Proceedings of ITC/ISHW2007

Impact of lithium-coated walls on plasma performance in the
TJ-II stellarator
F.L. Tabarés, I. García-Cortés, D. Tafalla, J.A. Ferreira, M.A Ochando, R. Balbín, F. Medina and the
TJ-II Team.
Laboratorio Nacional de Fusión. CIEMAT. Avenida Complutense 22, 28040 Madrid, Spaine-mail: tabares@ciemat.es
The TJ-II stellarator has been operated under several first wall conditions until now: full metallic scenario, fullboronized walls and, in the last campaign, lithium coating. Particularly conspicuous has been the change inrecycling associated to the different wall conditions, but also in impurity content. Lithium coating, tested for the
first time in a stellarator, has proven a very effective method for particle control in TJ-II. Changes in the shot byshot fuelling characteristics as well as in the total particle inventory compatible with good density control havebeen recorded after the Li deposition. Thus, a factor of 4 increase in the fuelling rate at constant density comparedwith the B-coated walls was recorded, and even a higher value was estimated for the allowed H inventory in the
puffing-controlled ECRH discharges. These changes were also mirrored in the radiation and edge radial profiles,
with increased electron temperatures. This led to enhanced interaction with the poloidal graphite limiters, which
had a deleterious effect on plasma performance. The lower instantaneous recycling also worked for the density
control under NBI heating scenarios. Record values of plasma energy content were measured at densities up to
4.5.1013 cm-3 under Li-coated wall conditions.

Keywords: plasma-wall interaction, density control, low Z components, wall coatings, low recycling, lithiumization,
TJ-II stellarator.

author’s e-mail: journal@jspf.or.jp

Proceedings of ITC/ISHW2007

1. Introduction
Plasma wall interaction issues are paramount inachieving fusion plasmas with high purity, controlleddensity and high confinement. Even when the selection ofplasma facing components, such as limiter and divertor
target materials, is made based on their ability to withstandthe very high particle and power fluxes characteristics ofpresent fusion plasmas, the interaction with the first wall,
reached by charge exchange neutrals, photons and somemore or less tenuous plasma, is considered to contribute to
the plasma impurity content as much as the PFCs do.
Therefore, a growing concern about proper conditioning of
the total inner wall of the fusion device has been taking
place in the last decades. Compared to tokamaks,
stellarator plasmas show distinct features in theirinteraction with the surrounding materials. On the goodside, the lack of disruptions and type I ELMs make themmore reliable for reactor operation. So it is the lack of
MHD-driven density limit. However, due to theirsignificantly higher aspect ratio, they offer a less favorable
area to volume ratio and also, a worse screening ofrecycled neutrals. This last parameter has obvious
implications in divertor design and, in general, in density
control by external sources. Although some specificdivertor concepts have been developed for stellarator withreasonable success for impurity and particle control, no
specific coating strategies for the first wall exist. However,
the application of those concepts with good performance intokamaks, such as boronization or Ti gettering, has also
improved machine operation in the stellarator community.
It must be said here that, due to the systematically higher
complexity in the vacuum vessel topography of stellarators,
homogeneous coatings are harder to produce, in particular
if line of sight deposition (i.e. Ti gettering) is sought. In thepresent work, the implementation of a system for the fullcoating of the inner walls of a stellarator with lithium (theTJ-II Heliac) is described for the first time. Compared toother low Z coating elements such as Be, C and B, lithium
is a very attractive element due to its very low radiation
power, strong H retention (leading to the formation of thevery stable hydride, LiH) and strong O getter activity andexcellent results have been achieved recently in tokamaks[1]
The structure of the paper is as follows: first, the innerwall conditioning history of the device is summarized.
Then, results achieved in terms of particle control by theuse of Li walls are presented. The performance of plasma
parameters associated to the change in wall conditions isthen addressed. Finally, the prospects for improvement andauthor’s e-mail: journal@jspf.or.jp

possible extension to other stellarators are described.

