Study of the velocity distribution in an intense pulsed hydrogen beam

Abstract We present the results of measurements of the velocity distributions of particles in a pulsed hydrogen beam obtained from a dissociator with a radio frequency discharge (duration 1.0 ms, repetition rate 1 Hz). It is shown that the hydrogen inside the dissociator is heated up to ∼2800 K, so the thermal dissociation of hydrogen molecules is essential. In order to cool the atoms, the gas was let through a pyrex channel 5 mm in diameter. The cooling channel walls being at room temperature and the channel having a length of 50 mm, we have obtained a supersonic beam of hydrogen atoms with a Mach number M | = 2.7±0.25. When the channel walls were cooled by the flowing liquid nitrogen and the channel was 70 mm long we obtained a beam of cooled atoms with a Mach number M | = 4.14±0.35. The velocity distribution of atoms depends on the power of the rf discharge inside the dissociator and on the gas consumption per pulse, and varies during the discharge pulse. For a temperature of the cooling channel walls T ch = 77 K, a gas consumption N = 3.3×10 17 molecules per pulse and a discharge power of 0.23 kW cm −3 , we have obtained an atomic beam with intensity I (0) = (2.8±0.8)×10 20 atoms sr −1 s −1 and a most probable velocity ν MP = (1.97±0.07)×10 5 cm s −1 .