Proceedings of ITC/ISHW2007

Investigation of effective cleaning method using ion cyclotronconditioning in LHD
Naoko ASHIKAWA, Kenji SAITO, Testuo SEKI, Akio SAGARA,Masayuki TOKITANI, Yoshihisa OHTAWA1), Masaki NISHIURA,
Suguru MASUZAKI, Kiyohiko NISHUMURA, Ryuhei KUMAZAWA,
Hiroshi KASAHARA, Takashi MUTOH, Naoaki YOSHIDA1)
and the LHD experimental group
National Institute for Fusion Science, Toki 509-5292, Japan
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan1)

The ion cyclotron conditioning (ICC) is one of the conditioning methods to reduce impurities and to remove tritium fromthe plasma facing components. Advantages of ICC are to be operated under strong magnetic field for fully torus area andcan be operated with high pumping efficiency. In particular, ICC in helical devices can be operated using the samemagnetic configuration for of main plasma and a shot by shot operation is easy. The advantages of ICC in LHD are thedurability of long-term operation and the flexibility of input power with pulse phases. Using material probe system,
effective cleaning area by ICRF conditioning is estimated. On the plasma facing area of material holder, a lot of helium
babble is observed by transmission electron microscope (TEM), but the gap area have no damage. It is suggested effective
particles come to the wall straight and a clearing for shadow area is difficult.
Keywords: LHD, Wall conditioning, RF plasma, PWI, T inventory

1. Inroduction
The ion cyclotron conditioning (ICC) is one of theconditioning methods to remove tritium from the wall
[1,2]. The operation of experimental devices for fusion
plasma with superconducting coils needs long time
duration for increasing and decreasing the magnetic fields.
As typical wall conditioning methods, a DC-glow
discharge cleaning is operated such as JT-60U and LHD[3,4] during non-magnetic field. In future, thesuperconducting reactors, which still will be operated
over long discharge duration and will deuterium/tritium
mixtures. Therefore, for future devices, a wallconditioning method in magnetic field will be required ingeneral.
As a general wall conditioning method to remove
impurities and retention gases, two type scenarios areplaned in the international thermonuclear experimental
reactor (ITER), one is an initial conditioning before main
experimental campaigns and second one is a day by day
conditioning during campaign. During experimental
campaign, a magnetic field will be kept in ITER, and thenthe ICC method during magnetic field is important. As
other reason, tritium retention is one of serious problem
author’s e-mail:ashikawa@lhd.nifs.ac.jp

in ITER and the ICC is considered as a tritium removal
method from the first wall and divertor target.
In the large helical device (LHD) [5], is providedwith a superconducting coil as confinement magnetic
field and DC-glow discharge cleaning are used in mostcase to operation the wall conditioning [4]. As other wallconditioning method, boronization and titanium gettering
are operated a few times during experimental campaign.
Ion cyclotron range of frequency (ICRF) heating
devices is developed for a high power and long pulse
discharge plasma heating in LHD [6,7]. Using same
ICRF antenna sets, ICC was operated in 2005 and a
successful operation was done as an initial operation [8].
From experiences in LHD and AUG, effectivecleaning areas by ICC are considered smaller than byglow discharge. But this RF plasma wall interaction is notinvestigated yet. In this paper, effective cleaning area by
ICRF conditioning was estimated using damages onmaterial probes for different area.

2. Experimental setup
For the ICC operation, one of the ion cyclotron
ranges of frequency was choosing and this frequency isProceedings of ITC/ISHW2007
the second harmonic heating for helium. This is a heating
method which takes advantage of finite Larmor radiuseffect and a heating efficiency at f = 38.47 MHz is about
40%. As a confined magnetic field, a magnetic field,Bt,=2.75T, and a magnetic axis, Rax=3.6m were used.
Hydrogen and helium partial pressure were measured byquadruple mass spectrometry (QMS) with a samplingtime of 1 sec. As plasma facing components the first wallconsists of stainless steel 304 and a divertor is made ofcarbon. A RF input phase of 3 sec and interval phase of
2 sec are used and it is determined by a stored duration
time of data acquisition system. An input power and ahelium pressure were scanning, but stable discharge was
kept during ICC operation.

3. Material probe experiment in ICC
During ICC in FY2006, stainless steel samples onhexahedron block holder is installed using the movablesample stage system at 4.5L port and this stage level was
set at just first wall as shown in Fig.1.

Sample holder has three kinds of facing on this block asfollowing 1) plasma facing area, 2) Vertical direction forplasma facing area and 3) Gap area. Samples installed onholder directory on plasma facing and vertical direction,and vertical area facing in-vessel component. The gap
area is demonstrated between tiles and thin plate isconnected to base holder with slit of 1.5mm. Samples
installed on holder, and then samples are not exposed toplasma directory.
This sample holder kept during ICC plasma andintegrated exposure time is about 4000 s. Stainless steel
samples is analyzed by transmission electron microscope(TEM) and these bright field images are shown in Figs.2.
From comparison number of damages due to helium
babbles on different position, only plasma facing area asshown in Fig.2(a) has large number of damages. For asample on the gap at position 3 in Fig.1, damage is notobserved by TEM analysis. From helium babbles are(a)(b)200nm
Helical CoilVacuum Vessel

Fig.2 Bright field images by TEM, (a) on plasma facing
are and (b) vertical area.

Plasmaproduced by an interaction between RF helium plasma
and target samples, large number of damages are
estimated as strong interaction in this experiment. These
results are suggested effective CX particles come to thewall straightly and a cleaning for shadow area will bedifficult.

Helical CoilProbe Head

4.5L-Port

2.Vertical direction

MaterialProbeSystem

4. Particle flux from RF conditioning plasma

1.Plasma Facing Area

3.Gap

Fig.1 material probe system and sample holder.

Using Natural Diamond Detector (NDD) [11] Charge
exchange neutral particle is measured as shown in Fig.3.
This NDD is installed at 4.5 lower side near the port andCX neutral particles by ICRF plasma can be detected on
this poloidal cross-section. Using NDD data at shot #70540, particle flux is estimated. Detected particle
thought out of slit of 2mm and this integrated time isabout 1.8 s. At shot #70840, using detected counts of fullenergy spectrum by NDD as shown in Fig.3 and area ofdetector, particle flux of charge exchange neutral iscalculated about 1.6x105~3.2x105 [s-1cm-2]. At thisexperiment, a particle flux can measured only RF input offrequency of 38.47 MHz, and it is to need changing anProceedings of ITC/ISHW2007
integrating time of time frequency for other frequency
and low power case.

stnuoc10510410

3102101#70840
4.5L NDD (0.5-2.3 s)background1Fig.3 Energy spectrum ofDetector

Natural Diamond

5. Summary
Using material probe system, effective cleaning area
by ICRF conditioning is estimated. From three kinds of200nm(a)(b)
area, number of helium bubble on samples is different.
On the plasma facing area of material holder, a lot of
helium babble is observed by transmission electron
microscope (TEM), but the gap area have no damage.
These results are suggested effective particles come to thewall straight and a clearing for shadow area is difficult.
During ICC, particle flux is measured about
1.6x105~3.2x105 [s-1cm-2] at port area.
In future works, using this estimated cleaning area
comparison of removal efficiency of tritium by different
method is important.

Acknowledgements
This work was supported by the NIFS budget
NIFS06ULPP515, the JSPS-CAS Core University
Program in the field of“Plasma and Nuclear Fusion”and
the Joint Research System of Research Institute forApplied Mechanics, Kyushu University. This work wasundertaken by IEA/ITPA joint experiment of DSOL-8“ICRF conditioning”.

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