2.Wall conditioning of the TJ-II stellarator
The TJ-II stellarator has been operated under diverse
first wall conditions since its beginning [2]. Under ECR
plasma generation and heating, density control is hamperedby the combination of low cut-off density and the large
surface/volume ratio of the vacuum vessel respect to theplasma. However, different origins of the problem wereidentified depending on wall conditioning and plasma
facing materials. For the initial scenario, a full metalmachine, desorption of high recycling He from the walls,
which was implanted during overnight GD conditioning,
either by the plasma or by direct interaction with themicrowave beams was the main responsible. The
systematic use of a short Ar GD period led to a significant
improvement of the control. Under boronized walls andgraphite limiters (low Z scenario), the improvement inplasma purity and the low recycling conditions at the
beginning of the operation (shortly after the depletion of H
from the film by He GD) provided a higher tolerance of theplasma to the external fuelling. Two complications,however, were found under boronized walls. First, thegradual loading of the C/B film by hydrogen deteriorates
the good recycling characteristics in a relatively shortperiod. After a total implantation dose corresponding to the
maximum uptake of the film of~1.1017cm-2, was achieved,
spontaneous density rise drove the discharge to cut-off
density, even in the absence of external puffing. The
application of a few dry discharges was required forrecovery, but this was only transiently obtained. A second
factor in play was the presence of the Enhanced Particle
Confinement (EPC) mode, characterized by a sudden
increase of particle confinement at a critical density in theorder of 0.6.1013 cm-3 [3] This mode, whose presence
seems correlated with edge collisionality and thedevelopment of a velocity shear layer at the edge [4], wasfound to strongly depend on fuelling pulse shape andamplitude [5] and, its development can be eventually
suppressed by proper tailoring of the gas puffing.
In the 2007 campaign, a low recycling, low Z wall has
been tested. For that purpose, an in situ lithium coating
technique was developed. It is based on evaporation undervacuum from four ovens, symmetrically spaced andoriented tangentially to the plane of the corresponding
flanges in the equatorial plane of the vacuum vessel, filledwith 1g of metallic Li each. A set of metallic resistances(Thermocoax) and thermocouples (K type, two per oven)are operated at pre-programmed temperatures of 500-600ºC by a central, PID-based power supply. Effusion fromProceedings of ITC/ISHW2007
the ovens creates an atomic beam aiming at the remoteregion opposed to the corresponding flange. Under HVoperation, the mean free path of the Li atoms is longenough to produce a thin layer at the vessel walls locatedmidway between adjacent ovens. The deposition pattern,
directly visible in the groove protecting the central coils,
matches the line of-sight flight of the Li atoms.
Alternatively, effusion under a He atmosphere was tried toenhance lateral diffusion of the beam, hence providing amore homogeneous coating of the walls in areas closer tothe ovens. A pressure of 10-5 mbar was chosen, based on
the experimentally determined mean free path for the
He-Li system at this pressure, !~ 70 cm and thecharacteristic length of the vacuum vessel, V/S ~ 20 cm(assuming pure cylindrical geometry). In order to extend
the lifetime of the Li coating, and due to the very highreactivity of this species with background gases (water, O2,
N2, CO…) a ~50 nm boron layer was deposited prior to theevaporation by conventional GD of an o-carborane/He
mixture. The B coating was depleted of H by a pure He
GD after its deposition. Also, He GD was applied every
day on the Li layer in order to remove hydrogen from theareas not fully covered by the coating. A total of 12 g of Li
were evaporated for the ~650 discharges performed in thisperiod.

