Session XP9: Poster Session IX: Supplemental and Post-Deadline Posters

An atmospheric pressure RF Helium plasma source for polymer surface modification

An atmospheric pressure plasma was generated by a RF capacitive discharge using either a gas mixture of helium and oxygen, or a mixture of helium and perfluorohexane (PFH). The modification of polyethyleneterephthalate (PET) surfaces by this plasma source was investigated. PET strips were exposed to plasma for different durations at the exit of the plasma source. Water contact angle measurements indicated that hydrophilic or hydrophobic PET surfaces were formed depending on the gas composition used in the plasma. The changes in the water contact angles on the modified PET surfaces were monitored as a function of time.   

On MHD stability of gravitating plasmas with field aligned flows

A previous stability condition (see Throumoulopoulos and Tasso, Physics of Plasmas 14, 122104 (2007)) for incompressible plasmas with field aligned flows is extended to gravitating plasmas including self-gravitation. It turns out that the stability condition is affected by gravitation through the equilibrium values only. A possible application of the condition in the framework of an idealized model could be the stability of the earth magma.   

Solid State Neutral Particle Analyzer in Current Mode on DIII-D Tokamak

A new three-channel solid state neutral particle analyzer (ssNPA) array, with the abilities of both active and passive charge exchange (CX) measurement, has been developed and successfully tested on the DIII-D tokamak. In active measurement mode, the three near-vertical viewing chords intersect with the footprint centerline of the closest near-tangential neutral beam at major radii of 1.50, 1.65, and 1.83~m; the outer and middle channels' sightline also cross the closest near-perpendicular neutral beam at major radii of 1.76 and 1.55~m, respectively. The inside channel is blocked for background detection during diagnostic commissioning stage. Directly deposited ultra-thin foils on the detector surface block stray photons below the energy of 1~keV and also bring about a 25~keV low energy threshold for deuterium particle detection. Operation of ssNPA in current mode provides an economical and simple approach for the diagnosing of the fast-ion distribution in energetic particle relevant experiments.   

Laboratory Measurements of Guide Field Reconnection

The presence of a guide field can significantly alter the dynamics of magnetic reconnection in laboratory and astrophysical plasmas. Experiments which vary the guide field from zero to several times the reconnecting field have shown that increasing guide field tends to reduce the reconnection rate. Contrarily, during sawtooth activity in fusion plasmas, reconnection occurs in the presence of very strong guide fields on very fast time scales. Here, we investigate guide field reconnection in the Magnetic Reconnection Experiment (MRX) and compare to previous measurements of reconnection in the Madison Symmetric Torus (MST) reversed field pinch. In MRX, guide field dynamics can be studied either by varying control currents within the primary drive circuits, or by specifically applying a guide field using a set of dedicated steady state guide-field coils. In MST fusion plasmas, reconnection occurs spontaneously and nonlinear effects dominate the plasma dynamics.   

Efficient Hybrid Methods for Coulomb Collisions in a Plasma

We report on a class of hybrid methods for the simulation of plasmas with Coulomb collisions, initially proposed by Caflisch et. al. These achieve significant efficiency at moderately small Knudsen number by combining a fluid solver to evolve the mostly dominant Maxwell part of the distribution function, and particle-in-cell and binary Monte-Carlo collision implementations to evolve the non-Maxwellian part of the distribution function. To represent the collisional interaction between the kinetic and Maxwellian components, particles are sampled from the fluid component and paired with the kinetic particles for collisions. Simulation particles must also be created at a rate that is sufficient to account for physical processes that drive the distribution function away from Maxwellian, and can be removed if collisions sufficiently drive the distribution function toward a Maxwellian. The performance these algorithms depends critically on the particular criteria for the exchange between the kinetic and fluid components and for the creation, destruction, and retention of the simulation particles.   

Microscopic instability and density limit in neutral magnetized plasmas

A microscopic model of a neutral magnetized plasma is proposed, which is a linearization of the classical one, where the electrons are considered to be subjected to Coulomb interactions among themselves and with a uniform positive neutralizing background. As no averaging over the individual particles is introduced, the model is dealt with as an actual many-body problem. Microscopic oscillatory modes are found, with wavelengths comparable to the mean interparticle distance. Such modes become unstable when the electron density exceeds a limit value proportional to the square of the magnetic field. When the full electromagnetic interactions are introduced in the model, dispersion relations are obtained which for long wavelengths reproduce the familiar ones of MHD while, for short wavelengths, reduce to those obtained in the purely Coulomb case. The density limit here found is of the same order of magnitude of the well known empirical Greenwald limit found in tokamaks.   

Current filaments in magnetized plasmas

We present direct experimental evidence of the presence of filamentary current structures in turbulent magnetized plasmas. Experiments have been performed in different devices. In the the reversed field pinch RFX-mod device, small scales turbulent intermittent structures, have been interpreted as Drift-Kinetic Alf\'en vortices, resulting from the non-linear coupling of drift and Kinetic Alfv\'en waves, with a bipolar current filaments associated to a vorticity perturbation. In thee ASDEX Upgrade tokamak evidences of monopolar current filaments travelling in the SOL, have been observed in correspondance with type-I ELMs. An evaluation of the current carried by individual ELMs is presented. Finally preliminary direct measurements of the 2D structure of the blob-induced parallel current using magnetic probes, as obtain in the simple magnetized plasma TORPEX, will be presented.   

Laser pulse bandwidth broadening via stimulated forward Brillouin scattering

One of the major limitations for implementing direct drive on the National Ignition Facility is the reduced bandwidth (220 GHz) now available for beam smoothing purposes. A bandwidth of around 1 THz is needed to reduce the Brillouin backscattering growth rate by 30\% for typical NIF parameters [1]. Here we present a novel method to increase the bandwidth of the incoming laser pulse via explicit stimulation of the Brillouin forward scattering process. A similar process has already been demonstrated for stimulated Raman forward scattering [2]. The incoming laser beam with frequency $\omega_0$ is accompanied by a low-intensity beam (1-2 \% of the main beam intensity) at the frequency of the first Stokes sideband for Brillouin forward scattering: $\omega_1 = \omega_0 - \omega_B$ with $\omega_B \sim 0.001\omega_0$. The beating between these beams drives the Brillouin forward scattering instability and causes several orders of (anti-)Stokes side bands to emerge, leading to an effective spectral broadening. In addition, the long wavelength ion-acoustic wave corresponding to Brillouin forward scattering will reduce the growth of the short wavelength ion-acoustic wave corresponding to Brillouin backscattering. [1] B. Brand\~ao, L.O. Silva, J.E. Santos, R. Bingham, Phys. Plasmas, submitted (2010). [2] R. Trines et al., Europhys. Lett. 66, 492 (2004).   

Anomalous Plasma Heating at the Center of ITER due to the Two Plasmon, (Gould-Trivelpiece Modes), Turbulence

The parametric excitation of two Gould-Trivelpiece modes with gyrotrons in electron cyclotron frequency\footnote{R. Prater et. al. \textit{Nucl. Fusion }48, No 3 (March 2008).}$^,$\footnote{V. Alexander Stefan, Bulletin APS-DPP, 2009; 2008.} domain is studied. The most powerful G-T modes counter propagate at the angle of 45$^{0}$ with respect to the toroidal magnetic field. Around 10 {\%} of the G-T modes energy is convected \footnote{M. N. Rosenbluth, \textit{Phys. Rev. Letters}, \textbf{29}, 565 (1972).}away toward the plasma edge, whereby it is dissipated via Landau damping. This generates suprathermal electrons and dragged by them accelerated ions. Based on a weak parametric turbulence theory\footnote{V. Alexander Stefan, \textit{Nonlinear Electromagnetic Radiation Plasma Interactions}, (S-U-Press, 2008).} the gyrotron dissipation rate is evaluated, showing strong bulk heating at the ITER center. The energy confinement time, in terms of gyrotron intensity, scales as I$_{o}^{-4/3}$.   

