Reduction of laser imprinting using polarization smoothing on a solid-state fusion laser

Boehly, T. R.; Smalyuk, V. A.; Meyerhofer, D. D.; Knauer, J.P.; Bradley, D. K.; Craxton, R. S.; Guardalben, M. J.; Skupsky, S.; Kessler, T. J.

doi:10.1063/1.369702

Cited by 209 publications

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“…The ontarget energies were $21 kJ in midintensity implosions and $26 kJ in high-intensity implosions. All of the experiments used distributed phase plates (DPP's) [14] and polarization smoothing (PS) [15], using birefringent wedges. The goal of these experiments was to measure target performance with glass ablators, hard x-ray signals produced by hot electrons from TPD instability in glass implosions and to compare them with those from plastic implosions.…”

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Implosion Experiments using Glass Ablators for Direct-Drive Inertial Confinement Fusion

et al. 2010

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Direct-drive implosions with 20-m-thick glass shells were conducted on the Omega Laser Facility to test the performance of high-Z glass ablators for direct-drive, inertial confinement fusion. The x-ray signal caused by hot electrons generated by two-plasmon-decay instability was reduced by more than $40Â and hot-electron temperature by $2Â in the glass compared to plastic ablators at ignition-relevant drive intensities of $1 Â 10 15 W=cm 2 , suggesting reduced target preheat. The measured absorption and compression were close to 1D predictions. The measured soft x-ray production in the spectral range of $2 to 4 keV was $2Â to 3Â lower than 1D predictions, indicating that the shell preheat caused by soft x-rays is less than predicted. A direct-drive-ignition design based on glass ablators is introduced. DOI: 10.1103/PhysRevLett.104.165002 PACS numbers: 52.57.ÀzThe goal of inertial confinement fusion (ICF) [1,2] is to implode a spherical target to achieve high compression of the fuel and high temperature in the hot spot to trigger ignition and maximize the thermonuclear energy gain. To achieve high compression, the shell entropy and temperature must remain low because it is easier to compress the fuel at a low temperature than at a high temperature [2]. The entropy is defined [3] by adiabat ¼ PðMbÞ=½2:2 ðg=ccÞ 5=3 , the ratio of the plasma pressure to the Fermi pressure of a fully degenerate electron gas [3]. In direct-drive spherical implosions, the target is driven by direct illumination with overlapped laser beams. To ignite DT fuel on the National Ignition Facility (NIF) [2] with a laser energy of E L ¼ 1:5 MJ and to achieve a gain of $40 to 50 will require high fuel compression with a total target areal density ( R) of $1500 mg=cm 2 [4]. As shown in Ref.[3], Rðmg=cm 2 Þ % 2600E L ðMJÞ 1=3 À0:6 so high areal densities require low-adiabat, 3 implosions. The fuel adiabat is determined by shocks launched at the beginning of the implosion [2]. The shock waves must be precisely tuned to set the inner portion of the shell on a low adiabat. Adiabat control is critical to achieving the desired R at peak compression [1,2]. Shock mistiming was an important cause of R degradation in direct-drive, cryogenic implosions on OMEGA and a subject of intensive research [5,6]. Another area of concern to ICF is the unstable growth of target modulations caused by hydrodynamic instabilities [1,2]. While the areal densities at peak compression have been shown to be relatively insensitive to hydrodynamic instabilities, the fusion neutron yields are very sensitive [7]. Shell preheat, another source of compression degradation, is caused by hot electrons generated by two-plasmon-decay (TPD) instability [8,9]. This preheat was shown to be virulent in DT and D 2 ablators [6,10] and was reduced by using plastic ablators [6,11]. The highest ignition-relevant areal densities with a shell R of $200 mg=cm 2 were achieved in cryogenic D 2 -ice implosions with plastic ablators, when the hotelectron preheat was suppressed, at a moderate laser-dr...

