J. A. Stamper

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.

Dependence of spontaneous magnetic fields in laser produced plasmas on target size and structure

Faraday rotation observations of magnetic fields generated by a high‐power (λ=1.06 μm) laser focused on various targets have been made. Results indicate that the fields produced on spherical targets may depend strongly on the target capacitance. Strongly structured fields were observed using large prepulse‐formed plasmas on wire targets. Using novel composition discontinuity targets and a UV probe beam, planar magnetic fields were noted and toroidal fields in the critical density region measured.

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.

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 Ndu2009:u2009glass 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 Ndu2009:u2009glass 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.

Interferometric measurement of asymmetric electron density profiles of laser-created plasmas from composite plane targets

Interferograms of composite plane‐target coronal plasmas have been analyzed by separate Abel inversion on each side of the interface between plasmas of different atomic number. The density difference measured across the interface changes sign near the central region of the interface. The absence of Faraday rotation of a visible probe beam in previous experiments is explained.

Ablative acceleration of laser-irradiated thin-foil targets

Ablative acceleration of thin foil targets at low laser irradiance (10/sup 12/ to 10/sup 13/ W/cm/sup 2/) are studied experimentally and theoretically. Ablative acceleration of oils up to approx. 10/sup 7/ cm/sec with good hydrodynamic efficiency (20%) have been achieved. These and other results are in good agreement with a simple rocket model.

Internal heat deposition in laser highly accelerated targets

A novel method is described from which estimates of the internal pressure or temperature of highly accelerated material can be made. The technique utilizes the relationship of the initial free expansion properties of a fluid boundary to internal pressure. The technique is applied to Nd‐laser ablatively accelerated targets; the results compare well with target rear surface temperature measurements using time‐resolved optical pyrometry.

Optical diagnostics of laser-fusion plasmas

Optical methods to diagnose the characteristics of laser fusion-like plasmas provide one of. the few possibilities to obtain sufficient spatial and temporal resolution in a density regime to be useful for laser fusion studies. We describe here four unusual approaches to this subject being pursued at NRL: 1) Spectral density analysis of scattered laser light; 2) Optical timeof-Night spectroscopy; 3) Spectroheliography in the UV; 4) The use of time-compressed harmonically and Raman frequency shifted laser probe light. The angular distribution of scattered laser light ( 1.06 pm) from planar targets irradiated by the NRL Pharos I1 Nd laser ( 100 psec, 1015-1016 W/cmZ) are Fourier-analyzed for target surface structure. Basically the scattering