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 structures under a three-dimensional (3D) boundary topology have rarely been directly measured. In this study, we directly measure the parallel electron-current density ( J_\texte,|| ) distribution at the plasma boundary of J-TEXT using a Directional Electron Probe (DEP). The positive direction of the J_\texte,|| is defined to be along the plasma current direction. Since electrons are negatively charged, J_\texte,||> 0 corresponds to an electron flow opposite to the plasma current direction. In a limiter configuration, a negative J_\texte,|| layer appears in the Scrape-Off-Layer (SOL). Its peak magnitude reaches approximately 35 kA/m². Inside the last closed flux surface (LCFS), the measured J_\texte,|| agrees with the Ohmic current density calculated by EFIT. Under a High-Field-Side (HFS) divertor configuration, the SOL J_\texte,|| is nearly zero because of the symmetric strike lines. An m/n = 3/1 magnetic island can be generated by a Resonant Magnetic Perturbation (RMP) field, where m is the poloidal number and n is the toroidal number. By rotating the island phase, the DEP measures the J_\texte,|| at different island locations. Results show that the J_\texte,|| inside the island is nearly zero and the electron pressure profile is flat. At the island boundary, a negative J_\texte,|| layer forms. As edge safety factor ( q_a ) decreases, the 3/1 magnetic island shifts outward, and the corresponding J_\texte,|| structure moves outward accordingly. These observations suggest that edge magnetic islands can lead to a non-axisymmetric current structure at the plasma boundary.