Ions Measurement at the Edge of HT-7 Tokamak

A reliable method of measuring ions and ion temperature in tokamak plasma is necessary, for which an omegatron-like instrument has been developed on the HT-7 tokamak. The basic layout of the omegatron-like instrument is shown in this article. The measurement of working gas ion has been performed in the last experimental campaign on HT-7 tokamak. The relations among ion current, the electron repeller voltage and trap voltage have been investigated. This omegatron-like instrument has also provided the edge-plasma ion temperature.     

The Spectroscopic Systems for the Study of Light Impurity Particle Transport in the HT-7 Tokamak

Three spectroscopic systems have been developed for the study of light impurity particle transport in the HT-7 tokamak. A visible multi-channel spectroscopic system （VIS） is used to obtain the brightness distribution of the line emission from ionized light impurities. The profile of Zeff（r） has been obtained from the visible multi-channel bremsstrahlung measurement （VB）. The system with a rotating hexahedral mirror for space-time resolved spectroscopy measurement from ultraviolet to visible （UV） can provide the brightness distribution of two different emission lines of the light impurities simultaneously. The emissivities by these multi-channel measurements can be obtained by Abel inversion. The measurement was performed in typical OH discharges in the HT-7 tokamak. The carbon particle transport was analyzed. The feasibility of these diagnostic systems for the impurity particle transport study is clearly demonstrated.     

Exposure of W-TiC/Cu Functionally Graded Materials in the Edge Plasma of HT-7 Tokamak

Six-layered W-TiC/Cu functionally graded materials were fabricated by resistance sintering under ultra-high pressure and exposed in the edge plasma of HT-7 tokamak. Microstruc- ture morphologies show that the TiC particles distribute homogeneously in the W matrix, strength- ening the grain boundary, while gradient layers provide a good compositional transition from W- TiC to Cu. After about 360 shots in the HT-7 tokamak, clear surface modification can be observed after plasma exposure, and the addition of nano TiC particles is beneficial to the improvement of plasma loads resistance of W.     

Radiation Losses in Ohmically Heated Plasma in the HT-7 Tokamak

Total radiation losses in ohmically heated plasma have been investigated in the HT-7 tokamak after different wall conditionings. Discharges after siliconization and boronization have been studied by using bolomitric measurements. Not only low-Z impurities but also high-Z impurities are reduced obviously after wall conditioning. The fraction of the total radiation loss to the ohmic input increases with increasing line-averaged central electron density. But it decreases with increasing the plasma current IP after wall conditionings. It is slightly lower for a boronized wall than that for a siliconized wall.     

Experiment Results about the Relationship of Edge Turbulence and Atomic Processes in the HT-7 Tokamak

Using a reciprocating Langmuir probe system, the boundary plasma behaviors were investigated before and after lithium/silicon coating. Accompanying the effective reduction of impurity radiation, strong shears of radial electric field and poloidal velocity came into being and the turbulence suppression and de-correlation were observed in the edge region of coated wall plasma. All these led to the reduction of the edge transport and improvement of plasma confinement. In the central line averaged density scanning experiments, an enhanced shear of the radial electric field was observed in the edge plasma with the increase of the density, which may account for the enhancement of the transport barrier and the improvement of particle confinement.The results suggest a close link between wall conditions and boundary plasma. In addition to the relationship, (～Te)/Te ～(～n)n/ne and θ_(～T)e(～n)e ～π, had been observed in the plasma edge region, which indicates the important role of the ionization and radiation in turbulence driving.     

Recent Progress of HT-7U Superconducting Tokamak

HT-7U is a superconducting tokamak. which is being constructed in Institute of Plasma Physics, Chinese Academy of Sciences. The mission of the HT-7U project is to develop a scientific and engineering basis of the steady state operation of advanced tokamak.The engineering design of the device has been optimized. The R&D program is going on. Short samples of the conductor and a CS model coil were tested. All the TF and PF coils will be manufactured and tested in Institute of Plasma Physics. Therefore, a 600-meter long jacketing line for cable-in-conduit conductors along with two winding machines, a set of VPI equipment and a test facility for the TF and PF coils are ready in ASIPP now. In this paper, the recent progress of the HT-7U is described.     

