中国环流器2号A托卡马克弹丸注入放电中空电流与反磁剪切位形

Hollow current and reversed magnetic shear configurations in pellet injection discharges on Huanliuqi 2A tokamak

具备弱剪切或负磁剪切和内部输运势垒的托卡马克运行方式被认为是提高聚变性能的最有前途的方法.中空电流密度剖面与反磁剪切位形是改进堆芯约束和形成内部输运垒的关键条件之一.在中国环流器2号A(HL-2A)弹丸注入实验中,成功地实现了维持时间约为100 ms的中空电流放电.伴随着中空电流剖面的形成,同时形成了反磁剪切位形.由于欧姆加热功率不太高,且没有外部辅助加热,只能在稳定的中空电流放电阶段看到内部输运垒形成的趋势.在弹丸注入后,电子热扩散系数显著降低,说明弹丸深度注入改善了能量约束.等离子体性能的增强:一方面是由于弹丸注入造成中心高度峰化的电子密度剖面;另一方面是由于等离子体中心存在负磁剪切.同时,中空电流位形有利于改善高密度等离子体的稳定性.结果还表明,在中空电流放电中,等离子体比压值是低的.为了提高β_(N)极限,可在等离子体边界附近放置导电壁.HL-2A弹丸注入实验的结果,为在限制器托卡马克上获得高参数放电提供了一种可能.

The tokamak with weak or negative magnetic shear and internal transport barrier(ITB)is considered to be the most promising approach to improving fusion performance.The hollow current density profile,as well as the reversed q profile(negative magnetic shear),is one of the key conditions for improving core confinement in advanced tokamak schemes.In the Huanliuqi 2A(HL-2A)experiment,a hollow current distribution with a discharge duration of about 100 ms is successfully achieved by injecting the pellets in the Ohmic discharge.The discharge is characteristic of circular equilibrium configuration and three frozen pellets are injected continuously at three different time moments.As a result,the hollow current profiles are formed in the plasma with weak hollow electron temperature in the core region.At the same time,the hollow currents are combined with the reversed magnetic shear profiles.Because the power of Ohmic heating is not so high and there is no external auxiliary heating,we can see only a trend of the formation of weak internal transport barrier in the stable hollow current discharge stage.However,the electron thermal diffusivity decreases significantly after the pellets have been injected.The deep injection of frozen pellets improves the energy confinement.The enhancement of plasma performance is due to the peaked electron density profile in the center,caused by pellet injection and the negative magnetic shear in the plasma center.It is concluded that the electron density profile peaked highly in the core plasma,caused by pellet injection,is beneficial to the improvement of particle confinement and plays an important role in enhancing the energy confinement.In addition,it is also demonstrated that,in general,during a hollow current discharge,the poloidal beta β_(P) value and normalized beta β_(N) value are both obviously low although the reversed magnetic shear is conducive to stabilizing ballooning modes and weakening the drift instabilities. However, comparing with the hollow current profile, the plasma with peaked current profile is very beneficial to the improvement of beta limit. In order to improve the β_(N) limit, a conductive wall is necessary to be placed near the plasma boundary. The results of HL-2A pellet injection experiments present a possibility of obtaining high parameter discharge on a limiter tokamak.