J. Plasma Fusion Res. SERIES, Vol.5 (2002) l1B-123

The lmpact of the New lsland Divertor on the Plasma
Performance in the W7-AS Stellarator
GRIGULL Peter, MccoRMICK Kent, BURHENN Rainer, BRAKEL Rudoli EHMLER Hartmut,
FENG Yuehe, GADELMEIER Franz, GIANNONE Louis, HILDEBRANDT Dieter, JAENICKE Rolf,
KISSLINGER Johann, KLINGER Thomas, KNAUER Jens, KoENIG Ralf, KUEHNER Georg,
NAUJOKS Dirk, NIEDERMEYER Helmut, PASCH Ekkehard, RAMASUBRAMANIAN Narayanan,
RUST Norbert, SARDEI Francesco, WAGNER Friedrich, WELLER Arthur, WENZEL Uweand the W7-AS TEAM
Max-Planck-Institut fiir Plasmaphysik, EUMTOM Ass., D-85748 Garching, Germany(Received: 11 December 2001 / Accepted: 15 May 2002)

Abstract
W7-AS has recently been equipped with ten open divertor modules in order to experimentally
evaluate this exhaust concept in view of future application to W7-X. The new divertor enables access toa new NBl-heated, very high density (up to n" = 3.5x1020 m-3) operating regime with promisingconfinement properties. The energy confinement time increases steeply with density. In contrast, theparticle and impurity confinement times decrease with increasing density. This allows full density control
and stable quasi steady-state operation also under conditions of partial detachment from the divertor
targets. Radiated power fractions are low to moderate in attached regimes and reach up to about gOVo in
detachment scenarios. The radiation mainly originates from lower ionization states of carbon and oxygenand stays always peaked at the edge.

Keywords:
stellarator, W7-AS, improved confinement, steady-state, divertor

1. Introduction
Divertor solutions for stellarators have to be

operation can be achieved, and on the type of divertor
scenarios which can be established with this geometry.
First results have been published in ref. [1]. The present
analysis gives an overview of the phenomenology andcompatible with the specific edge configurations. One
possibility is to utilize the flux diversion by inherent- or
externally induced magnetic islands at the edge in orderto establish edge plasma conditions and operational
scenarios as achieved e.g. in tokamak poloidal field
divertors. W7-AS is now being operated with such an(open) island divertor in order to experimentally
evaluate this plasma exhaust concept in view of futureapplication to W7-X. Initial efforts were focused mainlyon the impact of the new divertor on the core plasma

extends the parameter range studied.

2. Magnetic Field Gonfiguration andDivertor Geometry in W7-AS
W7-AS is a modular stellarator with five magnetic
field periods using non-planar field coils. The major andminor radii are R = 2 m anda < 0.16 m, respectively.
The magnetic field strength at the axis is B < 2.5 T. The
plasma cross section per period varies from elliptical toperformance, on whether adequate boundary conditions

at the upstream separatrix for high density divertor@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corre sponding author's e -mail : grigull @ ipp.mpg.dell8

Grigull P. et al., The Impact of the New Island Divertor on the Plasma Performance in the W7-AS Stellarator

triangular and back to elliptical. The rotational transformt can be varied between 0.25 and 0.6 by currents in an
additional set of planar coils; the magnetic shear is low.

coils affecting the 85.. perturbation field magnitude.
Present studies are performed at ru = 5/9 with a weaksuperimposed vertical field (100 G), which combination
provides a good compromise between sufficiently large
boundary islands and not too small main plasma cross

In cases with the local value of +u= 51- (m = 8,9, 10...)at the edge being resonant with the intrinsic B5,perturbation field component, the configurations arebound by macroscopic magnetic islands, which areutilized for the island divertor. The island topologicalparameters relevant for flux diversion can be varied
within a broad range [1] by means of special control

sections.

The divertor consists of ten identical modules - two
per field period - placed at the top and bottom of theelliptical cross sections, Fig. l. Apart from the size, thegeometry is similar in many aspects to that planned for
W7-X [1,2]. Each module is composed of an inertiallycooled target which intersects the islands, and of baffles
for neutral control. Two thirds of the target tiles are
instrumented with thermocouples; two opposite (top andbottom) modules are equipped with target-integrated,flush-mounted Langmuir probe arrays (for locations see
further below).