Edge Plasma behavior during Improved Confinement by Lower Hybrid Wave heating in HT—6M Tokamak

Lower hybrid heating (LHH) has been successfully carried out in the HT-6M tokamak.The H-mode has been obtained with a power threshold of 50kW under a boronized wall condition.Both energy and particle confinements have been improved along with a dropped edge plasma density and an increase electron temperature during the LHH phase.A negative Er well plays a key role of triggering and sustaining the good confinement.Both electrostatic fluctuation of the plasma potential and the density fluctuations dropped to an ultra-low level.The observation of an enhanced Er shear before the reduction in turbulence level is consistent with an increased Er shear as the cause of turbulence suppression.     

The Experiment of Modulated Toroidal Current on HT-7 and HT-6M Tokamak

The Experiments of Modulated Toroidal Current were done on the HT-6M tokamak and HT-7 superconducting tokamak. The toroidal current was modulated by programming the Ohmic heating field. Modulation of the plasma current has been used successfully to suppress MHD activity in discharges near the density limit where large MHD m = 2 tearing modes were suppressed by sufficiently large plasma current oscillations. The improved Ohmic confinement phase was observed during modulating toroidal current (MTC) on the Hefei Tokamak-6M (HT-6M) and Hefei superconducting Tokamak-7 (HT-7). A toroidal frequency-modulated current, induced by a modulated loop voltage, was added on the plasma equilibrium current. The ratio of A.C. amplitude of plasma current to the main plasma current △Ip/Ip is about 12% ～ 30%. The different formats of the frequency-modulated toroidal current were compared.     

Density Modulation Experiments to Determine Particle Transport Coefficients on HT-7 Tokamak

The particle diffusion coefficient and the convection velocity were studied based on the density modulation using D2 gas puffing on the HT-7 tokamak. The density was measured by a five-channel FIR interferometer. The density modulation amplitude was 10% of the central chord averaged background density and the modulation frequency was 10 Hz in the experiments. The particle diffusion coefficient （D） and the convection velocity （V） were obtained for different background plasmas with the central chord averaged density 〈ne〉 = 1.5×10^19m^-3 and 3.0×10^19 m^-3 respectively. It was observed that the influence of density modulation on the main plasma parameters was very weak. This technology is expected to be useful for the analysis of LHW and IBW heated plasmas on HT-7 tokamak in the near future.     

Alternate Data Acquisition and Real-time Monitoring System on HT-7 Tokamak

1 Introduction There are many tokamak devices over the world. Be- cause the tokamak device is based on the principle of a transformer, almost all tokamaks are pulse-operated and most of their discharge duration is within one or two minutes. A conventional data acquisition and anal- ysis system would be good enough for such devices. However, recent lower hybrid current-drive (LHCD) ex- periments such as those on the HT-7 and Tore Supra have generated discharges lasting several hundred sec- …     

The Study of Heat Pulse Propagation on HT—7 Tokamak

The propagation of heat pulses originating from sawtooth activity has been studied on the HT-7 tokamak.Two theoretical models are used for determining electron heat diffusivity from the experimental data measured by a diagnostic system of soft x-ray diode-array.The results show that one model caaled dipole model is more suitable for HT-7 tokamak.In order to improve the signal-to-noise(S/N) ratio of the original signals,over a few tens of sawteeth are averaged to generatc nicc wavcforms.The apace-time evolution is found to be diffusive in character,which is consistent with the theoretical model.The electron heat diffusivity determined from heat pulse propagation is larger than determined from background plasma power balance.The variation of Xe in different discharge phases has been also discussed.     

The Compression Algorithm for the Data Acquisition System in HT－7 Tokamak

HT-7 superconducting tokamak in the Institute of Plasma Physics of the Chinese Academy of Sciences is an experimental device for fusion research in China. The main task of the data acquisition system of HT-? is to acquire, store, analyze and index the data. The volume of the data is nearly up to hundreds of million bytes. Besides the hardware and software support, agreat capacity of data storage, process and transfer is a more important problem. To deal with this problem, the key technology is data compression algorithm. In the paper, the data format in HT-7 is introduced first, then the data compression algorithm, LZO, being a kind of portable lossless data compression algorithm with ANSI C, is analyzed. This compression algorithm, which fits well with the data acquisition and distribution in the nuclear fusion experiment, offers a pretty fast compression and extremely fast decompression. At last the performance evaluation of LZO application in HT-7 is given.     

