Neeraj Kumar’s PhD Thesis Defense

29 janvier 2021 par Eric Rostang
Neeraj Kumar, doctorate student in the DSC team, will defend his thesis on February 02 at 10 am, Paris time.
Supervisors:  S. Benkadda, C. Bourdelle, Y. Camenen and A. Loarte

Discipline: Physics and material sciences & Speciality: Energy, Radiation and Plasma

Jury composition: 
M. Clemente ANGIONI –  Max-Planck-institut fuer Plasmaphysik (IPP), Garching, Germany – Referee
Mme Paola MANTICA – National Research Council (CNR), Milano, Italy – Referee
M. Jonathan CITRIN – Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven, Netherlands – Examiner
M. Francis CASSON – UK Atomic Energy Authority (UKAEA), CCFE, Oxfordshire, UK – Examiner
M. Alberto LOARTE – ITER Organization, France – Examiner
Mr. Yann CAMENEN – National Center for Scientific Research (CNRS), Aix-Marseille University, France – Examiner

Mrs Clarisse BOURDELLE – IRFM, CEA M. Sadruddin BENKADDA – National Center for Scientific Research (CNRS), Aix-Marseille University, France – Thesis directorCadacache, France – Thesis co-director

One of the major goals of the ITER project is to demonstrate high fusion power gain in a tokamak. In ITER, metallic plasma-facing components are chosen for their low tritium retention and ability to sustain high heat loads. However, tokamaks operation with metallic plasma-facing components raises issues regarding the control of high-Z impurities since the accumulation of heavy impurities such as tungsten (Z=74) in the plasma core leads to significant radiation losses and deteriorates the energy confinement. Transport of tungsten (W) in the central part of ITER ( ρ < 0.3), is expected to be determined by neoclassical and turbulent processes, which strongly depend on the main ion density, temperature, and rotation profiles. Thus, a fine understanding of the dominant transport mechanisms in the central part is crucial to accurately predict W core accumulation. Previous studies mostly focused on the edge and core regions (ρ > 0.3), and the central part remains relatively unexplored so far.


In the central region, the gradients of density and temperature get smaller and, as a consequence, the level of turbulence may be reduced. In this region, a key question is therefore whether the plasma is linearly unstable. If yes, is turbulent diffusion sufficient to offset the neoclassical (inward) pinch of W, up to which radius and how sensitive is this to the background gradients? An auxiliary question is whether the quasi-linear approximation is valid in the inner core and up to which degree standard reduced quasi-linear models such as QuaLiKiz (QuasiLinear gyroKinetic) or TGLF (trapped gyro-Landau-fluid) can be used in the central zone. Understanding turbulent transport in the central region is crucial to predict core profile peaking that in turn will impact the fusion reactions and the tungsten neoclassical transport, in present devices as well as in ITER. The goal of this thesis is to address these questions and test the available turbulent transport models in the central region for existing tokamaks before applying them to evaluate turbulent transport in ITER.


Turbulent transport is investigated in the central region of the high-β JET hybrid H-mode discharge 75225 by means of linear and non-linear gyro-kinetic simulations using the gyro-kinetic code GKW in the local approximation limit. Compared to previous work, the analysis is extended towards the magnetic axis, ρ < 0.3, where the turbulence characteristics remain an open question. In contrast to the region ρ > 0.3 where Ion Temperature Gradient modes are the most unstable modes, the linear stability analysis indicates that Kinetic Ballooning Modes (KBM) dominate in the central region. A dedicated analysis performed at ρ = 0.15 reveals that the main parameters responsible for the destabilisation of KBMs in these hybrid H-modes are the high β and low magnetic shear values. The KBMs are driven by the main ion pressure gradient with little influence of the electron temperature gradient. Including fast-ions as a kinetic species in the simulations has a slight stabilising effect.  The study is then extended to the non-linear regime. It is found that the turbulence induced by these KBMs drives a significant ion and electron heat flux. Interestingly, linearly stable micro-tearing-modes (MTM) are excited non-linearly and drive a sizeable magnetic flutter electron heat flux. Standard quasi-linear models are compared to the non-linear results. The standard reduced quasi-linear models work reasonably well for the  E x B fluxes, but fail to capture magnetic flutter contribution to the electron heat flux induced by the non-linear excitation of the MTMs. An extension of the quasi-linear models is proposed allowing to better capturing the magnetic flutter flux.


Keywords: plasma, magnetic fusion, turbulence transport, quasi-linear, gyrokinetic, tokamak