J. Plasma Fusion Res. SERIES, Vol.l (1998) 19-26

W7-AS; Programme and Recent Results
KICK Manfred*, WAGNER Friedrich and the W7-AS TeamMax- P lanck- Institut fiir P lasmaphysik, E U RATO M Ass.

D-85748 Garching, Germany(Received: 30 September 1997/Acceptedi 22 October 1997)

Abstract
The programme of W7-AS concentrates mainly on topics related to the optimisation principles ofthe WENDELSTEIN stellarator line such as improved equilibria with reduced Shafranov shift, neoclassical transport in improved magnetic configurations including the effects of the ambipolar radial
electric field, confinement of high energetic particles, radial diffusion of trapped electrons generated by
ECR power deposition. But also topics.related to stellarator physics, more generally, are intensely
treated, such as the radiative limit at high densities, particle transport, GAE-modes, fluctuations andturbulence, the influence of shear on confinement, and the H-mode. Furthermore new diagnostics withrespect to long pulse operation of W7-X are developed. Optimum (neoclassical) confinement discharges
with energy confinement times (up to 50 ms) of more than 2.5 times larger than predicted by the International Stellarator Scaling, 1SS95, with {<1.5 keV are presented as well as discharges with Te=4
keV showing a strongly positive ECRH driven "electron root" feature of the radial electric field andhigh density high power NBI discharges leading to <fr>:1.8%. A 3D Edge code is developed tomodel edge transport and to optimise energy and particle deposition in a future island divertor. The roleof E x B drifts in islands is considered in detail. Heating scenarios with relevance to W7-X are finallyaddressed which are independent of cut-off densities and do not necessarily require magnetic resonanceconditions, such as the O-X-B heating scenario of ECR waves. The prospects of ICRH for long pulse
operation are explored.

Keywords
W7-AS, stellarator, optimum confinement, transport, ISS95, electric field, ion root, electron root,
equilibrium, heating scenarios

1. W7-AS

MW at 70 GI{z and P<1.2 MW at 140 GHz, ion cyclotron resonance heating, ICRH, with P<0.5 MW at

WENDELSTEIN 7-AS, W7-AS, is an Advanced
Stellarator [1] (torus radius 2 m, effective plasma

the antenna as well as neutral beam injection, NBI,
(P<3.5 MW, 50 keV) give access to net current freeradius <0.18 m) with 5 toroidal periods. The magnetic
configuration, with B.:2.5 T in the standard case, is
optimised by a reduction of the Pfirsch-Schltiter currents. It is generated by a system of 9 non-planar coils
in each period with 10 additional planar coils allowing avariation of the rotational transform, t, from 0.25 to
0.6. It has low shear and stability is given by a moderate magnetic well [2]. Different heating scenarios, electron cyclotron resonance heating, ECRH, with P<0.4*

plasmas in a wide range: electron densities, n" <3Xl02om-'[3], electron temperatures, Q<4 keV [4-6], ion
temperatures, {<1.5 keV [7-9], and energy confinement times, z.<50 ms [10]. All above results havebeen derived with a magnetic field on axis of Bo<2.5 T.

<B> =1.8% has been achieved at B,<t.25 T (seechapter 3.3). The theoretically predicted reduction of
01998 by The Japan Society of Plasma

Corresponding author's e-mail: kick@ ipp.mpg.de

Science and Nuclear Fusion Researcht9

KickM. et al., W7-AS; Programme and Recent Results

W7-AS is partly optimised in its field properties
and can therefore address some of the above issues.
The improvement in equilibrium is well documented[3,8]; the maximal beta is in the range <fr> -1.8"hand is well above that of a classical stellarator [3,9].
The bootstrap current is dominated by toroidal curvature in W7-AS and increases the rotational transformjust like in a tokamak. The measured bootstrap current
agrees with the expected one in a large range of colli-

the Shafranov shift by a factor of 2 has been proven experimentally already in an early stage of operation [3].
The bootstrap current of several kA can be compensated integrally either by a small ohmic current or more
locally by current drive by ECR [111. Current drive
also can be used to introduce shear for better confinement l4,l2l. With 0.4 MW ECRH at t40 GHz at highdensities H-mode transitions with the characteristics asknown from tokamaks could be achieved [13].
Plasma build-up can be carried out not only by
ECRH but also by non-resonant Rf at 900 MHz leading to a low density target for NBI thus allowing oper-

sionalities [11].

