Plasma Turbulence Studies Using Correlation Doppler Reflectometry on the ASDEX Upgrade Tokamak

One of the major goals of the tokamak fusion program is the
understanding and control of plasma turbulence. Turbulence causes
additional radial transport of heat and particles to the tokamak
vessel walls, thereby degrading the overall confinement of the
plasma. Diagnostics for the study of tokamak turbulence are
unfortunately scarce and limited in what they can measure. A new
diagnostic technique, Doppler reflectometry, has been developed for
measurements of plasma rotation profiles and turbulence properties.
It is a type of microwave radar technique which uses the
back-scatter of microwaves from a radial position in the plasma
where the refractive index equals zero. In this thesis work, the
technique is extended for turbulence correlation measurements by
adding a second Doppler reflectometer channel where two microwave
beams are launched into the plasma with a small frequency
difference. A radial correlation Doppler reflectometer system can
provide simultaneous measurements of the plasma radial electric
field Er and its shear (both parameters are believed to be
fundamental for suppressing turbulence) together with measurements
of the properties of the plasma turbulence, such as the radial
correlation length of the turbulence Lr. These measurements are
explored in this thesis work for a wide range of plasma conditions.
It was found that Er and its associated shear are indeed linked to
plasma confinement. Their absolute values increase with confinement
at the plasma edge. An increase in the absolute value of Er shear
was also detected at the same plasma edge region where a decrease
in radial correlation lengths of the turbulence was measured. This
observation is in agreement with theoretical models which predict
that an increase in the absolute shear suppresses turbulent
fluctuations in the plasma, leading to a reduction in Lr.
Measurements of Lr versus the perpendicular wavenumber of the
turbulence were also obtained, which prompted an investigation of
the correlation Doppler reflectometer response function using a
2-dimensional finite difference time domain (FDTD) code. The
simulation results show that the magnitude of the measured Lr is
dependent on the radial, poloidal and perpendicular wavenumbers of
the turbulence.