COMPUTATIONAL INVESTIGATION OF SINGLE-MODE VS MULTIMODE RAYLEIGH-TAYLOR SEEDING IN Z-PINCH IMPLOSIONS

A series of two-dimensional magnetohydrodynamic calculations have been carried out to investigate single and multimode growth and mode coupl ing for magnetically-driven Rayleigh-Taylor instabilities in Z pinches . Wavelengths ranging from 5.0 mm down to 1.25 mm were considered. Suc h wavelengths are comparable to those observed at stagnation using a r andom density ''seeding'' method. The calculations show that wavelengt hs resolved by less than 10 cells exhibit an artificial decrease in in itial Fourier spectrum amplitudes and a reduction in the corresponding amplitude growth. Single mode evolution exhibits linear exponential g rowth and the development of higher harmonics as the mode transitions into the nonlinear phase. The mode growth continues to exponentiate bu t at a slower rate than determined by linear hydrodynamic theory. In t he two and three mode case, there is clear evidence of mode coupling a nd inverse cascade. In addition, distinct modal patterns are observed late in the implosion, resulting from finite shell thickness and magne tic field effects. (C) 1998 American Institute of Physics. [S1070-664X (98)02112-0].