Experimental Investigation of Quasistationary Longitudinal Ion Streams in the L-L Stellarator

The results are presented of an experimental investigation of a quasistationary longitudinal ion stream detected in the L-l stellarator. The possibility of the existence of closed ion and electron streams is predicted by the theory of plasma equilibrium in toroidal traps formulated by Pfirsch, Schlüter, Sagdeev, Galeev and others. However, the directed ion and electron motion superposed on the random thermal movement of charged particles in toroidal systems had not until recently been investigated experimentally. An attempt to study experimentally streams of this kind was made using the L-l stellarator. With the help of a specially developed technique, the motion of the ion component in one direction along the axis of the toroidal chamber was recorded. The experimental results indicate an absence of either qualitative or quantitative agreement with the conclusions of the existing theory of plasma equilibrium in toroidal traps. In particular, the measured directed velocities considerably exceed the values predicted by the theory; the longitudinal stream detected in the experiment does not change its direction in the transverse cross-section of the chamber; the radial dependence of the directed velocities and their dependence on the rotational transform angle of the magnetic field lines - relationships which were found in the experiment - contradict the theoretical relationships. To explain the observed effect, the authors advance a hypothesis based on the existence of ''blocked'' particles in the toroidal trap. In the presence of radial electric fields, the longitudinal velocity distribution function of the particles becomes asymmetric, which leads to the appearance of an effective ion stream directed along the axis of the toroidal chamber. The relationships found in the experiment confirm the unique connection between the electric field and longitudinal ion stream values. Comparison of the experimental results with calculations performed on the basis of the suggested hypothesis confirms the validity of the explanation given of the mechanism whereby the ion stream occurs. (author)