The last few years have seen significant progress in constructing the atomic models required for non-local thermodynamic equilibrium (NLTE) simulations. Along with this has come an increased understanding of the requirements for accurately modeling the ionization balance, energy content and radiative properties of different elements for a wide range of densities and temperatures. Much of this progress is the result of a series of workshops dedicated to comparing the results from different codes and computational approaches applied to a series of test problems. The results of these workshops emphasized the importance of atomic model completeness, especially in doubly excited states and autoionization transitions, to calculating ionization balance, and the importance of accurate, detailed atomic data to producing reliable spectra. We describe a simple screened-hydrogenic model that calculates NLTE ionization balance with surprising accuracy, at a low enough computational cost for routine use in radiation-hydrodynamics codes. The model incorporates term splitting, {Delta}n = 0 transitions, and approximate UTA widths for spectral calculations, with results comparable to those of much more detailed codes. Simulations done with this model have been increasingly successful at matching experimental data for laser-driven systems and hohlraums. Accurate and efficient atomic models are just one requirement for integrated NLTE simulations. Coupling the atomic kinetics to hydrodynamics and radiation transport constrains both discretizations and algorithms to retain energy conservation, accuracy and stability. In particular, the strong coupling between radiation and populations can require either very short timesteps or significantly modified radiation transport algorithms to account for NLTE material response. Considerations such as these continue to provide challenges for NLTE simulations.

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Advances in NLTE Modeling for Integrated Simulations

Abstract In 2009 the National Ignition Campaign (NIC) gas-filled/capsule-imploding hohlraum energetics campaign showed good laser-hohlraum coupling, reasonably high drive, and implosion symmetry control via cross-beam transfer. There were, however, discrepancies with expectations from the standard simulation model including: the level and spectrum of the Stimulated Raman light; the tendency towards pancake-shaped implosions; and drive that exceeded predictions early in the campaign, and lagged those predictions late in the campaign. We review here the origins/development path of the “high flux model” (HFM). The HFM contains two principal changes from the standard model: 1) It uses a detailed configuration accounting (DCA) atomic physics non-local-thermodynamic-equilibrium (NLTE) model, and 2) It uses a generous electron thermal flux limiter, f = 0.15, that is consistent with a non-local electron transport model. Both elements make important contributions to the HFM’s prediction of a hohlraum plasma that is cooler than that predicted by the standard model, which uses an NLTE average atom approach, and a value of f = 0.05 for the flux limiter. This cooler plasma is key in eliminating most of the discrepancies between the NIC data and revised expectations based on this new simulation model. The HFM had previously been successfully deployed in correctly modeling Omega Laser illuminated gold sphere x-ray emission data, and NIC empty hohlraum drive. However, when the HFM was first applied to this energetics campaign, the model lacked some credibility/acceptance compared to the standard model, because it actually worsened the discrepancy between the observed hohlraum drive for the 1 MJ class experiments performed late in the campaign and the revised expectation of higher drive based on the HFM. Essentially, the HFM was making a prediction that the laser-hohlraum coupling was less than that assumed at that time. Its credibility was then boosted when a re-evaluation of the laser light losses from the hohlraum due to laser plasma interactions matched its prediction.

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The Role of a Detailed Configuration Accounting (DCA) Atomic Physics Package in Explaing the Energy Balance in Ignition Scale Hohlraums

We review the 5th non-LTE (non local thermodynamic equilibrium) kinetics code comparison workshop, held in November 2007. Both steady-state and time-dependent cases for elements ranging from carbon to gold were examined in detail. Calculations of radiative power losses and specific spectra were requested in addition to typical plasma quantities such as the ionization balance. Non-Maxwellian electrons, external Planckian radiation and opacity effects in spectra were also included in the comparisons. We discuss the organization of the workshop and present a set of representative results. The particular case of a tungsten plasma at tokamak operating densities was considered for the first time. Due to its importance to the ITER project, more detailed results of these comparisons will be published elsewhere.

Review of the NLTE-5 kinetics workshop

Investigations of soft matter using ultrashort high intensity pulses have been made possible through the advent of X-ray free-electrons lasers. The last decade has seen the development of a new typ ...

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Ultrafast Structural and Electron Dynamics in Soft Matter Exposed to Intense X-ray Pulses

We report on the performance of the highest radiant energy x-ray source at the LaserMegaJoule facility (LMJ) for testing material properties under irradiation. This radiation source was produced by the L-shell emission of xenon contained in a gas-pipe. The target was an epoxy ortho-cylinder tube with a diameter of 3 mm and a 50- μm thickness wall containing xenon gas at 1.2 atm. For this proof of principle shot, the LaserMegaJoule facility delivered a total energy of around 90 kJ of 351 nm laser and a total power of 30 TW. Thirty-two beams, divided into eight beamlets each, are arranged into two cones. The laser pulses were nearly flat, with a duration of 3 ns and a power range of 3.5–5 TW for each quadruplet. Two broadband spectrometers, DMX (DP4) and miniDMX (DP11), have been used to characterize the x-ray emission up to 10 keV. Radiant energy up to 0.5 kJ/sr has been recorded in the xenon L-shell band between 3 and 6 keV. This was the highest energy ever delivered for creating an x-ray source on the LMJ. Even though the target was axisymmetric, the strong asymmetric laser irradiation (nor axi or bottom/top symmetric) required full 3D radiation-hydrodynamic calculations to retrieve the radiant energy measured by broad band spectrometers. The main purpose of this work was to benchmark our 3D hydrorad code and the non-local thermodynamic equilibrium model by using newly developed x-ray spectrometers.

First high radiant energy xenon-pipe-based x-ray source on LMJ

We review the 9th NLTE code comparison workshop, which was held in the Jussieu campus, Paris, from November 30th to December 4th, 2015. This time, the workshop was mainly focused on a systematic investigation of iron NLTE steady-state kinetics and emissivity, over a broad range of temperature and density. Through these comparisons, topics such as modeling of the dielectronic processes, density effects or the effect of an external radiation field were addressed. The K-shell spectroscopy of iron plasmas was also addressed, notably through the interpretation of tokamak and laser experimental spectra.

Yu. Ralchenko

Papers

Review the 9th NLTE code comparison workshop