputs, spectral resolution, and spatial focusing properties, as well as lab evaluations of some spectrometer candidates will be presented.   

Performance of bent-crystal x-ray microscopes for high energy density physics research

We present calculations for the field of view (FOV), image fluence, image monochromaticity, spectral acceptance, and image aberrations for spherical crystal microscopes, which are used as self-emission imaging or backlighter systems at large-scale high energy density physics facilities. Our analytic results are benchmarked with ray-tracing calculations as well as with experimental measurements from the 6.151 keV backlighter system at Sandia National Laboratories. The analytic expressions can be used for x-ray source positions anywhere between the Rowland circle and object plane. This enables quick optimization of the performance of proposed but untested, bent-crystal microscope systems to find the best compromise between FOV, image fluence, and spatial resolution for a particular application.\\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND No: SAND2015-5977 A.   

VISRAD, 3-D Target Design and Radiation Simulation Code

The 3-D view factor code VISRAD is widely used in designing HEDP experiments at major laser and pulsed-power facilities, including NIF, OMEGA, OMEGA-EP, ORION, LMJ, Z, and PLX. It simulates target designs by generating a 3-D grid of surface elements, utilizing a variety of 3-D primitives and surface removal algorithms, and can be used to compute the radiation flux throughout the surface element grid by computing element-to-element view factors and solving power balance equations. Target set-up and beam pointing are facilitated by allowing users to specify positions and angular orientations using a variety of coordinates systems (e.g., that of any laser beam, target component, or diagnostic port). Analytic modeling for laser beam spatial profiles for OMEGA DPPs and NIF CPPs is used to compute laser intensity profiles throughout the grid of surface elements. We will discuss recent improvements to the software package and plans for future developments.   

Implementation and Optimization of a Plasma Beam Combiner at NIF

The seeded SBS process that is known to effectively amplify beams in ignition targets [1] has recently been used to design a target to combine the power and energy of many beams of the NIF facility into a single beam by intersecting them in a gas [2]. The demand for high-power beams for a variety of applications at NIF makes a demonstration of this process attractive. We will describe the plan for empirically optimizing a combiner that uses a gas-filled balloon heated by 10 quads of beams, and pumped by 5 additional frequency-tuned quads to amplify a single beam or quad. The final empirical optimization of beam wavelengths will be determined by using up to three colors in each shot. Performance and platform compatibility will also be optimized by considering designs with a CH gas fill that can be fielded at room temperature as well as a He gas fill to minimize absorption in the combiner. The logic, diagnostic configuration, and backscatter risk mitigation from two shots presently planned for NIF will also be described. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.\\[4pt] [1] R. K. Kirkwood et al Plasma Phys. Controlled Fusion 55, 103001 (2013).\\[0pt] [2] R. K. Kirkwood et al APS DPP 2012.   

Using xRage to Model Heat Flow for Experiments to Measure Opacities in HED Plasmas

We are developing a NIF proposal to measure opacities of C, N and O at temperatures and densities relevant to the base of the solar convection zone. Our proposed experiments would provide the first opacity measurements for these elements within this HED regime. A critical feature of our experimental platform is a super-sonic radiation front propagating within the targets. Under these conditions, density remains constant across the radiation front for a couple nanoseconds, enabling a window during which the opacities of the hot and cold target may be measured simultaneously. Afterwards, hydrodynamic effects create temperature and density gradients, which would obfuscate analysis of opacity data. We are using xRage to simulate heat flow within our targets in order to estimate the time scale over which temperature and density gradients evolve. These simulations will better inform our target design and diagnostic requirements. If successful, our experiments could yield the data necessary to validate existing opacity models or provide physical insights to inform the development of new opacity models. Accurate opacity models are essential to the understanding of radiation transport within HED systems, with applications ranging from astrophysics to ICF.   

A diffusive radiation hydrodynamics code, xRage, is implemented to compare radiation flow with experimental data from the Omega laser facility

A sound speed discrepancy between solar models and data collected using helioseismology exists. The sound speed discrepancy is the most pronounced at the base of the convective zone (CZ) for otherwise consistent solar models. One potential solution is that the opacity models for important elements such as carbon, nitrogen and oxygen are incomplete. At these high energy-density conditions few relevant opacity measurements exist to compare to the models. Only relatively recently have user facilities been able to reach the temperatures and densities that resemble the convective zone base. It is our long term goal to determine the opacities of carbon, nitrogen and oxygen at the relevant conditions. Preliminary testing has occurred at the Omega Laser Facility in Rochester, New York. Presented are the results of the shots taken on April 22, 2015. A half hohlraum was used to drive a supersonic radiation front through a dominantly carbon, CRF, foam. These results are compared to diffusive xRage simulations. (LA-UR-15-25495)   

Additions and Improvements to the FLASH Code for Simulating High Energy Density Physics Experiments

FLASH is an open source, finite-volume Eulerian, spatially adaptive radiation hydrodynamics and magnetohydrodynamics code that incorporates capabilities for a broad range of physical processes, performs well on a wide range of computer architectures, and has a broad user base. Extensive capabilities have been added to FLASH to make it an open toolset for the academic high energy density physics (HEDP) community. We summarize these capabilities, with particular emphasis on recent additions and improvements. These include advancements in the optical ray tracing laser package, with methods such as bi-cubic 2D and tri-cubic 3D interpolation of electron number density, adaptive stepping and 2nd-, 3rd-, and 4th-order Runge-Kutta integration methods. Moreover, we showcase the simulated magnetic field diagnostic capabilities of the code, including induction coils, Faraday rotation, and proton radiography. We also describe several collaborations with the National Laboratories and the academic community in which FLASH has been used to simulate HEDP experiments.   

Simulations of Laboratory Astrophysics Experiments using the CRASH code

Computer simulations can assist in the design and analysis of laboratory astrophysics experiments. The Center for Radiative Shock Hydrodynamics (CRASH) at the University of Michigan developed a code that has been used to design and analyze high-energy-density experiments on OMEGA, NIF, and other large laser facilities. This Eulerian code uses block-adaptive mesh refinement (AMR) with implicit multigroup radiation transport, electron heat conduction and laser ray tracing. This poster will demonstrate some of the experiments the CRASH code has helped design or analyze including: Kelvin-Helmholtz, Rayleigh-Taylor, magnetized flows, jets, and laser-produced plasmas.   

Modeling of Plasma Conditions and Spectral Properties of Radiation-Heated Matter

Opacity experiments at the Z facility provide important data for benchmarking opacity models and atomic data. The ability to accurately interpret the data obtained in these experiments increases the confidence in opacity calculations for a variety of astrophysical and laboratory problems. In the experiments, the Z dynamic hohlraum radiation source is used to both heat and backlight material samples. We will present the latest improvements to the simulation codes developed at Prism and how they affect the analysis of the experimental data. In particular, we will discuss angle-dependent radiation boundary condition recently implemented in the radiation-hydrodynamics code HELIOS. This improved modeling capability can potentially be important for studying behavior of plasmas driven by radiation sources that cannot be adequately described as neither directional nor Lambertian. We will also discuss atomic kinetics in radiatively heated samples and the possibility of its deviation from LTE. The effect of such deviation on both hydrodynamic evolution and radiative properties of these plasmas will be addressed.   

Spectroscopic and X-Ray Scattering Models in SPECT3D

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

Plasma kinetic effects on interfacial mix in settings relevant to inertial confinement fusion and laboratory experiments

Mixing of high-Z/low-Z interfaces in dense plasma media is a problem of importance for understanding mix in inertial confinement fusion experiments and recent experiments at the LANL Trident facility. In this presentation, we apply the VPIC particle-in-cell code [1] with a binary collision model [2] to explore kinetic effects of the atomic mixing. Comparisons are made to published analytic theory and hybrid modeling results [3] and conditions are identified under which plasma kinetic behavior may lead to anomalously rapid atomic mixing. \\[4pt] [1] Bowers et al., \textit{Phys. Plasmas} \textbf{15}, 055703 (2008).\\[0pt] [2] Takizuka and Abe, \textit{J. Computat.} \textit{Phys}. \textbf{25}, 205 (1977).\\[0pt] [3] Molvig et al., \textit{Phys. Rev. Lett}. \textbf{113}, 145001 (2014).   

Modeling the SNL-Z Opacity Platform

Driven by the need to validate computed opacity tables used for radiation hydrodynamic simulations, Los Alamos National Laboratory (LANL) and Sandia National Laboratories (SNL) have been involved in a collaboration to measure and characterize recorded opacities at the SNL-Z facility since 2009. The original success in measuring the spectral opacity of iron at a temperature of 156eV and at an electron density of 6.90x10$^{21}$cm $^{-3}$ (reported by J.E. Bailey et al. in PRL \textbf{99} 265002 2007) led to an interest in expanding iron measurements to higher temperatures and densities to conditions consistent with those at the base of the convection zone of the Sun. To obtain these higher temperature/density conditions, the tamper masses that sandwich the metal foil of interest were increased. Several disturbing discrepancies exist between the higher temperature/density opacity measurements and theory and continue to be largely unresolved for the past several years (J. E. Bailey et al, NATURE \textbf{517} 56 1 JAN. 2015). This continuing discrepancy has prompted LANL to perform detailed rad-hydro simulations of the SNL-Z opacity platform. In these simulations, both the dynamic hohlraum and the opacity target are modeled together. We report on the simulation methods and comparisons with dynamic hohlraum measurements that are used to assess the simulation fidelity.   

Self-Consistent Scattering and Transport Calculations

An average-atom model with ion correlations [1] provides a compact and complete description of atomic-scale physics in dense, finite-temperature plasmas. The self-consistent ionic and electronic distributions from the model enable calculation of x-ray scattering signals and conductivities for material across a wide range of temperatures and densities. We propose a definition for the bound electronic states that ensures smooth behavior of these measurable properties under pressure ionization and compare the predictions of this model with those of less consistent models for Be, C, Al, and Fe. \\[4pt] [1] C.E. Starrett and D. Saumon, High Energy Density Physics 10, 35 (2014).   

