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 radiofrequency (RF) power alone without Ohmic heating power (this is called the 'RF tokamak'). First, fast electrons in the range of several kilo-electronvolts 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 × 10 18 m −3 ). The fraction ∈ R (≤8%) of injected lower hybrid power is converted into poloidal magnetic field energy. For high densities ( e = (6−12) × 10 18 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 × 10 19 A·m −2 ·W −1 . The experimental values of ∈ R and η CD are explained by the direct loss of fast electrons.