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Implosion Experiments using Glass Ablators for Direct-Drive Inertial Confinement Fusion

et al. 2010

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“…In the direct-drive experiments described here, distributed phase plates, 23 polarization smoothing using birefringent wedges, 24 and 1-THz, two-dimensional smoothing by spectral dispersion 25 were applied to smooth the OMEGA laser beams in order to enhance implosion uniformity and the nuclear reaction rate. Three types of capsules were used to study implosions with a wide range of areal densities.…”

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Using nuclear data and Monte Carlo techniques to study areal density and mix in D2 implosions

et al. 2005

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Measurements from three classes of direct-drive implosions at the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] were combined with Monte-Carlo simulations to investigate models for determining hot-fuel areal density (ρR hot ) in compressed, D 2 -filled capsules, and to assess the impact of mix and other factors on the determination of ρR hot . The results of the Monte-Carlo calculations were compared to predictions of simple commonly used models that use ratios of either secondary D 3 He proton yields or secondary DT neutron yields to primary DD neutron yields to provide estimates ρR hot,p or ρR hot,n , respectively, for ρR hot . For the first class of implosions, where ρR hot is low (≤ 3 mg/cm 2 ), ρR hot,p and ρR hot,n often agree with each other and are often good estimates of the actual ρR hot . For the second class of implosions, where ρR hot is of order 10 mg/cm 2 , ρR hot,p often underestimates the actual value due to secondary proton yield saturation. In addition, fuel-shell mix causes ρR hot,p to further underestimate, and ρR hot,n to overestimate, ρR hot . As a result, values of ρR hot,p and ρR hot,n can be interpreted as lower and upper limits, respectively. For the third class of implosions, involving cryogenic capsules, secondary protons and neutrons are produced mainly in the hot and cold fuel regions, respectively, and the effects of the mixing of hot and cold fuel must be taken into account when interpreting the values of ρR hot,p and ρR hot,n . From these data sets, we conclude that accurate inference of ρR hot requires comprehensive measurements in combination with detailed modeling. a) Also visiting Senior Scientist,

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“…The individual laser beams were smoothed with distributed phase plates (DPPs), 34 two-dimensional smoothing by spectral dispersion with a bandwidth of 1.0 THz, 35,36 and polarization smoothing using birefringent wedges. 37 Two types of DPPs (SG3 and SG4) were used in the experiments described here, producing different beam intensity profiles. 38 Only 1-ns square laser pulses were used to directly illuminate the capsule.…”

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confidence: 99%

Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments

et al. 2006

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Radial profiles of nuclear burn in directly-driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7-MeV protons from the fusion of deuterium (D) and 3-helium ( 3 He) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median "burn radius" R burn containing half of the total D 3 He reactions. Different laser and capsule parameters resulted in burn radii varying from 20 to 80 µm. For example, reducing the D 3 He fill pressure from 18 to 3.6 atm in capsules with 20-µm-thick CH shells resulted in the mean burn radius changing from 31 µm to 25 µm; this reduction is attributed to increased fuel-shell mix for the more unstable 3.6-atm implosions rather than to increased convergence, because total areal density didn't change very much. Fuel-shell-interface radii estimated from hard (4-5 keV) x-ray images of some of the same implosions were observed to closely track the burn radii. Simulated burn radii produced with 1-D codes agree fairly well with measurements for glass-shell capsules, but are systematically smaller than measurements for CH-shell capsules. A search for possible sources of systematic measurement error that could account for this discrepancy has been unsuccessful. Possible physical sources of discrepancies are mix, hydrodynamic instabilities and/or preheat not included in the 1-D code. Since measured burn-region sizes indicate where fusion actually occurs as a consequence of all the complicated processes that affect capsule implosion dynamics, they provide exacting tests of simulations. a) seguin@mit.edu b) Also Visiting Senior Scientist, Laboratory for Laser Energetics, University of Rochester. c) Also Departments of Mechanical Engineering, Physics and Astronomy, University of Rochester.Byline: ICF burn region measurements 2

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Reduction of laser imprinting using polarization smoothing on a solid-state fusion laser

Cited by 209 publications

References 14 publications

Implosion Experiments using Glass Ablators for Direct-Drive Inertial Confinement Fusion

Implosion Experiments using Glass Ablators for Direct-Drive Inertial Confinement Fusion

Using nuclear data and Monte Carlo techniques to study areal density and mix in D2 implosions

Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments

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