Experimental study of non-inductive current in Heliotron J

It is important to control non-inductive current for generation and steady-state operation of high-performance plasmas in toroidal fusion devices. Helical devices allow dynamic control of non-inductive current through a wide variety of magnetic configurations. The reversal of non-inductive current consisting of bootstrap current and electron cyclotron driven current in electron cyclotron heating plasmas has been observed in a specific configuration at low density in Heliotron J device. By analyzing the non-inductive current for normal and reversed magnetic fields, we present experimental evidence for the reversal of bootstrap current. Our experiments and calculations suggest that the reversal is caused by a positive radial electric field of about 10 kV/m. Moreover, we show that the typical electron cyclotron current drive efficiency in Heliotron J plasma is about 1.0 x 1017 AW-1m-2, which is comparable to other helical devices. We have found that the value is about 10 times lower than that of tokamak devices. This might be due to an enhanced Ohkawa effect by trapped particles. (author)