  Fully non-inductive plasma current start-up without the central solenoid in
ECW plasma was used on EXL-50 Spherical Torus with a weak external vertical
field (Bv). Generally, the number of electrons leaving to the vessel wall by
the gradient Bt is larger than ions, and the positive potential was built up in
plasma. The relationship between floating potential and the plasma current was
studied using the Langmuir probes near the boundary. The results show that the
floating potential is positive (about 200V) and has a strong correlation with
plasma current. In open magnetic field, the plasma current is driven by the
high energy electrons in preferential confinement, the plasma current and
potential approximately positively correlated with total electron density.
After forming the closed flux surface, the plasma current consists mainly of
the ECW driven current, and potential is negatively correlated with plasma
current. By actively adjusting the Bv, it demonstrated that the positive
voltage is approximately inversely correlated with the Bv and plasma current
(Ip). Considering that the plasma temperature near the boundary is quite low
(~eV), the positive voltage near the boundary caused by the high-energy
electron loss. Therefore, the measurements of the boundary potential are
important for the study of high-energy electron confinement performance,
noninductive plasma current start-up and current driven.
