J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 40-45

Wall Conditioning with a High Magnetic Field
in HT-7 Superconducting Tokamak
LI Jiangang*, GU Xuemao, GAO Xiang, ZHANG Souying, JIE Yingxian,
YANG Xiaokang and HT-7 Team
Institute of Plasma Physics, Academia Sinica, 230031, Hefei, China(Received: 18 January 2000 / Accepted: 18 July 2000)

Abstract
ICRF wall conditioning techniques, which includes the hydrogen removal, impurity cleaning,boronization and siliconization, were described in this paper. This new technique has been demonstrated
to be very effective for wall conditioning, recycling, isotopic control and used daily during experiments.
The RF plasma parameters were measured as T"= 3-8 eV, Ti= 0.5-2 keY, n" = 0.3-5 x l0l7 m-3 bydifferent diagnostics. The nontoxic and nonexplosive solid carborane powder was used for the RFboronization. Energetic ions cracked the carborane molecule and the boron ions impacted and deposited
onto first wall. Comparing with GDC boronization, the B/C coating film shows the higher adhesion,
better uniformity and longer lifetime to the plasma discharges. Siliconization was carried out by using ahigh field side long RF antenna, which made the discharge more uniform. The ratio of SiFIa to helium isabout 5:95 at the pressure range of P" = 0.8-8 x 10-2 Pa. Compare with boronization, it showed quicker
recovery from a bad wall condition due to leakage of air to good wall condition. Plasma density could be
easily controlled after siliconization. But the lifetime is much shorter than that obtained by boronization.
Plasma performance has been improved after RF boronization and siliconization.

Keywords:
tokamak, wall condition, boronization, siliconization, ICRF

1. Introduction

future large superconducting tokamaks, such as ITER, in
the presence of the high magnetic field. The effects ofwall conditioning by applying RF plasma aredemonstrated. It shows much better results than thoseobtained by glow discharges and Taylor dischargecleaning. The details of the technique and its application
for processing different wall states are given in this

It is well known that the proper wall conditioning isessential to obtain good plasmas. With a high magnetic

field, traditional wall conditioning techniques would notwork. A new technique of wall conditioning, so called
RF conditioning, has been tested on TORE SUPERA[1,2], TEXTOR-94 [3] and HT-7 tokamak [4,5] byinjecting ion cyclotron resonant frequency (ICRF) wave
into plasma.
A RF conditioning technique has been successfully
applied on the HT-7 superconducting tokamak, which
includes hydrogen removal, impurity reduction,
recycling control, boronization and siliconization. This
special technique is aiming the wall conditioning for*

C o rre spondin g autho r' s e - mail : j-lipaper.

2. RF Plasma and Parameters
The theory of plasma production in tokamaks using
RF in the ICRF range has been investigated by someauthors [6,7]. To initiate the discharge, an antenna has to@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research@ mail. ipp. ac. cn

40Li J. et al., wall conditioning with a High Magnetic Field in HT-7 Superconducting Tokamak
3.

fi.21i0-10.12{ 4 0 1 I t216nUn

2# pEpiog ducrr (cm)

5WUt

Fig.1 The set-up of RF conditioning system on HT-7

Fig. 2 Electron temperature profile measured by ECE.
Br= 1.5T, P = 4x 10'2Pa, Par = 25 kW f= 30 MHz.

generate a RF electric field E1; parallel to the magnetic

plasmas by a factor of two with the same discharge

field lines. The 811 at the antenna is responsible forcondition. Electron density is in the range of 0.5-3 x
1017 m-3, which is measured by HCN interferometer andLangmuir probe. Ion temperature is as high as 0.5-2
keV with high-energy tail of a few tens of keV.
Since the presence of the harmonic resonant layersof the different particles, which are visible inside of theplasma, electron temperature and density are notuniform across the minor radius. Figure 2 shows the Z"
profile with an injection power of 25 kW. The
maximum T" is about 3 eV at the 5th harmonic layer ofhelium. A few other resonant layers also appear where
local T" is higher than the non-resonant regions. Central
ion temperature is about I keV. With the injecting RF
power increases, T" increases a little and Z; increases