Instability of plasma waves during relaxation of 2D turbulent flows

We observe strong excitation of novel low-frequency $z$-dependent plasma waves $(m_{\theta } =0,k_{z} =1)$, occurring during the nominally 2D relaxation of turbulent initial conditions (10 $-$100 interacting vortices) in strongly magnetized electron columns. This initial relaxation often results in ``2D vortex crystal'' states [1, 2]. Here we describe experiments showing the concomitant growth of ill-understood low-frequency plasma waves, probably due to ``leakage'' of 2D turbulent potential energy into $z$-dependent fluctuations. With plasma injection, the lowest regular $T$rivelpiece-$G$ould mode $(m_{\theta } =0,k_{z} =1)$ is observed at $f_{TG} (t)\approx 2.8$MHz and exponential decay time $\tau_{TG} \sim 1$msec. Also, we observe rapid exponential growth of a novel low-frequency mode with $f_{LF} (t)\approx 0.3$MHz, nominally also with $m_{\theta } =0,k_{z} =1$. In a few milliseconds (several tens of rotation times at $B=$10kG), the \textit{LF}-mode becomes highly nonlinear, developing up to a dozen temporal harmonics. When a \textit{LF}-harmonic resonates with the decaying \textit{TG}-mode, \textit{LF}-mode energy is transferred into the \textit{TG}-mode, and both modes remain at moderate amplitudes until the 2D turbulent relaxation abates (hundreds of rotation times). The ill-understood $f_{LF} $ is \textit{independent} of $B$, even though the growth and duration times follow scale as $B^{\mathrm{1}}$ from the 2D flows. \\[4pt] [1] K.S. Fine \textit{et al}., PRL \textbf{75}, 3277 (1995).\\[0pt] [2] D.Z. Jin and D.H.E. Dubin, PRL \textbf{80}, 4434 (1998).   

Flux-driven algebraic damping of m = 1 diocotron mode

Recent experiments with pure electron plasmas in a Malmberg-Penning trap have observed the algebraic damping of $m=1$ diocotron modes.\footnote{A.A. Kabantsev \textit{et. al.}, Phys. Rev. Lett. \textbf{112}, 115003, 2014.} Transport due to small field asymmetries produce a low density halo of electrons moving radially outward from the plasma core, and the mode damping begins when the halo reaches the resonant radius $r_{\mathrm{res}}$, where $f=mf_{E\times B}(r_{\mathrm{res}})$. The damping rate is proportional to the flux of halo particles through the resonant layer. The damping is related to, but distinct from spatial Landau damping, in which a linear wave-particle resonance produces exponential damping. This poster explains with analytic theory and simulations the new algebraic damping due to both mobility and diffusive fluxes. As electrons are swept around the ``cat's eye'' orbits of resonant wave-particle interaction, they form a dipole $(m=1)$ density distribution, and the electric field from this distribution produces an $E\times B$ drift of the core back to the axis, i.e. damps the $m=1$ mode.   

Numerical and Analytical Calculation of Bernstein Mode Resonances in a Non-Uniform Cylindrical Plasma

This poster presents theory and numerical calculations of electrostatic Bernstein modes in an inhomogeneous cylindrical plasma column. These modes rely on FLR effects to propagate radially across the column until they are reflected when their frequency matches the local upper hybrid frequency, setting up an internal normal mode on the column, and also mode-coupling to the electrostatic surface cyclotron wave (which allows the normal mode to be excited and observed using external electrodes). Numerical results predicting the mode spectra, using a novel linear Vlasov code on a cylindrical grid, will be presented and compared to an analytic WKB theory. A previous version of the theory\footnote{D.Dubin, Phys.Plasmas. \textbf{20}, 042120, 2013.} expanded the plasma response in powers of 1/B, approximating the local upper hybrid frequency, and consequently its frequency predictions are shifted with respect to the numerical results. A new version of the WKB theory uses the exact cold fluid plasma response and does a better job of reproducing the numerical frequency spectrum. The eventual goal is to compare the theory to recent experiments that have observed these waves in pure electron and pure ion plasmas.\footnote{M. Affolter \textit{et. al.}, Phys. Plasmas\textbf{22}, 055701, 2015.}   

Nonlinear Trivelpiece-Gould Waves: Frequency, Functional Form, and Stability

This poster considers the frequency, spatial form, and stability, of nonlinear Trivelpiece- Gould (TG) waves on a cylindrical plasma column of length $L$ and radius $r_{p}$, treating both traveling and standing waves, and focussing on the regime of experimental interest in which $L$/$r_{p\, }\gg $ 1. In this regime TG waves are weakly dispersive, allowing strong mode-coupling between Fourier harmonics. The mode coupling implies that linear theory for such waves is a poor approximation even at fairly small amplitudes, and nonlinear theories that include only a small number of harmonics (such as 3-wave parametric resonance theory) fail to fully capture the stability properties of the system. We find that nonlinear standing waves suffer jumps in their functional form as their amplitude is varied continuously. The jumps are caused by nonlinear resonances between the standing wave and nearly linear waves whose frequencies and wave numbers are harmonics of the standing wave. Also, the standing waves are found to be unstable to a multi-wave version of 3-wave parametric resonance, with an amplitude required for instability onset that is much larger than expected from three wave theory. For traveling wave, linearly \textit{stability} is found for all amplitudes that could be studied, in contradiction to 3-wave theory.   

Non-linear Frequency Shifts, Mode Couplings, and Decay Instability of Plasma Waves

We present experiments and theory for non-linear plasma wave decay to longer wavelengths, in both the oscillatory coupling and exponential decay regimes. The experiments are conducted on non-neutral plasmas in cylindrical Penning-Malmberg traps, $\theta $-symmetric standing plasma waves have near acoustic dispersion $\omega (k_{z} )\propto k_{z} -\alpha k_{z}^{2} $, discretized by $k_{z} =m_{z} (\pi /L_{p} )$. Large amplitude waves exhibit non-linear frequency shifts $\delta f/f\propto A^{2}$ and Fourier harmonic content, both of which are increased as the plasma dispersion is reduced. Non-linear coupling rates are measured between large amplitude $m_{z} =2$ waves and small amplitude $m_{z} = 1$ waves, which have a small detuning $\Delta \omega =2\omega_{1} -\omega_{2} $. At small excitation amplitudes, this detuning causes the $m_{z} =1$ mode amplitude to ``bounce'' at rate $\Delta \omega $, with amplitude excursions $\Delta A_{1} \propto \delta n_{2} /n_{0} $ consistent with cold fluid theory and Vlasov simulations. At larger excitation amplitudes, where the non-linear coupling exceeds the dispersion, phase-locked exponential growth of the $m_{z} =1$ mode is observed, in qualitative agreement with simple 3-wave instability theory. However, significant variations are observed experimentally, and N-wave theory gives stunningly divergent predictions that depend sensitively on the dispersion-moderated harmonic content. Measurements on higher temperature Langmuir waves and the unusual ``EAW'' (KEEN) waves are being conducted to investigate the effects of wave-particle kinetics on the non-linear coupling rates.   

Non-resonant Particle Heating due to Collisional Separatrix Crossings

We observe plasma heating when a pure ion column is ``sloshed'' back and forth across a trapping separatrix, with heating rate larger than expected from simple collisional viscosity. Here, an externally applied theta-symmetric ``squeeze'' potential creates a velocity separatrix between trapped and passing particles, and weak collisions at rate $\nu_{c}$ cause separatrix crossings. The trapped particles are repeatedly compressed and expanded (by $\delta L$ at rate $f_{sl}$) whereas the passing particles counter-stream and Debye shield the resultant potential variations. LIF diagnostics clearly show the separatrix energy $E_{sep} (r)$, in close agreement with $(r,z)$ Boltmann-Poisson equilibrium calculations. With $\nu_{c} \ll 2\pi f_{sl} \ll 2\pi f_{plas}$, simple bounce-averaged transport theory of the separatrix boundaries layer predicts heating scaling as $\dot{{T}}/T\propto (\delta L/L)^{2} f_{sl} \sqrt {\nu_{c} /f_{sl} } \quad V_{sq}^{2} /T^{2}$, distinct from bulk-viscosity heating scaling as $\nu_{c}^{1} $. Experiments corroborate the scalings with $f_{sl} $ (and hence $\nu_{c}$), with $\delta L$, and with $V_{sq} $, and give overall quantitative agreement with theory within a factor-of-two.   

Damping of Plasma Waves in Multi-species Ion Plasmas

The damping of Langmuir waves in multi-species pure ion plasmas is measured over four decades in temperature covering regimes of Landau, bounce harmonics, and interspecies drag damping. Thermal cyclotron spectroscopy determines the plasma composition. The plasma is predominantly Mg$^{+}$ resulting from a Mg electrode arc, with roughly 5-30{\%} other ions, typically H$_{3}$O$^{+}$ and O$_{2}^{+}$, arising from ionization and chemical reactions with the residual background gas. The plasma temperature is controlled with laser cooling of the Mg24 ions over the range $10^{-4}\le T\le 1$ eV. For $T\ge 0.1_{\, }$eV, the damping rates agree closely with Landau theory for $\theta $-symmetric standing waves, with discrete wavenumber $k_{1} =\pi /L_{p} $. At lower temperature $10^{-2}\le T\le 0.1$ eV the damping is not fully understood, but is most likely a result of Landau damping on higher $k_{z} $ bounce harmonics produced by the rounded plasma ends. For $T\le 10^{-2}_{\, }$eV, damping rates $10\le \gamma \le 10^{3}$ s$^{-1}$ are proportional to the ion-ion collisionality $\nu_{ii} \propto T^{-3/2}$, consistent with a theory prediction that includes interspecies drag. A decrease in $\gamma $ is observed at $T\le 10^{-3}_{\, }$eV, presumably due to strong magnetization, centrifugal separation of the species, and the collisionality approaching the mode frequency$f_{1} \approx 20_{\, }$kHz.   

