J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)516–518
000–000

Experimental Simulation of High Temperature Plasma Transport
Using Almost Dimensionally Similar Cold Plasmas
in the Compact Helical System
TOI Kazuo, IKEDA Ryousuke1, TAKEUCHI Masaki1, ITO Takafumi1, SUZUKI Chihiro,
MATSUNAGA Go2, SHOJI Tatsuo1, OKAMURA Shoichi and CHS Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan
Dep.t Energy Eng. and Science, Nagoya University, Nagoya 464-8603, Japan2
Institut fuer Plasmaphysik, Forschungszentrum Juelich, 52425 Juelich, Germany
1(Received: 9 December 2003 / Accepted: 18 Jun 2004)

Abstract
In the Compact Helical System (CHS), experimental simulation of high temperature plasma transport is attemptedby using cold plasma having similar dimensionless parameters such as electron-ion collision frequency normalized bybounce frequency v *ei , averaged toroidal beta value βt and the normalized gyro radius ρs*. The cold plasma is producedby 2.45 GHz electron cyclotron waves at very low toroidal field less than 0.1 T, and has v *ei ~ 0.05–1, βt < 0.02 % andρs* ~ 0.02–0.05. The radial profiles of fluctuation amplitude have similarity to those in a high temperature plasma. In
the cold plasma with low v *ei < 0.1, internal transport barrier is clearly formed in electron density and temperature
profiles when the radial electric field rapidly evolves to positive value.
Keywords:
plasma transport, dimensionally similar plasma, helical systems, radial electric field, turbulent fluctuations,internal transport barrier

1. Introduction

frequency), Bt (averaged toroidal beta), relative scale lengthsof density and temperature profiles and so on are the same intwo kinds of plasmas except for one parameter ρs* (ion gyroradius estimated with Te normalized with the plasma minorradius a), these two plasmas are “dimensionally similar” eachother and their plasma transport is expected to be similar[1,2]. Based on this hypothesis, an initial simulation
experiment using cold and tenuous plasma obtained with 2.45
GHz electron cyclotron heating (ECH) at very low Bt < 0.1 T
was carried out in CHS heliotron/torsatron [3]. Mainobjectives of the experimental simulation are as follows, (1)to compare fluctuation characteristics of both plasmas, (2) toobtain improved confinement regimes achieved in hightemperature plasmas and clarify physics mechanisms of
turbulence suppression, and (3) to establish realistic transportmodel in a toroidal plasma through detailed comparison
between experimental data and numerical simulations.

Energy and particle transport in a toroidal plasma is avery important and challenging issue in magnetic confinement
fusion research. In a tokamak or helical plasma where theplasma transport is controlled by electrostatic fluctuations, theturbulent particle and electron heat fluxes are respectively
expressed as Γturb = < ñe Ẽθ >/Bt and Qeturb = 3/2 (Te <ñe Ẽθ > +ne < T̃e Ẽθ > / Bt , where ñe , T̃e andẼθ are fluctuations of electrondensity, electron temperature and poloidal electric field, andBt is the toroidal magnetic field strength. Therefore,
correlation measurement among plasma fluctuations is crucial
to clarify underlying physics mechanisms in turbulent particle
and heat transport of a toroidal plasma. However, thecorrelation measurement is extremely difficult in a hightemperature plasma, except for the measurements withLangmuir probes (LPs) in the plasma edge region. If thetransport behaviors of high temperature plasma are simulated
by cold and low density plasma, detailed studies of turbulent
transport are possible by LPs and may provide a powerfulknob to clarify unknown transport mechanisms. When
relevant dimensionless plasma parameters such as v *ei(electron-ion collision frequency normalized by bounce

2. Main dimensionless plasma parameters
achieved
This simulation experiment has been carried out in CHS
using 2.45 GHZ ECH with much higher power (~ 20 kW).

©2004 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchCorresponding author’s e-mail: toi@nifs.ac.jp1516

Toi K. et al., Experimental Simulation of High Temperature Plasma Transport Using Almost Dimensionally Similar Cold Plasmas

with βt . The effective collision frequency v ei* has reached tofairly low value (v * = 0.05–1) that is comparable to that ofhigh temperature plasmas in LHD and CHS at higher field.
The averaged toroidal beta is considerably low βt (< 0.02 %).
The normalized gyro-radius ρs* is by a factor of 2–4 larger
than that in CHS plasma at high Bt .