3. Particle control on Li walls
Compared to former operation on boronized walls, thecontrol of plasma density by external puffing was verymuch upgraded upon lithiumization of TJ-II. Not only
the required puffing levels were significantly higher forthe same density, but also no sign of saturation wasobserved after a full day of ECRH operation. Of
particular relevance on machine performance is therecovery of pumping walls characteristics after shotswith densities above cut-off. Typically, one or two
purges (dry discharges) were required in B scenarios.
However, no such a need was found upon lithiumization,
the wall memory effect being basically washed out. In
order to quantitatively understand this effect, the particle
balance in a shot to shot basis was evaluated from theinjected hydrogen, and the desorbed one after theplasmas, the latter being measured by an absolutely
calibrated mass spectrometer. In figure 1 the results
for a typical operation day under both types of wallconditions are displayed. Two main differences between
B and Li walls are clearly seen from the figure. First, therequired particle injection for a give density issystematically higher in the Li case. Second, while the B
wall shows saturation at total retained inventory of H<
5.1020 (which, for the nominal saturation level of B films,

Fig.1. Particle balance under B and Li wall conditions. Top:
integrated particle injected per pulse. Bottom. Cumulative
retention of H in the walls. Black, Li. Blue and red, B.

implies an effective interaction area of less than 1 m2)
no sign of such saturation is seen for values up to 4 times
higher in the Li case.
shot # 16518
10

4

8

6

2

4

2

0

0

1000

1050

1100

1150

1200

1250

1300

Time(ms)

Fig.2. Time response of electron density (blue) andHa fluxes to the wall (red and brown) to a short gas puff
(green Ha and black manometer)

The dynamic behavior of plasma particles during thedischarge is shown in figure 2 for a characteristic shot.
First, gas puffing is injected for density built up. Then it
is abruptly interrupted and the evolution of cord density
and H" emission are recorded. For pure H plasmas, a
simple equation of the form
dN/dt= f. Qin-N/(!p/1-R)(1)

Proceedings of ITC/ISHW2007
was seen to decrease during the operation day. Figure 3
shows the shot by shot evolution of some relevant signals,
normalized to the line average density. As seen, after an
initial spike in all impurity monitors, associated to thevery first shots after Li deposition, a systematic decrease
takes place upon plasma operation. Of special relevanceis the strong decay of the bolometer signal, closely
related to the suppression of carbon impurities from theplasma. The strong reduction of carbon source in thepresence of Li is a well documented effect which has
been associated to the decrease of physical and chemical
sputtering of carbon by its coating with Li [7]. The
second day of operation is preceded by 30 min of He GD.
The impurity levels are seen to rise again, but they soon
recover to their low levels, the Li signal reaching levels
82

7

Li/ne, Prad/ne

6

1.5

5
4

1

CV/ne

can be applied. Here, Qin stands for the puffing rate (e-/s),#p is the particle confinement time, f is the fuelingefficiency and R the recycling coefficient. Applicationof eq.1 to the data of fig. 2 yields a f value near unity andan effective confinement time, #p/1-R, of ~30 ms.
Assuming no major changes in particle confinement
respect to the boron and metal cases (see below), a valueof ~0.65 is obtained for R. This value, although lower
than that deduced under boronized walls, is significantly
higher than expected for a fully absorbing wall and may
reflect the limited extension of the wall coverage by the
Li coating achieved.
Another important factor contributing to density
control by external puffing in the absence of
discontinuities in the confinement characteristics, as that
introduced by the transition to the EPC mode. In figure 3,
the dependence of line average density with particle
flows to the wall (H" signal) is shown for Li wall
operation. The location of the EPC transition for the B
and metal cases is also shown. For constant recycling
conditions and negligible contribution of impurities to Ne,
the linearity displayed in the figure obviously implies thatno change in the particle confinement characteristics ofthe plasma is taking place during the scan. This is also
confirmed through the ion energy confinement analysis
shown in section 5.

