Magnetic trapping of spin-polarized atomic hydrogen.

We have confined over 5x10 sup (12) atoms of hydrogen in a static magnetic trap. The atoms are loaded into the trap by precooling with a dilution refrigerator. At the operating densities of 1x 10 sup (13 cm sup (-3) the gas is observed to be nuclear polarized. The long lifetime of the trapped gas (over 10 minutes) suggests that it is thermally decoupled from the wall and has evaporatively cooled to a temperature of about 40 mK. The residual decay of the gas density is caused by spin relaxation induced by dipolar interactions between atoms.