Shiva and ICF
Shiva was never expected to reach ignition conditions, and was primarily intended as a proof-of-concept system for a larger device that would. Even before Shiva was completed, the design of this successor, then known as Shiva/Nova, was well advanced. The Shiva target chamber utilized high-resolution, high-speed optical and X-ray instruments for the characterization of the plasmas created during implosion.
When experiments with targets started in Shiva in 1978, compression was ramped upward to about 50 to 100 times the original density of the liquid hydrogen, or about 3.5 to 7 g/mL. For comparison, lead has a density of about 11 g/mL. While impressive, this level of compression is far too low to be useful in an attempt to reach ignition. Studies of the causes of the lower than expected compression led to the realization that the laser was coupling strongly with the hot electrons (~50 KeV) in the plasma which formed when the outer layers of the target were heated, via stimulated raman scattering. John Holzrichter, director of the ICF program at the time, said:
The laser beam generates a dense plasma where it impinges on the target material. The laser light gives up its energy to the electrons in the plasma, which absorb the light. The rate at which that happens depends on the wavelength and the intensity. On Shiva, we were heating up electrons to incredible energies, but the targets were not performing well. We tried a lot of stuff to coax the electrons to transfer more of their energy to the target, with no success.
It was earlier realized that laser energy absorption on a surface scaled favorably with reduced wavelength, but it was believed at that time that the IR generated in the Shiva Nd:glass laser would be sufficient for adequately performing target implosions. Shiva proved this assumption wrong, showing that irradiating capsules with infrared light would likely never achieve ignition or gain. Thus Shiva's greatest advance was in its failure, a not entirely obvious example of the null result.
ICF research turned to using an "optical frequency multiplier" to convert the incoming IR light into the ultraviolet at about 351 nm, a technique that was well known at the time but was not efficient enough to be worthwhile. Research on the GDL laser at the Laboratory for Laser Energetics in 1980 first achieved efficient frequency tripling techniques which were then used next (for the first time at LLNL) on Shiva's successor, the Novette laser. Every laser-driven ICF system after Shiva has used this technique.
On January 24 of 1980, a 5.5 magnitude earthquake at Livermore shook the facility enough to shear fist-sized bolts off Shiva; repairs were made and the laser was subsequently put back online a month later. Many experiments including testing the "indirect mode" of compression using hohlraums continued at Shiva until its dismantling in 1981. Shiva's target chamber would be reused on the Novette laser. Maximum fusion yield on Shiva was around 1010 to 1011 neutrons per shot.
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