3
2

0.5

1
0
16440

16480

16500

16520

16540

16560

16580

Series

1.2

Fig. 4. Shot to shot evolution of some impurity signals
normalized to the line density. Circles (left) CV,
triangles LiI, squares, Bolometer

1
-3

cm )

0
16460

line density (10

12

0.8
0.6
EPC Trans.
0.4
0.2
0
0

0.5

1
1.5
H! wall (a.u)

2

2.5

3

Fig.3. Line average density vs. flux to the walls for Li wallconditions. Line is a linear regression to the data

4. Impurity behavior and plasma parameters
Clean plasmas are routinely obtained in TJ-II ECR heatedplasmas under low Z scenarios, largely due to the strong
oxygen gettering effect of the B coatings and the use ofgraphite limiters [6] Although the Li coatings were notaimed at improving this situation, a significant effect has
been observed in that last campaign. So, carbon emission

even lower than before. Since its measurement is local,
the erosion of the initial layer at the observation window,
with subsequent spreading over other areas of the vessel,
could account for this effect. This will also account forthe concomitant decrease of impurity radiation, not to be
expected if simple removal of the beneficial effect was
taking place. After discharge 16540, systematic higher
contamination of the plasma is seen. This is associated tothe insertion of the graphite limiters one cm into theseparatrix during several shots. The interaction of thelimiters with the plasmas was seen to be stronger under
Li walls than in the B case, as monitored trough theincrease in total radiation and CV signal upon theirinsertion up to 2 cm. This is apparent contradiction withthe effect just described, and it was ascribed to the higher
edge temperature in Li-wall plasmas. A profile of edge
parameters, as deduced from the supersonic He beamdiagnostic, is displayed in figure 4. Although electron
densities are similar to those found under boronized walls,

Proceedings of ITC/ISHW2007

0.25

Metal
0.2

Boron
Lithium

0.15

0.1

0.05

1 1013150

figure. First, a very similar slope, directly giving afirst-order estimate of the #i value of ~5 ms, in agreementwith particle confinement times above mentioned, is foundin the three cases. Secondly, the discontinuity observed atcollisional frequencies of 10 s-1, corresponding to thetransition to the EPC mode is missing in the Li case, thus

Ti/(Te-Ti)

electron temperatures are higher by a factor between 1.5
and 2, tentatively ascribed to changes in dominant
impurity and edge power balance rather than to changes
in confinement. Since physical sputtering of carbon has
been proposed as the main contamination mechanism in
TJ-II ECRH plasmas [8], these changes are in line withthe observed increase.
Impurity radiation profiles are also seen to evolve uponchanging the wall material. Even when total radiationlevels are somehow lower in the Li case (clean
discharges), the analysis of SXR emission profiles
suggests central values of Zeff higher for the Li case. The
important issue of impurity accumulation in central
heated plasmas, however, deserves a more rigorous
analysis of the observed changes and it is out of the scopeof this work.

0

8 1012

0

5

10

15

20

-1

nu s

25

30

35

40

ei

100

50

-3

4 1012

Fig. 6. Ion energy balance analysis for three wall scenarios.
ne (cm )Te(eV)

6 10122 101200.7

0.75 0.8

0.85 0.9
r/a

0.95

1

1.05

0
1.1

confirming the results found through the fueling analysis of
figure 3. Thus, although central electron temperatures were
found to be higher under Li walls, which can be initiallyascribed to the higher performance of the ECRH system inthe last campaign, no changes in the ion channel areobserved as the wall material is changed, at least at
electron-ion collisionalities high enough to disregard CX
losses.

Fig. 5. Edge parameters under Li-wall conditions

Although no major changes in density profiles is seen by
the Thompson Scattering diagnostic, it is worth mentioningthat cut-off line densities are a 10% higher than in the B
counterpart, and a more detailed characterization of the
profiles is presently underway. Ion energy balance andconfinement has been analyzed through a simple
0-dimensional model for the metal and boron cases inprevious campaigns [9]. In its volume average notation, for
an ECRH plasma it can express in the form
$i-e.%Ti-e.ne- (Ti/#i+Kcxn0Ti).ni=0(2)where electron-ion collision frequency, ion energy
confinement time and CX losses are integrated in thesteady-state balance. Volume averaged electron
temperatures are used for Te. Previous estimate of CX
losses allow for neglecting this channel in eq. 2 under
ECRH conditions in TJ-II, which allows for the evaluationof the characteristic #i value by simple plotting the terms of
the equation, as displayed in figure 6. Two important
remarks must be made from the behavior displayed in the

