X-ray Opacity

Platform Purpose

The NIF x–ray opacity platform will enable detailed studies of the radiative properties of hot dense matter at previously unachievable temperatures, including absorption and emission spectroscopy over a wide photon energy range of 200 - 10,000 eV. The energy and precision of the NIF laser coupled with the NIF diagnostic suite will allow the study of radiatively driven hot dense plasmas in regimes previously inaccessible, allowing important advances in astrophysics, inertial fusion, radiation hydrodynamics, and related areas. Examples of experiments currently under study for this platform include: stellar core opacities to benchmark opacities used in the standard solar model and in stellar equilibrium codes (relevant to exoplanet habitability assessment) and absorption/emission spectroscopy of photoionized plasmas scaled to black hole and neutron star accretion-disk conditions. These experiments will be enabled by NIF´s capabilities to deliver very high radiation temperatures (photon fluxes) to produce samples with a wide range of densities and to characterize the resulting plasmas with excellent time–, space– and photon–energy resolution.

The x–ray opacity platform is under development but could be operational in late FY10.

Laser and Target Configuration

The laser and target configurations for this platform are summarized below:

Figure 1 shows a conceptual opacity target (right) with broad–spectrum capsule backlighter (left).

Figure 1: Experimental Configuration, Opacity (Left): Gas–filled plastic shell (could be directly or indirectly driven) used as a broad-spectrum backlighting source. (Right:) Sample inside laser–heated hohlraum.

The main target consists of a gold hohlraum similar to those used in the radiation transport and hohlraum energetics platforms, but potentially as small as 1.6 mm diameter, 3.0 mm long. The sample to be tested is placed in the center. Roughly 400 kJ (2/3 of the NIF beams) is delivered to the hohlraum in 2 ns, but 1/3 of the beams (200 kJ) go to a backlighter placed orthogonal to the hohlraum axis (to the left in the figure). The backlighter produces a bright x–ray flash that shines through the capsule to a gated x–ray spectrometer (to the right). Initial experiments to characterize the drive temperature suggest sample radiation temperatures above 300 eV should be achievable. The capsule backlighter can be directly driven, or indirectly driven using a second hohlraum. Prototype backlighters have been demonstrated at OMEGA using both spherically symmetric and polar direct-drive methods.

Spatial discrimination in the spectrometer allows simultaneous measurement of the sample X–ray emission and absorption, as well as the backlighter emission necessary to obtain normalized transmission spectra. The self-emission and transmission data can be used to benchmark opacity codes. Prototype instruments tested at OMEGA span the photon energy range from 250 to 9000 eV, the photon energy range relevant to Rosseland mean opacity measurements for plasma temperatures up to and beyond 600 eV. A second gated or streaked imager, orthogonal to the hohlraum and the main spectrometer, characterizes the sample size and thus, its density as a function of time.

Figure 2 shows an alternate configuration, the photoionization target geometry. By switching to a larger, lower–density sample outside the hohlraum (where it might be pre–expanded using a pair of NIF beams), the hohlraum radiation produces non–LTE conditions where photoionization is the dominant process. Photoionization parameters (4*pi*flux/density) in excess of 1000 erg/s/square cm may be achieved, comparable to black hole accretion disk conditions.

Figure 2: Experimental Configuration, Photoionization: This variation of the experimental configuration places the sample outside the hohlraum, resulting in a photoionization-dominated plasma.

In the photoionization platform, the diagnostics (and optional backlighter) are identical to the opacity platform. Both absorption and self–emission spectra can again be obtained over the band from below 250 eV to above 9000 eV.

Potential users are invited to consider packages of interest that might be driven using these sources and investigated using these diagnostics.

Table 1 summarizes the characteristics of the x–ray opacity platform.

Target	# of beams	Total energy	Laser energy/Pulse width
Gold hohlraum (no gas fill)	128 to 192	400 to 600 kJ	Above 300 eV for 1-2 nsec
Optional backlighter (Kr-filled CH shell)	0 or 64	0 to 200 kJ	350 kJ/ 2 nsec

Variants on this platform may be possible; contact the NIF User Office for further information.

Diagnostics

Standard diagnostics for this platform and their location (elevation, azimuth) are provided below. See the chamber geometry and diagnostic write-ups for additional information. For this platform, the primary diagnostics are one or two gated x-ray detectors (GXDs) and a streaked x-ray detector (SXD), with 2 of these used to characterize the opacity or photoionized "sample" and one to monitor the capsule backlighter implosion time. The hohlraum energetics diagnostics (Dante-1 (lower), Dante-2 (upper), FABS/NBI, FFLEX and SXI upper/lower) are also used to verify laser and hohlraum performance.

Diagnostic	Location
GXD/SXD with spectrometer "snout"	Equatorial DIM (90-45)
GXD/SXD with imaging "snout"	Equatorial DIM (90-315)
SXD/GXD with imaging "snout"	Polar DIM (0-0)
Dante-1	Fixed (143-274)
Dante-2	Fixed (64-350)
FABS/NBI	Q31B, Q36B
FFLEX	Fixed (90-110)
SXI, Top/Bottom	Fixed ((18-123),(161-326))

For further information on scientific opportunities at the NIF, please contact:
Dr. Christopher J. Keane, LLNL
PHONE: (925) 422-2179
E-MAIL: Contact us