Speaker
Description
Next italian high performance tokamak, Divertor Tokamak Test (DTT)[1], will be equipped with a electron cyclotron heating and current drive [2] (ECH/CD) capable of delivering to the plasma the power of 32 gyrotrons at 170 GHz, 1 MW. The main objectives [3] of this system are sustaining plasma current, EC assisted start up, bulk heating during the flat-top phase, and to control MHD and Neoclassical Tearing Modes (NTMs).
The design of the ECH/CD system is almost complete however some details can still be optimized as, for example, the focusing of the beams inside the plasma.
Uncontrolled NTMs (in particular the m/n = 2/1 and 3/2 modes) can substantially reduce plasma performance and lead to mode locking and disruption if it is not actively suppressed. ECH/CD is a well-established technique to stabilize NTMs by depositing localized power at the resonant surface. Four launchers are placed in an upper position of the tokamak to have access to the rational surface locations where NTM can develop, and will be equipped with the power of 8 out of the 32 sources. The injected power provides a stabilizing effect through both the direct current drive and the modification of the current profile induced by local heating. The effectiveness of these mechanisms depends on plasma parameters, launcher injection settings (toroidal/poloidal angles) and launcher optics (beam focusing).
In this work we study how the EC beam width affects the physics of stabilization: for narrow deposition the suppression is predominantly current–drive dominated, while for broader beams the stabilization increasingly relies on heating, with a corresponding increase of the power needed for the suppression. As a drawback, narrower beams will require improved resolution in the alignment of ECW deposition with the island. In order to evaluate how changes in beam width impact the NTM control, we use a reduced model based on the generalized Rutherford equation, constrained by parameters from integrated scenario simulations.
The resulting framework provides also a compact tool to simulate how different control strategies can operate in DTT, under different conditions. Final aim of this presentation is to investigate how different beam widths impact NTM control, through strategies as search and suppress, sweeping, and also applications of reinforcement learning. These results illustrate how an appropriate choice of focusing can optimize the NTM control, defining an effective range of EC beam widths that enhances desirable features of the suppression trajectory in the DTT full–power scenario.
[1] F. Romanelli et al. 2024 Nucl. Fusion 64, 112015.
[2] S. Garavaglia et al. 2026 Fusion Engineering and Design 222 115488.
[3] G. Granucci et al. 2024 Nucl. Fusion 64 126036
