S7-5
Diagnostic approach to the isotope effects
質量比・同位体効果の解明に向けた計測Masahiro Hasuo
蓮尾 昌裕
Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8540, Japan
京都大学大学院工学研究科 〒615-8540 京都市西京区京都大学桂Based on the isotope effects observed for the confinement improvement of magnetically confined toroidal
plasmas, diagnostic approach to investigate the contribution of recycling is discussed. For the purpose,
recent high dynamic range Balmar- spectroscopy to measure the radial density distribution of hydrogenfrom the edge to core regions is presented. Measurements necessary to evaluate the local density ratio of
the isotopes in a hydrogen-deuterium mixture discharge are also considered.

1. Isotope effects on magnetically confined
toroidal plasmas
A plasma is a typical complexity systemcomposed of astronomical number of interacting
ions, electrons and atoms together with electricand magnetic fields. A traditional reductionism
approach may be hard to be applied to understandits nature. However, it is not disorder but
universalistic phenomena sometimes appear in it.
One example is the confinement improvement by
the L-H transition of toroidal plasmas [1].
However, all the phenomena have not beenclarified such as the isotope effects on theconfinement improvement.
In tokamak plasmas, several isotope effects
caused by increase in the atomic mass number
have been reported such as reduction of thethreshold power for the H-mode transition,
improvement of the confinement saturation,
increase in the energy confinement time, increasein the stored energy in the pedestal region andreduction of the edge localized mode (ELM)
frequency [2-6]. These phenomena, however, areinconsistent with the prediction by the gyro-Bohm
scheme.
With the Large Helical Device (LHD) in
National Institute for Fusion Science (NIFS),
deuterium discharge experiments are scheduled tostart from 2016. Based on the experience withtokamak plasmas, further improvement in theplasma performance by the isotope effects andrealization of the LHD target plasma parameters areexpected. At the same time, since the isotopeeffects are thought to be universalistic phenomena
in toroidal plasmas and have been open question forcomprehensive understanding of the plasma physics,
the LHD experiments are expected to contribute tothis issue from a scientific point of view.

2. Isotope effects on recycling
It is known that the recycling from the edge
affects on the plasma confinement. For example,low recycling has been realized by adoption of lowZ element as the material of plasma facing walls,
and the enhancement of the energy confinement
and the reduction of the ELM have been observed[7,8]. In LHD, low recycling has been realized by
the wall-conditioning using the ion cyclotron rangeof frequency (ICRF) heating discharge, and a highion temperature plasma exceeding 7 keV has been
generated [9].
The relation between the recycling and theH-mode performance was analyzed with atheoretical model about 20 years ago [10]. In thepaper, a possible mechanism of the isotope effects
was also discussed; since the speed of heavier
isotope atoms is slower, the number of the atoms
penetrating into the plasma decrease, and then theenergy and momentum loss of ions by charge
exchange collisions will decrease.
3. Quantitative measurement of the recycling
Recently, it becomes possible to measurehydrogen atomic density distribution from the edge
to core regions by high dynamic range Balmar-
spectroscopy [11]. Its principle is rather simple;
since the velocities of a hydrogen atom and a proton
are exchanged by a charge exchange collision, thevelocity distribution of the atoms reflects the proton
temperature where the charge exchange collisions
take place. From the observed Doppler-broadened
Balmar- spectrum, especially its wings, with theinformation of the radial distribution of the proton
temperature, the radial distribution of the hydrogenatomic density can be deduced.
For LHD plasmas, the radial distributions of theelectron temperature and density are quantitatively

measured by the Thompson scattering method [12]and that of the proton temperature is measured by
the charge exchange spectroscopy with NBI [13].
With these data and the cross sections of electron
impact excitations and ionizations, proton impact
excitations and ionizations, and charge exchange
collisions, the hydrogen transport from the edge tocore regions can be also simulated [14].
4. Toward unified comprehension of the isotopeeffects
From the radial distribution of hydrogen(deuterium) atomic density, the NBI heating loss,
especially in the core region, and momentum loss,
especially in the pedestal region, of the bulk plasma
may be deduced.
However, as was written in section 1, unifiedcomprehension is indispensable because simple
linear summation of the contributions ofelementally processes is not applicable. For thepurpose, a unified model to simulate the overall
plasma with the input of all the measurement results
including the radial distributions of impurities andZeff should be necessary because the contribution ofcarbon impurities have been investigated in theconnection with the confinement improvement in
LHD [15]. Feedback from the simulation to themeasurement and repetitive improvement betweenthe simulation and measurement may be also
necessary. TASK3D, the integrated transport
analysis suite, should play a key role in this activity[16].
5. Issues to overcome in measurement of therecycling isotope effects
There are still not a few problems on theprecision measurement of the radial distribution ofhydrogen atomicdensity. For
example,contribution of proton impact excitations to thewings of the Balmar- spectrum becomes larger
than that of electron impact excitation when theproton temperature exceeds 5 keV. Such situation
will be realized in high ion temperature plasmas
with the ion internal transport barrier (ITB), in
which the isotope effects are expected to appear. In
the analysis of the Balmar- spectrum, thecontribution of the proton excitations should beincluded. Furthermore, the spatial distributions of
the plasma parameters, especially the iontemperature, in the scrape-off layers (SOLs) isdemanded because their accuracy is indispensablefor the quantitative estimation of hydrogen atom
flux into the confined region.
In the actual situation, a hydrogen-deuterium
mixture discharge will be generated in LHD.

Therefore, local density measurements for both thehydrogen and deuterium ions and those for both thehydrogen and deuterium atoms may be necessary.
Regarding the local density ratio measurement ofhydrogen and deuterium ions, the charge exchange
spectroscopy with NBI is expected be applicable.
On the other hand, the high dynamic range
Balmar- spectroscopy cannot be fully applicableto the local density ratio measurement of hydrogenand deuterium atoms because, in addition to thevery small wavelength shift of the Balmar- spectra
between hydrogen and deuterium atoms, theoreticaltreatment of velocity change caused by an elastic
collision between a hydrogen ion and a deuterium
atom, or between a hydrogen atom and a deuterium
ion, in the spectral profile analysis is not trivial.
Acknowledgments
In the preparation of this manuscript, the author
referred a document, which is in Japanese, on theplan for deuterium experiments in LHD of NIFS.
The author would like to thank Prof. M. Yokoyama
for valuable discussion. The author also thank Dr. K.
Fujii for the work on the radial distribution
measurement of hydrogen atomic density in LHD
plasmas with the high dynamic range Balmar-spectroscopy.
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