Langmuir probe measurements in the divertor plasma of the JET tokamak.
This thesis describes the design and application of a new array of Langmuir probes to measure
the plasma parameters in the divertor region of the Joint European Torus (JET) tokamak.
A novel probe tip design was developed to withstand the high heat fluxes in the divertor region
following thermal analysis and beam tests to identify the most suitable material. The probes
operate in the "very strong magnetic field" regime with a large projected area (4mm2) and
incidence angle to the magnetic field (> 10°) to avoid the sheath expansion effects associated
with flush mounted divertor probes. The probes may be remotely configured as single, triple
or floating probes. The combination of triple probe measurements and strike point sweeping
has provided unique high resolution profiles of plasma parameters across the divertor target.
Emphasis is placed upon the evaluation of probe interpretation assumptions and comparison
with non-disturbing diagnostics to assess the reliability of the measurements. The distortion
of the probe current-voltage characteristics is strongest for high recycling and detached
divertor conditions. Single probe analysis methods overestimate the electron temperature
while the virtual asymmetric double probe assumption matches well the functional form of
the current-voltage characteristics and is more consistent with independent measurements
and numerical modelling than single probe interpretation. However, the effects of plasma
resistivity remain difficult to evaluate quantitatively.
Analysis of the divertor plasma parameters over a wide range of conditions is presented. The
behavior of the ion fluxes and parallel gradients during high recycling and detached discharges
is studied and compared with predictions from simple analytical models of detachment. Numerical
models are used to study the impurity production from the divertor target plate. A
new technique of evaluating the plasma parameters both between and during Edge Localized
Modes (ELMs) using the triple probes is applied to study the influence of active pumping on
the divertor parameters.