The formation of polar ice caps and jets during implosions and the potential remedies*

Polar ice caps and jets have been observed in imploding capsules around stagnation, using Au-lined DU hohlraums, in experiments conducted at the National Ignition Facility. These jets can significantly reduce the yield of the implosions. Simulations indicate that early-time P4<0 radiation asymmetries result in lateral velocity along the shell, leading to mass accumulation in the polar and equatorial regions. The inward radial velocity is higher along the troughs of P4 <0 at the poles and the equator due to the higher inward momentum resulting from greater mass concentration at these locations. Conversely, the inward radial velocity is lower along the maxima (49.1° and 130.9°) of P4<0, as the stronger return shock reaches the maxima of P4<0 before reaching other locations, causing the shell locations at the maxima to be slowed down first. These conditions set the stage for rapid phase reversal for the shape of the shell from a4>0 to a4<0, shortly before ignition, creating the polar jets. The amplitudes of these jets are further enhanced by the Rayleigh-Taylor instability during deceleration. While P2 radiation asymmetries do not typically produce pronounced polar jets, they can generate polar ice caps that enhance the polar jets produced by P4 radiation asymmetries. Simulations using unlined DU hohraums show reduced polar jets.