Electrical control of nanostructures synthesis in the arc discharge

Arc synthesis of carbon nanotubes (CNT) is characterized by excellent production rate and high quality of synthesized nanotubes superior to many other methods. Our recent findings indicate that the plasma parameters play an important role in the CNT growth process. In this work we develop and apply the state-of-the-art single electrostatic probe technique for conditions of the anodic arc. The probe is equipped with fast electrically-controlled shutter able to operate on the millisecond time scale in order to prevent fast deposition of the probe and uncontrollable growth of collecting area. Two stages of the discharge are observed.~During the initial stage, when anode is cold (about second after arc initiation), the arc is supported by the cathode jets, and conventional plasma V-I characteristic of single probe is observed (with electron saturation current significantly exceeding the ion saturation current). Later, when the anode is sufficiently hot, the discharge is switched to the mode when arc is supported by the anode ablation. The saturation currents of positive and negative charge carriers of the single probe V-I characteristic are close~during this stage of arc. Currents detected by the probe at this stage are associated with various carbon nanostructures produced by the arc. Electrical charging of nanostructures by the discharge plasma enables possibility of in-situ electrical control of nanostructures~synthesis in the arc discharge.   

Two and Three Dimensional Electron-scale Structures in Collisionless Magnetic Reconnection

The development of two and three dimensional electron scale structures in collisionless magnetic reconnection is investigated using electron-magnetohydrodynamic simulations. In 2D, the Hall magnetic field develops nested structure of quadrupoles in the presence of multiple reconnection sites due to the interaction of inflow to the secondary site and outflow from the central dominant site. In the outflow regions, the current sheet (CS) gets bifurcated and then filamented limiting the length of the central reconnecting CS while at reconnection sites, triple peak structures form. In 3D, the X and O-points alternate in the third direction every half wavelength of the unstable mode (along the third direction) driving the reconnection. This leads to the formation of a curved X-line as the reconnection takes place in a current sheet which is undulating along the third direction with a period equal to the wavelength of the unstable mode. These structures have important implications for multi-spacecraft missions in Earth's magnetotail.   

Excitation of ion waves by charged dust beams in ionospheric aerosol release experiments

Ion waves may be excited by charged dust beams streaming across or along the geomagnetic field in the ionosphere during aerosol release experiments. The injection speed of the dust and gas is comparable to or larger than the ion thermal speed in the background plasma. The dust grains get charged by plasma collection from the ambient ionosphere, and can thus act as a heavy charged particle beam that excites instabilities in the background plasma. Wave frequencies larger than the ion gyrofrequency are considered, and collisions with neutrals are included. The theory is applied to relatively early times scales on the order of 0.1 -- 1 seconds in the dust-gas cloud expansion. Further issues will be explored, including the effects of photoemission on dust charging under daytime conditions, and possible instabilities associated with the generation of ion beams due to charge exchange of the exhaust neutrals with ambient ions.   

Thermal Stability Studies of Ignitor Plasmas

Conditions for the operation of a tokamak fusion reactor are studied considering a fiducial operating state and analyze variations about this state. We use a volume averaged 0-D two-temperature model. The variation of parameters such as the fusion power, the energy gain and the auxiliary heating power are represented in POPCON plots of density versus electron temperature, in order to determine the optimal operation point. We study the case of an ignited plasma with the characteristics of the Ignitor experiment [1]. Then the behavior of the burning plasma under the thermonuclear instability is studied, which may arise in some temperature ranges. The stability is analyzed by finding the eigenvalues of the equations resulting from a variation of the electron and ion temperatures using a linear approximation. For negative eigenvalues the system is stable but if a single eigenvalue is negative then the stability appears. We find that the thermonuclear instability may appear and the unstable regions in temperature and density are determined. Results are given by showing contours of marginal stability in POPCON plots.\\[4pt] [1] B. Coppi, et. al., Nuclear Fusion 41, 1253 (2001).   

The DC bias in complex plasma experiments involving thermophoresis

In recent complex plasma experiments involving thermophoresis at the Center for Astrophysics, Space Physics and Engineering Research (CASPER), it was observed that the natural DC bias on the powered lower electrode in a modified GEC cell changes with the applied temperature on the lower electrode. The levitation height of dust particles suspended above this electrode was observed to differ in these experiments from the levitation heights obtained employing a fixed DC bias. Using electronic data, Langmuir probe data in the plasma bulk, as well as passive optical emission data in the sheath, it is shown that the plasma characteristics, involving the real and imaginary part of the discharge impedance, change with the electrode temperature. These results, together with results from a self-consistent plasma fluid model in argon, show that the neutral gas density changes with electrode temperature, when running at constant pressure, due to the heating of the gas in the volume. It is therefore important to report the DC bias value in complex plasma experiments involving thermophoresis rather than simply reporting the value obtained at room temperature.   

Multi-keV X-ray Yields from High-Z Targets Fielded at the OMEGA Laser and the National Ignition Facility

We report on measurements and modeling of fluxes from X-ray source targets recently shot at the National Ignition Facility (NIF) and at the Omega laser. The targets were thin-walled pipes filled with mixtures of Xe and Ar gas at pressures of 1 to 1.5 atmospheres. The targets were irradiated with 3$\omega $ laser light, 20 kJ in 1 ns at Omega and 350 kJ in 5 ns at NIF. The emitted X-ray flux was monitored with multiple channels of X-ray-diode based DANTE instruments, and imaged with gated X-ray detectors. We compare predicted X-ray yields to measure yields. The current modeling appears to under-predict the yield of gas mixtures containing Ar. We also report on design and modeling of Fe foam-filled and stainless steel-lined targets.   

Study of Astrophysical Magnetorotational Instability in a Couette-Taylor Flow of Polymer Fluids

We report in detail a visco-elastic instability from experimental observations and numerical simulations of a Couette-Taylor flow of a polymer fluid in a narrow gap between two rotating concentric cylinders with a Kepelerian-like velocity profile, where the angular velocity decreases radially outward while the specific angular momentum increases radially outward. Under the considered parameters, the inertial Rayleigh instability and the purely elastic instability are not possible. It is proposed that this observed instability is analogous to the magnetorotational instability which plays a fundamental role in astrophysical Keplerian accretion discs.   

Terahertz radiation in an ultrashort-laser-induced discharge plasma in air

Radiation sources in the microwave to terahertz (THz) spectral region are of great interest, because of their potential applications as diagnostics to study properties of dielectric materials as well as ultrafast chemical and biological processes. Plasma-based radiation sources have the advantage of being frequency tunable, and of producing an ultrashort, high-power pulse without breakdown. We have demonstrated that the terahertz radiation can be generate by the burst current produced by a laser created ionization front, which is induced an optical-field-induced ionization (OFI) in air with a pulsed electric field. The peak frequency of the radiation spectrum depends on the rise time of the OFI. The central frequencies of the radiation are observed to be 0.2 and 0.04 THz at the pulse durations of 50 and 300 fs (FWHM), respectively. The power of the linearly polarized THz radiation is linearly increased with the square of the electrostatic field strength applied to the capacitors.   

Simulation of sputter deposition in dc magnetrons

Material sputter deposition has a multitude of industrial applications. Our goal at FAR-TECH, Inc., is a complete numerical simulation of a dc magnetron device. We intend to modify existing FAR-TECH, Inc. code to include flexible geometry manipulation, the most current atomic physics data, add transport of neutral atoms across the device, and model deposition on the substrate. Currently, dc magnetron simulation codes have limited geometry manipulation capabilities; however, this is important if design optimization is intended. Another uncommon feature in dc magnetron simulation codes is parallel performance. Since PIC simulations may take extremely long times (weeks), we are parallelizing our codes to achieve shorter run times. (Codes based on hybrid models perform faster, but have the disadvantage of having to know accurately the diffusion coefficients of electrons across the magnetic field lines.) We report preliminary results of this effort.   

