J. M. McMahon

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.

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.

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.

High-average-power operation of a Q-switched diode-pumped holmium laser

We have investigated high-peak- and high-average-power operation of diode-pumped, thulium-sensitized, holmium 2.1-microm lasers. Free-running laser powers of 14 W at 29 Hz have been demonstrated with 2.6% electrical efficiency. Q-switched operation produced average powers in excess of 11 W in a burst of short pulses. Preliminary optical parametric oscillator frequency conversion of the holmium laser to 4 microm is also reported.

The upgraded pharos II laser system

The Pharos II laser system has recently been upgraded for ablative acceleration studies at the Naval Research Laboratory (NRL). The design was optimized to produce uniform illumination of millimeter-size targets at intensities between 1013and 1014W/cm2in a multinanosecond pulse. Key elements of the design were the use of high-gain phosphate laser glass in disk amplifiers, and optimal relaying and beam shaping in the neat field. These design elements are discussed, as well as the overall performance of the completed system.

Power scaling of diode-pumped 2 micron lasers

Summary form only given. We have investigated high peak and average power operation of 2 /spl mu/m lasers as a pump source for parametric frequency conversion. Our goal is to produce a compact, reliable and tunable source of mid-IR radiation. A Tm,Ho:YAG Q-switched laser resonator system was developed to provide this pump source.<<ETX>>

High-power diode-pumped 2-μm lasers

We have investigated diode pumped thulium and holmium YAG lasers. Comparisons of laser performance for several different Tm/Ho concentrations and the results of a prototype Q- switched laser are reported.

Comparative Study of Diode-Pumped Two Micron Laser Materials

We have studied the characteristics of Thulium and Holmium/Thulium YAG lasers using high power laser diode pumping. Measurements of laser performance have been combined with spectroscopic measurements and compared to rate equation models for these materials.

Fusion Laser Technology Revisited

Laser research and development related to fusion began in the mid-1960’s in France (at the C.E.A laboratory at Limeuil and at the Companie Generale D’Electricite Research Center at Marcoussis), the U.S.S.R. (at the Lebedev and Kurchatov Institutes) and in the U.S.A. (at the Lawrence Radiation Laboratory and at Sandia). The brightness of lasers gave potential for producing high temperature and high density plasmas. The most notable work in laser development in this period was the development in a few years of very powerful neodymium glass lasers by CGE in France which by 1968 had attained energies of over 100 joules in pulses a few nanoseconds long. The discovery and demonstration of mode locking by DeMaria in 1966 provided a technique by which ultimately short and very well controlled temporal laser pulse shapes could be developed. After its invention by Patel in 1965 as a cw laser progress in developing high power pulsed carbon dioxide was also very rapid.

Diagnostics Of Laser Fusion Physics Experiments

Laser fusion involves the compression using very high power laser beams of a pellet containing fusionable fuel, such as a deuterium-tritium mixture, to such high densities and temperatures that it ignites and yields a net energy gain. The deposited energy causes a plasma to ablate from the target surface which drives the implosion. The physics issues to achieve success are numerous; they include: the laser absorption and pellet surface acceleration processes must be benign and efficient; uniform megabar pressures must be gen-erated by the ablating plasma to accelerate the target shell inward with a velocity over 150 km/sec and with about 1% accuracy; throughout this implosion the fuel must remain cold. To study these physics issues a number of novel diagnostics gre required. They involve measurements of photon and particle energies from 1 eV to 10 5 eV with subnanosecond time-resolution and micron spatial resolution. Many of these diagnostic techniques and their applications in the NRL laser fusion experiment are described.