gettered - National Institute for Fusion Science

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gettered - National Institute for Fusion Science

LITHIUM-GETTERED MOVING SURFACE PLASMA-FACING COMPONENTS

FOR PARTICLE CONTROL IN STEADY STATE MAGNETIC FUSION DEVICES

Yoshi Hirooka

Hirotsugu Ohgaki, Souichirou Hosaka,

National Institute for Fusion Science

Yusuke Ohtsuka and Masahiro Nishikawa

322-6 Oroshi, Toki, Gifu Osaka University

Japan, Zip code: 509-5292

2-1 Yamadaoka, Suita, Osaka,

Japan, Zip code: 565-0871

In our previous work, the first proof-of-principle

experiments were successfully conducted on the

particle control capability based on the concept of

moving-surface plasma-facing component (MS-PFC).

Over a continuously titanium-gettered rotating drum,

hydrogen recycling was found to be reduced down to

levels around 94% even at steady state. These

experiments on the MS-PFC concept have now been

extended to the second stage where lithium is

employed as the getter material, while using the same

rotating drum. These experiments are intended to

pilot the potential use of lithium as a flowing liquid

facing the edge plasmas in steady state reactors

beyond ITER. Reported in this paper are rather

dramatic findings that hydrogen recycling is reduced

down to levels around 76% and 86% at steady state

over the rotating drum at the lithium deposition rates

of 9.5/s and 7.3/s, respectively. These steady

state recycling data have been nicely reproduced by a

simple zero-dimensional particle balance model.

I. INTRODUCTION

Ever since the discovery of the supershot regime

in TFTR experiments 1 , it has been widely recognized

in the magnetic fusion research community that

reduced recycling over plasma-facing components is

a key edge condition to obtain high-performance core

plasmas. Therefore, wall conditioning techniques

such as boronization have been used in many plasma

confinement experiments.

However, by nature, conditioned walls will be

saturated with implanted particles, terminating

reduced recycling conditions. As such, the efficacy

of wall conditioning has finite lifetime, necessitating

the shutdown of plasma operation for re-conditioning.

One immediately predicts from these arguments that

the application of such wall conditioning techniques

will be limited for long-pulse devices, including ITER,

and no doubt for steady state power reactors. These

arguments clearly point to a need for enabling reactor

wall concepts development.

To provide a resolution to this technical issue, a

variety of wall concepts have been proposed over the

past decade, 2, 3, 4 most of which employ either a solid

or liquid plasma-facing material, but both are

continuously circulated for regeneration of surface

particle trapping capabilities. Proposed among these

is the concept of moving-surface plasma-facing

component (MS-PFC). 5, 6 The proof-of-principle

(PoP) experiments have been conducted using a linear

plasma facility 7 attached with a MS-PFC test unit

featuring a rotating drum that can be gettered with

titanium continuously for hydrogen trapping. 8, 9

Results indicate that even at steady state hydrogen

recycling can be maintained at levels around 94%.

Following this successful demonstration of the

unsaturable particle control capability of the MS-PFC

concept, the PoP experiments have been extended to

the second stage where lithium, more reactor-relevant

than titanium, is employed as the getter. In fact,

lithium was used as the wall conditioner in TFTR

10, 11

during the final stage of its DT-burning campaign.

Recently, the MS-PFC test facility has been

modified in such a way that standing liquids in a

horizontally placed crucible can be exposed to

vertically flowing plasmas, or that flowing liquids on

a slope can be bombarded with plasmas in inclined

directions. Standing liquid lithium experiments are

being conducted in parallel with the present work, and

the data have already been presented elsewhere. 11

The present work is intended to pilot the possible use

of lithium as a flowing liquid at the ultimate stage of

MS-PFC, referred to as the “liquid waterfall” concept.

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MOVING SURFACE PLASMA-FACING COMPONENTS

II. EXPERIMENTAL

III. RESULTS AND DISCUSSION

All the experimental details of the MS-PFC test

unit, employing a rotating drum, can be found in our

previous report. 7 Also, the modified MS-PFC test

facility named Vehicle-1 (for the Vertical and

Horizontal positions Interchangeable test stand for

Components and Liquids for fusion Experiments) has

recently been presented elsewhere. 11 In the present

work, Vehicle-1 is used in its horizontal position.

