The effects of fill tubes on the hydrodynamics of ignition targets and prospects for ignition

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The notion of using a narrow bore fill tube to charge an ignition capsule in situ with deuterium-tritium (DT) fuel is very attractive because it eliminates the need for cryogenic transport of the target from the filling station to the target chamber, and in principle is one way of allowing any material to be considered as an ablator. We are using the radiation hydrocode HYDRA [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] in two dimensions to study the effect of fill tubes on graded copper-doped Be ignition capsule implosions. The capsule is ∼1.1-mm radius and driven at ∼300eV. Fill tubes are made of glass and range in diameter from 10–20μm. These are inserted between 5 and 40μm into the ablator surface, and a glue layer around the capsule ∼2-μm thick is included. The calculations are unusually demanding in that the flow is highly nonlinear from the outset, and very high angular resolution is necessary to capture the initial evolution of the tube, which is complex. Despite this complexity, the net result is that by the time the capsule implosion takes off, a preferred, simple Bessel-like mode is set up that is almost independent of, and much larger than, the initial tube size, and close to the fastest growing mode for the capsule. The perturbation continues to grow during the unstable acceleration phase, and inverts as the capsule begins to stagnate, sending a spike of cold DT into the forming hot spot. In all cases studied the capsule ignites and gives close to clean one-dimensional yield. The principal seed of the perturbation appears to be shielding of the ablator in the close vicinity of the fill tube, and the growth is found to vary linearly with the diameter of the tube. The simulations and results are discussed.