Abstract
The Doppler-shifted cyclotron resonance (ω−kzvz=Ωf) between fast ions and shear Alfvén waves is experimentally investigated (ω, wave frequency; kz, axial wavenumber; vz, fast-ion axial speed; Ωf, fast-ion cyclotron frequency). A test particle beam of fast ions is launched by a Li+ source in the helium plasma of the LArge Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)], with shear Alfvén waves (SAW) (amplitude δ B/B up to 1%) launched by a loop antenna. A collimated fast-ion energy analyzer measures the nonclassical spreading of the beam, which is proportional to the resonance with the wave. A resonance spectrum is observed by launching SAWs at 0.3–0.8ωci. Both the magnitude and frequency dependence of the beam-spreading are in agreement with the theoretical prediction using a Monte Carlo Lorentz code that launches fast ions with an initial spread in real/velocity space and random phases relative to the wave. Measured wave magnetic field data are used in the simulation.
Highlights
Fast ions are ions with energies that are much larger than typical thermal energies of plasma constituents
Fast ions are found when a hot plasma merges with a colder background plasma, as when the solar wind collides with the magnetosphere
Resonant heating of fast ions by Alfvén waves well below the ion cyclotron resonance frequency might cause ion heating in toroidal fusion devices
Summary
Fast ions are ions with energies that are much larger than typical thermal energies of plasma constituents. Resonant heating of fast ions by Alfvén waves well below the ion cyclotron resonance frequency might cause ion heating in toroidal fusion devices.. The interaction of fast ions with waves and instabilities is challenging to study experimentally because of difficulties in diagnosing the fast-ion distribution function and the wave fields accurately, in either hot fusion devices or space plasmas. The approach of this work and the previous classical fast-ion transport study is to launch test-particle fast-ion beams with a narrow initial distribution function in phase space using plasma-immersible fast-ion sources.. With resonance overlap of fast ions and shear Alfvén wavesSAW, resonant beam transport, in addition to the well calibrated classical transport, is analyzed with good phase-space resolution.
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