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Inside the Nova Target Chamber: UCB Studies of X-Ray Ablation, Gas Dynamics and Deposition |
Graduates students Andy Anderson and John Scott with Professor Per Peterson inside the Nova laser target chamber at Lawrence Livermore National Laboratory. Nova has provided an important experimental capability to study x-ray ablation from various surfaces, to understand how the new National Ignition Facility target chamber will perform.
A 1.6-mm diameter Nova target explodes. Frequency conversion crystals change the red light of the Nova lasers to green and then blue before entering the chamber. While the blue light focuses inside tiny holes at the ends of the cylindrical target, or hohlraum, and heat its inside, the green light focuses beyond the target and illuminates the target, the stalk supporting it, and three instruments located less than two inches away, for a brief instant before the target vaporizes. The expanding target debris is initially hot enough to radiate visible light, forming the blue cloud around the location of the original hohlraum, but the target debris cools as it expands until it no longer emits. Simultaneously x-rays from the target have ablated material from the surfaces of the instruments. When this ablation debris begins to encounter high velocity target debris, it is heated enough to emit light. With the ablation debris's greater density, it pushes target debris back, ultimately stopping about 1/5 of the way to the target as it encounters the highest density target debris. Calculations with the UCB code TSUNAMI model this interaction well. The target and ablation debris end up depositing on surfaces around the inside of the target chamber. (hohlraum length 2.3 mm, laser entrance holes diameter 1.2 mm, wall thickness 20-25 microns, internal shields diameter 0.35 mm at 0.55 mm from center, laser energy 20 kJ, laser pulse 1-ns square, Credit Gary Stone, LLNL)
Numerous instruments inside the target chamber point at the location where the target positioner would suspend an ICF target. Laser light enters the chamber through two conical rings of five beam ports. On the opposite side of the target chamber beam dumps absorb the green light which focuses past the target. A small motor on the Wolter x-ray microscope, partially shielded from the target by the round face of the instrument, shows a sharp line between the region exposed to target x-rays, where x-ray ablation occurs leaving a shiny surface, and the shadowed region where target and ablation debris accumulate.
A round rail, below a circular handrail in the bottom of the target chamber, shows a clear interface between the portion shielded from target x rays by the handrail above, and the part maintained clean by x-ray ablation.
Instrument cabling inside the chamber casts an artificial shadow on the target chamber wall, providing a shaded region where target and ablation debris can accumulate without being remobilized by target x-rays. This observation led to the current design for the National Ignition Facility target chamber, where angled louvers will be attached to panels on the target chamber wall, so that material mobilized from the louver surfaces will deposit primarily on the back sides of adjacent louvers, rather than escaping into the target chamber. Experiments in Nova, supported by TSUNAMI calculations, suggest that over 90% of the mobilized ablation debris can be caught this way.