Toroidal plasma current startup and sustainment by lower hybrid waves in the WT-3 tokamak

In WT-3, lower hybrid waves are injected into a microwave discharge at the electron cyclotron resonance frequency to initiate the toroidal plasma current I_P and to sustain it by radio frequency (RF) power alone without Ohmic heating power (this is called the ‘RF tokamak’). First, fast electrons in the range of several kilo-electron volts are generated by electron cyclotron and lower hybrid power which initiate a weak toroidal plasma current. Then, fast electrons above 100 keV are produced by the lower hybrid waves, and I_P increases rapidly (ΔI_p/Δt ∼ 780 kA·s−1) to almost 13 kA. Subsequently the toroidal plasma current is ramped up slowly (ΔI_p/Δt ∼ 180 kA·s−1) to > 25 kA for low densities (̅_e 6 × 1018 m−3). The fraction ∈_R (≤8%) of injected lower hybrid power is converted into poloidal magnetic field energy. For high densities (̅_e = (6−12) × 1018 m−3), a quasi-steady-state discharge is obtained with I_P = 7−20 kA, and the figure of merit η_CD is about 0.05 × 10₁₉ A·m−2·W−1. The experimental values of ∈_R and η_CD are explained by the direct loss of fast electrons.