Limit-cycle oscillations (LCOs) close to the power threshold of L- to H-mode transitions are investigated in plasmas of ASDEX Upgrade. During this phase, referred to as I-phase, a strong magnetic activity in the poloidal magnetic field B ⋅ θ with an up–down asymmetry is found. In some cases, the regular LCOs during I-phase transition smoothly into a phase with intermittent bursts which have similar properties to type-III edge localised modes (ELMs). Indications of precursors during the intermittent phase as well as in the regular LCO phase point to a common nature of the I-phase and type-III ELMs. The LCO frequency measured in a set of discharges with different plasma currents and magnetic fields scales as f ∼ ( B t 1 / 2 I p 3 / 2 ) / ( n T ) .

Frequency swept reflectometry has reached the symbolic value of 1 μs sweeping time; this performance has been made possible, thanks to an improved control of the ramp voltage driving the frequency source. In parallel, the memory depth of the acquisition system has been upgraded and can provide up to 200 000 signals during a plasma discharge. Additional improvements regarding the trigger delay determination of the acquisition and the voltage ramp linearity required by this ultra-fast technique have been set. While this diagnostic is traditionally dedicated to the plasma electron density profile measurement, such a fast sweeping rate can provide the study of fast plasma events and turbulence with unprecedented time and radial resolution from the edge to the core. Experimental results obtained on ASDEX Upgrade plasmas are presented to demonstrate the performances of the diagnostic.

The ultra-fast swept reflectometer with the sweep time of 1 μs, inferior to the characteristic turbulent time scale, has provided the measurements of the fast density and density fluctuation evolution across major parts of tokamak plasma radius. The L-H transitions in a series of plasma discharges in ASDEX Upgrade have been studied with a high temporal resolution. The comparison of the density fluctuation behaviour in L- and H-mode is presented. The I-phase oscillation dynamics has been described in terms of the density fluctuation level, the radial electric field and the normalised electron pressure gradient. Indications for a phase shift between the turbulence and the radial electric field are observed in the beginning of the I-phase, where the turbulence grows first and the radial electric field increase follows. In the established I-phase the electric field and the turbulence are in phase.

The results on high frequency coherent modes developing at the plasma edge after L–H transitions are presented. These edge coherent modes (ECMs) observed in the I-phase, during ELM-free and ELMy H-mode phases have common characteristics, such as their frequency of 50–200 kHz, radial position and scaling of the frequency with the pressure gradient. The ultra-fast swept reflectometer with the sweep time of 1 μ s has provided measurements of the mode dynamics and localisation. The ECMs propagate in the electron diamagnetic direction in the laboratory frame and are localized in the region between the pedestal top and the steepest pressure gradient.

Transport codes are frequently used for describing fusion plasmas with the aim to prepare tokamak operations. Considering novel codes, such as SolEdge3X-HDG, synthetic diagnostics are a common technique used to validate new models and confront them with experimental data. The purpose of this study is to develop a set of synthetic diagnostics, starting from bolometer and visible cameras for the WEST tokamak, in order to compare the code results with the experimental data. This research is done in the framework of Raysect and Cherab Python libraries. This allows us to process various synthetic diagnostics in the same fashion in terms of 3D ray tracing with volume emitters developed specifically for fusion plasmas. We were able to implement the WEST tokamak model and the design of bolometer and visible cameras. Synthetic signals, based on full-discharge WEST plasma simulation, were used for to compare the SolEdge3X-HDG output plasma with experimental data. The study also considers the optical properties of the plasma-facing components (PFCs) and their influence on the performance of diagnostics. The paper shows a unified approach to synthetic diagnostic design, which will be further extended to cover the remaining diagnostics on the WEST tokamak.