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The investigation of impurity behavior in fusion plasmas is a critical issue in fusion plasma research. The effective charge (Zeff) profile is a widely used measure of the impurity levels in fusion plasmas. In this study, the visible bremsstrahlung emissivity profile is reconstructed using toroidal visible bremsstrahlung (TVB) arrays at Korea Superconducting Tokamak Advanced Research (KSTAR). KSTAR TVB arrays have recently been developed and calibrated using a halogen light source and an integrating sphere. The reconstruction algorithm has been developed using the Phillips–Tikhonov method, and the reconstruction accuracy is assessed with test profiles. Electron density and temperature profiles from Thomson scattering diagnostics are fitted for Zeff calculations. Subsequently, the Zeff profiles in the edge localized mode suppression experiment are reconstructed. In addition, line-averaged Zeff values in the 2020 KSTAR campaign are presented, which are mostly distributed from two to four. [ABSTRACT FROM AUTHOR]

A Thomson scattering (TS) system has been utilized to measure the electron temperature and density of the core region of Versatile Experiment Spherical Torus (VEST). Recently, the laser injection system is successfully upgraded adopting the burst laser with the repetition rate of 1 kHz and the energy of 2 J. Furthermore, improved collection optics with additional polychromators and a 32-channel fast digitizer are prepared to observe the fast time evolution of radial profiles. This improvement is essential to study fast phenomena such as internal reconnection event (IRE). We increase the TS signal and reduce the stray light by introducing new filters having better optical properties such as high optical density at 1064 nm, transmission, and reflectance. Moreover, the optimum reverse bias voltages are newly set to make the system operational independent of the background radiation. As a result, 1 kHz radial profiles of the core electron temperature and density are measured for the first time, showing characteristics of IREs in VEST. [ABSTRACT FROM AUTHOR]

A prototype real-time (RT) Thomson scattering system was tested in a KSTAR Thomson scattering diagnostic system [1]. For this RT-Thomson system, 155 channels of 5 GS/s digitizer were operated (31 polychromators, each has 5 signal channels used in the KSTAR Thomson scattering system). The 5 Giga Sampling / second (GS/s) digitizer restored the pulsed Thomson scattering signals, and these acquired digital signals were fitted using a pulse function and Fast Fourier Transformation (FFT) method to remove noise with high speed on the TS signals [2]. The pulse fitting parameters (pulse height, rising time, falling time and peak position) were applied to calculate plasma parameters like the electron temperature (T e). To reduce the T e calculation time, a neural-network (NN) method was introduced using graphic processor unit (GPU) calculations. These plasma parameters were transmitted to the KSTAR plasma control system (PCS)using a reflective memory (RFM) card. The delay time of this RT-Thomson before transfer to the RFM card measured less than 7 ms for a T e calculation of 31 positions, and the transfer time from the RFM card to the PCS was 6.8 ms. Therefore, at 80.31% of KSTAR long pulse plasma shot # 32,763 (84.15 s), the end-to-end treatment time was less than 50 ms. The uniqueness of this paper is that it uses 5GS/s digitizer, Fast Fourier Transformation FFT fitting method, and neural- network (NN) analysis system, and finally succeeds in transferring Te profile data to the KSTAR PCS system in real- time (80.31% within 50 ms delay) using RFM cards for KSTAR long pulse plasma (84.15 s) for the first time. [ABSTRACT FROM AUTHOR]

A Thomson scattering diagnostic system is designed for Versatile Experiment Spherical Torus (VEST) to measure the spatial profiles of the electron temperature. The system is carefully designed to collect a sufficient number of photoelectrons and to reduce the noise sources, since relatively low electron densities and temperatures are expected in VEST due to the limited power capacity at present. The target electron temperature and the density are 10–200 eV and 5 × 10 18 m −3 , respectively which are extrapolated from the data of triple Langmuir probes measuring the edge plasma parameters at R = 0.75 m by assuming a parabolic density profile. The collecting optics is designed to have a wide-view angle and low cost by using a commercial photographic lens of low f-number and high transmittance optical fiber bundle. The bandwidths of the interference filters in the polychromator are designed for reliable measurements within the target electron temperature range. As a photo detector which is coupled with the filters, an avalanche photodiode (APD) with a low dark current and an adequate quantum efficiency near the laser wavelength is selected for the high signal-to-noise ratio. The number of photons transferred to the polychromator and the number of photoelectrons in the APD are calculated. At the commissioning phase, an oscilloscope with a high sampling rate will be adopted to check the necessity of the noise reduction by multi-shot signal accumulation. [ABSTRACT FROM AUTHOR]

