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

Abstract

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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