To briefly mention the plasma characteristics in

Vehicle-1, the plasma density is typically of the order

of 10 10 cm -3 and the electron temperature is around

4eV. 12 The ion bombarding flux is thus of the order of

10 16 ions cm -2 s -1 . The ion temperature is believed to

be below 1eV unless associated with the Frank

-Condon dissociation process. However, the ion

bombarding energy to the rotating drum can be

controlled by applying a DC bias between the vacuum

chamber and the rotating target, respectively, at the

plasma and floating potentials.

The diameter of the plasma column is about 3.5

cm and that of the rotating drum is 15 cm. Also, the

rotation speed was set at 10 cm/s at the periphery.

Used as the hydrogen getter were lithium specimens

in the form of circular disk with the diameter of 2 cm

and the thickness of 0.2 cm. These disks are placed

in a molybdenum crucible with a built-in resistive

heater. Deposition rate measurements were conducted

in a separate experimental setup, employing a

commercially available quartz thickness monitor. To

avoid the radiation heat effects, the vendor requests

the distance from the evaporation source to sensor to

be longer than 30 cm in these measurements.

The hydrogen recycling behavior is observed

such that an optical fiber connected to a multi-channel

analyzer collects the volume integrated intensity of

H from the pre-sheath region (~1 cm in front) of the

rotating drum. More specifically, the recycling time

constants were measured at different ion bombarding

energies controlled by DC biasing. Also measured

were the steady state hydrogen recycling rates, i.e. H

intensities over the rotating target with and without

continuous lithium gettering onto it. For these

hydrogen recycling experiments, the plasma exposure

was controlled by a molybdenum shutter, the opening

time of which is about 20 ms.

Shown in Fig.1-(b) are the deposition rate data

plotted as a function of source-to-sensor distance with

the temperature of evaporation source as a parameter

10 2

(a)

r=30.8[cm] 10 17

r=31.8[cm]

r=32.8[cm]

10 1

r=33.8[cm]

r=34.8[cm]

r=35.8[cm]

10 16

r=36.8[cm]

10 0

r=34.8[cm] 10

10 3 15

10 14

Deposition rate [A/s]

o

Deposition rate [A/s]

O

III-A. Lithium Deposition Rate Measurements

The results of deposition rate measurements, with

the error of typically 10%, are shown in the Arrhenius

manner in Fig.1-(a) with the source-to-sensor distance

as a parameter varied from 30.8 to 36.7cm. The

activation energies obtained from the slopes of these

straight lines are scattered between 0.8 and 1 eV, all

smaller than the sublimation energy of 1.63eV. 12

One speculates from these data that the evaporation of

lithium might include clusters as well as mono-atomic

species although the details are unclear. However,

further discussion on the formation of lithium clusters

is completely beyond the scope of the present work.

10 -1

10 -2

0.001 0.0015 0.002 0.0025

1/T[K]

10 4 O

O

T=460[C ] T=331[C ] 10 18

10 3

O

O

T=428[C ] T=291[C ]

O

O

T=360[C ] T=236[C ]

10 2

10 17

10 1

10 16

10 0

10 15

10 14

10 -1

10 -2

(b)

0 10 20 30 40 50

Distance from surface [cm]

Fig. 1 Lithium deposition rate measurements:

(a) Deposition rates as a function of temperature; and

(b) Deposition rates as a function of evaporation

source-to-sensor distance.

Deposition rate [ion/cm 2 /s]

Deposition rate [ion/cm 2 /s]

704 FUSION SCIENCE AND TECHNOLOGY VOL. 47 APR. 2005


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MOVING SURFACE PLASMA-FACING COMPONENTS

varied from 236 to 460 o C. In the case of titanium

using the same experimental setup, 8 the deposition

rate was found to be proportional to 1/r 2 , where r is

the source-to-sensor distance. A similar trend is

expected for lithium. Although r was kept larger than

30cm for these deposition rate measurements, actual

hydrogen recycling experiments were done, setting

r=3.5 cm. The actual deposition rate on the rotating

drum was thus estimated from the extrapolation of the

1/r 2 curves obtained for lithium (see Fig. 1- (b)).

III-B. Time Evolution of Hydrogen Recycling

The time evolution behavior of the H intensity

as a measure of hydrogen recycling over the

lithium-deposited rotating drum was observed under

300W plasma exposures. The results are shown in

Fig. 2 with the ion bombarding energy, E, as a

parameter varied from the floating potential (F.P.)

