S7-3
Isotope effect in improved core confinement plasmas
改善コア閉じ込めプラズマにおける同位体効果	 
Takaaki Fujita
藤田隆明
Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学大学院工学研究科 〒464-8603	  名古屋市千種区不老町
Isotope effect in improved core confinement including internal transport barriers is reviewed for tokamak
plasmas. For ITB in reversed shear plasmas, isotope effect on confinement seems to be small. In positive
shear plasmas, more enhanced confinement is generally observed in higher mass number, except for JET
Optimized Shear plasmas. Possible reasons for these tendencies are discussed.

1. Introduction
The isotope mass number M of plasma ions is
one of the important parameters that would affectplasma behavior including cross-field transport
processes. Its effects on energy confinement in
OH, L-mode and H-mode plasmas have beenstudied extensively in many tokamak andstellarator devices. On the other hand, study on its
effects on improved core confinement includinginternal transport barriers (ITBs) seems to be €ratherlimited. This is partly because these plasmas have awider range of variety in the radial profiles and inconfinement properties and then evaluating theeffect of M correctly, eliminating effects of otherparameters, is not so easy as in OH, L-mode andH-mode plasmas. In this presentation, study on
isotope effects on improved core confinement
including ITBs is reviewed.

In TFTR supershot plasmas, isotope effects werestudied comparing deuterium NB injection andtritium NB injection. A strong isotope scaling was

2. Positive shear plasmas
Improved core confinement with peaked density
and/or temperature profiles is observed in positive
shear plasmas by some methods; counter NB modein ASDEX, I-mode in TEXTOR, supershot inTFTR, off-axis ICH in Alcator C-Mod, and pellet
injection in some devices.
In ASDEX, the counter NB mode was found only
in deuterium plasmas and the few attempts toachieve this in hydrogen plasmas were notsuccessful [1]. Also in ASDEX, strong peaking ofdensity profile was observed both in deuterium
pellet injection into deuterium plasma and inhydrogen pellet injection into hydrogen plasma,though higher confinement was observed indeuterium.
In TEXTOR I-mode plasmas, up to a 35%
increase in the confinement time was observed indeuterium plasmas over hydrogen plasmas, forfixed plasma current and NB power [2].

3. Reversed shear plasmas
Electron ITBs are observed with dominant
electron heating in reversed shear plasmas on manydevices. It is likely that the negative shear isbelieved to be the dominant factor for the formationof electron ITB and the heating power threshold isvery low. Very few attempts seem to be made toinvestigate isotope effect on this kind of ITB. It
seems that electron ITB was observed only indeuterium plasmas on FTU and TCV. In Tore Supra,electron ITB with ICRF minority heating was done
both in helium plasma and deuterium plasma, but
no systematic comparison is found.
Ion ITBs are also observed with NB heating inreversed shear plasmas on several devices. The iontransport can be reduced to the neoclassicaltransport level. Deuterium plasmas are used in mostdevices for this regime.
In JT-60U, the ion ITB was observed indeuterium and hydrogen reversed shear plasmas. Afoundχ

ttotisuch∝ M

as−2.6±0.50

τ Ethermal ∝ M

0.89±0.20

andfor fixed NB injection power[3].
In Alcator C-Mod ICH experiment, only€
deuterium plasma was used for H minority heating.
Internal transport barriers are observed in
NB-heated weak positive shear plasmas on JT-60U(high βp mode) and on JET (Optimized Shear
regime). In JET, comparison between DD plasmas
and DT plasmas indicates that core transport andthreshold power for ITB formation were similar [4].
In JT-60U, clear ITB was observed only indeuterium plasmas and attempts to obtain ITB inhydrogen plasmas were not successful.

'box-type' ITB with a very steep Ti gradient wasobserved in hydrogen plasmas as well as indeuterium plasmas. The threshold power for ITB
formation was compared in deuterium andhydrogen plasmas [5]. Though higher heating
power was required in hydrogen than in deuterium,
it might be attributed to the difference in electron
density, which was higher in hydrogen in order tosuppress shine through of hydrogen NB.
In TFTR reversed shear plasmas, isotope effects
were studied comparing deuterium NB injection
and tritium NB injection [6]. No difference on
global energy confinement time was observed.
More heating power was required for obtaining
transition from RS (Reversed Shear) regime to ERS(Enhanced Reversed Shear) regime in tritium thanin deuterium.
4. Discussion
For ITB in reversed shear plasmas, isotope effecton confinement seems to be small. This is probably
because the negative shear has a dominant role for
reducing the transport.
In positive shear plasmas, more enhanced
confinement is generally observed in higher mass
number, except for JET Optimized Shear plasmas.
The reduction of radial transport in this regime isattributed not only to weak magnetic shear but also
to density gradient, pressure gradient, ExB shearing
rate and so on. Therefore, the isotope effect ontransport in L-mode plasmas before entering theimproved confinement regime might have asignificant role. As is well known, the global
energy confinement in L-mode depends on M, forinstance in M0.5 in ITER89P scaling. It is found thatthe central density peaking factor ne0/<ne> is larger
in deuterium than in hydrogen [1] in ASDEX
Ohmic-heating plasmas. As for the momentum−0.8transport, strong dependence on M, χφ ∝ M eff
is found in ASDEX L-mode plasmas [7]. Thesmaller momentum transport may result in higherrotation and higher ExB shearing rate.

€References[1] M. Bessenrodt-Weberpals et al.: Nucl. Fusion 33(1993) 1205.
[2] J. Ongena et al.: in Plasma Phys. and Control. Nucl.
Fusion Research 1992 (Proc. 14th Int. Conf.
Wurzbürg, 1992), Vol. 1, IAEA, Vienna (1993) 725.
[3] S. D. Scott et al.: Phys. Plasmas 2 (1995) 2299.
[4] JET Team (prepared by C. Gormezano): Nucl.
Fusion 39 (1999) 1875.
[5] S. Ide et al.: Phys. Plasmas 7 (2000) 1927.
[6] S. D. Scott et al.: in Fusion Energy 1996 (Proc. 17thInt. Conf. Montreal, 1996), Vol. 1, IAEA, Vienna(1997) 573.
[7] A. Kallenbach et al.: Plasma Phys. Control. Fusion
33 (1991) 595.