Description
Turbulence at the plasma edge enhances particle and energy radial fluxes, which are transported through the scrape-off layer (SOL), with most of the exhaust deposited on the divertor surfaces. Filamentary structures in the SOL play a key role in this turbulent cross-field transport. For fixed separatrix conditions, longer decay lengths of electron temperature and density lead to increased heat loads. These decay lengths depend on the magnetic configuration and plasma parameters, collisionality, neutral dynamics, and can be strongly modified during edge-localised modes (ELMs). Understanding the evolution of electron density and temperature profiles during ELMs is therefore essential for predicting plasma–wall interactions in future fusion devices. Measurements at the divertor target, in particular, provide important insight into the parallel transport of ELM filaments.
Filament dynamics cannot be adequately measured using conventional Langmuir probe (LP) systems due to limitations in spatial and temporal resolutions. The Fast Langmuir Probe (FLP) system, based on a field-programmable gate array (FPGA), provides real-time measurements of three plasma parameters, electron temperature Te, ion saturation current Isat, and floating potential Vf, using a single electrode. This approach significantly improves spatial resolution compared to triple probes and temporal resolution compared to conventional Langmuir probes, with the conventional LP serving as a benchmark.
This new diagnostic system has been used for the first time on the MAST Upgrade (MAST-U) device to measure plasma edge parameters. We present results on the fluctuation statistics of Te, ne, and Isat in the SOL and private flux region (PFR) obtained using the FLP system in experiments on MAST-U and compare them with measurements from conventional LPs. In addition, ELM events at the divertor target are resolved. A sampling rate of 1.8 MHz for I-V curve acquisition is achieved, and, owing to the 125 MHz clock resolution of the Red Pitaya platform, significantly higher sampling rates are feasible. Currently, three bias voltages are applied at 30 kHz to obtain a single I-V curve using the same amplifier as the existing LP system, which constitutes the main bandwidth limitation of the FLP system. The FLP system enables the simultaneous acquisition of the three plasma parameters (Te, Isat, and Vf) together with full I-V curve data. This capability provides a new tool for investigating turbulent processes governing heat and particle transport across the SOL.