Proceedings of ITC/ISHW2007

Comparison of Impurity Transport in Different Magnetic
Configurations
Kieran J. McCarthya, R. Burhennb, K. Idac, H. Maassbergb, Y. Nakamurac, N. TamuracabLaboratorio Nacional de Fusión, Asociación EURATOM-CIEMAT, E-28040 Madrid, Spain
Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-17491 Greifswald, GermanycNational Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, Gifu-ken 509-5292, JapanThe goal of achieving steady state operation in stellarators brings into perspective the outstanding question of
successful management of impurities, in particular, the avoidance of core impurity accumulation during improved energy
confinement modes, with the resultant degradation of plasma energy due to radiation losses. Now, the dependence of
impurity transport on plasma parameters has been systematically investigated during recent decades (LHD, W7-AS, TJ-II),in particular with regard to the influence of density and heating power on confinement. While certain global trends havebeen found in the different devices other behaviours are machine specific.
Here, after summarising the techniques for studying transport, we review transport observations under various
stellarator plasma conditions in order to make comparisons, we draw attention to differing results for injected metallic andintrinsic impurities (core and edge), we identify those parameters that drive accumulation and discuss how observations
compare with model predictions. Finally, in the light of results from LHD and W7-AS, we consider recent features such as
the introduction of divertor modules in W7-AS that has allowed access to a high-density H-mode regime where radiation
profiles reach a steady state, or the onset purification of LHD plasmas at higher density.

