The measurement of ion acoustic turbulence and reduced thermal conductivity caused by a large temperature gradient in a laser heated plasma

Abstract

An extensive account is given of experiments in which an homogeneous preformed plasma of density 6*10¹⁶ cm^-3 was heated by 10.6 mu m radiation focused on 10¹¹ W cm^-2. Ruby laser scattering gave space and time resolved measurements of the resultant density and temperature, showing large temperature gradients up to lambda _e mod Delta T mod /T=0.5. Enhanced low frequency turbulence was observed. When compared with a one dimensional computer code, the observed temperature gradients imply that the heat flux was limited to 4+or-2% of the free streaming limit but that the absorption was 1.1+or-0.5 times the classical inverse bremsstrahlung absorption rate. The observed T_e/T_i and lambda _e mod Delta T mod /T were sufficient to excite heat flux driven ion acoustic turbulence, which explains the observed low frequency turbulence. This turbulence level is shown to be large enough to account for the observed thermal conductivity.