In JT-60, it has been found that the measurements of divertor radiation loss are subject to large errors (sometimes exceeding 100) due to the charge exchange neutral particles generated in the ripple loss process. This phenomenon occurs only in the neutral beam heated discharges performed under the condition that the direction of the ionB drift is towards the divertor and the neutral beam heating contains the perpendicular beam component. This influence is strong as the safety factor is large because both the ripple loss fraction and the neutral particle pressure near the divertor, as a target of charge exchange events, increase with the safety factor. The quantitative relationship between the bolometric measurements and the charge exchange loss is discussed, and the charge exchange loss, with about 10 or less of the ripple loss, is found to affect the bolometric measurements of the radiation loss. A method of avoiding this influence on the bolometric measurements is presented on the basis of the experimental results
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