1. ECRH assisted breakdown and heating experiments of SST-1 Tokamak

ECRH is a well-established auxiliary heating technique in which high-frequency microwave power - generated by a gyrotron is injected into a magnetized plasma and it is absorbed when the microwave frequency matches with the electron cyclotron frequency or its harmonics. This resonance condition causes electrons to gain perpendicular energy in a collisionless manner and efficiently heat the plasma.

The  42 GHz ECRH system has given menay interesting results on both the tokamaks SST-1 and Aditya-U. In SST-1, the need of ECRH is critical as the loop voltage of SST-1 is only ~3.5 V which is not enough to breakdown and plasma start-up. Hence ECRH-assisted start-up is mandatory for reliable plasma discharges at both fundamental harmonic (at 1.5 T) and second harmonic operation (at 0.75 T). The microwave power propagates through corrugated waveguides in HE₁₁ mode and is steered into the plasma by an electromechanical launcher system with steerable mirrors capable of movement across both poloidal and toroidal directions. Thus in SST-1, all the plasma shots are with ECRH assisted start-up, standard plasma discharges results are shown in following figure.

The Aditya-U tokamak which is an upgrade of the original Aditya tokamak with the replacement of rectangular vacuum vessel to circular one to accommodate the divertor coils for advance plasma experiments.  In Aditya-U, the normal loop voltage is around 22 V, but with ECRH assistance, successful plasma start-up was achieved at loop voltages as low as 6V and this saving in loop voltage helps to extend the plasma duration. In the Aditya-U, various ECH experiments at fundamental O-mode and second harmonic X-mode have been carried over a wide range of magnetic field 0.75T to 1.4T. In Aditya-U for breakdown, ECRH power was launched approximately 25 ms before the start of loop voltage, achieving a 40% reduction in peak loop voltage, with plasma currents of ~115 kA and discharge durations of ~250 ms demonstrated at ~12 V loop voltage. On Aditya-U, plasma heating has also been observed through an increase in soft X-ray (SXR) signal with ECRH power, with the bremsstrahlung-based SXR radiation indicating a rise in electron temperature in the fully ionized plasma. Notably, ECR heating has been demonstrated in deuterium plasma also, where a soft X-ray signal increase with ECRH power indicates effective EC wave coupling with the deuterium plasma.

Two pulse operation of ECRH system on SST-1 and Aditya- U tokamaks

The ECRH two-pulse experiments on tokamaks SST-1 and Aditya-U were enabled by upgrading the 42-GHz gyrotron system with a new advance anode modulator power supply with fast rise and fall time of 1 ms. The duration and ECRH power can be varied in both the pulses as per the requirement. The first pulse, kept below 150 kW, is used for plasma breakdown at the fundamental harmonic, while the second pulse with higher power ~200 kW is used for plasma heating. This separation is deliberate limiting power during breakdown minimizes the generation of high-energy electrons that can introduce impurities and degrade plasma quality.

In SST-1, where the loop voltage is below 5 V, ECRH-assisted start-up is mandatory for reliable plasma discharges; the first pulse triggers breakdown confirmed by immediate appearance of the Hα signal, and the second pulse of ~200 kW is launched after a 30 ms gap, spanning the current ramp-up to flat-top phase. While some increase in the electron cyclotron emission (ECE) signal was observed in SST-1 during the second pulse, conclusive evidence of heating remained elusive as the increase could partly be attributed to ohmic heating.

In Aditya-U, where the loop voltage (~20 V) is sufficient for breakdown, the two-pulse scheme offers the advantage of reduced volt-second consumption alongside simultaneous heating. The first pulse of 100 kW for 70 ms initiates breakdown, followed by a 150 kW second pulse for 50 ms; a clear increase in both SXR and ECE signals during the second pulse confirmed ECR heating in Aditya-U, making it the more conclusive demonstration of heating between the two tokamaks. Systematic follow-up experiments varying magnetic field, plasma density and synergy with ion cyclotron resonance heating ICRH and lower hybrid current drive (LHCD) are planned to further characterize the heating effects.