Abstract
In the Earth’s magnetosphere, sunspots and magnetic cusp fusion devices, the boundary between the plasma and the magnetic field is marked by a diamagnetic current layer with a rapid change in plasma pressure and magnetic field strength. First principles numerical simulations were conducted to investigate this boundary layer with a spatial resolution beyond electron gyroradius while incorporating a global equilibrium structure. The boundary layer thickness is discovered to be on the order of electron gyroradius scale due to a self-consistent electric field suppressing ion gyromotion at the boundary. Formed at the scale of the electron gyroradius, the electric field plays a critical role in determining equilibrium structure and plasma transport. The discovery highlights the necessity to incorporate electron gyroradius scale physics in studies aimed at advancing our understanding of fusion devices, the magnetosphere and sunspots.
Highlights
In many plasma systems, the plasma is surrounded by magnetic fields leading to a fascinating array of natural and manmade phenomena
The fully kinetic first principles simulation resolving electron gyroradius scale length reported here led to the discovery of a localized and self-consistent electric field that plays a critical role in the boundary layer marked by a diamagnetic current between the plasma and surrounding magnetic fields. This electric field arises from the ion density or pressure gradient at the boundary and its main role is to limit charge separation between electrons and ions
Extending experimental and theoretical tools toward electron gyroradius scale phenomena will help to take full advantage of the recently launched MMS mission
Summary
The plasma is surrounded by magnetic fields leading to a fascinating array of natural and manmade phenomena. Among examples of plasma diamagnetic effects are the magnetopause in the Earth’s magnetosphere, sharp boundary layers in magnetic cusp fusion devices, and the dark patches of sunspots (Hale, 1908; Chapman and Ferraro, 1930; Berkowitz et al, 1958; Braginskii and Kadomtsev, 1959; Sonnerup and Cahill, 1967; Spalding, 1971; Borrero and Ichimoto, 2011). In these systems, a diamagnetic current layer marks the boundary across which plasma penetration or loss to the magnetic field region is greatly reduced. The diamagnetic effect in these systems has been studied extensively, leading to the development of magnetohydrodynamics (MHD), the standard model for many solar, astrophysics and fusion plasmas over the past 50 years (Alfvén, 1942; Cowling, 1957)
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