Argonne Advanced Photon Source Research Articles

Liquid gallium metal cooling for optical elements with high heat loads

The intense photon beams from the insertion devices of the Argonne Advanced Photon Source (APS) will have very high total powers, which in some cases will exceed 10 kW, spread over a few cm 2. These high heat loads will require special cooling methods for the optical elements to preserve the quality of the photon beam. A set of finite element analysis calculations were made in three dimensions to determine the temperature distributions and thermal stresses in a single crystal of silicon with heat loads of 2–20 kW. Different geometric arrangements and different cooling fluids (water, gallium, oil, Na, etc.) were considered. These data were then used in a second set of calculations to determine the distortion of the surface of the crystal and the change in the crystal plane spacing for different parts of the surface. The best heat transfer, smallest surface distortions and smallest temperature gradients on the surface of the crystals were obtained when the cooling fluid was allowed to flow through channels in the crystal. The two best fluids for room temperature operation were found to be water and liquid gallium metal. In all cases tried, the variation in temperature across the face of the crystal and the distortion of the surface was at least a factor of two less for the gallium cooling case than for the water cooling case. The water cooling was effective only for very high flow rates. These high flow rates can cause vibrations in the diffraction crystal and in its mount that can seriously degrade the quality of the diffracted photon beam. When the flow rates were decreased the gallium cooling became 3–10 times more effective. This very efficient cooling and the very low vapor pressure for liquid gallium (less than 10 −12 Torr at 100°C) make liquid gallium a very attractive cooling fluid for high vacuum synchrotron applications. A small electromagnetic induction pump for liquid Ga was built to test this cooling method. A pumping volume of 100 cm 3/s was achieved. With no flow, a static head pressure of 8 psi was measured across the pump. With this flow rate of 100 cm 3/s and a ΔT = 10° C, the heat transfer would be 2.2 kW of power. This system worked well and based on the test data from this system, a second electromagnetic induction pump was built that developed four times the flow rate. The new system is portable, controls the output temperature of the Ga and can handle heat loads of 10 kW. This work was supported by U.S. Department of Energy, BES-Material Science under Contract No. W-31-109-ENG-38.