A technique using radio-frequency waves in the lower hybrid frequency range to drive toroidal current in a tokamak non-inductively — essential for extending pulse length toward true steady-state operation.
Lower hybrid current drive (LHCD) uses electromagnetic waves at frequencies between the ion and electron cyclotron frequencies — typically 1–8 GHz for modern tokamaks — to drive toroidal plasma current without relying on the central solenoid's transformer action. The injected waves interact with electrons traveling in one direction along the magnetic field, accelerating them and creating an asymmetric electron velocity distribution that produces a net toroidal current. This non-inductive current drive is a critical tool for sustaining tokamak plasmas beyond the inherent pulse-length limit set by the ohmic transformer.[1]
The lower hybrid frequency is defined as the geometric mean of the ion and electron cyclotron frequencies (modified by density effects), falling in the microwave range for typical tokamak parameters. Waves launched at this frequency propagate into the plasma as "slow waves" with their electric field component parallel to the magnetic field. Through Landau damping, these waves preferentially transfer momentum to electrons whose parallel velocity matches the wave's phase velocity. Because the waves are launched with a directed spectrum (asymmetric in the toroidal direction), the momentum transfer produces a net current.[2]
LH waves are coupled into the plasma using multijunction "grill" launchers — phased arrays of rectangular waveguides mounted flush with the vacuum vessel wall. The relative phasing between adjacent waveguides determines the launched wave spectrum and hence the parallel refractive index (N||), which controls where in velocity space the wave deposits its momentum. Modern designs such as the passive-active multijunction (PAM) launcher achieve excellent coupling over a wide range of edge plasma conditions.[1]
Current profile control: LHCD deposits current predominantly in the outer half of the plasma (r/a > 0.5–0.7), complementing bootstrap current and other current-drive sources that act closer to the core. This off-axis deposition is valuable for shaping the safety-factor profile and accessing advanced tokamak scenarios with internal transport barriers.
Limitations: At reactor-relevant temperatures and densities, lower hybrid waves may be absorbed before reaching their intended radial location due to parasitic damping on fast electrons or to spectral broadening effects. These issues have led some reactor designs — including ITER — to defer or downscope LHCD in favor of ECCD for current drive, though LHCD remains under active study for DEMO and other future devices.[2]