A high spatial resolution double-pulse Thomson scattering diagnostic; description, assessment of accuracy and examples of applications

A high spatial resolution (3 mm full width half maximum, i.e. 2% of the minor radius) double-pulse multiposition Thomson scattering system was in operation at the Rijnhuizen tokamak project RTP from March 1996 until September 1998. It upgrades the previously installed single-pulse Thomson scattering system. Two measurements of full electron temperature, T-e, and density, n(e), profiles can be performed in rapid succession during one plasma discharge, using a double-pulsed laser and a dual charge-coupled-device camera system. The temporal resolution is determined by the separation time between the two subsequent laser pulses and can be tuned in the range 20-800 mu s. Typical relative experimental errors are 3-5% for T-e and 2-4% for n(e) at n(e) = 5 x 10(19) m(-3). Plasma experiments show the application of the diagnostic; double-pulse Thomson scattering measurements have been performed on high-n(e) ohmic plasmas with large m/n = 2/1 magnetohydrodynamic activity, centrally heated plasmas with filaments (i.e. high T-e peaks) applying electron cyclotron heating, plasmas with a transient central T-e-rise after oblique pellet injection showing central filaments, and off-axis heated plasmas with off-axis sawtooth activity. The double-pulse feature of the diagnostic enables the study of the dynamics of T-e and n(e) profiles in these plasmas.

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