Influence of the excitation frequency on the RF power transfer efficiency of low pressure hydrogen ICPs

D Rauner,U Fantz,S Briefi

doi:10.1088/1361-6595/ab3d6a

Abstract

The influence of the excitation frequency on the RF power transfer of inductively heated hydrogen plasmas is investigated in the pressure range between 0.3 and 10 Pa. The experiments are conducted at a cylindrical ICP at frequencies in the range between 1 and 4 MHz and RF powers up to 1 kW. By applying a subtractive method which quantifies the transmission losses within the plasma coil and the RF network, the RF power transfer efficiency is determined. The key plasma parameters of the discharges are measured via optical emission spectroscopy and a double probe. By increasing the frequency from 1 to 4 MHz at a moderate RF power of 520 W, a significant enhancement of the RF power transfer efficiency is observed. It is most prominent at the presently considered low and high pressure limits and allows to reach high efficiencies of up to 95% at pressures between 3 and 5 Pa. While the AC loss resistance of the coil and the RF circuit only displays a relatively weak variation with the applied frequency due to the skin effect, the observed increase of the power transfer efficiency at higher frequencies is dominated by a considerable enhancement of the plasma equivalent resistance. This increased capability of the plasma to absorb the provided power is discussed against the background of collisional and collisionless heating of electrons. Thereby it is demonstrated that the observed behaviour can most likely be attributed to a decreasing difference between the angular excitation frequency and the effective electron collision frequencies. If the RF power is increased however, the RF power transfer efficiency increases globally while frequency induced differences tend to get less pronounced, as the plasma is generally capable of absorbing most of the provided power due to an increasing electron density.

Highlights

In analogy to the finite resistance of the components of the RF circuit leading to these power losses, the concept of the plasma equivalent resistance was introduced and is since commonly applied as a measure to quantify the capability of the plasma to absorb the provided RF power
In [18], the applicability of these simple approximations to describe the heating mechanisms in low pressure hydrogen Inductively coupled plasmas (ICPs) has been discussed. It was demonstrated at an excitation frequency of 1 MHz that a description of the power absorption according to equations (3) and (4) considering the introduced expressions for the electron collision frequencies yields a conclusive description of the relative dependences of η on the pressure and the RF
At a constant gas flow of 5 sccm, hydrogen discharges are generated in the pressure range between 0.3 and 10 Pa—which corresponds to the typical regime where the transition between local and nonlocal electron heating takes place in hydrogen ICPs [18]

Summary

Introduction

Coupled plasmas (ICPs) are one of the most thoroughly investigated radio frequency driven low pressure discharges due to their wide range of scientific and industrial application, reaching from processing plasmas to ion sources. The importance of ICPs operating light molecular gases has grown over the past years Their field of application ranges from hydrogen plasmas for material processing generated at moderate RF powers of a few hundred Watts [14, 15] to the high power regime (several tens of kW) required by RF driven ion sources for particle accelerators [16] and the neutral beam heating systems for fusion [17]—which are required to operate in deuterium as well. Up to date no comparable systematic investigations in light molecular gases like hydrogen or deuterium are reported and a direct transfer of the results obtained in noble gases is not straightforward since the typically achieved plasma parameters and electron collision probabilities which determine the plasma heating and power transfer mechanism generally depend on the gas type. The effect of a changed frequency on the crucial plasma parameters such as the electron density and temperature is measured, which allows for a discussion of the heating processes considered relevant for the observed behaviour of the RF power transfer efficiency

RF power transfer and heating mechanisms of ICPs

Experimental setup and diagnostics

RF power transfer at different exciation frequencies and varying pressure

Plasma parameters

Heating regime and discussion

Conclusion