Tokamak plasma edge studies by microwave short-pulse reflectometry and backscattering
Moving towards an increased understanding of the dynamic processes in the edge of
tokamak plasmas requires continuous innovation in diagnostic capabilities, pushing the
envelope in both spatial and temporal dimenstions. Plasma density and rotation, and
their respective fluctuations, are essential quantities for tokamak physics studies. The
present thesis describes the design, development, and first results of two new dedicated
diagnostics in the TCV tokamak of the reflectometry type: a heterodyne continuous-
wave Doppler backscattering (DBS) system and a short-pulse reflectometer (SPR). Both
of these diagnostics use a flexible quasi-optical launcher antenna and a pair of universal
polarizers allowing flexible coupling to either O or X mode and programmable polariza-
tion changes during the shot.
DBS is an active diagnostic technique that allows the study of electron density turbu-
lence via the scattering of a millimeter-wave (mmw) beam launched at oblique incidence
to a cutoff layer. This technique has been implemented using a fast arbitrary waveform
generator (AWG) as the main oscillator and commercial vector network analyzer ex-
tension modules as the main mm-wave hardware. It allows sweepable single frequency
operation featuring an I/Q mixer to analyze the heterodyne signal. An innovative multi-
frequency mode has been demonstrated. It leverages on the flexibility of the AWG source
by feeding a bi-frequency signal to a x6 varactor multiplier producing a 7-frequency out-
put that is directly measured via fast digital sampling in the receiver. Furthermore, a pair
of fast polarizers have been used to measure the magnetic-field pitch angle in the edge
of the plasma by monitoring the backscattered signal power as the probing beam output
linear polarization angle was varied at constant ellipticity. Ray-tracing simulations reveal
an accessible k¿ range between 3-16 cm¿1 with a resolution of 2-4 cm¿1 . Perpendicular
rotation velocity estimates compare well against ExB plasma poloidal rotation estimates
from charge exchange recombination spectroscopy.
In density profile reflectometry, the electron density can be inferred from the round-
trip group-delay of EM waves reflected from a plasma cutoff. Short pulse reflectometry
(SPR) consists of sending broadband mmw pulses (~1ns) and measuring their round-trip
group-delay using precise timing systems. This thesis describes the realization of a novel
approach to SPR, using low-frequency pulses from an arbitrary waveform generator and
producing short pulses with x6 varactor multipliers. The design offers unique flexibil-
ity regarding pulse output frequency and repetition rate, which allows the instrument
to overcome traditional SPR spatial sampling limitations while reducing hardware com-
plexity. In order to measure the group-delay of short pulses, both a traditional analog
and a novel digital sampling technique have been explored. A group-delay resolution of
14ps in average has been achieved with both approaches. The direct sampling technique
has the added advantage of measuring group-delays as well as reflected pulse amplitude
and width. Raw histograms of group-delay data show interesting qualitative changes
from L to H-mode. Frequency spectra of group-delay data allow the identification of
macroscopic density fluctuations as well as edge quasi-coherent modes during ELM-free
H-modes. Lastly, density profiles have been measured with microsecond time resolution,
effectively increasing the temporal resolution of density measurements in TCV by a fac-
tor of 103 , albeit over limited radial ranges. The output polarization flexibility allowsthe instrument to reveal densities in the 0.3-4.5 × 1019 m¿3 range.
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