Inhibition of turbulence in inertial-confinement-fusion hot spots by viscous dissipation

Weber, Christopher; Clark, Daniel; Cook, Andrew W.; Busby, Lee; Robey, H. F.

doi:10.1103/physreve.89.053106

Cited by 106 publications

(73 citation statements)

References 23 publications

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“…Nevertheless, the intuition developed here may still be useful; all else being equal, more ionization enhances TKE growth under rapid compression by weakening the viscosity growth. The present work also neglects magnetic field effects, in line with other studies of turbulence in 3D compressions [7,8]. This, too, limits the applicability to Z-pinch compressions, though there are also many instances in which the magnetic field need not dominate the dynamics in a Zpinch [6].…”

Section: Introductionsupporting

confidence: 58%

“…Note that while these gas-puff Z-pinches appear to have substantial non-radial TKE even at stagnation [6], turbulence in the hot spot of ignition shots at the National Ignition Facility (NIF) is expected to be dissipated by high viscosity [8]. The much higher temperatures in these hot spots create this high viscosity, but they are assisted by fuel of Z = 1, to the extent it is not contaminated by mix.…”

Section: Introductionmentioning

confidence: 99%

“…The plasma in magnetically driven [2,[4][5][6] or laser driven [7,8] compressions can be turbulent. There can be substantial reduction in viscosity growth due to increasing ionization state for a gas-puff Z-pinch.…”

Section: Introductionmentioning

confidence: 99%

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Compressing turbulence and sudden viscous dissipation with compression-dependent ionization state

Davidovits

Fisch

2016

Phys. Rev. E

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Turbulent plasma flow, amplified by rapid 3D compression, can be suddenly dissipated under continuing compression. This effect relies on the sensitivity of the plasma viscosity to the temperature, µ ∼ T 5/2 . The plasma viscosity is also sensitive to the plasma ionization state. We show that the sudden dissipation phenomenon may be prevented when the plasma ionization state increases during compression, and demonstrate the regime of net viscosity dependence on compression where sudden dissipation is guaranteed. Additionally, it is shown that, compared to cases with no ionization, ionization during compression is associated with larger increases in turbulent energy, and can make the difference between growing and decreasing turbulent energy.

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Section: Introductionsupporting

confidence: 58%

Section: Introductionmentioning

confidence: 99%

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Compressing turbulence and sudden viscous dissipation with compression-dependent ionization state

Davidovits

Fisch

2016

Phys. Rev. E

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“…The plasma motion in these compressions can be turbulent, whether magnetically driven [1][2][3][4] or laser driven [5,6]. However, rapid compression of this turbulent plasma, where the viscosity is highly sensitive to temperature, is demonstrated here to exhibit unusual behavior, where the turbulent kinetic energy (TKE) abruptly switches from growing to rapidly dissipating.…”

mentioning

confidence: 82%

Sudden Viscous Dissipation of Compressing Turbulence

Davidovits

Fisch

2016

Phys. Rev. Lett.

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Compression of turbulent plasma can amplify the turbulent kinetic energy, if the compression is fast compared to the viscous dissipation time of the turbulent eddies. A sudden viscous dissipation mechanism is demonstrated, whereby this amplified turbulent kinetic energy is rapidly converted into thermal energy, suggesting a new paradigm for fast ignition inertial fusion.Introduction.-Unprecedented densities and temperatures are now reached by compressing plasma using lasers or magnetic fields, with the objective of reaching nuclear fusion, prodigious x-ray production, or new regimes of materials. The plasma motion in these compressions can be turbulent, whether magnetically driven [1][2][3][4] or laser driven [5,6]. However, rapid compression of this turbulent plasma, where the viscosity is highly sensitive to temperature, is demonstrated here to exhibit unusual behavior, where the turbulent kinetic energy (TKE) abruptly switches from growing to rapidly dissipating. This behavior occurs in plasma, but is not predicted by studies of neutral gas compression [7][8][9][10][11][12][13]. In fact, it was observations of the dominant effect of the TKE in Zpinches, both in pressure balance [4] and in radiation balance [2,4], that stimulated the present study.Compression is rapid if the rate of compression is fast compared to the turbulent timescale τ = k/ with k the TKE and the viscous dissipation rate. In the initially rapid plasma compressions here, the viscous dissipation eventually grows such that the turbulent timescale τ shortens, and the plasma TKE suddenly dissipates. This dissipation is sudden in that it occurs over a time interval that is small compared to the overall compression time.This sudden dissipation mechanism now suggests a new fast ignition paradigm. Imagine an initially turbulent fusion fuel plasma where the majority of the energy is in the turbulent motion. This plasma is then rapidly compressed, causing both the TKE and thermal energy to grow, while the TKE retains most of the energy, as observed in certain Z-pinch experiments [1][2][3][4]. Since radiation losses (both synchrotron and bremsstrahlung) and nuclear fusion are dependent on thermal energy but not the TKE, those processes are minimized under such compression, since the plasma stays comparatively cool. However, late in the compression, the TKE suddenly dissipates viscously into thermal energy, thereby igniting the plasma without having undergone large radiation losses.In neutral gas, upon rapid compression, the TKE grows. In an isotropic 3D compression, it grows as 1/L 2 , where L is the (time dependent) side length of a box that is compressing with the mean flow along each axis. This is true for both the zero Mach case [7], where the TKE is solenoidal, as well as in the finite Mach case, where the TKE has both solenoidal and dilational components, which each grow in energy as 1/L 2 [12]. This is the same

