Proceedings of ITC/ISHW2007

Helium ion observation during 3rd harmonic ion cyclotron heating in Large
Helical Device
Tetsuo Ozaki, Pavel R. Goncharov, Evgeny A. Veshchev1), Shigeru Sudo, Tetsuo Seki,Hirofumi Kasahara, Yuichi Takase2), Takuya Ohsako2), High-Energy Particle Group andLHD Experimental Group
National Institute for Fusion Science, 322-6, Oroshi, Toki, Gifu 509-5292, Japan
1) Graduate University for Advanced Studies, Hayama, Kanagawa, 240-0193, Japan2) Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University ofTokyo, Kashiwa, Chiba, 277-8561, Japan(Received/ Accepted )In the higher harmonic ion cyclotron resonance heating (ICH) using the fast wave, the resonance layer of helium
appears near the plasma core. It is very important to measure the helium ion in order to investigate the confinementof α particle, which is produced by the nuclear reaction in ITER or fusion reactor. In the Large Helical Device(LHD), we try to observe the charge exchange helium particle by using the Compact Neutral Particle Analyzer(CNPA). The helium acceleration at lower than 5 keV, can be confirmed by comparing the signal ratio in adjusted
plate voltages of CNPA to helium and hydrogen. The successful helium measurement in LHD leads to thedevelopment of the α particle measurement.
Keywords: ICH, 3rd harmonics, helium, α particle, CNPA, LHD, plate voltage, resonance

1. Introduction
It is very important to investigate α particle heating
mechanism in future fusion reactor because α particle has amain role to heat the fusion plasma. High-energy particles
including α particle are emitted not only by the charge
exchange but also by the MHD instabilities in the fusion
reactor [1]. Their particles give damage to the plasma wall
addition to create a poor plasma confinement. Decelerated
α particle (or a helium ion) with the energy over 1 keVmakes bubbles and gives a serious damage on the wall
surface unlike hydrogen. In LHD [2], we find the helium
flux over 1019 m-2*s, whose energy is over 1.2 keV byusing the microscopic measurement of the irradiatedmaterial [3]. Therefore the suitable method for measuring
helium ion distribution should be established immediately.
It is very difficult to use spectroscopic methods or thepassive charge exchange neutral particle method forhelium ion. Helium ions are almost fully ionized except
near peripheral region. A few helium atoms are escapedfrom plasmas by the double charge exchange reaction
between the background helium neutral and the fully
ionized helium ion, whose cross section is too small.
Therefore the helium ion has not been observed until now
by the particle measurement. Here we describe that we
author’s e-mail: ozaki@nifs.ac.jp

succeed the observation of helium in higher harmonic ion
cyclotron resonance heating (ICH) [4].

Fig.1. Magnetic surface, ICH antenna and resonancelayers.

2. Higher Harmonic Ion Cyclotron ResonanceHeatingIt is one of most suitable method to use ICH in LHD
in order to obtain the accelerated helium ion. We have tried
ICH in He3/He4 mixture plasma. However the effective
heating has not been obtained probably due to theProceedings of ITC/ISHW2007
contaminated hydrogen acceleration rather than heliumacceleration. Here we propose the higher harmonic ICH
without the hydrogen resonance. This technique is utilized
in the electron heating using Landau damping [5]. To thispurpose, the ICH should not provide its power to thehydrogen ion. If the ions are accelerated, they cannot beconfined and their energy cannot be deposited due to thelow confinement magnetic field in high beta plasma. We
choose the suitable combination between the magnetic
field and the frequency of ICH so as there is no ion
cyclotron resonance for the hydrogen in the plasma coreregion. One of their combinations has the resonance for theHe4 around plasma core region. Therefore the hydrogengas should be used at this combination of the magnetic
field and the frequency in order to obtain high electron
heating efficiency. If we are interested in the helium
acceleration, the helium gas should be chosen in the samecombination.
If we detect the helium ion by using the charge
exchange neutral particle measurement, the hydrogen ion
always behaves as a noise because their masses andcharges are too close each other. Fortunately there is aFig.2(a). Efficiency for He-4 at the plate voltage of1/4.