Experimental Study of Diagnosis Neutral Beam for HT-7 Tokamak

Parametrical effect on plasma discharge and beam extraction in the diagnosis neutral beam （DNB） system for HT-7 tokamak was studied experimentally. Useful results with an improved beam quality were obtained.     

Turbulent Transport in the Tokamak Edge Plasma and SOL Region

In this work, the turbulent transport in the edge plasma and Scrape-Off Layer (SOL) region of IR-T1 tokamak at the presence of biased limiter has been investigated and analyzed. The time and radial evolution of floating potential, electric field and turbulent transport have been measured by using two arrays of the Langmuir probes in both the radial and poloidal directions. The analyses have been done by the Fast Fourier Transport method and spectral features of them are obtained with the help of the standard Auto-Correlation technique and modified covariance power spectral density estimate. The probability distribution function and actual transfer function magnitude of the radial and poloidal turbulent transport (Γ_r and Γ_p) have been investigated and compared in the edge plasma and SOL region. Also the histogram of turbulent transport has been analyzed and compared in the edge and SOL at presence of positive limiter biasing. The results show that in the edge plasma poloidal turbulent transport (Γ_p) is about of 60 % more than SOL region whereas radial turbulent transport (Γ_r) is about of 40 % less. During the application of positive biasing, it was found that Γ_r in the IR-T1 reduces by about 80 % in the edge plasma and 45–50 % in the SOL. Increase of Γ_p is about of 50 % after applied positive biasing in the edge while it increases 70 % nearly, in the SOL. Consequently, the improvement in confinement can be obtained for positive limiter biasing.     

The Design of the Polychromator for Thomson Scattering Measurements on HT-7 Tokamak

A five-channel polychromator, utilizing high performance interference filters, has been completed for Thomson scattering measurements on HT-7 tokamak. For our instrument, therange of electron temperature varies from 50 eV to 1.5 keV. According to this, the bandpass of the different interference filters are chosen. Unique features of the polychromator are high throughput,easy alignment, flexibility and compact size when compared with other alternatives. In this article,both the method of designing and the measured transmission curves for the polychromator are given.     

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.     

A Web-Based System for Remote Data Browsing in HT-7 Tokamak

HT-7 is the first superconducting tokamak device for fusion research in China.Many experiments have been performed on the HT-7 tokamak since 1994 with numerous satisfactory results achieved in the fusion research field. As more and better communication is required with other fusion research laboratories, remote access to experimental data is becoming increasingly important in order to raise the degree of openness of experiments and to expand research results. The web-based remote data browsing system enables authorized users in geographically different locations to view and search for experimental data without having to install any utility software at their terminals. The three-tier software architecture and thin client technology are used to operate the system effectively. This paper describes the structure of the system and the realization of its functions, focusing on three main points： the communication between the participating tiers, the data structure of the system and the visualization of the raw data on web pages.     

Electron Heating of LHCD Plasma in HT-7 Tokamak

Electron heating via lower hybrid current drive （LHCD） has been investigated in HT-7 superconducting tokamak. Experiments show that the central electron temperature Te0, the volume averaged electron temperature 〈 Te 〉 and the peaking factor of the electron temperature QTe = Teo/〈 Te 〉 increase with the lower hybrid wave （LHW） power. Simultaneously the electron heating efficiency and the electron temperature as the function of the central line-averaged electron density （ne） and the plasma current （Ip） have also been investigated. The experimental results are in a good agreement with those of the classical collision theory and the LHW power deposition theory.     

Simulation of Small Size Divertor Tokamak Plasma Edge Including Self-Consistent Electric Fields

The B2.SOLPES.0.5.2D code (Braams, Contrib Plasma Phys 36:276, 1996; Rozhansky and Tendler, Rev Plasma Phys 19:147, 1996) is applied for modeling SOL (Scrape off Layer) plasma in the small size divertor tokamak. Detailed distributions of the plasma heat flux and other plasma parameters in SOL, especially at the target plate of the divertor are found by modeling. The modeling results show that most of the electron heat flux and small part of ion heat flux arrive at target plate of the divertor, while, a large part of the ion heat flux and part of electron heat flux arrive at the outer wall. Also analysis of the role of poloidal E × B drifts in the redistribution of edge plasma is fulfilled.