The Onset of Inflation and the Effects of Seed Pulses on SRS

Using the PIC code OSIRIS, we performed 1D simulations with a continuous pump and a counter-propagating seed to study the onset of kinetic inflation in backward Stimulated Raman Scattering (SRS) for ICF-relevant plasmas. When we used a continuous seed, we found that the onset of inflation was sharply determined by the pump intensity and the seed wavelength. We performed further simulations using seed pulses of varying wavelength, intensity, and duration. When using pulses of short duration we observed a linear gain spectrum similar to that predicted by linear theory but slightly blue-shifted. This blue-shift is partly explained by relativistic and finite-size particle effects. We observed inflation-like behavior of the pump from the residual plasma wave created by the seed a sufficient time after the seed exited the system. As we increased the seed duration, the inflation-like behavior began to occur while the seed was still in the system.   

FLASH Capabilities, Architecture, and Future Directions

FLASH is a publicly available, high-performance, modular, extensible Eulerian hydrodynamic code. It has been used for simulating Type Ia supernovae, X-ray bursts, turbulence, cosmology and other applications. FLASH consists of inter-operable modules that can be combined to generate different applications. This flexible architecture allows many interchangeable co-existing alternative implementations of its components. Further, a simple mechanism exists for customization of code functionality without the need to modify the core implementation. A collection of unit and regression tests provides verifiability, and a rigorous software maintenance process enables efficient code distribution and management. Recently, we have been expanding the capabilities of FLASH to include high energy density physics and fluid structure interactions. In this poster, we present various directions for the growth of FLASH in the near and distant future, and its architectural features that make such expansion possible.   

Low Ion Velocity Slowing Down in a Demixing Binary Ionic Mixture

We consider ion projectile slowing down at low velocity Vp $<$ Vthe, target electron velocity, in a strongly coupled and demixing hydrogen-helium ionic mixture of mostly astrophysical concern. It is investigated in terms of quasi-static and critical charge-charge structure factor [1]. Non-polarizable as well as polarizable and partially degenerate electron backgrounds are successively given attention. The focussed low ion velocity slowing down [2] turns negative in the presence of long wavelength and low frequency hydrodynamic modes,thus signaling a first order critical demixtion. Such a process actually documents an energy transfer from target ion plasma to the incoming ion projectile, i.e a superelastic process. \\[4pt] [1] D.Leger and C.Deutsch, PRA 37, 4916, 4930 (1988)\\[0pt] [2] B.Tashev et al, PoP 15, 102701(2008)   

Spatial resolution test of a beam diagnostic system for DESIREE

A diagnostic system based on the observation of low energy ($\sim $10 eV) secondary electrons (SE) produced by a beam, striking a metallic foil has been built to monitor and to cover the wide range of beam intensities and energies for Double ElectroStatic Ion Ring ExpEriment [1,2].The system consists of a Faraday cup to measure the beam current, a collimator with circular apertures of different diameters to measure the spatial resolution of the system, a beam profile monitoring system (BPMS), and a control unit. The BPMS, in turn, consists of an aluminim (Al) foil, a grid placed in front of the Al foil to accelerate the SE, position sensitive MCP, fluorescent screen, and a CCD camera to capture the images. The collimator contains a set of circular holes of different diameters and separations ($d)$ between them. The collimator cuts out from the beam areas equal to the holes with separation $d$ mm between the beams centers and creates well separated (distinguishable) narrow beams of approximately same intensity close to each other. A 10 keV proton beam was used. The spatial resolution of the system was tested for different Al plate and MCP voltages and resolution of better than 2 mm was achieved. Ref.: 1. K. Kruglov \textit{et al}., NIM A 441 (2000) 595; 701 (2002) 193c, 2. MSL and Atomic Physics, Stockholm Univ.(www.msl.se, http://www.atom.physto.se/Cederquist/desiree{\_}web{\_}hc.html).   

Attosecond X-ray pulse generation from coherent relativistic nonlinear Thomson scattering of high power femtosecond laser with a nanowire array

The attosecond pulse generation from the relativistic nonlinear Thomson scattering (RNTS) was studied in a single electron case [K. Lee et al., Phys. Rev. E 67, 026502 (2003)]. However, an actual electron source (solid, plasma or electron bunch) includes many electrons which show collective behaviors different from single electron motion. Even though each electron in the source radiates attosecond pulses, the whole of the electrons radiates much longer pulse if the attosecond pulses are not coherent. In this presentation, nanowire array and mirror reflection scheme are proposed as a coherent condition and are verified by a series of particle-in-cell simulation. Even though a thin film target also satisfies the mirror condition, a nanowire array target is chosen because strong static field between charged particles in the thin film target disturbs coherence. The case of the thin film target is also studied for a comparison.   

The Greatest Physics Circus on Earth!

Scientists within the STEM (and particularly, the plasma) communities have long been active in the development of programs reaching out to levels K through 12th in an attempt to spark interest in science, technology, engineering and mathematics and the professional and financial rewards such fields bring. The CASPER Physics Circus (funded for the past eleven years by a grant from the United States Department of Education) is designed to not only attract students into a STEM field but also to act as a launching pad into other CASPER opportunities such as its High School Scholars program, REU/RET programs and undergraduate / graduate programs. At each step along this ``seamless'' pathway, CASPER Fellows work alongside research scientists, technicians and faculty as a valued member of an integrated research team. Specific results, curriculum and assessment data will be discussed.   

How large can the electron to proton mass ratio be in particle-in-cell simulations of unstable systems?

Particle-in-cell simulations are widely used as a tool to investigate instabilities that develop between a collisionless plasma and beams of charged particles. However, even on contemporary supercomputers, it is not always possible to resolve the ion dynamics in more than one spatial dimension with such simulations. The ion mass is thus reduced below 1836 electron masses, which can affect the plasma dynamics during the initial exponential growth phase of the instability and during the subsequent nonlinear saturation. The goal of this talk is to assess how far the electron to ion mass ratio can be increased, without changing qualitatively the physics. A criterion allowing to define a maximum ratio is explicated in terms of the hierarchy of the linear unstable modes. The criterion is applied to the case of a relativistic electron beam crossing an unmagnetized electron-ion plasma [1]. \\[4pt] [1] Bret A. and Dieckmann M., Phys. Plasmas, \textbf{17}, 032109, (2010)   

Exact relativistic kinetic theory of the full unstable spectrum of an electron-beam-plasma system with Maxwell-Juttner distribution functions

We present a detailed report of the entire unstable $\mathbf{k}$ spectrum of a relativistic collisionless beam-plasma system within a fully Maxwell-Juttner kinetic framework. The three competing classes of instabilities, namely, two-stream, filamentation, and oblique modes, are dealt with in a unified manner, no approximation being made regarding the beam-plasma densities, temperatures, and drift energies. We investigate the hierarchy between the competing modes [1], paying particular attention to the relatively poorly known quasielectrostatic oblique modes in the regime where they govern the system. The properties of the fastest growing oblique modes are examined in terms of the system parameters and compared to those of the dominant two-stream and filamentation modes [2]. \\[4pt] [1] Bret A., Gremillet L., B\'enisti D. and Lefebvre E., Phys. Rev. Lett, \textbf{100}, 205008, (2008)\\[0pt] [2] Bret A., Gremillet L. and B\'enisti D, Phys. Rev. E, \textbf{81}, 036402, (2010)   

Nonlinear kinetic description of Raman growth using an envelope code, and comparisons with Vlasov simulations

Using a nonlinear kinetic analysis, we provide a theoretical description for the nonlinear Landau damping rate, frequency, and group velocity of a slowly varying electron plasma wave (EPW). In particular, we show that the nonlinear group velocity of the EPW is not the derivative of its frequency with respect to its wave number, and we discuss previous results on the nonlinear Landau damping rate and on the nonlinear frequency shift of the EPW. Our theoretical predictions are moreover very carefully compared against results from Vlasov simulations of stimulated Raman scattering (SRS), and an excellent agreement is found between numerical and theoretical results. We use the previous analysis to derive envelope equations modeling SRS in the nonlinear kinetic regime. These equations provide very accurate predictions regarding threshold intensities for SRS and the growth time of SRS beyond threshold, provided that one uses the ansatz of self-optimization that we detail. Finally, we discuss saturation of SRS and, in particular, we derive growth rates for sidebands using a spectral method.   

