An Experiment to Transfer Angular Momentum from a Helical Low Energy Proton Beam to a Trapped Electron Plasma

摘要

As part of a continuing program of beam-plasma interaction studies, a low energy (2-10 keV) proton beam will be injected on a helical trajectory into a trapped electron plasma in a 1.6 T cryogenic solenoid. The proton source is a conventional duoplasmatron, but operated well below its design extraction energy. Beam tests over the desired energy range have established a mode with submillimeter beam focus and currents of a few μA. The beam will be transported into the high field, displaced, and then inflicted by a sudden impulse onto an offset helical trajectory of low pitch. The electron plasma trapping potential will provide a fine pitch control and will serve as an analyzer of the residual longitudinal momentum (helix pitch). Previous experiments in this laboratory employing proton beams of high energy (50-300 MeV) in a storage ring have shown that an electron plasma absorbs angular momentum and energy from the proton beam-for example exhibiting expansion through beam misalignment which breaks the trap azimuthal symmetry. The expectation is that a lower energy beam, traversing the plasma at velocity well below that of a typical wave mode, may be more effective in torque transfer. A possibility may exist for significant plasma compression with judiciously chosen settings of the helix position offset relative to the plasma surface. Progress in design and implementation of the low energy injection scheme will be presented.