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“…However, nominal Knudsen numbers calculated for the neutron-producing compression phase of these implosions are too small for the depletion to be significant at (1-3) × 10 −3 . Local Knudsen numbers might increase due to turbulence and mix, elevating the impact of this effect [37], but such smallscale turbulence is expected to be reduced by viscosity [51]. PHYSICAL Three-dimensional dynamics seeded by laser drive asymmetry, engineering features, ice-layer nonuniformity, etc., are believed to play a substantial role in these implosions [52][53][54][55].…”

mentioning

confidence: 99%

Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility

Johnson¹,

Knauer²,

Cerjan³

et al. 2016

Phys. Rev. E

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An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T ion are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T ion are observed and the difference is seen to increase with increasing apparent DT T ion . The line-of-sight rms variations of both DD and DT T ion are small, ∼150 eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T ion . Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T ion greater than the DD T ion , but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results. DOI: 10.1103/PhysRevE.94.021202 At the National Ignition Facility (NIF) [1], cryogenically layered capsules of deuterium (D) and tritium (T) fuel contained in 2-mm-diam carbon-based shells are imploded through laser irradiation of a surrounding high-Z hohlraum [2,3]. The imploding DT fuel assembles and "stagnates" in a configuration with a cold high-density shell surrounding a low-density hot spot. Efficient conversion of shell kinetic energy to hot-spot thermal energy is an essential requirement to achieving ignition at the NIF [4,5]. At peak convergence, this ideally results in a spherically symmetric, cold, dense DT fuel shell with an areal density ρR of ∼1.5 g/cm 2 surrounding a ∼5-keV hot spot with ρR ∼ 0.3 g/cm 2 . Although the word "stagnation" is often used for this phase of the implosion, it is inappropriate as the DT and DD neutron spectra indicate significant remaining kinetic energy. Neutron spectrometers [6][7][8][9][10][11][12][13][14][15] provide directional measurements of DT and DD neutron spectra from which yield, burn-averaged ion temperatures T ion and areal densities ρR are obtained. Neutron activation detectors (NADs) [16] measure the unscattered DT yield Y DT . In this paper we focus on the ion "temperatures" from a more extensive set of experiments than previously published [2] and conclude that the fuel assembly during burn in layered DT implosions is not well described by detailed one-dimensional (1D) physics models and simulations. The leading hypothesis for the observed discrepancy between the data and the 1D description is significant disordered motion and the highly 3D nature of the assembly at burn.For a homogeneous stationary DT plasma in thermal equilibrium at ion temperature T thermal , the variance of the * Corresponding author: gatu@psfc.mit.edu DT neutron spectrum (in units of neutron energy) is given bywhere E n is th...

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Inhibition of turbulence in inertial-confinement-fusion hot spots by viscous dissipation

Cited by 106 publications

References 23 publications

Compressing turbulence and sudden viscous dissipation with compression-dependent ionization state

Compressing turbulence and sudden viscous dissipation with compression-dependent ionization state

Sudden Viscous Dissipation of Compressing Turbulence

Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility

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