MHD Instabilities and Their Effects on Plasma Confinement in Large Helical Device Plasmas with Intense Neutral Beam Injection

MHD stability of the Large Helical Device (LHD) plasmas produced with intenseneutral beam injection is experimentally studied. When the steep pressure gradient near the edge isproduced through L-H transition or linear density ramp experiment, interchange-like MHD modes whoserational surface is located very close to the last closed flux surface are strongly excited in acertain discharge condition and affect the plasma transport appreciably. In NBI-heated plasmasproduced at low toroidal field, various Alfven eigenmodes are often excited. Bursting toroidalAlfven egenmodes excited by the presence of energetic ions induce appreciable amount of energeticion loss, but also trigger the formation of internal and edge transport barriers.

Observation evidence for the entropy switch model of substorm onset

The cause of substorm onset is not yet understood. Chen CX(2016) proposed an entropy switch model, in which substorm onset results from the development of interchange instability. In this study, we sought observational evidence for this model by using Time History of Events and Macroscale Interactions during Substorms(THEMIS) data. We examined two events, one with and the other without a streamer before substorm onset. In contrast to the stable magnetosphere, where the total magnetic field strength is a decreasing function and entropy is an increasing function of the downtail distance, in both events the total magnetic field strength and entropy were reversed before substorm onset. After onset, the total magnetic field strength, entropy, and other plasma quantities fluctuated. In addition, a statistical study was performed. By confining the events with THEMIS satellites located in the downtail region between ~8 and ~12 Earth radii, and 3 hours before and after midnight, we found the occurrence rate of the total magnetic field strength reversal to be 69% and the occurrence rate of entropy reversal to be 77% of the total 205 events.

Ion acceleration at dipolarization fronts associated with the interchange instability in Earth's magnetotail

It has been confirmed that dipolarization fronts(DFs)are the result of the interchange instability in the Earth's magnetotail.In this paper,we use a Hall MHD model to simulate the evolution of the interchange instability that produces DFs along the leading edge.A test particle simulation is performed to study the physical phenomenon of ion acceleration at the DF.The numerical simulation indicates that almost all particles move earthward and dawnward and then drift to the tail.The DF-reflected ion population at the duskside appears earlier as a consequence of the asymmetric Hall electric field.Ions that are distributed in a dawn-dusk asymmetric semicircle behind the DF tend to be accelerated to higher energies(>13.5 keV).These high-energy particles eventually concentrate in the dawnside.Ions experience effective acceleration by the dawnward electric field,while they drift through the dawn flank at the front,toward the tail.