Speaker
Jerry Hughes
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1092.pdf
A critical edge ion heat flux for L-H transition from combined analysis using
Alcator C-Mod and ASDEX Upgrade tokamaks *
J.W. Hughes1, M. Schmidtmayr2, F. Ryter3, E.A. Tolman1, N. Cao1, A.J. Creely1, N. Howard1,
A.E. Hubbard1, Y. Lin1, A. Mathews1, M.L. Reinke4, J.E. Rice1, E. Wolfrum3, S. Wukitch1,
Alcator C-Mod Team and ASDEX Upgrade Team
1
MIT Plasma Science and Fusion Center, Cambridge MA, USA; 2 Institute of Applied Physics,
TU Wien, Fusion@ÖAW, Vienna, Austria; 3 Max-Planck-Institut für Plasmaphysik, Garching,
Germany; 4 Oak Ridge National Laboratory, Oak Ridge TN, USA
Experimental studies of the transition from L-mode to H-mode confinement (L-H) on Alcator
C-Mod and ASDEX Upgrade (AUG) strengthen the basis for projecting power requirements for
future fusion devices. On C-Mod, L-H experiments at toroidal field 𝐵𝐵𝑇𝑇 of 4.0—7.8T reveal that
H-mode power threshold 𝑃𝑃𝑡𝑡ℎ accords roughly with projections from a scaling law used to
determine power needs for ITER. However, as on AUG, the scaling law does not capture the
experimental density dependence of 𝑃𝑃𝑡𝑡ℎ at low normalized density 𝑛𝑛�⁄𝑛𝑛𝐺𝐺 . Furthermore, at
higher line averaged density 𝑛𝑛� the inferred experimental 𝑃𝑃𝑡𝑡ℎ does not scale as strongly with 𝐵𝐵𝑇𝑇
as the scaling law indicates [1]. We can partially resolve these discrepancies by performing
transport and power balance analysis of C-Mod plasmas just prior to L-H transitions [2].
Analysis confirms and extends a key result found on AUG: a critical value of surface-integrated
ion heat flux per particle 𝑄𝑄𝑖𝑖 ⁄𝑛𝑛� is necessary to enable the transition from L-mode to H-mode [3].
The analysis of C-Mod data indicates that 𝑄𝑄𝑖𝑖 at the L-H transition not only increases linearly
with 𝑛𝑛� but also with 𝐵𝐵𝑇𝑇 . The 𝑛𝑛�, 𝐵𝐵𝑇𝑇 scalings are not necessarily reflected in the experimental
total L-H power because of changing balance of edge electron and ion heat fluxes, which
depends in turn on the auxiliary heating scheme and the strength of electron-ion equilibration.
Combining data from C-Mod and AUG yields a general expression for the edge ion heat flux at
the L-H transition, 𝑄𝑄𝑖𝑖 /𝑆𝑆 ∝ 𝑛𝑛�1.07 𝐵𝐵𝑇𝑇 0.76 , where 𝑆𝑆 is the plasma surface area. This result is
consistent with a critical shear in edge 𝐸𝐸 × 𝐵𝐵 being necessary for H-mode access, and can
explain the 𝑛𝑛�, 𝐵𝐵𝑇𝑇 , and 𝑆𝑆 dependencies in the 𝑃𝑃𝑡𝑡ℎ scaling law, providing an additional means of
extrapolating H-mode power requirements to ITER.
[1] E.A. Tolman, NF 58 (2018) 046004; [2] M. Schmidtmayr NF (2018); [3] F. Ryter, NF 54 (2014) 083003.
*
Supported by US Department of Energy awards DE-FC02-99ER54512, DE-SC0014264 and by National Science
Foundation Graduate Research Fellowship under Grant No. 1122374. This work has been partly carried out within
the framework of the EUROfusion Consortium and has received funding from the Euratom research and training
program 2014-2018 under grant agreement No 633053. M. Schmidtmayr was a fellow of the Austrian Marshall
Plan Foundation.