neutral gas ionization by highly accelerated electrons
during gas breakdown. To get a higher ionization ratio,
the harmonic resonance absorption layers should be
inside the plasma. The ion cyclotron frequency co is qB/
m. Where B is the magnetic freld, q the ionic charge andm the mass. The RF antenna for heating can give acertain ,E'11, which is also suitable for plasma production.
The hardware set-up for the RF conditioning is thesame as ICRF heating. Three kinds of RF antennaconfigurations are used. A l/4-turn antenna with
Faraday shielding is installed in the low field side of thetorus. A ll2 turn long antenna is mounted in the highfield side. An ion Bernstein wave (IBW) antenna is set

in the mid-plane of low field side. The arrangement ofwhole system is shown as Fig. 1.
RF plasma is easily produced by injection of the
ICRF power in the range from 5 kw to 50 kw for bothlinearly with the RF power. With a higher fillingpressure and RF power, the T" profile becomes flat

helium and deuterium working gas. Toroidal field (0.1 -temperature is much higher. Energetic ions are produced

2.5 T), gas pressure (8 x 104 Pa - 5 x 10-t Pa) and radioby RF plasma, which is expected to be produced in thefrequency (RF) (24 - 30 MHz) have been scanned to
optimize the cleaning efficiency. The basic RF plasma

resonant layer regions.

parameters produced by different antenna configurations

3. Particle Removal
As we know, two effects which govern the

without clear peaks. Compare with the plasma property

of GDC, electron temperature is similar but ion

are measured by a few diagnostics. Quadruple mass
spectrometer (QMS) analysis during and after the RF
conditioning is carried out. Electron temperature isestimated from visible line spectrum, Langmuir probe
and ECE. The difference of electron temperature bythese diagnostics is within a factor of three. Electron
temperature is in the range 3 eV - 10 eV within an
injection power of 35 kW. The electron temperature forhelium RF plasmas is higher than that in deuterium

conditioning efficiency are outgassing rate of particles

from the wall and the ionization rate of desorbed
molecules which induces a redeposition. For getting
higher outgassing rate, a higher particle flux to the wallwith suitable energy is favorable. For reducing
ionization rate, the pulsed-RF-mode with 0.3s RF power
and l.5s time interval is used. Helium RF discharge isused mainly for hydrogen removing. It is also very4l

LiJ. et al., wall conditioning with a High Magnetic Field in HT-7 superconducting Tokamak

effective for the replacement of deuterium (and its
isotopes). This makes plasma start up very easy andresults in particle recycling being controllable manner.

energetic and reactive particles bombard the first wall

The pulsed mode with higher filling pressure (0.05-0.1
Pa) increases the conditioning efficiency by controllingelectron density and temperature separately to minimize

efficiency of the deuterium RF discharge. QMS data in
Fig. 3 shows the results before ICRF wall conditioning
and 30 minutes pumping after 60 minutes pulsed l5 kW

the hydrogen redeposition probability. The partialpressure of hydrogen was increased by two order of

RF discharge. After ICRF wall conditioning, CO

and induce the desorption of molecular impurities.
These are main reasons for the good conditioning

reduced by a factor of three and H2O reduced more thancomes from the desorption from the wall. Hightwo times, while the working gas (H2 and D2) part
increased significantly, which was greater than 807o ofhydrogen removal rates have been obtained.

total pressure. Very effective impurity cleaning

The RF injection power level plays a very
important role in the helium RF wall conditioning

capability was demonstrated.

magnitudes after RF plasma was initiated, which mainly

Three years data shows that, comparing with GDCwith similar parameters in HT-7, the hydrogen removing
by RF cleaning is 20 time higher and particle-removing

experiment. In the lower power level (i.e. the power islower than 20 kW), the hydrogen removal rate goes up
quickly and reaches a saturation status when Pap is great

rates to H2O and CO is more than two order ofmagnitudes. The difference is due to the different
plasma properties. Very high ion temperature of RF
plasma may play key role to obtain high cleaning
efficiency. The energy of the neutral flux, which
induced by charge-exchange with energetic ions, isthan 30 kW. For effectively removing the impurities,
such as CO, CO2 and H2O, different optimized RF
powers are needed. The impurity removal is due to thebombarding of neutral energetic particles to the firstwall. An increase of filling pressure increases theplasma density and induced a reduction of electron

much higher than that of GDC, which results in is more
effective desorption of the impurity from the wall.

temperature. Higher plasma density (high fillingpressure) improves cleaning efficiency.

