(Excerpted from: “Inertial Confinement Fusion,” Energy & Technology Review, Lawrence Livermore National Laboratory, October 1982; with modifications)
The Shiva laser was finished, on schedule and within budget, in November 1977. It attained it’s desired 10-kJ performance, with a pulse duration shorter than 1 ns, and worked reliably and continuously until it was decommissioned at the end of 1981. Through those years, we added a great number of improvements, changes, and additional diagnostics. The Shiva laser and target facility was our workhorse, and it provided enormous amounts of data.
Shiva provided enough energy to permit us to begin experiments with ablatively compressed, directly driven targets and to investigate the problems associated with long-scale-length laser-plasma interactions. These experiments called for very sophisticated target fabrication techniques capable of depositing thick, extremely uniform layers of plastic (ablators) on thin-walled glass shells. The experimental measurements were greatly aided by high-resolution, high-speed optical and x-ray diagnostic instruments, remarkably sensitive radiochemistry systems capable of measuring the extremely low radioactivity level of 1 Bq (one decay per second), and accurate high-speed optical detectors for measuring preheating and the onset of stimulated scattering.
We were also able to demonstrate, with the Shiva laser, fuel compression to 100 times the density of liquid D-T using radiation-driven targets. Most important, we were able to diagnose the fuel density accurately by radiochemistry, using activation of silicon in the target.
In the late 1970s, it became clear that directly driven targets irradiated with 1-µm light were being preheated by energetic electrons generated by plasma instabilities. Because the 1-µm light is associated with a relatively low critical plasma density (n e = 1021/cm), it can excite large, relatively undamped waves in the plasma. Electrons accelerated in these waves to high energy (hot electrons) penetrate the target, preheating the target and fuel layers and making efficient compression difficult if not impossible.
Our realization of the extent of this problem caused us to investigate vigorously various means of controlling the generation of these hot electrons. One such method is to irradiate the target with a light of shorter wavelength (associated with a much higher critical plasma density). Using the two-beam Argus laser, we obtained the shorter-wavelength light by harmonically converting the light from the existing neodymium-glass lasers.