"'""n"r\3. Effects of the New Divertor
3.1 Plasma performance at high density

\

)o" I

,,-\

Titanium

The divertors enable access to a new NBI heated,very high density (up to 4 = 3.5x1020 m-3) operating
regime with promising confinement properties. Former
high-power NBI discharges in W7-AS with limiters
were mostly transient. Within the accessible density
range of up to about 2xl}2o m-3, the particle and im-/

evaporatorsa)purity confinement times were rather long and increased
Baffles

Probe arrayswith plasma density. The density could not be

Targetcontrolled, and both the density and the radiation fromthe core often quickly increased until the discharges
collapsed. The density 2", at the upstream separatrix
position increased less than linearly with the lineaveraged density, and lz"" values sufficiently high forfavourable divertor regimes could be achieved onlytransiently [3]. This situation has completely changed in
the new regime: above a certain threshold density, theenergy confinement time ?E steeply increases, whereas,in contrast, the particle and impurity confinement times't, and q^, strongly decrease with increasing density.

Consequently, high density, NBl-heated discharges,
including discharges with partial detachment (see Sec.
3.2) from the divertor targets, can now be quasi-steadily
maintained over many confinement times. Typical time

Fig. 1 (a) Elliptical plasma cross section with island

traces of plasma parameters are shown in ref. [1]. The

divertor modules. Such modules are installed at
the top and bottom of each of the five ellipticalplanes. The toroidal length of each module is 0.7
m. d, denotes the minimum separation between

regime exhibits strong similarity to the quiescent Hmode [4], but avoids impurity accumulation. For that it
is now termed High Bensity H-mode, HDH.
An overview of the existence range of the HDH
regime in the configurational space studied so far isdepicted in Fig. 2: Plotted are cE contours for NBI

x-points and targets.
(b) Photograph of a bottom divertor module. The
targets are made from CFC material, the bafflesfrom isotropic fine-grain graphite.
119

Grigull P. et al., The Impact of the New Island Divertor on the Plasma Performance in the W7-AS Stellarator

heated discharges (injected power Pysl = 2 MW,absorbed power PNsyabs = 1.4 MW) as functions of theline-averaged density n" and the plasma minor radius c-varied by control coil currents- for separatrix-bounded
configurations with ta = 519.Increasing plasma radii
correspond to decreasing minimum separations 1x
between x-points and targets (see Fig. la) and to
decreasing radial widths of the magnetic islands. With
the exception of the lowest densities, all data were

3.8

obtained from quasi steady-state discharges. The steep
increase of rs with density atne= l.7xl020m-3 is nearly
independent of the magnetic configuration. For A*> 2.3
cm and n"> 2.9-3.1x1020m-3 partial detachment fromthe divertor targets is observed. With increasing /*, theonset of detachment shifts towards lower densities.
Detachment extends the stable operation range to higher

densities. Maximum densities exceed the GREEN-

WALD limit [5] for an equivalent tokamak (elongation
K = 2, trianglJlarity 6 = 0.3) by a factor of about two.
The degradation of ?s in detached regimes is moderate.

separation A1 between x-point and target /cm
2.4

The subsequent analysis concentrates on discharges

3.3

in the configuration with a = 11.5 and /*= 3.9 cm(Standard Divertor Configuration, SDC).