Deuterium ICRF discharge is used for surface
cleaning and wall isotope control. The pulsed mode with
optimized pressure of I x l0-r Pa is used. The electron
temperature is lower (2-3 eY) compared with helium
discharge. Electron temperature changes a little within
the injection RF power of 30 kw. The low temperature

4. RF Boronization
The nontoxic and nonexplosive solid substancecarborane (C2Br0Hr2) was chosen for the boronizationmaterial for easy handling and exhausting [7].

minimizes the strong plasma reionization and wall
redeposition of neutral species. High temperature

According to the laboratory results from reference [8],the liner temperature was set around 150 - 200"C. The
wall was baked to 100'C. Toroidal magnetic field wasabout 1.8 T and the RF frequency was 30 MHz. With

deuterium ion and its enersetic tail are observed. Boththis arrangement, the fundamental hydrogen resonant
layer is located inside of the plasma.

12 rectangular samples of 10 mm x 10 mm

10dimension with different materials (graphite, stainlesssteal) were put into the different places of the machine
before boronization with same radius of liner. The

G'8

(L
I

o

56
G'

I
7-

Before RF conditioning
After RF conditioning

carborane container was heated and kept at 60'C. A
stable partial pressure of 0.2 Pa was provided. Heliumgas was regulated to the ratio of He: Caborane of l:1.
To improve toroidal uniformity of the coating, thepulsed RF power of 10 kW was coupled to the plasma

f

o
o

E4

0-

6
d2
L

with 2.4 seconds power on and 2 seconds power off.
Plasma color was gradually changed from the lightgreen to the pink, which indicated the injection ofcarborane vapor. Since several different species ofparticle existed in the plasma, their harmonic cyclotron
resonant layers were clearly observed from the RF
plasma, which was produced by the short l/4 turn010203040
m/e

Fig.3 QMS data before and after D, RF wallconditioning. Br= 1.5T, f = 30 MHz, P" = 0.15 Pa.
42

Li l. et al., Wall Conditioning with a High Magnetic Field in HT-7 Superconducting Tokamak

antenna. As the injection of the carborane vapor andstable RF discharge continued, the helium gas filling
was gradually reduced to the zero, which is favorable toget higher boron deposition rate.

Oxygen content increases from l0%o to 25Vo and boron
content decreases from 60Vo to 50Vo after 250 shots.
This demonstrates that boron film has very strong
oxygen capture capacity. The thickness of the film was

Carborane of 4 grams was used and theboronization process lasted for 65 minutes. The

reduced about 80 nm after 250 shots.

Since the anisotropy of RF discharge during thefirst boronization, which was indicated by the ion

carborane container was heated up to 120'C at the end
to evaporate all the carborane. After that, all the pumpducts were fully open. Helium RF plasma restarted andcontinued for 20 minutes but the RF power was reduced
to 3 kW to pre vent the heavy bombardment to the freshboron film. The purpose for the helium RF conditioning
after boronization was to remove the huge hydrogencontent that was absorbed in the film durins theboronization.
The residual gas analysis (RGA) was done during

cyclotron resonant layers, the film thickness capability
along the poloidal direction must not be uniform. The
film uniformity was not measured by the limitation of
the windows during the first boronization. The
uniformity along the poloidal direction was measured
with specimens on the top and horizontal windows

I *u iA;*lo'tt

boronization with RF plasma. The results showed aBll'different pattern with boronization using GDC on HT-7
and other devices. The group of mass 144(C2Br'H12)
was the only peak masses detected during GDCboronization in HT-7. The highest amplitude was given
with mass numbers 2 and 4 while very small amplitudes
of mass 18 and 44 were also observed during helium RF
clearing. The RGA pattern of the RF boronization isshown in Fig. 4. The partial pressure for the mass l1
and the groups of 22 were 50 times higher than that ofmass 144. This means that the carborane molecule iscracked into smaller pieces by the bombardment ofhigh-energy ions of RF plasma.
X-Ray-induced photoelectron spectroscopy (XPS)81*

Bltsu'

