Proceedings of ITC/ISHW2007

Observation of Ion Tail in ECH/ECCD Plasmas in Helical Devices
Shinji KOBAYASHI, Mitsutaka ISOBEa, Masaki OSAKABEa, Hiroyuki MATSUSHITAa,Masashi KANEKOa, Tetsuo OZAKIa, Pavel GONCHAROVa, Kazunobu NAGASAKI,
Yasuo YOSHIMURAa, Hiroe IGAMIa, Shin KUBOa, Takashi SHIMOZUMAa,Satoshi YAMAMOTOb, Kazuo TOIa, Yoshihiko NAGASHIMAc, Kenji SAITOa,Hiroshi KASAHARAa, Katsumi IDAa, Mikiro YOSHINUMAa, Sadatsugu MUTOa,Tohru MIZUUCHI, Hiroyuki OKADA, Katsumi KONDOd, Kiyoshi HANATANI,Fumimichi SANO and Shoichi OKAMURAa
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, 611-0011, Japana
National Institute for Fusion Science, Oroshi-cho, Toki-shi 509-5292, Japanb
Kyoto University Pioneering Research Unit for Next Generation, Kyoto University, Gokasho Uji 611-0011, Japanc
RIAM, Kyushu University, Kasuga 816-8580, Japan
dGraduate School of Energy Science, Kyoto University, Gokasho, Uji, 611-0011, JapanThe high energy ion tail, having the energy more than five times higher than the bulk temperature, has beenobserved in the electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) plasmas ofHeliotron J and CHS. The tail temperature of ion increases with decreasing the density and increasing the ECHpower. The ion tails appear under the condition of high tail temperature of electrons measured by the soft X-ray
charge-coupled-device system. The comparison of the fraction of the ion tail density between CHS and Heliotron J
shows that the tail ions are accelerated selectively in the parallel direction to the magnetic field. The high energy
electron is a key factor of the formation of the ion tail.
Keywords: Electron cyclotron heating, ion tail, electron tail, anomalous heating, neutral particle analysis

1. Introduction
The formation of the ion tail has been observed in theelectron heated plasmas, i.e. electron cyclotron heating(ECH) or electron cyclotron current drive (ECCD) plasmas,
in several torus devices [1-4]. The rise and decay times of
the ion tail have been much shorter than the collisionalenergy relaxation time between electrons and ions. Thisphenomenon has been considered to be due to (1) theacceleration by the strong Landau damping oflower-hybrid (LH) waves excited by the parametricthree-wave decay process [1] or (2) the anomalous
electron-ion coupling because of the slide-away regime ofthe electron energy distribution [5]. The high energy charge
exchange flux was observed experimentally in the X-mode
launch from high magnetic field side in the W7-A
stellarator [1]. The low-frequency decay wave
corresponding to the LH range of frequency was measured
with probe, which might be the cause of the ionacceleration. In the TCV tokamak, it was found that thehigh energy CX flux increased with the current drive
efficiency by ECCD [4]. The numerical calculation shows
the TCV experiments were in the slide-away regime. An
excitation of waves satisfying the dispersion relation of k·vi
= ω = k||ve has been expected due to the slide-away of theelectron energy distribution function in Ref. [5].
author’s e-mail:kobayashi@iae.kyoto-u.ac.jp

In a low magnetic shear helical device, Heliotron J [6],high energy CX flux has been measured in the 70 GHz 2nd
harmonic ECH plasmas [7], even the reflection of thelaunched EC waves has not been controlled. In this paper,
we describe the result on the ion tail measurements in theECH/ECCD plasmas in Heliotron J and Compact HelicalSystem (CHS) [8]. The rise time of the ion tail formation
is examined in Heliotron J. The dependence of the tailtemperature on the electron density is studied inHeliotron J and CHS. The density of ion tail isinvestigated from the viewpoint of the normalized ECHpower. The mechanism of the formation of the ion tail isdiscussed by the experimental results obtained in
Heliotron J and CHS.

2. Experimental results
2.1 Heliotron J
The 70 GHz ECH system has been installed in
Heliotron J, medium sized (R0/a = 1.2m/0.17m)
helical-axis heliotron device with L/M =1/4 helical coil,where L and M are the pole number of the helical coil andhelical pitch respectively [6]. The 2nd harmonic ECH
experiments have been carried out using two injection
methods; corner section and straight section launch [9]. In
the case of the corner section launch, the focused EC

Proceedings of ITC/ISHW2007
Heliotron J, 70 GHz ECH STD config.
Corner Section
Launch1020

ne=0.2×10 m

1019(a)191.1×10 m
181017-30.5

11.5
Energy[keV]

W2diane

11ECH01

Straight Section
Launch
0(a)

0(b)
Taili

10-3

T (keV)f(E) [a.u.]

