Evaporative Cooling of Spin-Polarized Atomic Hydrogen.

A gas of hydrogen atoms, confined in a static magnetic trap, has been evaporatively cooled to temperatures of a few millikelvin. The initial trap configuration held the gas at 38mK for as long as 5 hours. Evaporative cooling reduced the temperature to 3.0mK while maintaining the central density at 7.6x10 sup (12) cm sup (-3) . These values were determined by measuring the rate of electronic spin relaxation and are in agreement with model calculations. Further cooling to 1mK (inferred from the model) has been achieved. Measurements were made of the efficiency of the evaporative cooling process.