Thomson scattering diagnostics in the Globus-M tokamak

Specific features of the Thomson scattering diagnostics, its main characteristics and capabilities, and the results of its experimental testing in the Globus-M tokamak are described. A powerful multipulse neodymium-glass laser is designed for investigating both fast and slow processes in the tokamak plasma. The laser is capable of generating up to 20 pulses uniformly distributed in time during one tokamak discharge. In order to investigate fast transient processes, the laser repetition rate can be increased within a specified time interval. The possibility of varying the time interval between laser pulses from 0.5 ms to 1.0 s makes this diagnostics highly informative. The optical scheme developed in the course of these studies allowed one to simplify the power supply system and create a comparatively inexpensive laser system. The use of avalanche photodiodes and filtering polychromators with a high optical transparency provides high sensitivity of the diagnostics. A special software was designed that allows automatic processing of several hundred signals during one shot and provides data on the electron density and temperature immediately in the course of measurements. The diagnostics allows one to trace the time behavior of the spatial profiles of the electron temperature and density in both ohmic discharges and discharges with auxiliary heating, as well as in experiments with particle injection with a plasma gun.     

Tangential System of Thomson Scattering for Tokamak T-15

Two systems of Thomson scattering diagnostics, with vertical and tangential probing, are used in the D-shaped plasma cross section in tokamak T-15. The tangential system allows measuring plasma temperature and density profiles along the major radius of the tokamak. This paper presents the tangential system project. The system is based on a Nd:YAG laser with wavelength of 1064 nm, pulse energy of 3 J, pulse duration of 10 ns, and repetition rate of 100 Hz. The chosen geometry allows collecting light from ten uniformly spaced points. Optimization of the registration system has been accomplished. The collected light will be transmitted through an optical fiber bundle with diameter of 3 mm and quartz fibers (numerical aperture is 0.22). Six-channel polychromators based on high-contrast interference filters have been chosen as spectral equipment. The radiation will be registered by avalanche photodiodes. The technique of electron temperature and density measurement is described, and estimation of its accuracy is carried out. The proposed system allows measuring the electron temperature with accuracy not worse than 10% within the range of 50 eV to 10 keV on the pinch edge over the internal contour, from 20 eV to 9 keV in the plasma central region, and from 2 eV to 400 eV on the pinch edge over the outer contour. The estimation is made for electron density of not less than 2.6 × 10¹³ cm^–3.     

HL-2M 装置初始放电可见光辐射测量系统

HL-2M 装置初始放电实验期间采用了一套 8 通道可见光辐射测量系统,4 个通道用于测量 Hα辐射, 其余分别测量 HeⅡ线、CⅢ线、OⅤ线和轫致辐射。该系统采用新型的雪崩放大二极管(APD)模块和光电倍增管 (PMT)模块作为光探测器,这些新型的探测器由于集成度高、供电方式简单而使得整个系统的后端具有体积小、 操作简单及可靠性强等优点。在 HL-2M 装置初始放电期间,该系统为控制运行人员提供了基本的等离子体信息。     

通讯波段单光子探测器的研制

将InGaAs/InP雪崩光电二极管应用于盖革模式下,采用门脉冲模式淬灭雪崩,并使用魔T混合网络抑制尖峰噪声,实现了通信波段1550 nm的单光子探测.在APD工作温度为223 K时,测得暗计数率与探测效率的比值为0.035.     

Internal cancellation of spikes using two avalanche photodiodes in series for single photon detection

<正>We propose a method of improving the performance of InGaAs/InP avalanche photodiodes by using two avalanche photodiodes in series as single photon detectors for 1550-nm wavelength.In this method,the raw single photon avalanche signals are not attenuated,thus a high signal-to-noise ratio can be obtained compared with the existing results.The performance of the scheme is investigated and the ratio of the dark count rate to the detection efficiency is obtained to be 1.3×10~(-4) at 213 K.     

Low-threshold absolute parametric decay instabilities in experiments on electron cyclotron resonance heating in tokamaks

We have analyzed experimental conditions for the excitation of absolute parametric decay instabilities accompanying the electron cyclotron resonance heating (ECRH) of plasma at the second harmonic of resonance in tokamaks. It has been shown that, in the case of a nonmonotonic radial profile of the plasma density, when a heating beam passes near the equatorial plane of a tokamak, the parametric excitation of resonances of ion Bernstein waves accompanied by the generation of the backscattered microwave radiation can occur. The threshold of absolute instability thus developed is determined by the dissipation of an ion Bernstein wave and can be exceeded in current experiments on the ECRH of a plasma in tokamaks.     

