Magnetic Reconnection between Colliding Magnetized Laser-Produced Plasma Plumes

Abstract

Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counter-propagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8 -kJ, 2 -ns laser beams on the Omega EP Laser System. The interaction region between the plumes is pre-filled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an X-type null point geometry with B = 0 at the midplane and B = 8 T at the targets. The counter-flowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.PACS numbers: 52.27.-h, 52.35.Vd, 52.65. Rr, 52.72.+v, 94.30.cp Throughout the Universe, magnetic reconnection allows the magnetic field to change its topology and thereby allow an explosive release of stored energy [1][2][3]. Recently, a number of experiments have been carried out studying magnetic reconnection using laser-driven plasmas [4][5][6][7][8]. These experiments are in many ways complementary to traditional reconnection experiments with magnetized discharge plasmas [3]. Some notable features include the high plasma beta, strong inflows, and strong magnetic flux pile-up. This regime is very interesting as there are a number of space and astrophysical contexts where supersonic, magnetized flows collide, such as interactions of planetary magnetospheres with the solar wind [9], interaction of the solar wind with the interstellar medium at the heliopause [10,11], and pulsar windtermination shocks [12], to name only a few.Previous laser-driven experiments studied the reconnection of the self-generated (e.g., Biermann battery) magnetic fields between colliding laser-produced plasma plumes [4][5][6][7][8]. Magnetic field annihilation [5] has been observed, as well as plasma jets [4,[6][7][8] and electron energization [8]. This Letter presents, for the first time, results on reconnection of an externally applied magnetic field by counter-propagating, laser-driven colliding highenergy density (HED) plasmas. These experiments are based on new techniques for externally controlled magnetization of ablated plasma plumes. The geometry of this externally magnetized plasma experiment makes it amenable to end-to-end simulation with particle-in-cell codes modeling the entire progression of the experiment, including plasma formation and assembly of the current sheet. While previous results in HED plasmas could infer reconnection through annihilation of the magnetic field [5], this work is the first to observe clear stagnation of the counter-propagating magnetized ribbons and the formation of an extended reconnection layer. T...

Slowing of Magnetic Reconnection Concurrent with Weakening Plasma Inflows and Increasing Collisionality in Strongly Driven Laser-Plasma Experiments

“…The appearance of a stable, stationary current sheet, bearing qualitative resemblance to a Sweet-Parker current sheet in the collisional reconnection picture, is remarkably different than has been observed in similar experiments where the magnetic field structure has rapidly disrupted [17]. In the present experiments, the current sheet persists much longer than would be expected based on an annihilation of magnetic fields at the initial plasma flow velocity, which has been observed in previous experiments [16]-for a proton deficit region width of Δz ∼ 280 μm and an initial plasma flow velocity of V ∼ 700 μm=ns, the time for the plasma to cross the reconnection region unimpeded is only ∼0.4 ns; however, the current sheet persists nearly unchanged for at least 0.9 ns.…”

“…These experiments were conducted using laser-produced plasmas, a well-established platform for studies of high-β magnetic reconnection. Prior experiments have examined the annihilation of magnetic fields [14][15][16][17], thermal properties of the plasma [18,19], plasma jets [18,[20][21][22], and energetic electrons produced during reconnection [23]. Particle-in-cell simulations have predicted that, in addition to flux pileup, two-fluid physics plays a significant role in these strongly driven, quasicollisionless reconnection configurations [24].…”

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

“…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].…”

X-Point Effect on Plasma Blob Dynamics

“…Good quantitative agreement is found between the model and the experimental data in the initial accelerating phase of the blob dynamics. Magnetic field nulls (X points) are ubiquitous in space and laboratory plasmas [1][2][3]. The transport of particles and heat across them is of utmost importance for a variety of systems [4].…”