Initial lower hybrid current drive experiments with a passive active multijunction launcher in the ADITYA-U tokamak

Following the successful integration of a 3.7 GHz, 250 kW-1s passive active multijunction (PAM) launcher in the ADITYA-U tokamak, lower hybrid (LH) current drive experiments were conducted for the first time. The launcher was designed to launch lower hybrid waves (LHWs) with a parallel refractive index (N||) centered at −2.25 when adjacent modules were phased at 180°. Experiments explored phase variations from 30° to 270° with a toroidal magnetic field of up to 1.4 T. A maximum of ∼120 kW LH power was launched into the plasma, while achieving excellent coupling efficiency, with a reflection coefficient below 5% even when the launcher was positioned 20–30 mm behind the limiter. In a positive converter configuration, the PAM launcher partially sustained plasma currents, to extend the plasma pulse length up to ∼220 ms, demonstrating significant non-inductive current drive with zero loop voltage toward the end of the shot. To extend the duration of ohmic discharge, a negative converter was added, boosting the loop voltage to ∼330 ms and enabling the plasma current to be sustained for ∼430 ms. Enhanced photon counts from a CdTe detector and enhancement in the second-harmonic electron cyclotron emission signal confirmed the generation of suprathermal electrons due to LHW–plasma interactions, driving the plasma current non-inductively. However, sustainment of the plasma current was limited due to the counter-current drive direction of the launched waves. Future experiments will explore reversal of the wave direction to improve current sustainment.