H-mode grade confinement with L-mode edge in negative triangularity plasmas on DIII-D

Alessandro Marinoni

MIT

Friday, February 22, 2019

3:00pm

NW17-218

PSFC Seminars

 Abstract: Plasmas with Negative Triangularity (NT) shape on the DIII-D tokamak sustain H-mode level confinement and high normalized beta (H98,y2  = 1.3, ßN  = 2.6) for several energy confinement times, despite featuring edge pressure profiles typical of an L-mode plasma without Edge Localized Modes. This work builds upon previous results from the TCV tokamak, which showed that the energy confinement time of collisionless, L-mode plasmas subject to pure electron heating doubles when reversing triangularity with other parameters held fixed. The DIII-D experiments investigated NT plasmas using both pure electron (EC) and mixed ion-electron (EC-NB) heating, thus exploring for the first time a more reactor relevant regime where Te ~Ti . Compared to matched discharges at positive triangularity (PT), in both heating regimes NT plasmas feature 30% increase in stored energy and lower intensity of density and temperature fluctuations. A linear gyrokinetic analysis indicates that these plasmas are dominated by Trapped Electron Modes at ion scale but, unlike the TCV discharges, electron scale fluctuations are active in the core. Growth rates are predicted to decrease at NT at ion scales, with the largest decrease with EC only heating. In the high power phase, NT plasmas with L-mode edge maintain confinement levels comparable to those of PT plasmas that operate in an ELMy Hmode regime at the same heating power, corroborating their H-mode grade confinement. Additionally, the plasmas at NT produced 30% more neutrons than the PT counterpart, which a TRANSP analysis shows to be due to lower impuritycontent. These results indicate that NT may prove to be a promising candidate for reactor scenarios owing to high core confinement, low impurity content and ELM-free characteristics.

*Work supported by the US Department of Energy under DE-FG02-94ER54235 and DE-FC02-04ER54698

Bio: Dr.Marinoni is presently working as part of the Burning Plasma Physics at General Atomics, where his MIT appointment is located. His primary duties include operate the Phase Contrast Imaging diagnostic to detect density fluctuations on the DIII-D tokamak, data analysis and modeling, planning and support experiments, development of new diagnostic to detect high frequency RF waves as well as electron scale fluctuations. Prior to coming toMIT in 2011, Dr.Marinoni conducted his Ph.D. research at Ecole Polytechnique Fédérale de Lausanne (EPFL, Switzerland) under the supervision of S. Coda, developing advanced turbulence diagnostics to detect electron scale fluctuations and also studying the effect of negative triangularity on the non-linear gyrokinetic stability and transport of tokamak plasmas. In the latter work it was found that, due to a rather complex modification of the toroidal precessional drift of trapped particles in phase space, collisionless plasmas at negative triangularity feature lower intensity of fluctuations and associated thermal transport than matched discharges at positive triangularity, in agreement with experiments on the TCV tokamak. His doctoral thesis was awarded the 2011 IBMResearch Award for computational research at EPFL. After spending one year performing Monte-Carlo modeling of Generation IV fast spectrumreactors at the Paul Scherrer Institute in Switzerland, Dr.Marinoni decided to return tomagnetic fusion research by joiningMIT in 2011. Starting as a post-doctoral associate and then being promoted to staff scientist, his research focused onthe characterization of density fluctuations and thermal transport in advanced scenarios on the DIII-D tokamak, primarily by collecting and interpreting experimental data aided by gyro-kinetic modeling.