Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows

Abstract

We present experiments characterizing the detailed structure of a current layer, generated by the collision of two counterstreaming, supersonic and magnetized aluminum plasma flows. The antiparallel magnetic fields advected by the flows are found to be mutually annihilated inside the layer, giving rise to a bifurcated current structure-two narrow current sheets running along the outside surfaces of the layer. Measurements with Thomson scattering show a fast outflow of plasma along the layer and a high ion temperature (T i ∼ZT e , with average ionizationZ ¼ 7). Analysis of the spatially resolved plasma parameters indicates that the advection and subsequent annihilation of the inflowing magnetic flux determines the structure of the layer, while the ion heating could be due to the development of kinetic, current-driven instabilities. DOI: 10.1103/PhysRevLett.116.225001 The interaction of supersonic, counterstreaming plasma flows occurs in many astrophysical scenarios (e.g., astrophysical jets [1] and termination shocks [2,3]) and in laboratory experiments (e.g., colliding plasmas in inertial confinement fusion hohlraums [4]). The presence of frozenin magnetic fields advected by the colliding flows could play an important role in determining the structure of the interaction region in these systems. Collisions of magnetized plasmas with oppositely directed magnetic fields should eventually lead to annihilation of the flux via magnetic reconnection. In many astrophysical scenarios reconnection occurs in high beta plasmas and is strongly driven, with ram pressure significantly exceeding the magnetic pressure. The structure of the reconnection layer in these conditions is unknown, but is expected to adjust to accommodate the rate of magnetic flux delivered into the layer, where, for example, a pileup of the magnetic flux could contribute to controlling the reconnection rate [5,6]. A number of recent laser-driven, high energy density physics (HEDP) experiments [7][8][9][10] have investigated magnetic reconnection in the strongly driven regime, as well as the formation of astrophysically relevant collisionless shocks [11] and self-organized field structures [12]. Large-scale field structures produced by collisions between laser-driven plasma flows have, for example, been interpreted [11] as being due to the accumulation of advected toroidal magnetic fields generated via the Biermann battery mechanism at the laser spots [13]. Despite the importance of magnetic fields in defining the properties of shocks formed in HEDP plasmas, experimental information is still limited.In this Letter we present experimental data characterizing the structure of an interaction layer formed by the collision of two counterstreaming, supersonic (sonic Mach number M S > 3), magnetized plasma flows. These flows advect embedded magnetic fields (magnetic Reynolds number Re M > 30), orientated in opposing directions perpendicular to the flow, and their interaction is strongly driven [i.e., high dynamic beta regime,. The experiments provide detaile...

An experimental platform for pulsed-power driven magnetic reconnection

“…Aluminium wires produced super-sonic (M S = 3.1), super-Alfvénic (M A = 2.2) inflows, which advected a B = 2T magnetic field at a flow velocity of V in = 50 km/s. 26 These inflows formed as the result of oblique shocks between the ablation streams from adjacent wires, which redirected the flows and formed a dense, collimated jet which propagated radially outwards from the gap between adjacent wires ( Fig. 3a).…”

Interactions of magnetized plasma flows in pulsed-power driven experiments

“…As the interaction of the magnetized flows occurs over a relatively long timescale, with a continuous inflow of the magnetized plasma for many times the hydrodynamic timescale of the system (T ≫ L V flow ⁄ ), the power balance can be monitored as the reconnection process develops and the system evolves. The results have shown that there is a transfer of the kinetic and magnetic energy of the inflowing plasma to the kinetic and thermal energy inside the layer [25,[30][31][32].…”

“…Magnetic reconnection is a complex and multifaceted process which is an active topic of research across space, astrophysics and plasma physics [28,29]. One of the most notable applications of the inverse wire array setup therefore has been to create a platform for studying this process in the laboratory [25,[30][31][32][33]. In these experiments two identical inverse wire arrays are fielded side-by-side on the MAGPIE generator, connected in parallel and in the same polarity to each receive a 50% share of the current supply.…”

Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments

“…The pulsed-power-driven reconnection platform 12,13 uses two exploding wire arrays (Fig. 2a), In the experiments described in this paper, each array consisted of 16 carbon wires (Staedler Mars Micro Carbon B) with a diameter of 400 µm, evenly spaced around a circle with a 16 mm diameter.…”

“…In this paper we present a recently developed pulsedpower driven experimental platform, 12,13 which allows reconnection to be observed in a regime in which the magnetic, thermal and kinetic energies are in rough equipartition in the inflowing plasma. This platform converts initially solid wires into plasma using an intense pulse of electrical current, and then accelerates this plasma to high velocity.…”