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(a) The arrows represent a snap shot in time of 480 magnetic field measurements obtained on the basis of 24 highly reproducible plasma pulses. The magnetic field lines were inferred from the measurements. (b) Vacuum magnetic flux surfaces of the internal coils. (c) Resulting magnetic geometry and directions of the ExB-drift, when applying the reconnection drive.

MIT's Versatile Toroidal Facility

A new closed configuration in the VTF has been developed for our future study of collisionless magnetic reconnection. This configuration is realized by installing two internal loops made of copper tubing. The loops are supported in four locations by thin stainless steel wires. The loops are not connected electrically to power supplies; instead, their currents are induced inductively.

Plasmas have been produced with various values of the guide magnetic field. Pre-ionization with the RF system is required for breakdown during the reconnection drive, but the RF resonance condition does not have to be fulfilled. In the figure above, magnetic field measurements are shown for the closed cusp configuration. The plasma in the closed configuration is highly reproducible, and the magnetic profiles are accurately obtained by measurements from several discharges.

In order to observe fast reconnection in the closed configuration with a guide magnetic field, we have developed a drive scenario, which uses an additional internal coil set. By transferring the current from the inner loops to the outer loops a motion of the plasma will be generated horizontally toward and vertically away from the X-line. This drive is compatible with an ideal motion of the plasma where each flux surfaces will encircle a magnetic flux constant in time. Hence, for this motion the plasma frozen in condition need only be violated in a narrow region around the X-line. The reconnection drive is compatible with a reconnection rate up to 100 V/m.

New! The first tests of fast reconnection drive in the new closed configuration have been completed. Preliminary measurements support the suitability of this configuration for the study of collisionless reconnection. There is clear evidence for fast reconnection in this new configuration. Here is a movie showing the evolution of the current profile and the magnetic flux function in a fast reconnection event in an argon plasma. (The fast reconnection drive is turned on at t ~ 1060 microseconds and fast reconnection is observed for about 20 microseconds thereafter.) For more details, please visit the VTF home page.

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