Multi-instability plasma dynamics during the route to fully developed turbulence in a helicon plasma

Abstract

We report experimental studies demonstrating a controlled transition to fully developed broadband turbulence in an argon helicon plasma in a linear plasma device. We show the detailed dynamics during the transition from nonlinearly coupled but distinct eigenmodes at low magnetic fields to fully developed broadband turbulence at larger magnetic fields. As the magnetic field (B) is increased from B ∼ 40 mT, initially we observe slow smooth changes in the dynamics of the system (to B ∼ 140 mT), followed by a sharp transition (within ∼10 mT) to centrally peaked narrow density profiles, strong edge potential gradients and a pronounced bright, well-defined plasma core. At low magnetic fields, the plasma is dominated by drift waves. As the magnetic field is increased, a strong potential gradient at the edge introduces an E × B shear-driven instability. At the transition, another mode with signatures of a rotation-induced Rayleigh–Taylor instability appears at the central plasma region. Concurrently we also find large axial velocities in the plasma core. For larger magnetic fields, all the instabilities co-exist, leading to rich plasma dynamics and fully developed broadband turbulence at B ∼ 240 mT.