Magnetic shear effects on plasma transport and turbulence at high electron to ion temperature ratio in DIII-D and JT-60U plasmas. (30th March 2017)

• Record Type: Journal Article
• Authors: Yoshida, M.; McKee, G.R.; Murakami, M.; Grierson, B.A.; Nakata, M.; Davis, E.M.; Marinoni, A.; Ono, M.; Rhodes, T.L.; Sung, C.; Schmitz, L.; Petty, C.C.; Ferron, J.R.; Turco, F.; Garofalo, A.M.; Holcomb, C.T.; Collins, C.M.; Solomon, W.M.
• Is Part Of: Nuclear fusion. Volume 57:Number 5(2017:May)
• Publication Date: 2017-03-30
• Subjects: thermal transport -- electron heating -- magnetic shear -- rotation shear; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aa611e
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: Negative magnetic shear has been demonstrated in DIII-D and JT-60U to mitigate the confinement degradation typically observed with increasing the electron to ion temperature ratio ( T e / T i ). In recent experiments in DIII-D negative central magnetic shear (NCS) discharges, the thermal transport in the internal transport barrier formed around the radius of the minimum safety factor ( q min ) remained almost constant and modestly increased in the region outside of q min compared to the positive shear (PS) case, when T e / T i increased from about 0.8 to 1.1 through electron cyclotron heating (ECH). The benefit of NCS extending into the region outside of q min can be explained by the lower magnetic shear in the NCS plasma over the plasma radius relative to the PS plasma. Reduced confinement degradation at high T e / T i with NCS plasmas was commonly observed in DIII-D and JT-60U. The mechanism of the different transport responses between the NCS and PS plasmas has been assessed in terms of fluctuation measurements and gyrokinetic simulations in DIII-D; NCS gave a smaller rise in the low-wavenumber broadband turbulent fluctuations with the increase in T e / T i compared with the PS case. This is consistent with gyrokinetic simulations, which show a smaller rise in the growth rates of the ion temperature gradient mode in the NCS plasmas, with increasing T e / T i . Gyrokinetic simulations also showed a change in the stability of the electron modes with ECH applied, consistent with higher-wavenumber fluctuation measurements, although more detailed simulations are needed to give a quantitative explanation for the experimental observations. Control of q -profile and magnetic shear will allow confinement improvement in future machines with dominant electron heating.