RECENT RESEARCH PROJECTS

Our main goal is to develop a platform for the OMEGA-60 laser facility to study MHD effects in cylindrical implosions at regimes of large magnetic pressure and magnetization. 2-D simulations using the MHD code GORGON predicted that a seed B-field of 30 T can be compressed to ~30 kT owing to flux compression when imploding the cylindrical target. As a result, the characteristic conditions of the compressed core are expected to be modified by the large magnetization and magnetic pressure thereby reached at maximum compression. On a few previous magnetized cylindrical compressions at the OMEGA-60 facility, the effect of the compressed B-field was investigated using neutron measurements only. However, the neutron yield in cylindrical implosions is rather low (on the order of 10⁸-10⁹) and e.g. a small target variation can induce very large yield fluctuations, therefore strongly limiting a clear interpretation of the results. Alternatively, we propose to use Ar/Kr dopant(s) in the D₂ gas. As the core conditions are modified by the compressed magnetic field, we expect to observe systematic changes in the dopant emission spectra between unmagnetized and magnetized shots. Carrying out such magnetized implosion experiments will advance the modeling of B-field compression and diffusion, and the benchmarking of atomic kinetics and line shape calculations in magnetized plasmas relevant to complex ICF-related experiments with embedded B-fields. Simulations must include extended-MHD effects, which represent the transport of energy and magnetic flux in a plasma. Magnetized plasmas are thought to exhibit complex behavior in the electron population and, above resistive-MHD, extended-MHD additionally accounts for temperature gradient-driven transport—such as the Nernst term moving magnetic fields down electron temperature gradients—and electric-current-driven transport—such as the Hall term moving magnetic fields with the flow of charge. In this context, our goal is to obtain clear experimental data that will facilitate benchmarking of MHD codes and consequently help the modeling of magnetized HED experiments. Moreover, the interpretation of spectral emission measurements from complex magnetized experiments requires novel and accurate modeling of the detailed atomic and radiation transport physics. In this project, we are investing significant efforts to mature the spectroscopy diagnostic techniques for magnetized HED experiments.

Ion beams driven by short-pulse high-intensity lasers have been an increasingly active area of research as the beams are appealing for their potential, broad range of applications. Extensive computational and experimental studies have discovered various laser-driven ion acceleration mechanisms. Most mechanisms require ideal conditions of ultra-high laser intensity and sub-micron target thickness, to acquire high ion energy, yet there still exists difficulty in producing the ideal conditions resulting in limitation of proton energy below 100 MeV so far.

In this project, we investigate promising concepts to enhance the peak cutoff energy and flux of short-pulse laser-accelerated ions via the synergetic effects of target transparency and continuous field acceleration. Recent studies using multi-picosecond pulses with modest laser intensities have shown promising results. Experimentally demonstrated proton energies of up to 30 MeV at quasi-relativistic intensities and 50 MeV with the double pulses are far beyond energies predicted by the well-established TNSA model scaling and experimental results for the same laser intensity with sub-ps pulses. The laser pulse interacting with an expanding under-dense plasma for multi-ps duration generates a 'super-ponderomotive' electron population and this is key to sustain electric fields for proton acceleration. This continuous ion acceleration can be further improved with the effect of target transparency by using multi-ps pulses and ultra-thin foils, suggesting a new approach to efficient ion acceleration.

These hot electrons can preheat the target shell and degrade the implosion, and most previous investigations to date are carried out below the LPI thresholds to avoid the preheating effect. On the other hand, the advanced shock ignition (SI) fusion scheme with a slower implosion velocity and a reduced driver energy requires launching a high intensity ignitor spike pulse at the end of a fuel assembly phase to generate a strong shock to ignite the fuel. Copious hot electrons are expected in the SI-scale high intensity condition. It has been envisaged that moderate energy hot electrons (<200 keV) could further enhance the ablation pressure when depositing their energies in the high density compressed outer shell region and thus augment the ignitor shock, beneficial to SI. There is still lack of understanding of the physical processes in nonlinear LPI and subsequent hot electron generation and energy coupling in this new regime.