Modeling axisymmetric Bernstein modes in a finite-length non-neutral plasma

We have developed a 2-D PIC code to model high-frequency (near the cyclotron frequency) axisymmetric oscillations in a finite-length pure-ion plasma. We previously modeled these modes for infinite-length plasmas, where they are not detectable in the surface charge on the walls because of the axisymmetry and lack of z-dependence. This is not true in a finite-length plasma, however, because the eigenfunction of the oscillation has to have nodes a short distance beyond the ends of the plasma. This gives the modes a $\cos(k_z z)$ dependence, with a $k_z$ such that an integral number of half-wavelengths fit into the plasma. This $z$-dependence makes the mode detectable in the surface charge on the walls. We have modeled the plasma with different $k_z$ values and find that a larger value $k_z$ shifts the frequency downward by a small amount. The damping of the modes also increases as $k_z$ increases. The eigenfunction of the mode with the lowest-order radial dependence is linear in $r$, while higher-order radial modes behave as J$_1(k_r r)$. We will present the results of the properties of these different modes, along with a discussion of their dispersion relation and detectability.   

Cyclotron-Cavity Mode Resonant Cooling in Single Component Electron Plasmas

Generating cold (\textless 50 K) single component electron plasmas is of critical importance to many experiments. Examples include optimizing recombination rates for antihydrogen or Rydberg atom production and producing monoenergetic beams. Replacing a section of a Penning-Malmberg trap with a high-Q cavity resonantly enhances spontaneous emission of cyclotron radiation in the cavity through interaction with electromagnetic modes. This allows for rapid cooling of a single-component electron plasma confined in the high-Q cavity. ~We describe the observed effects of frequency detuning (lineshape), position dependence of the confined plasma, and saturation effects on both the cooling rate and equilibrium temperature as~the number of trapped electrons increases from 5$\cdot$10$^{6}$ to 3$\cdot$10$^{6}$. Prepared by LLNL under Contract DE-AC52-07NA27344. This research was supported by the Department of Energy, Grant DE-FG02-06ER54904.   

Production of Coherent Phase Space Islands in Trapped Plasma

Particles are coherently extracted from a cold Maxwellian distribution into phase space islands by applying a fixed-frequency RF drive while the plasma bounce frequency is swept downward by lowering the potential confining the plasma. These objects can appear spontaneously in pure electron and mixed ion plasma experiments during particle extraction when the noise power spectrum of the confining potential has peaks in the rf band, as is often the case in a laboratory environment. Interestingly, the particles in these islands have been observed to form tight energy distributions, making the mechanism potentially useful for low energy/monoenergetic plasma injection devices. In particular, these features would be useful for antimatter spectroscopy and mixing for antihydrogen formation. This work is supported by DoE, Grant DE-FG02-06ER54904.   

Improved Technique for Parallel Temperature Measurement of Cryogenic Non-neutral Plasmas

New hardware and software methods to optimize the parallel temperature diagnostic for nonneutral plasmas are reported. Plasmas are extracted onto an MCP-phosphor screen assembly in the UC Berkeley Cold electron research apparatus (CERES). Incident charges on the MCP are amplified and converted into light. Rather than measuring the charge directly on a Faraday cup, the light collected by photodetectors is used to measure the time of arrival of charges as they arrive at the MCP. Efficient light collection, using Fresnel lenses and nonimaging optics, are combined with enhanced light detection, with conventional and silicon photomultipliers, to act as an amplifier chain with effective single-electron resolution. Data from this detector is analyzed to obtain a parallel temperature using a suite of newly developed, GPU-accelerated software. Plasma temperature can be obtained in real-time without human input, reducing potential biases in these measurements. This research was supported by the Department of Energy, Grant DE-FG02-06ER54904.   

Novel Microwave Cavity for Resonant Cooling of a Lepton Plasma

A novel microwave cavity is described which can be used to cool lepton plasmas for potential use in creation of mono-energetic beams, and synthesis of antihydrogen. The cooling scheme represents an incarnation of the Purcell Effect; When plasmas are coupled to a microwave cavity, the plasma cooling rate is resonantly enhanced through increased spontaneous emission of cyclotron radiation. Geometric design considerations for a cavity with strong cooling power and low equilibrium plasma temperatures are discussed. A three electrode cavity forms a section of a Penning-Malmberg trap. It has a bulged cylindrical geometry with open ends aligned with the magnetic trapping axis. This allows plasmas to be injected and removed from the cavity without the need for moving parts while maintaining high quality factors for resonant modes. The cavity includes unique surface preparations for tuning the cavity quality factor and achieving anti-static shielding using thin layers of nichrome and colloidal graphite respectively. Preliminary data suggests that temperatures and cooling rates for these plasmas can be improved by at least a factor of 10 as described in an adjacent poster. This work is supported by DoE, Grant DE-FG02-06ER54904, and NSERC.   

Plasma Parking into Off-axis Storage Traps

Advanced uses of positrons benefit by the development of efficient techniques for particle accumulation, storage and delivery.\footnote{J. R. Danielson, et al., {\it Rev. Mod. Phys.} {\bf 87}, 247 (2015).} The multicell Penning-Malmberg trap is being developed as a way to obtain high-capacity antimatter traps.\footnote{J. R. Danielson, et al., {\it Phys. Plasmas} {\bf 13}, 125002 (2006).} The multicell test structure at UCSD consists of multiple aligned storage cells, with one cell on the magnetic axis, and three off-axis.\footnote{C. J. Baker, et al., {\it Phys. Plasmas} {\bf 22}, 022302 (2015).} Described here are tests of the process by which plasma, first located in a large diameter master cell, is autoresonantly excited into a large amplitude diocotron mode and then transferred into off-axis cells. Through the use of bounce-average orbits\footnote{N. C. Hurst, et al., {\it Phys. Rev. Lett.} {\bf 113}, 025004 (2014).} and other manipulation techniques, the plasma position during transfer can be controlled precisely, and the plasma can be ``parked'' at any radial or azimuthal location within a storage cell. Other experiments in the test structure, including plasma lifetime studies and experiments with large space charge, will also be described.   

Advancements toward matter-antimatter pair plasmas in the laboratory

APEX/PAX (A Positron Electron Experiment/Positron Accumulation Experiment) has as its overarching goal the creation and magnetic confinement of a laboratory electron-positron pair plasma, thereby enabling experimental investigations of a topic that has already been the subject of hundreds of analytical and computational studies. This goal involves several interdependent challenges: design and construction of a suitable magnetic confinement device, access to a sufficient number of sufficiently cool positrons, and refinement of methods for the transfer of the positrons (and an equal number of electrons) into the device. The latest results of the subprojects addressing these challenges will be summarized here. Highlights include efficient (40 percent) injection of the NEPOMUC (Neutron-Inducted Positron Source Munich) positron beam into the confinement region of a dipole magnetic field, characterization of the beam at energies from 5 eV to 1 keV, and hour-long electron plasma confinement in a high-field (2.3 Telsa) Penning-Malmberg trap.   

Unmatter Plasma

``Unmatter Plasma'' is a novel form of plasma, exclusively made of matter and its antimatter counterpart. An experiment (2015) on matter-antimatter plasma [or unmatter plasma] was recently successful at the Astra Gemini laser facility at the Rutherford Appleton Laboratory, Oxford, United Kingdom. The experiment that was made has produced electron-positron plasma. The positron is the antimatter of the electron, having an opposite charge of the electron, but the other properties are the same. Unmatter is considered as a combination of matter and antimatter. For example electron-positron is a type of unmatter. We coined the word ``unmatter'' (2004) that means neither matter nor antimatter, but something in between. Besides matter and antimatter there may exist unmatter (as a new form of matter) in accordance with the neutrosophy theory that between an entity and its opposite there exist intermediate entities.\\[4pt] [1] G. Sarri, K. Poder, J. Cole, W. Schumaker, A. Di Piazza, B. Reville, T. Dzelzainis, D. Doria, L.A. Gizzi, G. Grittani, S. Kar, C.H. Keitel, K. Krushelnick, S. Kuschel, S.P.D. Mangles, Z. Najmudin, N. Shukla, L.O. Silva, D. Symes, A.G.R. Thomas, M. Vargas, J. Vieira and M. Zepf, Generation of neutral and high-density electron--positron pair plasmas in the laboratory, Nature Communications 6:6747 (2015); DOI: 10.1038/ncomms7747.\\[0pt] [2] Florentin Smarandache, ``A New Form of Matter - Unmatter, Formed by Particles and Anti-Particles,'' EXT-2004-182 in CERN's web site, 2004.   

Density Correlations in Ultracold Neutral Plasmas

We present a model for the radial distribution functions (RDFs) in an ultracold plasma with two temperatures. If the temperature relaxation is slow, the RDFs can be approximated using methods of equilibrium statistical mechanics. Under various ansatzes for the cross temperature $T_{ie}$, we compute RDFs using the hypernetted chain approximation. To prevent Coulomb collapse, we model the electron-ion interaction using a Deutsch potential. Here we focus on the semiclassical regime. The strongly coupled ions arrange themselves similarly to a Yukawa OCP, with a Coulomb hole and long-range density oscillations. This can cause the electron-ion RDF to also display long-range order. Nontrivial electron-ion density correlations are significant because OCP theories seek to bundle all the electron physics into a single screening parameter $\kappa$ in a modified ion-ion interaction. We compare our two-component model to a YOCP on two fronts: (1) We compare our ion-ion RDFs to those from YOCP calculations, treating $\kappa$ as a fitting parameter to test the usual screening model. (2) We compute the electron-ion temperature relaxation rate in the effective potential theory using both two-component and YOCP effective potentials. Results are compared with molecular dynamics simulations.   

Numerical modeling of electron oscillation damping in an ultracold plasma

By using electric fields to apply an impulse to electrons in an ultracold plasma, it is possible to induce electron oscillations. These oscillations damp due to factors such as electron-ion collisions and the density inhomogeneity of the ultracold plasma. We present results from a numerical model of these electron oscillations that links their frequency and damping rate to ultracold plasma parameters such as density, electron temperature, charge imbalance, and applied electric field. We discuss the relationship between the electron-ion collision rate and the predicted electron oscillation damping time, as these two quantities have a non-trivial relationship. Finally, we discuss non-collisional damping mechanisms that dominate the damping rate at higher electron temperatures.   