Fig.2(b) Efficiency for scattered hydrogen at the plate
voltage of 1/4.

possibility of the helium observation because there is noacceleration of the hydrogen in this combination. Figure 1
shows the resonance layers at the magnetic field of 1.86 T
and the ICH frequency of 38.47 MHz, drawn on thevertical cross section of LHD magnetic surfaces [6]. The
plasma and the ICH antenna are also shown. The He4
resonance appears at ρ=1/3 in the 3rd harmonics of the ICH
frequency. Therefore the high efficient helium acceleration
can be expected. On the other hand, there are the hydrogen
resonance layers only at the peripheral region of theplasma. Unfortunately there are the electron cyclotron
resonance layers near the plasma edge in the configuration.
The electron heating efficiency is not high and is expected
to be strongly depended on the power deposition to heliumion.

3. Compact Neutral Particle Analyzer
The compact neutral particle analyzer (CNPA) [7] formeasurement of the charge neutral exchanged particle isinstalled perpendicular direction against LHD plasma
almost at the mid-plane. CNPA is a traditional E//B particle
analyzer with a diamond-like carbon film as a stripping foil,
the permanent magnet for the energy analysis of theparticle and condenser plates for the particle mass
separation. To precise detection in low energy region, there
is a particle acceleration tube of 10 keV. Therefore thehydrogen with the energy range from 0.8 to 168 keV canbe observed by 40 rectangular-shape channeltrons, which
is set on the position for the hydrogen measurement.
The spatial resolution is determined to be 5 cm by theseveral apertures in the neutral particle flight. Time
resolution is set to be 0.1 ms, which can cover the wholeplasma duration within the buffer memory with theCAMAC ADC. Data acquisition, data pre-process andanalyzing data display are routinely completed within 3
minutes discharge cycle.
If the condenser plate voltage is changed, the different
mass as helium can be observed in principle. According tosimple orbit calculation in CNPA, the beam spot of thehelium is different from the channeltron array, which isadjusted to the hydrogen even if the plate voltage is tuned[8]. Here we assume the single ionized helium ion after
translation of the carbon film of helium. The spot size isassumed to be determined by the aperture size (2 mmφ)and the geometric configuration of the plasma and thedetector. In low energy region, the spot size may beenlarged due to the scattering in the foil.
The helium beam spots do not correspond to thedetector array in higher energy channels when the plate
voltage is adjusted to a low energy channel because thedetector array position is adjusted to the proton. Now wecontinue accurate calculation for obtaining the detector

Proceedings of ITC/ISHW2007
efficiency of helium. The helium energy spectrum can beobtained because we are interested in lower energy helium
spectra in LHD experiments.
The calibration procedures are as follows;(1) Compare the simulation model [9] including accurate
orbit calculation and the experimentally calibrated
value in hydrogen.
(2) (1) is almost agreed. Therefore we believe thesimulation model and calculate the efficiency inhelium and the scattered hydrogen when the plate
voltage is set to be 1/4 for the hydrogen.
(3) The calculated efficiencies for the helium and thescattered hydrogen are shown in Figs. 2(a) and (b).

In order to obtain the high electron temperatureplasma, NBI#1, #2 and #3 are injected during 0.4 seconds
at the beginning of the discharge [10]. After that, theplasma is maintained by the NBI#2. During this phase, thepower of NBI#2 keeps low as the effect of ICH application
can be clearly seen. The line averaged plasma density of2x1019 m-3, the central plasma temperature of 2 keV can beobserved. ICH pulses are applied at two different timings.
The ECH is overlapped at the second ICH pulse in order toobtain high electron heating at the high electron
temperature. However the high electron temperature is notenough because the electron resonance region at thiscombination between the 2nd harmonic frequency of ECH
and the magnetic field, is off-axis. Typical stored energy
increment due to the ICH application of 1.55 MW, is 50 kJ
at Wp=300kJ. Temperature rising is small in hydrogenplasma. Main contribution of Wp increment may comefrom the density rising at the plasma edge.
We change the gas from the hydrogen to the helium inorder to study the electron heating reduction due to theFig.3(a). The time histories of the plasma
parameters and the neutral particle energy
spectrum in the voltage setting of hydrogen.

