Session CO8: HED: Magnetized Plasmas

Collision of two magnetized jets created by hollow ring lasers

In recent OMEGA laser experiments we have created narrowly collimated MG plasma jets by using 20 OMEGA beams from one hemisphere to form a hollow ring pattern on a flat CH target, and characterized the properties of these jets as a function ring radius d and target composition (pure CH vs. 2 percent Fe-doped CH). The strong MG poloidal magnetic field of these jets is created via the Biermann Battery (grad P$_{\mathrm{e\thinspace }}$\textbf{x }grad n$_{\mathrm{e}})$ mechanism by the collisions of individual laser blow offs and further compressed by the on-axis flow. The magnetic field gets stronger, more ordered and persists to greater distances from the target as d is increased from 0 to 1200 microns. Here we discuss the formation and evolution of magnetized high-beta shocks created by the collision of two such MG plasma jets, and the effects of changing the ring radius, target separation and composition. Results from 2D and 3D FLASH code simulations, and designs for future OMEGA experiments, will be presented. We will highlight the effects of electron thermal conduction on the shock structure and evolution. Potential applications of high-beta magnetized shocks to young stellar object outflows will also be discussed.   

Investigation of hydrodynamic instabilities in the presence of a background magnetic field at the LULI laser facility

Magnetic fields can play an important role in the evolution of hydrodynamic instabilities in many different physical systems, ranging from small inertial confinement fusion (ICF) experiments to astronomically large supernova remnants (SNRs), like the Crab Nebula. Of particular interest are the Richtmyer-Meshkov (RM), Rayleigh-Taylor (RT), and Kelvin-Helmholtz (KH) instabilities, as all three are relevant to magnetized ICF concepts and astrophysical systems, such as the interaction of shock waves with interstellar clouds and in the shells of SNRs. This talk will cover recent experiments performed at the Laboratoire pour L'Utilisation des Lasers Intenses (LULI) aimed at developing a platform to study B-field effects on blast-wave-driven hydrodynamic instabilities that will provide insight to the magnetic effects in high-energy-density (HED) plasma systems. Preliminary experimental results will be shown and discussed. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences High-Energy-Density Laboratory Plasma Science Program under Award Number DE-SC0018993.   

Direct observation of target material effects on high power laser-driven magnetic field generation

We report on experimental and computational observations of target material effects on magnetic field generation in high-power laser produced plasmas. Experiments performed with the OMEGA EP laser system compared nanosecond laser pulses focused to moderate intensity ($I_L = 2 \times 10^{14}$Wcm$^{-2}$) with multi-picosecond pulses focused to high intensity ($I_L > 10^{19}$Wcm$^{-2}$) interacting with foil targets. Proton radiography measured differences in the strength and spatial profile of self-generated magnetic fields as the target material was varied between plastic (CH), aluminum and copper. In the case of moderate intensity pulses, magneto-hydrodynamics (MHD) simulations including radiation transport reveal ionization dynamics in higher Z targets that initiate multiple regions of Biermann battery ($\nabla T_e \times \nabla n_e$) magnetic field generation. At high intensities, we observe enhanced filamentation in lower Z, insulator targets. These results should help inform magnetized high energy density (HED) and laboratory astrophysics experiments, such as laser-driven magnetic reconnection, where precise knowledge of the initial magnetic field topology is crucial.   

