"SPECIAL PLASMA SEMINAR Inertial Confinement Fusion (ICF) – One plausible approach to achieving ignition and energy gain in a laboratory"

 

 

 

Demonstrating ignition and energy gain in a laboratory is a scientific grand challenge that requires creating extreme states of matter under precise, highly controlled conditions. One plausible approach to achieve this is hot-spot ignition in an Inertial Confinement Fusion (ICF) implosion at the National Ignition Facility (NIF). Using this method it is necessary to maintain the deuterium-tritium (DT) fuel at low entropy during the implosion and to ensure that the shell kinetic energy is efficiently converted to hot-spot thermal energy upon stagnation. Analysis of NIF implosions performed to date indicates that the performance has improved almost two orders of magnitude since the first implosion taken in September 2010, and that we are about a factor of three away from conditions required for ignition. However, numerical simulations of the data for the hot spot and the surrounding main fuel indicate, first and foremost, that the stagnation pressure of the hot spot and fuel are significantly lower than predicted, even when the models are tuned to reproduce the shock timing and implosion velocity measurements.

 

The conjecture is that the pressure deficit is partly explained by the observed low-mode areal-density asymmetries, which may cause inefficient conversion of shell kinetic energy to hot-spot thermal energy at stagnation. It is also explained in part by higher than predicted mix of high-Z material into the hot-spot for high-convergence, low-entropy implosions. To address the detrimental effect of mix, higher-entropy, lower-convergence implosions that are more robust against Rayleigh-Taylor instabilities were conducted. This strategy gives up on theoretical high-gain in an ICF implosion to obtain better control of the implosion and bring experimental performance in-line with calculated performance, yet keeps the absolute implosion performance relatively high. With these implosions, areal densities around 1 g/cm2 and neutron yields of about 1e16 are now readily achieved. Although evidence for significant bootstrapping associated with alpha-particle self-heating is observed in these implosions, substantial low-mode areal-density asymmetries and  residual kinetic energy are still observed, which are not yet fully understood. Drive asymmetry is the most likely culprit for these observations.

 

In this presentation, I will introduce the concept of ICF; present the ignition experiments at the NIF; discuss the near-term efforts that will address current issues; and finally describe our efforts in understanding plasma stopping power that is relevant to alpha particle transport and self-heating of the hot spot. The work described here was supported in part by US DOE (Grant No. DE-FG03- 03SF22691), LLNL (subcontract Grant No. B504974) and LLE (subcontract Grant No. 412160-001G).

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