Containment of High-Density Plasma in a Pulsed Magnetic Well (M.T.S.E. II)

A new apparatus is now being used to study the containment of plasma trapped in a magnetic mirroror well. It follows the principle of our earlier experiment (M.T.S.E.I) in that plasma from a coaxial gun is guided along a converging static magnetic field and trapped between pulsed mirrors energized at suitable times. The trapped plasma has a density of about 1013ions/cm3 with a mean ion energy of 15 keV. Pulsed stabilizing fields(ℓ = 2 configuration) can also be applied to form a magnetic well: the field at the centre of the apparatus is 2 kG, under the mirror conductors 6 kG (maximum), and the maximum depth of the well is 2.0. The important new feature of this apparatus is a very large stainless steel vacuum vessel of volume 3000 litres, inside which are disposed the conductors producing the pulsed magnetic fields. This arrangement ensures that the plasma is not rapidly destroyed by the injection of neutral gas from the walls. The object of the experiment is to determine the critical β for hydromagnetic stability of a well, and to study the mode of decay of the trapped plasma. Plasma has been trapped in a simple mirror arrangement and its behaviour studied by a diamagnetic loop and several devices for detecting the flux of fast neutrals due to charge exchange, sensitive to different ratios of V⊥/V||. The results so far achieved show that a significant fraction of the plasma is trapped, giving β values of approximately 10%; the neutral particle detectors show clear oscillations of groups of ions between the mirrors; the containment time is about 80 μs, consistent with earlier results. Further measurements on plasma trapped in a well are proceeding. (author)