Cyclotron Instability in a Hot Electron Plasma with a Double‐Humped Velocity Distribution

• Published in: The Physics of fluids (1958) 1968-01, Vol.11 (2), p.388-399
• Main Authors: Perkins, W. A., Barr, W. L.
• Format: Article
• Language: English
• ISSN: 0031-9171; 2163-4998
• DOI: 10.1063/1.1691914

A high‐frequency instability occurring in a “hot electron” plasma (Te ∼ 25 keV; Ne ∼1011 cm −3 ) contained in a quadrupole‐mirror machine (i. e., “minimum‐B” field) has been studied experimentally and correlated with theory. The effects observed in the presence of instability bursts are: (1) Intense radiation is emitted at the electron‐cyclotron frequency; (2) a short pulse of electrons escapes along the field lines; and (3) the double‐humped energy distribution (characteristic of the electrons just before the instability) changes so that the distribution of the hot electron component becomes more nearly Maxwellian simultaneous with a marked increase in the 200‐eV component. The experimental observations are compared with the theoretical calculations of Hall, Heckrotte, and Kammash as modified to apply to the measured experimental distributions. In agreement with the observations, these calculations predict the occurrence of an instability at low plasma density, such that ω pe 2 /ω cp 2 ∼ 0.01 . In addition, the required presence of a cold electron component, the predicted growth rate of 2 × 10 7 / sec , and the prediction of instability frequencies at ω ce are all in agreement with the observations. The present picture is, therefore, that the instability derives from the “loss‐cone” nature of the distribution functions of the hot electrons (arising from the fact that they are contained in an open‐ended system where scattering into the loss cone is the main loss mechanism), coupled with the presence of a cold electron component.