Although the CEBAFCEBAF beam spot is intrinsically 0.1 mm0.1 mm, it will be necessary to defocus and raster the beam 5 mm to prevent target puncture. In addition, sufficient space on the frame is necessary to conduct heat away from individual foils when using external water cooling Chilled water will provide cooling for the target cells. It will flow through coiled tubes inside the chamber and into the spider extension near the spider and travel through drilled holes in the ladder and back to the extension. Tubes in the ladder will fit into the extension and help with positioning.
Calculations were made to determine if water could be used as the cooling medium for the solid targets. The heat deposited in several materials was determined with Lithium having the highest heat load at 780 watts. This is equal to 44.33 Btu/min. A modest temperature rise of 3.5 F would be required with water flowing through a 1/4 in diameter passage at 10 feet per second, near the optimum velocity. The flow would be 1.53 gallons per minute. Reducing the flow rate to 1 gpm would only increase the temperature rise to 5.35 F.
Additional calculations were made to see if the targets could tolerate the resulting conditions. The beam was assumed to be rastered in a 1 cm diameter and the target was assumed to be held at room temperature, 90 F, on a 1 in diameter circle. The materials considered were Lithium, Calcium and Aluminum. For all reasonably thin targets, the temperature at the edge of the rastered area did not approach the melting point of the material.