L. Buzi

First studies of ITER-diagnostic mirrors in a tokamak with an all-metal interior: results of the first mirror test in ASDEX Upgrade

In ITER, mirrors will be used as plasma-viewing elements in all optical and laser diagnostics. In the harsh environment, mirror performance will degrade hampering the operation of associated diagnostics. The most adverse effect on mirror performance is caused by the deposition of impurities. It is expected that the most challenging situation will occur in the divertor. With the envisaged changes to all-metal plasma-facing components (PFCs) in ITER, an assessment of mirror performance in an existing divertor tokamak with all-metal PFCs is urgently needed. Molybdenum and copper mirrors were exposed for nearly nine months in ASDEX Upgrade which has all-tungsten PFCs. Mirrors were located at the inner wall, under the dome and in the pump duct. During exposure, the mirrors were heated to temperature in the range 145?165??C. This was made to approach the expected level of heating due to absorption of neutrons and gammas on mirrors in the ITER divertor. After exposure, degradation of the reflectivity was detected on all mirrors. The highest reflectivity drop was measured on mirrors under the dome facing the outer strike point, reaching ?55% at 500?nm. The least degradation was detected on mirrors in the pump duct, where the reflectivity was preserved in the range 500?2500?nm and the largest decrease was about ?8% at 250?nm. On all contaminated mirrors carbon fraction did not exceed 50 at% while the major contaminants were metals and oxygen. The degradation of exposed mirrors underlines the necessity for urgent R&D on deposition mitigation and in situ mirror cleaning in ITER.

Numerical analysis of TDS spectra under high and low flux plasma exposure conditions

A recently developed numerical model, based on the dislocation-driven nucleation of gas bubbles, is used to analyse experimental results on deuterium retention in tungsten under ITER relevant plasma exposure conditions. Focus is placed on understanding the relation between exposure temperature and flux on primary features of thermal desorption spectra: peak positions and intensities of the desorption flux. The model allows one to relate the peak positions with the size of plasma induced deuterium bubbles and envisage exposure conditions (temperature and flux) for their formation. Based on the performed analysis, dedicated experimental conditions to validate the model are proposed.

Studies of protection and recovery techniques of diagnostic mirrors for ITER

In optical diagnostic systems of ITER, mirrors will be used to guide the light from plasma towards detectors and cameras. The mirrors will be subjected to erosion due to fast particles and to deposition of impurities from the plasma which will affect adversely the mirror reflectivity and therefore must be suppressed or mitigated at the maximum possible extent. Predictive modeling envisages the successful suppression of deposition in the diagnostic ducts with fins trapping the impurities on their way towards mirrors located in the end of these ducts. To validate modeling predictions, cylindrical and cone-shaped diagnostic ducts were exposed in TEXTOR for 3960 s of plasma operation. After exposure, no drastic suppression of deposition was observed in the cylindrical ducts with fins. At the same time, no detectable deposition was found on the mirrors located at the end of cone-shaped ducts outlining the advantages of the cone geometry. Analyses of exposure provide evidence that the contamination of exposed mirrors was due to wall conditioning discharges and not due to working plasma exposure. Cleaning by plasma sputtering was performed on molybdenum mirrors pre-coated with a 100 nm thick aluminum film. Aluminum was used as a proxy of beryllium. During exposure in electron cyclotron resonance-generated helium plasma, the entire coating was sputtered within nine hours, leaving no trace of aluminum and leading to the full recovery of the specular reflectivity without detrimental effects on the mirror surface.