100

Brtr,

cF'#io
crB

10

2 ll

2t 222321 tu

I37 139 I4I 143 t/ts

mass mr/e

was used to analyze the film properties of thespecimens. Some of them were taken out afterFig.4 The residential gas analysis during ICRF
boronization. Br = 1.8 T , PRF = 10 kW, f = 30 MHz

boronization. The other remained inside machine andexposed to the plasma discharge. XPS shows thatboronization produces a fine amorphous boron-carbon

films (a-ClB:H). Boron layer has a well-defined

The flm on C base aftsr250 shobcomposition, which is homogeneous in depth, as can beseen from the depth profile of graphite specimen shown

F60
6a-.----.81!-*.

E.ot

in Fig. 5. B/C ratio is about 3 up to a depth of 220 nm
for the fresh film shown in Fig. 5 (a). The oxygen
concentration is about llvo since the presence of H2O

t=---.

20(dand CO during the film deposition. This is because of
the continuous gettering of oxygen in the growing of thefilm during the plasma bombardment.
The prime advantage of boronization films is theirability to getter oxygen due to the incorporated boron
atoms and their superior resistance to chemical erosion
by thermal and energetic hydrogen species. The oxygengettering was observed when the film was exposed thebombardment of plasma discharge shown in Fig. 5 (b).

Fresh film on Cbase

460
6

5.0
C
20
0

0

I

00

200

300

400

500

DeDosition dePth (nm )Fig. 5 XPS analysis of boron film for the graphite
specimen (a) fresh film (b) film after 250 shots
43

Li J. et al., Wall Conditioning with a High Magnetic Field in HT-7 Superconducting Tokamak

higher efficiency in gettering oxygen of a-Si:H films
was observed than that of a-ClB:H film. Silicon layer
has a well define composition, which is homogeneous,amorphous and uniform both in depth and toroidal
directions shown as in Fig. 6.
Plasma performance quickly improved after eachsiliconization. Lower qo and wider stable operation
region were achieved compared with RF boronization.
Higher electron temperature was obtained for the same
plasma current and density in tokamak discharge.
Impurity evolution shot by shot after siliconization gave
good results. Oxygen content dropped by a factor of two
after siliconization and gradually recover after 100
shots, while carbon radiation reduced by more than threetime and kept at very low level even after 300 shots.

during next campaign. The film thickness along thepoloidal direction is different by the low field sideantenna. The long antenna was used. The RF plasma
produced by long high field antenna was nearlyhomogeneous along the both toroidal and poloidal
directions. Clear separation in ion cyclotron resonant
layers could not be observed during RF plasma
discharge. The film thickness of top window specimen
was about 20 nm thinner than those from the horizontal

window. which was less than lU%o of the film thickness.

The uniformity along the poloidal direction was
dramatically improved.

The plasma performance was immediately
improved after RF boronization. The metal impurity
lines were disappeared. The visible line of carbon andoxygen were reduced by a factor of three. The radiation
power significantly dropped from807o of input power to

XPS analysis to the bombarded film showed that a large
amount of SiC was formed. Silicon radiation increased
by a factor of 2.5. A weak hollow and wider density andthe level of 16%o on the condition of same plasma

electron temperature profiles were formed after

current and density. The Zaydropped close to the unity
at the density 5 x l0re m-3. Wider Hugill stability

siliconization. The edge safety factor could be as low as
2.45.The total radiation loss was reduced.
Compared with boronization, siliconization takes
much short time. It shows much fast recovery after aserious degradation of the condition due to leakage of
air to good wall condition. It takes only one hour after aminor leakage of the air. Plasma performance was backto a good state. The density can be easily controlled
after siliconization without the need of further helium
RF conditioning needed for reducing hydrogen content
in the film. But the lifetime is shorter than that obtainedby boronization due to thinner film (60 nm) compared

operation region was observed. The effect ofboronization was still remained after 1500 shots, which
was indicated by the decrease of Z"n, loop voltage andimpurity radiation. Generally speaking, the lifetime offile with 49 carborane is about 1500 shots.