Theoretical shapes of L$\alpha _{1 }$X-Ray Satellites spectra of $_{40}$Zr, $_{42}$Mo, $_{44}$Ru, $_{46}$Pd and $_{48}$Cd for lead as predicted by HFS calculations.

The X-ray satellite spectra arising due to 2p$_{3/2}^{-1}$3x$^{-1}$-3x$^{-1}$3d$^{-1}$ (x $\equiv $ s, p, d) transition array, in elements with Z = 40 to 48, have been calculated, using available HFS data on 1s$^{-1}$-2p$^{-1}$3x$^{-1}$ and 2p$_{3/2}^{-1}$-3x$^{-1}$,3x'$^{-1}$ Auger transition energies. The relative intensities of all the possible transitions have been estimated by considering cross - sections for the Auger transitions simultaneous to a hole creation and then distributing statistically the total cross sections for initial two hole states 2p$_{3/2}^{-1}$3x$^{-1}$ amongst various allowed transitions from these initial states to 3x$^{-1}$3d$^{-1}$ final states by CK and shake off processes. The calculated spectra have been compared with the measured satellite energies in L$\alpha _{1}$ spectra. Their intense peaks have been identified as the observed satellite lines. The peaks in the theoretical satellite spectra were identified as the experimentally reported satellites $\alpha _{3}$, $\alpha _{4}$ and $\alpha _{5}$, which lie on the high-energy side of the L$\alpha _{1}$ dipole line. On the basis of agreement between the computed spectra and measured satellites, it is observed that the satellite $\alpha _{3}$ is observed due to intense transition, $^{3}$F$_{4}-^{3}$F$_{4}$, in order of decreasing contribution of intensity. It has been found that the transition $^{1}$F$_{3}-^{1}$G$_{4}$ is the main source of the emission of the satellite $\alpha _{4 }$in the elements $_{42}$Mo to $_{48}$Cd. The line $\alpha _{5}$, observed in the spectra of elements with Z = 40-48, has been assigned to the $^{3}$D$_{3}-^{3}$F$_{4}$, $^{3}$D$_{2}-^{3}$F$_{3}$, $^{1}$P$_{1}-^{1}$D$_{2}$ and $^{1}$F$_{3}-^{1}$D$_{2}$ transitions.   

Laser driven fast electron collimation in targets with resistivity boundary

Fast Ignition of inertially confined fusion targets allows high energy gain factors to be achieved for comparably modest total input energy. We will discuss experimental results demonstrating that the relativistic electron beam in a dense plasma can be efficiently confined to 50$\mu $m radii and guided in structured targets irradiated by a laser pulse of about 1ps duration at an intensity of 10$^{20}$W/cm$^{2}$.   

Development of Azimuthal Asymmetries in $\Theta $-pinch FRC Preionization: Simulation vs. Experiment

Over the next few years at AFRL, FRCs will be formed, translated into and captured in imploding liners, and compressed to fusion conditions to investigate magnetized plasma compression. Recent experiments have shown smaller differences between vacuum and plasma shot magnetic probe signals than predicted by MACH2 2-d r-z simulation, implying smaller FRC radii and entrained mass. Understanding the causes of this apparent mass loss could lead to making substantially more robust FRCs. MACH2 2-d r-$\theta $ simulations show gross asymmetries developing during the multiple $\Theta $-pinches of the preionization phase caused by the magnetic Rayleigh-Taylor instability. The asymmetric plasma configurations in these simulations strongly resemble axial-view visible light images from previous preionization experiments on FRCX at Los Alamos. In the simulations, plasma flows to the wall and could be lost there. These simulations strongly suggest that 3-d simulations will be necessary for complete understanding of the full process, and they are, in part, preparation for those.   

Computational Support for FRCHX: Pre-shot and Post-Shot Interactions

Over the next few years, Los Alamos National Laboratory and the U. S. Air Force Research Laboratory Directed Energy Directorate will form, translate, capture, and compress a field reversed configuration (FRC) of magnetized deuterium plasma using an imploding solid liner to achieve magnetic fields more than 10$^{6 }$times that of the Earth and plasma pressures of 10$^{6 }$atmospheres. These experiments require multiple pulsed power events before FRC compression: formation of the FRC and its translation to and capture in a collapsing magnetic cavity. The FRC must be robust enough to have a long lifetime and yet be small enough to translate quickly and to enter the collapsing magnetic cavity. In early 2010 the team performed over 100 formation, translation, capture (FTC) experiments with a stationary liner and the first complete experiment with an imploding liner (FRCHX) in April. Working with the experimental team, NumerEx has performed integrated simulations of the FTC and FRCHX experiments to aid in the design of both, and to improve the simulations' fidelity. We will present comparisons of measurements from the simulations and experiments, as well as pre- and post-experiment analysis of the first FRCHX.   

Laser-plasma electron acceleration and X-ray production in capillary tubes

The dynamics of electron acceleration and X-rays production in capillary tubes are investigated numerically. An ultra-intense ($0.77 \times 10^{18}$ W/cm$^2$) and ultra-short (35 fs) laser pulse can be guided in cm long plasmas with electron densities of the order of $5 \times 10^{18}$cm$^{-3}$. Self-injected electrons are accelerated in the plasma wake, and undergo betatron oscillations giving rise to X-rays emission. We performed cm-long quasi-3D PIC simulations with CALDER-CIRC. The capillary tube was implemented through a non-uniform dielectric function. X-ray production was calculated a posteriori from electron trajectories by classical formulas. We compare numerical results with experimental ones. For cm long propagation, multiple electrons injection and acceleration are observed. The X-ray emission at the output of the tube is calculated in the conditions of the experiment. The effects of the plasma density and of the capillary edge on the X-ray beam divergence are discussed.   

High-frequency eigenmodes of a coaxial wave guide containing a relativistic annular electron beam with a coaxial wiggler

An analysis of the high-frequency eigenmodes of a coaxial wave guide containing a magnetized annular plasma column with a one-dimensional coaxial wiggler is presented. A transcendental equation is derived from the boundary conditions in the form of an eighth-order determinant equated to zero. Simultaneous solution of this determinantal equation and a polynomial equation derived from the wave equation yield the dispersion relations for the eigenmodes. By reduction of the order of the determinant the appropriate transcendental equation is easily obtained for some special cases, e.g., conventional wave guide containing an annular plasma column under electrostatic approximation. Numerical solutions are obtained for space-charge modes, and cyclotron modes. This treatment shows that dispersion curves are dependent on $d\gamma /dt$ and $B_w $, ignored in previous works. The difference between relativistic modes with effect of wiggler field and nonrelativistic cases is shown. This study is benefiting to improve the devices for generation of high-power electromagnetic radiation, charged particle acceleration, and other applications of plasma waveguide.   

