Research on transport barrier and two-fluid relaxation phenomenon based on radial electric field caused by an orbit loss of high energy electrons

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We are exploring ultra-high beta plasmas based on two-fluid relaxation theory proposed by Mahajan-Yoshida with an internal coild device Mini-RT, where the plasma is produced by 2.45 GHz microwave. The cut-off density of 2.45 GHz electromagnetic microwave is limited up to 7.4x10^<16> m^<-3>. Here we have considered to introduce an Electron Bernstein Wave (EBW) in order to overcome this density limit, because EBW has no constraint on the plasma density for wave propagation. We are studying an excitation of EBW through the mode-converstion from X-mode launched from the weak-field side.Up to now, in floating coil experiments, we have achieved the plasma density of 1.5x10^<17> m^<-3>, which is about 2 times higher than the cut-off density. To realize a mode conversion to EBW from X-mode efficiently, the steep density gradient should be prerequisite ; i.e., theory predicts that the product of the density scale length and the magnetic field strength should be 5x10^<-4> Tm. This means that the density scale length should be less than 1 cm in the Mini-RT case. We have therefore introduced the steep density gradient at the separatrix region, which is produced by the combination of the floating and levitation coils.There appears many higher harmonics of the electron cyclotron resonance in the internal coil device, because the field strength of the dipole magnetic field is rapidly decaying ; i.e., B 〜 R^<-3>. In addition, the mode-conversion takes place at the upper hybrid resonance region, which is sensitive to the plasma density. This might introduce the feasibility of heating profile control, by changing the position of the upper hybrid resonance region. Our preliminary experimental results show that the radial position of electron heating moves according to the change of the plasma density. These results might be useful for exploring ultra-high beta plasmas in the internal coil device.