S. E. Clark

Dynamics of exploding plasmas in a large magnetized plasma

The dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma was investigated with magnetic flux probes and Langmuir probes. Debris-ions expanding at super-Alfvenic velocity (up to MA=1.5) expel the ambient magnetic field, creating a large (>20 cm) diamagnetic cavity. We observe a field compression of up to B/B0=1.5 as well as localized electron heating at the edge of the bubble. Two-dimensional hybrid simulations reproduce these measurements well and show that the majority of the ambient ions are energized by the magnetic piston and swept outside the bubble volume. Nonlinear shear-Alfven waves (δB/B0>25%) are radiated from the cavity with a coupling efficiency of 70% from magnetic energy in the bubble to the wave.

Observation of collisionless shocks in a large current‐free laboratory plasma

We report the first measurements of the formation and structure of a magnetized collisionless shock by a laser-driven magnetic piston in a current-free laboratory plasma. This new class of experiments combines a high-energy laser system and a large magnetized plasma to transfer energy from a laser plasma plume to the ambient ions through collisionless coupling, until a self-sustained MA∼ 2 magnetosonic shock separates from the piston. The ambient plasma is highly magnetized, current free, and large enough (17 m × 0.6 m) to support Alfven waves. Magnetic field measurements of the structure and evolution of the shock are consistent with two-dimensional hybrid simulations, which show Larmor coupling between the debris and ambient ions and the presence of reflected ions, which provide the dissipation. The measured shock formation time confirms predictions from computational work.

Hybrid simulation of shock formation for super-Alfvénic expansion of laser ablated debris through an ambient, magnetized plasma

Two-dimensional hybrid simulations of perpendicular collisionless shocks are modeled after potential laboratory conditions that are attainable in the LArge Plasma Device (LAPD) at the University of California, Los Angeles Basic Plasma Science Facility. The kJ class 1053 nm Nd:Glass Raptor laser will be used to ablate carbon targets in the LAPD with on-target energies of 100-500 J. The ablated debris ions will expand into ambient, partially ionized hydrogen or helium. A parameter study is performed via hybrid simulation to determine possible conditions that could lead to shock formation in future LAPD experiments. Simulation results are presented along with a comparison to an analytical coupling parameter.

Laser-driven, magnetized quasi-perpendicular collisionless shocks on the Large Plasma Devicea)

The interaction of a laser-driven super-Alfvenic magnetic piston with a large, preformed magnetized ambient plasma has been studied by utilizing a unique experimental platform that couples the Raptor kJ-class laser system [Niemann et al., J. Instrum. 7, P03010 (2012)] to the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. This platform provides experimental conditions of relevance to space and astrophysical magnetic collisionless shocks and, in particular, allows a detailed study of the microphysics of shock formation, including piston-ambient ion collisionless coupling. An overview of the platform and its capabilities is given, and recent experimental results on the coupling of energy between piston and ambient ions and the formation of collisionless shocks are presented and compared to theoretical and computational work. In particular, a magnetosonic pulse consistent with a low-Mach number collisionless shock is observed in a quasi-perpendicular geometry in both experiments and simulations.

On the generation of magnetized collisionless shocks in the large plasma device

Collisionless shocks are common phenomena in space and astrophysical systems, and in many cases, the shocks can be modeled as the result of the expansion of a magnetic piston though a magnetized ambient plasma. Only recently, however, have laser facilities and diagnostic capabilities evolved sufficiently to allow the detailed study in the laboratory of the microphysics of piston-driven shocks. We review experiments on collisionless shocks driven by a laser-produced magnetic piston undertaken with the Phoenix laser laboratory and the Large Plasma Device at the University of California, Los Angeles. The experiments span a large parameter space in laser energy, background magnetic field, and ambient plasma properties that allow us to probe the physics of piston-ambient energy coupling, the launching of magnetosonic solitons, and the formation of subcritical shocks. The results indicate that piston-driven magnetized collisionless shocks in the laboratory can be characterized with a small set of dimensionless ...

Characterization of laser-produced carbon plasmas relevant to laboratory astrophysics

Experiments, analytic modeling, and numerical simulations are presented to characterize carbon plasmas produced by high-intensity ( 109−1013 W cm−2) lasers relevant to experimental laboratory astrophysics. In the large-scale limit, the results agree well with a self-similar isentropic, adiabatic fluid model. Laser-target simulations, however, show small-scale structure in the velocity distribution of different ion species, which is also seen in experiments. These distributions indicate that most of the plasma energy resides in moderate charge states (C+3–C+4), most of the mass resides in the lowest charge states, and the highest charge states move fastest.

Experimental study of subcritical laboratory magnetized collisionless shocks using a laser-driven magnetic piston

Recent experiments at the University of California, Los Angeles have successfully generated subcritical magnetized collisionless shocks, allowing new laboratory studies of shock formation relevant to space shocks. The characteristics of these shocks are compared with new data in which no shock or a pre-shock formed. The results are consistent with theory and 2D hybrid simulations and indicate that the observed shock or shock-like structures can be organized into distinct regimes by coupling strength. With additional experiments on the early time parameters of the laser plasma utilizing Thomson scattering, spectroscopy, and fast-gate filtered imaging, these regimes are found to be in good agreement with theoretical shock formation criteria.

Feasibility of characterizing laser-ablated carbon plasmas via planar laser induced fluorescence.

Planar laser induced fluorescence (PLIF) imaging can potentially assess ion distributions and coupling in the context of super-Alfvénic ablation plasma expansions into magnetized background plasmas. In this feasibility study, we consider the application of PLIF to rapidly expanding carbon plasmas generated via energetic laser ablation of graphite. By utilizing hydrodynamic and collisional-radiative simulations, we identify schemes accessible to commercially available tunable lasers for the C I atom, the C II ion, and the C V ion. We then estimate the signal-to-noise ratios yielded by the schemes under reasonable experimental configurations.

Magnetic field measurements in low density plasmas using paramagnetic Faraday rotator glassa)

Paramagnetic Faraday rotator glass (rare-earth doped borosilicate) with a high Verdet constant will be used to measure the magnetic field inside of low density Helium plasmas (T(e) ~ 5 eV, T(i) ~ 1 eV) with a density of n ~ 10(12) cm(-3). Linearly polarized light is sent through the glass such that the plane of polarization is rotated by an angle that depends on the strength of the magnetic field in the direction of propagation and the length of the crystal (6 mm). The light is then passed into an analyzer and photo-detector setup to determine the change in polarization angle. This setup can detect magnetic fields up to 5 kG with a resolution of <5 G and a temporal resolution on the order of a nanosecond. The diagnostic will be used to characterize the structure and evolution of laser-driven collisionless shocks in large magnetized plasmas.

Laboratory study of collisionless coupling between explosive debris plasma and magnetized ambient plasma

The explosive expansion of a localized plasma cloud into a relatively tenuous, magnetized, ambient plasma characterizes a variety of astrophysical and space phenomena. In these rarified environments, collisionless electromagnetic processes rather than Coulomb collisions typically mediate the transfer of momentum and energy from the expanding “debris” plasma to the surrounding ambient plasma. In an effort to better understand the detailed physics of collisionless coupling mechanisms, compliment in situ measurements of space phenomena, and provide validation of previous computational and theoretical work, the present research jointly utilizes the Large Plasma Device and the Raptor laser facility at the University of California, Los Angeles to study the super-Alfvenic, quasi-perpendicular expansion of laser-produced carbon (C) and hydrogen (H) debris plasma through preformed, magnetized helium (He) ambient plasma via a variety of diagnostics, including emission spectroscopy, wavelength-filtered imaging, and ...