Characteristics of a rotating plasma in a diverging magnetic field

The plasma centrifuge is one of plasma isotope separation methods which relies on mass-dependent radial ion transport in a rotating plasma under strong centrifugal force. To improve the isotope separation factor, it is primarily important to confine the rotating plasma in a magnetic field for longer than the column rotation period with keeping ion temperature low. We carried out a series of experiments in order to achieve increase in the rotational velocity and decrease in the ion temperature simultaneously. We tried to expand the rotating plasma in a diverging magnetic field for this purpose: if the electrical conductivity of plasma is high enough for the plasma potential to be constant along magnetic lines of force, the angular velocity ω of rigidly rotating plasma will become constant along the same lines of force. It is called Ferraro's isorotation law. Since rotational velocity υθ is written as υθ = γ, ω, υθ will increase in proportion to the radius γ. Furthermore, if the plasma is isolated thermodynamically from the wall in the diverging magnetic field, the ion temperature is expected to decrease by adiabatic plasma expansion. As a result of experiments, the plasma radius at the axial distance of 57 mm from the plasma gun expanded l.7 times as large as that at 12 mm. the rotational velocity increased 1.2 times, and ion temperature decreased to 1/3 compared with that for the uniform magnetic field. (author)