• Upgrade of Thomson scattering system on VEST has been performed. • The laser input power is increased up to 2.8 times by injection system improvement. • Optical fiber bundle replaces a single core fiber to increase the signal level 2.1 times. • Oscilloscope envelope mode is utilized to measure periodic large data for each burst. • Time resolution is improved to 1 ms with new burst laser and fast DAQ systems. A Thomson scattering diagnostic system was developed and successfully measured electron temperature and density of core plasma (R = 0.35 m) in Versatile Experiment Spherical Torus (VEST). However, the current system is able to measure only a single time point in a shot. Therefore, upgrade of the Thomson scattering system has been prepared and is in progress to obtain the time evolution. The upgrade is focused on three parts. First, the laser injection system is improved by introducing a burst mode laser with the energy of 2 J and repetition rate of 1 kHz for 10 ms. Furthermore, the laser injection optics is improved to minimize the optical loss. Second, minimum etendue limit of the collection optics is increased by employing a fiber bundle of three single-core fibers. Finally, a data acquisition system is prepared to record the burst laser signals for high sampling rate. Oscilloscope envelope mode is adopted and tested for measuring ten burst laser signal with 1.25 GS/s. Through the upgrade, the time evolution of electron temperature and density is expected to be measured in a single shot with enhanced signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

A Thomson scattering (TS) system has been developed on Versatile Experiment Spherical Torus (VEST) to measure the electron temperature and the density. In VEST, it is quite challenging to measure TS signals because the energy of the utilized pulsed laser and the target electron density are low, 0.85 J and ∼5 × 1018 m−3, respectively. Furthermore, the difficulties caused by filter specifications have to be resolved because of the use of commercial interference filters with broad bandwidth and poor blocking at the laser wavelength. Therefore, we especially optimize the placement order of the filter sets based on the evaluation of the relative errors to overcome the poor spectral resolution. In addition, the amount of the stray light that penetrating filters is reduced by vacuum components such as baffles, louvres, and a viewing dump. With the careful installation and the optimization of the system, we can develop an efficient Thomson scattering system. Recently, TS signals have been successfully measured from the ohmic plasmas, and the electron temperature and the density are determined by analyzing the signals with the Bayesian method. [ABSTRACT FROM AUTHOR]

The ITER Real-Time Framework (RTF) is a software suite designed and developed to facilitate the implementation of various real-time applications for ITER plant systems. The main driver to implement RTF was the Plasma Control System (PCS). RTF was designed as a base and development environment for PCS. However, due to its universal architecture, it can also be applied in other systems requiring real-time control or data processing. This paper presents the demonstration system to evaluate RTF in Thomson Scattering (TS) diagnostics. The system was developed to validate the integration of data acquisition hardware with RTF and check it in real-time data processing applications. The presented system covers the whole path of data acquisition, processing and archiving required for a typical Instrumentation and Control (I&C) system for ITER diagnostics. The main components of the RTF-based application developed for the system are device support for the pulsed giga-sample analog-to-digital converter, functional blocks implementing algorithms for analysing pulses from polychromator and calculating plasma electron temperature as well as interfaces for archiving raw data and publishing the measurement results. The overriding goal of the presented work was a detailed functional and performance evaluation of the implemented RTF-based system. The system was tested in laboratory using simulated data and in tokamak conditions at Korea Superconducting Tokamak Advanced Research (KSTAR). During the test campaign at KSTAR, the developed RTF-based application was integrated into the Thomson Scattering system and tested during regular operation of the tokamak with signals from a real polychromator. The results of measurements and performance evaluation are presented and discussed in the paper. • ITER Real-Time Framework (RTF) was evaluated for diagnostic I&C systems. • Integration of ITER RTF with high-speed switched capacitor digitizer is proposed. • Demonstration system for Thomson Scattering diagnostics is developed. • Initial results of functional and performance verification at KSTAR are discussed. [ABSTRACT FROM AUTHOR]