4. Experimental Results
LHD has a toroidal mode number of m=10, helicalmode number of l=2. The major radius and minor radius
are 3.9 m, 0.6 m, respectively. The helical ripple is 0.25
and a magnetic field is a maximum of 3 T. Although thestandard magnetic axis is 3.75 m, it can be changed from
3.4 m to 4.1 m by applying a vertical magnetic field.
There are three different heating systems of the electron
cyclotron resonance heating (ECH, 2 MW), the neutralbeam injection heating (NBI, 15 MW) and ICH (3MW).
As for electron temperature, a maximum of 10 keV isobserved by using a Thomson scattering and an electron
cyclotron emission. Electron density can be changed from
0.1 to 4x1019 m-3. The density profile is measured with amulti-channel interferometer.

Fig.3(b), The time histories of the plasma parameters
and the neutral particle energy spectrum in thevoltage setting of helium.
power absorption by the helium ion. The helium resonancelayer is around ρ=1/3 at the 3rd harmonics of ICH. As theresult, the Wp is obviously reduced by the helium gaspuffing. The reduction rate is depended on the amount ofthe puffing. Therefore the absorption to the electron may
be reduced because the injection power of ICH is partiallyabsorbed by the helium ion. The increasing rate of Wp islarge at the high electron temperature. This means that
effective electron heating by ICH can be obtained in thehigh temperature, especially at hydrogen plasma. On theProceedings of ITC/ISHW2007
contrary, in helium plasma, the temperature dependence of
the increasing rate of Wp is not significant. In hydrogenplasma, the ICH power is easily absorbed to plasma
electron because there is no resonance region against
hydrogen. However, in helium plasma, the power
deposition of ICH to the electron is not enough due to theexistence of the helium ion resonance layer.
To confirm the helium acceleration, we compare thespectra of the helium and hydrogen by using CNPA withdifferent plate voltages. Figures 3(a) and (b) show the ratio
between the signal of He and H in two similar shots. We
must remember that most signals are hydrogen even if we
set the plate voltage for helium. Therefore the ratio means
the ratio between the scattered hydrogen plus helium andthe real hydrogen. The large ratios at low and high energy
regions are due to the large scattering at the foil and theclose trajectory of the hydrogen beams, respectively. The
ICH is applied at 3.0 seconds, but not applied at 1.0 second.
He/H ratio lower than the helium energy of 5 keV at 3.0seconds is obviously larger than at 1.0 second. This means
the low energy helium ion is accelerated by the higher
harmonic ICH.
Higher harmonics heating provides the α particle
simulation experiment although it is not effective for theelectron heating. There is another candidate of the helium
acceleration as He3/He4, but it is too difficult due to thehydrogen contamination. By tuning the magnetic field andfrequency of ICH, higher acceleration energy of the helium
ion over 5 keV can be expected.
0.05
0.04375

1.0sRatio

0.03753.0sRatio

0.031250.025
0.01875
0.01250.0062501020(2.5)

(5)

30

4050(12.5)(7.5) (10)
H_Energy(keV)(He_Energy(keV))

60

(15)

Fig.4. Energy dependence of He/He ratio.

5. Summary
The helium acceleration experiment to study thefuture α particle measurement has been done. It is very
important to establish the α measurement because thehelium/ α makes a bubble and gives a serious damage on
the wall surface. The higher harmonic ICH without thehydrogen resonance is utilized in the electron heating using

Landau damping. There is the helium resonance layer at
ρ=1/3. In LHD, we can find the helium particle using thecharge exchange neutral particle method at this experiment.
By the helium acceleration, the electron heating efficiency
is reduced. This fact suggests the way of efficient heating.
At the same time, we can obtain the useful tool to develop
the α particle measurement.

Acknowledgements
This work was performed under NIFS-ULBB509, thegrants aid of No. 17540475 and 18035013.

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