Improving inner-cone beam transport in hohlraums by azimuthally staggered in time outer-cone beams at the National Ignition Facility

Hohlraums with low density gas fill (< 0.6 mg/cc) have been used frequently because of their high energy coupling. However, because of reduced tamping, the ablated wall material (bubble) penetrates further into the interior of the hohlraum and interferes with the inner cone beam transport at the end of the pulse. Thus, a low adiabat designs (requiring longer pulses) in a low gas-fill density hohlraum (having faster bubble growth) are not, in principle, compatible.

 

We propose an improvement to the inner-cone laser power propagation for a low-adiabat target design by reducing the number of beams assigned to the foot portion of the outer cone laser pulse; from the usual 128 to 32 beams.  This sparse assignment allows us to reduce the footprint of the outer cone beams by ~ 4x, while keeping the same x-ray drive by correspondingly increasing the laser power per beam during the foot.  Because the mass ablation rate of the wall material is a sublinear function of the laser intensity, it may be possible to delay the bubble expansion even with 4x more intense power due to the sparse arrangement of the outer cone beams during the foot.

 

We will discuss the potential of this new beam configuration and outline the proposed shot plan.