New generation of medium wattage metal halide lamps and spectroscopic tools for their diagnostics

A new generation of ceramic metal halide high intensity discharge (HID) lamps has achieved high efficiencies by implementing new design concepts. The shape of the ceramic burner is optimized to withstand high temperatures with minimal thermal stress. Corrosion processes with the ceramic walls are slowed down via adoption of non-aggressive metal halide chemistry. Light losses over life due to tungsten deposition on the walls are minimized by maintaining a self-cleaning chemical process, known as tungsten cycle. All these advancements have made the new ceramic metal halide lamps comparable to high pressure sodium lamps for luminous efficacy, life, and maintenance while providing white light with high color rendering. Direct replacement of quartz metal halide lamps and systems results in the energy saving from 18 up to 50{\%}. High resolution spectroscopy remains the major non-destructive tool for the ceramic metal halide lamps. Approaches to reliable measurements of relative partial pressures of the arc species are discussed.   

Low Power Driving Inertial Fusion Energy Concept

Main stream deuterium-tritium fusion energy schemes are confronted with serious technical challenges such as first wall material, energy extraction, etc., due to the high energy neutron flux. Low power driving inertial fusion energy concept takes full advantage of the energy concentration mechanism of implosion, using large radius, room temperature target and low but high total energy driving power to achieve fusion energy production. The implosion driving medium acts also as the first wall and the energy absorbing blanket. A high speed water jet driving inertial fusion energy concept is discussed.   

Microdroplet target synthesis for kilohertz ultrafast lasers

The difficulty of finding suitable solid targets for high rep-rate (up to kHz) lasers has been one of the major setbacks for the applicability of solid-target laser interaction experiments. In this work, we have developed a method for producing spatially stable micron-scale liquid targets of flexible shapes at kHz repetition rate for use in air and vacuum, by perturbing 5 and 30-$\mu $m diameter streams with fs laser pulses and monitoring the temporal development of the perturbation. Using water, we have produced features such as 2.1-$\mu $m diameter droplet and 1.3-$\mu $m diameter neck with less than $\pm $0.3-$\mu $m shot-to-shot variation, with prospects for further reduction in size and variability. The use of such micron-scale targets can be expected to prevent conductive heat dissipation, thus increasing the interaction temperature by more than two orders, enhance field strength for ion acceleration and allow spatially-deterministic laser-cluster experiments.   

A dynamic neutral fluid model for the PIC scheme

Fluid diffusion is an important aspect of plasma simulation. A new dynamic model is implemented using the continuity and boundary equations in OOPD1, an object oriented one-dimensional particle-in-cell code developed at UC Berkeley. The model is described and compared with analytical methods given in [1]. A boundary absorption parameter can be adjusted from ideal absorption to ideal reflection. Simulations exhibit good agreement with analytic time dependent solutions for the two ideal cases, as well as steady state solutions for mixed cases. For the next step, fluid sources and sinks due to particle-particle or particle-fluid collisions within the simulation volume and to surface reactions resulting in emission or absorption of fluid species will be implemented. The resulting dynamic interaction between particle and fluid species will be an improvement to the static fluid in the existing code. As the final step in the development, diffusion for multiple fluid species will be implemented. \\[4pt] [1] M.A. Lieberman and A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, 2nd Ed, Wiley, 2005.   

Ultrasonic Velocity, Viscosity and Refractive Index Investigation on Interacting Blend Solutions of PAA (Poly Acrylic Acid) and PVA (Poly Vinyl Alcohol) in Solvent DMSO (Di methyl Sulphoxide)

The present study provides a great insight into the major new research areas like Plasma research (which is yielding a greater understanding of the universe) and Nano Technology Research (which provides many practical uses like Drug Delivery System). The Ultrasonic Velocities, Viscosities and Refractive indices of Poly (Acrylic Acid) and Poly (Vinyl Alcohol) blends in DMSO solutions have been measured over a wide range of composition, concentration and at different temperatures. The variation of Ultrasonic Velocity, derived acoustical parameters, adiabatic compressibility, acoustic impedance, Rao number, molar compressibility and relaxation strength with composition of blend solution was found not linear. This non-linearity has been attributed to incompatibility in conformity with the earlier findings. This behavior was confirmed by Viscometric and interaction parameters studies, as well as by investigation of Refractive index studies. These investigations offer an entirely new and simple approach to the study of the compatibility of polymer blends which is in general obtained by sophisticated techniques of thermal dynamic mechanical and electron microscopic analysis.   

An overview of plasma-in-liquid experimental studies at the University of Michigan's Plasma Science and Technology Laboratory

Plasma production or plasma injection in liquid water affords one the opportunity to nonthermally inject advanced oxidation processes into water for the purpose of sterilization or chemical processing. Limitations of current injection approaches include limited throughput capacity, electrode erosion, and reduced process volume. Currently we are investigating two potential approaches to circumventing these issues. These include direct plasma injection using an underwater DBD plasma jet and the direct excitation of underwater isolated bubbles via a pulsed electric field. Presented here are results from these ongoing tests, which include a comparative study of the effectiveness of microdischarge, and plasma jet direct injection approaches on the decomposition of Methylene Blue dye. Additionally, an approach to excitation of isolated bubbles using pulsed electric fields is also discussed. Streamer propagation dynamics such as surface propagation and the observed excitation of surface waves on electrode-attached and free bubbles are also discussed.   

On the Meaning if Imaginary Part of Solution of Biquaternion Klein-Gordon Equation

In a preceding article we argued that biquaternionic extension of Klein-Gordon equation has solution containing imaginary part, which differs appreciably from known solution of KGE. In the present article we discuss some possible interpretation of this imaginary part of the solution of biquaternionic KGE (BQKGE). Further observation is of course recommended in order to refute or verify this proposition.   

Rayleigh-Taylor Instability Growth Control in HIF

Uniformity of heavy ion beam (HIB) illumination is one of key issues in HIB inertial confinement fusion (HIF): deviation from fuel implosion symmetry should be less than a few percent in order to compress a fuel sufficiently and release fusion energy effectively. In this paper a new mitigation method of the Rayleigh-Taylor (R-T) instability growth is presented in order to make a HIF target robust against a non-uniform implosion. In this study a new mitigation method of the R-T instability growth is proposed based on an oscillating perturbed acceleration, which can be realized by a rotating or oscillating HIB illumination onto a fuel pellet. The R-T instability analyses and fluid simulations demonstrate that the oscillating acceleration reduces the R-T instability growth significantly. In this paper a baseline steady acceleration g is perturbed by a perturbed oscillating acceleration g1, which is spatially non-uniform and oscillates in time (g $>>$ g1 ). An example result shows 84{\%} reduction of the R-T instability growth.   

Shock Ignition campaigns on OMEGA: comparison with experiments and future perspectives

Recently, Shock Ignition (SI) scheme has prompted both theoretical studies [1,2] and experimental campaigns [3]. SI experiments have been carried out at the OMEGA Laser Facility [4] in spherical geometry to study performances at intensities relevant to shock ignition. First restitutions from CHIC hydrodynamic calculations and PIC simulations allow a deeper understanding of critical parameters as implosion symmetry, shocks propagation, neutron production and light reflectivity. The analysis of results suggests several improvements of the pulse shape and target design for future experiments. \\[4pt] [1] R. Betti et al., Phys. Rev. Lett. \textbf{98}, 155001 (2007) \\[0pt] [2] M. Lafon et al., Phys. Plasmas \textbf{17}, 052704 (2010) \\[0pt] [3] W. Theobald et al., Phys. Plasmas \textbf{15}, 056306 (2008) \\[0pt] [4] W. Theobald\textit{, Private communication }(2010)   

Modeling Vacuum Arcs

We are developing a model of vacuum arcs to describe vacuum breakdown in 805 MHz systems, however the basic mechanisms at work should apply to other applications. The model assumes: 1) that arcs develop as a result of mechanical failure of the surface due to Coulomb explosions, 2) this is followed by ionization of fragments by field emitted currents and 3) the development of a small, dense plasma that interacts with the surface primarily through self sputtering and field emission, and 4) the plasma terminates as a unipolar arc capable of producing breakdown sites with high enhancement factors. We have attempted to produce a self-consistent picture of triggering, arc evolution and surface damage. We model these mechanisms using Molecular Dynamics (mechanical failure, Coulomb explosions, self sputtering), Particle-In-Cell (PIC) codes (plasma evolution), mesoscale surface thermodynamics (surface evolution), and finite element electrostatic modeling (field enhancements). We will present a variety of numerical results and identify where our model differs from other descriptions of this phenomenon.   

