Large laser facilities, such as the National Ignition Facility at Lawrence Livermore National Laboratory, are often at the forefront of laser-driven laboratory astrophysics[
High Power Laser Science and Engineering, Volume. 6, Issue 2, 02000e17(2018)
A platform for high-repetition-rate laser experiments on the Large Plasma Device
We present a new experimental platform for studying laboratory astrophysics that combines a high-intensity, high-repetition-rate laser with the Large Plasma Device at the University of California, Los Angeles. To demonstrate the utility of this platform, we show the first results of volumetric, highly repeatable magnetic field and electrostatic potential measurements, along with derived quantities of electric field, charge density and current density, of the interaction between a super-Alfvénic laser-produced plasma and an ambient, magnetized plasma.
1 Introduction
Large laser facilities, such as the National Ignition Facility at Lawrence Livermore National Laboratory, are often at the forefront of laser-driven laboratory astrophysics[
The Large Plasma Device (LAPD) at the University of California, Los Angeles (UCLA) addresses some of these shortcomings by providing a high-repetition-rate, highly repeatable, long-lived and well-diagnosed ambient plasma. Previous experiments on the LAPD have utilized commercially available, high-repetition-rate lasers to study the magnetic[
In this paper, we overview a new experimental platform at UCLA that combines the LAPD with a high-power, high-repetition-rate laser that is capable of on-target intensities in excess of . This platform allows new 3D volumetric data collection of the interaction between laser-driven plasma plumes and a magnetized ambient plasma. We present the first experimental results using this platform, and discuss its potential application to topics in laboratory astrophysics, including the study of both perpendicular and parallel low-Mach number magnetized shocks, the formation of magnetic instabilities and kinetic-scale magnetospheres.
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2 Experimental platform and setup
The experiments were carried out at UCLA utilizing a high-repetition-rate laser in the Phoenix Laser Laboratory[
The high-repetition-rate laser was originally designed[
In the experiments, a high-density polyethylene () plastic target, 30.5 cm long and 38 mm in diameter, was positioned 30 cm from the LAPD center axis. The target was mounted on a 2D stepper motor drive synchronized with the laser, which translated and rotated the target in a helical pattern. Each target position was repeated three times and then moved to provide a fresh surface. A single target could thus be used for up to laser shots. The laser was configured with a 14 ns pulse width (FWHM) and directed through an lens at an angle of relative to the target normal, which ablated the target with intensities up to . Due to the low ambient density, the laser had a negligible effect on the ambient plasma over the regions of interest. The target and laser were oriented so that the laser-ablated plasma was directed across the background field. The laser and target were synchronized to the LAPD, and they all operated at a repetition rate of 0.23 Hz to allow time for the diagnostics to position themselves between shots.
The position of the laser spot on the target defines as , with the background magnetic field directed along , the target surface normal directed along , and the vertical motion of the target directed along . The center of the LAPD is then at . Diagnostics was positioned in the planes defined by (– plane) or (– plane). The firing of the laser defines as .
The magnetic field was characterized using 1 mm diameter, three-axis magnetic flux (‘bdot’) probes[
The electrostatic plasma potential was measured using a resistively heated emissive probe[
Images of the interaction of the laser plasma and ambient plasma were acquired with a fast-gate (10 ns) intensified charge-coupled device (ICCD) camera. The camera was positioned either above the target or at the end of the machine to image the – and – planes, respectively. Data was acquired in 10 ns increments simultaneously with the measurements of the magnetic field or plasma potential. The camera collected light from broadband plasma self-emission and could be additionally filtered to isolate specific ion charge states. Results from the fast-gate imaging can be found in Ref. [
Additionally, swept Langmuir probes were employed to measure similar – and – planes of plasma electron density and temperature. These measurements were carried out in the absence of the laser plasma, and so provided the initial state of the ambient plasma at .
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3 Results
Magnetic field and electrostatic potential measurements were acquired in both the – and – planes. In the – plane, the magnetic probe scanned from to and to over a total of 1875 shots (where each position was repeated three times), while the emissive probe scanned from to and to using 945 shots. In the – plane, in order to avoid hitting the probes with the laser, the probes scanned several smaller regions that were stitched together into a final, roughly triangular region from to and to over approximately 7000 shots. For all measurements, the background magnetic field was and directed along , and the ambient plasma was composed of with an initial electron density of , electron temperature , and ion temperature (see Figure
The probe measurements were highly repeatable. Each location in a plane was repeated three times. The resulting time-dependent value (magnetic field or electrostatic potential) was found to be within 5% of the mean value at that time, i.e., . This held for each location that was measured.
Figure
Figure
We note that the cavity still collapses approximately an order of magnitude faster than the classical (Spitzer) or Bohm magnetic diffusion time , where is the gradient magnetic scale length on the order of a few cm and is the diffusivity. At the time of collapse, the laser plasma is mostly confined within the magnetic cavity and has a density , temperature , and average ionization [
Transverse components of were also measured. Figure
From our 2D planes of vector magnetic field and electrostatic potential , we can calculate the current density and electrostatic electric field , where we take (the electric fields are slowly changing relative to the rate at which signals are sampled). We can also estimate the charge density . The results are shown in Figures
These features are also reproduced in the – plane (Figure
4 Discussion and conclusions
The results of Section
This high-repetition platform thus provides a rich testbed for exploring the interaction between super-Alfvénic laser plasmas and magnetized ambient plasmas relevant to laboratory astrophysics. Indeed, the experiments presented here were modeled after previous low-repetition experiments on the LAPD that explored perpendicular magnetized collisionless shocks[
Other applications of this platform are currently being pursued. Fast-gate imaging of the laser plasma shows the formation and evolution of flute-like, Rayleigh–Taylor or large-Larmor-radius magnetic instabilities[
[7] I. F. Shaikhislamov, Y. P. Zakharov, V. G. Posukh, A. V. Melekhov, V. M. Antonov, E. L. Boyarintsev, A. G. Ponomarenko. Plasma Phys. Control. Fusion, 56(2014).
[25] D. B. Schaeffer, D. Winske, D. J. Larson, M. M. Cowee, C. G. Constantin, A. S. Bondarenko, S. E. Clark, C. Niemann. Phys. Plasmas, 24(2017).
[30] P. V. Heuer, M. S. Weidl, R. Dorst, D. B. Schaeffer, A. S. Bondarenko, S. Tripathi, B. Van Compernolle, S. Vincena, C. G. Constantin, C. Niemann.
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D. B. Schaeffer, L. R. Hofer, E. N. Knall, P. V. Heuer, C. G. Constantin, C. Niemann. A platform for high-repetition-rate laser experiments on the Large Plasma Device[J]. High Power Laser Science and Engineering, 2018, 6(2): 02000e17
Received: Dec. 1, 2017
Accepted: Feb. 6, 2018
Published Online: Jul. 4, 2018
The Author Email: D. B. Schaeffer (dschaeffer@physics.ucla.edu)