Control and acquisition for MAST Thomson scattering diagnostics

The MAST (mega-amp spherical tokamak) Thomson scattering (TS) diagnostics have been radically upgraded and expanded. Eight 30 Hz 1.6 J Nd:YAG lasers have been combined to produce a sampling rate of 240 Hz. The scattered signals are acquired by two spectrometer systems: core and edge. The core system has been built anew: collection optics, polychromators, digitizers, and control computers. It allows measurement of electron temperature and density at 130 spatial points with ∼10 mm resolution across the plasma.The Nd:YAG scattered light signals are registered in 650 channels as polychromator outputs; each channel is registered on two ADCs: at 1 GHz rate in a short interval around each laser pulse and at 100 kHz for background data. The fast ADCs are combined in 26 data acquisition units. Each unit is assembled in a 6 U PXI chassis with embedded controller and six 4-channel 1 GHz ADC cards. Some chassis contain a 96-channel slow ADC card with Ethernet control.The Ruby TS has been rebuilt with a new spectrometer and CCD camera to provide higher spatial resolution - 512 points; the laser has been modified to add double pulse capability.A new control and acquisition system has been developed; it has modular design allowing flexibility and seamless expansion. The system supports event-triggered and real-time operation (will be added in a later stage).A smart trigger device has been developed for TS timing and synchronisation. It provides complex pulse sequences for laser firing with resynchronisation on a number of digital and analogue inputs including plasma events. This device also triggers TS acquisition.The system is integrated by a TS master process running on the dedicated computer; it is represented as a standard MAST data acquisition unit. The Ruby TS is also implemented as a standard MAST unit linked with the Nd:YAG TS by MAST system services.     

Optimization of optical filters for ITER edge Thomson scattering diagnostics

In the ITER edge Thomson scattering measurement system, polychromators with optical band-pass filters and avalanche photodiodes are planned to be used. In this paper, we optimized the transmission wavelength ranges of the optical filters by a numerical method. Since measurements in a high electron temperature range are required for the measurement system, the Thomson spectrum could overlap a strong line emission of D_α when the electron temperature is rather high. It is shown that a filter whose transmission wavelength range is shorter than D_α becomes important to decrease the measurement error in the high temperature range. Moreover, it is found that a filter whose transmission wavelength range is above 1064 nm (laser wavelength) is useful to improve the measurement accuracy, in particular, when the number of filters is more than six.     

CO2 laser collective Thomson scattering diagnostics on the HT-7 tokamak

A CW CO2 laser collective Thomson scattering diagnostics was developed to measure plasma density fluctuations on the HT-7 tokamak. The design and construction of CO2 laser scattering apparatus is described. The laser source is a continuous-wave CO2 laser with a cavity length of 1.9 m and a power output of about 10 W at 10.6 μm. The k-resolution of the system is △k ≈ 3.2 cm^-1. The preliminary data from the diagnostic is presented.     

Design and development of a synchronized operation control system for Thomson scattering diagnostic on J-TEXT

A Thomson scattering diagnostic system is under construction at the Joint Texas Experimental Tokamak (J-TEXT).A 1064 nm Nd:YAG laser with 50 Hz repetition rate is used as the laser source.We have used a software for careful and precise control of the laser through serial communication.A time sequence operating system has been developed to synchronize the laser control and data acquisition system with the central control system (CSS).The system operates commands from the CSS of J-TEXT and generates triggers for the laser and data acquisition system in the proper sequence.It also measures an asynchronous time value that is needed for accurate time stamping.All functions are served by a field-programmable gate array development platform that is suitable for high-speed data and signal processing applications.Several embedded peripherals,including Ethernet and USB 2.0,provide communication with the CSS and the server.     

Upgrade of the COMPASS tokamak real-time control system

The COMPASS plasma control system is based on the MARTe real-time framework. Thanks to MARTe modularity and flexibility new algorithms have been developed for plasma diagnostic (plasma position calculation), control (shaping field control), and protection systems (central solenoid protection). Moreover, the MARTe framework itself was modified to broaden the communication capabilities via Aurora.This paper presents the recent upgrades and improvements made to the COMPASS real-time plasma control system, focusing on the issues related to precision of the real-time calculations, and discussing the improvements in terms of discharge parameters and stability. In particular, the new real-time system has given the possibility to analyze and to minimize the transport delays of each control loop.     

