Fast-Ion Generation by Ion-Acoustic Turbulence in Spherical Laser Plasmas

Abstract

Numerical hydrodynamic simulations which include absorption and transport modifications due to ion-acoustic turbulence have been performed. These simulations can reproduce the important features of the fast-ion velocity distributions as measured with biased charge collectors and a magnetic spectrograph in spherically illuminated microballoon experiments. Experiments with hemispherical targets show that a substantial amount of fast-ion energy is directed inward and may contribute to driving the implosion.Measurements of ion expansion energy from laser-heated plasmas show an anomalous fast-ion component. 1 Charge collectors biased to record ion current as a function of time show that in a typical experiment approximately half of the absorbed energy resides in only a few percent of the target mass. 2 This result implies that the absorbed energy is being coupled into a very thin layer at the target surface. This mode of coupling leads to poor momentum transfer between the expanding plasma and the compressed region of the pellet and could place a fundamental limitation on the efficiency of laser-driven ablative compression. Figure 1 presents data from two instruments used to obtain the ion velocity distribution. The target for this laser shot was a 54-jtim-diam spherical glass shell with 0.8-jum wall thickness filled with 10 atm of DT gas mixture. The laser energy on the target was 21 J at 1.06-/im wavelength in a 70-psec flat pulse. A lens-ellipsoidalmirror illumination system provided near-normal illumination of the pellet. 3 Figure 1(a) shows the voltage trace from a conventional charge collector. The initial sharp spike is produced by photoelectron emission from the collector surface due to the ultraviolet and x-ray flash when the laser pulse hits the target. The second peak results from the fast ions, and the broad feature represents the arrival of the bulk of the ionized target material. Using previously measured values for the mean ionic charge and secondary-emission coefficient, 4 this voltage trace can be converted into an ion-velocity spectrum. Because of uncertainty in the secondary-emission correction, the velocity spectrum above 2x 10 8 cm/sec is obtained with a small magnetic spectrograph which uses cellulose nitrate foils as detectors. 5 Figure 1(b) shows the ion-velocity spectrum. At high velocities, the spectrum is adequately 10 4 10

Explosion Dynamics of Rare Gas Clusters in Strong Laser Fields

“…However, the most energetic ions arise from Coulomb repulsion. Clearly, the observed quadratic charge dependence of the ion energies cannot be understood using common plasma corona theories [15][16][17][18][19]. Instead, we propose a simple Coulomb explosion model of a precharged cluster-sized plasma which is in good agreement with the present experimental results.…”

“…It should be mentioned that, after the pioneering work by Ehler [12], ion energies of several MeV͞amu have been reported from intense laser irradiation of solids for more than a decade [13,14]. Such energetic particles have been theoretically interpreted as a departure from ideal isothermal expansion due to overheated coronal electrons [15], rarefaction shocks [16], neutralizing return currents [17], ion acoustic turbulences [18], or self-focusing [19]. Most of the theoretical studies on fast ion generation have assumed that the energy distribution in the coronal plasma is described primarily by two temperatures for thermal and hot electrons.…”

Observation of Severe Heat-Flux Limitation and Ion-Acoustic Turbulence in a Laser-Heated Plasma

“…K, -wC 2 \ 2 n e *(l-C 3 n e \ 2 )-°' 5 g, (2) where s is a factor allowing for an artificial variation in the conductivity. When s = 1 the conductivity corresponds to Spitzer's value.…”

“…We have extended this technique to spatial and temporal resolutions of 200 p,m and 1 ns. Other experiments on laserproduced plasmas 5 diagnostics to measure thermal conductivity. Many other phenomena (magnetic-fieId generation, resonant absorption, atomic physics, fastelectron transport) complicate these experiments.…”

Anomalous Absorption in CO2-Laser-Target Interactions

“…If this is so, a simple calculation 16 balancing absorbed energy with that carried off by electrons and ions yields a flux factor /-0.04. If we now assume that ion turbulence is responsible for a possible inhibition of transport 16 ' 17 then it may be shown that ion fluctuation levels 6w/w~0.1 are required.…”