Rotation of a collisional Theta pinch plasma

Abstract

Research notes I81 that BEKEFI and HOOPER (1964), ROBERTS (1966), and others have reported observing cyclotron radia- tions from beam generated plasmas. These two investigators were mentioned because they observed similar phenomena dispite having quite different beam parameters; Bekefi had a very low power beam while Roberts used an electron gun from a high power klystron. In this phase of the experiment, the Rensselaer electron gun is fundamentally different from both of these in that while they both use cathodes that are shielded from the axial magnetic field, we have a cathode which due to the removal of magnetic shielding, is immersed in the field. The result is that they both have electron beams with s i m c a n t transverse energy due to magnetic lens effects (MI“, Thus we found that when the ‘yoke’ of soft iron was again used to partially shield our gun from the axial field, thereby forming a magnetic lens, electron cyclotron radiation did appew. The ‘t.ming’ curve for this radiation is shown in Fig. 3b. Of course, it was found that the beam generated plasma had the expected wide band noise which was absent for beam-cesium plasma operation under similar conditions of plasma density, beam current, and energy. In conclusion, these preliminary measurements indicate both the desirability and the feasibility of using such a cesium plasma system for the study of beam plasma interactions. Acknowledgments-The authors wish to acknowledge the help of Mr. W. R. STEWART in constructing the apparatus. This research was supported by the National Science Foundation under grant GK-1596. Rensselaer Polytechnic Institute Troy, New York U.S.A. R. L. GUNSHOR* J. L. MUNDY w. P. RAWSONT REFERENCES BEKEFI G. and HOOPER E. B. (1964) Appl. Phys. Lett. 4,135. FREDRICKS R. W. (1968) J. Plasma Phys. 2,365. GUNSHOR R. L. and MIHRAN T. 6. (1958) J. appl. Phys. 39,4635. MIHRANT. G. (1962)J. appl. Php. 33,1582. ROBERTS M. (1966) Ph.D. Thesis, Cornell University. * Present address : School of Electrical Engineering, Purdue University, Lafayette, Indiana 41907, U.S.A. t Present address: Hewlett-Packard, Colorado Springs, Colorado, U.S.A. Plasma Physics, Vol. 11, pp. 787 to 789. Pergamon Press 1969 Printed in Northem Ireland Rotation of a collisional Theta pinch plasma (Received 5 February 1969) TP----. LNIA phch plasmas have been observed I O rorate rapizy, iose rorationai symmetry and become turbulent. Recently, Diic~s (1968) has investigated rotation using a two-dimensional computer model for a collisional plasma. He finds appreciable rotation if a transverse magnetic field exists across the plasma. In recent experiments on collisional plasma (BENFORD, 1969) rotation has been found. This raises the question of whether a previous mechanism due to VELIKHOV (1964) for colli- sionless plasmas may be present here as well. VELIKHOV (1964) has treated the rotation of an axially symmetric theta pinch which arises from finite Larmor radius corrections to the ion diamagnetic currents. He considered the collisionless regime (n,~ 1, where 0, is the cyclotron frequency and T the momentum relaxation time for ion- ion collisions) and found that the plasma divided radially into oppositely rotating parts. T h i s note derives this effect for arbitrary Q-T. Followkg Velikhov, we consider a plane layer of plasma Uniform along the y-axis and moving along the x-axis, parallel to the imposed magnetic field B. (A description of this effect in cylindrical coordinates appears in a general review of theta pinch rotation by HAIm (1965).) The hydromag- netic equation of motion of the plasma is do m+n+ ;ii = -AP - A . en> + ,(v - v -)XB