J. Plasma Fusion Res. SERIES, Vol.5 (2002) 131-136

Global Particle Balance of Long Duration
Discharges on TRIAM-IM
SAKAMOTO Mizuki and TRIAM Group
Advanced Fusion Research Center, Research Institute for Applied Mechanics,

Kyushu University, Fukuoka 816-8580, Japan(Received: 18 December 2001 / Accepted: 15 May 2002)

Abstract
In the superconducting tokamak TRIAM-IM, the ultra-long discharge of the duration of 3 h 10 min
was achieved. In this discharge, the net wall pumping rate became zero (i.e. apparent wall saturation)around t - 30 min and after that the plasma was sustained without fueling until the end of the discharge.
The wall plays a role of the particle sink for the first 30 min and afterwards plays a role of the particle
source. The averaged wall pumping rate before the apparent wall saturation is evaluated to be about 2.4 x1016 atoms m-2 s-r and that after the wall saturation is about -8 x 10ls atoms m-2 s-1. The total amount ofwall-pumped hydrogen atoms was finally released from the wall until the end of the discharge. One
candidate of the causes of the apparent wall saturation is the increase in hydrogen release from thevacuum vessel due to the temperature increase.

Keywords:
particle balance, wall recycling, wall pumping, tokamak, ultra-long discharge

1. Introduction

from the wall in JET t8l, Tore Supra [9] and TRIAMlM [10]. The wall conditions continue to change with

The achievement of stable long pulse operation istime during the discharge. For example, strong radiation
damage, which provides a new particle-trap area, occurs

one ofthe requirements for the future fusion reactors. InTRIAM-IM, the study and development of long pulse
operation have intensively been carried out [1-6]. In

during the discharge [11]. It is reported that co-deposition of in-vessel elements, so calledorder to keep the steady state toroidal magnetic field, theM3Sn superconducting magnet system was successfully"tokamakium'0, also occurs during the discharge [12,131.
The co-deposited material of Mo with O can retain one
order larger amount of hydrogen than that of the normal

developed [1]. The lower hybrid current drive system,
the position and fueling control systems have also been

Mo [11,13]. The temperature of the first wall increases

developed and the ultra-long discharges have beenachieved [2-4]. Recently, ultra-long discharges with theduration longer than 3 hours (i.e. a new record) wereduring the discharge. The temperature increase causes
out-gassing from the wall.
In the longest discharge with the duration of 3 h l0min, the density control was lost after t - 30 min and theplasma density was sustained by the recycling particles
alone. In this article, the wall property of the longest
discharge is investigated via the global particle balanceachieved [7].

In order to achieve the stable steady state operation,understanding of the wall property during the long
duration discharge is indispensable. It was observed thatthe density control was lost in the final phase of the long

duration discharge due to the increase in the out-flux@20O2 by The Japan Society of Plasma

Corresponding author's e-mail: sakamoto@ triam.lqushu-u.acjp
131

Science and Nuclear Fusion Research

Sakamoto M. et al., Global Particle Balance of Lons Duration Discharses on TRIAM-IMin the vacuum vessel. In Sec. 2, TRIAM-IM device andthe long tokamak operation are presented. The gtobal
particle balance of the longest discharge are presented
and discussed in Sec. 3. Finally, a summary is given in

stainless steel supports cooled by the water. Note that

low Z material is not utilized as a plasma facing
component and low Z coating has never been carried out

at all. The wall conditioning is carried out just before
the experimental campaign. At first, extension ports areheated up to 1 10oC for 2 days. Electron cyclotron
resonance discharge cleaning is carried out for theconditioning of the plasma facing components for 2days, although the vacuum vessel cannot be heated toavoid the thermal load to the cryogenic system. It iseffective for removing oxygen and carbon |4,151.

Sec. 4.

