Zhong Guoqiang

Measurement of Neutron Flux at the Initial Phase of Discharge in EAST

Neutron emission in EAST was investigated by a time-resolved monitor system which consists of four 3He proportional counters and a 235U fission chamber. The D-D neutron flux increased approximately an order of magnitude during the 27 MHz ion-cyclotron radio frequency (ICRF) heating, demonstrating that the ICRF wave heated the plasma effectively. In addition in lower hybrid wave (LHW) experiment with higher plasma parameters D-D neutrons were also detected. However, masses of photoneutrons were generated in Ohmic discharges with low plasma density. Effect of plasma density on the production of photoneutrons was studied, and it is found that LHW can suppress the generation of the runaway electrons and reduce the share of the photoneutrons effectively.

Time Dependent DD Neutrons Measurement Using a Single Crystal Chemical Vapor Deposition Diamond Detector on EAST

A single crystal chemical vapor deposition (scCVD) diamond detector has been successfully employed for neutron measurements in the EAST (Experimental Advanced Superconducting Tokamak) plasmas. The scCVD diamond detector coated with a 5 μm 6LiF (95% 6Li enriched) layer was placed inside a polyethylene moderator to enhance the detection efficiency. The time-dependent neutron emission from deuteron plasmas during neutral beam injection (NBI) heating was obtained. The measured results are compared with that of fission chamber detectors, which always act as standard neutron flux monitors. The scCVD diamond detector exhibits good reliability, stability and the capability to withstand harsh radiation environments despite its low detection efficiency due to the small active volume.

Neutron Yields Based on Transport Calculation in EAST ICRF Minority Heating Plasmas

Neutron diagnostics, including flux and energy spectrum measurements, have been applied on the experimental advanced superconducting tokamak (EAST). The absolute calibration of neutron yields has been achieved by a calculation method using the Monte Carlo automatic modeling (MCAM) system and the Monte Carlo N-Particles (MCNP) code. Since the neutron yield is closely related with the ion density and temperature, it is a good measure of plasma performance, especially the wave heating effect. In ion cyclotron range of frequencies (ICRF) experiments, the increase in the ion temperature derived by the neutron yield indicates an effective plasma heating. Minority protons damp a large fraction of the total wave power, and then transfer part of the energy to deuterium by collisions. Neutron spectrum measurements also indicate that no tail is created by high energy deuterons during ICRF heating. However, the ion temperature derived by the neutron yield is consistent with the result by using a poloidal X-ray imaging crystal spectrometer (PXCS), showing a reliable transport calculation.

Neutron Flux Measurements in an ICRF Mode Conversion Regime Heating Plasmas on HT-7

Ion cyclotron resonance heating experiments using antenna in the high field side (HFS) have been carried out on HT-7 in different target plasmas. Unlike a standard-mode conversion heating scheme with dominant electron heating, anomalous ion heating and DD neutron fluxes higher than those estimated from thermal ions were observed in the present experiments with the ion-ion hybrid resonant layer near the center of plasma. The features of ion cyclotron range frequency (ICRF) antenna in HFS and experiments suggest that this is most probably due to the nonlinear 3/2 harmonic deuterium heating by the mode-converted ion Bernstein wave, which could produce a high energy tail on ion energy distribution.

Observation of Electron Energy Pinch in HT-7 ICRF Heated Plasmas

Inward energy transport (pinch phenomenon) in the electron channel is observed in HT-7 plasmas using off-axis ion cyclotron resonance frequency (ICRF) heating. Experimental results and power balance transport analysis by TRANSP code are presented in this article. With the aids of GLF23 and Chang-Hinton transport models, which predict energy diffusivity in experimental conditions, the estimated electron pinch velocity is obtained by experimental data and is found reasonably comparable to the results in the previous study, such as Song on Tore Supra. Density scanning shows that the energy convective velocity in the electron channel has a close relation to density scale length, which is qualitatively in agreement with Wangs theoretical prediction. The parametric dependence of electron energy convective velocity on plasma current is still ambiguous and is worthy of future research on EAST.

AC operation and runaway electron behaviour in HT-7 tokamak

Operation of HT-7 tokamak in a multicycle alternating square wave plasma current regime is reported. A set of AC operation experiments, including LHW heating to enhance plasma ionization during the current transition and current sustainment, is described. The behaviour of runaway electrons is analysed by four HXR detectors tangentially viewing the plasma in the equatorial plane, within energy ranges 0.3–1.2 MeV and 0.3–7 MeV, separately. High energy runaway electrons (~MeV) are found to circulate predominantly in the opposite direction to the plasma current, while the number of low energy runaway electrons (~tens to hundreds of keV) circulating along the plasma current is comparable to that in the direction opposite to the plasma current. AC operation with lower hybrid current drive (LHCD) is observed to have an additional benefit of suppressing the runaway electrons if the drop of the loop voltage is large enough.