Hot electron production and heating by hot electrons in fast ignitor research

Abstract

In an experimental study of the physics of fast ignition the characteristics of the hot electron source at laser intensities up to 10 " Wcm"2and the heating produced at depth by hot electrons have been measured. Efficient generation of hot electrons but less than the anticipated heating have been observed.The concept of isochoric fast ignition originated by Tabak et al. ' is of importance through its potential to give higher inertially confined fusion (ICF) gain than isobaric central spark ignition used in the more developed indirect and direct drive schemes 'and thereby to reduce the driver efficiency required for inertial fusion energy (IFE). The physics is new and challenging involving strongly relativistic laser plasma interactions and transport of energy by MeV electrons where electrostatic potentials and self generated magnetic fields may strongly modify the transport 3. Experimental and theoretical studies aimed at assessing the feasibilityof fastignitionas a newrouteto ICF arenowbeing carried out at many laboratories world wide including the Lawrence Livennore National Laboratory (LLNL), where the Nova laser facility has been adapted to generate petawatt pulses using chirped puke ampliilcation (CPA)4 . Experimental FaciIityTwo beam lines at Nova have been adapted for CPA operation and for experiments reported here, generated typically 20J and 500J pulses respectively of duration in the range 0.4 to 20 ps (maximum power up to 1 PW). Focusing of the two beams respectively was with an off axis f73parabolic mirror of focal length 42 cm in a focal spot of 15 yrn diameter 1 and an axial f14parabola of focal length 170 cm in an asymmetrical spot of 40 pm x 20SThe focal spots had a speckle ,pattern sub structure with a broad power spec& of intensity. Work is in progress to correct the wavefront using a deformable mirror. T&b eam line produced a power weighted average intensity on target in 0.45 ps pulses estimated at 210'9 Wcm"2. The 500J, beam line produced 10 N Wcm'2 in 1 PW, 0.45 ps pulses. A thick glass plate debris shield protected the parabola in the 500J beam line for longer pulse operation down to 5 ps. Non linear effects precluded the me of the debris shield for lPW shots and here a plasma mirror was used to reverse the beam direction thus projecting ablated target debris away from the un-protected parabola. The off axis parabola was used with a thin debris shield for all pulse lengths in the 20 J experiments. Targets in these experiments were exposed to ASE and leakage prepulses before the main pulse. ASE in a typically 3 ns period before the pulse varied in experiments reported here from 210-5 to 2104 of the main pulse energy. The energy of leakage pulses ranged from 104 to 10'2of the main pulse, occurring 2 ns or 4 ns before the main pulse but could be made as low as 104 with precise adjustment of Pockels cell gates. The hot electron sourceA central theme of the experimental work has been the characterization of the hot electron source produced at a solid target. Electrons directed into the t...

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