Experimental Simulation of High Temperature Plasma Transport
Using Dimensionally Similar Cold Plasmas in the Compact Helical System

Kazuo Toi, Ryousuke Ikeda1), Masaki Takeuchi1), Chihiro Suzuki, Go Matsunaga2), Tatsuo Shoji1), Shoichi Okamura and CHS Experimental Group


National Institute for Fusion Science, Toki 509-5292, Japan
1) Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan
2) Institut fuer Plasmaphysik, Forschungszentrum Juelich, 52425 Juelich Germany

Correlation measurement among plasma fluctuations is difficult but crucial to clarify underlying physics mechanisms in high temperature plasma transport. If the transport behaviors in such plasma is simulated by those in a cold plasma, detailed studies of plasma transport may be possible by the aid of Langmuir probes over edge to core plasma region. When key dimensionless plasma parameters such as ν*( normalized collision frequency) are the same except ρs*(normalized ion gyro-radius estimated with electron temperature) in two kinds of plasmas, these two plasmas are "dimensionally similar" each other and their plasma transport is expected to be similar. Moreover, in a helical system, characteristic transition related to neoclassical transport such as "electron root" is one of very interesting targets to be studied in such cold plasma.
Based on above-mentioned hypothesis, experimental simulation of high temperature plasma transport is being attempted using cold plasmas produced at very low field (Bt < 0.1T) in CHS, where 2.45GHz microwave up to 20 kW is employed for plasma production. The obtained dimensionless parameters are ν*> 0.08 , βt< 0.05% (toroidal beta) and ρs*~0.05, which are close to those in CHS at high Bt except ρs*. In these plasmas, radial profiles of relative amplitude of electron density(δne), temperature(δTe) and floating potential (δVf) fluctuations have similarity to those in high temperature plasmas. δne is dominated by low frequency components (<10kHz) in core region(r/a<0.6), but higher frequency components are also important in the edge. This character is different from those of δTe and δVf. Recent significant observation is that of the transition to "electron root" exhibiting clear jump in electron density and temperature without center-focused ECH deposition. Relaxation oscillations in electron density, temperature and potential are also observed in low density plasmas with more than 10 kW ECH.