Inertial Fusion: NIF hohlraum experiments yield first light
Initial “hohlraum” experiments on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL; Livermore, CA) have validated computer simulations and theoretical projections related to the plasma and x-ray environment necessary to achieve ignition, according to researchers.
Initial “hohlraum” experiments on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL; Livermore, CA) have validated computer simulations and theoretical projections related to the plasma and x-ray environment necessary to achieve ignition, according to researchers. (A hohlraum is small hollow sphere that converts laser light to x-rays to compress the fuel within it). The experiments, which involved activating the first four laser beams in NIF to measure laser-beam propagation in long-scale plasmas with lengths of gas-filled ignition hohlraums (gold-plated cylinders), demonstrated hohlraum performance limits consistent with simulations and analytic modeling for both low and high plasma-filling conditions.
One of NIF’s unique characteristics is the ability to provide long, steady laser drives with variable pulse lengths, up to 20 ns. In the hohlraum experients, 2-ns flattop UV laser pulses from the four beams (about 8 × 1012 W) were fired at a series of hohlraums of different sizes, each with a single laser entrance hole on one end, to test radiation temperature limits imposed by plasma filling (see figure). According to lead author Eduard Dewald, the experiments-which were performed in September 2004 but are described in a Nov. 18, 2005, Physical Review Letters article-produced radiation temperatures in agreement with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The measured x-ray flux showed signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraums.
Dewald and colleagues point out that their results can be extrapolated to higher laser energies by applying a simple analytic model for radiation-temperature limits. They believe that although the initial experiments were limited to the first four NIF beams-each of which only lasted about 9 ns-and only 1% of NIF’s ultimate 1.8-MJ output, future hohlraum experiments will be hotter, larger, and of longer duration when all of NIF’s 192 beams are fully operational. Eventually, the deuterium-tritium fuel capsule will be placed inside a larger hohlraum and all 192 beams will heat the interior of the hohlraum, through holes on both ends, creating x-rays that ablate and implode the capsule to ignition.
The initial series of experiments was the first hohlraum campaign on NIF and part of a larger four-campaign effort, coordinated by LLNL’s Nino Landen, and involving researchers from LLNL, Los Alamos National Laboratory, the United Kingdom’s Atomic Weapons Establishment, and General Atomics.