Keywords: Impurities, confinement, accumulation, transport,

1. Introduction
Impurities are an intrinsic component in fusion
plasmas. Indeed, impurities are acceptable at low
concentrations where they can be considered as being
beneficial. For instance, impurities can provide acontrolled manner to achieve radiative cooling at theplasma edge or their study, using spectroscopy, canprovide essential information about the plasma edge orcore. However, as we move towards advanced stellarators
it has become imperative to avoid core impurity
accumulation, in particular when operating in improved
energy confinement modes, where accumulation (ofhigh-Z impurities) can give rise to an overbalance ofequilibrium between radiation losses and heating powerwith a resultant degradation of plasma energy anddischarge termination.
The study of transport behaviour of intrinsic
impurity ions in fusion plasmas is a long-standing field of
research. Research on impurity accumulation and control
remains a key issue in fusion research and is anindispensable part of ongoing investigations. In tokamaks,
the appearance of, for example, edge-localized modes
author’s e-mail: journal@jspf.or.jp(ELMs) (which also exist in stellarators but difficult tocontrol) or sawtooth crashes, which originally are
confinement-degrading phenomena, can be employed toclear out impurities. However, stellarators are free ofexternally induced currents and, consequently cannot
utilize current-connected phenomena such as sawtoothingfor impurity reduction. Moreover, owing to the existenceof a magnetic topology that is quite different to that oftokamaks, additional transport regimes in thelong-mean-free-path regime (lmfp) exist in stellarators1
that might also affect impurity transport in the plateau or
Pfirsch-Schlueter regime (e.g. the influence on
temperature screening).
The continued study of impurity transport instellarators is therefore fundamental, in particular, as the
database in this area is smaller and less detailed than fortokamaks. A key question is to what extent observed
impurity behaviour can be described by theoreticalmodels developed for axisymmetric devices thatnowadays have been adapted for stellarators, in particular,
for the highly-collisional Pfirsch-Schlueter regime, where
the difference between tokamak and stellarator isProceedings of ITC/ISHW2007
assumed to be small. If good agreement with thetheoretical model can be found, then the observed
transport features are well understood and conclusions
can be drawn with respect to further improvements andextrapolations. However, in the case of disagreement withtheory, then this might point either to the domination of
turbulent transport or the disregard of stellarator-specifictransport contributions in the transport model. In this case,further interpretation becomes difficult and predictions
have to be substituted by measurements. Depending on
the collisionality regime of the impurities, theimplementation of such non-axisymmetric effects
self-consistently with the background plasma is anambitious task and is not yet available for analysis.
In this paper, after a brief summary of theory andexperimental techniques used for transport studies, we
review transport observations under various stellarator
plasma conditions in different stellarator devices in orderto make comparisons, we then identify parameters thatdrive accumulation. We also consider recent features
such as the introduction of divertor modules in W7-AS
that has allowed access to a high-density H-mode regime
where radiation profiles reach a steady state, or the onset
purification of LHD plasmas at higher density, that may
provide possible doorways to achieving improved
management of impurities in long pulsed discharges.
Finally, compare similarities and differences from thedifferent configuration and we review database needs.
2. BackgroundAt this point, it is useful to recall that the impurity
influx into the plasma from the chamber walls, or othersources, is governed by transport at the open magnetic
field and by retention whilst impurity transport inside theclosed magnetic surface is governed by diffusive andconvective terms. Indeed, impurity flux can be written
simply in terms of diffusive and convective parts as! = D " #n + v " n .
Here the convective part, v, is a strong function of ion
temperature and density gradients, which for analysis
purposes are often afflicted by certain errors dependingon the profile quality, as well as by the collisionality
regime of the background plasma. The diffusive part, D,
may be considered less susceptible to error and better forcomparisons.
Now, when the impurity ions are mostly in thehighly collisional (Pfirsch-Schluter) regime it would be
expected that the impurity transport in stellarators would
be sufficiently decoupled from the magnetic topology so
as to be similar to tokamaks. However, as stated before,
the convective flux and its sign are determined by density
and temperature gradients as well as the collisionality
regime of the background plasma. For tokamaks with thebackground plasma in the banana regime, this convective
flux can be outward directed and helps to flush out
impurities (“temperature screening”). In stellarators,
however, additional collisionality regimes for the
background gas appear in the lmfp region (ν-, 1/ν-,ν0.5-regime) due to the 3-dimensional magnetic topology2.
Unlike tokamaks, they are expected to lead to inward
directed convective impurity fluxes (accumulation) also
in the Pfirsch-Schlueter collisionality regime for
impurities. This would have important consequences forstellarators with respect to impurity radiation losses.
Therefore, a comparison of experimental data withtransport code predictions is a useful approach toelucidate the possible role of any neglected
stellarator-specific aspects.
In order to study impurity transport several tools areavailable. On the experimental side, the monitoring ofsoft x-rays, from highly ionized high-Z impurities in theplasma core, or broadband radiation (using bolometers),
is a common means of following impurity evolution.
Moreover active tools are also available. These include
laser blow-off, where a focused laser beam ablates asmall amount of tracer material (typically a non-intrinsic
metallic element) off a substrate, pellet injection where a
micro-ball of material is accelerated to 100’s m s-1 along
a pipe-gun3, and gas oscillation. The tracer material
released enters and traverses the plasma and the temporal
behaviour of the different ionization states of the tracer
ions in the plasma is monitored by spectrometers withcentral lines-of-sight. Thus, the impurity confinement
time, τimp, a global transport quantity, is derived from thetemporal decay of the radiation emitted by the highest
states. In order to extract local transport quantities acrossthe plasma minor radius from such data, iterative
procedures based on transport codes are employed.
Similarly, the evolution of an intrinsic impurity can beevaluated by following the radiation emitted by highly
ionized states and a model to reconstruct a steady-statesolution of the charge-state distribution for the plasma
with given density and temperature profiles. It should be
noted here, that whilst the transport of intrinsic impurities
is governed by transport both in the core and in the edge,
impurity screening effects play no role in determining thetransport of impurities injected using laser blow-off.

2. Experimental Impurity Transport
In this section we review results from a number of
impurity transport experiments carried out for a range ofplasma conditions in different stellarator machines. In
most cases a short summary of the experiment is given
and the most important results or findings are highlighted.
2.I W7-AS
Electron cyclotron radiation (ECR)-heated plasmas

Proceedings of ITC/ISHW2007
with a limiter configuration in W7-AS revealed a strong
dependence of impurity confinement time on several
plasma parameters, i.e., on minor radius ap, heating power
PECRH, and plasma density ne (τAl ~ ap2.4 ne1.2 B0.3 /PECRH0.8)4. See Fig. 1. Note: similar scalings were foundfor energy confinement and for particles5. The mostimportant impact of this scaling on impurity transportwith respect to consequences on machine performance isthe unfavourable density dependence: at high densities(e.g., 7x1019 m-3), the confinement time reached values
close to 1s or more. Stationary radiation levels could
usually be sustained at low density without problems.
However, beyond a central density of ~5x1019 m-3, theintrinsic impurity radiation, as well as the Zeff, started torise continuously throughout the pulse length (see Fig. x,
where all signals within the shaded region belong todischarges with densities >5x1019 m-3). Thence,
depending on the impurity flux level from the walls, thisled to a degradation of plasma energy by excessive
radiation losses.

