Thermionic Electron Beam Current and Accelerating Voltage Controller for Gas Ion Sources

Abstract

Thermionic emission sources are key components of electron impact gas ion sources used in measuring instruments, such as mass spectrometers, ionization gauges, and apparatus for ionization cross-section measurements. The repeatability of the measurements taken with such instruments depends on the stability of the ion current, which is a function, among other things, of the electron beam current and electron accelerating voltage. In this paper, a laboratory thermionic electron beam current and accelerating voltage controller is presented, based on digital algorithm implementation. The average value of the percentage standard deviation of the emission current is 0.021%, and the maximum electron accelerating voltage change versus the emission current is smaller than 0.011% in the full operating range of the emission current. Its application as a trap current or emission current-regulated ion source power supply could be useful in many measuring instruments, such as in microelectromechanical system (MEMS) mass spectrometers as universal gas sensors, where a stable emission current and electron energy are needed.

Digital Approach to Thermionic Emission Current to Voltage Conversion for High-Voltage Sources of Electrons

“…There are many vacuum devices that use thermionic electron sources operating under temperature or space charge limited mode, such as electron beam objects for high energy physics [17], water radiolysis [21], [23] integrated circuit manufacturing process monitors [16], X-ray photoelectron spectroscopes [22], devices producing rare gas excimers [3] or evaporators [15]. The stability of the electron emission current is one of the most important requirements for electron sources.…”

Conversion Method of Thermionic Emission Current to Voltage for High-Voltage Sources of Electrons

“…With regard to evaporators, there are also designs in which the electric power supplied to the cathode is controlled by the voltage directly proportional to the ion current [6]. In commonly used emission current control systems, the I e -V conversion is performed in the anode power supply circuit and its output signal is transferred as a negative feedback signal to the low-voltage cathode power control circuit [7][8][9][10]. For a relatively small value of the cathode-anode potential difference and, consequently, a relatively low electron energy, e.g., in mass spectrometers, ionization vacuum gauges, a galvanic connection can be used to implement the negative feedback loop.…”

“…For a relatively small value of the cathode-anode potential difference and, consequently, a relatively low electron energy, e.g., in mass spectrometers, ionization vacuum gauges, a galvanic connection can be used to implement the negative feedback loop. The works [7,9] present systems in which the negative feedback signal is transferred from the anode power supply circuit to the low-voltage cathode power supply circuit by means of a current mirror, a high-voltage differential amplifier used for this purpose is described in [8,10]. These controllers ensure high-quality stabilization of the emission current, however, the value of the electron accelerating voltage is limited by the breakdown voltage of semiconductor components.…”