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.

A Fast Ion Loss Detector for the TCV Tokamak

Lorenzo Stipani

A Fast Ion Loss Detector (FILD) was designed, assembled, installed and commissioned for TCV. This is a radially positionable, scintillator based detector that provides information on the 2D fast-ion velocity space lost at the probe's location. The collected particles are collimated inside the probe head and impinge upon a plate coated with a scintillator material that emits light. The photon flux is relayed to two acquisition systems: a sCMOS camera for high spatial resolution measurements of the emission locations in the scintillator, and fast photo-multipliers that allow time-correlation studies of fast-ion losses with high-frequency electromagnetic fluctuations. From its position with respect to the confined plasma and the fast-ion sources on TCV, FILD probes a small portion of the lost fast-ion phase space with high velocity-space and temporal resolution. Therefore, it naturally complements other available diagnostics such as neutral particle analysers, spectroscopy techniques for light emission from energetic particles following CX reactions, or neutron counters, which feature a broader, but less resolved, spatial coverage of fast-ion dynamics.The TCV-FILD design introduces some novelties for exploring new ranges of operation that may be adopted in similar systems for other Tokamaks, such as ITER. Two entrance slits can collect particles that circulate in co- and cntr-plasma current directions. This will be particularly useful when the second NBH system, injecting in the opposite toroidal direction, with particle energies that can excite strong Alfvénic modes, will be operated on TCV. A controlled pneumatic linear actuator radially positions the detector to expose the slits to the particle flux up to 9mm inward of the vessel wall. A plug-in design was conceived to facilitate diagnostic installation. The diagnostic was installed and commissioned during TCV experiments in 2020. The sensitivity of the detector to the local magnetic field line direction was investigated. The direction of the plasma current was found to select which of the two slits may be traversed by lost particles. The operational limits in discharges with NBH with total delivered energies up to 1MJ were assessed with the help of sensors monitoring the temperature of the probe head and cameras detecting visible light emissions resulting from the graphite shield heating by particle fluxes. Using FILD, fast-ion losses were detected for the first time on TCV in plasma discharges exhibiting strong MHD modes. Ejections of energetic ions were found correlated in time with Sawtooth crashes, as a sequence of individual events, and in strong phase coherence, as a continuous loss in time, with magnetic perturbations of a saturated and toroidally rotating magnetic island of a NTM. These observations, in addition to providing initial results on the relevant physics phenomena, stimulating further experiments and comparisons with theory, demonstrate the ability of TCV-FILD to provide valuable information in plasma discharges of interest for fast-ion studies. These measurements and additional information from other TCV diagnostics may now be combined to reconstruct, with tomographic inversion techniques, more of the fast-ion phase space. This will be used to identify the conditions for the excitation/suppression of magnetic instabilities, develop methods for their real-time control with heating and/or shaping actuators and investigate their dependencies on plasma parameters.

EPFL2021

A Fast Ion Loss Detector for the TCV Tokamak

Lorenzo Stipani

EPFL2021