Acceleration of blob driven by helical instability in a simple magnetic configuration in QUEST

Abstract Blob generation mechanisms have been studied in slab plasma produced by electron cyclotron waves in a simple magnetic configuration with vertical (Bz) and toroidal fields (Bt). Two dimensional high speed images are analyzed as plasma and fluctuation evolution. Near the outer boundary (source region) of the slab plasma helix-sinusoidal perturbations are exited, whose helix angle and vertical wavelength are consistent with pitch angle and turn distance of the magnetic field lines there. Two dimensional evolution of these helix instabilities are studied as a function of Bz/Bt. Steep gradient outside the source region triggers the plasma to expand radially as a blob ejection. Along the ridge line of the maximum inverse scale length a wave front of the helix perturbation moves outward. Radial ejection of blob with typical velocity of ∼1 km/s is recognized as non-linear evolution of these helix instabilities. Acceleration of a blob is found in the source-free region.