The ambipolar electric field and its impact ontransport will be discussed in detail below.
The confinement of NBI ions is complete in caseof slowing down times z. smaller than the energy confinement time ru. For cases with high I when r, can bemuch larger than the thermal zB, the measured slowing
down spectra cannot be modelled satisfactorily with theassumption of complete ion confinement. The discrepancy between measurement and the expectation fromation independently of the "electron cyclotron resonance fields" of 2.5 and 1.25 't, respectively. Also independent on resonant magnetic fields is the O-X-B
heating scenario [14,15] which recently has been successtully applied at W7-AS. ICRH has been operated
successfully with long pulses of up to 1s [16].

2. Programme

good confinement is also borne out by neutron flux

The major element of the optimisation is to reduce
the relative Pfirsch-Schltiter current I j,,'/ j ,') and for
W7-X to reduce the bootstrap current. Low PS currents
lead to improved equilibria with small Shafranov shift,high stability and strongly reduced neoclassical losses;

measurements in case of D into D-injection. The results
are still preliminary and the loss mechanisms are not yet

identified [171. With respect to transport of energetic
electrons the fast radial diffusion of trapped electrons,
heated by the interaction with ECRH, is observed in
power deposition measurements (fast ECRH modulation). The deposition profile deviates from that ex-

the minimisation of the bootstrap current avoids astrong disparity between high pressure vacuum field
equilibria so that the characteristics of the field design
are maintained towards high beta. The optimisation
rests on the reduction of average curvature by proper
plasma shaping, the repellence of trapped particles
from zones of high curyature by the mirror effect andfrom a proper match of toroidal and helical curvature.
Despite of the optimisation neoclassical transport displays the deleterious l/y collisionality scaling in thelong mean free pass, lmfp, regime. The presence of an
ambipolar field will further reduce the transport coefficients to a tolerable level. It is mandatory to measure
the ambipolar field and to assess the balance of radial
species flow. If the ambipolar electric field is deter-

pected from ray-tracing and absorption and shows
wings in the plasma periphery [18]. The widening of
the deposition profile is well modelled [19].
There are important topics in the development ofstellarators which are, at present, beyond the reach of arigorous optimisation like the high density operational
range 120j, turbulent transport [21,22], aspects of

MHD stability like the beam induced Global Alfv6n

waves, GAE, [22,231, and the exhaust conditions. Pres-ent stellarators can operate at high density (see chapter
1) which will ease the conditions for plasma exhaust. In
addition, stellarators can safely operate at high density.
The operational limit is not connected with a virulent
MHD phenomenon but is a radiative loss of plasmaenergy. Therefore, the collapse time is slow along theenergy confinement time, the plasma energy is homogeneously distributed by radiation and there is time tointerfere and to restore a stable plasma [20]. The confinement of W7-AS is determined by the superpositionof neoclassical transport in the plasma core and turbulent transport in the gradient region towards the edge.
The global confinement is determined by the turbulent
processes. The pattern of confinement of the low-shear
field system of W7-AS is rather capricious. Depending

mined by radial neoclassical fluxes - because thesuperimposed turbulent contributions are low or intrinsically ambipolar - various transport equilibria (roots)
might develop. Both negative and positive electric fields
reduce the neoclassical ion losses. The electron root
which develops at high electron temperature with thermally decoupled ions has the advantage of good electron and low impurity confinement. Stellarator optimisation is expected to improve also the confinement ofenergetic particles (in case of a reactor that of the aparticles).
20

Kick M. et al., W7-AS; Programme and Recent Resultson the choice of rotational transform r confinement canbe good or low. The energy content W shows a strong
variability with r. The following picture emerges: The
confinement is good, when the iota profile fits into a
iota range (edge to core) with few low-order resonances
as it is the case around the major resonances l/3 and1/2. Outside these intervals, shear is required for good
confinement [4,12]. Sufficient shear is provided by PS-

and bootstrap-current -100

NBl, te = 50ms

50

NBI+E
high TiahP10
lss95-.

if the plasma beta is large

data

enough. Outside the resonances free intervals, the development of good confinement depends on a cyclic
process because improved confinement enhances thepressure-driven currents which feedback positively on
confinement. If the pressure driven currents are enforced by operation at high beta or low collisionality,
the interplay between pressure driven currents and confinement is avoided and the sensitivity of W on t is lost.
Because of the complexity of the dependence of energy
content and field distribution not yet all details are satisfactorily understood.
The base-line confinement of W7-AS is provided
in the good confinement intervals. It resembles the