Magnetically Collimated Plasma Jets From Radial Foil Z-Pinch

\textbf{Abstract. } Collimated plasma jets are produced under numerous conditions within astrophysical environments. Experiments within the laboratory can be used to investigate the acceleration of such jets as well as dynamics when the jets are perturbed by obstacles to the flows. We create plasma jets by ablation of an Al foil driven by a 1 MA, 250 ns current pulse [1,2]. In this presentation we will show recent results collected using the MAGPIE pulsed power generator at Imperial College, London to drive a plasma from a radial foil. The JxB \quad force directs the plasma onto the central axis of the foil, and a collimated jet propagates outwards. The highly collimated formation is created from radial magnetic fields preventing the plasma from expanding outwards. Larger opening angles would be expected from these jets (M$=$5) if they were freely expanding [3]. Further downstream, in the jet, obstacles can be placed both directly into the flow and to block the flow completely, causing a pile-up of the plasma, and shocks to be formed. Diagnostics including Thomson scattering, laser interferometry, Faraday imaging and schlieren imaging are used to help characterise the flows and magnetic fields created. [1] F. Suzuki-Vidal \textit{et al.,} Astrophys. Space Sci. \textbf{322}, 19 (2009) [2] F. Suzuki-Vidal \textit{et al.,} HEDP, \textbf{9}, 141 (2013) [3] F. Suzuki-Vidal \textit{et al.,} PoP, \textbf{19}, 022708 (2012).   

Impact of self-generated magnetic fields on High Energy Density experiments

Mixing has been discussed among the inertial confinement fusion community as an explanation for decreased capsule performance. Understanding where and how mix occurs and accurately modeling mix is quintessential to developing future mix mitigation strategies and designing better performing implosions. Strong magnetic fields can be generated when plasma flows shear and go Kelvin-Helmholtz unstable. Strong magnetic fields can affect electron thermal transport and ion transport, and can have energy densities on the order of the turbulent energy, which could affect the mixing behavior. An experiment was conducted to study strong magnetic field generation as a result of shear flow from counter propagating shocks separated by a thin foil. Magnetic field location and strength was determined using proton radiography through the central sheared region. The location and morphology of the shock/shear region were measured using point projection backlighting x-ray radiography on the axis perpendicular to the protons. The presence of strong magnetic fields in a shock-shear platform may lead to a paradigm shift in the need for including extended magnetohydrodynamics effects to accurately model aspects of mix. (LA-UR-19-26166)   

\textbf{Exploring the effects of externally imposed B- field on shock-driven implosions}

The effect of externally imposed magnetic fields on shock driven inertial confinement fusion implosions are studied both experimentally and theoretically. The studies address kinetic effects in magnetized plasmas, and suppression of electron and ion Braginskii thermal conduction. In the experiments, where a 25T (0.25MG) external initial B-field is compressed to tens of MG, electrons are strongly magnetized, leading to suppression of thermal losses. These shock-driven implosions provide plasma conditions, with low ion density (10$^{\mathrm{22}}$-10$^{\mathrm{23}}$ /cm3) and high ion temperatures (\textgreater 10 keV), enabling studies of ion magnetization. As the ion gyro-radius is shorter than the ion mean free path, there is a Knudsen-number reduction for the ion species which is now determined by the former length scale. The results from 1D and 2D simulations and from the first exploratory experiments will be presented. The work was supported by DOE, NLUF, LLE, EPSRC grant EP/P010288/1, and Eurofusion Enabling Research ENR-IFE19.CEA-01.   

Modeling magnetic fields and synthetic radiographs for high energy density plasma flows in shock-shear targets

In HEDB experiments on the OMEGA laser, we use a shock-shear derived platform to maximize the magnetic field generation to determine the types of fields that are able to develop in such experiments. Radiation-magnetohydrodynamics simulations using FLASH code indicate that fields of tens of Tesla can be generated via Biermann battery effect due to vortices and mixing in the counter-propagating shock-induced shear layer. Monte Carlo simulations using the newly developed MPRAD code are carried out to study the interplay between the proton deflection by magnetic fields and the diffusive transport by Coulomb scattering. The synthetic proton radiography and X-ray framing camera images are in good agreement with experiment data. The magnetic fields are found to be of sufficient strength such that they may be able to change the dynamics of the small-scale evolution of vortices like those in a turbulent cascade, and affect our understanding of turbulence.   