Personnel protection during the operation of Thomson scattering laser system on COMPASS tokamak

A recently installed Thomson scattering diagnostic on COMPASS tokamak uses two high power lasers. The presented protection system ensures laser safety of the personnel. Protection covers three areas–laser laboratory, spectrometer laboratory and tokamak hall. Laser protection system inputs are controlling the covering of the laser beam path, the entrance doors, the beam shutters and the laser cooling. Six regimes are defined for the protection system, covering all operation of the laser system, including laser service and low power beam alignment. Hardware implementation of the protection is based on PLC. The system is controlled via PCs with a touch screen. Connection to the COMPASS personnel protection system is described.     

Design and development of five-channel interference filter polychromator for SST-1 Thomson scattering system

Five-channel interference filter polychromator is designed and fabricated for measuring electron temperature and density from Thomson scattered spectrum of SST-1 tokamak plasma. The instrument is designed for measuring electron temperature in the range of 20 eV-3 keV and electron density of 10¹⁸-10¹⁹ m⁻³. Optical ray tracing software, Zemax is used for simulating and optimizing the optical design. Each polychromator is a stand-alone unit with field programmable gate array (FPGA) based controller unit for easy operation, monitoring of the temperature variation of the instrument and communicating to a central personal computer (PC). The control unit also protects the avalanche photo diode (APD) detectors from damage due to high current flow, sets the slow channel gain and switches on the biasing power supply. Characteristics of the present polychromator design are relatively high signal throughput and variable bandwidth selection of filters combined with a stable, low cost and relatively simple configuration. Filter selection, arrangement order of filters, statistical error analysis, mechanical and optical design and estimation of electron temperature and density are discussed in this article. The fabricated filter polychromator is tested for its image quality, optical alignment, and integrated performance.     

Overview of the COMPASS CODAC system

This paper presents an overview of the Control, Data Acquisition, and Communication system (CODAC) at the COMPASS tokamak: the hardware set-up, software implementation, and communication tools are described.The diagnostics and the data acquisition are tailored for high spatial and temporal resolution required by the COMPASS physics programme, which aims namely at studies of the plasma edge, pedestal, and Scrape-off-Layer (SOL). Studies of instabilities and turbulence are also an integral part of the programme. Therefore, the data acquisition consists of more than 1000 channels, sampled at rates from 500 kS/s up to 2 GS/s.Presently, the feedback system controls the plasma position and shape, plasma current, and density and it includes 32 analogue input channels as well as 1 digital input/output channel and 8 analogue outputs. The feedback control runs within the Multi-threaded Application Real-Time executor (MARTe) framework with two threads, a 500 μs cycle to control slow systems and a 50 μs cycle to control the fast feedback power supplies for plasma position control.In this paper, special attention is paid to the links between the systems, to the hardware and software connections, and to the communication. The hardware part is described, the software framework is addressed, and the particular implementation – the dedicated software modules, communication protocols, and links to the database are described.     

汤姆逊X射线源激光系统的定时与同步诊断

清华汤姆逊X射线源中,束流动力学性能和对撞时间要求光阴极微波电子枪的触发激光脉冲与加速相位精确同步.本文分别介绍了超快紫外激光系统设计与实现、光阴极微波电子枪定时系统设计,以及同步系统的亚皮秒抖动测量.用高次谐波相位检波器法测量相位噪声功率谱密度可以获得精确的激光-射频相对均方根时间抖动,实验测定为小于200fs[1Hz,100kHz].     

Design of Thomson scattering diagnostic system on linear magnetized plasma device

In addition to the magnetic confinement fusion plasma, Thomson scattering has been applied to measure electron density and temperature of low-temperature plasmas. Based on a linear magnetized plasma device, a set of Thomson scattering diagnostic system is designed to diagnose the plasma with ${n}_{{\rm{e}}}={10}^{18}{\unicode{x02013}}{10}^{19}\,{{\rm{m}}}^{-3}$ and ${T}_{{\rm{e}}}=2{\unicode{x02013}}5$ eV. Due to low plasma temperature and density, this diagnostic system needs high spectral resolution and collection efficiency to meet the requirements of electron velocity distribution function measurements. Through the bench test, it is confirmed that the spectral resolution reaches 0.01 nm, and theoretical collection efficiency is high enough to obtain a Thomson scattering spectrum by 1000 accumulations.     

