R. J. Zhou

Generation and suppression of runaway electrons in EAST lower hybrid current drive experiments

In a lower hybrid current drive experiment on the EAST tokamak in the spring of 2010, the process in which fast electrons transform into runaway electrons was observed, and the way in which lower hybrid waves (LHWs) affect the behavior of runaway electrons was studied. It was found that when the power of LHWs is below a certain threshold, fast electrons excited by LHWs can be accelerated to runaway electrons due to the primary generation mechanism of runaway electrons, but when the power is high enough, fast electrons cannot transform into runaway electrons anymore. No matter how high the power was, the production of runaway electrons due to the secondary generation mechanisms was suppressed.

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.

Synchrotron radiation spectra and synchrotron radiation spot shape of runaway electrons in Experimental Advanced Superconducting Tokamak

A detailed analysis of the spectra of synchrotron radiation emitted by runaway electrons, and an analysis of synchrotron radiation spot shapes are presented for EAST runaway cases. Conditions required for the asymptotic expressions of synchrotron radiation spectra to be valid are studied for these EAST parameters. We provide the correct synchrotron radiation spectra in typical EAST discharges, and we show results of calculations of the shape of the synchrotron radiation spots emitted by runaway electrons. These shapes are detected by a visible light camera in EAST. Safety factor q(r), the horizontal displacement of electron drift surfaces with respect to the magnetic surfaces δe, pitch angle θp, and the position of the camera were taken into account. Our results indicate that the θp and q profiles can significantly affect the synchrotron radiation spot shape; it is simpler to record all synchrotron radiation if the camera is placed far from the plasma. An asymmetrical synchrotron radiation spot shape can ...

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.

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.

Effect of magnetic fluctuations on the confinement and dynamics of runaway electrons in the HT-7 tokamak

The nature of runaway electrons is such that the confinement and dynamics of the electrons can be strongly affected by magnetic fluctuations in plasma. Experimental results in the HT-7 tokamak indicated significant losses of runaway electrons due to magnetic fluctuations, but the loss processes did not only rely on the fluctuation amplitude. Efficient radial runaway transport required that there were no more than small regions of the plasma volume in which there was very low transport of runaways. A radial runaway diffusion coefficient of Dr≈10 m2s-1 was derived for the loss processes, and diffusion coefficient near the resonant magnetic surfaces and shielding factor ϒ=0.8 were deduced. Test particle equations were used to analyze the effect of magnetic fluctuations on runaway dynamics. It was found that the maximum energy that runaways can gain is very sensitive to the value of αs (i.e., the fraction of plasma volume with reduced transport). αs=(0.28−0.33) was found for the loss processes in the experime...

Investigation of runaway electrons in the current ramp-up by a fully non-inductive lower hybrid current drive on the EAST tokamak

The possibility of using a lower hybrid wave (LHW) to ramp up the plasma current (Ip) from a low level to a high enough level required for fusion burn in the EAST (experimental advanced superconducting tokamak) tokamak is examined experimentally. The focus in this paper is on investigating how the relevant plasma parameters evolve during the current ramp-up (CRU) phase driving by a lower hybrid current drive (LHCD) with poloidal field (PF) coil cut-off, especially the behaviors of runaway electrons generated during the CRU phase. It is found that the intensity of runaway electron emission increases first, and then decreases gradually as the discharge goes on under conditions of PF coil cut-off before LHW was launched into plasma, PF coil cut-off at the same time as LHW was launched into plasma, as well as PF coil cut-off after LHW was launched into plasma. The relevant plasma parameters, including H? line emission (Ha), impurity line emission (UV), soft x-ray emission and electron density , increase to a high level. The loop voltage decreases from positive to negative, and then becomes zero because of the cut-off of PF coils. Also, the magnetohydrodynamic activity takes place during the CRU driving by LHCD.

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.

Characterization of runaway-triggered spikes discharge in the EAST tokamak

In this paper, we investigate a regime of runaway discharges that is characterized by a strong enhancement of the relaxation oscillations seen in the electron cyclotron emission, Hα, visible bremsstrahlung and CIII emission, coupled with large spikes in the central line integral density, loop voltage, bolometer and Mirnov oscillations. In this regime, the background plasma is weakly ionized and low-temperature. The low-temperature background plasma can suddenly become heated by runaway electron beam–plasma instability. In this regime of runaway discharge, the primary mechanism of runaway electron generation is strongly suppressed because of the low temperature and low ionization of background plasmas.

Performance of fast-ion loss diagnostic on EAST

The scintillator-based detector for fast-ion loss measurements has been installed on EAST. To obtain high temporal resolution for fast-ion loss diagnostics, fast photomultiplier tube systems have been developed which can supply the complementary measurements to the previous image system with good energy and pitch resolution by using a CCD camera. By applying the rotatable platform, the prompt losses of beam-ions can be measured in normal and reverse magnetic field. The thick-target bremsstrahlung occurring in the stainless steel shield with energetic electrons can produce X-rays, which will strike on the scintillator based detector. To understand this interference on fast-ion loss signals, the effects of energetic electrons on the scintillator-based detector are studied, including runaway electrons in the plasma ramping-up phase and fast electrons accelerated by the lower hybrid wave.