Experiments on magneto-hydrodynamics instabilities with ECH/ECCD in FTU using a minimal real-time control system

Abstract

Experiments on real time control of magneto-hydrodynamic (MHD) instabilities using injection of electron cyclotron waves (ECW) are being performed with a control system based on only three real time key items: an equilibrium estimator based on a statistical regression, a MHD instability marker (SVDH) using a three-dimensional array of pick-up coils and a fast ECW launcher able to poloidally steer the EC absorption volume with dρ/dt = 0.1/30 ms maximum radial speed. The MHD instability, usually a tearing mode with poloidal mode number m and toroidal mode number n such that m/n = 2/1 or 3/2 is deliberately induced either by neon gas injection or by a density ramp hitting the density limit. No diagnostics providing the radial localization of the instabilities have been used. The sensitivity of the used MHD marker allows to close the control loop solely on the effect of the actuator's action with little elaboration. The nature of the instability triggering mechanism in these plasma prevents that the stabilization lasts longer than the ECW pulse. However when the ECW power is switched on, the instability amplitude shows a marked sensitivity to the position of the absorption volume with an increase or decrease of its growth rate. Moreover the suppression of the dominant mode by ECRH performed at high plasma density even at relatively low power level facilitates the development of a secondary mode. This minimized set of control tools aim to explore some of the difficulties which can be expected in a fusion reactor where reduced diagnostic capabilities and reduced actuator flexibility can be expected.

Physics conditions for robust control of tearing modes in a rotating tokamak plasma

“…Nonlinear formation of secondary islands, expected in systems with large amount of free energy, has not been often documented in tokamak experiments. However interesting observations have been recently reported in JET (without ECH) and COMPASS [60] and in FTU [61] in presence of ECH.…”

“…The variety of the signals to be acquired and processed for the control action, and their intrinsic uncertainties as well as those of the model predictions could be too high for the feedback specification, while the sensitivity is in principle infinite. The direct measurements are characterized instead by a lower uncertainty but with finite sensitivity due mainly to noise.Therefore it has been proposed [61] to use a combination of both using a probabilistic approach based on the Bayesian assimilation in real time of all the information available.…”

“…A practical steering of the rf beam across the target surface could probably be performed more easily developing a beam switch akin to the FADIS system [71] which as beeen used succesfully on ASDEX-U to switch the EC power transmission between the transmission lines connected to the upper and lower launchers, respectively, in synhronism with the mode rotation, so that the EC power could effectively be absorbed continuously close the O-point [72]. Another example of NTM amplitude control using a similar method is shown in Fig.11 [61]. Here NTMs are reduced in amplitude with an EC beam scanning the q = m/n surface from the low field side.…”

Review of SIRIO interferometer experimental results

“…Finally, we cannot fail to mention the numerous studies on plasmas injected with impurities [32][33][34][35], because it is crucial to determine the conditions that favor a strong increase of particle confinement while minimizing the amount of impurities needed, as well as to favor the so-called "plasma detachment". An increase of the electron density with a remarkable values of the peaking factor are reached in FTU doped pulses.…”

Overview of the FTU results

“…Experiments of real-time control of MHD instabilities using injection of electron cyclotron waves (ECWs) are being performed at toroidal field of 5.3-5.6 T and plasma current of 0.5 MA [9]. The control system was based on only three real time key items: an equilibrium estimator based on a statistical regression, an MHD instability marker (SVDH, ranging from 1 to 0 in presence of a growing mode) using a 3D array of pick-up coils [10] and the fast ECW launcher recently installed and tested on FTU [11].…”