A revisit to self-excited push pull vacuum tube radio frequency oscillator for ion sources and power measurements

Abstract

Self-excited push-pull vacuum tube oscillator is one of the most commonly used oscillators in radio frequency (RF)-ion plasma sources for generation of ions using radio frequency. However, in spite of its fundamental role in the process of plasma formation, the working and operational characteristics are the most frequently skip part in the descriptions of RF ion sources in literatures. A more detailed treatment is given in the present work on the RF oscillator alone using twin beam power tetrodes 829B and GI30. The circuit operates at 102 MHz, and the oscillation conditions, stability in frequency, and RF output power are studied and analyzed. A modified form of photometric method and RF peak voltage detection method are employed to study the variation of the oscillator output power with plate voltage. The power curves obtained from these measurements are quadratic in nature and increase with increase in plate voltage. However, the RF output power as measured by photometric methods is always less than the value calculated from peak voltage measurements. This difference is due to the fact that the filament coil of the ordinary light bulb used as load/detector in photometric method is not a perfect inductor. The effect of inductive reactance on power transfer to load was further investigated and a technique is developed to estimate the amount of power correction needed in the photometric measurement result.

Radiofrequency power dependence of an oscillator use in an ion source

“…The frequency of the RF signal is directly measured with a frequency counter FC 2400. As reported in our previous paper [6], the RF output power deteriorated by about 10% as a result of the coil oxidation. Therefore, the coils are freshly polished for each measurement.…”

“…It can be seen that as the plate voltage increases, the RF output power also increases accordingly. However, the variation of RF power with plate voltage is non-linear and can be described by a quadratic function, as also reported in our previous paper [6]. The maximum RF…”

“…The present RF oscillator follows the design used in an Oak Ridge type ion source reported by Moak et al [7]. A detailed description of the electronic component and its performance has been reported in our previous paper [6]. The operating frequency of this design is quite stable, and the efficiency of the oscillator is found to be ~37%.…”

“…The technique adopted for measuring the RF power is a modified form of the photometric method which has been discussed in detail in our earlier work [6]. In this method, the RF power is converted into light.…”

“…In our earlier work, L.R. Hlondo et al, [6] we have constructed an RF oscillator and studied the oscillation conditions, frequency stability and variation of RF output power with plate voltage. Two different techniques were adopted for measuring the RF power, namely, the peak voltage method and the photometric method.…”