5. RF Siliconization
Siliconization was carried out by using high fieldside long RF antenna, which made the discharge more
uniform. The ratio of SiHo to He is about 5:95 at thepressure range of P, = 0.8-2 x l0-2 Pa for the safetywith boron film (300 nm). Metal reduction graduallyhandling the exhausting silane gas from pumping
system. Pulsed mode was used with one-second power

recovered after 120 shots and oxygen reduction effects

on and one-second power off. The RF power was l0 kW

and the pre-cleaning was not needed. The wholeprocedure last normally for about 30 to 60 minutes.
Plasma discharge was fired after siliconization without
post helium RF cleaning to reduce the hydrogen content

in the film. Very good plasma discharges were

8oo

immediately obtained. But lifetime of the silicon film
was very short, which lasted for about 50 shots since thethickness of the film is only about 20-30 nm.
Higher silane to helium ratio (10:90) and fillingpressure (0.05-0.2 Pa) were used in the following RFsiliconization. The lifetime increased by a factor of five.
The thickness of the film was about 60 nm. It showed

o

o40
E

l=
r

01020304050607080

bright interference colors indicating their semitransparency. The Si/C ratio is around l-2. Silicon

film thicknoss (nm)mainly bonds with carbon. After exposing to tokamak
discharges, Si-O bond increased by a factor of three, aFig. 6 XPS analysis of silicon film for the fresh stainlesssteal specimen.

44

Li J. et al., wall conditioning with a High Magnetic Field in HT-7 Superconducting Tokamak

j

I

A
o

hefo re wall condith ningVafrer boronazation

technique is aiming future large superconducting
devices in the presence of high magnetic field.

afrer silaconization

Comparing with GDC and TDC, the hydrogen-removal
rate increased by a factor of 20 and cleaning efficiency

o fa.rAt

to impurity is as high as two order of magnitudes. BasicoCiloplasma parameters produced by different antennaconfiguration and their role to get best cleaning and

AF

N

p-rtl
w-

2

o
1

coating effects were studied. High cleaning efficiency is
attribute to the very high ion temperature, which induces
the high-energy neutral flux to the first wall and easily

f

vv

v t9

oo

012345tv

desorbs the impurities. RF boronization andsiliconization were carried out. The B/C:H and Si/C:Hn"(1013/cm3)

films show the high adhesion, uniformity and longer
lifetime to the plasma discharges than those obtained bynormal GDC method. The impurity level dropped day

Fig. 7 Comparison of impurity after the different wall

coatings.

by day by the different RF procedures. Plasma
performance has been improved after RF boronization

lasted for about 100 shots, while boron film lasted fornearly 800 shots. If longer preparation of siliconizationand the same thickness of the film were obtained, thelifetime of silicon film would increase. But it seems thatand siliconization. Higher lower hybrid wave current
driven and ICRF heating efficiency were obtained after
RF boronization. This new technique shows that it isvery effective and powerful for the quick and real-timewall condition for future large superconducting

the silicon coating with same thickness as boron one(300 nm) could not last for more than 500 shots. Further
experiments will be carried out in future to compare thelifetime of two kind films with the same thickness. The
uniformity of the film is as good as boron one. The main
reason for much thinner silicon film is due to the highratio of helium to silane gas. The reduction of the
impurity radiation of siliconization is not as good asboronization at the low-density case. Fig. 7 shows aquantitative compare of impurity reduction with density.
When the density is higher than 2 x lOre m-3, thereduction of the impurity is almost same. Plasma

magnetic fusion devices.

References

tll E. Gauthier et al., J. Nucl. Mater. 241-243, 553(1997).

l2l E.De La Cal, E. Gauthier, Plasma Phys. Contol.
Fus.39, 1083 (1997).
[3] H.G. Esser et al., J. Nucl. Mater. 241-243, 861(1997\.

[4] Y.P. Zhao, Chin. Phy. Lett. 14,916 (1997).
[5] J. Li et al., Nucl. Fusion 39,973 (1999).
t6l A.I. Lyssoivan et a/., Nucl. Fusion 32, 136l (1992).
[7] M.D. Carler et a/., Nucl. Fusion 30,723 (1990).
t8l M. Yamage et al., Jpn. J. Appl. Phys. 32, 3968

performance was improved by either RF-boronization or
RF-siliconization.

(l993).

6. Summery
RF wall conditioning technique has been tested,developed and routinely used in HT-7 tokamak. This45