Measurement of Magnetic Cavitation Driven by Heat Flow in a Plasma.

We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. Reconstruction of the magnetic field map from proton radiographs demonstrates that the field is advected by heat flow in advance of the plasma expansion with a velocity v_{N}=(6±2)×10^{5} m/s. Kinetic and extended magnetohydrodynamic simulations agree well in this regime due to the buildup of a magnetic transport barrier.

Observations of pressure anisotropy effects within semi-collisional magnetized plasma bubbles.

Magnetized plasma interactions are ubiquitous in astrophysical and laboratory plasmas. Various physical effects have been shown to be important within colliding plasma flows influenced by opposing magnetic fields, however, experimental verification of the mechanisms within the interaction region has remained elusive. Here we discuss a laser-plasma experiment whereby experimental results verify that Biermann battery generated magnetic fields are advected by Nernst flows and anisotropic pressure effects dominate these flows in a reconnection region. These fields are mapped using time-resolved proton probing in multiple directions. Various experimental, modelling and analytical techniques demonstrate the importance of anisotropic pressure in semi-collisional, high-ß plasmas, causing a reduction in the magnitude of the reconnecting fields when compared to resistive processes. Anisotropic pressure dynamics are crucial in collisionless plasmas, but are often neglected in collisional plasmas. We show pressure anisotropy to be essential in maintaining the interaction layer, redistributing magnetic fields even for semi-collisional, high energy density physics (HEDP) regimes.

The inadequacy of a magnetohydrodynamic approach to the Biermann battery.

Magnetic fields can be generated in plasmas by the Biermann battery when the electric field produced by the electron pressure gradient has a curl. The commonly employed magnetohydrodynamic (MHD) model of the Biermann battery breaks down when the electron distribution function is distorted away from Maxwellian. Using both MHD and kinetic simulations of a laser-plasma interaction relevant to inertial confinement fusion we have shown that this distortion can reduce the Biermann-producing electric field by around 50%. More importantly, the use of a flux limiter in an MHD treatment to deal with the effect of the non-Maxwellian electron distribution on electron thermal transport leads to a completely unphysical prediction of the Biermann-producing electric field and so results in erroneous predictions for the generated magnetic field. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Magnetic Field Generation in Plasma Waves Driven by Copropagating Intense Twisted Lasers.

We present a new magnetic field generation mechanism in underdense plasmas driven by the beating of two, copropagating, Laguerre-Gaussian orbital angular momentum laser pulses with different frequencies and also different twist indices. The resulting twisted ponderomotive force drives up an electron plasma wave with a helical rotating structure. To second order, there is a nonlinear rotating current leading to the onset of an intense, static axial magnetic field, which persists over a long time in the plasma (ps scale) after the laser pulses have passed by. The results are confirmed in three-dimensional particle-in-cell simulations and also theoretical analysis. For the case of 300 fs duration, 3.8×10^{17} W/cm^{2} peak laser intensity we observe magnetic field of up to 0.4 MG. This new method of magnetic field creation may find applications in charged beam collimation and microscale pinch.

Enhancement of pressure perturbations in ablation due to kinetic magnetized transport effects under direct-drive inertial confinement fusion relevant conditions.

We present kinetic two-dimensional Vlasov-Fokker-Planck simulations, including both self-consistent magnetic fields and ablating ion outflow, of a planar ablating foil subject to nonuniform laser irradiation. Even for small Hall parameters (ωτ_{ei}≲0.05) self-generated magnetic fields are sufficient to invert and enhance pressure perturbations. The mode inversion is caused by a combination of the Nernst advection of the magnetic field and the Righi-Leduc heat flux. Nonlocal effects modify these processes. The mechanism is robust under plasma conditions tested; it is amplitude independent and occurs for a broad spectrum of perturbation wavelengths, λ_{p}=10-100µm. The ablating plasma response to a dynamically evolving speckle pattern perturbation, analogous to an optically smoothed beam, is also simulated. Similar to the single-mode case, self-generated magnetic fields increase the degree of nonuniformity at the ablation surface by up to an order of magnitude and are found to preferentially enhance lower modes due to the resistive damping of high mode number magnetic fields.

Kinetic modeling of Nernst effect in magnetized hohlraums.

We present nanosecond time-scale Vlasov-Fokker-Planck-Maxwell modeling of magnetized plasma transport and dynamics in a hohlraum with an applied external magnetic field, under conditions similar to recent experiments. Self-consistent modeling of the kinetic electron momentum equation allows for a complete treatment of the heat flow equation and Ohm's law, including Nernst advection of magnetic fields. In addition to showing the prevalence of nonlocal behavior, we demonstrate that effects such as anomalous heat flow are induced by inverse bremsstrahlung heating. We show magnetic field amplification up to a factor of 3 from Nernst compression into the hohlraum wall. The magnetic field is also expelled towards the hohlraum axis due to Nernst advection faster than frozen-in flux would suggest. Nonlocality contributes to the heat flow towards the hohlraum axis and results in an augmented Nernst advection mechanism that is included self-consistently through kinetic modeling.

Superluminal sheath-field expansion and fast-electron-beam divergence measurements in laser-solid interactions.

We show that including a sufficient description of the target's rear surface significantly affects the interpretation of a wide range of laser-solid experiments. A simple Debye sheath model will be shown to be adequate. From this the sheath field responsible for ion acceleration has been shown to expand at superluminal speeds, leading to very large ion-emission regions on the target's rear surface; a new explanation for the dynamics of the ion-accelerating sheath field accounts for this observation and demonstrates the inaccuracy of measuring the angular divergence of the injected electron beam, crucial to fast ignition, from the lateral extent of the ion emission. However, it is shown that on careful probing the sheath field can provide unique insight into details of the fast electron's distribution function. The relative merits of probing other physical quantities has been examined. The width of the background temperature spot overestimates the divergence by a factor of 2 unless electron recirculation is prevented.

