Sawtooth Pacing by Real-Time Auxiliary Power Control in a Tokamak Plasma

Abstract

In the standard scenario of tokamak plasma operation, sawtooth crashes are the main perturbations that can trigger performance-degrading, and potentially disruption-generating, neoclassical tearing modes. This Letter demonstrates sawtooth pacing by real-time control of the auxiliary power. It is shown that the sawtooth crash takes place in a reproducible manner shortly after the removal of that power, and this can be used to precisely prescribe, i.e., pace, the individual sawteeth. In combination with preemptive stabilization of the neoclassical tearing modes, sawtooth pacing provides a new sawtooth control paradigm for improved performance in burning plasmas.PACS numbers: 52.35Py,52.50. Sw,52.55.Fa In high performance tokamaks, the plasma β (the ratio of plasma pressure to magnetic pressure) is often limited by metastable magnetohydrodynamic (MHD) instabilities known as neoclassical tearing modes (NTMs) [1]. These modes are of special concern for fusion-reactorgrade tokamak plasmas since the islands generated in the confining magnetic topology grow to a size, even at relatively low β, that they must be mitigated, or avoided altogether, to allow sufficiently economic power generation. Fortuitously, these modes will self-stabilize if the island is reduced below a certain size, and moreover they require a seed island to begin to grow. The main perturbation that triggers these modes in the standard tokamak scenario is the sawtooth core relaxation event [2,3] that can occur when the safety factor q is below one (q is the rate of change of the toroidal flux with the poloidal flux). At the "crash" of the sawtooth, particle and magnetic energy are redistributed from inside to outside the surface where q = 1 and the liberation of this energy can seed MHD modes at resonant surfaces where q = m/n is rational (m and n are the integer poloidal and toroidal mode numbers, respectively), in particular at the q = 3/2 and q = 2/1 surfaces. Large 2/1 modes can also lead to plasma current disruptions, which might damage a reactor.NTM avoidance is concerned with eliminating or reducing the triggering mechanisms. The scenario foreseen in ITER to reach the optimal fusion power, the socalled standard scenario, is a sawtoothing plasma with long sawtooth periods resulting from the strong fastpartical stabilization inherent in burning plasmas [4,5]. It has been shown in many tokamaks that the crashes of long-period sawteeth can trigger NTMs even at low β [6,7]. Therefore, the control of the sawtooth period is crucial for NTM avoidance and emphasis has been placed on shortening the period. It has been demonstrated in the tokamakà configuration variable (TCV) [8] and Tore Supra [9] that the sawtooth period can be controlled by feedback-positioning very-locally absorbed EC waves relative to the q = 1 surface. However, the dependence of the sawtooth period on the the absorption location is extremely non-linear. We propose a new approach for the control of the sawtooth period, that of sawtooth pacing by power control. The method provides a...

Real-time control of multiple MHD instabilities on TCV by ECRH/ECCD

“…In this case, control by changing the deposition location has its origin in varying the time rate of change of the magnetic shear. Sawtooth pacing [2], instead, relies on removing the stabilizing power at q = 1 during the sawtooth cycle. This causes the shear to increase more rapidly and the crash threshold to be attained sooner.…”

“…The powerful (4.5MW) and flexible (7 steerable launcher) EC system on TCV has recently been complemented by an equally flexible digital real-time control system with the aim of developing and testing integrated MHD control methods [1]. Sawtooth pacing is one such method [2]. The crash time of stabilized sawteeth can be precisely controled by removing the EC power at a given time after the last sawtooth crash, causing the crash to occur at a short and reproducible time thereafter.…”

Control of sawtooth via ECRH on EAST tokamak

“…According to the sawtooth model first proposed by Porcelli et al 11 and then modified in Tokamak a Configuration Variable (TCV) experiments, 8 the triggering condition of 1/1 internal kink mode that induces sawtooth crash can be written in the form…”

“…8 Using the safety factor profile provided by EFIT, two series of magnetic shear at q ¼ 1 surface is calculated at shot 54424. One at a time interval of 0.1 s (3.300 s, 3.400 s, and 3.500 s), and the other with a much shorter time scale (3.585 s, 3.595 s, 3.605 s, 3.610 s, and 3.615 s).…”

“…These profiles play an important role in the sawtooth behavior. [5][6][7][8][9][10] According to a widely accepted sawtooth model proposed by Porcelli,11 sawtooth crashes are triggered if magnetic shear s 1 ¼ r 1 dq/dr at the q ¼ 1 surface exceeds a certain critical value s 1,crit , where s 1,crit , defined at the same q ¼ 1 magnetic surface, is determined by ion/electron temperature, ion Larmor radius, electron/ion diamagnetic frequencies, and the collisionality regime. By changing the poloidal and toroidal injection angle of ECRH system within one single shot or between different shots, we can modify a) Email: lqxu@ipp.cas.cn the ECRH deposition position.…”