San Diego, California, 17-19 June 2002

Update for the drag force on an injected pellet and target fabrication for Inertial Fusion

T. Norimatsu, K. Nagai, T. Takeda, K. Mima and T. Yamanaka

ILE, Osaka University

norimatsile.osaka-u.ac.jp

1. Drag force

In past meetings, we discussed the influence of residual gas on the trajectory of an injected pellet basing on hydrodynamic approximation. This approximation was, however, inappropriate in two cases. First, if the pressure in a chamber is less than 0.01 Torr, the mean free path of gas molecules is larger than the pellet radius and the hydrodynamic model lose the validity. Second, if the gas is lead vapor like that in the wet-walled reactor KOYO, almost all of the impinging molecules are adsorbed on the pellet surface []. As the result, flow around the pellet is completely different form that without adsorption.

Recently we used kinetic model [] assuming 100% adsorption of vapor on the pellet. In this case, resultant resistance F acting on a sphere of radius a moving with velocity v through a gas of density r and average molecular velocity c is the same as a case with specular reflection of molecules on the surface. The force is given by F=(4/3) pr a²cv.

Figure 1 shows the drag force on a 2-mm-radius pellet moving with the velocity of 300 m/s through rarefied nitrogen gas at room temperature. The rigid line is calculated with the previous hydrodynamic model and the doted line was obtained with correction on the mean free path. The kinetic model assuming 100% adsorption gives the broken line. The difference is clear at the lower pressure region.

2. Fabrication

To demonstrate ignition and burn by fast ignition scheme, we are going to develop targets with a cone. We are going to use a low density foam to support the cryogenic fuel layer since the cone disturb the isothermal cooling that is required by most layering technique. The fuel is loaded through a capillary from a gas reservoir outside the chamber. This technique can skip a high-pressure diffusion-fill system and enables tritium facility around the chamber simple and convenient. This target has a flat top cone but a cone with a needle top is also studied because this structure helps focusing of laser and enhances self-pinch of the electron beam for core heating.