The distribution of the parallel electron-current at the boundary of plasma on J-TEXT with different plasma configurations

Currents at the plasma boundary play a crucial role in plasma stability and transport in tokamak devices. However, the current structure under a three-dimensional (3D) boundary topology has rarely been directly measured. In this paper, we directly measure the parallel electron-current distribution at the plasma boundary of J-TEXT using a Directional Electron Probe (DEP). The positive direction of the electron-current is defined as the direction of the plasma current. Under a limiter configuration, a negative parallel electron-current layer appears in the Scrape-Off-Layer (SOL). Its peak magnitude reaches approximately 35 kA/m². Inside the last close flux surface (LCFS), the measured parallel electron-current agrees with the Ohmic current calculated by EFIT. Under a High-Field-Side (HFS) divertor configuration, the SOL parallel electron-current is nearly zero because of the symmetric strike lines. An m/n = 3/1 magnetic island can be generated by RMP field, where m is poloidal number and n is toroidal number. By rotating the island phase, the DEP measures the parallel electron-current at different island locations. Results show the parallel electron-current inside the island is nearly zero and the electron pressure profile is flat. At the island boundary, a negative parallel electron-current layer forms. As edge safety factor (q_a) decreases, the 3/1 magnetic island shifts outward, and the corresponding parallel electron-current structure moves outward accordingly. These observations suggests that edge magnetic islands or stochastic magnetic field can lead to a non-axisymmetric current structure at the plasma boundary.