J. Plasma Fusion Res. SERIES, Yol.3 (200O) 67-72

Technical Aspects of Steady State Operation of
WENDELSTEIN 7.X
FEIST Jost-Henrich*, SnAUeR Torsten, ERCKMANN Volker, HARTFUSS Hans-Jiirgen,
HEIMANN Peter, NIEDERMEYER Helmut, RENNER Hermann, SAPPER J<irg,
SCHAUER Felix, SPETH Eckehart, WANNER Manfred and WESNER Fritz.
Max-Planck-Institut fiir Plasmaphysik, EURATOM Ass., D-17489 Greifswald, Germany(Received: 18 Januarv 20fi) / Accepted: 16 June 2000)

Abstract
The technical aspects of steady state operation of the W7-X stellarator are outlined. For cwoperation superconducting coils are necessary which are thermally insulated by a cryostat. The ECRH
system of W7-X consists of ten gyrotrons with a cw-power of I MW each. ICRH and NBI are used inpulsed mode but can be upgraded for longer pulses. The control system allows a flexible use of thedischarges. Diagnostics and data acquisition will handle the data with parallel computation and data
reduction before storing. 48 MW of electrical power will be supplied from the mains which can be
upgraded to 9l MW. The power supply for the heating is based on pulse step modulators with 3.3 MW
cw- or 6.5 MW pulsed-power. The magnets are fed by 20 kA, 30 V modules with a fast protection
system. The refrigerator is designed for 3.5 kW at 4 K. Large water reservoirs allow a pulse length of
30 min at full power.

Keywords:
WENDELSTEIN 7-X, steady state operation, magnet system, cryostat, plasma facing components,
ECRH, ICRH, NBI, control and data acquisition, power supply, cooling

1. Introductionenergy and particle handling together with plasma wallinteraction will not be discussed.

The WENDELSTEIN 7-X stellarator (W7-X) is thenext step device in the stellarator line of IPP Garching

and is being built in the new branch of IPP atTable 1: Main Parameters of W7-X

Greifswald. Start of operation is scheduled for 2006.
A schematic view of W7-X is shown in Fig. I, themain parameters are given in Table l. The physics
background and objectives as well as the technicalchallenges of W7-X have been described in a number ofpublications (e.g. [1], [2] and references therein). In thispaper, technical aspects of the different components of
W7-X proper, the heating systems, the diagnostic andMean major radius
Mean minor radius
Magnetic field on the plasma axis
Maenetic field on the conductor
Conductor current
Temperature of the magnet system
Stored magnetic energy

5.5 m
0.53 m
3.0 T
6.7 T

r8.2 kA
3.8 K
610 MJ

data acquisition as well as the periphery of theexperiment will be described with emphasis on steadystate aspects. Physics issues which mainly concern*Corresponding author's e-mail:@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Researchfeist@ ipp.mpg.de67

FEIST J. et al., Techfical Aspects of Steady State Operation of WENDELSTEIN 7-X

2. Components of W7-X and Steady State

from single strand measurements.

- A completely elastic behaviour up to I 14 Vo of the

Aspects

W7-X is a HELIcal Advanced Stellarator

nominal mechanical force.

- A quench characteristic in agreement with the(HELIAS) with strongly varying plasma cross section, 5

field periods and low shear. The main components are
the magnet system, the cryostat and the invessel

expectation.

Based on these results. the manufacture of the 70
coils has been released.

components.

2.1 The Magnet System
A detailed description of the magnet system isgiven in [3]. It comprises 50 non-planar and 20 planar
large magnet coils arranged in fivefold symmetry. For

2.2 The Cryostat
The cryostat consists of:plasma vessel, which follows the optimised shapeTheof the plasma. It provides the UHV-conditions for theplasma operation and space and mechanical support

steady state operation these coils have to besuperconducting. The conductor is a cable-in-conduitconductor composed of 243 copper stabilised NbTi
strands and enveloped by an aluminium alloy jacket.
With this conductor the individual winding packages arebuilt up, consisting of 6 or 3 double layers. For
mechanical stability, each winding package is
encapsulated by a stainless steel housing. The coils arefor the in vessel components.
- The thermal insulation based on radiation shields,
cooled to about 70 K, super insulation and vacuum.
- The 309 ports of sizes between 100 mm diameter and1000 x 400 mm2 cross-section for plasma diagnostic,
heating, vacuum pumping and cooling water supplyof the invessel components.
- The support of the cold mass.
- The outer vacuum vessel which encloses the whole

cooled by a flow of supercritical helium inside theconductor (37 Vo void fraction) and through the coolingloops attached to each coil casing. The coil system,

device.

including the rigid support structure to handle theLorentz forces of up to 3.6 MN per coil, has a weight ofappr. 400 t. Cool down is expected to last two weeks

The main influence of the cryostat on steady state
aspects are the thermal losses due to radiation andconduction. The following values have to be obtained:- Thermal load less than 6 Wm2 at 80 K and 0.1 Wm2and is foreseen only once a year.

