B. H. Ripin

Laser‐plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin‐foil targets irradiated by long, low‐flux Nd‐laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin‐foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

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.

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.

Laser‐produced‐plasma energy transport through plastic films

The transport of energy from a 1.06‐μm, 95‐psec laser pulse at an irradiance of 1015 W/cm2 through a thin layer of polystyrene into an Al substrate was studied by x‐ray, ion, and scattered‐light measurements. The intensities of the following quantities were measured as a function of polystyrene thickness: (1) x‐ray line radiation from the Al backing, (2) bremsstrahlung continuum from 3 to 88 keV, (3) ions of several keV energy, and (4) scattered laser light. The results indicate that a polystyrene thickness of no more than 0.5 μm is sufficient to inhibit substantial heating of the Al substrate.

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.

Electron–ion hybrid instability in laser‐produced plasma expansions across magnetic fields

High‐intensity laser irradiation of hollow glass cylinders immersed in a magnetic field results in plasma expansions strongly collimated in the direction transverse to both the initial flow and the magnetic field, but jetlike in the direction parallel to the initial flow. Magnetic fields from B=0 kG to B=10 kG produced plasmas with markedly different geometrical features. Fast framing camera photographs show the plasmas propagating across magnetic field lines and undergoing structuring indicative of transverse velocity shear‐driven instabilities. Comparison is made between the observed instability characteristics and predictions of Rayleigh–Taylor, classical Kelvin–Helmholtz, and the electron–ion hybrid instabilities. Only the electron–ion hybrid instability is consistent with the experimental results.

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.

Harmonic generation in Nd : laser‐produced plasmas

The fundamental and second through the fifth harmonic spectral lines have been observed from the plasma produced when a 75‐psec Nd : glass laser (∼1016 W/cm2) is focused onto a thick planar polystyrene target. Both line profiles and relative intensities of these harmonic are given.

Spectroscopic observation of fast ions from laser‐produced plasmas

Using a time‐of‐flight spectroscopic technique, measurements were made of the ion energy distributions of very fast ions and thermal ions produced when a 7–15‐J 100‐psec Nd : glass laser pulse (1.06 μm) strikes a (CH2)n slab target. Ion energies greater than 0.5 MeV have been observed for the first time with this technique of measurement. A simultaneous comparison is made between the signal of an ion charge collector placed 30 cm from the target and the intensity of the C VI 3434‐A ion line at 1 cm from the target.