110

100

1tSS95-!'tfl (ms)Fig. 1 Energy confinement times in W7-AS compared to|SS95-W7. Dashed line: lSS95 for all helical devices;solid line: |SS95-W7 for W7-AS data up to 1995;
open squares: pure ECRH; full triangles: ECRH
H-mode ; full circles: low power NBl. leading toz, = 50 mq o-pen circles: NBI with higher power; full(light) diamonds: combined NBI and ECRH, leading

to T,= 1.5 keV. Crosses for the same discharges taking the kinetic energy instead of the diamagnetic

L-mode confinement of tokamaks. In these r-ranges
and possibly with further restrictions, improved con-one. Both, circles and diamonds, show the increaseof r, by about a factor of 2.5 compared to lSS95.

finement regimes can develop. It is important that theconditions of external confinement as provided in stellarators are also capable of allowing states with improved confinement. An example which is well summarised in the literature is the H-mode of W7-AS [13].

profiles the gradients steepen and the radial electricfield E decreases in the gradient region to E <-300

V/cm [10]. Both, density and temperature show very
low values at the edge. Also in high power combined
NBI/ECRH discharges, narrow density profiles, steep
temperature gradients and large negative E (up to-700 V/cm) close to the plasma edge are found where

Below, another regime is described.

3. Recent Results
Selected topics from the W7-AS programme arebriefly discussed below.

locally n" becomes very small (Fig. 2). For this type ofdischarges with [ > { < 1.5 keV, optimum confinement
properties are found. z. exceeds the 1SS95 scaling by afactor of about 2.5 as in the purely NBI heated case(Fig. 1). The experimental particle fluxes as well as theion and electron energy fluxes are in good agreementwith the neoclassical predictions up to /eff:12 cm, see
Refs. [7-9]. The radial electric field, E, obtained fromthe ambipolarity condition of the neoclassical fluxes is
consistent with the experimental one deduced from thepoloidal rotation of tracers measured by active CXRS
in the plasma core (up to L2 cm) as well as by electron

3.1 Optimumconfinementdischarges

At W7-AS, purely NBI heated and combined
NBI/ECRH discharges at medium and high electron
densities, n" between 1.1x1020 and 0.5X1020 m-3, withT.> Ti= O.8-1.5 keV lead to high performance if goodwall conditioning and low recycling are provided. Energy confinement times up to z.:50 ms are determined. The experimental transport analyses in theplasma core are consistent with the neoclassical predictions from DKES code 124,251.
In purely NBI heated discharges with a power of
PNB'<400 kW, z.=50 ms has been determined [10],exceeding the 1SS95 scaling 126l by more than a factorof 2.5, see Fig. 1. During the gradual transition to good
confinement the density profile becomes narrower,
whereas the temperature profile broadens. In both

impact spectroscopy l27l and probe measurements [28]at the edge. The observation of neoclassical ambipolar
radial electric fields up to the very edge where the turbulent fluxes dominate the neoclassical ones indicatethat the anomalous particle fluxes are intrinsically ambipolar. Deviations in the intermediate range may be

2lKick M. et a/., W7-AS; Programme and Recent Results

Tr."
E

E

{o
a

1'O
E
o

5.s

-200

.c

uf -4ooc' .rl

-ooo

.2

-aoo

.o

Eto'

Eto"

..:2li-'€

€

d

lor

rot

o

'to
r

20

[cm]

Fig.2 Optimum confinement. Discharge #34313, combined ECRH and NBl, Pr.r"-750 kW, P"r,"u"=900 kW. at medium
density. 7l from Thomson scattering and ECE (dark symbols) and { (light symbols) from active CX-NPA and CXRS
(upper left); ," from Thomson scattering (upper middle), radial electric field (upper right) as deduced from active
CXRS, (dots) and passive spectroscopy (circles) compared to the neoclassically calculated ambipolar electric field(crosses). In the lower part the experimental particle and energy fluxes for ions and electrons, respectively. are compared with neoclassical predictions by DKES code taking the ambipolar electric field (upper right) into account.