5. NBI plasmas
High beta operation is one of the major goals of the
TJ-II program. Therefore, extensive NBI heating must be
coupled to the plasma and efforts in this direction havebeen made in the last years. It was early found, however,that proper density control was extremely challenging
under additional heating, partly due to the extra fuelling
term introduced by the beams and their interaction with thevessel walls. In the absence of divertor configurations, a
low recycling wall can be of great help in preventing plasma
collapse by excess density under finite heating power.
Although at present, the discharge duration under NBI
heating is still dominated by this problem, importantchanges have been detected in the evolution of the NBI
plasma parameters. In figure 7, a comparison of theseparameters under boron and lithium walls is shown.
Central electron temperature, edge and central radiations
and line average densities are shown in both cases. The

Proceedings of ITC/ISHW2007
6

300

rad

rad

P

5

250

4

200

3

150

2

100

1

50

0

1000

<Te>

central, P edge, density

Boron walls

1050

1100

1150

0

1200

300

rad

rad

P

250

4

200

3

150

2

100

1

50

0

1000

1050

1100

1150

<Te>

central, P edge, density

Lithium walls
5

References

G. Mazzitelli el al. Proc. 34th EPS Conference Warsaw(2007) to appear in Plasma Phys. Control Fusion and ref.
therein[2]
F.L. Tabarés, D, Tafalla et al. J.Nucl. Mater 266-269,1273(1999) Also, D. Tafalla and F.L. Tabarés, ibid. 290-293
1195 (2001)[3] F.L. Tabarés et al. Plasma Phys. Control. Fusion. 43,1023(2001)[4] L. Guimarais et al. These Proceedings[5] F.L. Tabarés et al. Proc. 28 th EPS Conference on Plasma
Physics and Controlled Fusion, Funchal, Madeira (2001)[6] D.Tafalla and F.L. Tabarés. Vacuum 67, 393 (2002)[7] S. Kato et al. J. Nucl. Mater 266-269, 411 (1999)[8] I. Garcia Cortés, F.L. Tabarés et al. J. Nucl. Mater 337-339,
441 (2005)[9] R. Balbin, F.L. Tabarés et al Proc. 14th Stell Workshop
Greifswald (2003)[10] M.A. Ochando, F. Castejon and A.P. Navarro. Nucl.
Fusion 37, 225 (1997)[1]time(ms)6

densities of 4.5 1013 cm-3, its limit possibly due to a purethermal collapse under the limited NBI power available atpresent. New techniques for improvement of film
homogeneity and careful control of limiter conditions areforeseen in order to improve the results.

0

1200

time(ms)

Fig.7. Plasma emissivity (W.cm-3, central and edge lines), line
density (1013 cm-3) and central electron temperature during
the injection of a 500kW neutral beam during the time
indicated by the squared pulse.

time of injection is shown by a squared pulse. Although
central emissivites are similar at the collapse, a higher line
density (4.6 vs. 3.4) is reached in the Li case. Interestingly,edge emissivities are significantly lower in the Li case.
Furthermore, the ratio of central to edge radiation behavesin a significantly different way in both cases. According to
the running models for radiation collapse in stellarators[10] and the different shape of the cooling rates for thedominant impurities at the edge, the results here presented
suggest the presence of radiative instability as candidate
for the B case, while a pure thermal collapse would be
limiting the plasma density in the lithium wall scenario,
with obvious implications in the future upgrading of the
NBI power in TJ-II.

6. Conclusions
The TJ-II has been operated under lithiated wall conditions,the first time that this technique has ever been applied to astellarator. Very encouraging results in terms of density
control and impurity level have been obtained; even when
only partial coverage was achieved. The strong
improvement in density control is not only due to lower
recycling of the walls but also to the inhibition of thetransition to the EPC mode by the higher puffing levels
required. NBI heating has been possible for record