Plasma initiation in mid-sized tokamaks, especially superconducting machines, requires a pre-ionization source having an electron density on the order of 1017–1018 m−3 that typically operates in the pressure range between 10−5 and 10−4 mbar. A spiral antenna excited by 13.56 MHz can provide these parameters as demonstrated in the APPEL device (Applied Plasma Physics Experiments in Linear device), which has an axisymmetric magnetic field over 4.0 m. A 3.5 m argon plasma column having a peak plasma density of approximately 1017 m−3 was formed corresponding to a maximum magnetic field strength of B = 0.41 tesla at 500 W radio-frequency (RF) power under an operating pressure range of 5 × 10−3–2.0 × 10−4 mbar. The maximum plasma density was observed at an axial position z = 1.0 m from the antenna, where the magnetic field was 0.25 T; a radial shift in the density peak was noted with decreasing pressure, becoming prominent near 3.5 × 10−4 mbar. Although the minimum pressure achieved remains slightly above typical tokamak startup conditions, this study provides valuable insight into RF plasma coupling in magnetized linear devices. A COMSOL Multiphysics simulation of the experimental setup shows qualitatively agreement with measured density profiles since, simulations are ideal conditions while experiments are real condition so the parameters are not exactly match but found similar trends. A qualitative explanation has been provided for the plasma produced by the spiral antenna in the axisymmetric magnetic field of the linear device.
This paper investigates the use of a spiral antenna to generate a pre-ionized plasma for tokamak startup. The experiments were conducted in the APPEL device, which has an axisymmetric magnetic field, and achieved a plasma density of 10^17 m^-3 at 0.25 T. The study provides insights into RF plasma coupling in magnetized linear devices and can help improve pre-ionization techniques for tokamaks.