Stabilization of a Simple Magnetized Toroidal Laboratory Plasma by a vertical magnetic field
Résumé
The stability of a Simple Magnetized Torus (SMT) is studied. It is well known that plasmas
created in SMTs are intrinsically turbulent. Most often the plasma is created with a heated
tungsten filament inserted in the small radius of the device. The emitted electrons are
accelerated by the voltage discharge and ionize the whole toroidal volume. Due to the
curvature of the magnetic field lines, the electrons undergo a vertical drift and are collected by
the conducting metal chamber. The obtained plasma is always turbulent. In our experiment, the
small radius of the torus is 10 cm and the large radius is 60 cm. We emphazise the importance
of using a circular loop filament (3 cm diameter) placed on the secondary axis of the torus as an emitter of the ionizing electrons. A toroidal symmetry is obtained for the ionization source. If no vertical magnetic field is imposed, the plasma is turbulent. But by progressively increasing the vertical magnetic field, the turbulent regime evolves towards a chaotic regime and finally the plasma presents a regular regime. A sinusoidal spatio-temporal fluctuation of the plasma density in the whole volume is recorded. The vertical magnetic field strength allowing this
regime The vertical magnetic field strength allowing this regime is determined by the total magnetic field angle which allows a field line length equal to the wavelength of the resonant ion acoustic wave along the helical field line from the bottom of the torus to the top collector
wall. This experiment shows for the first time that it is indeed possible to confine a stable toroidal magnetized plasma without imposing a twist on the magnetic field lines as in the tokamak geometry.