G Q Zhong

Investigation of ring-like runaway electron beams in the EAST tokamak

In the EAST tokamak, asymmetrical ring-like runaway electron beams with energy more than 30 MeV and pitch angle about 0.1 were investigated. Those runaway beams carried about 46% of the plasma current and located around the q = 2 rational surface when m/n = 1/1 and m/n = 2/1 MHD modes existed in the plasma. Those runaway beams changed from a hollow to a filled structure during the slow oscillations in the discharge about every 0.2 s, which correlated with a large step-like jump in electron cyclotron emission (ECE) signals, a big spike-like perturbation in Mirnov signals and a large decrease in runaway energy. Between those slow oscillations with large magnitude, fast oscillations with small magnitude also existed about every 0.02 s, which correlated with a small step-like jump in ECE signals, a small spike-like perturbation in Mirnov signals, but no clear decrease in runaway energy and changes in the runaway beam structure. Resonant interactions occurred between runaway electrons and magnetohydrodynamic instabilities, which gave rise to fast pitch angle scattering processes of those resonant runaway electrons, and hence increased the synchrotron radiation. Theoretical calculations of the resonant interaction were given based on a test particle description. Synchrotron radiation of those resonant runaway electrons was increased by about 60% until the end of the resonant interaction.

Operational region and sawteeth oscillation in the EAST tokamak

The first plasma discharges were successfully achieved on the experimental advanced superconducting tokamak (EAST) in 2006. The sawteeth behaviours were observed by means of soft x-ray diagnostics and ECE signals in the EAST. The displacement and radius of the q = 1 surface was studied and compared with the result of equilibrium calculation. The density sawtooth oscillation was also observed by the HCN laser interferometer diagnostics. The structure of the EAST operational region was studied in detail. Plasma performance was obviously improved by the boronization and wall conditioning. It was observed that lower qa and a wider stable operating region is extended by the GDC boronization.

Experimental observation of beta-induced Alfvén eigenmodes during strong tearing modes on the EAST tokamak in fast-electron plasmas

Beta-induced Alfv?n eigenmodes (BAEs) during strong tearing modes are investigated on the EAST tokamak systematically, and the relation between the BAE frequencies and plasma parameters such as electron density , ion temperature Ti, the profile of safety factor q(?) or the intensity of (the width of the magnetic island w) is given in detail during the injection of the power of lower hybrid wave (LHW) (or is also accompanied by the injection of ion cyclotron resonance frequency) comprehensively. All the conditions show that the values of BAE frequencies are in agreement with the generalized fishbone-like dispersion relation, and the activities of the BAEs have a strong interaction with the process of magnetic reconnection.The BAEs are formed during the injection of the power of LHW, and disappear immediately when the power of LHW is turned off on the EAST tokamak. The LHW plasmas or the runaway discharge in Ohmic plasmas can increase the population of fast electrons, which plays a role in the activities of BAEs and a possible excitation mechanism for the BAEs during the strong tearing mode activities.

Neutron flux in lower hybrid current drive plasmas on EAST

Diagnostics application of fusion reaction measurements on the Experimental Advanced Superconducting Tokamak (EAST) has been proved by fusion neutrons during deuterium plasma discharges. In lower hybrid current drive plasmas, fusion neutrons are dominant mainly due to a sufficiently low loop voltage, which suppresses the photo-neutrons caused by runaway electrons. For fixed plasma current and lower hybrid wave (LHW) power, the fusion neutron flux depends on ion density as , stronger than a power of two, caused by an increased ion temperature, which is due to an increased electron and ion energy exchange by more frequent collisions at higher ion densities. The fusion neutron flux depends on , at fixed LHW power and main ion density, which is mainly due to ion temperature affected by ohmic power and energy confinement time. Finally, the unchanged fusion neutron flux with LHW power indicates little variation in the main ion temperature, which is consistent with direct ion temperature measurements. This is explained by the fact that an increased energy transfer to ions by collisions is almost compensated for by less frequent collisions and decreased energy confinement time at higher LHW powers.

Preliminary observation of medium-Z impurities by means of the soft x-ray pulse height analysis diagnostic on EAST

A soft x-ray pulse height analysis (PHA) system, based on a silicon drift detector (SDD), has been successfully implemented on EAST to measure spectra of soft x-ray emission. Utilizing the soft x-ray PHA diagnostic, the behavior of intrinsic and injected medium-Z impurities is first investigated on EAST and the preliminary observations are presented. There are hardly any K? emission peaks of metallic impurities including Ti (Z = 22), Cr (Z = 24), Fe (Z = 26), Ni (Z = 28) and Cu (Z = 29) in the soft x-ray spectra for an ohmic heating plasma due to lower electron temperatures, while remarkable peaks can be seen in a lower hybrid wave (LHW) heated plasma. The profiles of K? intensities of these metallic elements have been obtained with about 7?cm spatial resolution. It is observed that the K? emissions come mainly from the core plasma column of ? ? 0.3 and found that their intensities are roughly proportional to the power of the LHW launched into the plasma. The Ar K? peaks can also be monitored when argon gas is puffed into the plasma. It is qualitatively confirmed that the K? lines of argon emitted from the lower ionic states are predominantly located in the outer plasma region, whereas the K? lines with higher energies are mainly from high ionization stages in the central region.

