High-power microwave beams used for heating and current drive in magnetically confined fusion plasmas can be broadened significantly by plasma turbulence, negatively impacting the efficiency of the machine. The dependence of this beam broadening on plasma and beam parameters is not yet fully understood, particularly where the dependence on one parameter is not separable from the dependence on the other parameters, meaning the dependence must be expressed via functions of linear combinations of parameters, rather than functions of single parameters. The aim of this work is to develop an empirical model for how the broadening depends on plasma and beam parameters, allowing for the easy estimation of beam broadening by turbulence without the need for computationally expensive full-wave simulations. In this paper, a microwave beam is simulated propagating through a turbulent layer of plasma using the 2D full-wave cold plasma code EMIT-2D. The dependence of beam broadening on background plasma density, fluctuation amplitude, turbulence correlation lengths in the radial and poloidal direction, thickness of the turbulence layer, and microwave beam waist are considered. The parameter scans are conducted in pairwise combinations of the parameters in order to determine the separability of the dependencies. We find that the dependence on the radial and poloidal correlation lengths are not separable from each other, and neither are the dependences on the fluctuation level and the background density, but all other dependencies are separable. Ignoring this inseparability in the correlation lengths will usually result in an over-prediction of the broadening in tokamak plasmas. An empirical formula for the beam broadening based on the turbulence and beam parameters is found for fusion-relevant scenarios, making prediction of the effect possible in microseconds, instead of the hours required for full-wave simulation. This could then be of use for integrated modelling of heating and current drive systems.
The UK’s Spherical Tokamak for Energy Production (STEP) reactor design program has recently taken the decision to use exclusively microwave-based heating and current drive (HCD) actuators for its reactor concepts. This is based on a detailed assessment considering all viable HCD concepts, covering the grid to plasma efficiency, physics applications, technology maturity, integration, maintenance, and costs. Of the two microwave techniques: Electron Cyclotron (EC) and Electron Bernstein Wave (EBW), EC was deemed the lowest risk and EBW is retained as a potential path to a more efficient, higher performing device. To assess the ECCD efficiency, the GRAY beam tracing code was employed to perform detailed scans of the launcher position, toroidal and poloidal launch angle, and frequency over the first 3 cyclotron harmonics. For EBW, GENRAY/CQL3D were used to estimate the CD efficiency, demonstrating promising results. To reduce the physics uncertainties in present models for EBW coupling and current drive, MAST Upgrade will install two dual frequency (28, 34.8 GHz), 900kW, 5s gyrotrons from Kyoto Fusioneering, as part of the MAST Upgrade enhancements package. This will be accompanied by a flexible 2D steering launcher system to allow midplane coand counter-CD and above midplane launch for co-direction off-axis CD. Coupling efficiency is quantified by measuring the heating induced by reflected (i.e. non-coupled) power to a plate inserted in the reflected beam path. The experiments will also include EBW driven solenoid-free start-up, increasing power and pulse length by a factor of 10 compared to previous MAST experiments. This presentation will discuss the STEP microwave studies and the MAST Upgrade physics design and capabilities.
We have demonstrated for the first time that turbulent plasma density fluctuations in the edge of the DIII-D tokamak are responsible for substantial broadening of an injected microwave beam by successful quantitative comparison between experimental observations and first principles 2D full-wave simulations. The broadening of the beam has important implications for control of tokamak discharges through localized electron cyclotron deposition needed for eliminating magnetohydrodynamic instabilities. This new predictive capability is mandatory to design & operate present & future tokamaks in such a way that microwave heating schemes achieve their intended objectives.
The original version of this article was published with an editor's name misspelled in the acknowledgements. The correct acknowledgement should read as below.
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