Results of measurements made on a vacuum arc centrifuge are reported. The rotational velocity of the plasma column has been deduced by cross correlating the floating potential detected by two Langmuir probes inserted into the plasma. The main result is that the rotational velocity is slightly lower than the Alfven critical velocity, for the four different elements used as cathode material to form the plasma (Mg, Zn, Cd, Pb). A tentative explanation for the results is proposed based on an extension of the global energy balance argument which describes a conventional plasma centrifuge.

Ever since conception of the vacuum arc centrifuge in 1980, periodic fluctuations in the ion saturation current and floating potential have been observed in Langmuir probe measurements in the rotation region of a vacuum arc centrifuge. In this work we develop a linearized theoretical model to describe a range of instabilities in the vacuum arc centrifuge plasma column, and then test the validity of the description through comparison with experiment. We conclude that the observed instability is a ''universal'' instability, driven by the density gradient, in a plasma with finite conductivity.

The azimuthal component of the force, which establishes rotation in vacuum arc centrifuges, is investigated. It is found that the design of the anode grid is one important factor influencing rotation. A range of tungsten-wire grids have been studied experimentally in a vacuum centrifuge operating with a magnesium cathode, and angular velocities have been determined by cross correlation of voltage probe signals. It has been verified that angular velocity increases when grids with higher effective electrical resistivity are used, as predicted theoretically. Grid heating during the 14 ms operating pulse increases resistivity and should also increase angular velocity; this effect has been observed experimentally and agrees with predictions.

Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron-Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L<5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

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