Capped probe for ion temperature diagnostic in a magnetized plasma

A simple ion temperature measurement method in a magnetic field using an electrostatic capped probe is presented and tested. The probe with the axis oriented parallel with the magnetic field is a small cylinder whose flat tip is insulated with a cap to inhibit electron inflow parallel to the field. The ion collector current, essentially less affected, becomes comparable to or larger than the saturation electron current, and the usual retarding potential technique can be used to measure ion temperature. Measurements were made in a diffused helium plasma with the density 1010 ∼ 1013 cm−3 at the gas pressure ∼mTorr and the field 0.8 ∼ 3.5 kG. Ion temperatures obtained are 0.8 ∼ 2 eV, which nearly equal electron temperatures, and are compared with data from the ion probe and Doppler profile measurements. Availabilities of the method and limiting conditions for the temperature determination are also discussed.