Origin and early history of the shiva program for high-power soft X-ray generation

Summary form only given. Back in 1970, the two principal sources of prompt X-radiation were high-voltage bremsstrahlung machines and the dense plasma focus. The former, operating at diode voltages above about a megavolt, provided X-ray spectra that were adequate above many tens of kilovolts, (and better at even higher energies). The latter device offered much softer spectra, but poor efficiencies for conversion of stored energy to X-ray output. In both cases, efficiencies were at much less than a few percent. We needed to do better, which required a new approach.The Shiva program, which began officially in February 1971, after initial calculations in December 1970, was based on the concept originally called the Electromagnetic Implosion Xray Source1. This concept is basically a large-radius z-pinch implosion of high-Z material. It derives from work that was directed toward electric propulsion for spacecraft2. The concept comprises the initiation of a multi-megampere, axial discharge through a thin cylindrical, metallic foil or wirearray, which quickly becomes a plasma; followed by the implosion of this high-Z plasma liner due to the radial Lorentz force, accumulating kinetic energy at speeds above 200 km/s; and then converting this energy, obtained over a few microseconds or less, into thermal energy and soft Xradiation in several nanoseconds. Operation is based on recognition that the dynamic z-pinch can absorb energy from the power source efficiently (>25%) by means of the timerate-of-change of inductance due to implosion. Successful behavior, however, required conquering the problem of heat transfer from the very high current discharge to the vacuuminsulator surface, especially the copious amounts of UV. At the time, c. 1971, the only sources of the necessary multimegampere currents were capacitor banks comprising many units connected in parallel at very low inductance, and explosively-driven, magnetic-flux compression generators. The Shiva program pursued both types of source, with the former leading over time to the Shiva Star capacitor bank, which has been used for many other applications. With the so-called Plasma Flow Switch, a megajoule of prompt, soft X-radiation was obtained on Shiva Star, completing thSummary form only given. Back in 1970, the two principal sources of prompt X-radiation were high-voltage bremsstrahlung machines and the dense plasma focus. The former, operating at diode voltages above about a megavolt, provided X-ray spectra that were adequate above many tens of kilovolts, (and better at even higher energies). The latter device offered much softer spectra, but poor efficiencies for conversion of stored energy to X-ray output. In both cases, efficiencies were at much less than a few percent. We needed to do better, which required a new approach.The Shiva program, which began officially in February 1971, after initial calculations in December 1970, was based on the concept originally called the Electromagnetic Implosion Xray Source1. This concept is basically a large-radius z-pinch implosion of high-Z material. It derives from work that was directed toward electric propulsion for spacecraft2. The concept comprises the initiation of a multi-megampere, axial discharge through a thin cylindrical, metallic foil or wirearray, which quickly becomes a plasma; followed by the implosion of this high-Z plasma liner due to the radial Lorentz force, accumulating kinetic energy at speeds above 200 km/s; and then converting this energy, obtained over a few microseconds or less, into thermal energy and soft Xradiation in several nanoseconds. Operation is based on recognition that the dynamic z-pinch can absorb energy from the power source efficiently (>25%) by means of the timerate-of-change of inductance due to implosion. Successful behavior, however, required conquering the problem of heat transfer from the very high current discharge to the vacuuminsulator surface, especially the copious amounts of UV. At the time, c. 1971, the only sources of the necessary multimegampere currents were capacitor banks comprising many units connected in parallel at very low inductance, and explosively-driven, magnetic-flux compression generators. The Shiva program pursued both types of source, with the former leading over time to the Shiva Star capacitor bank, which has been used for many other applications. With the so-called Plasma Flow Switch, a megajoule of prompt, soft X-radiation was obtained on Shiva Star, completing the early goal of the Shiva program3.e early goal of the Shiva program3.