This award is made in response to a proposal submitted to and reviewed under the NSF/DoE Partnership in Basic Plasma Science and Engineering joint solicitation NSF 08-589. This award provides funds to support undergraduate participation in the overall research effort, which is being funded separately by DoE.
The experimental high-energy laser-capability developed during the last two years at the Large Plasmas Device (LPD) will be used to create collisionless shocks in a controlled laboratory setting and study their structure, dissipation mechanisms, and effect on the particle velocity distribution in great detail. In addition, the dynamics of large diamagnetic cavities in the presence of an ambient plasma,will be studied, as well as their coupling to large amplitude shear Alfvén waves. Recent experiments with an energetic laser-produced plasma have already compressed the external magnetic field at the edge of a 20 cm large diamagnetic cavity by more than 50%. Ongoing upgrades to both the laser driver and the plasma source will extend the experiments to time-scales of the order of the ion-gyroperiod, providing enough time for a shock to form. Collisionless shocks are ubiquitous in space and influence energy transport and the particle distribution throughout the cosmos. These shocks are formed, for example, in supernova remnants, coronal mass ejections, or planetary bow-shocks, and are a source of cosmic rays. Unlike a hydrodynamic shock, which dissipates its energy through binary collisions, a collisionless shock forms in a tenuous plasma where the energy transfer proceeds through collective electromagnetic effects. The experiments will be modeled using a parallel, three-dimensional hybrid plasma code.
The broader impacts of this work are that, with the addition of the large ambient plasma, these laser-experiments will fill an important gap that has limited the relevance of previous laser laboratory astrophysics experiments in vacuum. This project will also help to establish laser-laboratory astrophysics as an important area of research. In addition, the synergism between basic plasma science and high-energy density physics will lead to new findings important to a number of scientific disciplines. It is envisioned that a new generation of researchers will be trained who will acquire a solid background in both fields.
The additional funding from NSF will permit increased student involvement, in this case specifically an undergraduate student, in this research.
We have performed several thousand high-energy laser shots in the LAPD to investigate the dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma. 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 observed field compressions of up to B/B0 = 1.5 at the edge of the bubble, consistent with the MHD jump conditions, 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 to super-Alfvenic speeds 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. While the data is consistent with a weak magnetosonic shock, the experiments were severely limited by the low ambient plasma densities (1012 cm-3). 2D hybrid simulations indicate that future experiments with the new LAPD plasma source and densities in excess of 1013 cm-3 will drive full-blown collisionless shocks with MA>10 over several c/wpi and shocked Larmor radii. In a separate experiment at the LANL Trident laser facility we have performed a proof-of-principle experiment at higher densities to demonstrate key elements of collisionless shocks in laser-produced magnetized plasmas with important implications to NIF. Simultaneously we have upgraded the UCLA glass-laser system by adding two large amplitude disk amplifiers from the NOVA laser and boost the on-target energy from 30 J to up to 1 kJ, making this one of the world’s largest university-scale laser systems. We now have the infrastructure in place to perform novel and unique high-impact experiments on collisionless shocks at the LAPD.
