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.

Sawtooth Activity in Ohmically Heated Plasma on HT-7 Tokamak

Sawtooth activity on HT-7 tokamak has been investigated experimentally mainly by using soft x-ray diode array and magnetic probes. Their behaviors and occurrences are correlated closely to the discharge conditions: the electron density Ne, the electron temperature Te, the safety factor qa on plasma boundary and wall condition etc. When central line-averaged electron density Ne(0) is over 2.0×1013cm-3 , major sawtooth activity emerges with a period of up to 6.5 ms and a fluctuation amplitude of up to 2-30 % of SXR radiation signal. In some cases such as the safety factor between 4.2-4.7 and Zeff=3.0-6.0, a monster sawtooth activity often emerges without apparent deterioration of plasma confinement and without major disruption. During these events, abundant MHD phenomena are observed including partial sawtooth oscillations. In this paper, the observed sawtooth behaviors and their dependence on the and their dependence density Ne and wall condition in ohmically heated plasma are introduced, the results are discussed and presented.

Experimental Results of MHD Suppression with Modulated LHCD on the HT-7 Tokamak

Modulation of lower hybrid current drive was used successfully to suppress MHD activity. This was achieved in discharges with MHD m = 2 tearing modes during the discharge conditions Ip = 110 kA, Bt = 1.75 T, ne0 - 1.1 × 1013 cm-3. The delivering time of LHCD pulse is less then 30 μs. The amplitude, interval and the period of LHCD modulation pulse can be adjusted very conveniently. The modulation LHCD can be delivered very fast at any time during the discharge. The modulation LHCD period was always much shorter than the plasma resistive time (Tη ≈100 ms). So the profile of plasma current is changed much faster than the plasma resistive time. The different forms of LHCD modulating can be proved.

H_α/ D_α Measurements Based on Photo Diode Array in the HT-7 Tokamak

Photo Diode Array (PDA) has been successfully applied in HT-7 tokamak experiments. The PDA system is almost free of electromagnetic interference from the machine. The system is compact and inexpensive, and it is convenient to be arranged in experiment. With the PDA system, the particle confinement time (τp) has been systematically investigated. The relations such as τp on the center-line-averaged electron density (ne), τp on plasma current (Ip), and τp, on the toroidal magnetic field (Bt) have been obtained. The particle confinement under the Ion Berstain Wave (IBW) Heating has also been measured and analyzed.

Recent Experimental Progress in the HT-7 Tokamak

Recent experimental campaign in the HT-7 toka-mak was scheduled to be ran from November 2002 toMarch 31, 2003, during which various issues were in-vestigated on getting high electron temperature, longpulse discharges and high performance plasma underquasi-steady-state condition. New systems includ-ing LHCD launcher, cryogenic compressor, real-timepoloidal control, water-cooled toroidal belt limiter,ferretic liner and several diagnostics were installedand operated to satisfy long-pulse operation require-ments. The maximum total injected power was over1 MW and the longest discharge duration was nearto 64 s.

Measurement of Magnetic Island Width by Multi-Channel ECE Radiometer on HT-7 Tokamak

A 16-channel electron cyclotron emission （ECE） radiometer has been employed to observe the （m, n） = （2, 1） magnetic island structure on HT-7 tokamak, where m and n represent the poloidal and toroidal mode number respectively. The results indicate that the island width is about 7 cm when the magnetic island is saturated during the m/n = 2/1 mode. The location of resonance surface can be determined by plotting the contour of ECE relative fluctuation. This method could be applied to the HT-7 and EAST campaigns in the future for the research of neoclassical tearing modes （NTMs）.

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.

Using Poloidal Flux as Feedback Control Signal for Injected Power of LHCD in Long Pulse Discharge on HT-7 Tokamak

Long pulse discharge is one of the important goals of HT-7 superconducting tokamak experiments. For ITER （International Thermonuclear Experimental Reactor） or a tokamak reactor, carrying out a steady operation is one of the main techniques. For long pulse discharges on HT-7 the poloidal flux is used as the feedback signal to control the injected power of LHCD （Low Hybrid Current Drive） system. Experimental results are presented.

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.

Multipulse Nd:YAG Laser Thomson Scattering Diagnostics on HT-7 Tokamak

A multipulse Nd：YAG （Neodym-yttrium aluminium garnet） laser Thomson scattering diagnostic system developed was recently applied on HT-7 tokamak to obtain more accurate electron temperatures. A CAMAC-based real-time computer system for laser control, data acquisition, analysis and calibration was investigated in detail. Furthermore, the reliability and accuracy of this diagnostic system were demonstrated by comparing the results with those of a soft-X ray diagnostic system.

