Physics – Plasma Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sm22a06f&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SM22A-06 INVITED
Physics
Plasma Physics
7800 Space Plasma Physics, 7807 Charged Particle Motion And Acceleration, 7831 Laboratory Studies, 7835 Magnetic Reconnection
Scientific paper
A new experiment has been developed on the MIT Versatile Toroidal Facility to investigate the plasma dynamics around a magnetic X-point and driven magnetic reconnection. The plasma is created by electron cyclotron resonance heating using a 50 kW microwave source at 2.45 GHz and confined in a toroidal magnetic cusp generated by external coils, forming a magnetic X--line at the center of the device. A toroidal magnetic field can be added to the cusp field to preserve the adiabaticity of the magnetic moment throughout the poloidal cross section and to explore the influence of the guide field on reconnection dynamics. Argon plasmas are created by ECRH with densities and temperatures in the range n~=1ṡ1017 m-3 and Te~=20 eV, corresponding to Lundquist numbers of the order of 500-1000. Collisional mean free paths are much larger than the dimensions of the reconnection layer. Reconnection is driven by the ExB drift resulting from the combination of the poloidal magnetic cusp field and the toroidal electric field induced by an ohmic transformer. The response of the plasma to the reconnection drive is measured over the whole poloidal cross-section in terms of the time evolution of plasma density, flows, poloidal magnetic flux and currents. Flows are obtained using multiple Mach probes and by measuring the time evolution of the density profile while the ECRH plasma production is pulsed. A multiple magnetic probe gives the poloidal flux evolution. The current distribution is reconstructed from the magnetic signals using a singular value decomposition method. Fast collisionless reconnection is observed for the first time in a controlled laboratory plasma. Despite a large toroidal loop voltage, a macroscopic current layer is detected only in the presence of very strong guide field. The role of single particle dynamics in driven magnetic reconnection is investigated experimentally and analytically. The trapping of particle orbits in the magnetic cusp is observed to allow fast reconnection in the absence of a macroscopic current layer, at a rate identical to that of vacuum, for relatively small ratios of guide to cusp field. The development of an electrostatic potential structure around the magnetic X--line during reconnection is predicted theoretically and observed experimentally. In the regime exhibiting a current layer, characterized by a strong guide to cusp field ratio, the sensitive dependence of the effective resistivity obtained from direct measurements of current and electric field upon such ratio is suggestive of particle orbit effects that produce an effective inertial resistivity. The dependence of the current layer characteristics upon plasma and magnetic field parameters will be reconstructed experimentally and used to clarify the origin of the observed anomalous resistivity. Experiments aimed at measuring electrostatic and magnetic fluctuation spectra and the ion response to the reconnection process using laser induced fluorescence will be discussed. This work is partly funded by DoE Junior Faculty Development Award DE-FG02-00ER54601.
Egedal Jan
Fasoli A. F.
Migliazzo Joshua
Nazemi J.
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