To test the concepts of winding, insulation,

at4K.
A- base vacuum of better than 10-6 mbar to haveembedding and electrical/mechanical stability a full-size
DEMO-coil was built. This coil was tested successfully
at Forschungszentrum Karlsruhe in the TOSKA facility.
The results are:
- A critical current of 19.2 kA at a magnetic field of
6.8 T and a temperature of appr. 5 K, as predicted

negligible gas conduction losses.
Thermalconduction losses through the support of thecold mass (10 poles made of Fibre Epoxy) of lessthan5Wat4K.
To test the concept of the cryostat and to gain
experience for its manufacture and assembly, a full-size

DEMO-cryostat was built, which represents l/8 sectorof W7-X and resembles all the features of W7-X except
the coils which are dummies. This cryostat is presently
under test at Garching.

2.3 The In-Vessel Components
A detailed description of the in-vessel components,
especially of the plasma facing components (PFC) is
given in [4]. Due to the steady state conditions at highheat loads and the 3D topology of W7-X, special
solutions have to be developed for the divertor (target
plates, control coils, baffles and cryopumps) and for the
first wall. The PFC of both elements are designed tohandle a cw-power of 10 MW each, with the total power
not exceeding 15 MW.

Fig. 1 Schematic.view of W7-X
68

FEIST J. et al., Technical Aspects of Steady State Operation of WENDELSTEIN 7-X

Ten divertor units are installed in W7-X, i.e. twounits per period. Each unit consists of two smooth setsof target plates and baffles, four cryopumps, a control
coil and in addition a mechanical pumping system
located outside of the device.
The target plates are designed to withstand a heat

the outside and can be heated up to 150"C.

To satisfy the particular requirements of the various
heating methods and to adapt the wall protection to the
local geometrical constraints, a combination of theseconcepts will become necessary in special areas.

Each divertor unit will be equipped with a turbomolecular pump (TMP) and four cryopumps. The TMPs

flux up to 10 MW/m2 with a critical heat flux of
25 MWlm2. One divertor unit consists of appr. 130

are the basic pumping system of W7-X. They will
provide an effective pumping speed for H2 of 420O lls

target elements, which span a 3D shaped area of 2.2 m2.
The single target elements are standardised with a widtheach. The base vacuum will be < l0-8 mbar. Problemsof 55 mm and a length between 270-500 mm. Flat
Carbon Fibre Composite (CFC) tiles will be brazed orwelded on a cooling structure which will consist of awith the TMP for long pulse operation are due to thestray magnetic field of W7-X. In a test with a TMP withnominal pumping speed of 5000 Vs, a field in excess of
3-5 mT reduced the frequency of rotation and increased

copper or a molybdenum alloy. The thickness of the
CFC (6-10 mm) is determined by the temperature limit
set at the surface of 1000'C, the thermal conductivity of
the CFC, the material of the cooling structure and thegeometry of the cooling channels. For the envisaged
operation of W7-X, erosion will not limit the lifetime of
the invessel components. The development of a divertor

the temperature of the rotor to unacceptable values. This

will be investigated further. Possible solutions are to put
the pump into a lower field region (this will reduce thepumping speed), to use smaller units (this will increase
the cost) or to provide a magnetic shielding. The foursystem for the next generation with steady state
operation is one of the main objectives of W7-X. The

cryopumps will have an overall pumping speed of
15.000 l/s for hydrogen. Regeneration will be necessary
after appr. 3 hours of operation due to saturation of thetarget plates will be actively cooled by pressurised water

pumps.

flow. The cooling system can provide a mass flow of3000 m3/h at a pressure drop of 14 bar. To suppress
boiling due to the high wall temperatures of the cooling

3. The Heating Systems
3.1 Electron Cyclotron Resonance Heating

channels a static pressure of l0 bar will be applied.

