Whistler mode flux ropes in a laboratory plasma

Summary form only given. A new nonlinear wave structure and its excitation mechanism are described. The wave consists of a whistler mode packet whose magnetic field configuration resembles that of an elongated spheromak or short flux rope. The wave packet is excited by drawing a short-duration pulse of electrons (I≃150 A, ΔT ≃0.1 μs) from a plane electrode (5-cm diam.) immersed in a large (1-m diam., 2-m length), uniform, quiescent afterglow plasma (n_e≃5 × 10¹¹ cm^-3, kT_e≃2 eV) with a uniform axial DC magnetic field (B_oz≃10 G). For essentially unmagnetized ions the field-aligned current penetrates into the plasma at the group velocity of whistlers, inducing antiparallel return currents for current closure. Due to the short duration of the applied pulse, the propagating plasma current detaches from the electrode and closes itself through cross-field polarization currents. The current vortex propagates free from boundaries along B_oz through the plasma. Its magnetic topology consists of helical, nested field lines (flux rope) whose projection in the r-z plane forms an elongated Hill vortex. The three-dimensional wave fields B_(x,y,z,t) were measured with magnetic probes throughout the plasma volume versus time from highly repeatable discharges