RF technologies for klystron based lower hybrid current drive, (LHCD) system at IPR

At IPR, two tokamaks, namely ADITYA-U and SST1 tokamaks are operational. The plasma current in tokamaks is initially driven inductively with the help of central solenoid Ohmic coils. However, the pulsed nature of operation and limited stored energy in the Ohmic coil prevents inductive plasma current drive beyond a limit and therefore to drive plasma current beyond this limit, lower hybrid current drive (LHCD) system is employed to drive plasma current non-inductively using high power electromagnetic waves. The LHCD system for both the tokamaks (2MW-CW system for SST1 and 250kW^-1sec for ADITYA-U) at 3.7 GHz is indigenously designed, developed, commissioned and is regularly operated to launch lower hybrid waves in to the tokamaks to drive plasma current non-inductively. The high power RF source for the LHCD system comprises of four klystrons, each rated for 500kW-CW RF power. Each klystron operates at -65kV/20A (1.3MW DC input power) and delivers 500kW rf power with a typical efficiency of ∼40%. Crowbar system protects the klystron in an event of arc. Klystrons are protected from high reflection with high power four port circulators. A VME/PXI based data acquisition and control (DAC) system is used to operate klystrons with other augmented sub-systems like auxiliary power supplies, HVDC PS, cooling system, fast/slow protection system, etc. in controlled manner in standalone mode or when interfaced with tokamaks control system. The SST1 LHCD system is very huge and complex network of standard (WR284) and non-standard rf transmission line and transmission line components like magic tees, folded magic tees, directional couplers, phase shifters, E/H bends, DC break, etc. and feeds the LH launcher, namely 'grill', which is placed inside tokamak UHV environment through a non-standard rectangular multi-channel rf vacuum window. The ADITYA-U LHCD system is relatively simpler as it employs a recently developed passive active multijunction (PAM) launcher. (author)