Description
This work presents the first use of real-time (RT) sensors provided by the eXtraordinary mode (X-mode) reflectometry for density and plasma position control. Density control using Ordinary (O-mode) reflectometry as a sensor was demonstrated in Asdex-Upgrade in 2010 [1], but requires an assumption on the density profile position. On the contrary, the X-mode reflectometry enables to detect the plasma edge radial position, which is used to start the density profile reconstruction and was planned on ITER [2].
A new acquisition system and a new algorithm dedicated to RT measurements were implemented on the WEST X-mode edge reflectometer. The world first electron density profiles were reconstructed in real-time during 2024, providing profiles every few ms [3].
Real-time density profiles can provide several sensors for plasma control. The first sensor is the local density at a fixed radial position. The second sensor is the edge plasma position deduced from the major radius of a fixed density. Thirdly, the plasma position knowledge allows to compute the density at given distance to the plasma edge, roughly the density at a fixed flux surface.
The first control experiments using X-mode reflectometry sensors have been performed on WEST in 2025. Density control is usually achieved using the interferometry line integrated density (every ms), the actuator is the gas injection. First, we replaced the line integrated sensor with the density at a fixed radial position, to perform control of the local edge density. Secondly, we replaced the density sensor by the density at a given relative position, allowing to control the local edge density during a plasma radial translation. These two control experiments were successful, without any specific issue. Lastly, we replaced the magnetic sensor used to control the plasma position by the radial position obtained from RT reflectometry. The actuators are the PF coils. The first experiments showed radial position oscillations due to a latency of about 30 𝑚𝑠 on the measurements, specific to reflectometry plasma position control. We improved all the RT data processing to shorten this latency to a few milliseconds, and the new plasma position control experiments were performed successfully without any radial oscillations.