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Phone: +46 31 7723180

Email: tunde_at_chalmers.se

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Dept of Applied Physics
Chalmers University of Technology
SE-412 96 Göteborg
Sweden

Transport in Impure Fusion Plasmas

The goal of magnetic confinement fusion is ultimately to sustain an enormous temperature difference between the core and the edge of the fusion plasma. For efficient fusion energy production, it is necessary to minimize the energy transport through the confining magnetic surfaces, and at the same time, control particle transport, so as to maintain the “burning” plasma by continuous fueling and remove helium-ash and impurities. Particles and heat are transported through collisional processes (that is called neoclassical transport) and plasma turbulence driven by small scale unstable waves (microinstabilities). These phenomena can be studied using gyro-phase averaged kinetic equations, with the so called drift kinetic- and gyrokinetic formalisms. When studying transport in fusion plasmas our group tries to find a balance between using both computationally demanding gyrokinetic simulation codes, such as the world-leading nonlinear tokamak microturbulence package GYRO, and analytic models that give insights, complement and assist the interpretation of simulation results.

Our group have two main areas of focus in transport theory; the transport of impurities (i.e. ions present in the plasma other than fusion fuel), and transport in transport barriers.

Impurities in the core can dilute the fusion fuel and more importantly they can lead to significant radiative energy losses and potentially to radiative instabilities and even plasma disruption, thus it is first priority in any fusion experiment to avoid their accumulation in the core. However their presence in the plasma edge might even be beneficial since they can help homogenizing the heat load on the plasma facing components by forming a radiative belt. Thus understanding, and as much as it is possible, actively controlling impurity transport is of high importance.

Transport barriers are regions in tokamaks where the turbulent transport is strongly reduced; these barriers are formed either spontaneously, as the so called H-mode pedestal, or being triggered artificially, as the internal transport barriers. Even though operation in improved confinement modes, such as H-mode, is considered to be crucial for achieving reactor relevant energy confinement, the physics of transport barrier formation and the transport within or in the vicinity of the barriers is far from being theoretically understood. One of the reasons for the theoretical difficulties lies in the fact that the typical radial length scales of plasma parameters (e.g. electron density and temperature) can be almost as small as the radial extent of the particle orbits.

Some of the specific problems addressed by the group are:

- What impact poloidal asymmetries in tokamaks have on impurity transport? Poloidally asymmetric electrostatic fields and impurity distributions appear in tokamaks due to centrifugal effects, neoclassical effects and under radio frequency heating.

- How is the collisional transport modified in transport barriers? Since turbulent transport is reduced in transport barriers, the role of collisional transport is more important there. Also, neoclassical flows and currents affect the micro- and macro stability of the barrier. To address related questions we use an improved kinetic formalism that can handle the steep gradients and high radial electrostatic fields present in the pedestal.

The projects are funded by Vetenskapsrådet via a Framework Grant on Strategic Energy Research "Research for future fusion reactors: using and avoiding impurities" (VR-grant 2014-5392, PI Tünde Fülöp) and the International Career Grant "Integrative modelling of transport barriers in fusion reactors" (VR-grant 2014-6313, PI Istvan Pusztai).

Project participants: István Pusztai, John Omotani, Sarah Newton, Stefan Buller, George Wilkie, Tünde Fülöp. Collaborators: Yevgen Kazakov (LPP/ERM Belgium), Jeff Candy (General Atomics), Peter Catto (MIT), Matt Landreman (University of Maryland), Per Helander (Max-Planck IPP, Greifswald).