Fast advection of magnetic fields by hot electrons.

Experiments where a laser-generated proton beam is used to probe the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target are presented. At intensities of 10(15) W cm(-2) and under conditions of significant fast electron production and strong heat fluxes, the electron mean-free-path is long compared with the temperature gradient scale length and hence nonlocal transport is important for the dynamics of the magnetic field in the plasma. The hot electron flux transports self-generated magnetic fields away from the focal region through the Nernst effect [A. Nishiguchi, Phys. Rev. Lett. 53, 262 (1984)] at significantly higher velocities than the fluid velocity. Two-dimensional implicit Vlasov-Fokker-Planck modeling shows that the Nernst effect allows advection and self-generation transports magnetic fields at significantly faster than the ion fluid velocity, v(N)/c(s)≈10.

Field compressing magnetothermal instability in laser plasmas.

The mechanism for a new instability in magnetized plasmas is presented and a dispersion relation derived. Unstable behavior is shown to result purely from transport processes-feedback between the Nernst effect and the Righi-Leduc heat-flow phenomena in particular-neither hydrodynamic motion nor density gradients are required. Calculations based on a recent nanosecond laser gas-jet experiment [D. H. Froula, Phys. Rev. Lett. 98, 135001 (2007)] predict growth of magnetic field and temperature perturbations with typical wavelengths of order 50 µm and characteristic growth times of ∼0.1 ns. The instability yields propagating magnetothermal waves whose direction depends on the magnitude of the Hall parameter.

Magnetic cavitation and the reemergence of nonlocal transport in laser plasmas.

We present the first fully kinetic Vlasov-Fokker-Planck simulations of nanosecond laser-plasma interactions including self-consistent magnetic fields and hydrodynamic plasma expansion. For the largest magnetic fields externally applied to long-pulse laser-gas-jet experiments (12 T) a significant degree of cavitation of the B field (>40%) will be shown to occur from the laser-heated region in under half a nanosecond. This is due to the Nernst effect and leads to the reemergence of nonlocality even if the initial value of the magnetic field strength is sufficient to localize the transport.

Effect of target composition on proton energy spectra in ultraintense laser-solid interactions.

We study how the proton density in a target irradiated by an ultraintense laser affects the proton spectrum, with analytical models and Vlasov simulations. A low relative proton density gives rise to peaks in the energy spectrum. Furthermore, a target with the protons confined to a thin, low density layer produces a quasimonoenergetic spectrum. This is a simple technique for producing proton beams with a narrow energy spread for proton radiography of laser-plasma interactions.

Magnetic reconnection and plasma dynamics in two-beam laser-solid interactions.

We present measurements of a magnetic reconnection in a plasma created by two laser beams (1 ns pulse duration, 1 x 10(15) W cm(-2)) focused in close proximity on a planar solid target. Simultaneous optical probing and proton grid deflectometry reveal two high velocity, collimated outflowing jets and 0.7-1.3 MG magnetic fields at the focal spot edges. Thomson scattering measurements from the reconnection layer are consistent with high electron temperatures in this region.

Resistive collimation of electron beams in laser-produced plasmas.

Intense relativistic electron beams, produced by high-intensity short-pulse laser irradiation of a solid target, have many potential applications including fusion by fast ignition. Using a unique Fokker-Planck code, supported by analytic calculations, we show that fast electrons can be collimated into a beam even when the fast electron source is not strongly anisotropic, and we derive a condition for collimation to occur.

Nonlocal magnetic-field generation in plasmas without density gradients.

Conventional theories of magnetic-field generation by laser pulses in collisional plasmas require the presence of density gradients or anisotropic pressure. Using the first two-dimensional Fokker-Planck code to self-consistently include magnetic fields, we find that magnetic fields can be spontaneously generated when a collisional plasma is nonuniformly heated even though inverted delta n = 0 and the pressure is purely isotropic. These magnetic fields, which can become strong enough to significantly affect transport, are attributed to nonlocal effects that are missing in the standard, local theories.

Large-amplitude plasma wave generation with a high-intensity short-pulse beat wave.

A short-pulse laser beat wave scheme for advanced particle accelerator applications is examined. A short, intense (3-ps, >10(18)-W cm(-2)) two-frequency laser pulse is produced by use of a modified chirped-pulse amplification scheme and is shown to produce relativistic plasma waves during interactions with low-density plasmas. The generation of plasma waves was observed by measurement of forward Raman scattering. Resonance was found to occur at an electron density many times that expected, owing to ponderomotive displacement of plasma within the focal region.

Phase modulation of intense ultrashort laser pulses reflected from steep, dense plasmas.

The phase modulation of intense ( I = 10(18) W/cm(2)) ultrashort laser pulses ( tau(p) = 70 fs) after reflection from steep, dense plasmas has been temporally resolved for the first time in particle-in-cell simulations. The position of the turning point from where the pulse reflects has been compared to the phase modulation, over a range of angles of incidence. At normal incidence or s polarization the phase modulation almost exactly represents the movement of the turning point due to the light pressure. As the angle of incidence is increased for p polarization, the simple Fresnel relationship between phase modulation and displacement of the reflection position, Delta phi(t) = -2k(0)Delta x(t)cos theta(0), increasingly breaks down.