Investigation of ECR formed Plasma launched from LFS using O and X mode in STARMA device

In the present study, we explore the production and behavior of X-mode plasma in the STARMA[1] (acronym for simple tight aspect ratio machine assembly) machine, when it is launched from low field side (LFS). The STARMA device is a small aspect ratio device with cm and cm with effective aspect ratio (A) ~1.5. A 2.45GHz, 6kW CW magnetron source is employed to produce plasma using ECR techniques. To form ECR plasma at fundamental mode for the 2.45GHz source, a magnetic field of 875G is required, which is realized using six toroidal field (TF) coils. The device provides a novel configuration in which fundamental harmonic (875 G) and second harmonic (437 G) ECR layer exists simultaneously. By varying the strength of the toroidal magnetic field, at the minor axis, by adjusting coil currents using super-capacitor [2] based power supply, the location of the ECR layer is varied radially and hence the plasma production location is controlled, thereby allowing us to tailor the radial density profiles of the plasma. The plasma is formed using the helium gas at the working pressure of mbar. The diagnostics employed in these studies mainly comprises of Langmuir probes (LP) to measure temporal and radial profiles of density and temperature. A horn antenna and Zero-bias Schottky diode detector are used for RF power measurements. The single LP is used to measure I-V characteristics of the plasma at a fixed radial location, using which electron density and temperature of the plasma is estimated. Triple Langmuir probe is also used to measure the temporal evolution of electron density and temperature at a given radial location. The probes are moved radially to measure the radial profile of the plasma parameter from shot to shot.

By varying the launching angles of the waves, we introduce a finite parallel refractive index for X-mode, allowing us to examine their distinct effects on the plasma behavior and its dynamics on plasma characteristics. Our measurements suggests that the X-mode launch from the LFS, in the presence of plasma, interacts with the right-hand (RH) cutoff, leading to reflection. Although, an evanescent region exists, between the upper hybrid resonance (UHR) and RH cutoff, the X-mode wave seems to tunnel through it, due to its wavelength being larger than the evanescent region. The radial profile at the equatorial plane suggests that plasma is formed with peak density 3x10¹⁶ m^-3 and electron temperature (Te ~ 25 eV) . The Budden parameter is calculated using the experimental density scale length (Ln) and toroidal magnetic field scale length (Lb) and based on this obtained Budden parameter, the wave tunneling effect responsible for the radial density profile is investigated. The temporal profiles of the X-mode produced plasma reveal significant fluctuations in reflected power and is attributed to the u-wave coupling effects manifested through plasma density fluctuations.