Interaction between a rotating electromagnetic field and a plasma contained in a strong constant magnetic field

Abstract

The paper presents theoretical calculations and the results of experimental studies of the behaviour of a plasma contained in a strong constant magnetic field when a rotating high-frequency field of dipole configuration with frequency Ω is superimposed upon it. It is shown that for υei > Ω (where υei is the frequency of collisions between electrons and ions) the high-frequency field excites an azimuthal electric current jφ on the plasma surface. The interaction between the current jφ and the constant magnetic field Hz generates a dynamic force, which forms the plasma into a column.The plasma is compressed into a column when the vector of the angular velocity of rotation of the high-frequency field and that of the constant magnetic field H⃗0 coincide in direction. When the plasma is compressed into a column, there is no contact with the walls of the vacuum chamber and the degree of ionization is close to total. Probe and microwave measurements have shown that the time of compression of a plasma into a column for Hz = 8 × 103 – 1 × 104 Oe is ∼ 300 μs and the rate 6 × 103 cm/s. The diameter of the column was of the order of 2–3 cm. The charged-particle concentration measured with the help of superhigh-frequency waves (λ = 2 mm, 8 mm and 32 mm) was greater than 2 × 1014 cm−3 and decreased by more than two orders at a distance of 1−1.5 cm from the column centre.When the vectors and H⃗0 are opposite in direction, the force is directed away from the axis, and in this case, the plasma in the form of a hollow cylinder is situated at the vacuum-chamber walls.The maximum strength of the rotating high-frequency field at 1.25 MHz was 100 Oe. The results obtained show good agreement with the theoretical calculations.