Observations of Instabilities in Electron‐Cyclotron Plasmas

Two types of instabilities have been studied in an electron‐cyclotron plasma confined by magnetic mirrors and heated by continuous wave microwave power at 10.6 Gc. One type of instability results in the loss of energetic electrons (Ee > 100 keV) across magnetic field lines, accompanied by oscillations in the 3–30 Mc range. Suppression of this instability is accomplished by increased background pressure. The second type is characterized by loss of fast electrons along field lines through the magnetic mirrors and accompanied by radio‐frequency oscillations at 5.3 Gc (=½ω ce ). The loss is also characterized by a threshold in magnetic field corresponding to locating the heating zones in the mirror throats. The first type of instability appears to be flute‐like and driven by ▿B drifts. The second type may be initiated by the mirror instability in the plasma.