Implementation of a 1-2 keV point-projection x-ray spectrometer on the National Ignition Facility

Abstract

A point-projection soft X-ray Opacity Spectrometer (OpSpec) has been implemented to measure X-ray spectra from ∼1 to 2 keV on the National Ignition Facility (NIF). Measurement of such soft X-rays with open-aperture point-projection detectors is challenging because only very thin filters may be used to shield the detector from the hostile environment. OpSpec diffracts X-rays from 540 to 2100 eV off a potassium (or rubidium) acid phthalate (KAP or RbAP) crystal onto either image plates or, most recently, X-ray films. A “sacrificial front filter” strategy is used to prevent crystal damage, while 2 or 3 rear filters protect the data. Since May 2017, OpSpec has been recording X-ray transmission data for iron-magnesium plasmas on the NIF, at “Anchor 1” plasma conditions (temperature ∼150 eV, density ∼7 × 1021 e−/cm3). Upgrades improved OpSpec’s performance on 6 NIF shots in August and December 2017, with reduced backgrounds and 100% data return using filter stacks as thin as 2.9 μm (total). Photometric noise is beginning to meet requirements, and further work will reduce systematic errors.

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

“…The RBS technique provides information on the areal density On the other hand, the overall general agreement between the two datasets at X-ray energies above 1200 eV provides encouragement that sample conditions are reasonably symmetric. Also, the similarity between this shot and the earlier work [13] suggests that reproducible measurements with low uncertainty should be possible. An upcoming experiment will repeat this measurement with a thicker sample to check whether the data follow the Beer-Lambert scaling, T(υ) = e − Lκ(υ) with = sample density, L = thickness (pathlength), and κ = opacity.…”

“…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. Inside OpSpec the X-rays propagate through three aluminized-plastic X-ray filters, Bragg-diffract off one of two convex rubidium acid phthalate (RbAP) crystals, then propagate past three more aluminized-plastic filters before being absorbed by one of two adjacent Fuji BAS-TR X-ray imaging plates [12,13]. After recent improvements [13] the spectrometer recently delivered the first analyzable transmission data [14].…”

“…Inside OpSpec the X-rays propagate through three aluminized-plastic X-ray filters, Bragg-diffract off one of two convex rubidium acid phthalate (RbAP) crystals, then propagate past three more aluminized-plastic filters before being absorbed by one of two adjacent Fuji BAS-TR X-ray imaging plates [12,13]. After recent improvements [13] the spectrometer recently delivered the first analyzable transmission data [14].…”

“…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. Inside OpSpec the X-rays propagate through three aluminized-plastic X-ray filters, Bragg-diffract off one of two convex rubidium acid phthalate (RbAP) crystals, then propagate past three more aluminized-plastic filters before being absorbed by one of two adjacent Fuji BAS-TR X-ray imaging plates [12,13].…”

“…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.…”