A new calibration implementation for Doppler Coherence Imaging Spectroscopy

Abstract

A new widely tunable, continuous-wave laser named C-Wave enables direct calibration at the unshifted, center-of-mass wavelength for most plasma lines in the range between 450 and 650 nm for Doppler Coherence Imaging Spectroscopy. It also provides the ability to scan around each desired wavelength over a small range of ±50 pm, thus enabling an easy way to translate the measured phase difference ∆Φ of the diagnostic to the desired Doppler line shift ∆λ, that corresponds to the physical parameter of interest, the particle velocity of an observed plasma species. A first wavelength scan of the C-Wave laser ±50 pm around a single plasma line of interest, C III at 464.881 nm, is presented and compared to a simulated wavelength scan. It demonstrates not only the C-Wave capability as a Doppler CIS calibration source, but also its potential to immensely simplify Doppler CIS analysis for nearly all visible plasma lines of interest.

Coherence imaging spectroscopy at Wendelstein 7-X for impurity flow measurements

“…The high velocity resolution, typical of CIS, sets a demanding accuracy level in order to have a reliable calibration. At W7-X, the high degree of accuracy is reached by using a continuous tunable laser (C-Wave, by Hübner photonics) [16]. The C-Wave emission comes from a diode-pumped solid-state laser (Nd:YAG, emitting at 532 nm) that passes through two cavities containing lithium niobate (LiNbO 3 ) crystals: a Optical Parametric Oscillator (OPO) and a Second-Harmonic Generator (SHG), both characterized by non-linear optical behaviors.…”

“…The C-Wave laser can also perform a direct measurement of wavelength shifts by fine scanning in the same range as the Doppler drifts expected by impurity ions in plasma discharges (±30 pm in 5 pm steps). This can highlight how the CIS systems are responding to different wavelengths and it is critical in accurately calibrating both the direction and the zero point of the impurity flow velocity [16].…”

“…Small-range wavelength scans are useful to characterize the CIS phase response and consequently be able to translate phase variations into flow velocity without relying on simulations. This type of dispersion measurement has been already presented in [16] for one line of interest, and it is here repeated for different central wavelength in order to highlight a wavelength dependence of the CIS response. The C-Wave laser has been tuned ±30 pm around 464.8811 nm (C III), 468.57 nm (He II), 486.135 nm (H β ) and 514.1856 nm (C II) in 5 pm steps.…”

“…This is possible by using two lasers: the C-Wave and a HeNe one (with emission at 632.816 nm). If the C-Wave is set in the blue/green range, the wavelengths of the two lasers are well distinguishable in the 2D FFT spectrum [16]. The chosen wavelengths for the C-Wave are 464.8811 nm (equivalent to the C III center of mass) and 514.1856 nm (equivalent to the C II center of mass).…”

“…These limitations are overcome with the W7-X laser, as it stands out for its fine (software) tuning capability (10 −2 pm) in a wide wavelength range (450-650 nm), which allows to emulate the wavelength shift expected during a plasma discharge (∼10 pm) and the exact, unshifted position of an investigated line for many different reference wavelengths. This is critical in accurately calibrating both the direction and the zero point of the impurity flow without relying on simulations [16]. This new level of accuracy, together with the 2D flow imaging capability of CIS, allowed to gain new information about the effects of the complex 3D geometry of W7-X.…”