For attached HDH discharges, the ts values lie well
above relevant scalings, such as the W7-AS internal
scaling (W7-95), the lnternational Stellarator Scaling

E 2.6(ISS95), the New Large Helical Device scalingsN(NLHDl, NLHD2) and the Lackner Gottardi gcaling(LGS) t6-81. This is exemplarily shown in Fig. 3 forthree different NBI heating powers (PNsr = 1,2 and 3.5
Mw, PNBfb' = 0.7, 1.4 and 2.5 MW, respectively). The
figure also demonstrates that - concomitant with the

o 2.O
lc
l..l

sudden jump of ?E - the impurity confinement time q-o

o,E

l'1.5

|

12.3 lJ.O

2.0

1

5.5

14,0

(measured by laser blow-off injection of aluminum)

1/+.5

drastically decreases to values close to the energy

Dlasma minor radius /cm

confinement time. This favourable density dependenceof ?i.p is, as mentioned above, in strong contrast to
previous experiments, which yielded a f,r.p * n.(O)ttzl

Fig. 2 Energy confinement times rE [ms] for NBI discharges (P"r,"b" = 1.4 MW) as a function of the line-

averaged density h" for various plasma minor
radii (varied separation between x-points and targets, see Fig. 1a). The dashed black line indicatesrollover of the particle flux over most of the target
area (tiles 7-17, see Sec. 3.2).

P08 scaling t9,101. As a consequence, the impurity
radiation from constant-density HDH discharges stays
constant after the build-up phase of the discharges.

lq
a

E
E 1n
iu

0t23012301234
line-averaged density n-" /1020 6-s
Fig. 3 Energy confinement times r. and impurity confinement times z,.o (from laser blow-off injection of aluminum) as
functions of the line-averaged density n" for NBl-heated discharges at different heating powers P"r,"b" ($tandard
Divertor Configuration, SDC). Shaded ranges indicate partially detached discharges. Comparison of r. with several
scalings (see text).
120

Grigull P. et al., Tlte Impact of the New Island Divertor on the Plasma Performance in the W7-AS Stellarator

camera) in Fig. 5. In contrast, the radiation in HDH
discharges originates mainly from lower ionization
states of carbon and oxygen and emanates always fromnear the edge. It should be noticed that the Abelinversion of the corresponding raw data could not treat

Radiated power fractions,f, = P.uolPNuub' (with P,u6 being
the radiated power) are low to moderate but reach up toabott9OVo in detached discharges as shown in Fig.4.

Radiative losses from normal confinement
discharges at lower density are dominated by core
radiation from higher ionization states of iron, oxygenand chlorine. Consistent with long impurity confinement
times, the losses strongly increase with the discharge
duration even in cases where the density can be kept
fairly constant as demonstrated by radiated power
density profiles (recorded by a 32 channel bolometer

*'0.8
o

E o.o

care, the tendencies are clear. With'stronger'
detachment (increasing density and radiated power
fraction), the raw data profiles become increasingly
asymmetric, which could point towards a benign
MARFE-like scenario.
With the transition from normal to HDH regimes.
the n" radial profiles typically change from centrally
peaked to flat over most of the radius, with steep
gradients at the edge, Fig. 6. The shape of the 7" profile
does not significantly change. Based on these profiles,

P"",uo" =E 2.5 MW

.t*

o 0.4

first SITAR code [11] simulations of the soft-X

on.H":f;r.*

o

E o.z

beyond the main plasma separatrix. Although local
values from this radial range have to be taken with someo 0.7 MW. 1.4 MWo
3,(E

the real island geometry at the edge. It was necessarily
performed with the assumption that flux surfaces extend

0f jrlemission of aluminum (injected by laser blow-off) havebeen performed. They yielded nearly unchanged particle

dF
t

#E -

ec€ o

diffusion coefficients for the transition from normal to
improved confinement, but a reduction of the inward
convection velocity at the edge by factors of four tofive.
Concomitant with broadened z" profiles, the density

1 1.5 2 2.5 3 3.5 4
ne

20

m-3

Fig, 4 Radiated power fractions f, as functions of the,?es near the main plasma separatrix steeply increases

torather high values, which is an important pre-condition
for favourable divertor operation, Fig. 7. With theline-averaged density n" for NBI-heated discharges (SDC). Gray shading indicates partiallydetached discharges.

I'^ 1..:..

E\ thin lin€s:
i normal confinement,u./ s-'..... \. unstationary

=

V2
o

-0.-6t3'-oo'5 s \o\. i.- i.. \'- ''. i' \thick tines:
stationary.

o$----)'ii.l:,

6

E

2.5

400

t, lPJll:slr",-..
'.. '\'..

