7. Project with Other Universities and Organizations

概要

7. PROJECT WITH OTHER
UNIVERSITIES
AND ORGANIZATIONS

– 141 –

NIFS Bilateral Collaboration Research Program on Heliotron J
The Heliotron J group at IAE, Kyoto University
has joined the Bilateral Collaboration Research Program managed by National Institute for Fusion Science (NIFS) since FY2004. This unique collaboration
program promotes joint research bilaterally between
NIFS and research institutes or research centers of universities that have facilities for nuclear fusion research.
Under this collaboration scheme, the facilities operated in the different universities are open to all fusion
researchers just as joint-use facilities of NIFS.
The main objective of the research in our Heliotron J group under this joint research program is to investigate experimentally/theoretically the transport
and stability of fusion plasma in the advanced helical
magnetic field and to improve the plasma performance
through advanced helical-field control in Heliotron J.
Picked up in FY2022 are the following seven key-topics; (1) magnetic configuration control for energy confinement, (2) Confinement improvement by hydrogen
pellet injection, (3) relation between structure formation and plasma fluctuations in the core and peripheral region, (4) physics mechanism of hydrogen pellet
ablation, (5) optimization of particle supply and heating scenario, (6) development of new technology in
experiment and analysis.
Two results from this collaboration research in
FY2022 are shortly reported below. The annual report
for all the collaboration subjects in this program will
be published by NIFS.
Magnetic configuration control for energy confinement: The energy confinement dependence on rotational conversion in Heliotron J shows a negative
trend, which is inconsistent with the ISS04 scaling. As
an indicator of neoclassical transport, the stored energy is studied against the effective helical ripple εeff.
Although a negative dependence on εeff appears, such
a dependence may be determined by a result at the
lowest εeff configuration. If we exclude this configuration result, we obtain a weak regression with a evaluation coefficient of R2~0.2, which means that the data
for most of the configurations cannot be explained by
εeff. The contribution of turbulent transport is under
evaluation.
Control of the rotational transformation can
change the position and width of magnetic islands produced at the rational surface. The period of the magnetic island structure (m/n=7/4, 8/4, 9/4) can also be
controlled by the rotational transform. When the magnetic island is shifted from the periphery to the core
region, the confinement degradation is clearly visible
in a certain range. A ultra-high-resolution ECE measured with a recently introduced ultra-fast oscilloscope
in the GHz band enable us to observe the response to

modulated ECH in the magnetic island configuration.
Probe measurements of the fluctuations at the magnetic island and electric field measurements using a
Doppler reflectometer are also in progress.
In the magnetic configuration control experiments,
a principal component analysis has been applied to
study the relation among the parameters that characterize the confinement. The bumpiness scan experiment can be summarized as follows: the first component (PC1) is the plasma volume, the second one
(PC2) is the rotational transform, and the third one
(PC3) is a parameter related to bumpiness. This means
that the rotational transform scan experiments can be
controlled independently of the bumpiness and aspect
ratio. This method will be a useful tool for analyzing
the results of configuration control experiments in
which many configuration parameters are interdependent.
Physics mechanism during hydrogen pellet ablation: The density of pellet ablation cloud can be measured from the Stark broadening of the emission lines
of the Balmer series, which is the emission of hydrogen. In the near-infrared region, the Zeeman effect on
the Doppler broadening and the Stark broadening are
relatively large compared to the visible region, and
this can be a particularly useful tool for medium-sized
devices such as Heliotron J. The polarization separation measurement of helium atoms by Zeeman spectroscopy has been proved to be a useful tool. For Stark
broadening, in addition to the conventional emission
of Hβ lines in the visible region, the Pa-α lines of the
Paschen series in the near-infrared region have been
used. The Stark broadening of the emission from the
dissolved cloud passing through the line of sight has
been measured using a small, simple near-infrared
spectrometer, and is found to be slightly above the
lower limit of the spectrometer (< 4×1021 m-3), which
is about two orders of magnitude smaller than a reported value (~1023 m-3) in LHD. This means that
measurements using multiple bandpass filters with
different transparent bands are not applicable, suggesting the need for a different approach. Currently, we are
developing i) a fast spectroscopy system to spatially
track the density of the dissolved cloud through fast
spectroscopy of the Stark broadening of the visible Hβ
line with increased resolution and using a high-speed
camera, ii) a high-dispersion near-infrared spectroscopic diagnostic to simultaneously determine the
Zeeman splitting, Stark broadening, and Doppler
broadening, and iii) a near-infrared emission line intensity ratios to estimate the electron temperature of
the ablation cloud. ...