In-flight observations of low-mode ρ R asymmetries in NIF implosions

• Published in: Physics of Plasmas
• Main Authors: Zylstra, A. B., Frenje, J. A., Séguin, F. H., Rygg, J. R., Kritcher, A., Rosenberg, M. J., Rinderknecht, H. G., Hicks, D. G., Friedrich, S., Bionta, R., Meezan, N. B., Olson, R., Atherton, J., Barrios, M., Bell, P., Benedetti, R., Berzak Hopkins, L., Betti, R., Bradley, D., Callahan, D., Casey, D., Collins, G., Dewald, E. L., Dixit, S., Döppner, T., Edwards, M. J., Gatu Johnson, M., Glenn, S., Grim, G., Hatchett, S., Jones, O., Khan, S., Kilkenny, J., Kline, J., Knauer, J., Kyrala, G., Landen, O., LePape, S., Li, C. K., Lindl, J., Ma, T., Mackinnon, A., Manuel, M. J.-E., Meyerhofer, D., Moses, E., Nagel, S. R., Nikroo, A., Parham, T., Pak, A., Petrasso, R. D.
• Other Authors: Fusion Science Center, Laboratory for Laser Energetics, National Laser Users Facility, National Science Foundation
• Format: Article in Journal/Newspaper
• Language: English
• Published: AIP Publishing 2015
• Subjects: Condensed Matter Physics; North Pole
• Online Access: http://dx.doi.org/10.1063/1.4918355; https://pubs.aip.org/aip/pop/article-pdf/doi/10.1063/1.4918355/13787184/056301_1_online.pdf

Charged-particle spectroscopy is used to assess implosion symmetry in ignition-scale indirect-drive implosions for the first time. Surrogate D3He gas-filled implosions at the National Ignition Facility produce energetic protons via D+3He fusion that are used to measure the implosion areal density (ρR) at the shock-bang time. By using protons produced several hundred ps before the main compression bang, the implosion is diagnosed in-flight at a convergence ratio of 3–5 just prior to peak velocity. This isolates acceleration-phase asymmetry growth. For many surrogate implosions, proton spectrometers placed at the north pole and equator reveal significant asymmetries with amplitudes routinely ≳10%, which are interpreted as ℓ=2 Legendre modes. With significant expected growth by stagnation, it is likely that these asymmetries would degrade the final implosion performance. X-ray self-emission images at stagnation show asymmetries that are positively correlated with the observed in-flight asymmetries and comparable in magnitude, contradicting growth models; this suggests that the hot-spot shape does not reflect the stagnated shell shape or that significant residual kinetic energy exists at stagnation. More prolate implosions are observed when the laser drive is sustained (“no-coast”), implying a significant time-dependent asymmetry in peak drive.