X-ray Sources Platform

Platform Purpose

Scientists from LLNL and elsewhere are collaborating to develop and validate a platform for repeatable high flux, high-fluence X-ray exposures of test objects with a specific spectral content and specific temporal waveforms in the incident X rays. The targets, or X-ray sources, in these experiments are low mass (≈15 mg) thin-walled (25 µm) gas pipes filled with a mixture of argon (Ar) and xenon (Xe) gasses. Studies will be done on target output as a function of Xe-to-Ar ratio, target fill pressure and laser-drive energy and power. Other sources, made from nearly all elements on the period table heavier than argon, are being designed for unique spectral characteristics and will be shot soon. In addition to test-object exposures, the bright X-ray sources can and will be used for radiography applications in other HED experiments and in laboratory studies of astrophysical environments.

The early experiments in this program will validate source-output reproducibility, demonstrate X-ray-output temporal pulse shaping, and quantify the debris from the X-ray source at different positions in the NIF chamber. For the first shots in developing this platform, a 4- x 4-mm target will be driven with 96 to 112 beams, using between 350 to 380 kJ of laser light, with a 5-ns-long flat-top pulse. A photo of one of the targets and two X-ray images from the target during an experiment are shown below. x-ray source figure 1 For this experiment, 70 TW of laser power was used to create a high-temperature plasma that optimized the output of L-shell Xe radiation. The emission is seen to be fairly uniform from the target’s whole volume by ≈1.5 ns into the 5-ns laser pulse. The gas fill pressure was 1.2 atm, which resulted in a plasma electron density in the target of ≈ 1021 cm^-3. This density is only 1/10th the critical density for the NIF laser light to be reflected from the target plasma. This means that the laser beams propagate supersonically through the gas in the target, heating the gas and ionizing material faster than the target can hydrodynamically respond, which leads to a highly efficient laser-to-X-ray conversion process. When the laser beams have reached the gas pipe wall, there is still some energy remaining in the beams, which results in the bright band seen in the middle of the target in the 1.87-ns image.

The NIF facility’s two X-ray diode-based Dante detectors determine the X-ray yield from the targets. There are enough channels in the Dante diagnostics that the spectral content of the X-ray emission can be reconstructed in specific spectral bands. The Dante detectors give the temporal waveform of the X-ray emission in different spectral bands. Hard X-ray emission (with energies > 20 keV) is measured with the Filter Fluorescer (FFLEX) detector. Spectral measurement of laser energy backscattered from the target is made with the Full Aperture Backscatter Station (FABS) and Near Backscatter Imager (NBI), which provide information on both the laser coupling and whether the desired plasma conditions (i.e., plasma electron temperature) have been achieved.

Since one of the purposes of this platform is to apply the resulting X-ray environments in the NIF chamber to interactions with test objects and scaled, surrogate astrophysical systems, the debris produced by the target and its trajectory in the chamber must be characterized and quantified. Witness films were placed at different orientations with respect to the target positioner axis on the equatorial plane in the chamber. The films were located between 34 and 50 cm from the X-ray source. No debris from the thin-wall gas pipes has been found on the witness films from any shot so far. Thus, it has been concluded that this platform will allow the study of X-ray interactions without the complication of target-debris interactions.

Diagnostic Configuration

Standard diagnostics for this platform and their location (elevation, azimuth) are provided below. See the Chamber Geometry and Diagnostics for additional information.

A summary of NIF diagnostics planned through FY2011 is shown below:

Diagnostic	Location
Dante-1	Fixed (143-274)
Dante-2	Fixed (64-350)
FABS/NBI	Q31B, Q36B
FFLEX	Fixed (90-110)
GXD	DIM 90-315
Target debris collection	DIM 90-45
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