A. Manzanares

A protection system for the JET ITER-like wall based on imaging diagnostics

The new JET ITER-like wall (made of beryllium and tungsten) is more fragile than the former carbon fiber composite wall and requires active protection to prevent excessive heat loads on the plasma facing components (PFC). Analog CCD cameras operating in the near infrared wavelength are used to measure surface temperature of the PFCs. Region of interest (ROI) analysis is performed in real time and the maximum temperature measured in each ROI is sent to the vessel thermal map. The protection of the ITER-like wall system started in October 2011 and has already successfully led to a safe landing of the plasma when hot spots were observed on the Be main chamber PFCs. Divertor protection is more of a challenge due to dust deposits that often generate false hot spots. In this contribution we describe the camera, data capture and real time processing systems. We discuss the calibration strategy for the temperature measurements with cross validation with thermal IR cameras and bi-color pyrometers. Most importantly, we demonstrate that a protection system based on CCD cameras can work and show examples of hot spot detections that stop the plasma pulse. The limits of such a design and the associated constraints on the operations are also presented.

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m3 high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries.

Fast visible camera installation and operation in JET

This article is a summary of the measurements of the recently installed wide‐view fast visible camera in the Joint European Tokamak JET. Here we limit ourselves to a description of the different phenomena and leave for forthcoming articles a more extensive analysis of every phenomenon.

On the operational specifications and associated R&D for the VIS/IR diagnostic for ITER

The measurement requirements which have been defined for ITER are being associated to specific diagnostics for which they are guidelines for the development - not technical specifications. As intermediate step we consider the definition of operational specifications and the associated R&D. This step is elucidated here for the example of the Visible and Infrared diagnostic at the equatorial plane of ITER. The measurement requirements are given in terms of temperature and spectral luminance ranges, spatial coverage and temporal and spatial resolution. The operational specifications have to take the harsh environment, the difficulties of the measurements and the operational role of the diagnostic into account. We find three distinct levels of operational performances in ascending order of difficulty to realize but arguably descending order of indispensability for operational purposes: qualitative imaging, quantitative imaging and the deliverance of precise temperature and flux data. This ordering helps us to prioritize the R&D work and the degree of required robustness and availability for certain functions of the diagnostic. The R&D ranges from first mirror and radiation hardness investigations of refractive optical material which is elaborated in some detail here and the development of cognitive image processing for the first level over the development of a multi-color approach and advanced calibration and maintenance methods for the second level to in situ measurements of optical and thermal properties and complex image processing including the use of data from other diagnostics for the third level. Photon-flux modeling and tokamak testing is important for all stages. It seems desirable that the diagnostic should comply from the onset with the second performance level. The third level could be reached later.