Correlated parameters in the quasi-classical treatment of atomic ground states using effective momentum dependent potentials for molecular dynamics simulation of strongly coupled plasmas

Ground state energies and configurations of N, F, Ne, Al, S, Ar and Ca are obtained using a quasi-classical treatment with Kirschbaum-Wilets potentials [1]. The effect of phase space parameters on the ground state energy is studied in detail and compared with Hartree-Fock values. The phase space parameters that resulted in ground state energies comparable to Hartree-Fock values are found to be correlated and follow a pattern with atomic number which led to identifying a predictive capability in the model. The change in ground state configurations for different phase space parameters is studied and correlated with the corresponding change in ground state energies. \\[4pt] [1] C.L. Kirschbaum and L. Wilets, Phys. Rev. A 21, 834 (1980).   

Study of vortex flows of dust particles in a plasma

Vortex motion of dust particles in a plasma has been studied both theoretically and experimentally. In the theoretical study, the ion drag force acting on the dust particle is found to be non-conservative and to have a finite curl because the gradient of \textbar \textbf{u}$_{\mathrm{i}}$\textbar and the gradient of $n_{i}$ are not parallel. The finite curl of the ion drag force acts as a source of vorticity; kinematic viscosity dissipates the generated vorticity. We confirm that vortex flows of micron size dust grains are observed where finite curls of the ion drag force are expected to exist in the Caltech ice dusty plasma experiment. The direction and velocity of the vortex flows are in good agreement with the values predicted by our model. We also found that vortex motion is only observed when the ion density exceeds a threshold value. Above the threshold value, the observed vorticity increases as the ion density increases as predicted by the theory. These observations support the conclusion that the vortex flows in the experiment result from the finite curl of the ion drag force (i.e., non-conservative force).   

Synchronization of the dust acoustic wave in a weakly-coupled dusty plasma system

A complex (dusty) plasma is a four-component system composed of ions, electrons, neutral particles and charged microparticles. The presence of the micro particles gives rise to new plasma phenomena, including collective modes such as the dust acoustic wave (DAW). The dust acoustic wave (also known as the dust density wave) is low-frequency, longitudinal mode that propagates through the dust component of the dusty plasma system and is self-excited by the free energy from the ion streaming through the dust component. In the laboratory setting, the majority of the self-excited dust acoustic waves that are observed are nonlinear, which allows for detailed studies of the nonlinear properties of waves at the kinetic level. One such nonlinear process is synchronization, where a self-excited wave or oscillations interacts with a driving force causing an adjustment of the wave or oscillation frequency. In this presentation, we report the results of an experimental study on the synchronization process of the naturally-occurring dust acoustic wave with an external modulation.   

Evolution of Self-organized Poloidal Dust Rotation with Neutral Pressure in a Direct Current Glow Discharge

Poloidal rotation of mono-dispersed dust particles in toroidally symmetric structures is obtained experimentally in an unmagnetized parallel plate dc glow discharge at high pressures, using a concentric metallic ring placed over surface of cathode. The poloidal rotation of dust particles is observed to be localized above the ring. A radial gradient in the ion drag force arising due to a radial density gradient above the ring is identified as the principal cause of dust rotation [1]. The evolution of this poloidal dust rotation with background gas pressure is studied. A transition from a filled-vortex (poloidal cross-section of the toroidal structure) to a vortex with void at the centre is observed with increase in fill-in gas pressure accompanied by a decrease in vortex height from cathode surface. The velocity of the dust particles is observed to increase with an increase in neutral gas pressure. This observation contradicts the obvious interpretation of slowing down of dust rotation due to an increase in neutral frictional force which increases with pressure. These experimental results with probable causes will be presented in details.\\[4pt] [1] Kaur et al., Phys. Plasmas 22, 033703 (2015).   

Externally and self-excited nonlinear waves in a dusty plasma.

Recently it has been shown that strongly coupled three-dimensional dust clouds can be easily levitated in the plasma sheath region of a glass box coated with a transparent yet conductive layer of indium tin oxide (ITO). Gradually reducing the neutral gas pressure below a critical value of $\sim $350 mTorr establishes self-excited waves within this system. In this paper, it will be shown that decreasing the ITO bias to $-$20 V allows waves to be externally induced within the lower region of the dust cloud. The underlying physics and synergistic effect of changing the pressure and/or ITO bias on these waves will be examined as will the onset of instabilities and the evolution of the dust density waves for ITO biases ranging from 0 to $-$40 V. Finally, the dust charge will be estimated by assuming the waves oscillate at the dust plasma frequency.   

Phenomenological Studies of an Indium-Tin-Oxide (ITO) Box in a RF Plasma

Studies using either a glass box or an Indium-Tin-Oxide (ITO) coated glass box, placed on the lower powered electrode of a GEC RF reference cell have become popular in the field of complex plasmas due to their ability to provide a more controlled environment. However, recent experimental data have shown two independent confinement regions within such boxes, impacting both dust-dust and dust-plasma interaction measurements. This study presents a series of potential maps created using a passive probe mounted on CASPER's S-100 nano-manipulator. The data collected has been correlated into potential field maps and is compared against dust behavior for various experimental operating conditions. The impact of these results on current complex plasma measurements will be discussed.   

A Consistent Model of Plasma- The Potential in a Glass Box

Numerical modeling has become a valuable diagnostic tool for experiments in the modern physical world. In modeling the dynamics of dust particles confined in a glass box placed on the lower electrode of a GEC cell, there are many interactions between the dust, plasma, and boundaries that need to be accounted for more accurately. The lower electrode affects the plasma conditions in the sheath, altering the electron and ion densities. These local variations in the plasma determine the charge accumulated on the surface of the glass box and the resulting electrostatic potential within it. This work describes the steps taken to build a consistent model of the relationship between the plasma conditions and the confining electric potential due to the glass box in order to more accurately model the charging and dynamics of dust clusters and strings.   

A Single Particle Deflection Experiment for MDPX

Complex plasmas contain, in addition to the usual electrons, ions, and neutral atoms, macroscopic electrically charged (nanometer to micrometer) sized ``dust'' particles. Based on the ratio of the electrostatic potential to kinetic energy, these micro-particles can exhibit gaseous, fluid, and crystal-like behavior. For this reason, complex plasmas are a unique testing ground to study multi-particle systems. In spite of the large charge that can be acquired by the dust grains, their charge-to-mass ratio can be quite low compared to other plasma particles. Thus, the direct impact of electric and magnetic fields on dust dynamics is relatively small, and as a result, direct measurements of the particle charge is quite difficult. However, a charge measurement using dust motion in magnetic field still seems possible - although challenging. In this presentation we discuss our initial efforts to perform a single particle g x B deflection measurement to determine the particle charge. We use the Magnetized Dusty Plasma Experiment (MDPX) with a magnetic field orientation perpendicular to gravity and observe the deflection of particles dropped vertically downward.   

A Study of Ion Drag for Ground and Microgravity Dusty Plasma Experiments

This presentation presents the results of a recent study of the interaction between charged dust particles and plasma ions through the ion drag force in a dc glow discharge plasma. Measurements of the dust particles motion are carried out using Particle Image Velocimetry (PIV). When an electrostatic perturbation is applied to the dust cloud, the particle motion, in response to the perturbation, is shown to reverse direction as the gas pressure is increased. An analysis of the dust particle motion and background plasma parameters suggests that there is a competition between the ion drag and electric forces on the particles. These forces are calculated for a range of pressures using detailed measurements of the plasma parameters carried out by a single Langmuir probe. The analysis of these measurements suggests that a change in the relative magnitude of the Coulomb collision ion drag compared to the electric force is a probable explanation for the observed reversal of direction of motion as the neutral gas pressure is increased. The application of these results to microgravity studies of dusty plasmas will be discussed. Support provided by NASA-JPL (JPL-RSA 1471384)   

Probe induced voids at high magnetic field

The presence of voids (dust free regions) in dusty plasmas has been considered for some time. Early studies include the observation of the ``great void mode'' in a laboratory experiment with growing dust grains and self-generated voids in microgravity experiments generated by a balance of an outward ion drag force and an inward electrostatic force acting upon the dust grains. In addition to self-generated void structures, there have also been studies of void regions formed around biased probes in dusty plasmas. In the presence of a magnetic field, it is anticipated that the ion drag force will become modified as the transport of ions in the plasma becomes constrained to magnetic field lines. As a result, the balance between the electrostatic and ion drag forces may be modified, leading to changes in void formation and geometry. This presentation will discuss an experimental study of the modification of the void region around a negatively biased probe in a dusty plasma at high magnetic field. A method for characterizing the void shape will be presented. The effects of the magnetic field, plasma generation, and biasing on void size and eccentricity are investigated.   

Imposed, ordered dust structures and other plasma features in a strongly magnetized plasma

The Magnetized Dusty Plasma Experiment (MDPX) device has been in operation for just over one year. In that time, the MDPX device has been operating using a uniform magnetic field configuration up to 3.0 Tesla and has successfully produced plasmas and dusty plasmas at high magnetic fields. In these experimental studies, we have made observations of a new type of imposed, ordered structure in a dusty plasma at magnetic fields above 1 T [E. Thomas, Jr., et al., Phys. Plasmas, 22, 030701 (2015)]. These dusty plasma structures are shown to scale inversely with neutral pressure and are shown to reflect the spatial structure of a wire mesh placed in the plasma. Additionally, recent measurements have been made that give insights into the effective potential that establishes the ordered structures in the plasma. In this presentation, we report on details of the imposed, ordered dusty plasma structure as well as filamentary features that also appear in the plasma and modify the confinement of the dusty plasma.   

Metastability of Intrinsic Fluctuations of Grain Charge Caused by Secondary Electron Emission

The effect of the secondary electron emission (SEE) on the grain intrinsic charge fluctuations was studied through a Markov approach in plasmas where a grain collects ions and electrons. Caused by SEE, the grain charge could have bistable macrostates. It was also shown that the fluctuations could be metastable, which is characterized by two time scales - one associated with the fluctuations around either macrostate and another associated with the random time intervals at which spontaneous transitions between the two macrostates occur. The study was conducted for various grain sizes, and plasma and grain charging parameters. \\ \\ The results were published at arXiv:1507.01013 and the work was supported by NSF through Award PHY-1414552.   

