Localized electron heating and current drive, like those produced by electron cyclotron heating (ECH) systems, are powerful tools for controlling the sawtooth period. They allow the direct modification of the plasma parameters which determine the sawtooth stability. In this paper we report a set of new experimental results obtained in the Tokamak à Configuration Variable (TCV) and a set of related simulations obtained applying a sawtooth period model in a transport code. The TCV device, equipped with a very flexible and powerful ECH system, is specifically suited for this kind of study. In previous works, the experimental behaviour observed in TCV and JET was found consistent with a sawtooth period model first proposed to predict the sawtooth period in burning plasmas. In this paper, new experimental results have motivated a set of simulations which allow the identification of the effects of localized heating and current drive separately. In particular, two heating locations exist at opposite sides of the q = 1 surface which allow most efficiently sawtooth stabilization and destabilization. Moreover, the modelling shows that counter- and co-current drive alone, without the presence of heating, have opposite effects on the sawtooth period at symmetrical locations as compared with the position of the q = 1 surface. The main features of the experimental behaviour can be explained as due to the modification of the local plasma parameters involved in the linear resistive stability threshold of the internal kink, in particular the dynamics of the magnetic shear at the q = 1 surface. However it is shown that the most effective locations to modify the sawtooth period are not exactly at q = 1.

Abstract.A new paradigm is presented to reconstruct the plasma current density profile in a tokamak in real-time. The traditional method of basing the reconstruction on real-time diagnostics combined with a real-time GradShafranov solver suffers from the difficulty of obtaining reliable internal current profile measurements with sufficient spatial and temporal accuracy to have a complete picture of the profile evolution at all times. A new methodology is proposed in which the plasma current density profile is simulated in real-time by solving the first-principle physics-based equations determining its evolution. Effectively, an interpretative transport simulation similar to those run today in post-plasma shot analysis is performed in real-time. This provides realtime reconstructions of the current density profile with spatial and temporal resolution constrained only by the capabilities of the computational platform used and not by the available diagnostics or the choice of basis functions. The diagnostic measurements available in real-time are used to constrain and improve the accuracy of the simulated profiles. Estimates of other plasma quantities, related to the current density profile, become available in real-time as well. The implementation of the proposed paradigm in the TCV tokamak is discussed, and its successful use in plasma experiments is demonstrated. This framework opens up the possibility of unifying q profile reconstructions across different tokamaks using a common physics model and will support a wealth of applications in which improved real-time knowledge of the plasma state is used for feedback control, disruption avoidance, scenario monitoring, and external disturbance estimation.

In the TCV tokamak, the m/n = 2/1 island is observed in low-density discharges with central electron-cyclotron current drive. The evolution of its width has two distinct growth phases, one of which can be linked to a "conventional" tearing mode driven unstable by the current profile and the other to a neoclassical tearing mode driven by a perturbation of the bootstrap current. The TCV results provide the first clear observation of such a destabilization mechanism and reconcile the theory of conventional and neoclassical tearing modes, which differ only in the dominant driving term.

In a fusion device, the so-called sawtooth instability can lead to the triggering of confinement limiting neoclassical tearing modes. On the other hand, the existence of sawteeth is desirable for the removal of one fusion product, i.e. helium ash from the plasma core. This has led to great interest in the control of sawteeth. The sawtooth period can be changed drastically by local modification of the q-profile. In this paper, the influence of the beam line geometry of the neutral beam injection in the ASDEX Upgrade tokamak will be presented as well as the effect of local electron cyclotron current drive. Systematic scans in the electron cyclotron current deposition from the high-field side to the lowfield side resolve areas with sawtooth stabilization and destabilization. These observations will be discussed, including modelling of the main results, with the ASTRA transport code constrained by experimental data.

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...