Validation of fast-ion D-alpha spectrum measurements during EAST neutral-beam heated plasmas

To investigate the fast ion behavior, a fast ion D-alpha (FIDA) diagnostic system has been installed on EAST. Fast ion features can be inferred from the Doppler shifted spectrum of Balmer-alpha light from energetic hydrogenic atoms. This paper will focus on the validation of FIDA measurements performed using MHD-quiescent discharges in 2015 campaign. Two codes have been applied to calculate the Dα spectrum: one is a Monte Carlo code, Fortran 90 version FIDASIM, and the other is an analytical code, Simulation of Spectra (SOS). The predicted SOS fast-ion spectrum agrees well with the measurement; however, the level of fast-ion part from FIDASIM is lower. The discrepancy is possibly due to the difference between FIDASIM and SOS velocity distribution function. The details will be presented in the paper to primarily address comparisons of predicted and observed spectrum shapes/amplitudes.

The behavior of neutron emissions during ICRF minority heating of plasma at EAST

Ion cyclotron radio frequency (ICRF) wave heating is a primary method to heat ions in the Experimental Advanced Superconducting Tokamak (EAST). Through neutron diagnostics, effective ion heating was observed in hydrogenminority heating (MH) scenarios. At present, investigation of deuterium–deuterium (DD) fusion neutrons is mostly based on time-resolved flux monitor and spectrometer measurements. When the ICRF was applied, the neutron intensity became one order higher. The H/H + D ratio was in the range of 5–10%, corresponding to the hydrogen MH dominated scenario, and a strong high energy tail was not displayed on the neutron spectrum that was measured by a liquid scintillator. Moreover, ion temperature in the plasma center (T i) was inversely calculated by the use of neutron source strength (S n) and the plasma density based on classical fusion reaction equations. This result indicates that T i increases by approximately 30% in L-mode plasma, and by more than 50% in H-mode plasma during ICRF heating, which shows good agreement with x-ray crystal spectrometer (XCS) diagnostics. Finally, the DD neutron source strength scaling law, with regard to plasma current (I P) and ICRF coupling power (P RF) on the typical minority heating condition, was obtained by statistical analysis.

Status of neutron diagnostics on the experimental advanced superconducting tokamak

Neutron diagnostics have become a significant means to study energetic particles in high power auxiliary heating plasmas on the Experimental Advanced Superconducting Tokamak (EAST). Several kinds of neutron diagnostic systems have been implemented for time-resolved measurements of D-D neutron flux, fluctuation, emission profile, and spectrum. All detectors have been calibrated in laboratory, and in situ calibration using 252Cf neutron source in EAST is in preparation. A new technology of digitized pulse signal processing is adopted in a wide dynamic range neutron flux monitor, compact recoil proton spectrometer, and time of flight spectrometer. Improvements will be made continuously to the system to achieve better adaptation to the EASTs harsh γ-ray and electro-magnetic radiation environment.

First results obtained from the soft x-ray pulse height analyzer on experimental advanced superconducting tokamak

An assembly of soft x-ray pulse height analyzer system, based on silicon drift detector (SDD), has been successfully established on the experimental advanced superconducting tokamak (EAST) to measure the spectrum of soft x-ray emission (E=1-20 keV). The system, including one 15-channel SDD linear array, is installed on EAST horizontal port C. The time-resolved radial profiles of electron temperature and K(alpha) intensities of metallic impurities have been obtained with a spatial resolution of around 7 cm during a single discharge. It was found that the electron temperatures derived from the system are in good agreement with the values from Thomson scattering measurements. The system can also be applied to the measurement of the long pulse discharge for EAST. The diagnostic system is introduced and some typical experimental results obtained from the system are also presented.

IBW heating experiments in HT-7 tokamak

The Experiments with Ion Berstein wave (IBW) heating has been carried out in HT-7 superconducting tokamak. Core electron heating and peaking profile of electron temperature can be observed in the beginning 40 ms after injection of IBW. The temporary heating efficiency of IBW increases with the toroidal magnetic field (BT). The electron temperature drops gradually with the increasing electron density after 40 ms of IBW. The electron density increases by a factor up to 2.4 and the profile of electron density becomes wide. Improved particle confinement induced by IBW is one reason for the density increase. Impurity release during IBW heating is also a contribution for the density increase. Suppression of MHD by IBW is also observed. The fluctuation and transport in the SOL of HT-7 tokamak have been investigated using a Langmuir probe array. The polodial shear flow induced by IBW maybe the reason for improved particle confinement.