J. Plasma Fusion Res. SERIES, Vol.7 (2006) 77-80

Measurement of the Energetic Particle in NBI Plasmas of Heliotron J
KANEKO Masashi∗ , KOBAYASHI Shinji1 , SUZUKI Yasuhiro, MIZUUCHI Tohru1 ,NAGASAKI Kazunobu1 , OKADA Hiroyuki1 , NAKAMURA Yuji, HANATANI Kiyoshi1 ,MURAKAMI Sadayoshi2 , KONDO Katsumi and SANO Fumimichi1
Graduate School of Energy Science, Kyoto University, Uji 611-0011, Japan
1 Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan
2 Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan（Received: 4 October 2004 / Accepted: 22 November 2005）

Abstract
The behavior of energetic particles is investigated with a charge-exchange neutral particle energy analyzer(CX-NPA) system in Heliotron J. In the tangential neutral beam injection (NBI) experiments, the high energy
tail component up to 20 keV is observed in proton energy spectrum. The dependence of the energetic particle
confinement on the magnetic configuration is studied with regard to one of the Fourier spectrum of the magnetic field, bumpiness. The decay time of CX-flux after the termination of NBI decreases with increasing thebumpiness component.
Keywords:
Heliotron J, particle confinement, neutral beam injection, charge-exchange neutral particle measurement,
configuration eﬀect
1．Introduction
In magnetized fusion plasmas, study of the energetic
particle confinement and the ripple transport is important subject not only for the production of high performance plasma but also for the reduction in damage
to the wall materials through plasma-wall interactions.
From the viewpoint of the wave-plasma interactions,the energetic particle transport is closely related to themagnetohydrodynamic (MHD) instabilities [1].
The energetic ion behavior in helical systems
strongly depends on the magnetic configuration due toits asymmetry. In the planar axis heliotron configuration, such as Heliotron E, particle confinement can be
improved when the magnetic axis is shifted inwardly.
However, this magnetic configuration has a rather magnetic hill and clear MHD instabilities are observed [2].
In a helical-axis heliotron configuration device, Heliotron J, a high level compatibility between the goodparticle confinement and the MHD stability has beenstudied to explore the concept of helical-axis heliotron
configuration [3]. The MHD stability in low shear helical systems is mainly provided by a magnetic well. In
Heliotron J, a vacuum magnetic well is formed in theentire plasma region. On the other hand, the theoreticalanalysis predicts that the variety of the magnetic fieldstrength along the toroidal direction (toroidal mirror ratio), bumpiness component in Boozer coordinates [4],plays an important role in the energetic particle transport in the helical-axis heliotron configuration [5]. In
Heliotron J, the bumpiness component is comparable
to main helical field component but with the opposite
sign, which is considered to be eﬀective in suppressing
neoclassical ripple transport. Then, the investigation of
the eﬀect of the magnetic field configuration, in particular bumpiness, on the energetic particle transport isan important subject. In this paper, we report the experimental results on the behavior of energetic particles
produced by neutral beam injection (NBI) in Heliotron
J plasmas [6]. The confinement of the energetic particles is discussed in the bumpiness control experiments
in conjunction with numerical calculation.
2．Experimental set-up
Heliotron J is a medium sized plasma experimental device with an L/M = 1/4 helical coil (R0 /a =1.2 m/0.17 m, B0 < 1.5 T). The magnetic configuration can be controlled over a wide range by changing
the current ratios in the coil system of the device. Twosets of toroidal field coils with diﬀerent coil currents areequipped to control the bumpiness component.
The experimental set-up of the neutral beam injection (NBI) and charge exchange neutral particle analyzer (CX-NPA) systems is shown in Fig. 1. Hydrogen

Corresponding author’s e-mail: kaneko.masashi@lhd.nifs.ac.jp∗ Present address: National Institute for Fusion Science, Toki 509-5292, Japan77

C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchMeasurement of the Energetic Particle in NBI Plasmas of Heliotron J

Fig. 2 Energy spectra measured with CX-NPA in NB injection into deuterium plasma.

Fig. 1 Experimental set-up of NBI and CX-NPA systems.

beams are injected in the tangential direction with themaximum acceleration voltage of 30 kV and the maximum power of 0.7 MW. The ion energy distributionfunctions are measured with a CX-NPA system [7]. The
CX-NPA system is an E//B type one which can measure
the hydrogen and deuterium neutral atoms separately
and has energy ranges from 0.4 to 80 keV (hydrogen)and from 0.2 to 40 keV (deuterium), respectively. In thestandard configuration of Heliotron J (rotational transform l/2π = 0.56 at last closed flux surface), the chordof CX-NPA crosses the plasma center. The pitch angleof detective particle on the magnetic axis is 68 degree
that corresponds to a passing particle in the standardconfiguration of Heliotron J.

