Speaker
Alessandro Marinoni
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1111.pdf
Reduced fluctuations in high confinement plasmas at negative
triangularity on DIII-D∗
A. Marinoni1 , M.E. Austin2 , M.L. Walker3 , E.M. Davis1 , A.W. Hyatt3 , C.C. Petty3 ,
M. Porkolab1 , J.C. Rost1 , O. Sauter4 , K.E. Thome5 and the DIII-D Team
1 Plasma Science & Fusion Center, MIT, Cambridge (USA)
2 University of Texas-Austin, Austin (USA) 3 General Atomics, San Diego (USA)
4 Swiss Plasma Center, EPFL, Lausanne (CH)
5 Oak Ridge Associated Universities, Oak Ridge (USA)
Plasmas with negative triangularity (δn ) shape on the DIII-D tokamak are characterized by
high confinement and normalized beta (H98,y2 = 1.2, βN = 2.6), despite featuring edge pressure
profiles typical of an L-mode plasma without Edge Localized Modes.
This work was inspired by previous results from the TCV tokamak [1], where the energy con-
finement time of collisionless (νeff ∼ 0.2) L-mode plasmas subject to pure Electron Cyclotron
Heating (ECH) was shown to double when reversing δ , with other parameters held fixed. The
experiments on DIII-D investigated δn plasmas at moderate collisionalities (νeff ' 0.5) in both
pure ECH and mixed ion-electron (ECH/NBI) heating regimes, thus exploring for the first time
a more reactor relevant regime where Te ∼ Ti . For both heating schemes, plasmas at δn show up
to 30% increase in stored energy compared to discharges with matched actuators but positive
δ (δ p ). In the high power phase, during which δ p plasmas develop an H-mode pedestal, the δn
matched discharges produced 30% more neutrons than the δ p counterpart, which a TRANSP
analysis shows to be due to a reduced main ion dilution owing to lower impurity content. The
intensity of density fluctuations measured by the Phase Contrast Imaging (PCI) diagnostic [2]
is seen to decrease by 50% and 30%, respectively, in the two heating phases at δn . The relative
intensity of fluctuations at positive and negative wavenumbers is seen by the PCI to depend
on the shape. A linear gyro-kinetic analysis indicates that, in both heating schemes, discharges
are dominated by Trapped Electron Modes at ion scale but, unlike the TCV discharges, strong
electron scale fluctuations are present in the core. Growth rates are seen to decrease at δn in the
spectral region kθ ρs < 2, with the largest decrease seen in the ECH-only phase.
References
[1] Y. Camenen et al., Nucl. Fusion 47 (2007) 510
[2] J.R. Dorris et al., Rev. Sci. Instrum. 80 (2009) 023503
∗ Work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using
the DIII-D National Fusion Facility under Award DE-FC02-04ER54698, and under Award DE-FG02-94ER54235.