~5V to 100eV by applying negative DC voltages to

the rotating drum. The H intensity data acquisition

was initiated at t=0 after which point in time the

shutter was opened by the operator at an arbitrary

time, which can be recognized by the rise in

H intensity. These experiments were done on lithium

films with the initial thicknesses above 3m, far

thicker than the hydrogen implantation ranges for

these ion bombarding energies even as the sputter

erosion effect is taken into account. In between

hydrogen plasma exposures, the surface was refreshed

with helium plasma bombardment for about 1 minute

with the biased bombarding energy at E=50 eV.

These time evolution curves have been fitted with

the following empirical equation: 14

I


I

{1 exp( t )}, (1)

H H r

where I H is the H light intensity at steady state

(i.e., t = ), and is the recycling time constant.

Notice that in Fig. 2, increases with increasing ion

bombarding energy. Clearly, this is because the

depth of implantation increases, which then requires

hydrogen atoms to go through a longer path returning

to the surface where reemission occurs. The detailed

kinetic analysis of these recycling time constant data

12, 15

will be published elsewhere.

Also notice that, as the bias voltage becomes

more negative, i.e., higher ion bombarding energy, the

H intensity increases. This is most likely because

of the enhanced reflection of electrons, including the

Maxwellian and characteristic hot components, which

increased A&M reaction rates for excitation as well as

ionization. The rotating drum is actively cooled

with water, so that the surface remains at around room

temperature even under plasma bombardment. 8, 9

Therefore, we believe that the increase in H intensity

is not due to the rise in surface temperature.

H

intensity [a.u]

340

E= -100[V] , r =60.6[ms]

320

300

280

260

240

E= -50[V] , r

=49.5[ms]

E= -20[V] , r

=45.8[ms]

E= F.P.[V] , r

=37.7[ms]

220

0 500 1000 1500 2000 2500

Time [ms]

Fig.2 Time evolution of the hydrogen recycling

behavior over a lithium film with the ion

bombarding energy varied as a parameter.

III-C. Reduced Recycling at Steady State

After the steady state recycling conditions were

achieved in two runs both with no DC bias, lithium

deposition was resumed to see how the H intensity

would behave, and the results are shown in Fig. 3.

Normalized H

intensity [a.u.]

Normalized H

intensity [a.u.]

Normalized H

intensity [a.u.]

Normalized H intensity [a.u.]

Normalized H intensity [a.u.]

Normalized H intensity [a.u.]

Normalized H intensity [a.u.]

Normalized H intensity [a.u.]

Normalized H intensity [a.u.]







100

75

50

25

Li Li deposition starts

starts

24%

reduction

14%

reduction


0 50 100 200

Time (sec)

Fig. 3 Steady state hydrogen recycling behavior

over a continuously lithium-gettered rotating

drum. Data taken at deposition rates: 9.5/s

and 7.3/s, leading to a 24% and 14%

reduction in hydrogen recycling, respectively.

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MOVING SURFACE PLASMA-FACING COMPONENTS

Notice that the level of steady state recycling is

reduced down to around 86% and 76%, as opposed to

100% if there was no continuous lithium gettering.

In these experiments, the lithium deposition rates

were set 7.3/s and 9.5/s by changing the position

of the evaporation source to the rotating drum.

These steady state recycling data will be analyzed by

the zero-dimensional particle balance model to be

described in the next section. It is important to note

here that because no DC bias was applied during

these two runs, the effect of lithium sputtering is not

necessary to be considered in this model.

III-D. Particle Balance Modeling

The zero-dimensional particle balance model

analysis to be performed here was once applied to

analyze the steady state hydrogen recycling behavior

over a continuously titanium-gettered rotating drum

and was found to reproduce nicely the experimental

data. 8 The key concept in this model is that if the

hydrogen reemission flux is difficult to be estimated

directly, one can then set it equal to the net trapping

flux, so long as the steady state is maintained.

In our recent work, 9 although noticeably reduced

steady state hydrogen recycling was observed over a

continuously lithium-gettered rotating drum, the lack

of data on hydrogen sticking coefficient to lithium

prevented us from performing the particle balance

analysis. Meanwhile, as mentioned earlier, the

MS-PFC test facility was modified to Vehilcle-1 and

extensive efforts have been made to measure sticking

coefficients for molecular hydrogen and also for

hydrogen plasma species, i.e., H, H + , H + 2 , and H + 3 . 12

Interestingly, it has been found that the intensity

of H in the pre-sheath region was rather closely

related to the density of molecular hydrogen than that

of atomic hydrogen, the latter of which one would

expect is more important, though. 8, 9, 12, 15 The same

trend is assumed in the present work as well, meaning

that the estimation of molecular reemission fluxes is

the key in particle balance modeling.

