Metal foil targets were irradiated with 1 mum wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10;{19} W cm;{-2}, giving values of both Ilambda;{2} and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K_{alpha} emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with Ilambda;{2} and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.

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...

Fast ignition is a two-step inertial confinement fusion concept where megaelectron volt electrons ignite the compressed core of an imploded fuel capsule driven by a relatively low-implosion velocity. Initial surrogate cone-in-shell, fast-ignitor experiments using a highly shaped driver pulse to assemble a dense core in front of the cone tip were performed on the OMEGA/OMEGA EP Laser [

A beam of multi-MeV helium ions has been observed from the interaction of a short-pulse high-intensity laser pulse with underdense helium plasma. The ion beam was found to have a maximum energy for He2+ of (40(+3)(-8)) MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations show that the ions are accelerated by a sheath electric field that is produced at the back of the gas target. This electric field is generated by transfer of laser energy to a hot electron beam, which exits the target generating large space-charge fields normal to its boundary.

Laser-driven magnetic reconnection is investigated using proton deflectometry. Two laser beams of nanosecond duration were focused in close proximity on a solid target to intensities of I∼1×1015 W cm−2. Through the well known ∇ne×∇Te mechanism, azimuthal magnetic fields are generated around each focal spot. During the expansion of the two plasmas, oppositely oriented field lines are brought together resulting in magnetic reconnection in the region between the two focal spots. The spatial scales and plasma parameters are consistent with the reconnection proceeding due to a Hall mechanism. An optimum focal spot separation for magnetic reconnection to occur is found to be ≈400±100 μm. Proton probing of the temporal evolution of the interaction shows the formation of the boundary layer between the two expanding plasma plumes and associated magnetic fields, as well as an instability later in the interaction. Such laboratory experiments provide an opportunity to investigate magnetic reconnection under unique conditions and have possible implications for multiple beam applications such as inertial confinement fusion experiments.