Fig.1. Energy and impurity confinement times vs.
electron density for NC limiter ECRH plasmas(pink shaded region) and NBI plasmas (green
shaded region) including the HDH transition
during divertor operation. The gap between ECRH
and NBI plasmas in the overlapping region is due
to differences in the chosen plasma size.
These findings gave rise to several importantquestions. For instance, whether this effect was aconsequence of changes in transport – as would beindicated by the long confinement times – or was theresult of temporally increasing impurity sources from thewall (e.g. heating up of inboard tiles). This wasinvestigated in an experiment using a constant fluorine
gas puff (CHF3) applied throughout the pulse duration ofa high-density discharge (Ref. 6) to simulate a stationary
impurity source. The fluorine radiation in the plasma corerose continuously until the end of the pulse, thisindicating an accumulation of fluorine in the core plasma.
This supported the idea of a change of impurity transport

being responsible for the impurity accumulation at highdensity – rather than growing impurity source rates as aconsequence of the heating up of inboard tiles.

Fig.2. Transport coefficients for high (7x1019m-3(1)) and low density (3.5x1019m-3 (2)) ECRH
plasmas and model predictions (shaded area).
Another puzzling factor is the nearly missing
dependence of the impurity confinement time on
magnetic field strength B (i.e. ~B0.3 (+/-0.2)). In order toelucidate two nearly identical plasmas with 1.25 and 2.5
T central magnetic field strengths were compared (Ref 6).
It was revealed that the local impurity diffusion
coefficient, D, in the plasma core was significantly higher(x 3 or 4) in the case of 1.25 T for radial positions withcomparable densities. However, the inward convection v
was also increased, so that, overall, the global impurity
confinement time did not change. Although it is unclear
whether this was a general characteristic, in thePfirsch-Schluter regime, v may be directly related to D
via some terms containing gradients in the backgrounddensity and temperature profiles so the simultaneous
increase of both transport coefficients may not contradict
theory and may explain the unchanged confinement time.
It was also found that there was no electron
temperature dependence. Furthermore, using laser blow-off
into plasmas with different absolute densities, but very
similar density profile shapes, it was found that theconfinement time was not radial electric field dependent.
This was further confirmed when comparing electric fields,
as measured by charge-exchange, for different electron
densities.
Now from a study comparing the intrinsic impurity
radiation evolution in ECRH plasmas with different
densities it was found that the different time behaviors of
the intrinsic impurities could be explained by changes intransport alone7. For this, local transport coefficients werecompared for a medium (3.5 x 1019 m-3) and a high (7 x1019 m-3) central density discharge7. See Fig. 2. From
analysis, it was seen that the centrally peaked D reduced bya factor ~2 to 3 at high density whilst v‘s were similar
except in the centre where the differences with respect toProceedings of ITC/ISHW2007
the large diffusion coefficients were considered of minor
importance for overall confinement. It is interesting toremark here, that similarly peak D profiles and density
tendency were observed in the TJ-II, albeit at lower
densities8. This brings up the question as to whether thepeaked profiles in both devices are connected with ECRH
deposition. This is a point for further investigation. Now,
returning to W7-AS, model predictions (shaded areas)
with different background impurity concentration
assumptions suggests that the fast onset of a stationary
radiation level can be well described by high-diffusivetransport whilst the increase of radiation at high density
was compatible with lower diffusion coefficients, inparticular at the edge, where low diffusion coefficients
govern the time constant for transport. Moreover, reduced
impurity fluxes caused a longer time scale for achieving
stationary conditions within the discharge duration (it waspredicted that stationarity could be achieved after ~1.7 s
in the high density case)9. This is noteworthy as it implies
that stationary radiation levels depend on impurity
sources, i.e., small impurity sources may be acceptablefor high-density plasmas. In such a case, wallconditioning comes to play an important role inaccumulation prevention.
The scaling law also shows a degradation of
impurity confinement with increased heating power. Thiswas manifested by shorter decay times and by enhanced
D’s, when laser blow-off was performed into plasmas
with similar central densities, magnetic field strength andedge rotation transform but different injected ECRH
power9. The difference in D was considered to be anindication for enhancement of turbulent transport atincreased heating power in agreement with the observed
power degradation of energy confinement. In contrast, thedifferences in v were relatively small. A degradation of
impurity confinement with ECRH power (τ ~P-3) was
found also in the TJ-II10. Indeed, it turned out to bestronger than the one observed in the global energy
confinement (P-0.6). Again, the manifestation of similartendencies in different stellarator devices may be anindication of common underlying physics for impurity
transport.
The investigation of impurity transport at densities
higher than the ECRH cut-off frequency in W7-AS(1.2x1020 m-3) was restricted to neutral beam injection(NBI) heated plasmas. These were also characterized bylong impurity confinement times, low diffusion
coefficients and high inward convection. However, thepreviously observed density dependence on confinement,
as well as the absolute values of transport coefficients,
were not compatible with and often fell below
neoclassical (and Pfirsch-Schlueter) predictions of thetransport model for axisymmetric devices. Indeed,differences in transport between H-mode and normal
confinement-mode (NC) plasmas were difficult to extract
because at higher densities where the H-mode usuallyappeared, the impurity confinement time of the NC
plasma was already large.