汤姆逊散射初步实验设计

基于汤姆逊散射的X射线源具有一些现有X射线源所不具备的优点，因而具有广泛的应用前景。我们将利用现有的16MeV返波型行波加速器与纳秒激光器进行汤姆逊散射初步实验。本文给出了即将进行的该初步实验的理论计算及整体方案，并对在实验中可能出现的问题的解决方案进行了讨论。     

Development of real-time plasma analysis and control algorithms for the TCV tokamak using Simulink

One of the key features of the new digital plasma control system installed on the TCV tokamak is the possibility to rapidly design, test and deploy real-time algorithms. With this flexibility the new control system has been used for a large number of new experiments which exploit TCV's powerful actuators consisting of 16 individually controllable poloidal field coils and 7 real-time steerable electron cyclotron (EC) launchers. The system has been used for various applications, ranging from event-based real-time MHD control to real-time current diffusion simulations. These advances have propelled real-time control to one of the cornerstones of the TCV experimental program. Use of the Simulink graphical programming language to directly program the control system has greatly facilitated algorithm development and allowed a multitude of different algorithms to be deployed in a short time. This paper will give an overview of the developed algorithms and their application in physics experiments.     

Characterization of heat transport dynamics in laser-produced plasmas using collective Thomson scattering: Simulation and proposed experiment

We propose an experiment in which the collective Thomson scattering lineshape obtained from ion acoustic waves is used to infer the spatial structure of local heat transport parameters and collisionality in a laser-produced plasma. The peak-height asymmetry in the ion acoustic wave spectrum will be used in conjunction with a recently developed model describing the effects of collisional and Landau damping contributions on the low-frequency electron density fluctuation spectrum to extract the relative electron drift velocity. This drift arises from temperature gradients in the plasma. The local heat flux, which is proportional to the drift, can then be estimated, and the electron thermal conductivity will be inferred from the relationship between the calculated heat flux and the experimentally determined temperature gradient. Damping of the entropy wave component at zero mode frequency is shown to be an estimate of the ion thermal conductivity, and its visibility is a direct measure of the ion-ion mean free path. We also propose to measure thermal transport parameters under dynamic conditions in which the plasma is heated impulsively by a laser beam on a fast (50 ps) time scale. This technique will enable us to study heat transport in the presence of the large temperature gradients that are generated by this local heating mechanism. Deviations of the inferred local thermal conductivity from its Spitzer-Härm value can be used to study the transition to a nonlocal heat transport regime. We have constructed a simple numerical model of this proposed experiment and present the results of a simulation.This work was performed under the auspices of the U.S. Department of Energy by Sandia National Laboratories under Contract DE-AC04-94AL85000 and by Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.     

Upgrade of plasma density feedback control systemin HT-7 tokamak

The HT-7 is a superconducting tokamak in China used to make researches on the controlled nuclear fusion as a national project for the fusion research. The plasma density feedback control subsystem is the one of the subsystems of the distributed control system in HT-7 tokamak (HT7DCS). The main function of the subsystem is to control the plasma density on real-time. For this reason, the real-time capability and good stability are the most significant factors, which will influence the control results. Since the former plasma density feedback control system (FPDFCS) based on Windows operation system could not fulfill such requirements well, a new subsystem has to be developed. The paper describes the upgrade of the plasma density feedback control system (UPDFCS), based on the dual operation system (Windows and Linux), in detail.     