A detailed description of the ECRH system is given

The main purpose of the control coils is thecorrection of error fields and the modification of thein [5]. 10 MW of heating power are required to achieve
the envisaged plasma parameters. The standard heating
scenario is 2nd harmonic extraordinary-mode with 140
GHz at 2.5 T launched perpendicularly to the confining
magnetic field from the low field side. This scenario hascomplete absorption of the irradiated microwaves in asingle transit through the plasma at all relevant plasma
parameters. This is of high importance for steady state

plasma boundary. If the power deposition will become

critical for cw-operation, they can be used to distributethe power to the target plates on a larger surface bysweeping the deposition area with a frequency of up to
20 Hz. This will reduce the time averaged power density

without producing cyclic stresses. The time constants ofthe target elements are in the order of 3 s. The
deposition area can be enlarged by a factor of two.
The baffle elements, covering 3 m2 per divertor
unit, are designed to withstand a power density of lessthan 0.5 MWm2. Two concepts are being studied andoperation from the point of view of protection andcooling of the inner vessel wall.
The steady state aspects concern the gyrotrons, thewindows, the elements of the optical transmission line,
the inner wall and the conditions alone the transmission

prototypes are manufactured for testing: Stainless steel

path.

quilted panels with surface layers of BaC (low Z-

The total ECRH power is generated by l0
gyrotrons with I MW output power in cw-operation
each. The gyrotrons will have a depressed collector forelectron beam energy recovery, which results in anmaterial, reduced gas inventory, controlled surface
temperature and gas release during long pulse operation)or clamped C tiles on water cooled support structures.
The same concepts as for the baffles will be used for the
first wall elements, which have to cover an area of 100
m2 and will be designed for a power density of less thanO.2 MW/m2. The temperature of the plasma vessel
behind the PFC is controlled by water cooling loops on

efficiency of > 45 Vo.Prototype gyrotrons are presently
being developed with industries. For the windows, large
diameter (106 mm) edge cooled diamond disks will be
used with very low loss-tangent.
Based on the excellent experience at W7-AS
69

FEIST J. et al., Technical Aspects of Steady State Operation of WENDELSTEIN 7-X(140 GHz/0.8 MW) an optical transmission line for W7-

be designed for cw-operation from the very beginning.

X was chosen. It is the most simple, reliable and costeffec-tive solution with respect to steady state operation.
The advantages are: Very high power capability due to

3.3 Neutral Beam Injection
The neutral beam injection system for W7-X is

low power density, low diffraction loss of unwanted
higher-order free-space modes, low Ohmic losses, noneed for special waveguide sections to compensate
thermal expansion of the waveguide, and minimum
number of parts. The millimetre wave power will betransmitted via two multi-beam waveguides (MBWG)

based on the injection system for the ASDEX-Upgrade

tokamak which is described in [7]. It will consist of two
beam lines in counter-directed positions, which can beequipped with four ion sources each, delivering up to 20
MW to the plasma. In the first stage only one ion source
is used per beam line which can produce 95 A at 55 kV

with a length of about 50 m which combine fivefor hydrogen, 80 A at 60 kV for deuterium. In thesecond stage eight sources with 65 A, 100 kV areenvisaged. To stop the beams for conditioning of theinjector and to handle the residual ions during theindividual rf-beams each. Central part of the MBWG are
the various mirrors. To reach the theoretical predictionsof high transmission efficiency, the design of thesemirrors [6] must guarantee a stable surface under
different heat loads imposed by the millimetre waves.
The design finally chosen consists of a 6G-70 mm thickhoneycomb structure from stainless steel and a thinsheath of copper on the mirror surface. The coolingchannels are situated directly below the copper and form
one or several spirals. Width, depth and the number ofspirals is varied to adapt it to the different mirror shapes
and dimensions and to match the local heat transfer toinjection phase, panels are used which can handle power
densities of more than 25 MWm2. Ion sources andpanels are actively cooled, i.e. they can run in cw-mode.
However, for long pulse operation, sputtering of the ion
dump may limit the lifetime.
Other parts of the beam line like the neutraliser andthe beam scrapers are designed for l0 s operation only
and have to be replaced by actively cooled structures.

The main limitation in pulse length is the pumping

the distribution of the heat load. With this design
thermal expansion and deformation is small. The
increase of the copper surface temperature withsystem which is based on titanium evaporation pumps.
These pumps will saturate after l0 s. For longer pulses,

they have to be replaced by conventional chevron type
cryopumps which fit into the geometry of the titanium
pumps (same size and pumping speed). The refrigerator
plant for W7-X already includes the additional power
load by these pumps.
For both heating systems (ICRH and NBI) the highvoltage power supply (5.2) and the cooling system (5.5)

microwave loading (typ. 2 kW per mirror) can be kept
below 20 "C and the deviation from the original surfaceof < I pm in steady state is negligible.
The transmission of the power along the 50 m of
the MBWG calls for special attention to the humidity or
cooling of the air. At standard conditions, 500 kW will
be lost due to absorption by the water in the air. This
has either to be cooled away by an appropriate air
conditioning system or it has to be reduced by reductionof the humidity, which may be difficult to obtainbecause of the water content of the concrete. A
combination of both measures will be used.

have to be upgraded.

