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.

Coda, Stefano
Porte, Laurie
Lausanne, EPFL

 Record created 2019-08-14, last modified 2019-08-20

Download fulltext

Rate this document:

Rate this document:
(Not yet reviewed)