Study of laser driven magnetic fields in the coil target

Laser driven magnetic fields in coil targets were studied with the MTW laser at the Laboratory for Laser Energetics, University of Rochester. The magnetic field in coil targets was generated by the laser beam with energy of 25 J and pulse durations of 2.4 ns and 70 ps. The longitudinal magnetic field was measured by the Faraday rotation of the CW laser beam at the wavelength of 405 nm in the small glass disc. Axial magnetic fields of 10-20 T were measured in coils. An increase of intensity in the short pulse regime by a factor of 30 resulted in the increase of the magnetic field in 1.6-2 times. A pulse of the magnetic field showed a short 0.3-2ns rising edge and long sub-microsecond falling edge. A long falling edge can be produced by relaxation of the magnetic energy accumulated in the coil through the plasma-filled capacitor. The work was supported by the DOE grant DE-SC0016500. The MTW Facility is supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Current Transients in Laser-Driven Coils

The familiar isothermal plasma expansion model is applied to gain some insights into laser-driven coils, where a laser is used to eject electrons from one end of a metal coil to produce a magnetic field. The initial formation of the electron sheath draws a current from the target in the form of an electron rarefaction wave traveling at the electron thermal velocity. A circuit model should be applicable only following the passage of the electron rarefaction wave around the coil. The outward acceleration of the ions by the electrons then leads to an ion rarefaction wave that produces a second transient current pulse traveling at the ion sound speed. For parameters typical of published laser-driven coil experiments, these transient current pulses could be significant and could explain the disagreement between different diagnostics of the magnetic field. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Three-Dimensional Modeling of Laser-Plasma Confinement in a Strong Magnetic Field

Plasmas created by laser pulses in strong magnetic fields generated by pulsed-power machines can be confined into different shapes (discs or jets) depending on the magnetic field and laser orientations.\footnote{ V. V. Ivanov \textit{et al.}, Plasma Phys. Control. Fusion \textbf{59}, 085008 (2017).}$^{\mathrm{,}}$\footnote{ V. V. Ivanov \textit{et al.}, Phys. Plasmas \textbf{26}, 062707 (2019).} Experiments performed at the Zebra Facility at the University of Nevada, Reno, coupled a $\lambda $~$=$~1.06-mm laser pulse of intensity 3~\texttimes ~10$^{\mathrm{15}}$ W/cm$^{\mathrm{2}}$ to a rod with an axially driven current generating a 3-MG azimuthal magnetic field. The generated plasma was confined in the axial direction and expanded in the azimuthal direction following the field lines to form a plasma disc. Two-dimensional modeling has previously shown the axial confinement of the plasma with expansion in the radial direction. We now present 3-D modeling results using \textit{HYDRA} that show axial confinement of the plasma along with its azimuthal expansion, leading the plasma to move along the field lines of the external magnetic field. The effects of different terms in Ohm's law on the structure and dynamics of the plasma are discussed.   

Towards a New Platform for Magnetized HED Physics

A developing application of laser-driven currents is in generating magnetic fields of picosecond-nanosecond duration with magnitudes up to \textasciitilde 600T. Single loop and helical coil targets can direct the discharge current along wires to generate spatially-uniform, quasi-static magnetic fields on the millimetre scale. Here, we report on simultaneous proton imaging measurements across both axes of a single loop coil ranging from 1 to 5mm in diameter. Comparison with proton tracking and magnetic models show that fields measured via proton deflectometry are the result of kiloampere currents in the coil and electrostatic charges on the coil surface. We demonstrate how magnetic fields can then be used to engineer states of highly-magnetized matter by studying the dynamics of an exploding foil.   

Plasma Transport with Higher-Moment Models in PERSEUS.

