Special gas heater for the TESLA test facility

A special heater has been designed and is under construction at Fermi National Accelerator Laboratory for use at the TESLA test facility at the Deutches Electronen Synchrotron (DESY). This heater is used to warm up the 2.2 K helium gas coming from a dewar which is being pumped down to a pressure of 1600 Pa in order to keep the liquid in the test dewar in the superfluid state. This heater must warm the gas from 2.2 K to 300 K and must have a sufficiently low flow restriction that the pressure drop is less than 100 Pa across the unit with a flow of 10 grams/second. The heating element is constructed of 304 SS tubes with diameter .0127 m and wall thickness 0.254 mm. Each tube is 1 meter long and there are 648 tubes arranged in a square bundle. The tubes are heliarc welded to 3.176 mm thick stainless bars which make the electrical connections to the tubes. The gas is heated primarily inside the tubes with electrical current. The tubes are in a series parallel electrical arrangement which requires a power supply current of 500 amperes. A test has been performed which determines the performance of a single tube for a family of values of flow and heating which bring helium gas from 77 K to 300 K. The results of the test will be compared with a computer simulation of the performance of a single tube