Detection of high k turbulence using two dimensional phase contrast imaging on LHD

High k turbulence, up to 30 cm − 1 , can be measured using the two dimensional CO2 laser phase contrast imaging system on LHD. Recent hardware improvements and experimental results are presented. Precise control over the lens positions in the detection system is necessary because of the short depth...

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Bibliographic Details
Published in:Review of scientific instruments 2008-10, Vol.79 (10), p.10E724-10E724-5
Main Authors: Michael, C. A., Tanaka, K., Vyacheslavov, L. N., Sanin, A., Kharchev, N. K., Akiyama, T., Kawahata, K., Okajima, S.
Format: Article
Language:English
Subjects:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
CARBON DIOXIDE LASERS
DEPTH
DETECTION
ECR HEATING
LHD DEVICE
PLASMA
PLASMA DIAGNOSTICS
SIGNALS
STELLARATORS
TURBULENCE
TWO-DIMENSIONAL CALCULATIONS
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Summary:High k turbulence, up to 30 cm − 1 , can be measured using the two dimensional CO2 laser phase contrast imaging system on LHD. Recent hardware improvements and experimental results are presented. Precise control over the lens positions in the detection system is necessary because of the short depth of focus for high k modes. Remote controllable motors to move optical elements were installed, which, combined with measurements of the response to ultrasound injection, allowed experimental verification and shot-to-shot adjustment of the object plane. Strong high k signals are observed within the first 100 - 200 ms after the initial electron cyclotron heating (ECH) breakdown, in agreement with gyrotron scattering. During later times in the discharge, the entire k spectrum shifts to lower values (although the total amplitude does not change significantly), and the weaker high k signals are obscured by leakage of low k components at low frequency, and detector noise, at high frequency.
ISSN:0034-6748
1089-7623
DOI:10.1063/1.2968266