Experiments with the l = 1 diocotron mode

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1988

OSTI ID:5905635

Fine, K S

Experiments are presented on a particularly interesting oscillatory mode in pure electron plasmas. The plasmas studied are columns contained inside conducting cylinders in an axial magnetic field. Traditional theory models diocotron modes as surface density perturbations; the mode of interest has azimuthal model number l = 1 and is essentially independent of axial position. At large amplitude the mode is more correctly viewed as a dynamical equilibrium in which the plasma column is offset of a displacement, D, and drifts around the cylindrical axis. The frequency, f, of the mode is observed to vary with amplitude as {Delta}f {proportional to} D{sup 2}. This frequency shift arises because (1) the plasma is closer to its image charge than a linear model assumes, and (2) the plasma distorts from a circular shape. From measurements of n(r,{theta}) it is seen that at large amplitudes the column shape becomes elliptical with elongation in the {theta}-direction. The small amplitude frequency of the diocotron model in an infinite length column, f{sub d}, depends only on the charge per unit length. Measurements of frequency have been made versus plasma length, L{sub p}, and plasma radius, R{sub p}. The mode frequency is found to be up to 200% higher in short plasmas, and the fractional frequency shift increases as L{sup {minus}1}{sub p} and decreases with R{sub p}. Finally, the effects of two field perturbations have been studied. An axisymmetric electrostatic field induces exponential damping in the diocotron mode. The other is a small (10{sup {minus}3} rad) tilt of the magnetic field that induces mode damping, as well as particle transport towards a square, low-noise density profile.

• Research Organization: California Univ., San Diego, CA (USA)
• OSTI ID: 5905635
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
700108* -- Fusion Energy-- Plasma Research-- Wave Phenomena
AMPLITUDES
CYLINDERS
DAMPING
DATA
DENSITY
ELECTRONS
ELEMENTARY PARTICLES
EQUILIBRIUM
EXPERIMENTAL DATA
FERMIONS
FLUCTUATIONS
INFORMATION
LEPTONS
MAGNETIC FIELDS
NUMERICAL DATA
PERTURBATION THEORY
PHYSICAL PROPERTIES
PLASMA
PLASMA WAVES
TRANSPORT THEORY
VARIATIONS