Physics and DT implosion results from the high-Trad hohlraum, thick-ablator, (“HiT”) experimental effort

The principal function of all ICF implosions is to transform kinetic energy into internal energy in as small a volume as possible to have the best chance of reaching burning plasma conditions. Since kinetic energy is a product of shell mass and the square of velocity, a potential trade-off in the design exists between shell thickness and velocity. At fixed peak hohlraum radiation temperature (Trad) and laser pulse duration, a thicker implosion will naturally slow down. Velocity is key for obtaining high ion temperatures in the hotspot of an implosion, but high velocity risks more ablation-Rayleigh-Taylor (A-RT) instability. A thicker shell implosion is less prone to the damage done by Rayleigh-Taylor instability because of reduced in-flight aspect ratio and is less sensitive to energy-sapping asymmetries, according to piston-model theories of ICF implosions. The velocity of a thicker shell can be maintained using higher ablation pressure resultant from a hotter radiation temperature (Trad) hohlraum by reducing hohlraum surface area (i.e. smaller case-to-capsule ratio, CCR). This has the added benefit of increased ablative stabilization to the A-RT. However, symmetry control becomes much more difficult with a smaller CCR hohlraum. To find an optimum of these trade-offs given the realities of our ability to control implosion physics, we conducted an experimental campaign that achieved a burning plasma on the first deuterium-tritium (DT) cryogenic layered implosion test this past year. This talk will overview the physics and capsule fabrication motivations for HiT, the ideas supporting piston-model theory, and the experimental results to date. A number of other supporting experimental results will also be discussed, along with how this learning is being integrated into future experimental campaigns.