Tentative First Observations of Quasi-Stationary Zonal Flows in a Spherical Tokamak & Improvements in Image-Velocimetry Workflows

Turbulence drives anomalously high cross-field transport rates in tokamak plasmas.
This drastically decreases the energy efficiency and presents a major issue in fusion
research because the underlying plasma dynamics are not yet fully understood.
Phenomena known as ‘zonal flows’ directly suppress cross-field transport while
siphoning energy from the turbulence, making them highly beneficial for enhancing
plasma confinement. Despite the theoretically predicted ubiquity of zonal flows in toka-
mak plasmas, experimental measurements, particularly in spherical tokamaks, are re-
markably scarce. This scarcity is largely attributed to a notorious difficulty in obtaining
accurate velocity field measurements. To address this gap, this thesis presents im-
provements to velocity field inference techniques and preliminary measurements of
zonal flows. To this end, the Beam Emission Spectroscopy (BES) diagnostic on the
upgraded Mega-Ampere Spherical Tokamak (MAST-Upgrade) was utilised.
Image-velocimetry techniques, which are used to infer velocity fields from imag-
ing diagnostics such as BES, were extensively performance tested for the first time.
The two primary techniques, Cross-Correlation Time-Delay Estimation (CCTDE) and
Dynamic Time-Warping (DTW), were investigated across the majority of reasonably
achievable experimental conditions. Strongly nonlinear behaviour was identified in both
techniques, indicating that testing was required for reliable velocimetry. Tests uncov-
ered the effects and mitigation techniques of the barber pole illusion, the impact of
sheared flows, optimal operational velocimetry parameters, the effect of varying the
number of spatial channels, and more. Results were numerous, nuanced, and often
unexpected; highlighting that thorough reading is recommended to anyone aiming to
perform image-velocimetry analysis.
Velocimetry analysis of BES data from the second MAST-U campaign was per-
formed. The aforementioned testing results were used to guide and optimise the ve-
locimetry workflow while preemptively avoiding complications. However, precision of the inferred velocities fell short of expectations, a discrepancy later attributed to an in-
correctly calibrated optical filter in the BES diagnostic. Consequently, only background carbon II emissions reached the BES detector in most shots, rendering velocimetry of
the main species impossible.
Instead, attempts were made to perform velocimetry using the carbon background emissions, which may appear impossible at first glance due to localisation issues. Af-
ter calibrations using a synthetic diagnostic, results demonstrated agreement with the Charge-Exchange Recombination Spectroscopy (CXRS) diagnostic, implying accurate
velocimetry. Notably, this marked the first instance of successful CII velocimetry using
BES, opening a promising new avenue for investigating impurity dynamics.
A single useful shot, #46459, emerged unaffected by the preceding optical filter
issues. The data was predicted to be suitable for accurate velocimetry based on the
velocimetry testing results. Upon comparison with CXRS, temporal velocity trends showed agreement with CCTDE velocimetry. CCTDE velocities were inferred with typi-
cal standard errors below 1 km/s at a temporal frequency of 4 kHz. The results unveiled radially localised velocity structures, intermittently drifting and re-forming on timescales of 1-10 ms. Further analysis identified this as a coherent localised shear mode featur-
ing a characteristic frequency of 60 Hz and a radial wavelength of approximately 10 cm. The observation of this mode closely aligns with the expected attributes of quasi-
stationary zonal flows, pointing to the first zonal flow detection in a high-performance spherical tokamak.