Prototype of 10 Tesla Water Cooled Bitter-type Magnet System

A 1 Tesla water cooled Bitter-type magnetic system has been designed and is under construction at the Dusty Plasma Laboratory of the University of Maryland, Baltimore County (UMBC). It is a scaled version of a 10 T Bitter-type magnet that will be used in dusty plasma experiments where dust larger than 500 nm diameter will be strongly magnetized. We present here the design methods used for both magnets, and discuss the design parameters that drive the magnet cooling and power storage bank subsystems. The pressure vessel and plasma vacuum chamber subsystems are then built with the aforementioned subsystems as constraints. To validate our design, magnetic field and temperature measurements within the prototype magnet are compared to finite element analysis (FEA) and analytical methods used for preliminary designing. This knowledge will be used to finalize the 10 T magnet design. Once operational, the 10 T magnet will be programmable to be on for at least ten seconds to several minutes, with up to 20 plasma events planned per day.   

Thermal Design of a Bitter-Type Electromagnet for Dusty Plasma Experiments: Prototype Design and Construction

For the purpose of analyzing magnetized dusty plasma at the University of Maryland Baltimore County (UMBC) Dusty Plasma Laboratory, we are designing a resistive water cooled Bitter-Type electromagnet. When completed, the magnet will be programmable to generate fields of up to 10 T for at least 10 seconds and up to several minutes. An analytic thermal design method was developed for establishing the location of elongated axial cooling passages. Comparisons with finite element analysis (FEA) data reveals that the thermal design method was capable of generating cooling channel patterns which establish manageable temperature profiles within the magnet. With our analytic method, cooling hole patterns can be generated in seconds instead of hours with FEA software. To further validate our thermal analysis as well as manufacturing techniques of our magnet design, we are now constructing a prototype electromagnet. The prototype is designed to operate continuously at 1 T with a current of 750 A, and has four diagnostic ports that can accommodate thermocouples and optical access to the water flow. A 1.25 inch diameter bore allows for axial field measurements and provides space for small scale experiments. Thermal analysis and specifics of the electromagnet design are presented.   

Viscosity and Shear Flows in Magnetized Dusty Plasmas

Magnetized dusty plasma experiments are planned at the Dusty Plasma Laboratory of the University of Maryland, Baltimore County (UMBC), to investigate E x B rotation with dust of at least 500 nm in diameter. At this size, individual particles can be tracked and viscosity, shear flow, and temperature can be measured directly using a methodology similar to that used for linear shear flow configurations [Feng et al. PRL 109, 185002 (2012)]. The experiments are planned with a specially designed Bitter-type magnet that can be configured to achieve up to 10 T for at least 10 seconds, to minutes, with much longer operation times at lower fields also possible. At the highest field, the dust will be fully magnetized and thus we aim to achieve direct E x B rotation of the dust (and not just by ion drag). The motivation for these experiments comes from observations of electron and ion temperatures in excess of 100 eV in E x B rotating plasmas [R. Reid et al. Phys. Plasmas 21, 063305 (2014)]. The experimental setup and planned diagnostics for the magnetized dusty plasma are presented.   

Fine Particle Charging Rate Limit Modification to Grain Dynamics in Abrupt and Gradual Inhomogeneities

Gyro-phase drift is a guiding center drift that is directly dependent on the charging rate limit of dust grains. The effect of introducing a gyro-phase-dependence on the grain charge leads to two orthogonal components of guiding-center drift. One component, referred to here as grad-q drift, results from the time-varying, gyro-phase angle dependent, in-situ-equilibrium grain charge, assuming that the grain charging is instantaneous. For this component, the grain is assumed to be always in its in-situ-equilibrium charge state and this state gyro-synchronously varies with respect to the grain's average charge state. The other component, referred to here as the gyro-phase drift, arises from any non-instantaneous-charging-induced modification of the grad-q drift and points in the direction associated with increasing magnitude of in-situ-equilibrium charge state. Gyro-synchronous grain charge modulation may arise from either abrupt or gradual inhomogeneity in plasma conditions. This work assesses the feasibility of observing gyro-phase drift in Auburn's MDPX, and how gyro-phase drift might be used to test dust grain charging models in an experiment.   

Heavy ion acceleration driven by THE INTERACTION BETWEEN ultraintense Laser pulse AND sub-micron foils

For ion acceleration at the intensity exceeding 10$^{21}$W/cm$^{2}$, Radiation Pressure Acceleration (RPA) could offer advantages over Target Normal Sheath Acceleration (TNSA) and Break-Out Afterburner (BOA). In this ultra-relativistic regime, target electrons become highly relativistic and the results are sensitive to many parameters. Especially for heavy ions acceleration, the understanding of the most important parameter effects is limited due to the lack of experiments and modeling. To further understand the key parameters and determine the most suitable regimes for efficient acceleration of heavy ions, we have carried out two-dimensional Particle-in-Cell simulations with the epoch code. In the simulations, effects of preplasma and optimal targets thicknesses for different laser pulse have been studied in detail. Based on the understanding of ion RPA, we propose some new target parameters to achieve higher ion energy. This work was performed with the support of the Air Force Office of Scientific Research under grant FA9550-14-1-0282.   

Proton acceleration from short pulse lasers interacting with ultrathin foil

Two-dimensional particle-in-cell simulations using 50 nm Si$_{3}$N$_{4}$ and DLC foils are compared to published experimental data of proton acceleration from ultra-thin foils (\textless 1 $\mu $m) irradiated by short pulse lasers (30-50 fs), and some underlying physics issues pertinent to proton acceleration have been addressed. 2D particle-in-cell simulations show that the maximum proton energy scales as $I^{2/3}$, stronger than Target Normal Sheath Acceleration for thick foils (\textgreater 1 $\mu$m), which is typically between $I^{1/3}$ [1] and $I^{1/2}$ [2]. Published experimental data were found to depend primarily on the laser energy and scale as $E^{2/3}$. The different scaling laws for thick (\textgreater 1 $\mu $m) and ultra-thin (\textless 1 $\mu$m) foils are explained qualitatively as transitioning from Target Normal Sheath Acceleration to more advanced acceleration schemes such as Radiation-Induced Transparency and Radiation Pressure Acceleration regimes. This work was performed with the support of the Air Force Office of Scientific Research under grant FA9550-14-1-0282. \\[4pt] [1] F. N. Beg, et. al., Phys. Plasmas \textbf{4}, 447 (1997)\\[0pt] [2] K. Krushelnick, et. al., Plasma Phys. Control. Fusion \textbf{47}, B451 (2005)   

Fundamental Studies on the Use of Laser-Driven Proton Beams for Fast Ignition

A short-pulse-laser-driven intense proton beam remains a candidate for Fast Ignition heater due to its focusability and high current. However, the proton current density necessary for FI in practice has never been produced in the laboratory and there are many physics issues that should be addressed using current and near-term facilities. For example, the extraction of sufficient proton charge from the short-pulse laser target could be evaluated with the multi-kilojoule NIF ARC laser. Transport of the beam through matter, such as a cone tip, and deposition in the fuel must be considered carefully as it will isochorically heat any material it enters and produce a rapidly-evolving, warm dense matter state with uncertain transport and stopping properties. Here we share experimental measurements of the proton spectra after passing through metal cones and foils taken with the kilojoule-class, multi-picosecond OMEGA EP and LFEX lasers. We also present complementary PIC simulations of beam generation and transport to and in the foils. Upcoming experiments to further evaluate proton beam performance in proton FI will also be outlined.   

Laser-driven ion dynamics using multiple ultra-high intensity laser beams

Ion acceleration from foils irradiated by a laser pulse at relativistic intensity is dominated by target rearside electron dynamics of the foil. Simulations show that focusing a second, similar intense laser beam onto the foil, one can produce ion beams with interesting spectral features with respect to angular distribution and higher cut-off energies or can even initiate another acceleration phase depending on the temporal delay. In this contribution, we report on a series of recent experiments adressing the ion acceleration utilizing two ultrashort (30fs), high intensity (10$^{20}$ W/cm$^2$) and high contrast (10$^{-10}$) laser beams. Both beams were focused and spatially overlapped onto 5 micron titanium targets. The main goal was to investigate the impact of temporal delaying of the two laser pulses on the maximum proton and/or ion energy. Extensive studies show an proton energy enhancement by factor 1.5 and clear impact on carbon ion spectra. In addition, we characterize the rearside plasma expansion with a temporal and spatial resolved interferometer (TASRI) and recflectometry using a chirped optical probe to obtain the evolution of electron temperatures and densities in a 20 ps time window for each shot.   

Initial design of a beamline for ultra-intense laser-matter interactions at the BELLA-i PW laser user facility

BELLA, the Berkeley Lab laser accelerator center hosts a 1 PW Ti:sapp laser with 1 Hz repetition rate, where electron acceleration to 4.5 GeV was demonstrated recently [1]. For electron acceleration, irradiances of up to 10$^{19}$ W/cm$^{2}$ are desired and these are implemented with a long focal length laser beamline and beam spots of w$_{\mathrm{0}}=$52$\mu $m. Much higher irradiances of 10$^{22}$ W/cm$^{2}$ can be achieved when the laser beam is focus more tightly, to a spot of w$_{\mathrm{0}}$\textless 5 $\mu$m in a shorter focal length beamline. A key requirement for many application of laser-matter interaction in this regime, such as laser-ion acceleration or the generation of relativistic surface high harmonics is the ultra-high intensity contrast of the laser pulse. We will describe our design for a short focal lengths beamline, BELLA-i, including multiple plasma mirrors for ultra-high contrast in the laser pulse. The resulting laser pulses will enable reliable access to many exciting aspects of high energy density laboratory physics and laser-matter interactions in the relativistic regime for a community of users.\\[4pt] [1] W. P. Leemans \textit{et al.,} PRL \textbf{113}, 245002 (2014)   

Generation of thin, near critical density gas targets for laser plasma interaction experiments

We present the design and characterization of a thin (~200µm FWHM), high density pulsed gas jet which we use to study near critical and overcritical laser plasma interactions. We show that cryogenic cooling of the pulsed jet provides the necessary density enhancement for reaching overcritical plasma densities at 800 nm (>1.7*?10?^21 ?cm?^(-3)) with pure hydrogen gas at plenum pressures below 1000 psi. Further, we present 2D and 3D PIC simulations showing the interaction of femtosecond pulses with our experimentally measured near critical gas density profile. The simulations show electron and ion acceleration at drive pulse energies as low as a few tens of millijoules.   