Fig. 3 The energy spectra at 1 – 2 ms, 4 – 5 ms after termination of NBI. The energy spectra during NBI
are also shown.

and from 4 to 5 ms after the NBI turned oﬀ. The line
averaged electron density is 0.5 × 1019 m−3 . The decayof CX-flux in energy range from 2 to 6 keV is fasterthan in the other energy ranges. In this energy range,
the 1/e decay times of CX-flux are about 1 ms. On theother hand the decay time of CX-flux at 7 keV is about
2.5 ms. However, the non-collisional orbit calculationof guiding center predicts that the loss time of the ionsin energy range from 2 to 6 keV is longer than that ofhigher (> 7 keV) energy ranges. For example, the loss
times averaged in detection area of CX-NPA are 1.8 and
0.8 ms for ions having energy of 5 keV and 7 keV, respectively. The calculation result is not consistent withthe experimental result. The detail explanation of thecalculation is described in the next subsection. One candidate of the mechanism of faster decay in energy range
from 2 to 6 keV is the eﬀect of radial electric field. In
helical systems, radial electric field plays an importantrole in the particle confinement in this energy range. If
a radial electric field exist and poloidal drift due to theEr × B drift cancel the poloidal motion of the passing

3．Experimental results
3.1 NBI heating experiment

The injection experiment of the hydrogen neutralbeam into deuterium plasmas has been carried out. Figure 2 shows the ion energy spectrum measured with
the CX-NPA system in the case that the beam energy
and injected power are 28 keV and 0.51 MW, respectively. In the hydrogen spectrum, a clear high energy
tail component up to 20 keV is observed. In the deuterium spectrum, energetic particles up to 5 keV are observed and the bulk ion temperature is estimated to be
0.3 keV. Since the present CX-NPA system can not detect the CX-flux having the same pitch angle as the injected neutral beam, the full and one half energy components of the injected beam are not observed clearly.
However a small ”shoulder” in the hydrogen spectrumin 7 to 9 keV. This ”shoulder” may be attributed to one
third energy (9.3 keV) component.
Figure 3 shows the energy spectra from 1 to 2 ms,

78

Measurement of the Energetic Particle in NBI Plasmas of Heliotron J

particles due to the rotational transform, passing particles will drift vertically out of the plasma because of thetoroidal curvature drift. This eﬀect is called ”toroidalresonance” [8]. For instance, for toroidal resonance of
2 keV ions with pitch angle of 70 degree, a potential of
about 0.8 kV at the plasma center is required. The measurement of the radial electric field experimentally andfurther analysis with regard to the numerical calculation
for the energetic particles taking account of the eﬀect on
the radial electric field are needed for the future work.
3.2 Bumpiness control experiment

In order to investigate the dependence of the energetic ion confinement on the strength of the bumpiness component, the behavior of the CX-flux has beenstudied by changing the bumpiness component, B04 /B00
from 0.04 to 0.15. Here Bmn is the Fourier component ofmagnetic field strength in the Boozer coordinates wherethe subscript m/n denotes poloidal/toroidal mode numbers. In these configurations, the magnetic axis position (R = 1.2 m), plasma volume (V p = 0.7 m3 ) androtational transform (l/2π = 0.56) at LCFS are almost
fixed. Figure 4 shows (a) energy spectra during NBIand (b) the time evolution of CX-flux with energy of
7.5 keV for several configurations with diﬀerent bumpiness component. The electron density is almost kept
constant in these discharges, then the ion collisionality is considered to be almost the same. In the caseof B04 /B00 > 0.08, clear changes are observed in boththe energy spectra and behavior of the CX-flux. In theenergy spectra, high energy tail components decrease
with increase the bumpiness component. The 1/e decay time of CX-flux after the NBI turned-oﬀ decreases
with increase the bumpiness component. In the caseof B04 /B00 < 0.08, no clear diﬀerences can be seenin the energy spectra and behavior of the CX-flux. To
understand the experimental result, the non-collisional
orbit calculation for ion guiding center is carried out.
In this code, the drift orbit equation is solved in real
coordinate. Figure 5 shows the loss time of the test
particles launched along the chord of CX-NPA as functions of major radius and pitch angle, where the loss
time is defined by the flight duration of the test particle to wall. At B04 /B00 = 0.15, the main part of thedetection area of CX-NPA is dominated by the directloss particles which have the loss time less than 0.5 ms,
while at B04 /B00 = 0.04 and 0.08 CX-NPA observes
the passing particles which have the loss time more than
10 ms. Then the decrease in the decay time with bumpiness component can be considered to be attributed tothe change in the loss cone. However further analysis isneeded with regard to the numerical calculation for theFig. 4 (a) energy spectra during NBI and (b) the time
evolution of CX-ﬂux with energy of 7.5 keV measured in the bumpiness control experiment.

high energy particles taking account of the pitch angle
scattering for the future work.
4．SummaryThe behavior of the energetic particle is investigatedin Heliotron J.
When the hydrogen NB is injected into deuterium
plasmas, high energy protons up to 20 keV and deuterium up to 5 keV are observed, while the bulk (D)ion temperature is estimated to be 0.3 keV. The decayof CX-flux having energy ranges from 2 to 6 keV after
the termination of NBI is faster than in the other energy ranges. The faster decay may be due to the radial
electric field. The dependence of the energetic particle
confinement on the bumpiness component is examinedin the range of B04 /B00 from 0.04 to 0.15. The high energy tail components (E > 5 keV) in the energy spectra decrease and the 1/e decay time of CX-flux after
the NBI turn-oﬀ decreases with increasing the bumpiness component. The non-collisional orbit calculation
for ion guiding center predicts that the main part of thedetection area of CX-NPA is dominated by the directloss particles in the high bumpiness case, while CXNPA can observe the passing particles in the low bumpiness case. Since the behavior of the energetic particles
79Measurement of the Energetic Particle in NBI Plasmas of Heliotron JFig. 5 Result of non-collisional orbit calculation for proton having energy of 7.5 keV in the case of B04 / B00 = 0.04, 0.08, and0.15. Detection angle of CX-NPA is also shown.

has strong dependence on the pitch angle, observationof the pitch angle distribution is necessary for more detailed discussion.

[3]
[4]
[5]
[6]

Acknowledgements
The authors are grateful to members of the Heliotron
J Group for useful discussions and support of this work.

[7]
[1]

[2]

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