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Title: Initial Computational Study of a New Multi-Hole Hohlraum (the "Midraum")

Abstract

Existing cylindrical hohlraums with two oppositely positioned laser entrance holes (LEHs) have multiple constraints. Their goal is to produce radiation sources distributed over the sky, as visible from the spherical implosion capsule, with most of the deposition near the zeroes of the fourth Legendre polynomial in cosine of the polar angle. This requires some of the laser light to propagate across the hohlraum to positions near the hohlraum symmetry plane. The ratio of case spherical radius to capsule spherical radius should exceed 3 so that the light doesn’t pass through over-dense ablator plasma. Radiation transport can smooth higher radiation modes. For capsules that demand long pulse lengths, hohlraum walls can blow in and change the position where light is absorbed. This changes the radiation symmetry in a time dependent fashion. This affects both P2 and P4. This wall motion can be reduced by introducing fill gas into the hohlraum. The gas provides back pressure and tamps the wall motion. Adding the fill gas comes at some cost. It leads to increased absorption of laser light along the path. The fill gas adds heat capacity to the system, ultimately requiring more laser energy to meet the radiation flux goals, both inmore » total and particularly in the amount of radiation coming from the vicinity of the capsule waist. Given the existing beam pointing at NIF energy from the outer beams must be transferred into the inner beams. Cross beam energy transport (CBET) is accomplished via a plasma instability. This transfer is not perfectly predictable. In addition, the higher intensity required to make up for the losses along the long path can lead to stimulated backscatter as well as the generation of suprathermal electrons. The inner beams will pass through the plasma ablated from the capsule toward the end of the pulse. Heating this plasma acts as another parasitic loss. In addition, the light passing through the turbulent blow-off can be refracted in unpredictable directions.« less

Authors:
 [1];  [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1409994
Report Number(s):
LLNL-TR-741288
DOE Contract Number:  
AC52-07NA27344
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION

Citation Formats

Tabak, M., and Jones, O. S. Initial Computational Study of a New Multi-Hole Hohlraum (the "Midraum"). United States: N. p., 2017. Web. doi:10.2172/1409994.
Tabak, M., & Jones, O. S. Initial Computational Study of a New Multi-Hole Hohlraum (the "Midraum"). United States. doi:10.2172/1409994.
Tabak, M., and Jones, O. S. Thu . "Initial Computational Study of a New Multi-Hole Hohlraum (the "Midraum")". United States. doi:10.2172/1409994. https://www.osti.gov/servlets/purl/1409994.
@article{osti_1409994,
title = {Initial Computational Study of a New Multi-Hole Hohlraum (the "Midraum")},
author = {Tabak, M. and Jones, O. S.},
abstractNote = {Existing cylindrical hohlraums with two oppositely positioned laser entrance holes (LEHs) have multiple constraints. Their goal is to produce radiation sources distributed over the sky, as visible from the spherical implosion capsule, with most of the deposition near the zeroes of the fourth Legendre polynomial in cosine of the polar angle. This requires some of the laser light to propagate across the hohlraum to positions near the hohlraum symmetry plane. The ratio of case spherical radius to capsule spherical radius should exceed 3 so that the light doesn’t pass through over-dense ablator plasma. Radiation transport can smooth higher radiation modes. For capsules that demand long pulse lengths, hohlraum walls can blow in and change the position where light is absorbed. This changes the radiation symmetry in a time dependent fashion. This affects both P2 and P4. This wall motion can be reduced by introducing fill gas into the hohlraum. The gas provides back pressure and tamps the wall motion. Adding the fill gas comes at some cost. It leads to increased absorption of laser light along the path. The fill gas adds heat capacity to the system, ultimately requiring more laser energy to meet the radiation flux goals, both in total and particularly in the amount of radiation coming from the vicinity of the capsule waist. Given the existing beam pointing at NIF energy from the outer beams must be transferred into the inner beams. Cross beam energy transport (CBET) is accomplished via a plasma instability. This transfer is not perfectly predictable. In addition, the higher intensity required to make up for the losses along the long path can lead to stimulated backscatter as well as the generation of suprathermal electrons. The inner beams will pass through the plasma ablated from the capsule toward the end of the pulse. Heating this plasma acts as another parasitic loss. In addition, the light passing through the turbulent blow-off can be refracted in unpredictable directions.},
doi = {10.2172/1409994},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Nov 02 00:00:00 EDT 2017},
month = {Thu Nov 02 00:00:00 EDT 2017}
}

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