LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996 Part S1 - Quantum Fluids and Solids: Liquid Helium Reducing Gravity at the Superfluid Transition in Helium-4* Melora Larson, Feng-Chuan Liu, and Ulf E. Israelsson Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109-8099, USA There are two intrinsic experimental limitations in measurements near Tx for traditional ground based exper- iments: the gravitationally induced pressure variations present in any macroscopic helium sample limit how closely the transition can be approadmd, and the onset of convection in a sample with T >Tx limits the range of heat values that can be used. To overcome these limitations, we have built a low gravity simulator consisting of a superconducting magnet with a magnetic field profile shaped to provide a magnetic force opposite that of gravity. Preliminary measurements have shown a decrease in the range of reduced temperatures across a sample but not the expected suppression of the onset of convection. 1. Introduction The h-transition in 4He provides an almost ideal system for testing theories of critical phenomenon against experiment. Recent experiments[2, 1] on the influence of a heat current on the superfluid tran- sition in 4He have shown interesting new behavior while showing notable disagreement with existing theories[3, 4]. Experimental studies of the X-transition are lim- ited intrinsically only by gravitationally induced pressure variations that limit how closely the tran- sition can be approached. It was proposed over a decade ago that the gravitationally induced pressure variations in a column of 4He could be counteracted by applying a suitably shaped static electric or mag- netic field[5]. It was shown that the chemical po- tential of liquid helium is coupled to electromagnetic fields in the same way that pressure is coupled into the system. Therefore, the application of electromag- netic fields on a helium sample should not dmnge the nature of the transition. 2. Cancelling Gravity on the Ground Our recent work has focused on building a low gravity simulator using an applied magnetic field. To create a diamagnetic force that counteracts gravity, the magnetic force on a volume of liquid must be equal and opposite the gravitational force over that volume. This balance of forces will occur when the component of the magnetic field times the magnetic field gradient in the direction opposite gravity is: B x OB/Oz = 21T2/cm, (1) The low-gravity simulator reported on here includes an Oxford Instruments 17Tesla magnet run in per- sistent mode capable of achieving B x OB/Oz > 23T2/cm. This magnet allows the cancellation of gravity to 1%, 0.01g, over a sample volume 0.5cm in diameter and 0.5cm high. An interesting feature of the simulator is that it allows the effective grav- ity on the sample to be any value between lg and -0.1g by varying the magnetic field trapped in the superconducting magnet. 3. Results The initial measurements using the low-gravity simulator have been optimized to show the decrease in the distance between T~(z=0) and Tx(z=L) as the magnetic field increases. To this end, a thermal conductivity cell of diameter 0.5cm and height L = 0.55cm was placed at the maximum in B • OB/Oz of the magnet. The cell had high conductivity cop- per end caps and 0.01cm thick stainless steel side- walls. Helium-4 melting curve thermometers were connected to monitor the temperature of the bottom cell end cap, Thor, and to regulate the temperature of the top end cap, Ttop. A constant heat current, Qbot, was applied to the bottom of the cell and removed at the top of the cell. * This work was supported by NASA-MSAD Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. S 1 ]'79