The role of the sheath in magnetized plasma turbulence and flows
Controlled nuclear fusion could provide our society with a clean,
safe, and virtually inexhaustible source of electric power production.
The tokamak has proven to be capable of producing large amounts of
fusion reactions by confining magnetically the fusion fuel at
sufficiently high density and temperature, thus in the plasma state.
Because of turbulence, however, high temperature plasma reaches the
outermost region of the tokamak, the Scrape-Off Layer (SOL), which
features open magnetic field lines that channel particles and heat
into a dedicated region of the vacuum vessel. The plasma dynamics in
the SOL is crucial in determining the performance of tokamak devices,
and constitutes one of the greatest uncertainties in the success of
the fusion program. In the last few years, the development of
numerical codes based on reduced fluid models has provided a tool to
study turbulence in open field line configurations. In particular, the
GBS (Global Braginskii Solver) code has been developed at CRPP and is
used to perform global, three-dimensional, full-n, flux-driven
simulations of plasma turbulence in open field lines.
Reaching predictive capabilities is an outstanding challenge that
involves a proper treatment of the plasma-wall interactions at the end
of the field lines, to well describe the particle and energy losses.
This involves the study of plasma sheaths, namely the layers forming
at the interface between plasmas and solid surfaces, where the drift
and quasineutrality approximations break down. This is an
investigation of general interest, as sheaths are present in all
laboratory plasmas.
This thesis presents progress in the understanding of plasma sheaths
and their coupling with the turbulence in the main plasma. A kinetic
code is developed to study the magnetized plasma-wall transition
region and derive a complete set of analytical boundary conditions
that supply the sheath physics to fluid codes. These boundary
conditions are implemented in the GBS code and simulations of SOL
turbulence are carried out to investigate the importance of the sheath
in determining the equilibrium electric fields, intrinsic toroidal
rotation, and SOL width, in different limited configurations. For each
study carried out in this thesis, simple analytical models are
developed to interpret the simulation results and reveal the
fundamental mechanisms underlying the system dynamics. The
electrostatic potential appears to be determined by a combined effect
of sheath physics and electron adiabaticity. Intrinsic flows are
driven by the sheath, while turbulence provides the mechanism for
radial momentum transport. The position of the limiter can modify the
turbulence properties in the SOL, thus playing an important role in
setting the SOL width.
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