Development of a novel spiral antenna system for low loop voltage current start up atSteady State Superconducting Tokamak(SST-1).

In general, superconducting tokamaks require low loop voltage current start up for the safety purpose of its poloidal field coils. The loop voltage inside the vacuum vessel of Steady-state Superconducting Tokamak (SST-1) is low in nature since its central solenoid is located outside the cryostat. The low loop voltage current start up of the SST-1 is routinely performed by Electron Cyclotron Resonance (ECR) method at the toroidal magnetic field Bt=1.5T(first harmonic) and 0.75T(second harmonic). Recently, an alternative RF based plasma current start up system had been planned for operating the machine specially for higher toroidal magnetic field regime 1.5T ≤ Bt ≤3T. The system is already developed based on an antenna system, made of series combinations of two at spiral antenna, to assist plasma current start up at lower inductive electric field. It is already tested and installed in SST-1 chamber. The system testing had been performed without background magnetic field within frequency regime 35-60MHz at present. The test results show that it can produce electron density ne ≈1016m-3 measured by the Langmuir probe in expense of 500W RF power. The spectroscopy results indicate that its capability to produce plasma density higher than 1013 m-3 and electron temperature Te = 2 -6eV. In addition, it also shows that the presence of turbulent electric field of the order of 106V/m at antenna center and finite anomalous temperature of neutral particles. Calculations show that the obtained density is enough for SST-1 low loop voltage plasma breakdown. The antenna system is also capable to produce plasma at higher frequencies. This article will discuss the development of the prototype and the installed antenna system along with their test results in detail.