Experimental simulation of high temperature plasma confinement using dimensionally similar low temperature plasma

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Better understanding of transport phenomena such as transport barrier formation in high temperature plasma is a key issue for realization of fusion-burn plasma. However, the phenomena are very complex and the understanding is still preliminary. The main reason is difficulty to measure various plasma parameters and turbulent fluctuations with high time and spatial resolutions. To resolve this difficulty, this project aims at simulating transport in high temperature plasma using "dimensionally similar" low temperature plasma where several dimensionless parameters such as normalized collision frequency, plasma toroidal beta and so on are the same as those in high temperature plasma, except normalized ion gyro radius evaluated with electron temperature. In the low temperature plasma, Langmuir probes can be applied for measurements of equilibrium plasma parameters, fluctuations and correlations among various fluctuations with high time and spatial resolutions. This experiment was carried ou … More t in a small sized heliotron/torsatronl device CHS, having two main research objectives : (1) to produce and heat low temperature and density plasmas at low toroidal field condition by launching 2.45GHz electron cyclotron waves (ECWs), and (2) to measure electrostatic fluctuations in this low temperature plasma and clarify turbulent plasma transport. (1) : Plasma production was performed by 2.45GHz ECW launching in the range of the toroidal field from 175 G to 1050 G Over-dense plasmas exceeding the cut-off density for ordinary mode (O-mode) were successfully and routinely generated. In these over-dense plasmas, collisionality regimes of MHD (collisional), plateau and 1/v-regimes were realized over a substantial fraction of a plasma minor radius. This plasma production and heating is thought to be done by electron Bernstein waves (EBWs) converted from launched ECWs. Power absorption profile measured with ECW power modulation technique is concentrated in interior region beyond X-mode cutoff and upper-hybrid layers. This result is consistent with mode-converted EBW heating. (2): When launching microwave power exceeds a certain threshold, electron density profile exhibits a clear transition to a very peaked profile having steep gradient from a usual parabolic one. This indicates formation of internal particle transport barrier. As a new trial, we attempted to heat up a plasma by ramping ECW power from very low(~1kW) to high power (~20kW). In the initial phase with very low heating power, turbulent particle flux is very low but quickly increases toward strongly turbulent state with the increase in WCW power. However, turbulent particle flux immediately saturates at a certain level. With the increase in turbulent particle flux, fluctuations of radial electric field grow clearly, suggesting generation of sheared poloidal flow. Detailed analysis is under way.In parallel to these researches, we also studied characteristics of edge transport barrier formed in high temperature plasmas in CHS and the large helical device LHD. These results support the progress of this project. Less