Spectral Dependence of Stratified Electrothermal Instability in Tamped Aluminum 6061 with Current in a Skin Layer

The stratified electrothermal instability (ETI) was recently observed on the surface of thick aluminum 6061 pulsed with rapidly rising lineal current density ($3\times 10^{15}$\,A\,m$^{-1}$s$^{-1}$) for 70\,ns.\footnote{T.M. Hutchinson, T.J. Awe, B.S. Bauer, K.C. Yates, E.P. Yu, W.G. Yelton, and S. Fuelling, `Experimental Observation of the Stratified Electrothermal Instability on Aluminum with Thickness Greater than a Skin Depth,’ submitted (2017) to Phys. Rev. Lett. Also see Invited Talk by T.J. Awe at this APS-DPP meeting.} A transparent 70-$\mu $m-thick Parylene-N coating tamped the aluminum expansion and suppressed surface plasma. The evolution of the aluminum surface emission pattern was recorded with time-resolved microscopy (3-$\mu $m resolution). The images were converted into a series of blackbody surface-temperature maps. Analysis of these temperature maps provides information on the evolution of temperature fluctuations, as a function of axial wavelength and azimuthal width. Perturbations with axial wavelength longer than 20\,$\mu $m grow, while those with axial wavelength shorter than 10\,$\mu $m decay. Comparing the spectral dependence of growth/decay rates with MHD simulations could test the modeling of ETI positive feedback and of damping by thermal conduction.