2. Experimental Setup and the Long
Tokamak Operation
Figure I shows a bird's-eye view of TRIAM-lM.
Sixteen superconducting toroidal field coils, which are
made of M3Sn, are installed. Toroidal magnetic field isup to 8 T. The plasma vacuum vessel has a D-shaped
cross-section with the horizontal length of 0'.26 m andthe vertical length of 0.38 m. The poloidal field coils,which are made of normal conductor Cu, are mounted
on the vacuum vessel. The major radius of the center ofthe vacuum vessel is 0.84 m. The whole machine isinstalled inside a bell-shaped vacuum vessel (i.e. a belljar) for thermal insulation. Extension ports connect
between the plasma vacuum vessel and the bell-jar.
Plasma facing components consist of high Z
materials only, i.e. a stainless steel vacuum vessel,3 setsof molybdenum poloidal limiters and molybdenum
divertor plates as shown in Fig. 2. The divertor plates
are installed at the bottom of the vacuum vessel. The
position of the surface of the divertor plates is the sameas that of the poloidal limiter. The vacuum vessel isdirectly cooled by the water. Molybdenum plates for thepoloidal limiter and the divertor are cooled via theq,Side view
Poloidal Limiter(Motybdenum).- ......rDivertor Plate(Molybdenum)

Vacuum Vessel(Stainless Steel)t

Vacuum Vessel(Stainless Steel)Top view

Divertor Plate(Molybdenum)
Poloidal Limiter(Molybdenum)"{'Fig. 2 Cross-sectional view of the plasma facing components.

Fig. 1 Bird's eye view of TRIAM-1M.
132

Sakamoto M. et al., Global Particle Balance of Long Duration Discharges on TRIAM-IM

Table 1 Parameters of the heating and current drive
systems in TRIAM-IM.
Frequency
Max. power
Pulse duration
(GHz)
(kw)
LHCD

ECH

1.00

c.9o

Eo

2.45

50

CW

8.2200CW<J 0.998.2200CW.=

170200

5 seco

ctr

ooo

tr

0.98
40

^30
|-20-e ro0Jun.
1988

Oec.

1989

May1990

Time(sec)

apr.

Jun.
i995

2001

ssp.

Fig. 4 Time evolution of the recycling coefficient of 2.452001(6lmin) I l00min)

0102030405060

GHz LHCD plasma. The line averaged electron
density is about 0.2 x 101e m-3.

20mln) (lSsmln)

01234
Time (hour)
where N" is the total number of electrons in the plasma,Fig. 3 Progress of the long tokamak operation in TRIAM1M. The plasma current is sustained by 2.45 GHz

4 the fueling efficiency, [u"y the fueling rate by the piezoelectric valve, R the recycling coefficient (i.e. the ratio of the hydrogen influx to the hydrogen out-flux on
the plasma surface) and zo the particle confinement
time. The value of the left hand side of eq. (1) can beobtained from the density profile measurement by theinterferometer. The fueling efficiency 4 is obtained fromthe ratio of the increment of N" to the total amount of
the additional gas puff. The fueling rate is estimatedfrom the voltage applied to the piezoelectric valve calibrated with the actual particle source rate. The sum of
the first two terms of the right hand side of eq. (1) implies the influx of hydrogen atoms to the plasma and its
amount is controlled by the piezoelectric valve. It can beevaluated from the radial profiles of the Ho line intensity
and the electron density. Using eq. (1) and the value of(4fru"r + R N"/tp), two unknown parameters, R and ro,can be estimated. In eq. (1), the effect of impurity is nottaken into account, namely, Z"tr = | is assumed. In thesteady state condition (i.e., dN. /dt = 0 and no impurity

lower hybrid current drive.
The heating and current drive systems in TRIAM-lM are summarized in Table l. The ultra-long dischargeof which the density range is less than about 0.2 x 101e
m-3 are sustained by the 2.45 GHz LHCD system. The

high density and long duration discharges are sustainedby the 8.2 GHz LHCD systems. Steady state and long
duration discharges can be sustained by LHCD poweralone.

Figure 3 shows the progress of the steady state
tokamak operalion in TRIAM-lM. It can be divided into
3 steps concerning from a viewpoint of the operation;
the 3 min discharge (Jun. 1988), I hour discharges (Dec.
1989, May 1990) and longer than 2 hours (since 1995).
In each step, the plasma position and fueling control

systems were developed [6]. As far as the fueling
control is concerned, the control system using the Holine intensity has been developed. The gas feed isfeedback controlled by a piezoelectric valve so as tokeep the Ho line intensity at the central chord constant.
The Ho line intensity is related with the number ofhydrogen atoms that are ionized per unit time, i.e.
particle influx to the plasma [16]. Thus the particle

accumulation), however, the estimation of R from eq.
(l) is considered to be valid, because the equation canbe written by

44o"r+R4u,-4ut=0,influx to the plasma is controlled during the discharge.

