PhD in Mitigating the plasma contamination by high-Z impurities produced close to radio-frequency antennas in fusion reactors

Supervisor: Dr. J. Moritz, Université de Lorraine, Institut Jean Lamour, Fusion plasmas team

Co-supervisor: Dr. G. Urbanczyk, Université de Lorraine, Institut Jean Lamour, Fusion plasmas team

Partner Manager: Dr. V. Bobkov, Max-Planck-Institut für Plasmaphysik, Garching, Germany

Contact: jerome.moritz@univ-lorraine.fr

Thesis Title

Mitigating the plasma contamination by high-Z impurities produced close to radio-frequency antennas in fusion reactors.

Thesis Subject

For fusing a mix of Deuterium - Tritium (D-T) in the core of fusion reactors, a plasma temperature of about 150 million degrees has to be reached and maintained in order to overcome the Coulomb repulsion of positive ions. This can be achieved thanks to different heating systems disposed around the vacuum vessel. Ion cyclotron resonance heating (ICRH) is one efficient way to heat ions by launching electromagnetic waves from the plasma edge at a frequency which can be absorbed by ions located in the core plasma. These waves are in the radio spectrum (30-100MHz) and are produced by large antennas. For ITER, the world largest tokamak in construction in south-east France, an ICRH antenna will deliver up to 20MW of power to the plasma.

For maximizing the coupling with ions in the core of the reactor, ICRH antennas have to be placed as close as possible to the plasma, which also makes them interact strongly with the plasma edge. This interaction leads to a positive electrostatic biasing of the plasma driven by ICRH fields, called “sheath rectification”, of up to several hundred Volts. Ions accelerated in such a potential drop can overcome the sputtering threshold of the antennas materials and pollute the plasma often with high-Z metallic impurities, which is very detrimental for the reactor operation if the concentration of these impurities is larger than 10-4.

Two possible ways to mitigate the plasma pollution due to the RF sheath rectification are to i) inject a neutral gas at the antenna proximity in order to increase the plasma density and reduce the electron temperature, ii) modify the antennas design and excitation to reduce their interaction with the plasma.

During the two first years of the thesis, the candidate will participate to the development of a new hybrid particle-in-cell / Direct Simulation Monte Carlo code in order to simulate and understand the role of light impurities very often present in tokamak discharges such as Oxygen, Carbon, Boron and even Helium with different charge states on the sputtering of antennas and wall materials (in particular tungsten [W]). As raised in point i), the effect of the neutrals injection and plasma temperature will be clearly investigated for the first time and the role of ionized W redeposition evaluated. This work will be carried out at Institut Jean Lamour in the Fusion plasmas team which has a long experience in particle-in-cell and gyrokinetic simulations of sheath, edge and core plasmas. The simulation work will be complemented by an experimental counterpart at Max-Planck-Institut für Plasmaphysik in Garching (Germany) where the tokamak “ASDEX Upgrade” is operated. With its full W first wall, flexible ICRH system and mature diagnostics – in particular spectroscopy and charge exchange monitoring most impurities – ASDEX Upgrade is a perfect experimental device to investigate RF sheath physics and induced plasma pollution. Several stays by the candidate at IPP Garching are planned during the three years of the thesis in order to select experimental data for input parameters and benchmarking the simulations. During the third year of the thesis, the candidate will work on the determination of equivalent RF sheath impedance by means of particle-in-cell simulations. This work will allow the calculation of specific boundary conditions for electromagnetic codes such as Petra-M, RAPICASOL, SSWICH-SW and for different plasma conditions (temperature, density, magnetic field strength and angle with the antenna’s surface). As stated in ii), these electromagnetic codes used to optimize the design of ICRH antennas in current and future fusion devices will benefit of a better understanding of the plasma edge behavior with respect to RF fields.