Conceptual design of initial opacity experiments on the national ignition facility

• Published in: Journal of plasma physics 2017-02, Vol.83 (1), Article 595830103
• Main Authors: Heeter, R. F., Bailey, J. E., Craxton, R. S., DeVolder, B. G., Dodd, E. S., Garcia, E. M., Huffman, E. J., Iglesias, C. A., King, J. A., Kline, J. L., Liedahl, D. A., McKenty, P. W., Opachich, Y. P., Rochau, G. A., Ross, P. W., Schneider, M. B., Sherrill, M. E., Wilson, B. G., Zhang, R., Perry, T. S.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION; ASTRONOMY AND ASTROPHYSICS; ATOMIC AND MOLECULAR PHYSICS; Design; Ions; Iron; Opacity; Plasma physics; Solar physics; Solved and Unsolved Problems in Plasma Physics: Nathaniel J. Fisch Symposium; X-rays
• ISSN: 0022-3778; 1469-7807
• DOI: 10.1017/S0022377816001173

Accurate models of X-ray absorption and re-emission in partly stripped ions are necessary to calculate the structure of stars, the performance of hohlraums for inertial confinement fusion and many other systems in high-energy-density plasma physics. Despite theoretical progress, a persistent discrepancy exists with recent experiments at the Sandia Z facility studying iron in conditions characteristic of the solar radiative–convective transition region. The increased iron opacity measured at Z could help resolve a longstanding issue with the standard solar model, but requires a radical departure for opacity theory. To replicate the Z measurements, an opacity experiment has been designed for the National Facility (NIF). The design uses established techniques scaled to NIF. A laser-heated hohlraum will produce X-ray-heated uniform iron plasmas in local thermodynamic equilibrium (LTE) at temperatures ${\geqslant}150$  eV and electron densities ${\geqslant}7\times 10^{21}~\text{cm}^{-3}$ . The iron will be probed using continuum X-rays emitted in a ${\sim}200$  ps, ${\sim}200~\unicode[STIX]{x03BC}\text{m}$ diameter source from a 2 mm diameter polystyrene (CH) capsule implosion. In this design, $2/3$ of the NIF beams deliver 500 kJ to the ${\sim}6$  mm diameter hohlraum, and the remaining $1/3$ directly drive the CH capsule with 200 kJ. Calculations indicate this capsule backlighter should outshine the iron sample, delivering a point-projection transmission opacity measurement to a time-integrated X-ray spectrometer viewing down the hohlraum axis. Preliminary experiments to develop the backlighter and hohlraum are underway, informing simulated measurements to guide the final design.