ECRH assisted plasma experiments on tokamaks SST-1 and Aditya

The 42GHz Electron Cyclotron resonance Heating (ECRH) system is used in tokamaks Aditya and SST-1 to carry out ECRH related experiments at fundamental and second harmonic. The Gyrotron capable to deliver 500kW power for 500ms duration is commissioned in SST-1 tokamak hall and ~75m long transmission line is laid to connect both the tokamaks. The corrugated waveguide (φ63.5mm) based transmission line at normal atmospheric pressure is used to transmit power in HE11 mode. The special feature of this line is that it consists of two waveguide switches, which facilitates to transmit the power in either tokamak without changing any hardware. The total loss in the line is less than 1.1dB. In SST-1, the 42GHz ECRH system is used for tokamak start-up at fundamental (1.5T) and second harmonic (0.75T). Since the loop voltage of SST-1 is low (~3.5V), the ECRH assisted start-up is mandatory for reliable plasma discharges. Since the first pass absorption of ECRH is not 100% and in this case the reflected beam is used to drive the plasma current from high field side. In order to achieve this, a profiled reflector is installed on the inboard side wall of the tokamak. This reflector is adjusted such that ECRH beam is reflected to ~17°, which helps in driving the ECCD. The ECH power (~180kW to 300kW) is launched and pulse duration is varied from 100ms to 400ms. The systematic experiments are carried out to monitor the effect of ECCD in SST-1 plasma at fundamental and second harmonic. It is observed that IECCD contribution is ~ 5 to 10kA, which is a significant contribution as it supports the plasma current (I _P ) and long pulse shot with higher plasma current are achieved with longer ECRH pulse. In Aditya, the second harmonic ECRH assisted low loopvoltage plasma startup experiments have been carried out systematically. The ECRH power in extraordinary (X ₂ ) mode is launched from low field side of tokamak. The ECH power is launched in tokamak around 25ms before the loop_voltage and pulse duration is varied from 75ms to 125ms. The normal operating loop voltage of Aditya is ~22V and with ECRH assisted breakdown, the normal discharges (Ip~100kA, plasma duration ~150ms) are achieved at loop voltage as low as 7V, which shows significant (~70%) reduction in the loop_voltage.