Research Of Runaway Electron Behavior In The HL-2A Tokamak

Since energetic electron experiences lower collisional drag force in a discharge of tokamak, it is easily accelerated to run away from bulk plasma by E-field. When runaway electron is continuously accelerated, its energy can reach tens of MeV. As large flux energetic electrons could seriously damage the device, it is essential to do experimental research of runaway electron behavior and explore effective method of suppress the runaways.Base on the analysis of large quantity of discharge data on HL-2A, we have done these three aspects of study below:Firstly, through deep analyses of primary generation process and others relative critical runaway electron theories, we learned that electric density and electric field(or loop voltage) were two of the main factors that affect generation of runaway electrons. Results of the statistics of threshold conditions for runaways generation during flat-top phase on HL-2A tokamak showed that due to others runaways loss mechanisms, the experimental onset E-field of runaways generation was 12-58 times of Conner-Hastie theoretical E-field, the critical electric density for suppressing runaways was significantly lower than the theoretical calculations. The statistically analysis of growth rate of hard x-ray intensity indicated that the production rate of runaways under the primary generation mechanism was proportional to the loop voltage and inversely proportional to electric density. Decrease of loop voltage and increase of electric density were confirmed to be an effective measure to retard or suppress the generation of runaway electron.Secondly, via the study of plasma electron velocity distribution during electron cyclotron resonance heating(ECRH), we found that large quantity of thermal electrons and energetic electrons appeared in plasma during ECRH and they were slowing down after ECRH shutdown. Observes of hard x-ray decreasing during ECRH indicated that electron cyclotron wave had an ability of suppressing runaways. It was because that the increasing of electron temperature could cause the critical energy for electron runaway rises. And we studied the influence of power and duration of ECRH on the effect of runaway electron suppression.Finally, we studied the runaway electrons and runaway current during plasma disruption. Statistical study of the relationship between toroidal magnetic field and runaway electrons indicated that there were no runaway current generating under the condition of low toroidal magnetic field(1.2-1.5 T) during plasma disruption on HL-2A. Explanation was that magnetic fluctuation affect the transport of runaway electron, which load to the decelerating of runaway electrons. Then we introduced the feature of each phase of plasma disruption that has runaway current appeared in device, and investigated generation of runaway electron and evolution of runaway current with a typical discharge. At last we summarized the main characteristic of plasma disruption accompanied with runaway electrons generation.