HL-2A 装置可见光诊断方法初探

等离子体H_α 径向分布的测量是等离子体实验中的重要内容。在HL- 2A 实验中, 等离子体成像系统中的CCD 图片记录了等离子体辐射信息, 特别是H_α  线辐射信息( 在相机前加H_α 滤光片) 。因为光测量是辐射的线积分, 数据的处理和分析显得十分必要的。利用等离子体可见光成像系统, 采用Abel 变换, 得到H_α  在HL- 2A 装置中的径向分布, 提供了一项可靠的测量手段。     

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为了检测到散射信号且有较高的信噪比,使用硅雪崩二极管(APD)探测器探测汤姆逊散射光,采用电荷敏感运算放大器减小电子噪声。同时,设计了放大电路,并采取了一些抗干扰措施。在实验中,此激光汤姆逊散射系统成功地测量出HT-7等离子体的电子温度。     

用于红外单光子探测的雪崩光电二极管传输线抑制电路模型的理论分析

结合利用雪崩光电二极管(APD)进行红外单光子探测电路模型的工作原理和特点以及传输线瞬态电脉冲产生的过程,提出了将传输线瞬态过程脉冲发生电路模型用于APD雪崩抑制的一种新方法,该方法可以实现利用APD门模工作方式进行红外单光子探测的过程.主要从理论上计算了红外单光子信号入射APD时,传输线脉冲发生电路模型中负载电阻输出电脉冲的特点,讨论了传输线终端不同边界条件对输出电脉冲的影响,通过理论计算确定了这种利用APD进行红外单光子探测新模型的电路结构与参数,证明了该电路模型用于红外单光子探测APD门模工作方式的 关键词： 红外单光子探测技术 雪崩光电二极管(APD) 抑制电路 传输线瞬态过程     

On the plasma mechanism of developing the early stages of breakdown in gases

The time evolution of the initial stages of breakdown in high-pressure gases is theoretically investigated. The following three stages are considered: initial: the origination and development of an ionization avalanche, the change of this avalanche to a plasma avalanche, and the change of the plasma avalanche to a plasma streamer. The streamer ionizes the gas via its radiation, causes new avalanches propagating in an enhanced discharge field, and bridges the discharge gap. Simple formulas are derived for the times of transition between various breakdown stages and for the ionization front velocity at separate stages.     

Measurement and analyses of the mean effective ion charge in the centre of tokamak discharges

There are two different definitions for specifying the mean effective ion charge Z_eff in plasmas: a) from the Spizer electrical resistivity of the plasma and b) from bremsstrahlung radiation losses of the plasma. In this paper Z_eff in the centre of tokamak ohmic discharges has been determined from information on sawtooth-relaxations of the steady state plasma, based on the analysis for the power balance of the plasma electrons in the plasma centre during the period of recovery after the sawtooth crashes. This method is found to supply reliable results for tokamak parameters. While its application requires some efforts in data analysis, it can provide a reliable determination of Z_eff, independent of the information from bremsstrahlung radiation losses of the plasma.     

Numerical Modelling of Impurity Retention in the ITER Tokamak Scrape-off Layer Plasma

The hydrogen plasma and carbon ions flows generated at the target plates in the ITER tokamak scrape-off layer are numerically investigated. A 2D model of hydrogen plasma and impurity ions is presented. The model is based on the electron-ion Braginski fluid equations [1] for hydrogen plasma and rate equations for impurities. Arbitrary level of impurity ions concentration is assumed. Recycling of hydrogen, sputtering and self sputtering of carbon atoms at the target plates are taken into account. Equations of the model are solved in the slab geometry using 2D-multifluid numerical code EPIT [2]. Problems of impurity ions retention and radiation in the divertor volume are analyzed. Results of calculations for ITER tokamak boundary plasma are presented, showing that poor retention is likely at high impurity concentration in the divertor volume. The radiation power can be a significant part of ingoing energy.     

Density profiles in tokamaks from electron cyclotron radiation spectra

Measurement of the line shape of optically thick and optically thin lines in the electron cyclotron radiation spectrum emitted by a tokamak plasma may yield both electron temperature and density profiles. Currently temperature profiles are routinely extracted from optically thick lines. Consequently, this paper is addressed to the density profile problem. Algorithms for extracting density profiles are outlined in the case of uncontrolled reflection and controlled reflection of the cyclotron radiation within the tokamak vacuum chamber.     

First results of movable limiter experiments and its effects on the tokamak plasma confinement

Limiters play a number of roles in the tokamak operation. It serves primarily to protect the wall from the plasma when there are disruptions, runaway electrons, or other instabilities and also the limiters localize the plasma–surface interaction. In this research, we presented the first results of movable limiter experiments and its effects on the tokamak plasma confinement. For this purpose, we designed, constructed, and installed a movable localized poloidal limiter, and then measured the effects of limiter position on the time intervals of plasma parameters such as plasma density, temperature, and energy confinement time. The results of effects of the movable limiter experiments on plasma confinement.     

