Asian Journal of Physics Vol 32, Nos 9 – 12 (2023) 583-591

Development of liquid stub tuner and liquid phase shifter for antenna-plasma impedance matching for high power RF experiments

Raj Singha, Vishant Gahlautb, Joydeep Ghosha, Varuna, Mumtaz A Ansaria, Amardas Allia and P K Atreya
aInstitute for Plasma Research, P. O. Bhat, Distt. Gandhinagar-382 428, India
bDepartment of Physical Sciences, Banasthali Vidyapeeth, Banasthali- 304 022, India
Dedicated to Prof B N Basu

Tokamak plasma discharges using radio frequency (RF) power are being very extensively used for plasma pre-ionisation, breakdown, heating and current drive and wall conditioning. For RF heating of plasma, impedance matching plays an important role. Impedance of plasma consists of resistive as well as inductive part. Resistance varies in the range of 0.5 to 5 Ohm, while susceptance part varies from 10 to 200 Ohm. The impedance of RF generator is normally 50 Ohm. This necessitates the matching of plasma impedance with the source impedance for maximum power transfer. The conventional matching is done using conventional stub tuners and conventional phase shifters. At high RF power to the plasma where current is of the order of kA for Ion Cyclotron Resonance (ICR) range of frequencies, conventional stub tuner that have an electric short called plunger, moving between inner and outer conductor of coaxial rigid transmission line develops an arcing. In a similar way as in case of stub tuner, the phase shifter also has finger contacts between the moving inner and outer conductors. Theses finger contacts have spring action in order to make movement feasible between inner and outer conductors. Due to inherent ovality in copper tubes, used for inner and outer conductors, the contact between inner and outer conductors becomes non-uniform and hence many a time gets spark developed for high value of RF current flowing through these finger contacts between inner and outer conductors. In order to avoid these sparks, liquid stub tuner (LST) and liquid phase shifters (LPS) are developed where there is no metallic moving parts. In LST and LPS only liquid moves. As the dielectric constant (∈r) of liquid is different from the air, hence moving liquid changes the wavelength of RF wave being passed in LST and LPS. This in turn changes relative length (l/λ), where l is length and λ is wavelength of the medium in which RF wave is passing. The medium may be air or the liquid being used. This concept is used in LST and LPS for impedance matching at high power ICR frequencies. Here, in this paper we present design and simulation results of LST and LPS. © Anita Publications. All rights reserved.
Keywords: Liquid stub tuner, liquid phase shifter, plasma, impedance matching.

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