RF power transfer efficiency and plasma parameters of low pressure high power ICPs

D Zielke,U Fantz,S Briefi

doi:10.1088/1361-6463/abd8ee

D Zielke, U Fantz + Show 1 more

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https://doi.org/10.1088/1361-6463/abd8ee

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Abstract

Inductively coupled radio frequency (RF) ion sources operating at 1 MHz under the condition of a low gas pressure of 0.3 Pa are the basis of negative hydrogen/deuterium ionbased neutral beam injection systems of future fusion devices. The applied high RF powers of up to 75 kW impose considerable strain on the RF system and so the RF power transfer efficiency η becomes a crucial measure of the ion source’s reliability. η depends on external parameters such as geometry, RF frequency, power, gas pressure and hydrogen isotope. Hence, η along with the plasma parameters are investigated experimentally at the ITER prototype RF ion source. At only 45%–65% in hydrogen and an increase of around 5% in deuterium, η is found to be surprisingly low in this ion source. The power that is not coupled to the plasma is lost by Joule heating of the RF coil (∼26%) and due to eddy currents in the internal Faraday screen (∼74%). The matching transformer adds up to 8 kW of losses to the system. The low values of η and the high share of the losses in the Faraday screen and the transformer strongly suggest optimization opportunities. At high power densities well above 5 W cm−3, indications for neutral depletion as well as for the ponderomotive effect are found in the pressure and power trends of η and the plasma parameters. The comprehensive data set may serve for comparison with other RF ion sources and more standard inductively coupled plasma setups as well as for validating models to optimize RF coupling.

Highlights

The applied high radio frequency (RF) powers of up to 75 kW impose considerable strain on the RF system and so the RF power transfer efficiency η becomes a crucial measure of the ion source’s reliability. η depends on external parameters such as geometry, RF frequency, power, gas pressure and hydrogen isotope
The RF power transfer efficiency η has been measured at an NNBI ion source under the conditions of low gas pressures of around 0.3 Pa, a low RF frequency of 1 MHz and high RF generator powers of up to 75 kW
The losses are distributed between the Faraday screen and the RF coil, where the former absorbs 74% of the losses through Joule heating by eddy currents and the latter 26% of the losses by direct Joule heating

Summary

Introduction

An example at a low radio frequency (RF) of 1 MHz, a high generator power of up to 75 kW and a low gas pressure between 0.2 and 0.6 Pa with a focus on 0.3 Pa is the prototype ion source for ITER at the BATMAN Upgrade (BUG) testbed [4, 5]. In this ion source, which is operated in pulsed mode with a maximal pulse length of around 5 s, plasma is generated in a compact, half-closed cylindrical vessel of around 8 l—called the driver—where high electron temperatures around 10 eV and plasma densities around 1018 m−3 are reached at a gas pressure of 0.3 Pa. The driver’s axial length is around 17 cm and its inner radius is around 12 cm, i.e. its low aspect ratio L/R is around 1.4. For further information about the expansion region (inclusive magnetic filter, bias plate and plasma grid), the modular design of NNBI ion sources, H− production and beam formation, see [4, 5, 7]

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