Christian Gabriel Theiler | EPFL Graph Search

Christian Theiler obtained his Master’s degree in physics from ETH Zurich in 2007 and his PhD from EPFL in 2011. He then joined MIT as a postdoctoral associate to work on the Alcator C-Mod tokamak. In 2014, he returned to EPFL as a EUROfusion fellow, to join the TCV tokamak team. Two years later, he was named Tenure Track Assistant Professor in Plasma Physics at EPFL. Christian’s research focuses on tokamak boundary physics and related diagnostic techniques. He has contributed to the understanding of the formation, propagation, and control of turbulent plasma structures, called blobs, and gained new insights on the structure of transport barriers in the plasma periphery in different high-confinement regimes. His current research focuses on detachment physics and turbulence characteristics in conventional and alternative divertor magnetic geometries.

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Basil Duval, Holger Reimerdes, Christian Gabriel Theiler, Kevin Henricus Annemarie Verhaegh

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms has been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium, indicating a molecular ion density (D-2(+)) that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate (D-2 + D+ -> D-2(+) + D) for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the D-2(+) content by >x100. By disabling ion isotope mass rescaling: (1) the total ion sinks are more than doubled due to the inclusion of MAR; (2) the additional MAR causes the ion target flux to roll-over during detachment; (3) the total Da emission in the divertor increases during deep detachment by roughly a factor of four; (4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50% increase in neutral pressure; (5) total hydrogenic power loss is increased by up to 60% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over. Extrapolating simplified scalings of divertor molecular densities (TCV & MAST-U) to reactor-relevant densities suggests the underestimation of molecular charge exchange could strongly impact divertor physics (neutral atom density, ions sinks) and hydrogen emission (which has implications for detachment control) in deeply detached conditions, warranting further study.

IOP Publishing Ltd2023

Model-based impurity emission front control using deuterium fueling and nitrogen seeding in TCV

Basil Duval, Cristian Galperti, Christian Gabriel Theiler

This paper presents the first result using nitrogen-seeded exhaust feedback control of the NII impurity emission front in TCV. The NII emission front position is consistently located below its commonly used CIII counterpart, indicating the NII emission front is representative of a colder plasma region. We demonstrate control of the NII impurity emission front position for two cases: (a) using nitrogen seeding as the sole actuator, and (b) using deuterium fueling as an actuator while injecting a small amount of nitrogen that remains a trace impurity. For sole nitrogen actuation, peak target current density is significantly reduced when the NII emission front approaches the x-point (approximate to 50% for the NII front at the halfway point). When actuating with deuterium, peak target current density is less affected, which is explained by changes in fueling engendering a different scrape-off-layer plasma density. Perturbative (system identification) experiments show that nitrogen actuation induces a stronger, but slower, response of the NII emission front than deuterium actuation. Moving the NII emission front back to the target after pushing it towards the x-point has proven difficult, where both the NII front position and total radiated power do not reach pre-seeding conditions within the discharge time following termination of nitrogen injection. This result highlights the need to account for impurity retention for such seeded discharges in exhaust control strategies.

IOP Publishing Ltd2023

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Basil Duval, Holger Reimerdes, Christian Gabriel Theiler, Kevin Henricus Annemarie Verhaegh

IOP Publishing Ltd2023