Experiments with Atomic Hydrogen in a Magnetic Trapping Field

We describe the loading of a minimum-$B$-field trap with neutral atomic hydrogen cooled through thermal exchange with liquid-helium surfaces. We monitor the gas during filling and decay by observing the atoms which are ejected from the trap as a result of spin exchange and magnetic dipolar relaxation. The stability of the gas is unaffected if we change the wall coverage from pure ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to dilute ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures. The maximum trapped density is 3 × ${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$ at about 100 mK. The limiting decay rate is attributed to dipolar relaxation and found to agree with theory for both magnitude and field dependence.