| Summary: | I present particle-in-cell (PIC) simulations of minority energetic protons in deuterium plasmas, which demonstrate a collective instability responsible for emission near the lower hybrid frequency. The simulations capture the lower hybrid drift instability in a parameter regime motivated by tokamak fusion plasma conditions, and show further that the excited electromagnetic fields collectively and collisionlessly couple free energy from the protons to directed electron motion. This results in an asymmetrically populated tail in the velocity distribution of electrons antiparallel to the magnetic field. Obliquely propagating modes are spontaneously excited by energetic ions, whose ring-beam distribution is motivated by population inversions inferred from observations of ion cyclotron emission from JET and TFTR, in a background plasma with a temperature similar to that of the core of a large tokamak plasma. A fully self-consistent electromagnetic relativistic PIC code representing all vector field quantities and particle velocities in three dimensions as functions of a single spatial dimension is used to model this situation, by evolving the initial antiparallel travelling ring-beam distribution of 3 MeV protons in a background Maxwellian deuterium plasma with realistic ion to electron mass ratio. These simulations provide a proof-of-principle for a key plasma physics process that may be exploited in future alpha channelling scenarios for magnetically confined burning plasmas. A simple argument states that some natural symmetry breaking may occur in physical systems. However, it is not possible to determine the sign or magnitude of the current drive effect from these initial simulations. Scans in parameter space elucidate the pervasiveness of this instability, suggesting that opportunities for wave lower hybrid wave amplification at the expense of alpha energy may be widespread. Future alpha channelling scenarios may rely on stimulated emission of LH wave energy from the alpha particles thereby improving LHCD efficiency. Protons exhibit phase space bunching which confirms that resonant energy transfer occurs at the gyrophase angles at which the velocity of an energetic proton on its cyclotron orbit precisely matches the phase velocity of the lower hybrid wave in the direction of propagation. Electron spatio-gyrophase oscillations determine the wavelength of the propagating lower hybrid wave, and thereby govern the spatial distribution of gyrobunching of the energetic protons that drive the instability. These results deepen fundamental understanding of the LHDI, which is of interest to fusion and space plasma research communities. A reduced model is constructed to investigate the concept of stimulated emission of lower hybrid waves from populations of fusion ions that are unstable to the lower hybrid drift instability in tokamaks. Extrapolations are made from PIC code data which, with the reduced model, show that stimulated emission may be a fruitful avenue of research.
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