Engineering the Alcator C-Mod MSE diagnostic: Solutions to reactor-relevant diagnostic challenges
Deploying precise optical diagnostics in harsh reactor environments poses significant engineering challenges including limited access, large forces, time-varying systematic errors, reflections, low signal-to-background levels and a need for in-situ calibration. Fielding a polarization-based Motional Stark Effect (MSE) diagnostic on Alcator C-Mod has proven to be difficult due to similar challenges, which has led to the development of solutions that enable polarization angle measurements with an accuracy of 0.2 degrees.
A motional stark effect instrument to measure q(R) on the C-mod tokamak
A motional Stark effect (MSE) instrument to measure q(r) using the Torus Experiment for Technology (TEXT) neutral beam on the Alcator C-Mod tokamak has been designed and installed. The neutral beam is on the midplane, aimed radially, and designed to operate in hydrogen at 50 keV. The MSE optics view the outer half of the plasma −0.3<(R−R0)/a<1.05. This geometry results in a spatial resolution of 1 (edge)–3 (center) cm from a beam apertured horizontally to 2 cm. Estimates of the signal levels indicate for Hα that photoelectron statistical errors in the measurement of Bθ/Bφ can be less than 0.2% for ne0<2.0×1014 cm−3 similar to other MSE instruments. The collection optics reside within the vacuum chamber reflecting and imaging the neutral beam through a vacuum window in the neutral beam port. The optics are designed so that beam emission spectroscopy can be carried out simultaneously with MSE measurements. Optical signals are transmitted to remote detectors by fiber optics. Low Verdet glass and dielectric m...
Recent Heating and Current Drive results on JET
An overview is presented of the results obtained on JET by the Heating and Current Drive Task Force (TF-H) in the period May 2000—March 2001. A strongly improved Lower Hybrid (LH) coupling was achieved by optimizing the plasma shape and by controlling the local edge density via the injection of CD4. Up to 4 MW have been coupled in type III ELMy H-mode and/or on Internal Transport Barrier (ITB) plasmas with reflection coefficients as low as 4%. Long lasting quasi steady-state ITBs have been obtained by adding the LH current to the bootstrap and beam driven components. Furthermore the use of LH in the pre-heat phase results in electron temperature in excess of 10 keV, deep negative magnetic shear and strongly reduced power threshold for ITB formation. Preliminary results on ICRF coupling are reported including the effect of CD4 injection and the commissioning of the wide band matching system on ELMy plasmas. IC CD scenarios have been studied in H and 3He minority and used to modify the stability of the sawt...