Upgrade of the tangentially viewing vacuum ultraviolet (VUV) telescope system for 2D fluctuation measurement in the large helical device

Abstract

A high-speed tangentially viewing vacuum ultraviolet (VUV) telescope system, using an inverse Schwarzschild-type optic system was developed to study fluctuations in the Large Helical Device (LHD). However, for the original system, the sampling rate was restricted to below 2000 Hz due to the low signal to noise (S/N) ratio in the experiment. In order to improve the S/N ratio, upgrade of the system was made. With this upgraded optical system, the maximum framing rate is improved to 6000 fps with a similar spatial resolution. Rotation of the m = 2 structure caused by the magnetohydrodynamic (MHD) instability is measured by the upgraded system. The spatial structure of the image is consistent with the synthetic image assuming the interchange mode type displacement of the flux surfaces.

Two-dimensional vacuum ultraviolet images in different MHD events on the EAST tokamak

“…Synthetic images are made to estimate the mode structure by comparing the synthetic images and experimental data. Synthetic images are made by following the method mentioned in references [10,14]. In general, the observed VUV light contributed by CVI line emission I VUV can be expressed as I x…”

“…Synthetic images are made to estimate the mode structure by comparing the synthetic images and experimental data. Synthetic images are made by following the method mentioned in references [10,14] In this work, the radial plasma displacement ξ r is assumed to be a function of averaged minor radius ρ, ξ r = A f exp , w 0 2 r r r ´r -r r -( ) ( ) where, A 0 , ρ r , and w are the amplitude, the peak location and the radial width of the mode, respectively. In this case, ξ r is assumed to be an odd function (tearing-type), f(ρ)=1 (ρρ r ), f(ρ)=−1 (ρρ r ).…”

“…A tangentially viewing VUV imaging system was developed in the Large Helical Device (LHD) in 2008 [9], and it was upgraded in 2010 [10]. The 2D structure of the m/n=1/1 (m is the poloidal mode number and n is the toroidal mode number) interchange mode in LHD plasma has been obtained using this system.…”