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Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 Podcast
Electron temperature and pressure at the edge of ASDEX Upgrade plasmas

Electron temperature and pressure at the edge of ASDEX Upgrade plasmas

Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05 vor 13 Jahren

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Understanding and control of the plasma edge behaviour are
essential for the success of ITER and future fusion plants. This
requires the availability of suitable methods for assessing the
edge parameters and reliable techniques to handle edge phenomena,
e.g. to mitigate 'Edge Localized Modes' (ELMs) --- a potentially
harmful plasma edge instability. This thesis introduces a new
method for the estimation of accurate edge electron temperature
profiles by forward modelling of the electron cyclotron radiation
transport and demonstrates its successful application to
investigate the impact of Magnetic Perturbation (MP) fields used
for ELM mitigation on the edge kinetic data. While for ASDEX
Upgrade bulk plasmas, straightforward analysis of the measured
electron cyclotron intensity spectrum based on the optically thick
plasma approximation is usually justified, reasonable analysis of
the steep and optically thin edge region relies on full treatment
of the radiation transport considering broadened emission and
absorption profiles. This is realized in the framework of
integrated data analysis which applies Bayesian probability theory
for joint analysis of the electron density and temperature with
data of different independent and complementary diagnostics. The
method reveals that in regimes with improved confinement
('High-confinement modes' (H-modes)) the edge gradient of the
electron temperature can be several times higher than that of the
radiation temperature. Furthermore, the model is able to reproduce
the 'shine-through' peak --- the observation of increased radiation
temperatures at frequencies with cold resonance outside the
confined plasma region. This phenomenon is caused by strongly
down-shifted radiation of Maxwellian tail electrons located in the
H-mode edge region and, therefore, contains valuable information
about the electron temperature edge gradient. The accurate
knowledge about the edge profiles and gradients of the electron
temperature and --- including the density information --- the
electron pressure allows a detailed study of plasma edge phenomena
like ELMs or the transition from 'Low-confinement mode' (L-mode) to
H-mode. It is shown how the application of non-axisymmetric MP
fields acts on the edge profiles of electron temperature, density
and pressure in H-modes with type-I and mitigated ELMs and during
the L-H transition. Compared to type-I ELMs, mitigated ELMs tend to
occur at higher edge densities, lower edge temperatures and reduced
edge pressure gradients. This parameter regime can be achieved by
strong gas fuelling. MP fields might support ELM mitigation by
shifting the threshold between type-I and small ELMs towards
slightly higher edge temperatures. The application of MPs in
L-modes results in a degradation of the pressure gradient due to
increased heat transport. At the L-H transition, the pressure
gradient and the radial electric field shearing seem to exhibit the
same value with and without MPs, while its required heating power
is increased in the presence of MPs.