Observation of the guiding center negative mass instability in the drift orbit cyclotron

The guiding center negative mass instability was observed in the Drift Orbit Cyclotron, which is an axisymmetric mirror machine with the magnetic field radial profile tailored so that the delapprox.B drift frequency is independent of radius. Electrons are continuously injected in the radial direction to form a magnetically confined, azimuthally delapprox.B drifting annulus with radius 23 cm, thickness 0.65 cm and height 7.85 cm. As the density of the annulus is increased, the negative mass instability causes the radial boundaries of the annulus to deform. The temporal and spacial character of the deformation of the annulus is studied experimentally near the threshold density for instability. A linear theory is used to describe the threshold density for instability and a theoretical model is presented to explain the saturation of the instability near the instability threshold