Multibeam Seeded Brillouin Sidescatter in Inertial Confinement Fusion Experiments

We present the first observations of multibeam weakly seeded Brillouin sidescatter in indirect-drive inertial confinement fusion (ICF) experiments. Two seeding mechanisms have been identified and quantified: specular reflections (“glint”) from opposite hemisphere beams, and Brillouin backscatter from neighboring beams with a different angle of incidence. Seeded sidescatter can dominate the overall coupling losses, so understanding this process is crucial for proper accounting of energy deposition and drive symmetry. Glint-seeded scattered light could also be used to probe hydrodynamic conditions inside ICF targets.

Figure 1

This illustrates seeded SBS in a hohlraum. A seed may be amplified by one or more quads in a near-backscatter geometry prior to exiting the hohlraum. The direction of the scattered light is fixed by the direction of the seed. It is subsequently observed using the 30° cone scattered light diagnostics. The seed source, direction, and overlap region with incident outer beams are shown approximately, overlaid with the material boundary surfaces predicted by hydrodynamic simulations for a shot and time—N140429-004-999 at 5.5 ns—analyzed throughout the Letter.

Figure 2

(a) A typical SBS spectrum observed with the 30° FABS. There are two distinct features: (i) that which is redshifted with respect to the incident light has been determined to be mostly sidescatter seeded by SBS backscatter, and (ii) that which is blueshifted is sidescatter seeded by glint. (b) Normalized lineouts through each feature plotted with the upper hemisphere normalized incident power. The glint-seeded feature turns off with the upper hemisphere beams whereas the SBS-seeded feature continues until the end of the lower hemisphere.

Figure 3

A lineout through the SBS is overplotted with the truncated lower outer beam power and the full length 30° pulse. The SBS drops to about $7\%\pm 2\%$ of the peak value just prior to truncation, indicating the outers were reamplifying the inner SBS by about $14\times$. The accompanying illustration shows the inner SBS seed (red dashed) from lower inner beams getting reamplified by lower outer beams before exiting the hohlraum.

Figure 4

(a) The calculated intensity of a 23.5° quad after reflection ($R\approx 3\times {10}^{-6}$) off of the $0.16n_c$ density contour provided by HYDRA for shot N140429-004-999 at 5.5 ns. (b) The calculated distribution in theta-phi space when the light reaches the chamber wall, plotted with yellow quadrangles that indicate the positions of the 30° NBI scatter plates. (c) The same thing from the face-on view of NBI, and (d) shows the actual distribution of light on the NBI plates on that shot. The data are slightly broader in theta and narrower in phi than the simulation.

Figure 5

A lineout through the glint is overplotted with the truncated lower outer beam power. The glint drops by a factor of $\approx 2$, indicating the outers were providing about half the energy. The accompanying illustration shows glint from upper inner beams intercepting the lower outers entering the hohlraum in a region (with outward sonic flow) that supports energy transfer from the incoming to the outgoing light.

Figure 6

The apparent “reflectivity” inferred from the data is plotted with the incident laser pulse shape, which uses the time history from FABS, total energy from the NBI plate, and ray tracing to determine a multiplier with which to extrapolate a total signal. The early time reflectivity decays rapidly but at peak power glint returns and at 5.5 ns (the time corresponding to the hydrodynamic simulation in Fig. 4), the reflectivity exceeds the value at time zero.