US Patent 4792763: Hot cathode ionization pressure gauge

A hot cathode ionization gauge with which neutral gas densities in the ra 1017 to 1021 m-3 can be measured, in particular under the conditions typical of fusion-orientated plasma experiments (magnetic field strength from 0 to more than 3 tesla, magnetic field direction varying by up to +/-20 degrees or more and strong plasma-induced noise background), and which is of robust design, reliable in operation and very compact and has a high sensitivity which depends in a reproducible manner on the magnetic field strength and is independent, in a certain degree which is predetermined and can be influenced by the construction, of the field direction and, within a wide range, of the gas density comprises the following electrodes arranged in mutually spaced relationship in the stated order along an axis: a cathode consisting of a tungsten wire at least 0.5 mm thick and which has a middle, active portion, a plane control electrode of diaphragm form arranged at a close distance from and parallel to the active portion of the cathode and having an elongated opening with which the active portion of the cathode is in alignment, a plane accelerating electrode arranged at a distance from and parallel to the control electrode and having a series of parallel slit-shaped apertures extending perpendicularly to the longitudinal direction of the active cathode portion, and a plane, plate-shaped ion collector electrode parallel to the control electrode.