UCLA

Highly energetic electrons produced naturally or artificially can be trapped in the Earth's radiation belts for months, posing a severe danger to valuable space satellites. Concepts that can lead to radiation belts mitigation have drawn a great deal of interest. This dissertation investigates the loss of fast electrons from a magnetic mirror trap when irradiated by a shear Alfvén wave. The experiment is performed in the quiescent after-glow plasma in the Large Plasma Device (LaPD). The background magnetic field is programmed to include a magnetic mirror section (mirror ratio ≈ 2, length = 3.5 m). A trapped fast electron population is generated in the mirror section by X-mode high power microwave pulses. Different power levels, pulse durations and heating schemes lead to different electron energies. Two distinct electron energy ranges are studied separately in this work, which are around 100 eV and 100 keV respectively. Shear Alfvén waves of arbitrary polarization are launched externally by a Rotating Magnetic Field (RMF) source (δB/B0 ≈ 0.1%, λ_|| ≈ 9 m). It is demonstrated that the shear Alfvén wave can effectively de-trap energetic electrons from the magnetic mirror field in both electron energy ranges.

Due to grad-B and curvature drift, the highly energetic electrons (above 100 keV) form a hot electron ring in the magnetic mirror. Experimental evidence indicates that the ring is deformed in the right-handed (RH) circularly polarized shear Alfvén wave field. Electron losses are observed in both the radial and the axial direction of the mirror field, with modulation at the Alfvén wave frequency. The periodical loss continues even after the termination of the wave.

A test particle simulation is performed, which confirms that the single particle motion of the energetic electrons in the Alfvén wave field is not adequate to explain experimental observations, and the hot electron collective behavior must play a role in the de-trapping effect.

It is proposed that the hot electron ring is deformed due to E_Alfvén×B₀ drift in the Alfvén wave field. The ring deformation grows when the electron azimuthal drift matches the rotation of the RH shear Alfvén wave pattern, which generates a collective mode of the ring that degrades its confinement and leads to electron loss from the magnetic mirror.