AVS 45th International Symposium
    Vacuum Technology Division Monday Sessions
       Session VT-MoM

Paper VT-MoM6
Effect of Background Neutral Pressure on the m=1 Diocotron Mode Amplitude in a Pure Electron Plasma@footnote 1@

Monday, November 2, 1998, 10:00 am, Room 329

Session: Vacuum Gauging, Outgassing and Leak Detection
Presenter: S.F. Paul, Princeton University
Authors: E.H. Chao, Princeton University
R.C. Davidson, Princeton University
S.F. Paul, Princeton University
Correspondent: Click to Email
The word "diocotron" was first used to describe instabilities in hollow electron columns which had shear in the angular flow velocity. These instabilities can occur in propagating nonneutral electron beams and layers and in low-voltage microwave generation devices such as magnetrons, traveling-wave tubes, and ubitrons. We use the word to generally refer to low-frequency electrostatic oscillations perpendicular to the magnetic field and have studied experimentally the mode with azimuthal mode number m=1. The diocotron mode is studied in a pure electron plasma confined in a Malmberg-Penning trap. The frequency of the mode is generally on the order of 100 kHz while the plasma frequency is on the order of 10 MHz and the electron cyclotron frequency is 100 MHz. The frequency of the m=1 diocotron mode in an infinite length column was predicted theoretically by Levy@footnote 2@, however, we find better agreement when the finite column length theories@footnote 3,4@ are used which predict an upward frequency shift from the infinite length case. The mode amplitude is affected by wall resistance as well as by the background neutral pressure. The resistive wall destabilization of the m=1 diocotron mode was predicted and experimentally verified by White@footnote 5@. Our measurements of the growth rate agree reasonably well with theoretical predictions. The m=1 diocotron mode is also predicted to be driven unstable in the presence of collisions with background neutrals@footnote 6@. However, we have found experimentally that increasing the background neutral pressure causes the amplitude of the m=1 diocotron mode to decrease as the column expands. @FootnoteText@ @footnote 1@Research supported by the Office of Naval Research. @footnote 2@R.H. Levy, Phys. Fluids 11, 920 (1968). @footnote 3@S.A. Prasad and T.M. O'Neil, Phys. Fluids 26, 665 (1983). @footnote 4@K.S. Fine and C.F. Driscoll, Phys. Plasmas 5, 601 (1998). @footnote 5@W.D. White, J.H. Malmberg, and C.F. Driscoll, Phys. Rev. Lett. 49, 1822 (1982). @footnote 6@R.C. Davidson and E.H. Chao, Phys. Plasmas 3, 3279 (1996).