Hot ion mode of small bore plasmas in JET

Ion temperatures in excess of 20 keV and high D-D neutron rates have been achieved in JET, by the injection of 80 keV neutral beams into low density, low volume plasmas. Two small bore configurations were tried, one limited on the inner wall and the other on the beltlimiters (low field side). Both configurations had plasma current of 3 MA and toroidal field of 3.4 T at the vessel axis. The small plasma minor radius aids the beam penetration while the low density increases the power per particle and beam slowing down time. The inner wall configuration has several advantages; it can have a smaller volume because of the lower major radius and higher toroidal field, the neutral beams are closer to tangential, and the inner wall is known to pump deuterium effectively. The limiter configuration, however, allows the use of combined RF and NBI heating systems and could benefit from geometrical scaling of confinement. (author) 2 refs., 5 figs.