Spectroscopic Study of Initial Stage of Pulsed DC Discharges in Noble Gases

The relative intensities of spectral lines at 419.8 and 420.1 nm near the beginning of a pulsed dc glow discharge in Ar were measured by optical emission spectroscopy. The direct excitation cross-sections to the excited state energy level in the 3p manifold, 3p5 and 3p9 respectively, from ground state Ar atoms are very similar for the two transitions. However the 3p9 excited state level is also populated by and thus sensitive to the Ar metastable state density. Due to this, the intensity ratio of the two spectral lines can serve as a convenient probe of the relative metastable and electron densities in a discharge. Measurements of the spectral line intensity ratios, as a function of time, changes significantly near the beginning of the dc pulse. Depending on the reduced electric field, the fast growth of electron density can occur before or after the growth of metastable density. These spectral measurements can be used as a simple probe of the plasma species densities. Data will also be presented on similar investigations conducted in Ne and Xe.   

Comparative calculations of plasma ionization balance, collisionality and resistivity using various models in application to z-pinch physics

High energy density plasmas produced by the imploding wire array loads, including single- and multi-planar wire arrays, has been extensively studied for the past few years at the University of Nevada, Reno at 1.7 MA Zebra facility. Various modeling tools such as the magnetohydrodynamic (MHD) codes and non-LTE atomic kinetic models have been applied to analyze plasma dynamics and radiation features. In this work the results of the aforementioned models are compared with the average atom model (Thomas--Fermi and Ziman approximations). The analysis is accomplished for low (Al) and moderate (Cu) atomic number elements in broad ranges of T$_{e}$ and n$_{e}$. The advantage of application of such approach to the analysis of z-pinch experiments is discussed.   

Relationship between ELM size and its transport speed through the scrape-off layer in NSTX

Edge localized modes (ELMs) are of concern for future devices because they damage plasma facing components, due to high particle and heat fluxes. ELMs are believed to release energy in a narrow poloidal layer near the outer midplane, where the magnetic field is weakest and the ballooning instability is strongest. Here we characterize transport of the ELM fluxes through the scrape-off layer (SOL), the region of open field lines just outside the magnetic separatrix. One way to quantify this is by measuring the in-out delay, the difference in time between the ELM flux reaching the outer and inner diverter strike points. This measure is correlated with the average speed of the ELM through the SOL. We compare these sppeds to various measures of the ELM magnitude, e.g. increase of D-alpha emission, and decrease of plasma density and stored energy. We find that as the size of an ELM increases, the average speed with which it travels through the SOL tends to increase, qualitatively consistent with an expectation that larger ELMs dump hotter particles into the SOL, yielding faster SOL transport rates.   

Non-LTE equation of state for simulations of laser-plasma interaction

In this work we present equation of state (EOS) for the free and bound electrons in non-LTE (Local Thermodynamic Equilibrium) conditions typical for laser-plasma interaction. In order to characterize the plasma we solve stationary rate equations for the populations of the bound electrons (since the Saha-Boltzman distribution is not adequate in non-LTE). For this purpose we use screened hydrogenic average atom approximation with l-splitting. Having found the bound state populations and ionization degree, we evaluate the pressure and internal energy. The bound electrons pressure is calculated using the quantum mechanical stress tensor formula and the free electrons are treated as free fermions. This electronic EOS is added to the cold and ionic terms of QEOS model and then implemented to a 1D hydro-radiative code. The simulation results are compared to those with other EOS models.   

Design of a flyer plate driven hydrodynamics experiment for Z

We present the preliminary design of a Z experiment intended to observe the growth of several hydrodynamic instabilities (RT, RM, and KH) in a high-energy-density plasma. These experiments rely on the Z-machine's unique ability to launch cm-sized slabs of cold material (known as flyer plates) to velocities of several times 10 km/s. During the proposed experiment, the flyer plate will impact a cm-sized target with an embedded interface that has a prescribed sinusoidal perturbation. The flyer plate will generate a strong shock that propagates into the target and later initiates unstable growth of the perturbation. The goal of the experiment is to observe the perturbation at various stages of its evolution as it transitions from linear to non-linear growth, and finally to a fully turbulent state.   

Electronic-Polarization Diagnostics of Anisotropic Electron Velocity Distribution Function

The electron velocity distribution function (EVDF) in anisotropic plasma has been investigated by both probe and magnetic-polarization Hanle techniques. In a helium beam-plasma discharge, the EVDF momentums have been measured. The depolarization of the 4$^{1}$D$_{2}$ -- 2$^{1}$P$_{1}$ helium atom spectral line was analyzed for a charged particle density of roughly 10$^{11}$ cm$^{-3}$ and the rate constant was determined for the misalignment of helium atoms in the 4$^{1}$D$_{2}$ state due to collisions with charged particles. A method for the remote investigation of anisotropic properties of plasma particles not readily amenable to contact diagnostics was tested experimentally. An advantage of the proposed method is the possibility of directly measuring the EVDF anisotropy in distant astrophysical and geophysical plasma objects. The experimental results and remote investigative technique will be presented.   

Toroidal flow: Increasing complexitiy in the interaction between fast particles and magnetohydrodymanical waves

In many tokamak experiments, neutral beam injection is used for additional heating, with the injected particles inducing a net rotation of the plasma. This rotation plays an important role in the stability of the system and in the interaction between the fast particles and the bulk plasma. However, in the next generation of tokamaks, the plasma rotatation is expected to be significantly slower and therefore the influence of rotation in present devices needs to be investigated and understood in detail. We present a fully consistent model of the bulk plasma, the fast particles and their interaction. The bulk is described by the MHD equations, whilst for the fast particles a kinetic description is used. The equilibria are computed using the FINESSE code and their stability is analysed using the PHOENIX code. Both codes take arbitrary toroidal flow into account. The HAGIS code, extended to consistently include flow, is used to simulate the fast particles. Model results for tokamaks with circular cross-section and JET-like equilibria show a complex dependence of the plasma stability upon the rotation.   

Development of X-ray imaging microscopes for LMJ

For the future Laser Megajoules French facility (LMJ), our laboratory develops time-resolved X-ray Imaging systems to diagnose laser produced plasma. In this presentation, we describe the design of these imagers which combine grazing X-ray microscope and camera. A first set of three imaging diagnostics will give basic measurements during all the life of the facility : two twelve-image microscopes focalize X-rays from the target on a framing camera. The third one produces an image on a streak camera. These microscopes also contain refractive lenses to extend the spectral range up to 15 keV. A second set of diagnostics will consist of advanced high resolution X-ray imaging systems. Imaging studies performed with a microscope composed of three concave toroidal mirrors are presented. This microscope, working at 0.6 degrees grazing incidence, has a focal length longer than 80 cm. About the imaging performances, we have achieved a spatial resolution of about 6 microns for the sagittal dimension and around 10 microns for the tangential dimension within a field of 1 mm. To increase the bandwidth of reflectivity of all these mirrors until 10 keV, multilayer coatings have been deposited.   

