Areal Density Measurement of Imploded Cryogenic Target by Energy Peak Shift of DD-Produced Protons

Abstract

DD-produced protons are used to measure the imploded fuel-shell areal density pAR. Twelve green beams of the GEKKO XII laser directly imploded a cryogenic foam-shell target containing a liquid or solid D2 fuel. The proton emission is detected through a dipole magnet on a CR-39 plate, whose energy shift from 3.02 MeV yielded the maximum pAR of 10 mg/cm' or the density of 6 g/cm', in agreement with that from the secondary neutron method within a factor of 2. The proton energy spread is sensitive to the laser energy imbalance. PACS numbers: 52.70.Nc, 52.25.Tx, 52.50.Jm, 52.58.Ns If fuel is compressed to about 1000 times the DT liquid density and a central region is heated more than 5 keV during the final implosion stage, a fuel ignition and a net energy gain will occur. Directly driven targets have reached imploded density of 200 times the liquid DT with glass microballoons [1] and 600 times with plastic hollow shell targets [2]. A high gain target will be a cryogenic DT hollow shell with a central cavity [3]. Since a central cavity of the cryogenic target has no residual gases, it achieves high density compression.We are studying density and areal density of imploded cryogenic D2 targets. Very light plastic foam shell is used to help liquid or solid fuel make a uniform hollow shell. The secondary neutron method is used to determine the compressed density [4]. Neutrons from our cryogenic target, however, are few so we developed the other method to cross-check the areal density of the compressed shell. Fusion produced protons are used to measure the areal density.The target that we used is a low density plastic foam hollow shell sphere. The foam layer contains a liquid or solid D2 fuel, making a hollow shell cryogenic fuel target [5,6]. The foam is made of trimethlylolpropanetrimethacrylate (typically H, 4C, OO4) of 10~1.3 p, m thickness and 220~28 mg/cm density (20% weight of solid plastics) and is coated with a 4 pm thick polyvinylphenol ablator, both to increase laser absorption and to reduce preheating [7]. The shell radius is within RQ 303~24 p, m. The mass is 7.9~1.4 p, g. D2 density at 21 K is 170 mg/cm'. The target is sustained vertically with 7 p, m thick glass fiber at the center of the vacuum chamber and is cooled down to a presumed temperature in a retractable liquid-He-cooled shroud [8].Once D2 gas becomes liquid or solid in the foam layer, the shroud is retracted, and in 10 ms, 12 laser beams illuminate the target. At each exit of beams from the GEKKO XII system, there are a random phase plate and a single potassium dihydrogen phosphate crystal for second harmonics.The beams of 32 cm in diameter are focused in tetrahedral configuration on the target by f/3 15 asph. eric lenses. The focusing depth d/Ro = The total energy (527 nm) on target is from 3.5 to 12.1 kJ in 2.05~0.25 ns quasi-Hat-top pulse with 0.94~0.014 ns rise. Energy imbalance o ", between 12 beams is (3.7~1.7)% rms for all of the shots.In the final stage of implosion, a colder but denser main fuel-plastic mixed plasma layer (the second la...

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