The traditional approach to deriving plasma transport coefficients is to perturb a near-Maxwellian distribution function and solve a Fokker-Planck or similar equation to first order in the perturbation parameter, typically the Knudsen number. This not only assumes high collisionality but also steady state solutions for non-equilibrium variables such as the heat flow and stress tensor, which destroys the hyperbolicity of the system of equations. While these near-Maxwellian transport coefficients are accurate in their appropriate regime, this technique provides parabolic equations that are computationally expensive to solve and does not extend itself well to plasmas of low collisionality. By expanding the distribution function in terms of the moments, one obtains hyperbolic equations that do not have the same restrictions on Knudsen number, and thus may provide more accurate transport coefficients in a broader parameter regime. We present our implementation of a regularized 13-moment model in the PERSEUS code as well as results from validation tests in both the high and low Knudsen number regimes.   

Comparison of STA calculations to measured spectra from hot and dense Ge

We report emissivity and opacity results from Super-Transition-Array (STA) calculations for hot, dense Ge plasma in order to assess the viability of the model against both experimental spectra and opacity. We show STA model reproduces the emission spectra from the 2p-3d, 2s-3p and 2p-4d transitions measured in a short laser-pulse experiment [1]. Considering the temperature and density gradients in our model, we find that the plasma temperature and density to be Te$=$600 eV and $\rho =$2.0 g/cc, respectively, which are close to the values obtained from LTE GRASP2K, CASSANDRA and DAVROS opacity codes. Result from the collisional-radiative code FLYCHK also shows a good fit to the observed spectra, but indicates a plasma Te$=$800 eV and $\rho =$1.5$+$/-0.5 g/cc. In addition, we also examine and compare the STA opacity results for a broad range of Ge plasma conditions covering the L- and M-shell spectral range with detailed calculations from the hybrid LTE opacity SCO-RCG code [2]. The sensitivity of the opacity results between the codes to the plasma temperature and mass density is discussed. [1] Hoarty et al., HEDP, 6, 105 (2010); Harris et al., HEDP, 6, 95 (2010). [2] Porcherot, et al., JQSRT, 65, 91 (2000)   

Experiments to understand the interaction of stellar radiation with molecular clouds

Enhanced star formation triggered by local hot and massive stars is an astrophysical problem of interest. Radiation from the local stars act to either compress or blow apart gas clumps in the interstellar media. In the optically thick limit (short radiation mean free path), radiation is absorbed near the clump edge and compresses the clump. In the optically thin limit (long radiation mean free path), the radiation is absorbed throughout, acting to heat the clump. This heating explodes the gas clump. Careful selection of parameters, such as material density or source temperature, allow the experimental platform to access different hydrodynamic regimes. A stellar radiation source is mimicked by a laser-irradiated, thin, gold foil, providing a source of thermal x-rays around 80-eV. The gas clump is mimicked by low-density CRF foam. We plan to show preliminary results, in the optically thick limit, where the shock is radiographed at various times. This work is funded by the U.S. DOE NNSA Center of Excellence under grant number DE-NA0003869, and the NLUF Program, grant number DE-NA0002719, and through the LLE, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-NA0003856. This work is funded by the LLNL under subcontract B614207.   

Laser-driven amplification of a seed magnetic field by electrons carrying OAM

Creation of quasi-static magnetic fields exceeding 1 kT is challenging and their use in laser-plasma interactions is further complicated by the plasma diamagnetic response. In this talk, we show how a seed magnetic field of 100 T can nevertheless be successfully amplified by more than an order of magnitude in a laser interaction with a thin foil. The new amplification mechanism involves the motion hot electrons towards the laser axis that causes them to gain orbital angular momentum (OAM) due to the presence of the seed axial magnetic field. The resulting azimuthal current amplifies the seed magnetic field. The mechanism is demonstrated using 3D kinetic simulations for a thin foil irradiated by a 600 fs laser pulse with a peak intensity of $10^{17}$ W/cm$^2$. The simulations show that the amplified field persists for hundreds of femtosecond after laser-plasma interaction. This mechanism of the magnetic field amplification may be relevant to the applications that rely of charged beam collimation and hot electrons creation.