lower hybrid current drive is driving 📝plasma current with radio waves at the lower hybrid frequency.
The lower hybrid frequency is a natural resonance of magnetized 📝plasma that sits in the gigahertz range, between the gyration frequencies of ions and electrons. Waves launched there from a phased waveguide array — a "grill" antenna mounted at the plasma edge — travel in one toroidal direction and are absorbed by electrons moving with them, pushing that population forward like surfers caught by a wave. The asymmetric push is a net electric current, sustained without any transformer action from the 📝central solenoid.
Among the techniques of non-inductive 📝current drive, lower hybrid holds the record for efficiency — amperes driven per watt spent — which made it the workhorse of long-pulse experiments: Tore Supra in France and EAST in China have sustained discharges for minutes on lower hybrid power. Its limitation is accessibility. The waves are absorbed toward the cooler outer region of the plasma and struggle to penetrate the dense, hot core of reactor-grade machines, so lower hybrid is best suited to driving off-axis current that shapes the current profile, while other methods and 📝bootstrap current carry the core.