L. A. Jones

The interaction of a high irradiance, subpicosecond laser pulse with aluminum: The effects of the prepulse on x‐ray production

The conversion efficiency into kilovolt line radiation for 248‐nm light at 1017 W/cm2 on an aluminum target is measured. The x‐ray yield is found to increase with the scale length of the target plasma. The interaction is modeled as resonance absorption, and the plasma scale length is determined from the prelase energy and irradiance.

X‐ray generation by high irradiance subpicosecond lasers

We have studied the interaction of 290‐fs, 308‐nm laser pulses with aluminum targets at irradiances exceeding 5×1018 W/cm2. The x‐ray spectrum is dominated by the H‐ and He‐like lines from aluminum, with the brightest lines radiating 0.8% of the incident laser light energy. This fraction is close to that measured at 50 times less irradiance, but occurs at a slightly higher ionization stage. The x rays are emitted from a region of subcritical electron density at 3–6×1021 W cm−3. The radiance of the 1.73‐keV Lα line is measured to be 4×1012 W/cm2/sr.

Effects of the internally produced nonthermal electrons on the temperature diagnostics of a hollow gas shell Z pinch

Experimental evidence is presented indicating that a nonthermal electron energy distribution occurs in a collapsing gas shell Z pinch at the time of pinch. It is estimated that these results can be explained if less than 10% of the current is associated with an energetic (∼4 keV) electron beam. Interpreting the spectroscopic diagnostics consistently with the nonthermal electron energy distribution indicates that the temperature of the plasma ‘‘hot spots’’ could be as low as ∼400 eV instead of the 1 keV obtained by assuming a thermal distribution.

A laser‐initiated, gas‐embedded Z pinch: Experiment and computation

Measurements done on a new plasma device, a laser initiated, gas‐embedded Z pinch, are compared with magnetohydro dynamics(MHD) calculations. The measurements, which include two‐foil x‐ray absorption, absolute continuum emission, and schlieren shadowgraphy, are in good agreement with predictions made by a two‐dimensional MHD calculation, which indicates that the peak electron temperature in the plasma is 200 eV and the peak electron density is 1×1020 cm−3.

X-ray streak camera diagnostics of picosecond laser-plasma interactions

An x‐ray streak camera is used to diagnose a laser‐produced Al plasma with time resolution of ∼10 ps. A streak record of filtered emission and a time‐integrated transmission grating spectrum reveal that the plasma radiation is dominated by emission from He‐ and H‐like resonance lines.

Experimental investigation of a magnetic gate as a multimegampere, vacuum opening switch

We report on the first experiments to demonstrate the vacuum operation of a magnetic gate opening switch. Currents greater than 1 MA have been transferred in approximately 1 μs. A model has been developed to describe the switch’s operation. The model and the experimental data are in excellent agreement.

Atomic processes in plasmas under ultra‐intense laser irradiation

Lasers delivering subpicosecond pulses with energies of a fraction of a Joule have made it possible to generate irradiance levels approaching 1020 W/cm2. We presently operate two such systems, a KrF based excimer laser capable of producing a few 1017 W/cm2 at 248 nm with a repetition rate of 3–5 Hz and a XeCl based excimer laser capable of producing mid 1019 W/cm2 at 308 nm and 1 Hz. We will discuss some experimental results and the theory and modeling of the interaction of such intense laser pulses with aluminum. Because of a small ASE prepulse the high intensity interaction is not at the solid surface but rather at the ne=2×1022 cm−3 (KrF) laser critical density of the blowoff plasma generated by the ASE. The transient behavior of the plasma following the energy deposition by the intense subpicosecond pulse can be viewed as the energy‐impulse response of the plasma. Experimental results and modeling of the x‐ray emission from this plasma are presented.

Experimental tests of a moving foil as a high current vacuum opening switch

We report on the first experimental investigation of the use of a magnetically accelerated foil—moving foil—as a vacuum opening switch. One megampere, 60% of the total current, was transferred to a low inductance path in 800 ns.

Long-term operation of ground-based atmospheric sensing systems in the tropical Western Pacific

Three semi-autonomous atmospheric sensing systems were installed in the tropical western Pacific region. The first of these Atmospheric Radiation and Cloud Stations (ARCS) began operation in 1996. Each ARCS is configured as a system-of-systems since it comprises an ensemble of independent instrument systems. The ARCS collect, process, and transmit large volumes of cloud, solar and thermal radiation, and meteorological data to support climate studies and climate-modeling improvements as part of the U.S. Department of Energy’s Atmospheric and Radiation Measurement (ARM) Program. Data from these tropical ARCS stations have been used for satellite ground-truth data comparisons and validations, including comparisons for MTI and AQUA satellite data. Our experiences with these systems in the tropics led to modifications in their design. An ongoing international logistics effort is required to keep gigabytes per day of quality-assured data flowing to the ARM program’s archives. Design criteria, performance, communications methods, and the day-to-day logistics required to support long-term operations of ground-based remote atmospheric sensing systems are discussed. End-to-end data flow from the ARCS systems to the ARM Program archives is discussed.

High-intensity subpicosecond XeCl laser: laser physics and x-ray generation

Experiments performed with a terawatt-class laser system which is based on amplification in XeCl discharge amplifiers are described. Energy extraction in XeCl discharge amplifiers is examined experimentally and theoretically using 0.25-ps nearly transform limited pulses and both 16- and 75-ps strongly chirped pulses of the same bandwidth. We find that while the saturation fluence for the 16-ps pulse is comparable to that of the 0.25-ps pulse, stretching the pulse to a width of 75 ps increases the saturation fluence by 35%. A study of the interaction of the terawatt laser pulses with aluminum targets at irradiances exceeding 5 X 1018 W/cm2 is also presented.