In indirect-drive inertial confinement fusion, multiple laser beams are converted to soft x-ray to implode the capsule in the center of a hohlraum. A critical issue for achieving ignition is the drive symmetry on the capsule. As the expanding of bubble plasma, laser spot moves toward the hohlraum axis and the LEH. The laser spot motion will enhance the polar drive while reducing the equatorial drive, ultimately leading to an oblate hotspot. A common method to mitigate the laser spot motion is shifting the laser beams inward, with the initial laser spot position closer to the equator of the capsule. However, the blowing capsule plasma may block the transmission of inner rings, which will also lead to degeneration of the drive symmetry.
  In our previous experiments, the hotspot P2 was -15% when the inner rings were shifted inward by 100 μm. Conversely, the hotspot P2 increased to -1% when the inner rings were shifted outward by 200 μm, which was different with the intuitive understanding that shifting the inner rings outward will reduce the hotspot P2. The deflection of the inner beams in the blowing capsule plasma may be the main reason why the hotspot P2 decreases when shifting the inner rings inward. After considering the deflection effect, the hotspot P2 is -13% when the inner rings are shifted inward by 100 μm and -4% when the inner rings are shifted outward by 200 μm, which is consistent with the experimental results. This work quantifies the impact of laser deflection in the blowing capsule plasma on the hotspot distortion, which provides the basis for precise control on the drive symmetry.

inertial confinement fusion,drive symmetry,laser spot motion,laser deflection