A FIELD-FREE DENSE-PLASMA PRODUCED BY ELECTRON-CYCLOTRON RESONANCE IN A LINEAR MULTIPOLE FIELD

A description is given of characteristics of a plasma produced by electron cyclotron resonance (ECR) in a linear hexadecapole device. The field configuration generated by eight straight current rods circumferentially located in a glass chamber has a wide magnetic field-free region inside the rods and a strong peripheral field region between the rods and the chamber wall. Plasma was produced by ECR in the strong peripheral field using 2.45 GHz of radio frequency (rf) power and was effectively transferred into the field-free central region along radial field lines. Radial distribution of the ion saturation current I(s) was measured by a Langmuir probe, being remarkably flat in the field-free region and showing extremely low fluctuations of I(s). The electron density at the chamber center increased in proportion to the rf power and significantly exceeded the cutoff density determined by the rf frequency. The production mechanism for the overdense plasma, the influence of the rf electric field direction upon the electron density profile, and the effect of ion drift motion due to the field line curvature are also discussed.