Abstract
The Drive campaign [D A Callahan et al., this conference] on the National Ignition Facility (NIF) laser [E. I. Moses, R. N. Boyd, B. A. Remington, C. J. Keane, R. Al-Ayat, Phys. Plasmas 16, 041006 (2009)] has the focused goal of understanding and optimizing the hohlraum for ignition. Both the temperature and symmetry of the radiation drive depend on laser and hohlraum characteristics. The drive temperature depends on the coupling of laser energy to the hohlraum, and the symmetry of the drive depends on beam-to-beam interactions that result in energy transfer [P. A. Michel, S. H. Glenzer, L. Divol, et al, Phys. Plasmas 17, 056305 (2010).] within the hohlraum. To this end, hohlraums are being fielded where shape (rugby vs. cylindrical hohlraums), gas fill composition (neopentane at room temperature vs. cryogenic helium), and gas fill density (increase of ∼ 150%) are independently changed. Cylindrical hohlraums with higher gas fill density show improved inner beam propagation, as should rugby hohlraums, because of the larger radius over the capsule (7 mm vs. 5.75 mm in a cylindrical hohlraum). Energy coupling improves in room temperature neopentane targets, as well as in hohlraums at higher gas fill density. In addition cross-beam energy transfer is being addressed directly by using targets that mock up one end of a hohlraum, but allow observation of the laser beam uniformity after energy transfer. Ideas such as splitting quads into “doublets” by re-pointing the right and left half of quads are also being pursued. LPI results of the Drive campaign will be summarized, and analyses of future directions presented.
Highlights
The radiation environment for an indirect drive capsule at the National Ignition Facility (NIF) [1] is the hohlraum (Figure 1)
The beams propagate to the hohlraum wall where they are converted to x-rays, which drive the capsule implosion
To improve hohlraum performance, we have modified the hohlraum fill composition (He replaced by C5H12, shot at room temperature), hohlraum fill gas density, and the hohlraum shape
Summary
The radiation environment for an indirect drive capsule at the National Ignition Facility (NIF) [1] is the hohlraum (Figure 1). To improve hohlraum performance (both “hot” and “round”), we have modified the hohlraum fill composition (He replaced by C5H12, shot at room temperature), hohlraum fill gas density (increased by > 1.5 for the HiFoot design, and decreased to near-vacuum for 2-shock HDC capsules), and the hohlraum shape (cylinder to rugby) (c.f. Figure 1). During 2011-2012, over the time when shock-timed implosions were fielded at NIF as part of the National Ignition Campaign (NIC), the measured SRS light from the 230 and 300 cones (16 quads in all) was nearly constant at ~ 150 kJ.
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