Experimental study of density gradient-driven micro-instabilities and the confinement degradation during H-mode in EAST

A broadband(BB)mode is observed by collective Thomson scattering diagnostics in repeatable shots of EAST and analyzed for the first time.This BB mode usually grows during L–H transitions,featuring a BB quasi-coherent mode with increasing frequency.During H-mode operations,it is characterized by steady-state BB in the high-frequency range(f~200–2000 k Hz),at the electron scale(k_(θ)ρ_(s)=1–2),mainly driven by the density gradient,and is sensitive to the value ofηein the region of interest(ρ=0.4–0.8),wherehe=(R/L_(Te))/(R/L_(ne))is the ratio of the normalized electron temperature gradient and density gradient,and the regionρ=0.4–0.8 usually has a relatively low collisionality(v_(eff)<5).The frequency of BB is found to be dependent on the electron temperature and density gradient,which is a typical feature of electron-driven turbulence.A negative correlation between the energy confinement and the intensity of the BB turbulence during H-mode has been found,which indicates a strong electron thermal transport induced by the BB turbulence.The BB significantly decreases the electron temperature and causes flatter electron temperature profiles in the region of interest(ρ=0.4–0.8),thus makingηedecrease and the BB destabilize further.These characteristics of BB are related to the theoretical density gradient-driven trapped electron mode.It should be noted that this mode is not observed by other diagnostics in EAST,and shows very different features to the coherent modes in the edge.

Kinetic micro-instabilities in the presence of impurities in toroidal magnetized plasmas

The theoretical and numerical studies on kinetic micro-instabilities,including ion temperature gradient（ITG） driven modes,trapped electron modes（TEMs） in the presence of impurity ions as well as impurity modes（IMs）,induced by impurity density gradient alone,in toroidal magnetized plasmas,such as tokamak and reversed-field pinch（RFP） are reviewed briefly.The basic theory for IMs,the electrostatic instabilities in tokamak and RFP plasmas are discussed.The observations of hybrid and coexistence of the instabilities are categorized systematically.The effects of impurity ions on electromagnetic instabilities such as ITG modes,the kinetic ballooning modes（KBMs） and kinetic shear Alfvén modes induced by impurity ions in tokamak plasmas of finite β（=plasma pressure/magnetic pressure） are analyzed.The interesting topics for future investigation are suggested.

High Wavenumber Density Fluctuation Measurement in the HT-7 Tokamak

A three channel CO2 laser forward scattering diagnostic system was set up to study the core plasma density fluctuation in the HT-7 tokamak. The range of the wavenumber measurement in the poloidal direction was kθ= 12 cm^-1 to 34 cm^-1 and the wavenumber resolution was △k = 2.5 cm^-1. The wavenumber spectrum, the coexisting modes and the micro-instability eruption in experiments are presented.

Effects of trapped electrons on the ion temperature gradient mode in tokamak plasmas with hollow density profiles

The ion temperature gradient(ITG)mode in the presence of impurity ions and trapped electrons(TEs)is numerically investigated in tokamak plasmas with hollow density profiles,using the gyrokinetic integral eigenmode equation.It is found that in the inverted density plasma,the increase of the ITG enhances the growth rate and frequency of the ITG,and the density gradient plays an important role in the ITG modes.For weak density gradient situations,the trapped electron effects increase the instability of the ITG,while the impurity has an obviously stabilizing effect.For the strong density gradient cases,both the impurities and trapped electrons enhance the ITG instabilities.In addition,it is shown that the growth rate of the ITG decreases with positive magnetic shear s while the real frequency increases with positive magnetic shear.The growth rate of the ITG increases with negative magnetic shear s while the real frequency decreases with negative magnetic shear.The length of the calculated mode structure in the positive and negative magnetic shear intervals is also presented.