Real-time Toroidal Velocity (rtVphi) Diagnostic on KSTAR

A real-time toroidal velocity (rtVphi) system has been installed on KSTAR as a part of the real-time diagnostics for real-time Disruption Event Characterization and Forecasting Research (DECAF) [1]. The overall specifications and connection scheme of the rtVphi system are presented. The new 32 channel system currently has a data acquisition rate of 0.4 kHz. Data that was successfully obtained during the 2023 campaign will be analyzed and compared with data from the KSTAR CES. We will further optimize this system to achieve the targeted value of 1 kHz in the 2024 campaign.

Due to better resolution of the rtVphi system than the KSTAR CES diagnostic, the change in the plasma flow and ion temperature profile near the pedestal during events such as the ELM can be monitored better with our system. We observe that the toroidal velocity at the pedestal recovers slowly during the ELM suppression period. The ion temperature pedestal is also degraded during the suppression period, exhibiting the pedestal broadening by RMP-induced transport. This observation clarifies the response of pedestal confinement by the 3D RMP with fine temporal resolution. Based on data from the ECEI system, the change in the pedestal confinement is related to the suppression of turbulence initiated by drift waves. We have been developing a linear model to calculate the dispersion relation of drift waves at the pedestal. This is based on the existing model for lower hybrid drift waves inside the reconnection current sheet [2, 3], but additional terms from the temperature gradient have been added.