S7-2
Hydrogen isotope effect on H-mode in JT-60U
JT-60UのHモードにおける水素同位体効果
Hajime Urano
浦野 創
Japan Atomic Energy Agency, Naka Fusion Institute801-1, Mukouyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構 那珂核融合研究所 〒311-0193 茨城県那珂市向山801-1
The effects of hydrogen isotope on H-mode confinement and edge pedestal structure in JT-60U aresummarized. The thermal energy confinement time is longer by 20-30% for deuterium than for hydrogen at
a given absorbed power. At a given stored energy, spatial profiles of electron density, electron temperature,and ion temperature become identical between these two cases. However, the power required to sustain theidentical profiles is twice larger for hydrogen than for deuterium, leading to larger heat diffusivity at theplasma core. Ion temperature gradient scale length for deuterium is smaller than for hydrogen. Hydrogenisotope enables us to investigate the physics picture which cannot generally be drawn as far as experiments
are conducted with only one ion species. The characteristics of dimensionless parameters, isotope massdependence of which exists only in , can break the strong correlation between  and at the pedestal.
This approach revealed that the pedestal width was not varied by but determined by 0.5.

1. Introduction
The knowledge of the influence of plasma
isotopic composition on heat conduction in steady
H-mode plasmas has important consequencesfrom both physics and engineering points of view.
The effects of the isotope mass M on energy
confinement have been extensively studied in thelate 1980s and the early 1990s. While many
experimental results have focused particularly onohmic and L-mode plasmas, there are also severalresults on the isotopic comparison for the case ofH mode plasmas. In ASDEX, the ratio of theenergy confinement time of deuterium tohydrogen becomes 1.5–2 in H-mode plasmas[1,2]. In DIII-D, the energy confinement timescales as M/Zi2, where Zi denotes the ionic charge,indicating the superiority of deuterium
confinement to hydrogen in H-mode plasmas [3].
InTFTR,
theH-modes
producedindeuterium–tritium
plasmashaveenergy
confinement enhancement larger by a factor of ∼1.25 than the corresponding deuterium plasmas[4]. In JET, the energy confinement time in
ELMy H modes shows very weak massdependence (th  M0.03) [5].
For all discharge types, energy confinement
increases with isotope mass th  M with theexponent  greater than 0. However, little is
known about the process responsible for energy
confinement through variation of the isotopic
composition. Herein, therefore, the effect ofhydrogen isotopes on heat transport and thepedestal structure is characterized using hydrogenand deuterium H-mode plasmas in the JT-60U
tokamak [6,7].
In the present understanding, H-mode
confinement is characterized by separation into
pedestal and core components. In many cases,
both components are strongly correlated. The
pedestal component plays a role as a boundary
condition in determining the core heat transport
through profile stiffness. On the other hand, ahigher p improves stability of the plasma edge inthe low magnetic field side. On the basis of thisbackground, this study presents dependence ofheat transport on the hydrogen isotope mass inconventional H-mode plasmas from the viewpointof the TG scale length of ion temperature profiles
at the plasma core.
2. Energy confinement in JT-60U tokamak
Dependence of confinement quality on isotopic
composition was investigated for conventional
H-mode plasmas in JT-60U. The thermal energy
confinement time th increases by a factor ofapproximately 1.2-1.3 for deuterium compared tothat for hydrogen at a given heating power PL [6].
When the thermal stored energy Wth was fixed,
profiles for electron density ne, electron
temperature Te, and ion temperature Ti became
identical for both cases, whereas a higher heating
power was required for hydrogen. The ion
conductive heat flux Qi for hydrogen became
approximately two times that for deuterium,Fig.1: Schematic view of the influence of hydrogenisotopic composition on energy confinement inconventional ELMy H-mode plasmas involving therelation between edge pedestal structure and coreheat transport.

corresponding to a required heating power tosustain the same Wth value that was two times aslarge for hydrogen. Hence, the ion heatdiffusivity i values for hydrogen were higher,
explicitly throughout the minor radius, than thosefor deuterium at the same ion temperature
gradient scale length LTi. The Ti/Ti, or theinverse of LTi, required for a given i increased bya factor of approximately 1.2 for deuterium
compared to that for hydrogen. These results leadto the conclusion that LTi decreases as thehydrogen isotope mass increases (hydrogen todeuterium) in H-mode plasmas [8].
The self-regulating physics mechanism
determining the overall H-mode confinement wasalso investigated. The relationship between the
total poloidal beta pTOT and the pedestal poloidal
beta pped was nearly identical, irrespective of thedifference in isotope species, suggesting that agreater pedestal pressure observed for deuterium
H-modes be obtained from a greater pTOT. In
addition to a smaller i or LTi for deuterium
contributed by the thermal component, the fastion energy determined by the slowing down timeof high energy ions s  M1/2T3/2/n is one of thekey factors leading to a greater pTOT for
deuterium [7].
3. Dimensionless parameter dependence ofpedestal width
While the understanding of the physics behindthe constraint of the edge pressure gradient has
progressed, what determines the pedestal width has
also intensively been addressed from theperspective of the dimensionless parameters.

Various models of the pedestal width, based on
E×B shearing suppression of the edge turbulence,
predict an unfavorable size scaling of the pedestal
width which becomes larger with the ion poloidal
gyro radius pi [9]. However, the existence of ELMs
can be a constraint to performing the dimensionlessanalysis on the pedestal width. Assuming thepedestal pressure is limited by ELMs to scale with
Ipξ, the trajectory along a given normalized pedestal
collisionality  (~n/T2) reduces p,ped (~nT/Ip2) toIpξ−2 and p (~T1/2/Ip) to Ipξ/6−1. Empirically, theexponent ξ ranges 1.0–1.5 in many devices. Then,the strong correlation of p  p0.67–1.2 has led in
the past to difficulties in testing these models
experimentally.
In order to solve the problem of the strong
correlation generally existing between p and p,
one method is the experiment for the edge
dimensionless analysis using hydrogen isotope
species of hydrogen and deuterium. This approachis based on the isotope mass dependence that existsonly in pwhile the other dimensionlessparameters are independent of the isotope mass, andthus the correlation between p and pis broken.
The experiment performed in JT-60U has shownthat the pedestal width is unvaried in matchedhydrogen and deuterium discharges with the same
p, indicating that the pedestal width is not variedwith p [10]. Similar experiments weresubsequently conducted in DIII-D, showing nearlyno dependence of the pedestal width on p,
consistent with the JT-60U findings [11].
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33 1205.
[3] Schissel D.P. et al 1989 Nucl. Fusion 29 185.
[4] Sabbagh S.A. et al 1995 Proc. 15th Int. Conf. on
Plasma Physics and Nuclear Fusion Research 1994
(Seville, Spain, 1994) vol 1 (Vienna: IAEA) p 663.
[5] Cordey J.G. et al 1999 Nucl. Fusion 39 301.
[6] Urano H. et al 2012 Nucl. Fusion 52 114021.
[7] Urano H. et al 2013 Nucl. Fusion 53 083003.
[8] Urano H. et al 2012 Phys. Rev. Lett. 109 125001.
[9] Onjun T. et al 2002 Phys. Plasmas 9 5018.
[10] Urano H. et al 2008 Nucl. Fusion 48 045008.
[11] Groebner R.J. et al 2009 Nucl. Fusion 49 085037.