Argon gas puffs have produced 330kJ ± 9% of x-ray radiation above 3keV photon energy in fast z-pinch implosions, with remarkably reproducible K-shell spectra and power pulses. This reproducibility in x-ray production is particularly significant in light of the variations in instability evolution observed between experiments. Soft x-ray power measurements and K-shell line ratios from a time-resolved spectrum at peak x-ray power suggest that plasma gradients in these high-mass pinches may limit the K-shell radiating mass, K-shell power, and K-shell yield from high-current gas puffs.
This Letter presents results from the first fully integrated experiments testing the magnetized liner inertial fusion concept [S.A. Slutz et al., Phys. Plasmas 17, 056303 (2010)], in which a cylinder of deuterium gas with a preimposed axial magnetic field of 10 T is heated by Z beamlet, a 2.5 kJ, 1 TW laser, and magnetically imploded by a 19 MA current with 100 ns rise time on the Z facility. Despite a predicted peak implosion velocity of only 70 km/s, the fuel reaches a stagnation temperature of approximately 3 keV, with Te ≈ Ti, and produces up to 2e12 thermonuclear DD neutrons. In this study, X-ray emission indicates a hot fuel region with full width at half maximum ranging from 60 to 120 μm over a 6 mm height and lasting approximately 2 ns. The number of secondary deuterium-tritium neutrons observed was greater than 1010, indicating significant fuel magnetization given that the estimated radial areal density of the plasma is only 2 mg/cm2.
The refurbished Z machine at Sandia National Laboratories has been successfully configured to drive gas-puff Z pinches. A series of experiments using Ar loads produced K-shell yields of 330 ± 9% kJ, with highly reproducible K-shell spectra and power pulses. Using spectroscopic and power data, we are able to constrain the properties of both the cold, non-K-shell radiating mass as well as the hot K-shell component of the pinch plasma. As in previous gas-puff shots on the original version of Z, only about 1/4 to 1/3 of the load mass was heated to temperatures sufficient to produce K-shell x-rays.
Radiation magnetohydrodynamic r-z simulations are performed of recent Ar shots on the refurbished Z generator to examine the effective ion temperature as determined from the observed line width of the He-γ line. While many global radiation properties can be matched to experimental results, the Doppler shifts due to velocity gradients at stagnation cannot reproduce the large experimentally determined width corresponding to an effective ion temperature of 50 keV. Ion viscous heating or magnetic bubbles are considered, but understanding the width remains an unsolved challenge.