Jacob Grun

Characteristics of ablation plasma from planar, laser-driven targets

The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd‐laser pulses (4 ns,<1014 W/cm2) are measured and the dependence of ablation parameters upon absorbed irradiance is determined. Large laser spots are used in these experiments so that the results are not sensitive to boundary effects.

Channeling of terawatt laser pulses by use of hollow waveguides

Subpicosecond laser pulses at power levels in excess of 1 TW were channeled through hollow microcapillary tubes by use of a combination of grazing-incidence dielectric and plasma-wall reflection mechanisms. Maximum input and output intensities were 10(17) and 10(16) W/cm(2) through 50-microm radius by 3-cm-long glass microcapillary tubes with as few as two waveguide modes being excited. 133-microm radius tubes as long as 13 cm resulted in successful channeling with an extinction coefficient of 0.2 cm(-1) and a plasma-wall reflectivity of 80%.

Sub‐Alfvénic plasma expansion

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub‐Alfvenically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower‐hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free‐streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are ...

Ablative acceleration of planar targets to high velocities

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.

Efficient multi-keV X-ray sources from laser-exploded metallic thin foils

A set of materials—titanium, copper, and germanium—has been experimented with at the OMEGA laser facility [Boehly, Opt. Commun. 133, 495 (1997)] by irradiating thin foils with a prepulse prior to a main pulse with variable delay, in order to design efficient x-ray laser-sources for backlighting, material testing, and code validation. This concept led to increasing factors from 2 to 4 comparing to cases without prepulse, in the experimental conditions. As a result, high multi-keV x-ray conversion rates have been obtained: 9% for titanium around 4keV, 1% for copper around 8keV, and 2.5 to 3% for germanium around 10keV, which places these pre-exploded metallic targets close to the gas with respect to their performance, with wider energy range. A good agreement with hydroradiative code FCI2 [Schurtz, Phys. Plasmas 7, 4238 (2000)] calculations is found for titanium and copper on all diagnostics, with nonlocal-thermal-equilibrium atomic physics and, either nonlocal thermal conduction taking self-generated B-fie...

Dynamical overstability of radiative blast waves: the atomic physics of shock stability.

Atomic-physics calculations of radiative cooling are used to develop criteria for the overstability of radiating shocks. Our calculations explain the measurement of shock overstability by Grun et al. [Phys. Rev. Lett. 66, 2738 (1991)]] and explain why the overstability was not observed in other experiments. The methodology described here can be especially useful in astrophysical situations where the relevant properties leading to an overstability can be measured spectroscopically, but the effective adiabatic index is harder to determine.

Laboratory laser-produced astrophysical-like plasmas

Laser-produced plasmas have many properties similar to, or which can be scaled to, those encountered in space, magnetospheric, ionospheric, and astrophysical situations. We describe several such experiments performed with the PHAROS III Nd-laser facility at NRL.

Identification of explosives with two-dimensional ultraviolet resonance Raman spectroscopy.

The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.

Multi-keV x-ray conversion efficiencies of laser-preexploded titanium foils

In the context of target design for multi-keV x-ray laser-produced experiments, the concept of exploding metallic thin foils by two laser pulses delayed in time has been tested at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasma 3, 2108 (1996)]. The first laser pulse creates an underdense plasma (ne∕nc≈0.2), and the second laser pulse heats the plasma plume which produces strong line emission from the titanium K shell (Heα at 4.7 keV and Hα at 4.9 keV). Six OMEGA beams (500-ps duration) for the prepulse and nine beams (1-ns duration) for the heating pulse irradiate one side of the foil. Different experimental conditions have been investigated in order to optimize the conversion efficiency enhancement on titanium foils. The influences of the foil thicknesses (5 and 6 μm), the delays (3, 4, and 5 ns) between the laser pulses, and the laser intensities (1.3 and 2.2×1015Wcm−2) have been tested. The absolute output power was measured by a set of filtered x-ray diodes, gi...

Magnetic field compression and evolution in laser‐produced plasma expansions

The evolution of a magnetic bubble resulting from the expansion of Nd‐laser‐generated plasma into a photoionized magnetized background plasma is examined experimentally and is compared with theory and computer simulations. The initial laser‐produced plasma speed is greater than the plasma sound and Alfven speeds and is energetic enough to be unmagnetized; the background plasma is effectively magnetized and its density is varied from the collisionless to the collisional regimes. The data support theoretical predictions that the initial expansion of the magnetic bubble is dominated by the uncoupled laser‐produced plasma.