Simulations of Ion Acceleration in Thin Dense Gas Jets

We present particle in cell simulations of the interaction of intense femtosecond lasers with thin near-critical density gas jets. 2D simulations show the production of ion beams using as little as 50 mJ of laser energy in a 40 fs laser pulse. The introduction of a transverse density gradient is shown to deflect the laser as well as the accelerated electrons and ions away from the region of high density. 3D simulations show the generation of multi-MeV proton beams in good agreement with the 2D simulations.   

Ultrabroadband Relay Imaged GRENOUILLE as a Time-Resolved Diagnostic for Relativistic Hole Boring

In a highly intense laser-solid interaction, the surface of the resultant plasma is pushed into the interior of the target at a significant fraction of the speed of light as a result of the intense radiation pressure from the focused laser beam. This is known as hole boring. During the hole boring process laser interactions with electrons at the receding target surface generate light at frequency harmonics of the incident laser. The frequency shift of these harmonics is proportional to the velocity of the target surface. In previous experiments at the Texas Petawatt we observed red-shifts in the 351nm harmonic up to 513nm, corresponding to a recession velocity of 0.18c. We designed an ultra-broadband GRENOUILLE to conduct time resolved measurements of spectral shifting of second harmonic light over the duration of the incident laser pulse. This GRENOUILLE is relay imaged from the target plane to prevent spectral splitting, and is an all reflective design to reduce pulse broadening and chromatic aberrations. With an f/3.15 optic focusing into a thick BBO crystal, the system accepts wavelengths from 526nm to 766nm with 4.8nm spectral resolution and 5.6fs temporal resolution.   

Effects of Local Field Modulation on the Laser-Driven Rayleigh-Taylor Instability, plasmonic effects and 3D structures

The acceleration of dense targets driven by the radiation pressure of high-intensity laser may lead to a Rayleigh-Taylor instability with rippling of the interaction surface. Using a simple model it is shown that the self-consistent modulation of the radiation pressure caused by a sinusoidal rippling affects substantially the wavevector spectrum of the instability depending on the laser polarization. In particular, the strong enhancement of the local field when the rippling period equals the laser wavelength explains why the latter is the dominant instability scale observed in several simulations. The nonlinear evolution is investigated by three dimensional simulations which show the formation of stable structure with ``wall paper'' symmetry.\\[4pt] [1] S. Sgattoni, {\it et al.}, {\it Phys. Rev. E}, {\bf 91} 013106 (2015)   

A high repetition rate laser-heavy water based neutron source

Neutrons have numerous applications in diverse areas, such as medicine, security, and material science. For example, sources of MeV neutrons may be used for active interrogation for nuclear security applications. Recently, alternative ways to generate neutron flux have been studied. Among them, ultrashort laser pulse interactions with dense plasma have attracted significant attention as compact, pulse sources of neutrons. To generate neutrons using a laser through fusion reactions, thin solid density targets have been used in a pitcher-catcher arrangement, using deuterated plastic for example. However, the use of solid targets is limited for high-repetition rate operation due to the need to refresh the target for every laser shot. Here, we use a free flowing heavy water target with a high repetition rate (500 Hz) laser without a catcher. From the interaction between a 10 micron scale diameter heavy water stream with the Lambda-cubed laser system at the Univ. of Michigan (12mJ, 800nm, 35fs), deuterons collide with each other resulting in D-D fusion reactions generating 2.45 MeV neutrons. Under best conditions a time average of $\sim$ 10$^5$ n/s of neutrons are generated.   

Tabletop laser driven shock-ion acceleration in near-critical plasmas

An intense laser pulse interacting with near-critical density plasma can drive an electrostatic shock capable of accelerating quasi-monoenergetic, high-energy ion beams in the laboratory. Experimental plans using the T-cubed laser ($1.053 \; \mu \rm{m}$, 15 TW, 6 J in 400 fs) will be discussed. The target parameters requirements for this experiment are investigated using quasi-1D particle-in-cell simulations. To determine and optimize the formation of a shock and the subsequent proton beam acceleration, the simulation plasma scale lengths and density profiles were varied. The resulting electron heating, shock formation and proton acceleration will be presented and discussed.   

Laser Absorption by Over-Critical Plasmas

Absorption of high intensity laser light by matter has important applications to emerging sciences and technology, such as Fast Ignition ICF and ion acceleration. As such, understanding the underlying mechanisms of this absorption is key to developing these technologies. Critical features which distinguish the interaction of high intensity light - defined here as a laser field having a normalized vector potential greater than unity - are that the reaction of the material to the fields results in sharp high-density interfaces; and that the movement of the electrons is in general relativistic, both in a fluid and a thermal sense. The results of these features are that the absorption mechanisms are qualitatively distinct from those at lower intensities. We will review previous work, by our group and others, on the absorption mechanisms, and highlight current research. We will show that the standing wave structure of the reflected laser light is key to particle dynamics for normally incident lasers.   

Analysis of Fast Electron Energy Distribution by Measuring Hard X-ray Bremsstrahlung

Characterization of intense, short-pulse laser-produced fast electrons is important for fundamental understanding and applications. We carried out an experiment to characterize the fast electron energy distribution by measuring angular-dependent high-energy bremsstrahlung x-rays. A 100 $\mu $m thick metal foil (Al, Cu, and Ag) mounted on a plastic backing was irradiated by the 0.35 ps, 15 J Leopard Laser at the Nevada Terawatt Facility. The bremsstrahlung x-rays and the escaping electrons from the target were recorded using differential filter stack spectrometers at 22$^{\circ}$ and 45$^{\circ}$ off laser axis and a magnet-based electron spectrometer along the laser axis. The electron spectrum inferred from two different diagnostics had single slope temperature of $\sim$ 1.5 MeV for the Cu foil. The results were compared to an analytic calculation and a 2-D Particle-in-cell code, PICLS. The analysis of the electron energy distribution and angular distribution will be presented.   

Delaying wave field filamentation in focusing Kerr media

Coherent wave packets can traverse focusing Kerr-like media at powers smaller than the critical power of self-focusing. However, at powers much larger than the critical power, wave packets tend to break into many filaments for times not much exceeding the self-focusing time. This work shows how this filamentation can be significantly delayed by proper randomizing of the wave packet Fourier components.   

Recent Target Campaigns Fabricated at the University of Michigan

Conducting high-energy-density physics campaigns often requires the fabrication of sophisticated targets designs. At the University of Michigan we have been fabricating these highly complex targets for over a decade. One such recent experiment required accurate, repeatable placement of two large, 14.8 kGauss magnets positioned among several other components fielded on Titan at Lawrence Livermore National Laboratory. We present this target here, along with several of our recent target campaigns and techniques used to fabricate them in a highly cost effective and repeatable way.   

Creation of Pure Frozen Gas Targets for Ion Acceleration using Short Pulse Lasers

A system for shooting interchangeable frozen gas targets was developed at the University of Texas and will be tested at Los Alamos National Lab. A target holder which can hold up to five substrates used for target growing was cryogenically cooled to temperatures below 14 K. The target substrates consist of holes with diameters ranging from 15$\mu $m-500$\mu $m and TEM grids with micron scale spacing, across which films of ice are frozen by releasing small amounts of pure gas molecules directly into the vacuum target chamber. Frozen gas targets comprised of simple molecules like methane and single element gasses like hydrogen and deuterium will provide novel target configuations that will be compared with laser plasma interaction simulations. The targets will be shot with the ultra-intense short-pulse Trident laser. Accelerated ion spectra will be characterized using a Thomson Parabola with magnetic field strength of 0.92T and electric field strength of 30kV. Hydrogen targets will be additionally characterized using stacks of copper which become activated upon exposure to energetic protons resulting in a beta decay signal which be imaged on electron sensitive imaging plates to provide an energy spectrum and spacial profile of the proton beam. Details of target creation and pre-shot characterization will be presented.   

Generation of Solid Density Ar Fiber Targets for High-Repetition Intense Laser Pulse Interaction with Overdense Plasma

Recently the interaction of high-intensity laser pulses with matter has been studied in the purpose from understanding basic physical sciences to nuclear fusion energy source application. Solid thin foil targets are generally used to create overdense plasma. However, these thin foil targets are favorable for single shot experiments, and the target surface condition is not uniform over shots. By contrast, atomic or molecular clusters with solid intra-particle densities can be used for multi-shot or high-repetition-rate experiments, but those targets generate underdense plasma within the laser focal volume. Here, we present an experimental study of new type of solid targets, solid-density Ar fibers ($\sim$ 50 micron diameter) continuously generated from cryogenically cooled capillary nozzles, for high-intensity laser experiments at a 1 kHz repetition rate.   

QED multi-dimensional vacuum polarization finite-difference solver

The Extreme Light Infrastructure (ELI) is expected to deliver peak intensities of 10$^{23}$ -- 10$^{24}$ W/cm$^2$ allowing to probe nonlinear Quantum Electrodynamics (QED) phenomena in an unprecedented regime. Within the framework of QED, the second order process of photon-photon scattering leads to a set of extended Maxwell's equations [W. Heisenberg and H. Euler, Z. Physik 98, 714] effectively creating nonlinear polarization and magnetization terms that account for the nonlinear response of the vacuum. To model this in a self-consistent way, we present a multi dimensional generalized Maxwell equation finite difference solver with significantly enhanced dispersive properties, which was implemented in the OSIRIS particle-in-cell code [R.A.Fonseca et al. LNCS 2331, pp. 342-351, 2002].~ We present a detailed numerical analysis of this electromagnetic solver. As an illustration of the properties of the solver, we explore several examples in extreme conditions. We confirm the theoretical prediction of vacuum birefringence of a pulse propagating in the presence of an intense static background field [arXiv:1301.4918 [quant-ph]]. We also show the generation of high harmonics from the vacuum when two counter-propagating pulses interact for realistic beam setups in agreement with a theoretical calculation performed. By considering the finite structure of the fields, the results obtained serve as an important benchmark for experiments aimed at detecting nonlinear QED processes resorting to ultra intense lasers.   

