Non-inductive current drive at zero loop voltage using LHCD PAM launcher on ADITYA-U

For the first time, the ADITYA-U tokamak has sustained a plasma current of ~40 kA purely through lower hybrid waves (LHWs) using a passive active multijunction (PAM) launcher, without any loop voltage. While inductive discharges are limited to ~150 ms by the available loop voltage, injection of LHWs at 80 ms extended the plasma pulse to 320 ms at a peak parallel refractive index of −2.5. Efficient coupling was achieved, with the reflection coefficient remaining below 5% in agreement with ALOHA predictions, and a figure of merit of ~0.2 × 1019 A/m²W was obtained during the non-inductive phase. Suprathermal electron generation, evidenced by bremsstrahlung emission and hard X-ray spectra (20–200 keV) measured with a CdTe detector, confirmed effective power transfer from LHWs. These first LHCD results with a PAM launcher on ADITYA-U demonstrate reliable wave–plasma coupling, extended discharge duration, and non-inductive current drive capability.