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

Abstract

The new JET ITER-like wall (made of beryllium and tungsten) is more fragile than the former carbon fiber composite (CFC) wall and requires active protection to prevent excessive heat loads on the plasma facing components (PFC). Analogue 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 (VTM). 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-colour 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.

In-vessel calibration of the imaging diagnostics for the real-time protection of the JET ITER-like wall

“…1 Analog monochrome CCD cameras (HITACHI KP/M1AP) operating in the near infrared wavelength region are used to measure the surface temperature of the PFCs. This type of camera demonstrates an acceptable reliability in the JET environment because of their operation robustness under high magnetic field and their resistance to neutrons and gamma radiation.…”

“…The protection system based on imaging diagnostics has been implemented on JET in 2011 1,2 and was at this time calibrated in the laboratory by using a calibrated blackbody. Then the calibration was corrected taking into the account the transmittance of the in-and ex-vessel optical components.…”

Plasma operation with an all metal first-wall: Comparison of an ITER-like wall with a carbon wall in JET

“…In addition, an integrated protection system for ITER-like Wall (PIW) was implemented which uses CCD cameras operating in the near infra-red to trigger context determined stops [43]. Experiments carried out to verify the power handling limits set by the bulk W divertor with its lamella structure [44] and wall geometry have shown that the design process has delivered the expected performance.…”

Experiences With Tungsten Plasma Facing Components in ASDEX Upgrade and JET

“…In order to achieve PFC temperature measurements in real time, a comprehensive set of bi-color pyrometers (8) and monochrome video cameras (7) with adjustable filters and thermo-couples was installed. These measurement were used to establish a map for the energy deposition at all relevant PFCs in order to allow a fast reaction of the discharge control preventing their overload [18]. In the first year of operation the protection system worked reliably, although Be melting was found in unobserved areas of the high field side Be limiters, revealing toroidal asymmetries in the main chamber power load which must be taken into account in the wall protection strategy [19].…”