Bound-Free Transitions to GeV Energy via Optical Tunneling

Many laser plasmas are created through the mechanism of tunneling ionization. For weakly to moderately relativistic laser amplitudes ($a = eA/mc \approx 1$), the photoelectron spectrum can extend to the MeV range, with the electron gaining approximately the ponderomotive potential at the position where the bound-free transition occurred. When $a \approx 100$, a new regime of acceleration appears, in which ultrarelativistic energy is obtained in a fraction of an optical cycle. We compute photoelectron characteristics based on relativistic tunneling ionization rates, and advanced particle tracking simulations, utilizing state-of-the art computer hardware. It is found that using near-term multi-petawatt lasers, free space acceleration from rest to GeV energy is possible. The effect of radiation reaction is also examined.   

Comparative analysis of theories of relativistic photoionization

Laser-plasma experiments routinely rely on photoionization for plasma formation. For large laser intensities or for high-Z atoms relativistic effects become important. We investigate a unique regime of relativistic photoionization from high-Z atoms where relativistic effects modify both the bound and continuum electronic states. Theories of photoionization are based on the imaginary time method and the S-matrix method, amongst others. We compare the results of these approaches for both the Dirac and the Klein-Gordon equations. Analytical results for the momentum distribution of ejected electrons and ionization rate are presented and compared with those from numerical solutions.   

Hamiltonian and Lagrangian dynamics of charged particles including the effects of radiation damping

The effects of radiation damping (radiation reaction) on accelerating charged particles in modern high-intensity accelerators and high-intensity laser beams have becoming increasingly important. Especially for electron accelerators and storage rings, radiation damping is an effective mechanism and technique to achieve high beam luminosity. We develop Hamiltonian and Lagrangian descriptions of the classical dynamics of a charged particle including the effects of radiation damping in the general electromagnetic focusing channels encountered in accelerators. The direct connection between the classical Hamiltonian and Lagrangian theories and the more fundamental QED description of the synchrotron radiation process is also addressed. In addition to their theoretical importance, the classical Hamiltonian and Lagrangian theories of the radiation damping also enable us to numerically integrate the dynamics using advanced structure-preserving geometric algorithms. These theoretical developments can also be applied to runaway electrons and positrons generated during the disruption or startup of tokamak discharges.   

Enhanced electron-positron pair production by irradiation of a thin foil target with two ultraintense laser pulses

In this presentation, a novel scheme for enhanced QED production of electron-positron pair sources is reported, which uses two ultraintense lasers irradiating a thin foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of gamma-rays and the Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced gamma-ray emission and pair production. Further, the created pairs are later collected and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in formation of dense electron-positron pairs. 2D QED-PIC simulations show that an unprecedented positron density of 10$^{28}$m$^{-3}$ can be achieved at laser intensities $3.4 \times 10^{23}$W/cm$^{2}$.   

Laser pulse scattering in the transition from the classical to the quantum regime

At ultra-high intensities, laser pulse scattering on electron beams can lead to significant energy loss through radiation damping, for example in all-optical configurations. At small $\chi$ parameters, the radiation damping can still be modeled classically. The radiation emission can then be obtained in simulations by combining radiation damping in the particle dynamics and introducing quantum corrections in the classical emissivity formula due to the recoil of the emitting electron. With the formula implemented in the post-processing code jRad, this approach is checked through the comparison of the energy captured by the code in the detector with the energy that the particle is observed to lose by direct inspection of its trajectory. Results are shown for the scattering of circularly polarized plane waves of increasing intensities by an electron. In this work, the spectrum from the scattering of ultra-high intensity laser pulses (up to a0 $\sim$ 30) by relativistic electrons (Lorentz factors of 1000s) is investigated. From laser intensities of a0 $\sim$ 10 to 30 a significant change in the spectrum shape is observed and spike-like features emerge. The origin of such features is investigated for different pulse intensities and durations.   

Hydrodynamic simulation of Irradiation of ultra-intense laser on the inner surface of shell

We have conducted the hydrodynamic simulation of irradiation of ultra-intense laser on the inner surface of imploding CD shell to generate the high temperature hot spark. In spite of the conventional core heating by the fast electrons in the fast ignition,we propose to use relatively longer pulse of 100ps, and it directly irradiates the inner surface of imploding shell. The laser intensity is ranging from 10$^{17}$ W/cm$^2$ to 10$^{18}$ W/cm$^2$. In this irradiation. In this intensity region, the laser absorption fraction is relatively low and most of the irradiated laser light reflects multiply, and heats of the inner surface of the shell. Also, fast electrons with moderate energy ranging from 50keV to 100keV are generated and preheats the inner part of imploding shell. Then, the preheated shell generates the hot spark.In order to confirm this concept, we have conducted the preliminary experiment by using 1.06 micron wavelength and 100ps duration beams of GXII laser system. We observed that high temperature region of keV in the central part of the target.Also we have conducted the hydrodynamic simulations to confirm this concept. We will show the preliminary calculated results and possibility as a alternative heating method in the fast ignition.   

Pellet ignition using shock-accelerated ions in the corona

Recently we have suggested that fast ignition with ions might be possible using a scheme in which, towards the end of the compression phase in inertial fusion, a sequence of intense short pulses is used, first to heat the corona to a high temperature then to launch a shock wave to accelerate ions into the compressed core. This is in contrast to other ion fast ignition schemes in which a separate target is envisaged for the generation of the ions. Initial estimates of the range of energetic ions moving into the core suggest that ions in the 1-10 Mev range will deposit their energy when the density reaches $10^{25}-10^{26}$ cm$^{-3}$. We will report on detailed studies to identify the range of corona temperatures and shock Mach numbers needed to produce ions of the energy necessary to produce core heating. With the aid of computer simulations of the heating of the corona and production of shock waves in the resulting high electron temperature plasma we will study the requirements for laser systems to make this scheme viable.   

Effect of Laser Wavelength and Ablator Material on Hot Electron Generation in High Power Laser Plasma Interaction at Shock Ignition High Intensity Conditions

Hot electrons with energies \textless 100 keV have been found to augment ablation pressure leading to Gbar shocks in strong spherical shock experiments on OMEGA$^{\mathrm{\ast }}$. To study this potential benefit at shock ignition-relevant high intensities ($\sim $10$^{\mathrm{16}}$ W/cm$^{\mathrm{2}})$, we have conducted an experiment using the high-energy OMEGA EP laser system to examine the effect of laser wavelength, intensity and ablator material on hot electron generation and energy coupling. Targets are multilayered planar foils consisting of Cu and Al layers with an ablator made of either plastic (CH) or lithium. The target is first irradiated by multi-kJ UV beams at low intensity to produce a long scalelength, hot plasma, as is the case in the shock ignition regime. Correspondingly, this is followed by the injection of the high intensity UV or IR main interaction pulse. The resultant energy, spectrum and angular distributions of the hot electrons are measured via their induced Cu fluorescence emission and the bremsstrahlung radiation. Details of the experiment and results will be presented. $^{\mathrm{\ast }}$W. Theobald et al., Phys. Plasmas 22, 056310 (2015).   

Plasma heating with crossing relativistic electron beams

Plasma heating by relativistic electron beams is a powerful tool with applications including the heating of inertial confinement fusion targets and the study of matter in extreme conditions. We discuss the use of two relativistic electron beams to efficiently heat the plasma ions where the beams cross by using beam-plasma instabilities and non-linear wave coupling between Langmuir and ion-acoustic waves. Energy from the electron beams is coupled to the plasma ions as the beams become unstable and drive Langmuir waves which couple non-linearly to ion-acoustic waves which are then damped . Results of linear growth rate calculations are presented for the system of two crossing electron beams demonstrating a broad spectrum of unstable modes. Relativistic Vlasov-Maxwell simulations in two space and two momentum dimensions have been performed which demonstrate the non-linear coupling of the electron beam energy into ion-acoustic waves and the energy cascade to the background ions. Time-frequency analysis is applied to analyze the non-linear coupling between Langmuir and ion-acoustic waves in wave phase space. Structural properties of the strong turbulence produced at late times are analyzed.   

Nonlinear Amplification of the Whistler Wave in a Magnetized Relativistic Beam-Plasma Interaction

We have been investigating a relativistic electron beam--plasma interaction under a strong magnetic field using a hybrid simulation code. In an initial stage, the electron beam drives a return current in a background plasma and such a two beam state causes a longitudinal two stream instability and a transverse Weibel instability. The application of a strong magnetic field is proposed for the suppression of the beam instabilities. When a sufficiently strong magnetic field is applied along the beam propagation, the Weibel instability is well suppressed and electrons flow laminarly. When the magnetic field strength is not large enough, however, electrons stagnate and the total number of beam electrons is largely reduced. Our detailed analyses show that a strong whistler wave is excited during the interaction and the wave stops the beam electrons. Since the whistler wave is composed of transverse electromagnetic fields, there should be a mechanism to convert the transverse field to a longitudinal one. In order to investigate this problem, we have performed a lot of simulation runs for a simple geometry. Then we found the amplified transverse modulation of the background plasma due to the Weibel instability plays an important role for the amplification of the whistler wave.   

Relativistic Plasma Polarizer: Impact of Temperature Anisotropy on Relativistic Transparency

3D particle-in-cell simulations demonstrate that the enhanced transparency of a relativistically hot plasma is sensitive to how the energy is partitioned between different degrees of freedom. We consider here the simplest problem: the propagation of a low amplitude pulse through a preformed relativistically hot anisotropic electron plasma to explore its intrinsic dielectric properties. We find that: 1) the critical density for propagation depends strongly on the pulse polarization, 2) two plasmas with the same density and average energy per electron can exhibit profoundly different responses to electromagnetic pulses, 3) the anisotropy-driven Weibel instability develops as expected; the timescales of the growth and back reaction (on anisotropy), however, are long enough that sufficient anisotropy persists for the entire duration of the simulation. This plasma can then function as a polarizer or a wave plate to dramatically alter the pulse polarization [1].\\[4pt] [1] D.J. Stark, C. Bhattacharjee, A.V. Arefiev, T. Toncian, R.D. Hazeltine, and S.M. Mahajan. Phys. Rev. Lett. 115, 025002 (2015).   

