We present findings from an experimental tuning campaign aimed at igniting larger DT cryogenic layered implosions using a dual frustum shaped hohlraum, denoted “frustraum”. The frustraum's distinctive shape reduces hohlraum wall losses while concurrently enhancing minimum capsule clearance with the hohlraum wall and sensitivity to pointing changes. Compared to current cylindrical hohlraum (6.4 × 11.24 mm), the frustraum has a wall area approximately 20 % smaller, resulting in a measured improvement in efficiency of around 12 %. Consequently, 12 % less laser energy is required to implode a capsule within the same acceleration timeframe. Conversely, directing the same laser energy into the frustraum yields higher ion temperatures within symmetry capsules, along with increased radiation temperatures and reduced implosion acceleration times compared to current cylindrical hohlraums.
Cylindrical implosion experiments are used to directly measure instability growth in a convergent geometry, providing a wealth of data for model validation. Double cylinders are a natural extension of the platform and enable measurements at a classically unstable interface, the outer surface of the inner cylinder, which experiences no ablative stabilization from the laser drive. However, the utility of this platform relies upon maintaining adequate axial uniformity of the inner cylinder during the implosion. Although previous smaller-scale double cylinder experiments exhibited acceptable levels of axial uniformity, radiation-hydrodynamics simulations of larger-scale double cylinders predict more axial non-uniformity induced by the impedance mismatch as the shock wraps around the axial ends of the inner cylinder. A mechanism to reduce axial non-uniformity in these larger double cylinder implosions is presented, and preliminary experimental data confirms the efficacy of the selected mitigation approach.
For many decades the running joke in fusion research has been that “fusion” is twenty years away and always will be. Yet, in 2023 we find ourselves in a position where we can talk about the milestones of burning plasmas, fusion ignition, and target energy gain greater than unity in the past tense – a situation that is remarkable! This paper tells some of the story of the applied physics challenges that needed to be overcome to achieve these milestones and the strategy our team followed. Things did not always go well and some practical lessons learned are part of this story. The data shows, getting to a burning plasma in late 2020 and early 2021 was a key tipping-point, after which ignition (August 8, 2021) and target gain (December 5, 2022) were rapidly achieved.
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