due to the violation of the condition of small electricfields for neoclassical theory [18].
In all discharges with optimum confinement themeasured density profiles at outer radii are very similar
for all central densities from 0.5 to 1.1X1020 m-3 andheating powers between 0.4 and 1.3 MW. Also, the {and { profiles are very similar in this region [18]. Prerequisites for this type of discharges are good wall conditioning and very low recycling in order to obtain narrow density profiles and to provide global density control even for high NBI power levels with the related
strong particle sources of up to 2.5X1020 s-1. All discharges have been carried out at r= t/3 at B":2.5 T

strong positive E, of up to *600 V,/cm close to theplasma centre with peaked electron temperature

with a vertical field, B"=O.0LxBo. As a consequence

figurations of W7-AS where a significant part of the
ECRH power is absorbed by ripple trapped electrons
close to the axis [4-6,18]. These features are lost if the
ECRH power is decreased or if the configuration ischanged such that the toroidal ripple is reduced. The
evidence, that ripple-trapped suprathermal electrons induced by ECRH generate the strong positive E in thecentre, is supported by Monte Carlo simulations (in 5D

profiles and T"(0)=4 keY, 4(0)-0.: keV and flat
density profiles (n"(0)=0.2xL020 m-3) have beenmeasured [4-6], see Fig. 3. 1" in the intermediate radial

range agrees with the neoclassical prediction for theslightly positive E (ion root), in the centre it is larger
than calculated for the neoclassical (thermal) electronroot but much lower than neoclassically predicted if E
is assumed to be zero, see Fig. 3. These large positiveelectric fields, correlated with strongly peaked electron
temperature profiles have only been found at X-mode

ECRH with powers above 400 kW in magnetic con-the plasma is shifted towards the inboard limiters thus
influencing the recycling behaviour but also the magnetic configuration is improved with respect to neoclassical transport, because the magnetic ripple at thelocation of strong (grad B) is reduced [29]. Once theabove described conditions are fulfilled optimum confinement is reproducibly attained. During the next experimental campaign the question of attaining optimum

phase space) [L9].

confinement under separatrix conditions around t= 7/2
will be a major topic of investigation.

3.3 Equilibrium and stability
For a discharge heated by about 2.2 MW of NBI

at Bo:1.25't with B,:0.026x4 with n"(0):2v1g2s
m-3 and T = T":0.35 keV the free boundary equili-

3.2 ECRH driven "electron root" feature
In low density discharges, heated by ECR (PECRH:400 kW, 2nd harmonic X-mode at 140 GHz), verybrium code NEMEC [30] predicts a central Bo of 4o/"22

KickM. et al., W7-AS; Programme and Recent Results

l5+
I

J2

o

and an averaged < F> of 1.8%. This is in excellent
agreement with the B profile evaluation from Thomson

rui.- lg'1.-''-''--,-l'.i?&estimated neglecting the effect of Z.o which is expected
to be small for this discharge. Furthermore the shift of, ),J

1,=+';:hjrscattering data, see Fig. 4, where the ion contribution is

r

[cm]

the magnetic axis as deduced from the maximum of the
B profile agrees very well with the NEMEC code result.
Though the Shafranov shift of W7-AS is reduced by afactor of 2 compared to classical stellarators [3] it is still
considerable. Nevertheless, the equilibrium B limit has
not been reached, so far. In the upper part of Fig. 4 thesurfaces of constant soft X-ray emission intensity are
shown as are obtained by tomographic reconstruction
using 2 soft X-ray cameras [8]. They are in good agreement with the NEMEC results. Though in high B configurations with Bo:4"/" the magnetic well is reduced
and NEMEC predicts resistive interchange unstable regions at the plasma edge no instabilities have been observed. so far.

I*l

?#

t'nl . Er=ot&r'tt"lo f'f--t-..*i.'. ,,fs*,Jm''h*liii-'"of
t

0

-m

ffi

r

lcnl

Fig.3 "Electron root" feature. Low density ECR heated,
Pr..r=750 kW, discharge with strongly peaked f(upper left). The strongly positive electric field fromCXRS is compared to the neoclassical electron root
close to the centre and slightly positive ion root atintermediate radii (lower left). The lower right pic-

ture shows the experimental heat conductivity(dash-dotted line) compared to neoclassical predictions if f, = 0 is assumed (dotted line) and if electron
and ion roots of E are taken into account (crosses).

