Progress in long scale length laser–plasma interactions

Abstract

The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2 × 1015 W cm−2. The targets were filled with 1 atm of CO2 producing up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 1020 cm−3 and temperatures of Te = 2 keV. The high energy in an NIF quad of beams of 16 kJ, illuminating the target from one direction, creates unique conditions for the study of laser–plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keV x-rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ∼1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (∼2 mm), increasing to 10–12% for ∼7 mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modelling of the laser–plasma interactions at ignition-size scale lengths.

Absolute x-ray yields from laser-irradiated germanium-doped low-density aerogels

“…33,34 The two instability channels by which the long scale length, underdense plasma may reflect laser energy are by stimulated Brillouin scattering ͑SBS͒ or stimulated Raman scattering ͑SRS͒. 35 We measured the energy backscattered from our targets back into the optics of one cone 2 and one cone 3 beam ͑beams 25 and 30, respectively͒.…”

The National Ignition Facility: enabling fusion ignition for the 21st century

“…In late summer 2003 16 kJ of 3 light in four beams was delivered to the target chamber for experiments. 40 We plan to complete a campaign of increasing energy 3 shots through 2004 culminating in a full energy shot with 40 kJ of energy in four beams, thereby demonstrating NIF's maximum 3 performance requirements.…”

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Absolute instability modes due to rescattering of stimulated Raman scattering in a large nonuniform plasma

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