=

8t

51881,5t890

'i\

\ !'.

2.0
300

Dart. delached
\

E

R t.s

o

-

E

200

c 1.0

100

0

normalconfinemento246810',t2't4
eff€ctive minor radius refi /cm

0L
0Fig. 5 Radiated power density profiles (from bolometer812

normalcontinement

0.5
0

04812

effective minor radius rsff /cm

camera) for NBI heated discharges (Prne,"b" = 1.4
MW, SDC). Temporal evolution (thin lines) for adischarge with normal confinement at nearly constant t" == 1.7x1020m-3, and for stationary HDH discharges (thick lines) at ne = 2.4x1lzo p-3 (attached)and 3.2x1020 m-3 (partially detached).

Fig. 6 Comparison of typical electron temperature 7.

and density n" radial profiles for NBI heated discharges (P"","0" = 1.4 MW, SDC) in the normal
confinement and HDH regimes. The separatrix
position is at r"o = 11.5 cm.

t2l

Grigull P. et al., The Impact of the New Island Divertor on the Plasma Performance in the W7-AS Stellarator
0.8

a n€s (Li boam) tr T"" (Thomson
I n€e (Thomson sc.)

0.6

160

'120

E

-l

"5 0.4

80 a'

0.2

40

0r234
6" /1020 m€
Fig. 7 Electron densities n"" (from Li beam and Thomson

Fig. 8 View of a divertor target. Ho traces show that in-

scattering) and temperatures 4s (from Thomson
scattering) near the main plasma separatrix asfunctions of the line-averaged density h" (Prr,"b"=1.4 MW, SDC). Lines are for guiding the eye only.

teraction with the plasma is concentrated at two
helical stripes. The numbers denote target tiles.
The watershed (tile 12) is closest to the main
plasma. The separation from the main plasma increases with increasing toroidal distance from tile
12. Tile 9 is slightly more retracted in order to allow diagnostic openings. Regions A and D show
different features with respect to rollover and detachment (see text).

The gray shaded range indicates partially detached discharges.

transition to partial detachment nes shows rollover anddecreases with increasing fr". Corresponding electron
temperatures 2", (measured by Thomson scattering at
the inboard side of a triangular cross section) typically

70

oR
oE

start to decrease already at smaller Z" values.
D

3.2 Divertor plasma regimes

tr-H

E.'

The leading features of the divertor regimes werestudied with diagnostics concentrated at two opposite(top and bottom) divertor modules. The interaction of
the plasma with the targets typically concentrates mainly
at two helical stripes. An example of Ho patterns isshown in Fig. 8, in which the target tile and probe
position terms used in the following are also defined.

t.

I nedtopl3. n.bott.li
eoo rlltopraGoT-_bofi.1
€d

a-

50

+ Ted top 5

40 -{

+

"*41

3

r;i4'[-i-#%.ft$rr*

30P
20

TF

10

12

Within the strike zones, the thermal load typically shows
strongest maxima at tlle 12, which is closest to the main
plasma, and at tiles further away from the main plasma
but with steeper inclination to the field lines (tiles 5 or
6). Concerning rollover and detachment of the energyand particle-fluxes, Ho-, thermography- and Langmuirprobe-data indicate a rather uniform behavior over most

34
n

20

0

m3

Fig. 9 Peak downstream densities n"o and temperatures[o measured with the probe array at tile 13 (representative for target region D. see Fig. 8) for NBI
discharges (P*r,"o" = 1.4 MW, SDC). The shaded
area indicates detachment from region D. Peaktemperatures measured at tile 5 (top target, region A) for comparison. Lines are for guiding theeye only.

of the target plates (tiles 7-17, rcgion D) with the
exception of the 'wing' region (tiles 4-6, region A)
which behaves differently. Downstream parameters
measured with the probe array at tile 13 (representative
for region D) are exemplarily shown in Fig. 9 whichplots peak densities n"6 arrd temperatures 2"6 versus i"for NBI (2 MW) discharges. With increasing n" theexhibits -in agreement with Ho data- rollover down todetachment. The threshold density for rollover increases
with decreasing distance /* between x-points and targets(see Fig. 2). Rollover coincides with decreasing upstream temperature and, hence, decreasing upstream

