Physics – Plasma Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sm42b05e&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SM42B-05
Physics
Plasma Physics
7800 Space Plasma Physics, 7807 Charged Particle Motion And Acceleration, 7831 Laboratory Studies, 7835 Magnetic Reconnection
Scientific paper
The question of which mechanisms can allow rapid changes of the magnetic field topology in collisionless plasmas is addressed in a laboratory experiment, based on the MIT Versatile Toroidal Facility (VTF). In this presentation we focus on mechanisms that are present at the level of single particle orbits, for cases in which the strength of the magnetic field perpendicular to the reconnection plane (the guide field) is comparable to, or smaller than that of the field on the reconnection plane (the cusp field). Similarly to space plasmas, the electron mean free path is much longer than the characteristic length of particle orbits, considered for the VTF case of the order of the dimensions of the plasma cross-section. Hence, particle orbits are expected to influence the macroscopic plasma dynamics during the development and sustainment of reconnection. We find analytically and demonstrate experimentally that reconnection driven by an electric field along the X--line can take place in a collisionless plasma, in a magnetic cusp with a relatively weak guide field without a macroscopic current layer, consistently with the effect of particle orbits. This study led to three main conclusions. First, because particles are mirror trapped, a reconnection electric field, Ez, does not provide acceleration of the particles in its direction parallel to the X--line. This allows the reconnection to proceed at the externally imposed rate, the same as in vacuum. Second, in the limit of massless particles, an electrostatic potential, Φ , is developed self-consistently by the plasma on the reconnection plane to avoid charge separation. This potential was calculated analytically and has been measured experimentally for the first time. Third, particles drift across the plane perpendicular to the X--line at the Ware--pinch velocity (Ez/Bxy). This drift speed is independent of the electrostatic potential Φ . This ``frozen in law'' at the single particle level is expected to break along the separatrix where different mechanisms can remove the steep gradients in Φ . High resolution measurements of the structure of the electrostatic potential are being performed to identify such mechanisms, and will be discussed at the conference. This work is partly funded by DoE Junior Faculty Development Award DE-FG02-00ER54601.
Egedal Jan
Fasoli A.
Migliazzo Joshua
Nazemi J.
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