192(x10-4 Wb)

21

dia

10

Heliotron J 70GHz ECH#9810 Standard Config. 2

W

22ne(x1019m-3)

101.5(b)

Bulk
0160

Ti (keV)

Tail

240
time (ms)280

1.0Fig.2
0.5
Bulk
0.0
0.0

0.5

1.0
1.5
19 -3n (×10 m )2.0

eFig.1200(a) Energy spectra of charge exchange flux
obtained in 70 GHz ECH plasmas of Heliotron
J and (b) tail and bulk temperature of ion as afunction of line-averaged electron density.

waves using mirror system were launched from the outertorus side of the corner section, where the tokamak-likemagnetic field was formed. The non-focused EC waves
were launched at the straight section. The gradient of themagnetic field strength at the straight section is small
owing to the formation of the quasi-omnigenous magneticfield.
Figure 1(a) shows the CX energy spectrum measured
by the CX neutral particle analyzer (CX-NPA) [10]obtained in the standard configuration of Heliotron J. The
CX-NPA is E//B type one, whose energy range is from 0.4
to 80 keV for hydrogen and from 0.2 to 40 keV for
deuterium, respectively. CX-NPA detected the ions withthe pitch angle of 110 degree to the magnetic field in thestandard configuration of Heliotron J. In the experimentsof the corner section launch under the low density
condition at the line-averaged electron density ne of 0.2 ×1019 m-3, the high energy ion flux was found in the energy
range more than five times higher than the bulk iontemperature. The bulk temperature estimated to be 210 eVfrom the slope of the CX spectrum in the energy range
from 0 to 1 keV. A folded spectrum more than 1 keV shows
the tail temperature is 710 eV. In the higher density at ne =1.1 × 1019 m-3, no clear tail was observed, which shows thetail formation depends on the electron density. The ray

Time evolution of plasma parameters in ECH
plasmas of Heliotron J, (a) line-averaged density
and stored energy and (b) tail and bulk
temperature of ion.

tracing calculation of the EC waves shows the absorption
profile in the corner section launch is localized at theresonance layer [11]. In the straight section launch, on thecontrary, ion tail has not been measured clearly even in thelow density condition at ne = 0.2 × 1019 m-3. A relatively
long-path absorption profile was expected by the ray
tracing calculation in the straight section launch [9]. It wasalso found that the tail temperature depended on theinjection power of ECH (PECH) in the case of the cornersection launch, that is, the tail temperature was 840 eV atthe electron density of 0.4 × 1019 m-3 with PECH of 310 kW,
while that at PECH = 240 kW was 250 eV. These results
indicate the focused strong EC waves are favorable to theformation of the ion tail.
Figure 1(b) shows the dependence of the bulk and tailtemperatures on the line-averaged electron density. These
data were obtained in the ECH injection power of 310 kW
in the corner section launch. The tail component wasobserved in the case that the density was lower than 1.0 ×
1019 m-3. The tail temperature increased up to 1.5 keV withdecreasing the density. The bulk ion temperature, on theother hand, was not so sensitive to the density in thedensity range below than 2 × 1019m-3.
Figure 2 (a) and (b) show the time evolution of theline-averaged density, stored energy and tail and bulk ion
temperatures measured by the CX-NPA system. The tailtemperature increased up to 640 eV by 10 ms at thebeginning of the discharge. The decay of the tailtemperature was very quick according to the change in theelectron density. The electron temperature measured by theSX absorber foil method was about 1 keV. The energy
exchange time from electron to ion was about 0.3 sec

Proceedings of ITC/ISHW2007
CHS, 53GHz ECH, R =94.9cmaxCo-ECCD, #122207-122270

6105n = 0.2x104100.3x10with Ion Tail-3

19-3m

210110100123Energy (keV)

1010910(b)8v#116976-116988
T =420 eV

710Tail

6104103100with Ion Tail

10Fig.4||v
280eV
123Energy (keV)

4(a) CX energy spectra in 54 GHz ECH
plasmas of CHS in ne = 0.2, 0.3 and 0.4 × 1019
m-3 and (b) comparison of CX energy spectra
between parallel and perpendicular velocity .

under the condition, which shows that the formation of ion
tail cannot be explained by the classical collision process.

2.2. Compact Helical System
CHS is a planner axis helical device (R0/a =1.0m/0.2m) with L/N=2/10 helical coil, where N is thetoroidal period number [8]. The 53 GHz ECH system inCHS has three steerable mirrors and polarizer to control
the injection angle of the EC waves in both toroidal andpoloidal directions and the polarity [12]. In CHS, the effectof the X-mode launch from high field side is negligible.
The ECCD experiment using the 53 GHz ECH system inCHS is described in Ref. [13].
The CX energy spectrum measured with fast neutral
analysis diagnostics (FNA) [14] is shown in Fig. 3 (a)obtained in the ECH plasmas at the line-averaged electron
density of 0.2, 0.3 and 0.4 × 1019 m-3, respectively. TheECH injection angle was oblique one and the toroidal
current due mainly to ECCD was about 3 kA in theCo-direction under the experimental conditions, which
increased the rotational transform. The folded spectra wereobserved in the energy range more than 0.8 keV. The tailtemperature increased with decreasing the density. Thedensity dependence will be discussed later by comparing
with the results obtained in Heliotron J.
To examine the velocity distribution of tail ions, theCr Fe

100410105

Ti

0.4x1019 m-3-1f(E) (A.U.)

mef(E) (A.U.)

