Crossed Beam Energy Transfer in the NIF ICF Target Design
In the National Ignition Facility (NIF) ICF point design, the cylindrical hohlraum target is illuminated by multiple laser beams through two laser entrance holes on the ends. According to simulations by LASNEX and HYDRA plasma created inside the hohlraum will stream out of the LEH, accelerate to supersonic speeds and then fan out radially. Inside the hohlraum, flows are subsonic. Forward Brillouin scattering can transfer energy between pairs of laser beams (0 and 1) if the following frequency matching condition is satisfied: {omega}{sub 0} - {omega}{sub 1} = (k{sub 0} - k{sub 1}) {center_dot} V + |k{sub 0} - k{sub 1}| c{sub s} (1) where {omega}{sub 0.1} and k{sub 0.1} are the frequencies and wave-numbers of the two laser beams, V is the plasma flow velocity and c{sub s} is the local ion sound speed. In the nominal case of equal frequency beams, this requires the component of the plasma flow velocity transverse to the bisector of the beam directions to be sonic, with the resulting transfer being to the downstream beam. In the NIF beam geometry, this is from the outer to inner cones of beams. The physics of this transfer is the same as in beam bending; the difference being that in the case of beam bending the effect is to redistribute power to the downstream side of the single beam. Were significant power transfer to occur in the point design, the delicately tuned implosion symmetry would be spoiled. To directly compensate for the transfer, the incident beam powers would have to be adjusted. The greatest vulnerability in the point design thus occurs at 15.2ns, when the inner beams are at their peak power and are at their nominal design power limit. In this situation, some other means of symmetry control would be required, such as re-pointing. At 15.2ns, the envelope focal intensities of the outer and inner beams are approximately 10{sup 15} and 6.7 10{sup 14} W/cm{sup 2} respectively. There is little absorption or diffractive spreading of the beams in the crossing region, so these intensities are also representative there. The outer beams are at higher intensity, despite their lower power, because of their smaller spot-size required to clear the LEH at their steeper angle of incidence. The approximate locations of the crossing beams are shown in Fig. (1). There are four cones of NIF beams entering each LEH. The eight quads at 50{sup o} and 44.5{sup o} to the hohlraum axis are termed the outer beams; four quads at 30{sup o} and 23{sup o} form the inner beams. Each cone is arranged symmetrically around the hohlraum axis, staggered in azimuthal angle. Condition (1) is thus a constraint on the 3D plasma flow velocity that differs modestly, depending on precisely which pair of overlapping beams is considered. In order for the correct component of the flow to be sonic, the total flow in general will be supersonic. The potential resonant surfaces therefore lie outside the LEH.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15004921
- Report Number(s):
- UCRL-JC-152016; TRN: US0305208
- Resource Relation:
- Conference: 2003 Third International Conference on Inertial Fusion Sciences and Applications, Monterey, CA (US), 09/07/2003--09/12/2003; Other Information: PBD: 27 Aug 2003
- Country of Publication:
- United States
- Language:
- English
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