Abstract
A hybrid early time x-ray drive followed by conventional laser direct drive obtained by utilizing a thin high-Z overcoat on plastic ablator targets has proven to be very effective at reducing laser imprint in experiments driven by the Nike krypton-fluoride laser. An inherent low level laser prepulse from that laser system heated and expanded the coating prior to the main pulse. Here, we report on results using a frequency tripled Nd:glass laser system (Omega EP) where the inherent prepulse was several orders of magnitude smaller. By applying a separate prepulse, these experiments allowed us to test if the prepulse is important for mitigating imprint with the high-Z layer. The results show that the high-Z coating is much more effective at reducing laser imprint when it is pre-expanded, in this case, by an externally generated low level soft x-ray prepulse of ∼10 J/cm2. With the x-ray prepulse, laser imprint at spatial wavelengths below 100 μm was reduced by an order of magnitude, similar to that observed with the laser prepulse on Nike. Furthermore, the experiments here establish that high-Z coating is effective in reducing imprint even in the case of a fixed speckle pattern. Rayleigh–Taylor-amplified laser imprint and high-Z layer dynamics were measured using through-foil and side-on x-ray radiography. Pre-expansion times from 30 down to 6 ns were effective, potentially compatible with laser prepulse generation using existing NIF and OMEGA front ends; however, temporal beam smoothing appears to be necessary for the laser prepulse that directly illuminates the high-Z coating. The highest imprint reduction is observed for Pd and Au coatings of at least 400 Å thickness; thicknesses down to 200 Å show a reduction in imprint with an adjusted prepulse.
Highlights
Direct drive inertial confinement fusion1,2 requires highly uniform illumination to achieve the symmetric and hydrodynamically stable implosions required for high target performance
A hybrid early time x-ray drive followed by conventional laser direct drive obtained by utilizing a thin high-Z overcoat on plastic ablator targets has proven to be very effective at reducing laser imprint in experiments driven by the Nike krypton-fluoride laser
The results show that the high-Z coating is much more effective at reducing laser imprint when it is pre-expanded, in this case, by an externally generated low level soft x-ray prepulse of $10 J/cm2
Summary
Direct drive inertial confinement fusion requires highly uniform illumination to achieve the symmetric and hydrodynamically stable implosions required for high target performance. Various beam smoothing schemes have been developed to achieve uniform illumination on a ns timescale. Most present ICF lasers have a THz or less bandwidth, and so the beam smoothing is not as effective on a ps to tens of ps timescale. Both simulations and experiments have shown that unacceptable imprint can occur at early time before a smoothing plasma forms. Simulations and experiments show the need to mitigate imprint for low adiabat direct drive, for example, on OMEGA implosions.. Simulations and experiments show the need to mitigate imprint for low adiabat direct drive, for example, on OMEGA implosions. Various imprint mitigation strategies have been proposed, including the use of foambuffered targets, preformed plasma, and high-Z doped ablators.
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