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

Abstract

In WT-3, lower hybrid waves are injected into a microwave discharge at the electron cyclotron resonance frequency to initiate the toroidal plasma current IP 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 IP increases rapidly (ΔIp/Δt ∼ 780 kA·s−1) to almost 13 kA. Subsequently the toroidal plasma current is ramped up slowly (ΔIp/Δt ∼ 180 kA·s−1) to > 25 kA for low densities (n̄e < 6 × 1018 m−3). The fraction ∈R (≤8%) of injected lower hybrid power is converted into poloidal magnetic field energy. For high densities (n̄e = (6−12) × 1018 m−3), a quasi-steady-state discharge is obtained with IP = 7−20 kA, and the figure of merit ηCD is about 0.05 × 1019 A·m−2·W−1. The experimental values of ∈R and ηCD are explained by the direct loss of fast electrons.