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在HT-7装置上建立了一套高速CCD可见光成像诊断,测量了边界等离子体的可见光辐射成像.在HT-7装置放电中,首次观察到在等离子体边界区域存在一条极向旋转的可见光辐射带,由CCD诊断系统得到其极向旋转的频率为858Hz.根据多道Hα阵列测量得到极向旋转频率为952Hz.多道磁探针信号测量发现,等离子体内部存在m/n=3/1的电磁模,该模的旋转频率为972Hz.从电子回旋辐射诊断系统得到的电子温度剖面发现该模的磁岛宽度约为2.5cm.     

Effect of optical radiation in a photo-DOVATT

Avalanche photodetectors are very important solid-state detectors currently used in long-distance and wide-band optical communication systems, due to their faster speed of response compared to other solid-state photodiodes. Furthermore, it has been found that by using heterostructures one can improve both multiplication gain and quantum efficiency of such a device. DOVATT is a heterojunction impatt device in which there is one avalanche zone followed by two drift zones at different scattering limited velocities. The device is very useful for generation of high power in the X-band.The present paper examines the effect of optical radiation on such a device. Studies have been made on the frequency-response characteristics of the device. The results show that the device has the potentiality of becoming a powerful photodetector in optical communication systems.     

Avalanche photo-diodes for high frequency applications

The aim of this paper is to encourage the use of avalanche photo-diodes in optical communication systems, especially if high speeds are required. The basic theory of the device is first briefly reviewed. This is followed by a few examples of the possible performance compared to that of a PIN photodiode. Some frequency response measurements of recent avalanche photodiodes are then presented.     

Investigation on the Hard X-ray Radiations of the IR-T1 Tokamak Plasma: Electric and Magnetic Perspectives

In this experiment, the effect of magnetohydrodynamic (MHD) fluctuations in the hard X-ray radiation from the IR-T1 tokamak plasma is investigated. To reach this goal, the main parameters of plasma such as plasma current and loop voltage are measured. Also, the rake and poloidal Langmuir probes are used to calculate the radial and poloidal electric fields. To detect the hard X-ray radiation, a NaI-scintillator detector is used. To study on the MHD fluctuations, an array of 12 Mirnov coils is used. The obtained data are analyzed by using the singular value decomposition (SVD) algorithm. The wavelet spectrum of the dominant principal components of Mirnov coils is drawn. The results of wavelet and SVD analysis show that the hard X-ray radiation is increased with increasing the fluctuations of the dominant principal components (at the same time). It is also shown that the rate of hard X-ray radiation emitted from the tokamak plasma increased with increasing the MHD fluctuations. The energy of the system is wasted and reduced by these radiations. This an increase in the particle pressure of the plasma.     

应用高速CCD诊断研究HT-7装置边界等离子体旋转

在HT-7装置上建立了一套高速CCD可见光成像诊断,测量了边界等离子体的可见光辐射成像。在HT-7装置放电中,首次观察到在等离子体边界区域存在一条极向旋转的可见光辐射带,由CCD诊断系统得到其极向旋转的频率为858Hz。根据多道H_α阵列测量得到极向旋转频率为952Hz。多道磁探针信号测量发现,等离子体内部存在m/n=3/1的电磁模,该模的旋转频率为972Hz。从电子回旋辐射诊断系统得到的电子温度剖面发现该模的磁岛宽度约为2.5cm。     

Pioneering experiments on atomic-beam-assisted generation of drag currents in the Globus-M spherical tokamak

Research data for drag currents in the Globus-M spherical tokamak are presented. The currents are generated by injecting atomic beams of hydrogen and deuterium. Experiments were carried out in the hydrogen and deuterium plasma of the tokamak. It has a divertor configuration with a lower X-point, a displacement along the larger radius from–1.0 to–2.5 cm, and a toroidal field of 0.4 T at a plasma current of 0.17–0.23 MA. The beam is injected into the tokamak in the equatorial plane tangentially to the magnetic axis of the plasma filament with an impact diameter of 32 cm. To provide a 28-keV 0.5-MW atomic beam with geometrical sizes of 4 × 20 cm (at a power level of 1/e), an IPM-2 ion source is used. The generation of noninductive currents is detected from a rise in the loop current and a simultaneous dip of the loop voltage. The injection of the hydrogen and deuterium atomic beams into the deuterium plasma results in a noticeable and reproducible dip of the loop voltage (up to 0.5 V). Using the ASTRA transport code, a model is constructed that allows rapid calculation of noninductive currents. Calculations performed for a specific discharge confirm that the model adequately describes the effect of drag current generation.