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.     

Spectroscopic Measurements of Impurity Spectra on the EAST Tokamak

Ultraviolet (UV) and visible impurity spectra (200～750 nm) are commonly used to study plasma and wall interactions in magnetic fusion plasmas. Two optical multi-channel analysis (OMA) systems have been installed for the UV-visible spectrum measurement on EAST. These two OMA systems are both equipped with the Czerny-Turner (C-T) type spectrometer. The upper vacuum vessel and inner divertor baffle can be viewed simultaneously through two optical lenses. The OMA1 system is mainly used for multi-impurity lines radiation measurement. A 280 nm wavelength range can be covered by a 300 mm focal length spectrometer equipped with a 300 grooves/mm grating. The Dα/Hα line shapes can be resolved by the OMA2 system. The focal length is 750 mm. The spectral resolution can be up to 0.01 nm using a 1800 grooves/mm grating. The impurity behaviour and hydrogen ratio evolution after boroniztion, lithium coating, and siliconization are compared. Lithium coating has shown beneficial effects on the reduction of edge recycling and low Z impurity (C,O) influx. The impurity expelling effect of the divertor configuration is also briefly discussed through multi-channels observation of OMA1 system.     

Impurity Emission Behavior in the Soft X-Ray and Extreme Ultraviolet Range on EAST

Spectroscopy in the soft X-ray and extreme ultraviolet(XEUV) region is very important in magnetic fusion research.Recently,two flat-field spectrometers that utilize a varied line spacing grating to image the spectra of 1-13 nm and 5-50 nm were installed on EAST for core impurity emission monitoring and impurity transport study.The instruments were proven to be capable of observing spectral lines from low-Z impurities(Li,C,O,N,Ar,etc.) and highly ionized medium- and high-Z impurities(Fe,Cr,Ni,Cu,Mo,etc.).For example,spectra in the wavelength intervals of 1-2 nm and 5-13 nm contained strong metal lines,especially molybdenum lines during H-mode phases.Argon and nitrogen lines were also observed,which were injected for diagnostic purposes.Impurity lines were identified and compared to measurements on other magnetic fusion research devices.Detailed measurements of radial emission profiles from various impurity line emissions were also presented.     

Sawtooth Activity in Ohmically Heated Plasma on HT-7 Tokamak

1. IntroductionSawtooth oscillations, characterized by periodiccollapses of the pressure or internal disruptions inthe plasma core, belong to the most typical forms ofmagnetohydrodynamic (MHD) activity. Since sawtooth oscillations were discovered in 1974 [l], theyhave been widely studied in many tokamaks and havecontinuously stimulated extensive investigations anddevelopments in experiments and theory. Experimental results in many tokamaks have convincinglyshown that sawtooth instability plae…     

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 hydrid wave on HT-7 superconducting tokamak have been performed and analyzed.A ramp-up rate of over 300kA/s is obtained and a conversion efficiency of over 10% has been achieved during the ramp-up phases.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.     

Test of a High Throughput Detector on the X-ray Crystal Spectrometer of the EAST

An attempt was made to improve the spatio-temporal resolution of the tangen- tial X-ray crystal spectrometer (XCS) on the Experimental Advanced Superconducting Tokamak (EAST) by evaluating experimentally the applicability of a novel X-ray photon detection tech- nology for measuring the satellite spectra of Ar XVII with a high counting rate. High-resolution experimental data on the profiles of ion temperature and plasma rotation velocity facilitate the studies of the mechanisms underlining important physical phenomena, such as plasma heating, L-H transition and momentum transport. Based on silicon diode array and single-photon counting technology, a relatively small area (83.8×33.5 mm 2 ) two-dimensional detector was successfully in- stalled and tested in the recent EAST campaign. X-ray photon counting rate higher than 20 MHz was observed for the first time, and high quality satellite spectra were recorded for ion temperature and plasma rotation measurement, indicating that the new technology is suitable for the next-step high-resolution XCS on EAST, and the deployment of a detector array with a much larger X-ray sensing area is planned for better plasma coverage.     

Behaviors of Electron Heat Transportation in HT-7 Sawtoothing Plasma

It is found that in HT-7 ohmic plasma, main energy loss comes from electron heat conduction, hence quantitative data of electron heat diffusivity is a very important issue for investigation of electron heat transportation behavior in different target plasmas so as to get high performance plasma. A time-to-peak method of the heat pulse propagation originating from the sawtooth activity on the soft x-ray intensity signal has been adopted to experimentally determine electron heat diffusivity χe^HP on the HT-7 tokamak. Aiming to improve the signal-to-noise (S/N) ratio of the original signal to get a stable and reasonable electron heat diffusivity χe^HD value, some data processing methods, including average of tens of sawteeth, is discussed. The electron heat diffusivity χe^HP is larger than χe^PB which is determined from the balance of background plasma power. Based on variation of the measured electron heat diffusivity χe^HP , performances of different high confinement plasmas are analyzed.     

Vertical Instability in EAST： Comparison of Model Predictions with Experimental Results

Growth rates of the axisymmetric mode in elongated plasmas in the experimental advanced superconducting tokamak （EAST） are measured with zero feedback gains and then compared with numerically calculated growth rates for the reconstructed shapes. The comparison is made after loss of vertical position control. The open-loop growth rates were scanned with the number of vessel eigenmodes, which up to 20 is enough to make the growth rates settled. The agreement between the growth rates measured experimentally and the growth rates determined numerically is good. The results show that a linear RZIP model is essentially good enough for the vertical position feedback control.     

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.     

Enhanced Lower Hybrid Current Drive Experiments on HT-7 Tokamak

Effective Lower Hybrid Current Driving (LHCD) and improved confinement experiments in higher plasma parameters (Ip > 200 kA, ne > 2 ×1013 cm-3, Te≥ 1 keV) have been curried out in optimized LH wave spectrum and plasma parameters in HT - 7 superconducting tokamak. The dependence of current driving efficiency on LH power spectrum, plasma density ne and toroidal magnetic field BT has been obtained under optimal conditions. A good CD efficiency was obtained at higher plasma current and higher electron density. The improvement of the energy confinement time is accompanied with the increase in line averaged electron density, and in ion and electron temperatures. The highest current driving efficiency reached ηCD = IpneR/PRF≈1.05×1019 Am-2/W. Wave-plasma coupling was sustained in a good state and the reflective coefficient was less than 5%. The experiments have also demonstrated the ability of LH wave in the start-up and ramp-up of the plasma current. The measurement of the temporal distribution of plasma