High Repetition Rate Exploration of the Biermann Battery Effect in Laser Produced Plasmas Over Large Spatial Regions

Abstract

Self-generated magnetic fields occur in astrophysical systems throughout our universe. The Biermann battery is one possible source of such magnetic fields. This effect is caused by non-parallel temperature and density gradients within a plasma, represented in the magnetohydrodynamic framework by the baroclinic term in the induction equation, c en e ∇T e × ∇n e . In this paper we present a high repetition rate laboratory astrophysics experiment examining the spatial structure and evolution of Biermann generated magnetic fields in laser produced plasmas. We have extended the work of prior experiments, which spanned over the millimeter scale, by measuring fields spatially at approximately 1.0 centimeter and larger. Our measurements show azimuthally symmetric magnetic fields with peak values of 60 G in our closest measurements, 0.7 cm from the target surface. Optical Thomson scattering measurements give values for the electron temperature and density of the laser produced plasma (LPP) on axis, of T e = 10 ± 2 eV, and (5.5 ± 1) × 10 16 cm −3 , respectively. The velocity of the LPP's front has been estimated to be ∼ 330 km s −1 by measuring the time and position of the maximum magnetic fields measured. The expansion rate of the magnetic fields, and current density within the system are also mapped and examined. Furthermore, we have initiated a simulation campaign using the FLASH code to determine the veracity of the Biermann mechanism in our experiments, as well as the plasma's characteristics.

Time-resolved velocity and ion sound speed measurements from simultaneous bow shock imaging and inductive probe measurements

“…In laser-driven plasmas, which exhibit rapidly-evolving Biermann-generated or externally-imposed fields, multi-axis probes are used to reconstruct spatially-and temporallyresolved 3D magnetic fields. [4][5][6][7] Similarly, b-dot probes are routinely fielded in pulsed-power-driven plasmas to measure the ∼ 1 − 10 T fields advected by the plasma flows. 8,9 The magnetic Reynolds number R M = UL/ η represents the relative importance of magnetic field advection over resistive diffusion.…”

“…For large Reynolds numbers R M 1, advection dominates and the magnetic field becomes frozen into the flow. In magnetized plasmas with frozen-in flux, inductive probes can provide an estimate of the bulk flow velocity 5,10 -a quantity which is typically measured using other techniques, such as optical Thompson Scattering (OTS), [11][12][13] Mach probes, 14 or from the Doppler shift of spectral lines. 15 Inductive probe measurements have previously been used to estimate the ex-pansion velocity of laser-driven R M ∼ 10 4 plasma bubbles, 5 as well as inflow and outflow velocities in subsonic magnetic reconnection experiments.…”

“…In magnetized plasmas with frozen-in flux, inductive probes can provide an estimate of the bulk flow velocity 5,10 -a quantity which is typically measured using other techniques, such as optical Thompson Scattering (OTS), [11][12][13] Mach probes, 14 or from the Doppler shift of spectral lines. 15 Inductive probe measurements have previously been used to estimate the ex-pansion velocity of laser-driven R M ∼ 10 4 plasma bubbles, 5 as well as inflow and outflow velocities in subsonic magnetic reconnection experiments. 10,16 Flow velocity can be inferred by placing multiple probes along a velocity pathline, and measuring the transit time of a magnetized fluid parcel between the probes.…”

“…However, in situations where the first probe significantly perturbs the velocity or magnetic field seen by the second probe, it may not be possible to position multiple probes along the same pathline. There are several ways to overcome this issue -(1) a single probe can be used, and its location can be varied between repeated shots, 5 (2) in symmetric systems, probes can be positioned on different pathlines along the direction of symmetry, and (3) the magnetic field can be measured at the source, such as in the transmission line of a pulsed-power machine, and a probe can be placed in the flow to measure the advected field.…”