Experimental observation of the stratified electrothermal instability on aluminum with thickness greater than a skin depth.

A direct observation of the stratified electrothermal instability on the surface of thick metal is reported. Aluminum rods coated with 70 mu m Parylene-N were driven to 1 MA in 100 ns, with the metal thicker than the skin depth. The dielectric coating suppressed plasma formation, enabling persistent observation of discrete azimuthally correlated stratified thermal perturbations perpendicular to the current whose wave numbers, k, grew exponentially with rate gamma(k) = 0.06 ns(-1) - (0.4 ns(-1) mu m(2) rad(-2)) k(2) in similar to 1 g/cm(3), similar to 7000 K aluminum.