In the last few decades, experiments on several medium-size tokamaks (TCV [1], DIII-D [2] and AUG [3]) have shown that giving the plasma a Negative Triangularity (NT) poloidal crosssectional shape can cause a reduction of turbulent transport with respect to Positive Triangularity (PT) plasmas, while also inhibiting the transition to H-mode. This allows a NT plasma to have an L-mode-like edge pressure profile and H-mode-like core pressure level. This is a promising configuration for a future reactor, where detrimental edge localized modes have to be avoided and high confinement times are needed. In order to explore the feasibility and the limits of NT, the Divertor Tokamak Test (DTT) facility, a novel superconducting tokamak under construction in Italy, is also considering a Negative Triangularity option for the full power scenario. Within an extensive framework of preliminary studies that involve both numerical modelling and experiments, during the 2022/2023 EUROfusion WPTE campaign two experimental sessions on TCV have been dedicated to testing the feasibility of such a scenario in DTT and tuning the parameters in order to optimize it. The reference PT and NT magnetic equilibria envisioned for DTT have been reproduced in TCV and three different heating mixes, i.e. NBI, NBI/ECRH and ECRH, have been applied in order to access different turbulent regimes. In order to perform a thorough comparison, within a fixed heating mix three scenarios have been considered: a NT-PT L-mode pair with the same injected power and a high power PT H-mode scenario. Independently of the heating mix, NT L-mode discharges proved to perform much better compared to PT L-mode ones with the same heating power and were also able to reach the central values of thermal pressure similar to those of PT H-mode shots. Looking at the logarithmic gradients of the temperature and density profiles, it can be seen clearly that all the beneficial influence of NT is limited to a radial interval = [0.8 - 1.0], where the gradients are indeed very large and where the absolute value of triangularity is still sufficiently high. This suggests that the reduction of turbulent transport in a NT plasma is not strongly affected by the nature of the turbulent regime, i.e. whether it is ITG or TEM dominated. Finally, preliminary predictive simulations performed with ASTRA-TGLF [4, 5] and local gyrokinetic ones performed with GENE [6] at a fixed radial location ( = 0.85) seem to be able to reproduce the beneficial effect of NT on confinement. More extensive gyrokinetic simulations are still ongoing and will be compared with previous predictive simulations performed for DTT by the authors.
Experiments and gyrokinetic simulations of TCV plasmas with negative triangularity in view of DTT operations
Mantica P;Innocente P;
2023
Abstract
In the last few decades, experiments on several medium-size tokamaks (TCV [1], DIII-D [2] and AUG [3]) have shown that giving the plasma a Negative Triangularity (NT) poloidal crosssectional shape can cause a reduction of turbulent transport with respect to Positive Triangularity (PT) plasmas, while also inhibiting the transition to H-mode. This allows a NT plasma to have an L-mode-like edge pressure profile and H-mode-like core pressure level. This is a promising configuration for a future reactor, where detrimental edge localized modes have to be avoided and high confinement times are needed. In order to explore the feasibility and the limits of NT, the Divertor Tokamak Test (DTT) facility, a novel superconducting tokamak under construction in Italy, is also considering a Negative Triangularity option for the full power scenario. Within an extensive framework of preliminary studies that involve both numerical modelling and experiments, during the 2022/2023 EUROfusion WPTE campaign two experimental sessions on TCV have been dedicated to testing the feasibility of such a scenario in DTT and tuning the parameters in order to optimize it. The reference PT and NT magnetic equilibria envisioned for DTT have been reproduced in TCV and three different heating mixes, i.e. NBI, NBI/ECRH and ECRH, have been applied in order to access different turbulent regimes. In order to perform a thorough comparison, within a fixed heating mix three scenarios have been considered: a NT-PT L-mode pair with the same injected power and a high power PT H-mode scenario. Independently of the heating mix, NT L-mode discharges proved to perform much better compared to PT L-mode ones with the same heating power and were also able to reach the central values of thermal pressure similar to those of PT H-mode shots. Looking at the logarithmic gradients of the temperature and density profiles, it can be seen clearly that all the beneficial influence of NT is limited to a radial interval = [0.8 - 1.0], where the gradients are indeed very large and where the absolute value of triangularity is still sufficiently high. This suggests that the reduction of turbulent transport in a NT plasma is not strongly affected by the nature of the turbulent regime, i.e. whether it is ITG or TEM dominated. Finally, preliminary predictive simulations performed with ASTRA-TGLF [4, 5] and local gyrokinetic ones performed with GENE [6] at a fixed radial location ( = 0.85) seem to be able to reproduce the beneficial effect of NT on confinement. More extensive gyrokinetic simulations are still ongoing and will be compared with previous predictive simulations performed for DTT by the authors.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
