Turbulence in Alcator C-Mod and Wendelstein 7-AS plasmas during controlled confinement transitions
N.P.Basse, S.Zoletnik1, M.Endler2, E.M.Edlund, C.L.Fiore, M.J.Greenwald, A.E.Hubbard, J.W.Hughes, J.H.Irby, G.J.Kramer3, L.Lin, Y.Lin, A.G.Lynn4, E.S.Marmar, D.R.Mikkelsen3, D.Mossessian, P.E.Phillips4, M.Porkolab, J.E.Rice,
J.A.Snipes, J.L.Terry, S.M.Wolfe, S.J.Wukitch, K.Zhurovich MIT Plasma Science and Fusion Center, Cambridge, USA
1 KFKI-RMKI, EURATOM-Association, Budapest, Hungary
2 IPP-Greifswald, EURATOM-Association, Greifswald, Germany
3Princeton Plasma Physics Laboratory, Princeton, USA
4University of Texas at Austin, Austin, USA
The role of plasma turbulence in confinement transitions, both induced and spontaneous, is currently being investigated in most magnetic confinement fusion devices. This paper is part of that continual effort, and focuses on controllable confinement transitions in tokamaks and stellarators.
At certain values of the edge rotational transform,ιa, the confinement quality of plasmas in the Wendelstein 7-AS (W7-AS) stellarator was found to react very
sensitively to small modifications of ιa. Since ιa could be changed reproducibly by e.g. a small plasma current, these transitions provided a method to perform a systematic investigation of differences in turbulence during ’good’ and ’bad’ confinement phases [1].
The macroscopic changes of confinement in W7-AS were attributed to the presence of low-order rationalιa-values in the plasma and the fact that W7-AS had small magnetic shear. Due to the larger shear in the Alcator C-Mod tokamak, confinement transitions associated with low-order rational qa-values would therefore be expected to be local instead of global.
In this paper we present a comparative study of turbulence observed during controlled confinement transitions in the Alcator C-Mod tokamak and the W7-AS stellarator. In both cases, the transitions were created by ramping the plasma current. Fluctuations in the electron density of Alcator C-Mod plasmas were measured using the recently
upgraded reflectometry [2] [3] and phase-contrast imaging (PCI) [4] diagnostics. This data was in turn correlated with magnetic, Dα and electron temperature fluctuations.
References
[1] S.Zoletnik, N.P.Basse, M.Saffman et al., Plasma Phys. Control. Fusion 44, 1581 (2002)
[2] Y.Lin, J.Irby, P.Stek et al., Rev. Sci. Instrum. 70, 1078 (1999)
[3] N.P.Basse, Y.Lin, G.J.Kramer et al., 45th APS-DPP Conference (2003) [4] A.Mazurenko, M.Porkolab, D.Mossessian et al., Phys. Rev. Lett. 89, 225004
(2002)
