Lower hybrid instability driven by mono-energy α-particles with finite pitch angle spread in a plasma

Abstract

A kinetic formalism of lower hybrid wave instability, driven by mono-energy α-particles with finite pitch angle spread, is developed. The instability arises through cyclotron resonance interaction with high cyclotron harmonics of α-particles. The α-particles produced in D-T fusion reactions have huge Larmor radii (∼10 cm) as compared to the wavelength of the lower hybrid wave, whereas their speed is an order of magnitude smaller than the speed of light in vacuum. As a result, large parallel phase velocity lower hybrid waves, suitable for current drive in tokamak, are driven unstable via coupling to high cyclotron harmonics. The growth rate decreases with increase in pitch angle spread of the beam. At typical electron density of ∼1019 m−3, magnetic field ∼4 Tesla and α-particle concentration ∼0.1%, the large parallel phase velocity lower hybrid wave grows on the time scale of 20 ion cyclotron periods. The growth rate decreases with plasma density.