Initial results of producing RELAX-tokamak plasmas for investigating two-fluid states in tokamak as well as reversed-field pinch*

The RELAX machine was upgraded [1] to investigate two-fluid toroidal plasmas in low-aspect-ratio circular tokamak and RFP plasmas. The strength of the toroidal magnetic field can reach approximately 0.2 T, which is about four times higher than before the upgrade. The safety factor at the plasma edge of RELAX-tokamak plasmas is approximately in the range between 2.5 and 3.5. The flattop-time is 2.5 ms, which is more than a thousand times longer than the Alfvén transit time. On the other hand, preliminary data for the plasma current and loop voltage are 20 kA and <10 V, respectively. This result indicates that more optimal control of the plasma is required. Currently , plasmas will be controlled by vertical and horizontal magnetic fields and active feedback control systems using saddle coil arrays. The electron density, which is so far estimated from the filling gas pressure, is of the order of 10¹⁸ m^-3. Thus, the ion skin depth for hydrogen plasmas results in ~10 cm. This is comparable with the typical scale length of plasmas formed in the RELAX. Therefore, theoretically, a two-fluid state is expected to appear in this case. Comparative studies in the RELAX will provide insight into the two-fluid plasma physics, especially transport mechanisms by internal probes and passive measurement instruments. In this conference, we present the initial results of the comparative studies of tokamak and RFP in the relatively lower-density regime.