Energy Distribution of Positive Ions Extracted from a Hot Cathode Gas Discharge Ion Source

Abstract

An electrostatic parallel plate energy analyzer was used for measuring the energy distribution of Ar+ ions in a beam extracted from a nonmagnetic, hot cathode, gas discharge ion source under various conditions of source pressure, anode voltage, and anode current. A large number of energy distribution curves were obtained as X-Y recordings. The curve shape depends upon source conditions, though not strongly when the pressure is greater than 3×10−3 Torr with an anode voltage above 50 V (for argon) and an anode current of at least 200 mA. When conditions are optimum, the ion energy distribution curve is a narrow peak with a tail that gradually diminishes towards lower energies over an interval of approximately 30 eV. The actual value of the optimum full width at half maximum was found to be 1.1 eV by extrapolation to zero beam energy. The shape of the curve is such that more than 81% of the beam ions have energies within a 10 eV interval. The source can be used to increase beam currents greatly in certain types of magnetic deflection mass spectrometers without extreme loss of mass resolution due to ion beam energy spread.