I.I. Orlovskiy

Progress in research and development of mirrors for ITER diagnostics

Metallic mirrors will be used as plasma-viewing elements in all optical and laser diagnostic systems in ITER. In the harsh environment of ITER, the performance of mirrors will decrease mainly because of the erosion of their surfaces and deposition of impurities. The deterioration of the optical properties of diagnostic mirrors will directly affect the entire performance of the respective ITER diagnostics, possibly leading to their shutdown. Therefore, R&D on mirrors is of crucial importance for ITER diagnostics. There is a coordinated worldwide R&D programme supervised by the Specialists Working Group on first mirrors of the International Tokamak Physics Activity, Topical Group on Diagnostics. This paper provides an overview of new results in the field of first mirrors, covering the manufacturing of ITER mirror prototypes, investigations of mitigation of deposition and mirror cleaning and the predictive modelling of the mirror performance in ITER. The current status of research on beryllium deposition—a new critical area of mirror research—is given along with an outlook for future activities.

MHD-mode locking by controlled halo-current in the T-10 tokamak

Experiments on a non-disruptive halo-current influence on the m = 2 mode behaviour at the flat-top stage of a tokamak discharge are presented. The halo-current in the rail limiter—plasma—vacuum vessel—external circuit—rail limiter loop was used. An EMF source controlled with a preprogrammed signal or with a feedback m = 2 signal was introduced into the external part of the halo-current circuit. The EMF source has generated the oscillating halo-currents with an amplitude of up to 500 A in the frequency range 0–20 kHz. In the case of the preprogrammed control signal the switching on of the EMF source resulted in the shift of the m = 2 mode frequency to the frequency of the halo-current oscillations. In particular, the rotation of the m = 2 mode stopped under a pulse of zero-frequency halo-current. In the tokamak discharges, when the mode rotation spontaneously stopped before the switching on of the oscillating halo-current, the mode rotation was restored at the halo-current frequency. In the case of the halo-current feedback control by the m = 2 mode signal, the effect depended on the choice of the phase shift in the feedback loop. Some increase or decrease of the m = 2 mode amplitude as well as some variations of the mode frequency were observed at different values of the phase shift. The halo-current effect on the m = 2 mode behaviour can be attributed to a coupling between the m/n = 2/1 magnetic islands and the halo-current magnetic field. The experiment was simulated on the assumption that the tearing mode is affected by the halo-current magnetic field component with the same helicity. In the calculations for the T-10 conditions, the mode behaviour under the effect of the halo-current was similar to the experimental observations.

Main challenges for ITER optical diagnostics

The review is made of the problems of ITER optical diagnostics. Most of these problems will be related to the intensive neutron radiation from hot plasma. At a high level of radiation loads the most types of materials gradually change their properties. This effect is most critical for optical diagnostics because of degradation of optical glasses and mirrors. The degradation of mirrors, that collect the light from plasma, basically will be induced by impurity deposition and (or) sputtering by charge exchange atoms. Main attention is paid to the search of glasses for vacuum windows and achromatic lens which are stable under ITER irradiation conditions. The last results of irradiation tests in nuclear reactor of candidate silica glasses KU-1, KS-4V and TF 200 are presented. An additional problem is discussed that deals with the stray light produced by multiple reflections from the first wall of the intense light emitted in the divertor plasma.

Tearing-mode natural frequency behaviour under mode-locking in T-10 tokamak

The results of the m = 2, n = 1 tearing mode natural frequency measurements in the course of the mode-locking by a controlled halo-current are presented. The halo-current in the rail limiter?plasma?vacuum vessel?external circuit with a controlled EMF source?rail limiter loop was used. The halo-current represented a superposition of a direct-current pulse for the mode-locking and an oscillating-current component for the natural frequency measurements by the resonant response of the mode. In the experiment, the mode signal oscillations at the frequency of the halo-current oscillating component were observed even if the mode rotation stopped under the pulse of the halo-current. The maximum amplitude of these mode oscillations was achieved when the frequency of the halo-current oscillating component was equal to the frequency of the mode before switching on the halo-current. It means that under the conditions of the T-10 experiment, there was no variation of the mode natural frequency upon termination of the mode rotation by the halo-current. This mode behaviour can be attributed to the well-known effect of frequency locking in an externally driven non-linear oscillator. The experiment was simulated with a model of a rotating non-linear tearing mode in the presence of the halo-current.

Issues And Options For Mirrors Of CXRS And H‐alpha ITER Diagnostics

The present status of investigations on the first mirror (FM) problem is discussed on the basis of the last experimental results obtained in modern tokamaks and laboratory devices. Engineering solutions that are necessary for protection of Fms for CXRS and H‐alpha diagnostics are considered. Future plans for R&D that has yet to be done are presented. Results of neutron irradiation and heating tests of multilayered dielectric mirrors are given briefly.