GRAVITATIONAL EFFECTS ON NONLINEAR HEAT-TRANSPORT NEAR THE SUPERFLUIDTRANSITION IN HE-4

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux Q, in the range 0.1 < Q < 2 erg/(s cm(2)). While Haussmann and Dohm (HD) accurately predi ct the initial departure of the thermal conductivity, kappa, from the linear response region, kappa, is greater than expected very close to T-lambda. We anticipate that the nature of the thermal conductivity's nonlinearity may depend upon Earth's gravity in the low heat flux limi t (Q < 0.5 erg/(s cm(2))). Comparison of our data to similar data to b e taken in a microgravity laboratory will provide an experimental dete rmination of the effect of gravity on nonlinear heat transport near th e superfluid transition. The microgravity measurements will also permi t the first experimental test of theories that do not consider gravita tional effects, such as those by HD.