(2)

where [u, is the hydrogen out-flux from the plasma and3. Experimental Results and Discussion

it is the same as the hydrogen influx.
The time evolution of the recycling coefficient in

The particle balance equation inside the plasma is
written bydN"ldt = 4l-iu"r + R N"l rD - N"/ ro,the steady state discharge is shown in Fig. 4. The
recycling coefficient is rather high and increases withtime. It means that the most of the hydrogen flux comes(1)

133Sakamoto M. et al., Global Particle Balance of Long Duration Discharses on TRIAM-1M

from the wall, and the contribution of the externalfueling (i.e., q4,"J decreases with time and it becomes- 0.1 Vo of the total influx to the plasma at r - 60 s.
Although the change of R in Fig. 4 is - 2 Vo, rll6a

TRIAM-IM. Therefore. the wall seems to release theretained hydrogen. The piezoelectric valve automatically

stopped fueling and afterwards did not supply thehydrogen gas at all until the end of the discharge. Thetime evolution of the neutral gas pressure at thepumping duct (Fig. 5 (e)) suggests that the neutralpressure in the main chamber also increased.

decreases by a factor of twenty.

In this study, we note that the particle balance inthe vacuum vessel. It can be written by
dNr9/dr+ dNfi/dr= lru"r- 4u-p- Quu,

(3)

where Nf is the total number of hydrogen neutral atoms

40*70/i75

in the vessel, Nrl the total number of hydrogen ions in
the plasma, Ilo-p the pumping rate by the external
pump-unit and (u" the net wall pumping rate, which^30!zoomeans the balance between the absorption rate and the10release rate of the wall. Namely, ftun is positive when(a)0

the total number of hydrogen atoms absorbed by thewall is larger than that of hydrogen release from thewall. This means that the wall plays a role of the particle
sink. When Qun is negative, the wall plays a role of theparticle source. From a viewpoint of the global particle
balance, fl"uu = 0 corresponds to be the apparent wall
saturation. The first term of the left hand side of eq. (3)is deduced from the data of the ionization gauge at thepumping duct. Nfi is assumed to be same as N". Ilo-o isthe product of the neutral pressure and the pump speedof the external pump-unit at the ionization gauge. Theunknown parameter 1}4r can be obtained from the5,t-4?3(g-o2I(b)
02.5
2.0fd

1.5

lrr,ls@1.0o 0.5 (c)above equation.

Now, we will discuss about the particle balance inthe longest discharge. The waveforms of the discharges
are shown in Fig. 5. The plasma current was almost0.0
1.00.8constant of - 20 kA during the discharge and wassustained by 2.45 GHz LHCD. The RF power decreased
from 7.5 kW to 6.9 kW at t - 46.6 min to reduce theheat load to the limiter. Until t - 30 min, the Ho line
intensity followed the reference level indicated by theIE0.6

0.4-o o.20.0
2.0

chain line in Fig. 5 (b). The reference level was
manually decreased about 6.5 Vo at t - 27 min (denotedby an arrow) for the test of the wall condition. The Holine intensity spontaneously increased around t - 30 minand reached to 1.6 times higher level. Moreover, thetoroidal profile of the Ho line intensity was measured atfour different positions (including a limiter position). AlloFa0o(9o--

1.5(e)

1.0

0.50.050of the signals increased at the same rate after the100

150

200

Time (min)apparent wall saturation. This suggests that the increasein the Ha line intensity (i.e. influx to the plasma) wascaused by the contribution from the whole toroidal

Fig. 5 Waveforms of the 3 hours discharge; (a) plasma
current, (b) H" line intensity (solid line) and refer-position. The OV and MoI line intensities were almost
constant during the discharge as shown in Fig. 5 (c) and(d). Oxygen and Molybdenum are major impurities inence level for the feedback control (chain line),(c) OV line intensity, (d) Mol line intensity and(e) neutral gas pressure at the pumping duct.
134

