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Effects of Plasma Radiation on the Thomson Scattering Diagnostic Installed on the Large Helical Device

Kazumichi NARIHARA, Ichihiro YAMADA, Hiroshi HAYASHI and Hisamichi FUNABA

National Institute for Fusion Science, Toki 509-5292, Japan

(Received 13 December 2006 / Accepted 19 June 2007 / Published 20 November 2007)

Abstract

Recently we modified the Thomson scattering diagnostic (TS) installed on LHD so that DC levels (V_DC) of all avalanche photodiodes (APD) used for detecting scattered light can be registered every 1 ms, which enabling us to make validity check on TS data taken under very intense plasma radiation. In the line of this task, we first examined how the pulse-performance of an APD degrades as the intensity of continuous light (J_DC) incident to the APD increases. We found two effects are involved in deteriorating the pulse-performance of the APD: (1) the responsivity of the APD to a pulsed light drops as J_DC increases, causing a systematic errors on the deduced electron temperature (T_e) and density (_ne); (2) the frequency response of the APD and the following circuit drops as J_DC increases, which deforms the pulse shape. The bias voltage applied to the APD (V_b) has large influence on these behaviors, showing the best overall performance for a high J_DC around V_b ∼ 0.5V_r, where V_r is the recommended voltage giving responsivity of 675 kV/W at 1060 nm. Considering these effects together, we set a conservative validity criterion for the pulse APD performance in term of the V_DC: V_DC < 0.5 V. The V_b = 0.5 V_r setup gives much reliable T_e-profiles without a collapse in T_e-profile for a much wider range of plasma radiation intensity. With this criterion, we check the validity of T_e- and n_e-profiles of two example data.

Copyright (c) 2007 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

Thomson scattering diagnostic, high-density plasma, validity check, LHD, APD

DOI: 10.1585/pfr.2.S1107

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This paper may be cited as follows:

Kazumichi NARIHARA, Ichihiro YAMADA, Hiroshi HAYASHI and Hisamichi FUNABA, Plasma Fusion Res. 2, S1107 (2007).