Fusion product driven lower hybrid electron current in tokamaks
Cook, James William S. (2011) Fusion product driven lower hybrid electron current in tokamaks. PhD thesis, University of Warwick.Full text not available from this repository.
Official URL: http://webcat.warwick.ac.uk:80/record=b2581285~S1
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
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.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||Q Science > QC Physics|
|Library of Congress Subject Headings (LCSH):||Deuterium -- Mathematical models, Protons -- Mathematical models, Plasma dynamics, Tokamaks|
|Official Date:||December 2011|
|Institution:||University of Warwick|
|Theses Department:||Department of Physics|
|Supervisor(s)/Advisor:||Chapman, Sandra C. ; Dendy, R. O.|
|Extent:||xiii, 139 leaves : ill., charts|
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