Fig.3. Top: transport coefficients for an HDH(a) and an NC (b) plasma with 1 MW NBI
heating power. The shaded region is theconfinement region of the plasma withseparatrix around 14cm. bottom: density andtemperature profiles of HDH and NC plasmas.
Finally, when operation with new island divertor
modules began densities up to 4 x 1020 m-3 wereachieved 11. Moreover, it was found that beyond a certain
power dependent threshold density (1.5 to 2.1x1020 m-3),
the unfavourable density scaling of impurity confinementin the NC regime changed suddenly. See Fig. 1. Within anarrow density interval, the plasma entered the Highdensity H-mode regime where the impurity confinement
time dropped by more than a magnitude to values
comparable to the energy confinement time, which
simultaneously increased by nearly a factor of 212,13. As aresult, quasi-stationary operation at densities up to 4x1020
m-3 was possible.
The drop in impurity confinement in HDH was inagreement with the observed reduction of inward
convection in the core (this being related to the flatteningof the density profile) but is in contradiction to the strong
inward convection at the plasma edge. Note; thetemperature profile is very similar in both cases.
Comparison with the H*-mode, with nearly similar
profiles to HDH but a large impurity confinement time,
indicates the importance of edge transport. Now, in order
to compensate for this large inward convection at theProceedings of ITC/ISHW2007
plasma edge, a high edge diffusion pedestal is needed toachieve the short confinement times observed in HDH.
See Fig. 3. This would highlight more the importance ofthe plasma edge for impurity transport.
2.II LHD
Impurity transport in the LHD device has beenongoing for several years. Indeed, both intrinsic andinjected impurity transport has been studied in constantdensity and density ramp-up long pulse discharges (up to
several 10’s) heated by NBI14,15. One of the most notablefindings has been the observation of intrinsic metallic
accumulation in a narrow density window (<ne> ~1 to
3x1019 m-3) in hydrogen discharges. This was observed
both for constant density and density ramp-up situations. In
a constant density experiment, where the plasma density
was kept almost constant at 2.9x1019 m-3, this coreaccumulation (the main metallic impurity being iron)manifested itself by an increase of central radiation power(the edge radiation power remaining almost constant) witha long timescale15. The increase in central radiation gave
rise to a significant decrease in the central electron
temperature. However, there was no radiation collapse of
the plasma. In another experiment, with a slow density
ramp-up, a dramatic alteration of impurity behaviour wasobserved. In the first instance, the central radiation
increased rapidly with density. See Fig. 4. Then, as thedensity was increased further, the central radiation began toreduce steadily. From analysis of the evolution of thecentral radiation power and highly-ionised iron line
emission evolutions using the one-dimensional impurity
transport code (MIST)16 an impurity accumulation window
was deduced. Here, the central iron density increased
steadily when <ne> was above ~1019 m-3, reaching a peakat about <ne> ~2.7 x 1019 m-3. When the plasma density
increased beyond this density turning point the central ironFig. 4. Time evolution of LHD plasma parametersand central iron density in a density ramp-up
discharge.