Iterative noise removal from temperature and density profiles in the TJ-II Thomson scattering

TJ-II Thomson Scattering diagnostic provides temperature and density profiles of plasma. The CCD camera acquires images that are corrupted with some kind of noise called stray-light. This noise degrades both image contrast and measurement accuracy, which could produce unreliable profiles of the diagnostic. So far, several approaches have been applied in order to decrease the noise in the TJ-II Thomson scattering images. Since the presence of the noise is not global but located in some particular regions of the image, advanced processing techniques are needed. However such methods require of manual fine-tuning of parameters to reach a good performance. In this contribution, an iterative image processing approach is applied in order to reduce the stray light effects in the images of the TJ-II Thomson scattering diagnostic. The proposed solution describes how the noise can be iteratively reduced in the images when a key parameter is automatically adjusted during the iterative process.     

Development of ITER divertor Thomson scattering support structure design on the basis of engineering analyses

The support structure for divertor Thomson scattering equipment – the front diagnostic rack, which actually plays plugging role of the divertor port, should be designed to sustain the severe ITER conditions. Meeting the requirements of multifield analyses (which often contradict each other) results in an iterative design process. A number of design variants based on engineering analyses results were developed in 2011–2012. We study here the close to the final design of the diagnostic rack for consistency to electromagnetic, thermal and seismic loads. The specific ITER environment imposes a restricted list of materials and requires a careful design of optical elements to accommodate their thermal expansion. Special attention is focused on the mirror deformed shape under operating loading conditions and its effect on optical system performance, which is vital for all optical systems with mirrors specially designed for the ITER.     

HT-7超导托卡马克MHD实时检测与处理系统的设计与实现

磁流体不稳定性（MHD）是影响HT-7长脉冲放电的一个重要因素,通常可以通过调制低杂波电流驱动的方法来抑制MHD。实验发现全程调制低杂波电流驱动会降低低杂波的驱动效率。我们采用了主动反馈调制低杂波电流驱动的方法,设计并实现了MHD的实时检测与处理系统。研究结果表明,该系统既可以实时地抑制MHD,又不降低低杂波驱动效率。     

Influence of heating effect in Thomson scattering diagnosis of laser-produced plasmas in air

A state diagnosis of laser-produced plasma in air generated by a 1064 nm pulse laser was investigated by the Thomson scattering(TS) method. The evolutions of the electron temperature and electron density were obtained as a function of the time delay which ranged from300–3200 ns. The heating effect produced by the 532 nm probe beam with different energies on the air plasma at different interaction times was further studied using a time-resolved optical emission spectroscopy technique. The influence of the probe beam on the electron density was found to be negligible, whereas its influence on electron temperature is evident. In addition, the heating effect of the probe beam on the plasma strongly depends on the energy of the probe beam, and gradually weakens with increasing time delay. Our results are helpful for further understanding the TS method and its application in plasma diagnostics.     

Design of Amplifier Circuit for the HT-7 Tokamak Thomson Scattering System

Thomson scattering diagnostic is important for measuring electron temperature and density profiles. To improve the signal-to-noise ratio, a silicon avalanche photodiode （APD） with high quantum efficiency, high sensitivity, and high gain up to 100 was adopted to measure the Thomson scattering spectrum. A preamplifier, which has low noise, high bandwidth, and high sensitivity, was designed with suitable transimpedance. Using AD8367 as the post-amplifier, good performance of the APD readout electronics have been obtained. A discussion is presented on the performance of the amplifier using a laser diode to simulate the Thomson scattering light. The test results indicate that the designed circuit has a high amplifying factor and fast rising edge. So reduction of the integral gate of the CAMAC ADC converter can improve the signal-to-noise ratio.     

The timing system on the J-TEXT tokamak

This paper describes the timing system designed to control the operation time-sequence and to generate clocks for various sub-systems on J-TEXT tokamak. The J-TEXT timing system is organized as a distributed system which is connected by a tree-structured optical fiber network. It can generate delayed triggers and gate signals (0 μs–4000 s), while providing reference clocks for other sub-systems. Besides, it provides event handling and timestamping functions. It is integrated into the J-TEXT Control, Data Access and Communication (J-TEXT CODAC) system, and it can be monitored and configured by Experimental Physics and Industrial Control System (EPICS). The configuration of this system including tree-structured network is managed in XML files by dedicated management software. This system has already been deployed on J-TEXT tokamak and it is serving J-TEXT in daily experiments.