  We describe the design and performance of a large magnetic trap for storing
and cooling of atomic hydrogen (H). The trap operates in the vacuum space of a
dilution refrigerator at a temperature of 1.5 K. Aiming at a large volume of
the trap we implemented the octupole configuration of linear currents (Ioffe
bars) for the radial confinement, combined with two axial pinch coils and a 3 T
solenoid for the cryogenic H dissociator. The octupole magnet consists of eight
race-track segments which are compressed towards each other with magnetic
forces. This provides a mechanically stable and robust construction with a
possibility of replacement or repair of each segment. A maximum trap depth of
0.54 K (0.8 T) was reached, corresponding to an effective volume of 0.5 liters
for hydrogen gas at 50 mK. This is an order of magnitude larger than ever used
for trapping atoms.