Theoretical design of an energy recovering divertor

An energy recovering divertor (ERD) is a device for converting thermal to electrical energy in the divertor channel of a tokamak. Because ERD's are a type of heat engine operating at plasma temperatures, they have the thermodynamic potential for extremely high efficiencies. An ERD offers several important benefits to a tokamak fusion reactor. First, any energy recovered by the ERD is subtracted from divertor heat load, thus circumventing materials limitations. Second, energy recovered by the ERD is available for auxiliary heating, thus allowing the reactor to break even at a lower Lawson parameter. Third, an ERD can be used to power auxiliary current drive, thus reducing dependence on bootstrap current. We will present a design for an ERD based on amplification of Alfven waves in a manner analogous to a free-electron laser. While its projected efficiency falls short of the thermodynamic potential for this class of device, it nonetheless demonstrates the theoretical viability of direct power conversion in a tokamak divertor. We will also present potential approaches towards higher efficiency devices of this type. Work supported by the U.S. DOE under grant DE-FG02-97ER54392.   

Temperature and Density Measurements in Low-Density, Laser-Driven Magnetized Plasmas Using Thomson Scattering

We present electron temperature and density data from Thomson scattering measurements on recent collisionless shock experiments on the Trident laser at Los Alamos National Laboratory. A graphite or CH target was placed inside a static magnetic field ($\sim1$ kG) created by a $50$ cm-diameter Helmholtz coil and was ablated by two sequential laser pulses at $1053$ nm. The first pulse created an ambient low-density, magnetized plasma while the second pulse created a super-Alfv\'{e}nic (M$_{A}$ $\sim10$) plasma to shock the ambient plasma. A seperate laser beam at $527$ nm was used for Thomson scattering to characterize the ambient plasma $3 - 19$ cm radially from the target and $0.5 - 9.7$ $\mu$s after the first ablation. The electron temperature was found to be $10 - 50$ eV and, combined with Rayleigh scattering, the electron density was found to be $10^{13}- 10^{15}$ cm$^{-3}$. Several carbon emission lines were also observed in the Thomson spectrum and were compared to FLYCHK simulations to characterize the ambient plasma charge state.   

Design and preliminary characterization of a micro-scale dense plasma focus

Experimental results are presented for a sub-millimeter dense plasma focus (DPF). Reducing the DPF from cm to $\mu $m sizes should allow for unique applications such as portable neutron based detectors. With smaller size efficiency is expected to increase, but total neutron emission will be lower. Challenges will be maintaining various scaling parameters and generating sub-ns rise time voltage pulses. Scaling suggests higher pressure operation though there may be limitations at high pressure in confinement due to increased collisions. Two DPF devices have been designed: a 100 $\mu $m radius and a 750 $\mu $m radius anode. A pulse generating system capable of up to 4 J/pulse with peak voltages of 20 kV and controllable voltage rise rates up to 20 kV/ns has also been assembled and tested. The DPF device will operate at pressures of 10-1000 Torr in hydrogen though initial experiments with the 750 $\mu $m indicate that trace air contamination significantly affects the pinch process and a UHV chamber is being assembled. Voltage, current, imaging, nanosecond time resolved optical emission spectroscopy and x-ray detection measurements have been used to diagnose the DPF device.   

Scaling Laws for Pellet Ablation in Tokamaks

We investigated by numerical simulations the effect of various physical parameters on the pellet ablation rate and analyzed the scaling laws. Our study on the non-rotational ablation cloud indicated that the ablation rate is a function of the toroidal magnetic field and the warm-up time. Our recent numerical simulation of the rotational ablation cloud confirmed the effect of the magnetic field, however, it showed that the ablation rate of a rotational cloud is insensitive to the warm-up time. The ablation rate has weak dependence on the effective longitudinal shielding distance. Although pellet penetration measurements from JET, ASDEX and Tore Supra seemed to be in good agreement with the NGS scaling laws, deviation from the NGS scaling laws has been reported by the ASDEX upgrade team [1]. We will present our systematic study of the effects of the physical parameters and their scaling laws. The validation of our MHD code against experimental data from DIII-D tokamak pellet launches will also be presented. The pellet lifetime and density profiles reproduced by simulations will be compared to the experimental measurements [2]. [1] K. Gal, et al, Nucl. Fusion, 48, 085005 (2008). [2] P.B. Parks and L.R. Baylor, Phys. Rev. Lett. 94, 125002 (2005).   

PIC simulations of beam energy enhancement by density gradient in a laser wakefield accelerator

Using a newly developed PIC code, we revisit the problem of density tapering to prolong the dephasing length and enhance the accelerated beam energy in laser wakefield electron accelerator. In the new code, all the standard schemes of PIC's were followed. The benchmark against the well-known PIC such as XOOPIC showed excellent agreement. We investigate the density gradient effect on beam energy increasing in a bubble regime of the LWFA. The wavelength shrinkage of the wakefield by the increasing plasma density helps phase-lock the beam and the accelerating field. From a series of simulations we found that, at least for the plasma frequency of order wp/w $\sim $ 0.05, the relativistic lengthening of the plasma wavelength tends to restore the dephasing between the beam and field very quickly. Even though we observed energy enhancement from 500 MeV to 800 MeV from 3 mm propagation for a parabolically increasing density, the relativistic effects seem to put a fundamental restriction on the energy enhancement by the density gradient.   

Low frequency magnetic fluctuation in a current sheet in plasma merging experiment

Low frequency magnetic fluctuation accompanied by magnetic reconnection has been investigated in the TS-3 torus plasma merging experiment. Strong magnetic fluctuations with frequency near the ion cyclotron range were observed within a current sheet at the initial phase of magnetic reconnection under a considerable guide magnetic field $B_{X} \sim B_{\parallel}$, the reconnecting magnetic field. A temporary drop of the guide field strength was observed just before the initiation of the fluctuation. However, the fluctuation disappeared when the guide field was increased to $B_{X} \sim1.5 B_{\parallel}$. Our results indicate that the excitation of the low frequency fluctuation strongly depends on the initial reconnection angle and the guide field reduction. The effect of the magnetic fluctuation on both the reconnection rate and ion heating will be presented.   

A Dispersion Free Methodology for Modeling Intense Charged Particle Beams

We show a novel dispersion free 3-D method for modeling the space-charge fields of intense charged particle beams in a circular conducting pipe. The dispersion free aspect of this method is obtained from the use of time-dependent Green's functions for computing the fields. This leads to highly accurate representations of time-dependent space-charge fields in intense beams, compared to those found when using traditional FDTD methods where typical numerical grid dispersion errors can be important. We show how this method compares to the FDTD method, and how it can be parallelized for high-performance computing.   

Super-Alfv\'{e}nic Magnetic Field Fluctuations Generated from Low-Density, Magnetized Laser-Plasma Expansions

In recent experiments at the Trident Laser Facility, at the Los Alamos National Laboratory (LANL), the three beam configuration and a pulsed Helmholtz coil were utilized to investigate laser-driven, magnetized shocked plasmas. The $56$ cm, $4.2$ kJ pulsed Helmholtz coil was used to create a $0.1-1.0$ kG magnetic field over an experimental volume of $\sim4\times10^{3}$ cm$^{3}$. Two sequential laser pulses, spaced $1.0-10.0$ $\mu$s apart, were used to ablate a CH or graphite target that was imbedded in the field. The first laser pulse created an ambient magnetized plasma and the second laser pulse created a debris plasma to shock the ambient plasma. The third laser pulse was frequency-doubled and employed for Thomson scattering measurements to characterized the ambient plasma density ($10^{13}-10^{15}$ cm$^{-3}$) and electron temperature ($10-50$ eV). An array of single-axis, $1$ mm b-dot probes were used to measure magnetic field compression, expulsion, and fast-diffusion inside and around the diamagnetic cavity formed by the laser-plasma expansion. A magnetic field compression pulse in the shocked plasma was observed to separate and propagate away from the leading edge of the diamagnetic cavity at an Alfv\'{e}nic Mach number on the order of $10$ (M$_{A}\sim10$).   

