Changzheng Li

Refluxing of fast electrons in solid targets irradiated by intense, picosecond laser pulses

The propagation of fast electrons produced in the interaction of relativistically intense, picosecond laser pulses with solid targets is experimentally investigated using K-alpha emission as a diagnostic. The role of fast electron refluxing within the target, which occurs when the electrons are reflected by the sheath potentials formed at the front and rear surfaces, is elucidated. The targets consist of a Cu fluorescence layer of fixed thickness at the front surface backed with a propagation layer of CH, the thickness of which is varied to control the number of times the refluxing fast electron population transits the Cu fluorescence layer. Enhancements in the K-alpha yield and source size are measured as the thickness of the CH layer is decreased. Comparison with analytical and numerical modelling confirms that significant refluxing occurs and highlights the importance of considering this phenomenon when deriving information on fast electron transport from laser-solid interaction experiments involving relatively thin targets.

Density limits investigation and high density operation in EAST tokamak

Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a H → L transition. The L-mode and H-mode density limit has been investigated in EAST tokamak. Experimental results suggest that density limits could be triggered by either edge cooling or excessive central radiation. The L-mode density limit disruption is generally triggered by edge cooling, which leads to the current profile shrinkage and then destabilizes a 2/1 tearing mode, ultimately resulting in a disruption. The L-mode density limit scaling agrees well with the Greenwald limit in EAST. The observed H-mode density limit in EAST is an operational-space limit with a value of . High density H-mode heated by neutral beam injection (NBI) and lower hybrid current drive (LHCD) are analyzed, respectively. The constancy of the edge density gradients in H-mode indicates a critical limit caused perhaps by e.g. ballooning induced transport. The maximum density is accessed at the H → L transition which is generally caused by the excessive core radiation due to high Z impurities (Fe, Cu). Operating at a high density () is favorable for suppressing the beam shine through NBI. High density H-mode up to could be sustained by 2 MW 4.6 GHz LHCD alone, and its current drive efficiency is studied. Statistics show that good control of impurities and recycling facilitate high density operation. With careful control of these factors, high density up to 0.93 stable H-mode operation was carried out heated by 1.7 MW LHCD and 1.9 MW ion cyclotron resonance heating with supersonic molecular beam injection fueling.

Snake perturbation during pellet injection in the EAST tokamak

The pellet-induced snake oscillation was observed by soft x-ray (SXR) diagnostic in EAST for the first time after a fueling-sized pellet penetrated the q = 1 surface. The snake phenomenon has a long lifetime with a helicity of m = 1 and n = 1. Basic behaviors of the snake, including the triggering condition, interaction with the sawtooth and snake rotation frequency, were discussed in detail by multiple core diagnostics. The snake location was also analyzed through observation of the vertical SXR arrays and raw SXR brightness profiles. It is clear that the snake resided in a broad region between the magnetic axis and the q = 1 surface derived from equilibrium reconstruction. This investigation is beneficial for the understanding of the snake formation for EAST and future devices, like ITER and DEMO.

The influence of neutral particles on boundary turbulence and confinement in the HT-7 tokamak

An effective technique for wall conditioning by injecting an ion cyclotron resonant frequency wave, RF coating, was developed in the HT-7 tokamak. Under different wall conditions, namely before and after lithium/silicon coating, the boundary plasma behaviour was investigated using a reciprocating Langmuir probe system. On reduction of impurity radiation, strong shears of the radial electric field and the poloidal velocity accompanied by turbulence suppression and de-correlation were observed in the edge region of the coated wall plasma. This led to a reduction in edge transport and improvement in plasma confinement. An enhanced shear of the radial electric field was observed on increasing the electron density, which may account for the improvement in particle confinement. The results suggest a close link between wall conditions and boundary plasma. The features of ionization and radiation driven turbulence, e/Te~ne/ne and θ_ene~π, had been found in the plasma edge region. The interplay between atomic processes in the boundary plasma and the formation of the radial electric field shear may be responsible for this behaviour.

Boundary plasma behavior under different wall conditions on HT-7 tokamak

The boundary plasma behaviors under different wall conditions on the HT-7 tokamak were investigated using a reciprocating Langmuir probe system. The ion cyclotron radio frequency (ICRF) boronization has become a routine wall conditioning technique on HT-7. After ICRF boronization, the turbulence de-correlation and radial electric field shear in the plasma edge region accompanied the strong reduction of impurity radiation, which resulted in the enhancement of edge transport barrier. The central chord average density scanning experiment had been carried out on HT-7. Accompanying the increase of density an evident increase of radial electric field shear could be found in the plasma edge region, which could be responsible for the turbulence de-correlation and improved particle confinement. The results presented here suggest a link between wall conditions and boundary plasma physics; especially interplay between atomic processes and turbulence through the formation of radial electric field shear in the plasma edge region.

Radio frequency siliconization: An approach to the coating for the future large superconducting fusion devices

Radio frequency (rf) siliconization has been carried out on the HT-7 superconducting tokamak in the presence of a high magnetic field, which is a try on superconducting tokamaks. Three different procedures of rf siliconization have been tested and a very promising method to produce high quality silicon films was found after comparing the film properties and plasma performance produced by these three different procedures. The Si/C films are amorphous, semitransparent, and homogeneous throughout the layer and adhere firmly to all the substrates. The advantages of silicon atoms as a powerful radiator and a good oxygen getter have been proved. An outstanding merit of rf siliconization to superconducting devices is its fast recovery after a serious degradation of the condition due to the leakage of air to good wall conditions. A wider stable operation region has been obtained and plasma performance is improved immediately after each siliconization due to significant reduction of impurities. Energy confinement ...