Study of MARFE Phenomena on HT-7 Tokamak

MARFE phenomena on HT-7 tokamak is studied in this paper. A best correlation' has been found between the total input ohmic power and the product of the .edge line average density and Zeff. MARFEs which is strongly correlated with impurity density, always occurs at Zeff = 3~8 ohmic discharges. In HT-7 tokamak high Zeff discharges, it is found that the MARFEs usually occur at values of : in the range of 0.5~0.7, where p = In the case of good wall condition (Zeff= 1 ~2), the onset of MARFEs have not been observed before reaching the Greenwald density limit' on HT-7. An improved confinement mode plasma which is induced by the MARFE is observed, the global particle confinement time increases 1.9 times. The relaxation time between the MARFE event trigger and the L-U transition is about 1.4 ms, the followed L-H transition time is 1.9 ms, the improved confinement mode phase is maintained for about 40 ms. MARFE cools the plasma edge, and the electron density profileis observed to become more narrow and peaking.

A Study of Energy Conversion Efficiency Versus Plasma Density by Lower Hybrid Current Drive in HT-7 Tokamak

Ramp-up experiments by means of lower hybrid wave on HT-7 superconducting tokamak have been performed and analyzed. A ramp-up rate of over 300 kA/s is obtained and a conversion efficiency of over 1.0% has been achieved during the ramp-up phase. The study of the dependence of conversion efficiency on plasma density shows that the conversion efficiency is affected by the driven current, which is mainly dominated by the competition of impurity concentration with wave accessibility condition. In addition, the effect of current profile may play an important role in determining the conversion efficiency.

Feedback Control Systems on HT-7 Tokamak

Through a detailed analysis of the construction of HT-7, a decoupling model for heating field and vertical field is given. This paper presents a simple introduction of its basic structure. And with a practical design, this model has been successfully applied to the device, procuring a very good controlling of the plasma parameters of HT-7. The performance of individual feedback systems is also analyzed in detail.

Upgraded Poloidal Field Control System for HT-7 Tokamak

This paper reviews how we have designed the upgraded poloidal field control system on HT-7 superconducting Tokmak by applying advanced computer technologies and Multivariable decoupling control theory. This paper emphasizes on designing the method of the control system,including system components and their functions.

Soft X-ray Tomography and Sawtooth Oscillations during IBW in HT-7 Tokamak

Tomographic reconstructions of soft x-ray emissivity in the HT-7 tokamak have been obtained using 86 miniature soft-x-ray detectors. Because there are two-detector arrays which are distributed in horizontal and vertical directions on one poloidal plane, two-dimensional images of x-ray emissivity are obtained without having resort to a rotational model. The algorithm of Fourier-Bessel expansion was employed. The data analysed so far have been used principally to study MHD instabilities, and the illustrative examples of the sawtooth crash in IBW-heated plasma are described.

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-7 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, a great 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.

Analysis of Charge-Exchange Spectrum during Lower Hybrid Current Drive on HT-7 Tokamak

A perpendicular neutral particle analyzer (NPA) is used on HT-7 tokamak experiment to provide the measurements of neutral particle flux and ion temperature. The measured results were compared with calculated data by Monte-carlo method for ohmic heating. In lower hybrid current drive (LHCD) experiments, the ion heating of LHCD was identified. The chargeexchange neutral particle flux enhanced was observed. The energy spectrum clearly showed a high-energy ions tail. The bulk ion temperature increased by 0.4-0.5 keV after the onset of LHCD. Efficient ion heating of 1 eV. kW-1 was obtained.

Temperature of the Limiter Surface Measured by IR Camera in HT-7 Tokamak

Temperature measurement by IR （infrared） camera was performed oll HT-T tokamak. particularly during long pulse discharges, during which the temperature of the hot spots on the belt limiter exceeded 1000℃. The heat load on the surface of the movable limiter could be obtained through ANSYS with the temperature measured by IR-camera. This work could be important for the temperature measurement and heat load study on the first wall of EAST device.

The Realization of Low Loop Voltage Start-up of HT-7 Tokamak Discharge with the Assistance of Lower Hybrid Wave

Low voltage start-up was realized in HT-7 discharges under the assistance of lower hybrid (LH) waves. The use of a strong LH wavet which can Change its N// spectrum in a large range of several milliseconds, made the loop voltage for start-up reduce from around 20 V to less than 5 V. It means that the electric field for HT-7 start-up decreases from 2.5-3 V/m to 0.6 V/m. Some physical phenomena such as the consumption of magnetic flux in start-up phase and the radiation from the initial plasma were observed in this kind of low voltage start-up discharges.

Control System of Pellet Injector on the HT-7 Tokamak

In this paper the control system of the pellet injector is introduced in detail and the system mainly includes two parts: the present and the remote control system. The present control system controls the injector and provides the interface to the remote system. And the remote control system has acquired present signals with analog input card and perform the actions through digit output card, it also has an interface for Windows programming easily used by the operators when carrying out the pellet injection experiments. Through several HT-7 campaigns, the remote control system has been validated to be feasible and reliable and has made successful shots for studying the interactions between the pellets and plasma.