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Notes: The x-ray beams for the next generation of synchrotrons will contain much more power (1–10 kW) than is available at present day facilities. Cooling the first optical components in these beam lines will require the best cooling technology that one can bring to bear. Argonne continues to pioneer the use of liquid metals as the cooling fluid and has adopted liquid gallium as the liquid metal of choice. Its low melting point, 29.7 °C and its very low vapor pressure make it an easy fluid to handle and its high thermal conductivity and heat capacity make it an excellent cooling fluid. A series of experiments were performed during April 1991 with the wiggler beam at the F2 station of the CHESS facility at Cornell to investigate the cooling of large areas of high power. Two types of cooling crystal geometries were tested, one where the cooling channels were core-drilled just below the surface of the crystal and a second where slots were cut into the crystal just below the surface with a diamond saw. Both crystals performed well with beam powers up to 1050 W and power densities of up to 14.5 W/mm2 at normal incidence.An infrared camera was used to measure the variation in the temperature of the top layer of the silicon crystals. For the core-drilled crystal the peak temperature measured at the center of the beam at a power density of 12.3 W/mm2 was 15 °C hotter than the crystal surface outside of the beam with a flow of liquid gallium of 2 gpm (gallons per minute) and was 10 °C with a flow of 4 gpm. The maximum distortion of the crystal surface distortion of the core drilled crystal was about ±2.0 arcsec for the 2 gpm case with a maximum power density of 10.9 W/mm2 and about 5% of the expected beam intensity was lost at peak power of 14.5 W/mm2. For the slotted crystal the peak temperature difference for a peak power of 10.9 W/mm2 was 3.5 and 2.0 °C for liquid gallium flows of 1 and 2 gpm, respectively. No intensity loss was measured for the maximum power density of 14.5 W/mm2. The fact that the peak temperature differences on the surface of both crystals was decreasing with increased flow of liquid gallium suggests that even higher power densities can be accommodated with higher flows of liquid gallium. This work is supported by the Department of Energy, BES-Materials Sciences, under Contract W-31-109-Eng-38.