3.4 First steps to an island divertor
The W7-AS "high iota" configurations (iota > 0.5),
bounded by "natural" islands of considerable size, havethe basic edge topology required for an island divertor.
The presently installed target arrangement (L0 inboard
target plates, preserving both the fivefold periodicity
and up,/down symmetry of the configuration) is the firststep on the way to an optimised divertor, which will
start operation within the next two years.
Extensive measurements of the island edge structures by plasma spectroscopy and target calorimetry are

in excellent agreement with predicted vacuum andequilibrium configurations, which are available up to
central B values of = Io/o. Analysis of high density NBI

t:5/9 gives strong indications of stablehigh recycling conditions for (n") >1020 m-3 [31].
The observations are reproduced by the 3D plasmaedge transport code EMC3 [32] which has been
coupled selfconsistently to the EIRENE [34] neutraldischarges forgas code.

For low density ECRH discharges, poloidal asymmetries in the island SOL have been observed for=5/9 configurations, with the islands deeply cut by
the inboard plates through the O-point (nig. 5(a)) [33].
Higher densities from Langmuir probe array data were
measured for the lower or upper island fans, dependingon the B field direction €ig. 5(b)). The asymmetries
can be explained by the effects of an E X B drift
driven by the radial temperature gradient in the island
which leads to a radial gradient of the electrostatic
potential. The poloidal density gradient also implies an

Fig.4 Tomographic reconstruction of soft X-ray emission
compared to magnetic flux surfaces calculated by
NEMEC code for high B discharge heated by about
2.2 MW of NBI at Bo=1.25 T with 8.=0.026 8",ne(Ol=2x1020 m-3 and T= T.=0.35 keV. The shift ofthe magnetic axis agrees well with that resulting
from the B prolile calculated from Thomson scattering.

23

Kick M. er a/., W7-AS; Programme and Recent Results

VTe

+B

t75

la6

R (crn)(b)
Fig. 5 (a) left: Poloidal cross section of the t = 5/9 configuration at the poloidal plane of an inboard plate, right: Flow to target
plate resulting from the E, x B drift (schematic).

(b) Density contour from probe array (curves) compared to EMC3/EIRENE results for positive and negative B-field.
For positive (negative) B the deposition maximum is shifted downward (upward).

ECRH is restricted in density because of the cut-off

imbalance of the power and particle fluxes to the plates,
which is consistent with calorimetric and Ho measure-condition. Non resonant plasma build-up has been successfully carried out with turbulent Rf heating at a frequency of 900 MHz in combination with NBI at various
magnetic fields between 0.6 and 2.5T (thus "non resonant") starting from a thin cold plasma (n"( 1018 m-3,
7: < 100 eV). Another scenario which is neither restricted to a certain fixed magnetic field nor by an
upper limit of the density is heating by electron Bernstein waves. EBW. Since these waves cannot be excitedfrom outside the plasma they have to be generated bymode conversion from electromagnetic waves, e.g. in aso called O-X-B process. An O-mode wave launchedwith an optimum angle oblique to the magnetic field
vector is converted into an X-mode at the O-mode cutoff density. This X-wave propagates towards the upper

ments. The drift velocities resulting from estimatedradial temperature gradients were inserted into the

EMC3/EIRENE code, leading to the same phase shiftof the density contours as experimentally observed (Fig.
5(b)).

3.5 Heating scenarios with relevance for W7-X
The operation of W7-AS using ECR waves forheating (pure or in combination with NBI) as well as
for start up of the plasma desires a proper adjustmentof the magnetic field to meet the resonance condition:
B":2.5 T for 70 GHz fundamental O-mode and 140
GHz second harmonic X-mode and A": L25 T for 70
GHz second harmonic X-mode waves. Furthermore
z4

Kick M. el a/., W7-AS; Programme and Recent Results

ing to about 400 eV some time after the beginning of
the purely ICR heated phase recovers to about 800 eVat the end of the ICRH pulse whereas the line integrated electron density remained constant at about
0.16x1020 m-3. During the whole ICRH pulse the impurity radiation remained low. In order to increase thepower deposited in the plasma a double strap antenna

c
.v,

ot

will be installed during the next experimental campaign.

o

=.1
r!'

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gth Int. Conf. on Plasma Phys. and Control. Nucl.

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kW and P""t = 360 kW have been achieved, so far.
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ion cyclotron resonance heating, ICRH, is foreseen.
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26