Feedback stabilization of axisymmetric modes in the TCV tokamak using active coils inside and outside the vacuum vessel

Abstract

A new vertical position control system, including an internal active coil, has become operational on TCV. The new system has made it possible to stabilize plasmas with open loop growth rates up to 4400 s −1 , currents up to 1.0 MA and elongations up to 2.58. The closed loop stability of the new system has been analysed with a numerical model in which the plasma is assumed to be undeformable, and the power supply outputs are delayed with respect to their inputs. Model predictions agree with the main experimental results.

Experimental and Theoretical Stability Limits of Highly Elongated Tokamak Plasmas

“…The TCV tokamak (tokamak à configuration variable) can operate routinely up to k 2.5 [15]. In recent experiments, we have measured current limits in the range 2.0 , k , 2.5, and, in this Letter, we present the first experimental evidence for a saturation of the normalized current when k .…”

“…We also present tentative evidence for a b limit at high elongation, and we show that the experimental stability limits are consistent with ideal and resistive MHD stability calculations. Figure 1 summarizes recent experimental results in TCV at high elongation [15]. In this figure, we plot the normalized plasma current I N I p ͑MA͓͒͞a͑m͒B͑T͔͒ against elongation for D-shaped plasmas with a fixed horizontal minor radius, a 0.25 m. The discharges shown here are all Ohmically heated, and b values are typically between 1.5% and 2.5%.…”

“…Ideal MHD stability limits are computed with ERATO [26] and KINX [27]. The equilibria considered here all have relatively low values of q s , which is a necessary condition for axisymmetric stability at high elongation [15]. Consequently, the volume inside the q 1 surface is large, and the reconstructed q profiles usually have q 0 , 1.…”

Using APCS for Plasma Vertical Control at TCV

“…The initial analogue vertical position control loop for the internal fast coils was composed of a observer (from integrated magnetic probe signals) and a D (derivative) controller [7]. When migrating to the digital control system the D algorithm run on the digitized observer signal proved to be too noisy.…”

“…Higher elongation plasmas make stability margins smaller, demanding more accurate and faster control systems. TCV uses an almost unique shape and vertical control system composed of slow active coils outside the vessel and fast active coils inside the vessel, with sub millisecond response time power supplies [2], [7].…”