The first cryogenic DT layered, beryllium capsule implosion at the National Ignition Facility

TL;DR

This paper reports the first cryogenic DT layered beryllium capsule implosion at NIF, analyzing drive conditions, discrepancies in X-ray flux predictions, and performance factors affecting implosion yield.

Contribution

It presents the first successful cryogenic DT layered beryllium capsule implosion at NIF and analyzes the impact of drive multipliers and asymmetries on performance.

Findings

Achieved 29% of the predicted yield for Be capsule implosion.

Discrepancies in X-ray flux predictions affect implosion dynamics.

Performance limited by drive asymmetries and capsule instabilities.

Abstract

NIF experiments with Be capsules have followed a path of the highly successful "high-foot" CH capsules. Several keyhole and ConA targets preceeded a DT layered shot. In addition to backscatter subtraction, laser drive multipliers were needed to match observed X-ray drives. Those for the picket (0.95), trough (1.0) and second pulse (0.80) were determined by VISAR measurements. The time dependence of the Dante total x-ray flux and its fraction > 1.8 keV reflect the time dependence of the multipliers. A two step drive multiplier for the main pulse can match implosion times, but Dante measurements suggest the drive multiplier must increase late in time. With a single set of time dependent, multi-level multipliers the Dante data are well matched. These same third pulse drive multipliers also match the implosion times and Dante signals for two CH capsule DT. One discrepancy in the calculations is the X-ray flux in the picket. Calculations over-estimate the flux > 1.8 keV by a factor of ~100, while getting the total flux correctly. These harder X-rays cause an expansion of the Be/fuel interface of 2-3 km/s before the arrival of the first shock. VISAR measurements show only 0.2 to 0.3 km/s. The X-ray drive on the DT Be capsule was further degraded by a random decrease of 9% in the total picket flux. This small change caused the capsule fuel to change from an adiabat of 1.8 to 2.3 by mistiming of the first and second shocks. With this shock tuning and adjustments to the calculation, the first NIF Be capsule implosion achieved 29% of calculated yield, comparable to the CH DT capsules of 68% and 21%. Inclusion of a large M1 asymmetry in the DT ice layer and mixing from instability growth may help explain this final degradation. In summary when driven similarly the Be capsules performed like CH capsules. Performance degradation for both seems to be dominated by drive and capsule asymmetries.