Ionlzation of fast hydrogen atoms in a strong magnetic field

To determine the fraction of 30 keV hydrogen atoms that were ionized by the Lorentz force in a magnetic field up to 60 kG, we performed a direct measurement, unlike the experiment of Sweetman who evaluated the neutral flux following charge exchange. The beam of neutral hydrogen atoms was obtained from charge exchange of a 30 keV proton beam on a supersonic jet of carbon dioxide in a vacuum of residual pressure 5 × 10−7 torr; the beam strength, measured with a small inertial calorimeter (thermocouple) calibrated by an ion beam, was nearly constant during the magnetic pulse. Electrons were removed by a weak electric field. The ions were measured with three collectors (before, in and after the central magnetic field of 2.2 cm radius); at fields above 25 kG all the ions appeared at the central collector that had a sensitivity 0.5 nA. At fields of 55 to 60 kG, the fraction (5 to 4) × 10−4 of the hydrogen atoms were ionized by Lorentz ionization, that is an ion current about 0.05 μA and an equivalent neutral current about 100 μA. From 32 to 51 kG, ionization is from the state n=9, for larger fields from n=8.