P. A. Jaanimagi

Direct‐drive laser‐fusion experiments with the OMEGA, 60‐beam, >40 kJ, ultraviolet laser system

OMEGA, a 60‐beam, 351 nm, Nd:glass laser with an on‐target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct‐ and indirect‐drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct‐drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct‐drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high‐gain, direct‐drive capsules. Details are presented of the OMEGA direct‐drive experimental program and initial data from direct‐drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficienc...

High-speed gated x-ray imaging for ICF target experiments (invited)

We describe the use of gated microchannel‐plate detectors as high‐speed framing cameras in laser‐driven inertial‐confinement‐fusion experiments. Using an array of pinholes to image the target, detectors capable of generating up to 16 individual frames with ∼90 ps resolution on a single laser shot are now in routine use. The detectors have been used to study the development of intentionally applied perturbations in laser‐driven targets. In off‐line tests new detectors have demonstrated time resolutions better than 40 ps.

Experimental investigation of smoothing by spectral dispersion

Measurements of smoothing rates for smoothing by spectral dispersion (SSD) of high-power, solid-state laser beams used for inertial confinement fusion (ICF) research are reported. Smoothing rates were obtained from the intensity distributions of equivalent target plane images for laser pulses of varying duration. Simulations of the experimental data with the known properties of the phase plates and the frequency modulators are in good agreement with the experimental data. These results inspire confidence in extrapolating to higher bandwidths and other SSD configurations that may be suitable for ICF experiments and ultimately for direct-drive laser-fusion ignition.

Characterization of a subpicosecond x-ray streak camera for ultrashort laser-produced plasmas experiments

We present results of the characterization of an ultrafast x-ray streak camera, based on Photonis (formerly Philips Photonics) P860 tube, developed for use in ultrashort laser-produced plasma research. The streak camera presented here (called PX1) has been extensively characterized with continuous and pulsed x-ray sources. Time resolution of 350 fs in the keV x-ray range has been achieved, while maintaining a high spatial resolution of 40 μm along a direction perpendicular to the time dispersion axis. It is shown that the streak camera response is lower when the photocathode is illuminated by a pulsed source than when used with a continuous one. This effect seems to be related to a change in the phosphor response. The camera has been used to achieve high-resolution subpicosecond time-resolved spectroscopy of ultrashort laser plasmas allowing the measurements of K-shell line emission durations of 700 fs.

New constraints for plasma diagnostics development due to the harsh environment of MJ class lasers (invited)

The design of plasma diagnostics for the future MJ class lasers (LMJ–Laser MegaJoule—in France or NIF—National Ignition Faciliy— in the USA) must take into account the large increased radiation field generated at the target and the effect on the diagnostics components. These facilities will focus up to 1.8 MJ ultraviolet laser light energy into a volume of less than 1 cm3 in a few nanoseconds. This very high power focused onto a small target will generate a large amount of x rays, debris, shrapnel, and nuclear particles (neutrons and gamma rays) if the DT fuel capsules ignite. Ignition targets will produce a million more of 14 MeV neutrons (1019 neutrons) by comparison with the present worldwide most powerful laser neutron source facility at OMEGA. Under these harsh environmental conditions the survivability goal of present diagnostic is not clear and many new studies must be carried out to verify which diagnostic measurement techniques, can be maintained, adapted or must be completely changed. Synergies ...

Characterization of direct-drive-implosion core conditions on OMEGA with time-resolved Ar K-shell spectroscopy

Direct-drive-implosion core conditions have been characterized on the 60-beam OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system with time-resolved Ar K-shell spectroscopy. Plastic shells with an Ar-doped deuterium fill gas were driven with a 23 kJ, 1 ns square laser pulse smoothed with 1 THz smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges. The targets are predicted to have a convergence ratio of ∼15. The emissivity-averaged core electron temperature (Te) and density (ne) were inferred from the measured time-dependent Ar K-shell spectral line shapes. As the imploding shell decelerates the observed Te and ne increase to 2.0 (±0.2) keV and 2.5 (±0.5)×1024 cm−3 at peak neutron production, which is assumed to occur at the time of the peak emissivity-averaged Te. At peak compression the ne increases to 3.1 (±0.6)×1024 cm−3 and the Te decreases to 1.7 (±0.17) keV. The observed core conditions are close to those predicted by a one-dimensional h...

Performance of 1-THz-bandwidth, two-dimensional smoothing by spectral dispersion and polarization smoothing of high-power, solid-state laser beams

Laser beam smoothing achieved with 1-THz-bandwidth, two-dimensional smoothing by spectral dispersion and polarization smoothing on the 60-beam, 30-kJ, 351-nm OMEGA laser system is reported. These beam-smoothing techniques are directly applicable to direct-drive ignition target designs for the 192-beam, 1.8-MJ, 351-nm National Ignition Facility. Equivalent-target-plane images for constant-intensity laser pulses of varying duration were used to determine the smoothing. The properties of the phase plates, frequency modulators, and birefringent wedges were simulated and found to be in good agreement with the measurements.

Cylindrical implosion experiments using laser direct drive

Direct-drive cylindrical-implosion experiments are performed to study perturbed hydrodynamic flows in convergent geometry. Two experimental campaigns have been conducted, to demonstrate the advantages of direct over indirect drive and to validate numerical simulations of zeroth-order hydrodynamics and single-mode perturbation growth. Results and analysis of three unperturbed-target shots and two perturbed-target shots are discussed in detail. For unperturbed-target implosions, positions of inner and outer shell edges agree between simulation and experiment during the laser pulse. However, observed shell thickness is greater than simulated in unperturbed targets during deceleration and rebound; the effect appears only at the shell’s exterior edge. For perturbed-target implosions, growth factors ∼10–14 are observed, whereas growth factors near 30 are expected from simulation. Rayleigh–Taylor growth appears to differ between simulation and experiment. Observed zeroth-order flow at the exterior edge of implod...

A self-calibrating, multichannel streak camera for inertial confinement fusion applications

Self-calibrating, multichannel UV streak cameras have been designed, and six units have been deployed on the OMEGA laser system. These instruments acquire 12 channels simultaneously on a low-noise, charge-coupled-device camera in single-shot operation. The instruments can discern temporal features out to a bandwidth of 11 GHz, and the peak signal-to-noise ratio in each channel is 200:1. The unique feature of this system is the self-calibration ability built into it. The geometric distortions, flat field, and sweep speed of each channel can be measured and adjusted on a routine basis. By maintaining a strick regime of weekly calibrations, accurate power-balance measurements on the OMEGA laser can be obtained. These cameras represent a cost-effective solution for power balancing the OMEGA laser system.

Space and time resolved measurements of the heating of solids to ten million kelvin by a petawatt laser

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32–0.73 PW and intensities of up to 4×1020 W cm−2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ~107 K (kT/e~ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15–30%, and tends to increase with laser energy.