3.1 The Penning gauge
The working principle of this type of gauge is to generate a discharge between two metal electrodes
(anode and cathode) by applying a DC high voltage. The discharge current is pressure dependent and
serves as measurand for pressure. The lower measurement limit lies around 1 Pa, since at lower
pressures the gas density is too low to generate enough charge carriers to maintain the discharge.
To extend this limit, a magnetic field crossing the electrical field is used. This magnetic field
greatly increases the path length of the electrons from cathode to anode, so that the electron can
generate another electron by impacting on a gas molecule to maintain the discharge (Penning
discharge). Owing to their higher mass the ions are only slightly affected in their trajectories by the
magnetic field and travel directly to the cathode. Secondary electrons released when the ions hit the
cathode (cathode sputtering) support the discharge.
In crossed field gauges, the ion current vs. pressure relation follows the equation