density fell off, this being interpreted as impurity clear- or
flush-out processes. In another similar study, it was also
found that accumulation could be avoided in thisaccumulation window by performing a fast density
ramp-up. It was then found that metallic impurities would
not accumulate as they passed through the accumulation
window before accumulation could occur.
In order to further understand this impurity behaviour,
radial electric field profiles, Er, derived from poloidal
rotation velocity measurements made using charge
exchange spectroscopy of neon, were also considered17. It
was found that Er near the plasma edge changed frompositive (electron root) to negative (ion root) as density
increased, the transition occurring near the density where
the accumulation window begins (1019 m-3 with Ti ~2 keV).
Indeed in LHD where the density profile is flat in the core
and the temperature profile is almost parabolic fordischarges maintained with gas puffing, it was determinedthat the impurity flux is determined by the convection
velocity for the metallic impurities. In the region of hightemperature and low density, high-Z accumulation was
avoided due to this positive Er. towards the edge. In thehigh density and low temperature region (highcollisionality), accumulation was not seen, this being
attributed to the dominant contribution of the temperature
gradient in the PS regime (temperature screening effect).
Here, the qualitative behaviour of neon penetration into theplasma core could be accounted for by a large convection
contribution due to the radial electric filed, this being
explained from neoclassical impurity transport for the v-1
regime for a nonaxisymmetric which predicted a large
convective component proportional to Er. On the whole,the qualitative features were considered consistent withneoclassical impurity transport. However, the inward
impurity flux (accumulation) is not found in a simple
analysis with neoclassical theory, even when including an
expression for axisymmetric torus plasmas.
Pellet injection experiments using titanium have also
been perform for a range of constant density plasmas18. At
low density, the titanium exited the plasma and there was
no accumulation. Furthermore, the associated Ti emission
decay time, which is closely connected to impurity
confinement time, could be explained by the diffusioncomponent only in an impurity transport analysis. At
higher density, an abrupt increase in the Ti emission decay
was observed right across the accumulation window region
implying long confinement. Indeed, it is not unrealistic toconsider that the impurity confinement time could continue
to increase when above the accumulation window density
region. Moreover, if the τ and Er were to continue to rise athigh density, then the purification window would meanthat there is a decrease of impurity influx but still longProceedings of ITC/ISHW2007
confinement times. If so then this may be similar to HDH
on W7-AS.
3. Similarities and Differences
In summary, there are indications for anomalous/
turbulent transport at low and medium densities in allmachines and a tendency to approach neoclassics at highdensity. In addition, it has been indicated that there is an
improvement of impurity core confinement withincreasing density. Moreover, it is important to determine
if impurity screening mechanisms at high density are
similar or not in W7-AS and LHD. Indeed, many featuresare qualitatively consistent with traditional neoclassics
but not quantitatively consistent with traditional
neoclassics. Finally there is a need to implement
non-axisymmetric neoclassical theory in order to achievea better understanding of the underlying physics.
4. Database
For an impurity transport database, new data, or thesystematic reviewing and cataloguing of existing data, arerequired in order to improve scaling laws for the different
machines. For instance, this might include the influenceon τimp of magnetic field strength, density, temperature,iota, heating power and type, radial electric field.
Furthermore, diffusion and convective coefficients at 2 ormore radial points would also be needed in order toevaluate the contribution of local transport. Furthermore,such data would help to achieve a better understanding of
the physics of impurity transport and aid comparison.
Also, in order to attain a basis for better understanding it
will be necessary to consider stellarator specific featuresin neoclassical models, for instance, 3-D magnetic
topology, gradB-drift, etc. Finally, a crucial point for
future predictions will be to determine when can impurity
transport be described by a neoclassical model and whenis it anomalous or turbulent.

Acknowledgements
This work contributes to the International Stellarator
Profile Database under the auspices of the IEA
Implementing Agreement for Cooperation in theDevelopment of the Stellarator Concept. The authors
acknowledge the contributions of the W7-AS, LHD and
TJ-II teams to this work.

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