Real-time control of the period of individual ELMs by EC power on TCV

Abstract

The period of individual type-I edge-localized modes (ELMs) in TCV H-mode plasmas is controlled by real-time controlled application of electron cyclotron (EC) power close to the plasma pedestal. An ELM pacing algorithm, closely related to sawtooth pacing (Goodman et al (2011 Phys. Rev. Lett. 106 245002)) has been implemented in the TCV control system. This algorithm switches the EC power to a low level after detecting an ELM, and subsequently increases the power to a higher level after a pre-set time interval, stimulating the advent of the next ELM. While the mean ELM period is observed to depend only on the mean power applied, ELM pacing is shown to significantly regularize the ELM period with respect to the case of continuously applied power. It is also shown that the ELM period can be changed from one ELM to the next on time scales shorter than the global energy confinement time. These results present a challenging benchmark to physics-based pedestal models and can point towards obtaining a deeper understanding of the physics of individual ELM cycles.

Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

“…The first consists of driving non-linearly saturated modes at the edge that, being coupled to the most unstable peeling-ballooning mode, prevent the onset of type-I ELMs; techniques based on this approach are the Quiescent High (QH) mode regime [8] and the Resonant Magnetic Perturbation (RMP) [9]. The second approach is based on triggering ELMs when the pedestal has lower energy than that at the peeling-balloning limit, so that the resulting heat fluxes are below the damage threshold of PFCs; such techniques, known as ELM pacing, generate small and frequent ELMs and are routinely executed via pellet [10] and gas [11] injection, vertical stabilization control [12], or by means of modulated Electron Cyclotron Heating absorbed near the plasma edge [13]. A third technique aims at developing confinement scenarios that inherently provide necessary energy confinement without the presence of a particle confinement barrier, and thus avoiding the necessity for ELMs for impurity control while simultaneously keeping plasma edge below the peelingballooning stability limits.…”

An optimal real-time controller for vertical plasma stabilization

“…The vertical stabilization controller was implemented and tested using one of the hardware modules with parallel digital signal processing capabilities of the Advanced Plasma Control System [31]. For further testing of the controller it is envisaged the use of an ELM detector [32] capable of signaling the error and unavailability of plasma position observer. It is also planned the controller implementation in a newer control hardware based on FPGA [33] to study and compare the performance of both systems.…”

Real-time control of ELM and sawtooth frequencies: similarities and differences

“…The bursting nature of sawteeth and ELMs means that two distinct control strategies can be considered. In pacing/locking control, exemplified by pellet ELM triggering [30][31][32][33], Radial Field 'kicks' [34][35][36][37][38] and ELM pacing using pulsed ECRH [39], the plasma is perturbed periodically at the desired frequency. Continuous control schemes, exemplified by Electron or Ion Cyclotron heating control of sawteeth [40][41][42][43][44][45][46][47][48][49][50][51][52][53], modify the underlying plasma parameters which determine the collapse frequency.…”