30Fig.3

19e

1010CHS 53GHz ECH #106072-106113(a)f (E) [A.U.]

101000w/o Ion Tail
0246Energy (keV)

810Energy spectrum of electron measured by theSX-CCD system obtained in ECH plasmas ofCHS.

parallel and perpendicular velocity components of CX flux
to the magnetic field were measured by changing thetoroidal angle of FNA. Figure 3 (b) shows the comparisonof the CX energy spectra between the parallel andperpendicular directions. These data were obtained in theCo-ECCD plasmas at the electron density of 0.2 × 1019 m-3.
The CX flux in the parallel velocity direction was higher
than that of the perpendicular one. The bulk and tailtemperatures in the parallel direction were 170 and 420 eV,
respectively, while those for the perpendicular direction are
160 and 290 eV, respectively. The fraction of ion taildensity is defined by the following formula,
5×Ec

∫f

Ec

Tail

5×Ec
⎞⎛ Ec( E ) dE / ⎜⎜ ∫ f Bulk ( E ) dE + ∫ f Tail ( E ) dE ⎟⎟ (1)Ec⎠⎝0where fBulk(E), fTail(E) and Ec are the energy spectra of thebulk and tail ions and the critical tail energy, respectively.
When Ec is given by 1 keV, the tail density fraction is
1.3 % for the parallel direction, while that in theperpendicular case is 0.7 %. These results suggest the tailions are accelerated selectively to the parallel direction.
When the injection angle of ECH was varied from Co- tocounter direction in CHS, no clear difference was seen in
the ion tail flux on the ECH injection angle.
Figure 4 shows the electron energy distribution
measured using the pulse height analysis technique withthe soft X-ray charge-coupled device (SX-CCD) system in
CHS. [15]. An existence of the high energy electron morethan 2 keV was confirmed in the low density ECH plasmas.
The slope of the electron tail becomes flat when the ion
tails are formed, that is, the ion tails appear under thecondition that the tail temperature of electron is higher than
4 keV. Thus the high energy electron has a key factor for
the formation of the ion tail.

Proceedings of ITC/ISHW2007
heating on the ion tail formation, we are planning aX-mode launch from high field side using the ECH systemof Large Helical Device [16].

5(a)Heliotron JCHS v||CHS v

Bulk

43Ti

Tail/Ti

4. Summary2
1000.51
19-3n (x10 m )

1.5

e-1

Tail density fraction(b) 10-2

10-3

10We confirmed the formation of the ion tail under thelow electron density conditions in ECH plasmas ofHeliotron J and CHS. The ion tail formed under thecondition that the high energy electron was produced bythe focused EC wave. The LH decay wave heating is notconsidered as the main source of the ion acceleration in
Heliotron J and CHS because the effect of the X-mode
launch from high field side can be neglected. Although thephysical mechanism of the selective acceleration of ions tothe parallel direction is still unknown, the high energy
electron produced by the focused strong ECH is key factor
for the ion tail formation.

Acknowledgement

-4

1010

12ECHFig.5

31010-19
P /n V (10 kW)
e104p(a) Temperature ratio of tail and bulk ions as afunction of electron density and (b)dependence of fraction of ion tail density on
the normalized ECH power obtained inHeliotron J and CHS.

3. Discussions
In this section, we compare the measurement resultsof ion tail in Heliotron J and CHS. Figure 5 (a) shows thetemperature ratio of tail to bulk ions in Heliotron J andCHS as a function of the electron density. The closed andopen squares show the temperature ratios determined by
the CX spectra with parallel and perpendicular velocity
components in CHS, respectively. In Heliotron J, theCX-NPA has set almost perpendicularly. The temperature
ratios in Heliotron J and in the parallel component in CHS
increase with decreasing the density. The temperature ratio
in the case of the perpendicular component of CHS islower than the Heliotron J results. This is attributed to thedifference in the injection power of ECH between CHSand Heliotron J.
Figure 5 (b) shows the fraction of ion tail density as afunction of the ECH injection power normalized by thetotal electron numbers (electron density times plasma
volume Vp). The tail density fraction in CHS and Heliotron
J increases with the normalized power. A slightly higher
fraction of the tail density is obtained at the parallel
component in CHS than the others. The perpendicularcomponent in CHS, on the other hand, is in the same orderof Heliotron J results.
In order to clarify the effects of the LH-decay wave

One of the authors (S.K.) wishes to thank theexperimental members of Heliotron J, CHS and LHD
groups. This work is partly supported by the NIFS
Collaborative Research Program. (NIFS04KUHL003,
NIFS05KUHL007 and NIFS05KLBB003)

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