In this project, we are studying LPI and the dependence of hot electron generation and energy coupling on high power laser intensity, wavelength, and target ablator material in nonlinear laser-plasma interaction at SI scale high intensity. Experiments are conducted using the high energy OMEGA-EP and OMEGA-60 lasers at the Laboratory for Laser Energetics in the University of Rochester with the beam time awarded via DOE NNSA's National Laser User's Facility (NLUF) program. The temperature and energy of hot electrons generated by LPIs are measured and the hot electron spatial energy deposition is imaged in experiments. Advanced diagnostics including backscattering and angular filter refractometer are used to diagnose the region and the mechanism of the main LPI present in SI conditions. Simulations are planned to be performed on NERSC with the particle-in-cell (PIC) code OSIRIS, a state-of-the-art, fully explicit, multi-dimensional, fully parallelized, fully relativistic PIC code which we have access to. The simulation results can be compared with the results from experiments to have a precise understanding of the LPI mechanisms. This project is expected to have a broad impact on the resolution of basic nonlinear laser-plasma interaction physics issues and particularly important for fusion related HEDLP applications, specifically for the SI scheme.

The dynamics involved become more complex as the laser energy and resulting beam current increase, and as beam pulse length increases. We have been awarded OMEGA EP laser time at the Laboratory for Laser Energetics to use high energy short pulse laser beams to extend previous proton focusing/transition studies to 10 ps pulse duration. We will be collecting detailed information about the forces that affect such beams, utilizing the second EP beam to produce protons to probe conditions set up by the first EP laser beam. Energy deposition and heating of a solid foil (dense plasma) by the focused proton beam will also be characterized by measuring its induced K-shell fluorescence emission via x-ray imaging and spectroscopy techniques. Our goal is to extend our understanding of the High-Energy Density physics involved facilitating optimal source designs for various medical and energy applications and eventually experiments like NIF integrated proton fast ignition experiments with the Advanced Radiographic Capability (ARC) beam.

Experiments have been conducted at the major national high-energy-density (HED) science facilities (OMEGA-EP, NIF, and Z) in a synergistic relationship with a National Nuclear Security Administration (NNSA) initiative for short pulse HED science. The Center brings academic scientists from around the country into a collaboration that fosters rapid progress in this exciting field. A major long-term goal is to study fast ignition as a potential future energy source. UR Fusion Science Center

The FI is expected to achieve a higher gain comparing to the conventional laser fusion (i.e. central hot spot ignition). For the success of FI, The fast electron transport in hot dense plasma needs to be addressed. In the on-going National Laser Users Facility (NLUF) project of our group, the transport of fast electrons generated by 1 kJ level short pulse beam (10 ps duration) will be studied in a hot plasma (Te ~ few 10's eV and Ne ~ 1022 /cm3) with a volume of ~ 0.2 mm3 on the Omega EP laser facility at the Laboratory of Laser Energetics (LLE), the University of Rochester.

This Consortium addresses physics related to controlling the laser-induced generation of large electron currents and their propagation through high density plasmas. These issues are important for a wide range of HED phenomena, including high energy ion beam generation, isochoric heating of materials, the developement of high brightness backlighters and fast ignition inertial confinement fusion.

The aim of the 5 year project is to more directly design and carry out experimental studies of plasma jets which have relevance to astrophyical jet formation and propagation, particularly those emanating from Young Stellar Object (YSOs). The project is unique in several ways. For the first time, pulsed power experiments will be carried out (Imperial, UCSD) with computational support from both the plasma (Imperial) and astrophysics (Rochester) communities, and will be guided by observational needs (Rice). The project has also secured shot time at the 26 MA Z machine at Sandia National Laboratories which will generate jets of unprecedented density and Reynolds number, and push experiments significantly further into the turbulent and astrophyically relevent regimes.
Visit the collaboration website for updates at its Wiki Site.

This 3 year project, led by the UCSD HEDP Group and carried out the Nevada Terawatt Facility (NTF) , will for the first time use proton probing to diagnose B-field structures in pulsed-power driven exploding wire experiments. NTF has both a MA scale pulsed power generator and a high power, short pulse laser facility, and the proximity of these allows coupling of the two systems. The project will examine the broad space and time evoluion of the B-field in wire plasmas, and address outstanding issues in the distribution of current in ablation and imploding wire array z-pinches.