Z"oF* values steadily decrease (as expected), aod n*Peak122

Grigull P. et al., The Impact of the New Island Divertor on the Plasma Performance in the W7-AS Stellarator

in view of future application to W7-X. The divertor
2.O

enables access to a new NBI heated, very high density

b 1.s

operating regime with high energy- and low particle andimpurity confinement times (HDH regime). This allows
for full density control and stable quasi steady-state
operation, also under conditions of partial detachmentfrom the divertor targets. Radiated power fractions are
low to moderate in attached regimes and reach up toql

3=g 1.0

about 9OVo in detachment scenarios. The radiation
originates mainly from lower ionization states of carbon
and oxygen and emanates always from the edge. The
HDH regime is robust against changes of the magnetic

o
o-

0.5

field configuration, whereas stable detachment isrestricted to configurations with sufficiently large
clearances between x-points and targets. The physics

Fig. 10 Thermal load on a bottom target (from thermog-

background of the new regime is not yet clear, but areduction of the inward pinch for impurities by
flattening of the density profiles (due to broadened NBI
deposition and strong recycling particle sources at theedge) seems to be a key ingredient. The access to the
HDH regime is probably enabled by the strongly
enlarged coverage of the plasma surface by carbon tiles
which improves transient particle pumping and screens
the plasma more efficiently from unfavourable
interaction with the vessel wall.

raphy) for attached and partially detached NBI
discharges (P*g,'b" = 1.4MW, SDC). The plasma attile 5 stays attached during partial detachment,
but at reduced thermal load.

plasma pressure. In contrast to tokamak divertors, it
starts already at Z"o > l0 eV where losses of parallel
momentum by charge exchange neutrals are notexpected to be efficient. Detachment from the strikezones is partial in the sense that the particle flux remains
finite and 2"4 stays above 2 eV (from Ho/H, line
intensities, not shown), where volume recombination is

References[]

not yet effective. An important result is that theapproach to this type of detachment is continuous. All

P. Grigull et al., accepted for publication in Plasma
Phys. Control. Fusion 43 (2001).

t2l H. Renner et al.,Ntcl. Fusion 40, 1083 (2000).
t3l K.McCormick et al.,Plasma Phys. Control. Fusionintermediate states including partial detachment can bequasi-steadily maintained. One of the potentiallystabilizing effects is that detachment is partial also in a
spatial sense: the plasma in region A stays always
attached, Fig. t0. Maximum temperatures and densities
measured with probe arrays at tiles 5 (see ref. [])41, 8285-8304 (1999).

t4l K. McCormick et al., accepted for publication in
PRL.
t5l M. Greenwald et al.,Nttcl. Fusion 28,2199 (1988).
t6l U. Stroth er al., Nucl. Fusion 36,1063 (1996).

t7l K. Lackner and E. Gottardi, Nucl. Fusion 30,767

indicate a complex interplay between top and bottom
targets, which changes with density and the level of theexternal gas feed; the peak temperatures stay, however,
above 20 eV even at the highest density. This isinconsistent with the rather low upstream temperatures

t8l K. Yamazaki et al.,LAEA Conf. on Fusion Energy,

shown in Fig. 7 and could point again to an
inhomogeneous temperature distribution near theContr. Fusion and Plasma Physics, Bournemouth,(leeO).
Sorrento, Italy, FTP2I 12 (2000).

t9l R. Burhenn et al., Proc. 22nd Europ. Conf. on
UK, 1995, Vol. 19c Part III, p.145.

separatrix and MARFE-like developments.

[0] R. Burhenn et al.,Proc.24s Europ. Conf. on Contr.

4. Summary and Conclusion

Fusion and Plasma Physics, Berchtesgaden, Germany, 1997, Vol. 21a Part IV, p.1609.
[11] WVII-A Team, NI Group, Nucl. Fusion 25,1593

W7-AS is operated with ten open divertor modules
in order to experimentally evaluate this exhaust concept(1985).

LZJ