Electron Transport in the Plasma Edge with Rotating Resonant Magnetic Perturbations at the TEXTOR Tokamak
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Beschreibung
Small three-dimensional (3D) magnetic perturbations can be used as a tool to control the
edge plasma parameters in magnetically confined plasmas in high confinement mode (”Hmode”)
to suppress edge instabilities inherent to this regime, the Edge Localized Modes
(ELMs). In this work, the impact of rotating 3D resonant magnetic perturbation (RMP)
fields on the edge plasma structure characterized by electron density and temperature
fields is investigated. We study a low confinement (L-mode) edge plasma (r/a > 0.9)
with high resistivity (edge electron collisionality ν∗
e > 4) at the TEXTOR tokamak. The
plasma structure in the plasma edge is measured by a set of high resolution diagnostics:
a fast CCD camera (Δt = 20μs) is set up in order to visualize the plasma structure in
terms of electron density variations. A supersonic helium beam diagnostic is established
as standard diagnostic at TEXTOR to measure electron density ne and temperature Te
with high spatial (Δr = 2 mm) and temporal resolution (Δt = 20μs). The measured
plasma structure is compared to modeling results from the fluid plasma and kinetic neutral
transport code EMC3-EIRENE.
A sequence of five new observations is discussed: (1) Imaging of electron density variations
in the plasma edge shows that a fast rotating RMP field imposes an edge plasma
structure, which rotates with the external RMP rotation frequency of |νRMP | = 1 kHz. (2)
Measurements of the electron density and temperature provide strong experimental evidence
that in the far edge a rotating 3D scrape-off layer (SOL) exists with helical exhaust
channels to the plasma wall components. (3) Radially inward, the plasma structure at
the next rational flux surface is found to depend on the relative rotation between external
RMP field and intrinsic plasma rotation. For low relative rotation the plasma structure
is dominated by a particle and energy loss along open magnetic field lines to the wall
components. For high relative rotation indications for a magnetic island acting as locally
confining sub-volumes are found. (4) For high relative rotation, the entire measured edge
plasma structure is shifted by π/2 toroidally with respect to the position modeled in vacuum
approximation. The latter two experimental findings are compatible with modeling
results of the underlying magnetic topology including plasma response obtained by a 4-
field drift fluid transport model. (5) A smaller shift is measured in front of the RMP coils.
This gives direct experimental evidence that the near field plasma structure is governed
by the competition between the RMP near field and the local plasma structure at the
next inward rational flux surface.
The results obtained are essential input for benchmarking models, which include plasma
response, in order to extrapolate the RMP imposed 3D plasma structure toward the next
step fusion experiment ITER. The measurements of the plasma structure presented indicate
that the underlying magnetic topology is rotation dependent and may therefore
stimulate direct measurements of the components of the magnetic field in future
Autor*in
Henning Stoschus