Thermal electron transport in direct-drive laser fusion

Delettrez, J. A.

doi:10.1139/p86-162

Cited by 79 publications

(34 citation statements)

References 18 publications

(25 reference statements)

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“…As will be shown, even the shock burn duration, when compared to ion diffusion times (inferred from this collection of experimental measurements), is insightful and is an excellent figure of merit for understanding the transition from hydrodynamic to strongly kinetic regimes (see Table I). It is demonstrated that standard and well-known hydrodynamic simulations [18][19][20] are increasingly discrepant with experimental results as the ion mean-free path becomes larger than the minimum shell radius.…”

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confidence: 84%

“…In a broader context, this near single-parameter study allows for a quantitative assessment of long meanfree-path effects in a regime comparable to the early phases of cryogenically layered hot-spot ignition implosions, in which an M ∼ 10-50 shock converges in a DT gas of initial density 0.3 mg=cm 3 [1,22]. Radiation-hydrodynamic simulations were performed using three well-known and benchmarked hydrodynamic simulation codes: LILAC [18], HYADES [20], and DUED [19]. All three gave very similar predictions in this campaign and, as a representative case, the one-dimensional (1D) version of the two-dimensional Lagrangian DUED code [19,23] is utilized herein; it includes flux-limited electron thermal transport with a flux limiter of f ¼ 0.07 and non-LTE opacities.…”

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confidence: 99%

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Exploration of the Transition from the Hydrodynamiclike to the Strongly Kinetic Regime in Shock-Driven Implosions

et al. 2014

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Clear evidence of the transition from hydrodynamiclike to strongly kinetic shock-driven implosions is, for the first time, revealed and quantitatively assessed. Implosions with a range of initial equimolar D 3 He gas densities show that as the density is decreased, hydrodynamic simulations strongly diverge from and increasingly overpredict the observed nuclear yields, from a factor of ∼2 at 3.1 mg=cm 3 to a factor of 100 at 0.14 mg=cm 3 . (The corresponding Knudsen number, the ratio of ion mean-free path to minimum shell radius, varied from 0.3 to 9; similarly, the ratio of fusion burn duration to ion diffusion time, another figure of merit of kinetic effects, varied from 0.3 to 14.) This result is shown to be unrelated to the effects of hydrodynamic mix. As a first step to garner insight into this transition, a reduced ion kinetic (RIK) model that includes gradient-diffusion and loss-term approximations to several transport processes was implemented within the framework of a one-dimensional radiation-transport code. After empirical calibration, the RIK simulations reproduce the observed yield trends, largely as a result of ion diffusion and the depletion of the reacting tail ions. Inertial confinement fusion implosions, whether for ignition [1] or nonignition [2,3] experiments, are nearly exclusively modeled as hydrodynamic in nature with a single average-ion fluid and fluid electrons [4,5]. However, in the early phase of virtually all inertial fusion implosions, strong shocks are launched into the capsule where they increase in strength and speed as they converge to the center and abruptly and significantly increase the ion temperature in the central plasma region. In this process, and in the rebound of the shock from the center, which initiates a burst of fusion reactions (i.e., the fusion shock burn or shock flash [6]), the mean-free path for ion-ion collisions can become, especially for lower-density fueled implosions, sufficiently long that both the shock front itself and the resulting central plasma are inadequately described by hydrodynamic modeling. This process and the transition of regimes from hydrodynamiclike to strongly kinetic are the focus of this Letter.Recent kinetic and multiple-ion-fluid simulations have begun to explore deviations from average-ion hydrodynamic models, particularly during the shock phase of implosions when such effects are potentially paramount. For example, in an effort to explain observed yield anomalies in multiple-ion fuels of D 3 He, DT, and DT 3 He [7][8][9], researchers have investigated multiple-ionfluid effects [10][11][12] as well as utilized a hybrid fluidkinetic model [13,14]. Other modeling work has included ion viscosity and nonlocal ion transport [15] in order to reduce discrepancies with shock-generated nuclear yields. Very recently, a model for Knudsen layer losses of energetic ions [16], based in part on earlier work [17], was explored for a variety of plastic capsule implosions with relatively thick walls, all largely ablatively driven (not shock driven) a...

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confidence: 84%

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confidence: 99%

Exploration of the Transition from the Hydrodynamiclike to the Strongly Kinetic Regime in Shock-Driven Implosions

et al. 2014

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“…Equation (11-3) provides a smooth transition, but sometimes a "sharp cutoff" is used, given by Q ¼ minðQ SH ; f B Q fs Þ: As pointed out by Delettrez,585 the difference between the two forms can be significant in some regimes of interest. Shearer recognized that his upper limit was probably too high since it neglected electric-field effects, so Eq.…”

Section: Thermal Transportmentioning

confidence: 99%

“…Their model can describe the preheating effects beyond the base of the heat front and has the merit of being far easier to implement in a hydrodynamics code than a Fokker-Planck model. Delettrez 585 pointed out some limitations of this approach. An extensive mathematical analysis of the convolution method was given by Luciani et al 604 Delettrez 585 provided a comprehensive review of many experiments, especially layered-target-burnthrough and charge-collector measurements of mass ablation, that have typically indicated effective values of f in the range of 0.03-0.1.…”

Section: Thermal Transportmentioning

confidence: 99%

“…Delettrez 585 pointed out some limitations of this approach. An extensive mathematical analysis of the convolution method was given by Luciani et al 604 Delettrez 585 provided a comprehensive review of many experiments, especially layered-target-burnthrough and charge-collector measurements of mass ablation, that have typically indicated effective values of f in the range of 0.03-0.1. There were some exceptions, but Delettrez pointed out that the conclusions of some of these experiments, carried out before beam smoothing (Sec.…”

Section: Thermal Transportmentioning

confidence: 99%

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Direct-drive inertial confinement fusion: A review

et al. 2015

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The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

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