Radiobiology Using Laser Driven Protons

The advantage of using ion beams in radiotherapy is easily understood in terms of the Bragg peak effect if compared to widely used x-ray irradiation systems. There is therefore a large literature about cell irradiation using ions from conventional accelerators. Employing the TARANIS Terawatt laser at Queen's University, the effect of proton irradiation of biological cells, on timescales orders of magnitude shorter than with conventional accelerators, has been investigated. The laser driven MeV proton beam has been energy dispersed by using a magnetic system prior to the irradiation, allowing simultaneous irradiation of a number of cell spots with different doses on a ns timescale. Consistent lethal effects on V-79 cancer cells have been observed.   

Temporal Evolution and Cross-correlations in Spectral Parameters of GRB Prompt Emission

Prompt emission from gamma-ray bursts (GRBs) exhibits very rapid, complicated temporal and spectral evolution. This diverse variability in the lightcurves reflects the complicated nature of the underlying physics, in which inter-penetrating relativistic shells in the outflow are believed to generate strong magnetic fields that vary over very small scales. We simulate full GRB lightcurves and spectra, with the aforementioned assumptions. The framework is unbiased to any particular choice of radiation mechanism. The effects of various source geometries, viewing angles, and bulk Lorentz factor profiles of the radiating outflow jets on the spectral features and evolution of these light-curves are explored. We utilize the anisotropic jitter radiation mechanism, among others, as test cases, and extract spectral correlations as seen in observations. We report that even with a simplified model, we are able to duplicate certain features observed in the GRB prompt emission spectra, such as the occurrence of hard, synchrotron-violating spectra, the ``tracking'' of observed flux with spectral parameters, and spectral softening below peak energy within individual episodes of the light curve. We highlight predictions that can be made as a result of our simulations, in the light of recent advances in the observational sphere of GRB physics.   

Hot Electron Confinement in High Intensity Laser-Matter Interactions

High-intensity ($>$10$^{18}$ W/cm$^{2})$ lasers can produce relativistic electrons ($\sim $MeV) when focused onto solid density targets. We present measurements of escaped relativistic electron lifetimes in short pulse laser-irradiated solid experiments. Electron durations measured were significantly longer than the laser pulse length, suggesting the presence of phenomena which confine high energy electrons within the target-plasma volume. Investigating the confinement time of high energy electrons exceeds the limits of any simple plasma expansion models. Utilizing the implicit hybrid particle-in-cell code LSP [D. R. Welch \textit{et al.}, Phys. Plasmas \textbf{13}, 063105 (2006)], experimental conditions were simulated to explore the physics of hot electron confinement in laser-irradiated materials. *This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Computational study of stationary shock wave instability toward a laboratory experiment

We have investigated standing accretion shock instability (SASI) in core-collapse supernova cores by multi-dimensional simulations with steady-state initial conditions for examining its effects on the explosion mechanism so far. Numerical results positively suggested that SASI assists a shock revival due to a net heating increase in SASI-induced turbulence behind the shock wave for not only two- but also three-dimensional flows. However, a numerical instability on a stationary shock wave, so-called carbuncle phenomenon is well known and still a painful problem for shock capturing scheme. Laboratory experiments simulating a stationary shock wave in an accretion flow may directly prove the actual existence of SASI. Some attempts to explore possible laboratory experiment with high-power laser have been conducted by radiation hydrodynamics simulations for observing the SASI growth on the ground. Moreover, three-dimensional simulations for bow-shock instability have been performed and may help the design of the laboratory experiment and understanding the mechanism of SASI.   

Progress towards laser-driven Particle Therapy accelerators

Recent successes in laser-ion acceleration have motivated research towards laser-driven compact accelerators for medical therapy. Realizing laser-ion acceleration for medical therapy will require adapting both the laser-ion acceleration to the medical requirements, as well as the treatment methodology to the foreseeable laser constraints. Three key scientific and technological challenges are identified: increasing laser-accelerated proton energies to 250 MeV; developing compact, strong field magnetic beam manipulation systems; and development of real-time in-vivo dosimetry to enable pulse-by-pulse active feedback and control. Progress in each of these key areas are reviewed, with special emphasis on the prospect of increasing the energy of laser accelerated protons by modifications of the Target Normal Sheath Acceleration process.   

Theoretical understanding of record proton energies from laser acceleration with cone targets and future prospects

The laser-acceleration of protons to 67.5 MeV has recently been observed at the LANL Trident laser using novel cone targets [1]. The measured enhancement in proton energy is understood from collisional Particle in Cell simulations, which show that the hot electron temperature, responsible for the Target Normal Sheath Acceleration at the cone-top, is significantly increased when the laser grazes the cone wall. This is due to the extraction of electrons from the cone wall by the laser electric field, and their boost in the forward direction by the vxB term of the Lorentz force. This is in contrast to previous predictions of optical collection and wall-guiding of electrons in angled cones [2]. This new mechanism should enable new and more robust target designs for reaching high laser-accelerated proton energies.\\[4pt] [1] S.A. Gaillard, invited talk NI3.00004.\\[0pt] [2] Y. Sentoku et al, Phys. Plasmas 11, 3083 (2004).   

Energy scaling of laser accelerated protons, and performance of reduced mass targets

Proton acceleration from thin foils and reduced mass targets is studied with the 150 TW Ti:Sa DRACO Laser at FZD. DRACO has $\sim$30 fs pulses, with up to 5 J at 10 Hz, and a contrast of 1e-10 in the ps regime, and 1e-9 to 1e-10 in the ns regime. Proton spectra are measured in radiochromic film stacks and magnetic spectrometers. Flat metallic foils exhibit a near-linear scaling of the maximum proton energy with laser power, consistent with [1] in the limiting case of ultrashort laser pulses [2]. Despite the high laser contrast, a slight deformation of the target rear surface results in a reproducible deflection of the emission of energetic protons away from the target normal direction [2]. The mass limited targets of 2 $\mu$m thick Si, were fabricated by MEMS techniques and ranged from 20x20 $\mu$m$^2$ to 100x100 $\mu$m$^2$ lateral size. Significant influence of the target edge and supporting stalks is observed, which depending on size can both both increase or decrease the maximum proton energy in comparison to a flat foil.\\[4pt] [1] J. Schreiber \textit{et al}., \textbf{PRL 97}, 045005 (2006).\\[0pt] [2] K. Zeil \textit{et al}., \textbf{NJP 12}, 045015 (2010).   

Radiobiological applications of ultrashort pulse laser-accelerated proton beams

Ultrashort pulse laser proton acceleration is demonstrated to yield energies hitherto only accessible with high energy lasers. Up to 20 MeV protons are observed with the FZD Draco Ti:Sa laser with ~30 fs pulses and only 2 J. This proton energy range allows for first well controlled applications. The radiation dose per shot observed for energies above 10 amounts to few Gy and thus provides excellent starting conditions for the irradiation of in vitro tumour cells with the aim of determining dose dependent biological damage. A first experiment demonstrates the availability of all components indispensable for systematic radiobiological studies: A laser-plasma accelerator providing stable proton spectra with maximum energy exceeding 15MeV over hundreds of pulses and applicable doses of a few Gy within few minutes, a beam transport and filtering system, an in-air irradiation site, a dedicated dosimetry system providing both online dose monitoring and a precise absolute dose information applied to the cell sample, and the full infrastructure for analysing radiation induced damage in cells.\\[4pt] [1] S.D. Kraft, K. Zeil, et al., \textit{New J. Phys}. \textbf{12}, 085003 (2010).