The microwave power is launched as perpendicularly and/or
tangentially against the toroidal field line. The toroidal
magnetic field at the magnetic axis is varied from 0.105 T to0.06 T in order to study various ECH scenarios. The magnetic
axis position is adjusted to inward-shifted (Rax = 92.1 cm) oroutward shifted (Rax = 97.4 cm) configuration. Plasma
parameters are measured with a triple Langmuir probe, 2 mm
microwave interferometer, bolometer and visible spectrometer.
The line averaged electron density is in the range of about
1017 m-3 for hydrogen plasma and the central electron
temperature is 5–20 eV. Ion temperature is expected to bevery low (<< 1 eV) if charge exchange loss is dominant. Two
important dimensionless parameters v ei* and ρs* achieved inthese cold and tenuous plasmas are plotted on Fig.1, together

3. Characteristics of electrostatic
fluctuations
In this cold and tenuous plasma, fluctuations can bemeasured even in the plasma core region with LP, as shownin Fig.2(a). All relative amplitudes of fluctuations ñe /ne , T̃e /Teand Ṽs /Te increase rapidly from ρ ~ 0.8 toward the last closed
flux surface (LCFS), of which features are similar to those inhigh temperature plasmas and their edge [4-6]. In the coreregion of ρ ≤ 0.8, they stay almost a constant level that is
considerably enhanced for that in 2.45 GHz ECH plasma of0.4 kW [3]. Spectral power of ne-fluctuations in the plasmaedge ( ρ ~ 1.0) extends up to 100 kHz, and that in the coreregion is concentrated in lower frequency less than 10 kHz(Fig.2(b)). On the other hand, the spectra of T e and V sfluctuations extend to relatively high frequency range morethan 100 kHz in the core region as well as the edge. Turbulent
particle flux is concentrated within the core region of ρ ≤0.8, while the flux is localized in the plasma edge on the very
low power case [3].

4. Transport barrier formation in outwardshifted plasma with low collisionality

Fig. 1Comparison of v *ei and ρs* in 2.45 GHz ECH plasmas
shown in the upper part of this figure with those in
CHS and LHD plasmas obtained at high Bt .

(a)Fig. 2As mentioned above, cold plasma with low v *ei has
already been obtained using 2.45 GHz ECH, although Bt isvery low and ρs* is fairly large. In this plasma, internaltransport barrier is clearly formed in electron density andtemperature profiles ( ρ < 0.6 ), where the barrier formation
starts from t ~ 150 ms in Fig.3(a). On the other hand, the(b)(a) Radial profiles of relative fluctuation amplitude of electron density, electron temperature and plasma potential, where Bt =0.079 T and Rax = 0.92 m. (b) Frequency spectra of electron density fluctuations at various radial locations.

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Toi K. et al., Experimental Simulation of High Temperature Plasma Transport Using Almost Dimensionally Similar Cold Plasmas(a)Fig. 3(b)(a) Temporal evolution of electron density ne , electron temperature Te and plasma potential Vs measured at ρ = 0.35, where Bt= 0.061 T and Rax = 0.97 m. (b) Radial profiles of electron density and radial electric field just before (t = 150 ms) and after thetransition (t = 180 ms).

electron density and temperature outside the barrier region
start to decrease at the transition. It should be noted that thefluctuation amplitude even increases after the transition. As
shown in Fig.3(b), radial electric field Er evolves from ~ 0 to
the positive value of ~ 0.6 kV/m in the core region after thetransition, which corresponds to the high poloidal velocity of~ 10 km/s because of very low Bt . However, the poloidalvelocity shear seems to be marginal to suppress micro
turbulence (~ 1×105 s-1). Formation mechanism of this internaltransport barrier (ITB) is under investigation. A possible
candidate mechanism is the reduction of neoclassical transport
by the electron root regime [7,8]. However, energy andparticle transport in this plasma is dominated by anomaloustransport. Detailed study of Er profile is necessary to clarify
the ITB formation. In this discharge condition, the layer offundamental electron cyclotron resonance is located justoutside LCFS, and the second harmonic cyclotron layer is inρ ~ 0.6. Moreover, the electron density exceeds the cutoff
density (~ 7×1016 m-3). This suggests electron heating byelectron Bernstein waves mode-converted from launched Xmode.

successfully carried out in CHS. The plasma fluctuations
seem to behave similarly to those in high temperatureplasmas. One significant example is to achieve the formationof ITB in this cold plasma. This new experimental project
would contribute to clarifying important transport mechanisms
in high temperature toroidal plasmas.
They appreciate Dr. F. Zacek in Institute of Plasma
Physics, Prague, for his guidance in initial ECR experiments.
This work was supported in part by the Grant-in-Aid for
Scientific Research from Japan Society for Promotion ofScience, No. 15206107.

References[1] B.B. Kadomtsev, Sov. J. Plasma Phys. 1, 295 (1975).
[2] R.E. Waltz, J.C. DeBoo and M.N. Rosenbluth, Phys. Rev.
Lett. 65, 2390 (1990).
[3] K. Toi, S. Kawada, G. Matsunaga et al., in Proc. 29th
EPS on Plasma Phys. Control. Fusion (Montreux, 2002)
P-4.061.
[4] B.A. Carreras, IEEE Trans. Plasma Sci. 25, 1281 (1997).
[5] Ch.P. Ritz et al., Phys. Rev. Lett. 62, 1844 (1989).
[6] K. Ohkuni, K. Toi et al., Phys. Plasmas 8, 4035 (2001).
[7] H. Sanuki et al., J. Phys. Soc. Japan 69, 445 (2000).
[8] M. Yokoyama et al., J. Plasma Fusion Res. 79, 816(2003).

5. Summary
Experimental simulation of high temperature plasmatransport using almost dimensionally similar cold plasma
produced by 2.45 GHz ECH at low toroidal field has been

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