Sakamoto M. et al., Global Particle Balance of Long Duration Discharges on TRIAM-lM

In the steady state condition, the wall pumping rate
[11 can be written bylloarr = liuet- 4o-osurface temperature also rapidly increased for 5 min andreached a quasi-steady state. The range of the surface
temperature measured is about 600 - 800"C provide thatthe emissivity e of the molybdenum plate is unity. The
real surface temperature is expected to be rather higher,
since t of the molybdenum plate seems to be low. The(4)since the two terms of the left hand side of eq. (3) isnearly zero and it seems to be no impurity accumulation.
The averaged wall pumping rate from t - l0 to 20 min
is estimated to be about 2.4 x l0t6 atoms m-2 s-r formeq. (4). In this period, the wall played a role of the particle sink. The whole surface area (S - 5 m') of the wallis used for the calculation of the wall pumping rate. After t - 30 min, (4 = -ilu.p - -8 x 1015 atoms m-'s-1,
since the fueling was stopped (4o"r = 0). In this period,
the hydrogen lelease rate from the wall exceeded theabsorption rate, therefore the wall played a role of theparticle source and the recycling coefficient would be
more than unity. The total number of hydrogen atoms

estimation of e is a future work. The temperature andthe characteristic time of the temperature increase, 4,depend on the balance between the heat load and thecooling ability, i.e. the distance of the measuring pointfrom the cooling pipe. The value of 6 is a few minutes
for the limiter and a few tens of minute for the vacuum
vessel and the divertor plate. It is considered that thelimiter does not contribute the apparent wall saturation,
since q of the limiter is much less than the time of theapparent wall saturation t. (- 30 min). On the other
hand, z1 of the vacuum vessel is similar to /r. Moreover,the increase in the Ho line intensity after the apparent
wall saturation is considered to be caused by the overall

supplied by the piezoelectric valve is estimated to beabout 4 x 1020. And the total number of hydrogen atoms

contribution from the whole toroidal position as

pumped by the extemal pump-unit during the discharge

described above. So, one of causes of the apparent wall
saturation in the longest discharge is considered to be
the increase in the hydrogen release from the vacuum
vessel due to the temperature increase.

is almost the same as that of the supplied hydrogen.
Namely, it was observed that J4.", Or - F]"-o dr. F.oeq. (4), this means that the total amount of wall-pumped

hydrogen atoms was finally released from the wall until
the end of the discharge, i.e.

4. Summaryfi;"1ar - O.

The detailed investigation of the mechanism of theapparent wall saturation during the discharge is a future

In the superconducting tokamak TRIAM-lM, thelongest discharge with the duration of longer than 3
hours was achieved. The global particle balance of thisdischarge was investigated. From the viewpoint of the
global particle balance, the net wall pumping rate
became zero around t - 30 min. After that the plasma
was sustained without fueling until the end of thedischarge. The averaged wall pumping rate is about 2.4

work, but one candidate is the increase in the wall
temperature, which is related to the out-gassing. The
time evolution of the temperature of the plasma facing
components is shown in Fig. 6. The temperature ismeasured by thermocouples. An infrared camera alsomeasured the surface temperature of the interacting
point between the plasma and the limiter plate. Thex 1016 atoms m-2 s-l before the apparent wall saturation
and after that it becomes -8 x 1015 atoms m-2 s-r. The
wall plays both roles of the particle sink and the particle
source. The total amount of wall-pumped hydrogenatoms was finally released from the wall until the end of
the discharge. One of the causes of this apparent wall
saturation is considered to be the increase in thehydrogen release from the wall due to the temperature

250.o 2ooo1so5(sb 1ooincrease.

CLCb50Acknowledgement
00This work is partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,

50 100 150 200 250 300time (min)Culture, Sports, Science and Technology.

Fig. 6 Time evolution of the temperature of the limitersupport, divertor plate and the vacuum vessel.
135

Sakamoto M. et al., Global Particle Balance of Long Duration Discharses on TRIAM-IM

171 S. Itoh, FURKU Report Ol-O4 (73), (2OOl) lin

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