Conceptual design of initial opacity experiments on the national ignition facility

Abstract

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 150 eV and electron densities 7 × 10 21 cm −3. The iron will be probed using continuum X-rays emitted in a ∼200 ps, ∼200 µm 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 ∼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.

Iron X-ray Transmission at Temperature Near 150 eV Using the National Ignition Facility: First Measurements and Paths to Uncertainty Reduction

“…Section 7 draws summarizes and discusses future work. Figure 1 (left) illustrates the experimental approach used at NIF [3,4]. Opacity experiments have many detailed requirements [7,8], but the heart of the measurement is a fully calibrated X-ray transmission spectrum obtained through a uniform sample with known composition, areal density, electron density, and temperature.…”

“…Several years' research by various teams into possible systematic errors in the experiment or inadequate approximations in various opacity theories have not resolved the disagreement. To address this divergence, an entirely different opacity experimental 'platform' has been developed [3,4] on the National Ignition Facility (NIF) laser system [5,6]. The current goal of the NIF campaign is to independently replicate the Z measurements at 156 eV, including quantification of the uncertainties, to validate the NIF platform.…”

“…The backlighter X-ray flash passes through a 750 × 1200-micron rectangular slot in a silicon-coppersilicon layered collimator, and then transits the hohlraum by passing either through or around the rectangular sample [3]. The resulting backlighter and absorption spectra are projected into the NIF opacity spectrometer (OpSpec), an open-aperture time-integrated crystal spectrometer [12,13] pointed downward from the upper pole of the NIF target chamber.…”

“…This type of opacity experiment requires the accurate measurement of X-ray transmission through a sample of known areal density at known density and temperature [3,7,8]. The areal density is measured prior to shot as detailed in Section 2.…”

“…The X-ray transmission measurement itself is produced using point-projection X-ray spectroscopy as illustrated in Figure 1 (left), following previous design and implementation work [3,7]. Specific details of the opacity spectrometer (OpSpec) setup for shot N171214-001 are given below, but in brief the underlying symmetry of the experimental platform enables OpSpec to record two sets of spectra per shot, one per image plate, with each side of the spectrometer observing half of the sample in transmission, together with the backlighter spectrum passing around the sample on one side [3,12,13]. The two image plate detectors each record spectra from ~1000 to 2000 eV in one dimension, spatially resolving the spectra from the sample absorption (region U), backlighter (region V), sample self-emission (region E), and various instrument backgrounds in the other dimension.…”