San Diego, California, 17-19 June 2002

Wall-Thickness Dependence of Cooling-Induced Deformation of Polystyrene Spherical Shells

 

T. Endo, N. Kobayashi, K. Goto, M. Yasuda, and Y. Fujima

Center for Integrated Research in Science and Engineering, Nagoya University

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan

endocirse.nagoya-u.ac.jp

High-gain inertial fusion needs cryogenic targets with extremely high sphericity: typically higher than 99%. This requirement should be fulfilled at cryogenically cooled conditions, although the sphericity of target shells is usually inspected at room temperature. If target shells are deformed during cooling, we will have to inspect the sphericity of them after cooling. From this point of view, the authors have been working on deformation of target shells induced by cooling. In the 13th Target Fabrication Meeting held at Catalina Island, California in 1999, we first reported that some polystyrene shells were deformed in the order of 1% of the radius by cooling only. We thought this deformation, which we call ‘cooling-induced deformation (CID)’, as a result of nonuniformity of the thermal-expansion coefficient. Recently, we experimentally evaluated experimental errors very carefully, and have clarified that any polystyrene shell shows CID to one degree or another. And we have also recognized that some shells show poor repeatability and poor monotonicity about CID. We reported these experimental results in the 14th Target Fabrication Meeting held at West Point, New York in 2001.

In this paper, we report the experimental results on wall-thickness dependence of CID. We used polystyrene (PS) hollow spherical shells as test samples. We fabricated the PS shells by the density-matched emulsion method using the hand-shaken microencapsulation technique. The number-averaged and weight-averaged molecular weights of the PS were M_n=1.1x10⁵ and M_w=4.0x10⁵, respectively. The diameter of the PS shells was about 400-550 m m. In order to investigate the wall-thickness dependence of CID, the wall thickness of the PS shells was varied between 5 and 60 m m. The PS shells were cooled using liquid nitrogen from room temperature to -190°C. By using a reflection-type confocal scanning laser microscope, the shell images at 0°C and -190°C were recorded. By comparing the shapes of a shell at these two temperatures, we quantitatively determined the deformation induced by cooling. It has been found that thinner shells show larger deformation; particularly, the shells thinner than approximately 10 m m show untolerable deformation, that is, almost 1% of the radius.