All CCGs utilize crossed electric and magnetic fields to trap electrons. The high voltage
ranges from 2-6 kV and the magnetic field 1-2 kG. The electron plasma, responsible for
ionization, originates from the random release of an electron at the cathode caused
directly, or indirectly, by a cosmic ray, field emission, a photon, radioactivity or some
other event. A discharge slowly builds inside the ionization volume to the point where the
entry of new electrons into the plasma is limited by space charge repulsion. At pressures
below 10-4 Torr, the discharge is practically a pure-electron plasma. The electrons move
in cycloidal jumps, circling about the anode, and during part of each jump they have
sufficient energy to ionize gas molecules through electron impact ionization. The
probability of collision is proportional to the gas density. The slow ions generated, are
quickly captured by the cathode. The current generated by this ion collection process is
measured and used as an indirect indication of gas density and pressure.