At steady state, the molecular reemission flux

from a standing lithium surface, R e , can be set equal

to the net trapping flux and this may be expressed by:

l L

R 2 ( , (2)

e H2 Li H2

H Li H H Li H

)

where H2, H and H + are the incoming fluxes of

molecular hydrogen, atomic hydrogen, and ionized

hydrogen, respectively, H2->Li, H->Li, and H + ->Li are

the sticking coefficients for molecular hydrogen,

atomic hydrogen, and ionized hydrogen to lithium,

respectively, and the ratio of l/L is the flux dilution

factor, 2 where l is the width, i.e. the diameter, of the

plasma-exposed area and L is the total periphery

length of the rotating drum.

As opposed to this case, if the surface is moving,

and gettered continuously with fresh lithium, the

reemission flux, MS R e , will be expressed as follows:

R 2 l

(


L


), (3)

MS e H2Li H2

HLi H

H Li H

LiLi Li

where Li is the incoming flux of evaporated lithium,

and Li->Li , is the self-sticking coefficient of lithium.

The last term in eq. (3) describes the gettering effect

due to lithium hydride formation (LiH), which then

reduces the reemission flux of molecular hydrogen.

From our recent work, 12 the molecular hydrogen

sticking coefficient and the “compound” sticking

coefficient for plasma species have been evaluated to

be 0.0032 and 0.37, respectively, both measured for

100 0 C lithium surfaces. Also, for the plasma species

composition modeled for T e ~4 eV, where T e is the

electron temperature, i.e., 60%H + 3, 30% H + 2, 10%

H + , 16 and under the assumption that half the flux of H

is directed towards the rotating drum, the following

approximate relation has been derived: 12

2.55 .

(4)

H Li H

H Li H HH H H Li H



2


3


Substituting the experimental data such as those from

Langmuir probe measurements into H + , the ratios of

reemission fluxes with/without continuous lithium

gettering is calculated to be MSR e /R e =0.74 and 0.85, a

26% and 15% reduction, respectively. Despite the

simplicity of the model, these are in excellent

agreement with the experimental data shown in Fig. 3.

This again supports our hypothesis that the H

intensity in the presheath region near the surface is

closely related to the molecular reemission flux and is

consistent with the results of recent modeling work. 17

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MOVING SURFACE PLASMA-FACING COMPONENTS

IV. CONCLUSION

In the present work, it has been demonstrated that

steady state hydrogen recycling over a continuously

lithium-gettered moving surface can be maintained at

levels significantly lower than 100%. From these

data, one immediately expects that if the lithium

deposition rate is further increased, hydrogen

recycling can be reduced down to the level, around

~50%, at which DT-supershot confinement was

achieved with the lithium-painted walls in TFTR. 10, 11

Unfortunately, with the current experimental

setup it is not readily possible to increase the lithium

deposition rate for the two reasons: (1) lithium

deposits on the rotating drum will be heated not to

getter hydrogen if the evaporation source was

positioned any closer than now; and (2) prior to each

run lithium needs to be thermally depleted of H 2 and

H 2 O which, otherwise, would lead to an increase in

H signal. The finite amount of lithium loaded in

the crucible limits the time for steady state

evaporation at any higher temperatures than now.

Needless to say, these difficulties must be improved

in our future experiments.

One might still find a path to reach a tentative

conclusion in the following manner. Using the

Vehicle-1 facility, liquid lithium and steady state

plasma interaction experiments have been conducted

and large values of sticking coefficients even for

low-energy hydrogen plasma species have been

obtained for standing liquid lithium. 12 In the present

work, it has been found that steady state hydrogen

recycling is reduced significantly over moving-solid

lithium. The combination of these findings allows

us to expect that liquid lithium can be used as a

plasma-facing material, i.e. liquid metal waterfall,

with reduced recycling at steady state.

Nonetheless, it is awaited for more direct PoP

experiments to be conduced in the magnetic fusion

research community for the sake of future high-power

and high-density reactors development beyond ITER.

Currently, a “seesaw”-type flowing liquid lithium

setup is under design for Vehicle-1 in which two

heated reservoirs of lithium are connected with a

bridge for liquid to flow across over under steady

state hydrogen plasma bombardment. The details of

this experimental setup will be presented elsewhere 15 .

ACKNOWLEDGEMENTS

The financial supports from the Future Energy

Research Association and Japanese Government

Research Grant (#13680574) are greatly appreciated.

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