Collisionless shock waves mediated by Weibel Instability

Relativistic collisionless shocks are common events in astrophysical environments. They are thought to be responsible for generating ultra-high energy particles via the Fermi acceleration mechanism. It has been conjectured [1] that the formation of collisionless shocks is mediated by the Weibel instability that takes place when two initially cold, unmagnetized plasma shells counter-propagate into each other with relativistic drift velocities. Using a PIC code, VLPL [2], which is modified to suppress numerical Cherenkov instabilities, we study the shock formation and evolution for asymmetric colliding shells with different densities in their own proper reference frame. Plasma instabilities in the region between the shock and the precursor are also investigated using a moving-window simulation that advances the computational domain at the shock's speed. This method helps both to save computation time and avoid severe numerical Cherenkov instabilities, and it allows us to study the shock evolution in a longer time period. Project is supported by US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945. \\[4pt] [1] M. V. Medvedev et al., ApJ 526, 697-706 (1999)\\[0pt] [2] A. Pukhov, J. Plasma Phys. 61, 425-433 (1999)   

Study of the Electric Field Screening Effect on Low Number of Carbon Fiber Field Emitters

Field emitter arrays have the potential to provide high current density, low voltage operation, and high pulse repetition for radar and communication. It is well known that packing density of the field emitter arrays significantly affects the emission current.$^{1}$ Previously we conducted experiments using two- and four-cathode configurations. Here we extend our previous work and present experimental results for nine cathodes in a square and cylindrical configuration. The experiments used nine cathodes with variable spacing to investigate the effect of electric field screening on current emission. Emission characteristic is compared for the case of two, four and nine field emitters with different spacing. Particle-in-cell simulations are performed to compare with the experiments.   

Electron emission due to femtosecond laser assisted photo-emission from tungsten and carbon cathodes

Electron emission under the influence of an ultrashort pulsed laser for single and double tipped carbon fiber and tungsten cathode field emitters has been studied to characterize the effect of electric field screening. Each cathode tip is illuminated by a 50fs, 800nm laser pulse and the emitted current is measured as a function of applied DC bias voltage and laser energy. In addition, emission current is also measured as a function of the time delay between femtosecond laser pulses for the double tip experiments. The single tip experiments show the emission mechanism changes from multiphoton emission to single photon assisted tunneling emission as laser energy increases. For the double tip measurements, our previous work showed that electric field screening between the cathodes plays a significant role in the emitted current characteristic under DC conditions. Here, we study the effect of the electric field screening on the ultrashort time scale, where the light transit time between the two cathodes is longer than the duration of the laser pulses. Theoretical analysis is performed for comparison with experiments.   

Electric Field Screening by the Proximity of Two Knife-Edge Field Emitters of Finite Width

Field emitter arrays have the potential to provide high current density, low voltage operation, and high pulse repetition for radar and communication. It is well known that packing density of the field emitter arrays significantly affect the emission current [1]. Previously we calculated analytically the electric field profile of two-dimensional knife-edge cathodes with arbitrary separation by using a Schwarz-Christoffel transformation [2]. Here we extend this previous work to include the finite width of two identical emitters. From the electric field profile, the field enhancement factor, thereby the severity of the electric field screening, are determined. It is found that for two identical emitters with finite width, the magnitude of the electric field on the knife-edge cathodes depends strongly on the ratio $h/a$ and $h/r$, where $h$ is the height of the knife-edge cathode, \textit{2a} is the distance between the cathodes, and 2$r$ represents their width. Particle-in-cell simulations are performed to compare with the analytical results on the emission current distribution.\\[4pt] [1] L. Nilsson, et al., Appl. Phys. Lett. 76, 2071 (2000).\\[0pt] [2] W. Tang, D. Shiffler and K.L. Cartwright, J. Appl. Phys. 110, 034905 (2011).   

Harmonic Generation in the Multifrequency Recirculating Planar Magnetron

The Multifrequency Recirculating Planar Magnetron (MFRPM) is a high power microwave source adapted from the Recirculating Planar Magnetron$^{a}$, currently under investigation at the University of Michigan. The device features 2 dissimilar periodic structures allowing for the generation of (L-band) 1- and (S-band) 2-GHz high power microwave pulses simultaneously. These distinct frequencies offer the potential for variable coupling for defense applications, such as counter-IED. Experiments have been performed on the RPM, driven by the Michigan Electron Long Beam Accelerator with a Ceramic insulator (MELBA-C) using a -300kV, 1-10 kA, 0.3-1.0 us pulse applied to the cathode. Using the Mode Control Cathode$^{b}$ and a coax-to-waveguide extraction system, the MFRPM has demonstrated simultaneous production of 20 MW at 1 GHz and 10 MW at 2 GHz. The L-band oscillator also produced both 2- and 4-GHz oscillations when the S-band oscillator turns on. These harmonics persist after the S-band oscillator turns off. Ongoing work will attempt to isolate these harmonics to measure the power accurately and confirm these observations. [a] R.M. Gilgenbach, Y.Y. Lau, D.M. French, B.W. Hoff, J. Luginsland, and M. Franzi, ``Crossed field device,'' U.S. Patent US 8 841 867B2, Sep. 23, 2014. [b] M.A. Franzi, R.M. Gilgenbach, Y.Y. Lau, B.W. Hoff, G. Greening, and P. Zhang, Phys. Plasmas 20, 033108 (2013).   

3D Hot Test Simulations of a 220 GHz Folded Waveguide Traveling Wave Tube Using a CFDTD PIC Method

Millimeter or sub-THz wave sources centered at 220 GHz is of interest due to the potential for its commercial and military applications including high resolution radar, remote sensing, and high-data-rate communications. It has been demonstrated via 3D cold test finite element method (FEM) simulations that a folded waveguide traveling wave tube (FWTWT) can be designed and optimized at this frequency range with a small signal gain of 18 dB over a comparatively broad (-3 dB) bandwidth of $\sim$ 10{\%} [1]. On the other hand, 3D hot test simulations of a V-band ladder TWT have been successfully demonstrated using a conformal finite-difference time-domain (CFDTD) particle-in-cell (PIC) method [2] for center frequency of 50 GHz. In the present work, the 220 GHz FWTWT designs have been reviewed and studied. 3D Cold test simulations using both the CFDTD and FEM methods have been carried out and compared with each other as basis for 3D hot test CFDTD PIC simulations. The preliminary simulation result shows that the gain-bandwidth features at 220 GHz are achievable while carefully avoiding beam interceptions. Our study shows that the interaction characteristics are very sensitive to the operating beam parameters. Detail simulation results and discussions will be presented. \\[4pt] [1] R. Zheng and X. Chen, J. Infrared Milli. Terahz Waves 30, 945--958 (2009). \\[0pt] [2] C. R. Douglas, M. C. Lin, P. H. Stoltz, D. Smithe, J. S. Lee, H. Song, and S. H. Lee, IEEE Trans. Electron Dev. 57, 3500-3507 (2010).   

Simulation of a distributed current source in a linear format CFA

A fundamental limit on Crossed-Field Amplifiers (CFA) gain is beam to RF power ratio. With too much beam power, the RF signal on the slow wave circuit is ``swamped.'' It is proposed here that a controllable, distributed cathode source can be used to tailor current injection and optimize gain. In this work a linear format CFA with a meander line slow wave circuit is tested experimentally and numerically using Vsim. Simulations of the original design, which operates at 900 MHz, shows $<$ 1dB gain at beam currents $>$100 mA. This beam current is higher than the capabilities of the Field Emitter Array cathodes available to the group; therefore no experimental gain was observed. To be able to compare simulation to experiment, the CFA model under study was changed to the experiment used at Northeastern University in 1991, which also uses a meander line circuit and an injected beam configuration. Direct comparisons between the simulation and this experiment are performed to validate the model. Additional simulations study the effect of different current distributions on gain, bandwidth, and efficiency. Practical considerations such as how to control the energy of the beam separately from the sole potential in order to minimize lost current to sole are also examined.   

Target Chamber Manipulator

A system has been developed to allow remote actuation of sensors in a high vacuum target chamber used with a particle accelerator. Typically, sensors of various types are placed into the target chamber at specific radial and angular positions relative to the beam line and target. The chamber is then evacuated and the experiments are performed for those sensor positions. Then, the chamber is opened, the sensors are repositioned to new angles or radii, and the process is repeated, with a separate pump-down cycle for each set of sensor positions. The new sensor positioning system allows scientists to pre-set the radii of up to a dozen sensors, and then remotely actuate their angular positions without breaking the vacuum of the target chamber. This reduces the time required to reposition sensors from 6 hours to 1 minute. The sensors are placed into one of two tracks that are separately actuated using vacuum-grade stepping motors. The positions of the sensors are verified using absolute optical rotary encoders, and the positions are accurate to 0.5 degrees. The positions of the sensors are electronically recorded and time-stamped after every change. User control is through a GUI using LabVIEW.   

Interactions of plasma waves and lasers with electron beams resulting in a line focus

An electron beam, injected perpendicularly across co-propagating plasma waves and laser beams, has been shown in numerical trajectory simulations to be focused to a line. The numerical trajectory simulations solve the equation of motion of the electron interacting with the electric fields of a plasma waves and up to two co-propagating laser beams. The combination of these fields appears to have a similar effect on the electron beam as a cylindrical lens. The effects on the focus, due to variations of the electron beam energy, plasma wave amplitude, and laser wavelengths are studied. A PIC code has been modified to model this interaction also. A comparison with other plasma wave focusing schemes is made. An experimental test to observe this focusing has been designed.   

Ion Beam Scattering by Background Helium

The presence of background gases can cause charged particle beams to become more diffuse due to scattering. Calculations for the transport of an ion beam have been performed using Aleph, a particle-in-cell plasma modeling code, and verified against a general envelop equation for charged particle beams. We have investigated the influence of background helium on the coherence and transmitted current of the ion beam. Collisions between ions and neutral particles were calculated assuming isotropic elastic scattering. Since this tends to predict larger scattering angles than are expected at high energies, these are conservative estimates for beam scattering.