Characterizing counter-streaming interpenetrating plasmas relevant to astrophysical collisionless shocks

Abstract

A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH2 foils have been irradiated with a laser intensity of ∼10 16 W/cm 2 . The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2,000 km/s, an electron temperature of ∼110 eV, an ion temperature of ∼30 eV, and a density of ∼10 18 cm −3 were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ωpi ∼0.16 mm, the interaction length, int, of ∼8 mm, and the Coulomb mean free path, λ mf p ∼27 mm. With c/ωpi int < λ mf p we are in a regime where collisionless shock formation is possible.

Experimental evidence for collisional shock formation via two obliquely merging supersonic plasma jets

“…Related studies include counter-streaming laser-produced plasmas supporting hohlraum design for indirect-drive inertial confinement fusion [17][18][19] and for studying astrophysically relevant shocks, [20][21][22][23][24] colliding plasmas using wire-array Z pinches, 25,26 and applications such as pulsed laser deposition 27 and laser-induced breakdown spectroscopy. 28 Primary issues of interest in these studies include the identification of shock formation, the formation of a stagnation layer [29][30][31] between colliding plasmas, and the possible role of two-fluid and kinetic effects on plasma interpenetration.…”

Filamentation Instability of Counterstreaming Laser-Driven Plasmas

“…The ion distribution function consists of two counterstreaming beam components with flow speeds ±V near 8 × 10 5 m/s, with single-stream ion temperatures near 150 eV. Under such conditions the ion-ion inter-plume interaction is quasi-collisionless, with the C 6+ -C 6+ mean-free-path between the opposing streams of order 10 cm, and the C 6+ -electron mean-free-path at least 4 mm (and likely longer if the electrons are heated in the interaction region [24]). Meanwhile, the electron collision frequency is faster than the dynamics we consider, therefore the electrons form a single thermalized population.…”

“…3(a), the results are remarkably constant over orders of magnitude variation in ion temperature, demonstrating that V /λ is the dominant scaling. Experimentally, this is important, as recent experiments have observed complex heating dynamics of both electrons and ions in similar counterstreaming conditions [24,25]. Instead, this dominant scaling enables straightforward comparison of observations and the linear theory: the wavelengths are measured in the radiography, and the interaction speed V ≈ C s + L/t is well-constrained due to the simple nature of the ablative flow.…”

Thomson scattering diagnostic for the measurement of ion species fraction

“…We show that it is possible to measure the ion species fraction to better than ±0.06 for a range of non-magnetized plasmas using Thomson scattering. A direct measurement of ion species fraction can be useful when studying a wide range of multi-ion species plasmas, a few exampeles include ion species separation in a capsule ablator [6] or collisionless shock creation from counterstreaming plasma flows [7].…”

“…The electron feature [ Fig. 2 (a)] is use to measure the electron temperature and density [7] which range from 250 eV to 80 eV and 1.7 × 10 18 cm −3 to 5.5 × 10 18 cm −3 respectively. The electron temperature and density are measured with an uncertainty of ±15%.…”