4. Gontrol, Diagnostic and Data Acquisition
4.1 The Gontrol System
The main objective of the control system for steadystate operation is to ensure a most flexible use of thelong pulses for a number of different experiments during

3.2 lon Cyclotron Resonance Heating
At present, ICRH will be used only in the pulsed

one discharge. The configuration of the experiment,
including the diagnostic and the data acquisition will be
determined within periods of time with variable length(segments) by software programmes, defining the set ofexperimental and technical parameters for each segment.
Special segments will be used for start up, transitionmode. Two generators with 2 MW each will beavailable with a pulse length of max. l0 s every 5 minin a frequency range from 30 to I 15 MHz. To upgrade
the generators and the transmission lines for cwoperation would need development work especiallybetween segments and shut down. Initially, the order ofthe segments will be prescribed before the discharge
starts. In a later stage an automatic optimisation isplanned, based on the experimental results and theavailable resources of the experiment.

concerning the cooling of ceramics and feedthroughs.
The other components of the ICRH system like theantenna loops, the limiters, the faraday screens etc. are
more subject to heat load frorn the plasma and have to70

FEIST J. et al., Techr.ical Aspects of Steady State Operation of WENDELSTEIN 7-X

Every component (heating systems, power supplies,
refrigerator etc.) has its own control system for local andindependent operation. The coordinated action of theamount of channels must be designed to reduce data
before archiving. Reduction is mainly done on physicalbasis or by event dependent change on sample rate.
When necessary the reduction can be done in parallel bymultiple CPU's.

components is guaranteed by the central control
computer which triggers the start of the different
segments and controls the necessary interlocks to avoid

5. The Peripheral Installations
5.1 The Main Supply for the Experiment

dangerous mutual effects.

The units for operation and display are separated
from the real time tasks and are connected via Internet.
This increases the safety and flexibility of the system
and allows remote operation.

The distribution system for the electrical power for
the institute and the experiment is very flexible. The
power can be drawn from two different sources: The

Industrial type parts will be used whereverbasic supply for the institute is provided by the
Greifswald utilities, delivering a power of 5 MVA at a

possible. For ambitious real time tasks which can not be
handled with PLCs, PCs will be installed, using the real

20 kV level. This power has the standard stability of theutilities and is mainly used for the infrastructure of thetime operation system VxWorks. Signals within acomponent will be exchanged by Profibus, between
components via dedicated Ethernet connections. All
systems will be synchronised by a central clock.

institute and some basic installations of the experiment.
The electrical power for the heating is drawn from themain 110 kV grid of Mecklenburg-Vorpommern. A
power of up to 43 MVA can be used for an unlimited
time. This power has a stability of +10/-15 Vo and is

4.2 Diagnostic and Data Acquisition
A detailed description of the diagnostic is given in

stepped down to the 20 kV level. Provision is made to
double this power later for stage II heating. The powert8l. A first goal of W7-X is the experimental
verification of the major design elements: Selection of
iota = I with moderate shear, improved equilibrium,

supplies for the cooling systems, the refrigerator and themagnets power supply can be connected to both supply

particle drift optimisation, energetic particle

nets. This scheme makes it possible to have sufficient
power all the time for basic supply and to have theconfinement, negligible bootstrap current and the role ofthe drift optimisation on anomalous transport. There will
be about 50 different plasma diagnostics at different

possibility to run the experiment at reduced parameters
if one of the supplies fails.

stages. The steady state aspects of the diagnostic mainly
concern the thermal stability of the parts exposed to the

5.2 Power Supplies for the Heating Systems
These power supplies have to energise the threeheating systems of W7-X. The parameters of thesesystems, as seen by the power supplies are listed inplasma, the effect of the ECRH microwaves on
dielectric material and the large amount of data.
All diagnostics must be able to collect and store

acquired and reduced data continuously. An object
oriented approach for software design will be used.
Since the relation to the actual time is so important in acontinuously operating system, all collected data will bemarked by a timestamp, which can have a resolution of

Table 2.
The power supply will be modular. Eight modules,
rated 65 kV, 50 A (cw) or 130 A (15 s) will be used for

ECRH or NBI(I). Always two modules can be put inseries to get the necessary voltage of < 130 kV for NBI(II). Two modules, rated,32.5 kV, 100 A (cw) or 130 A(15 s) will be used for ICRH or in the series connection
for the remaining two ECRH gyrotrons. The modules
will be based on the pulse step modulator principle,

up to l0 ns. It is planned to have no additionalclassification of the data corresponding to pulse
numbers. The global system time is distributed in a starlike network from a central timer system. The data will
be archived in an object oriented database.

The continuous operation of the experiment also
requires the monitoring of several important signals
from the experimental control system as well as from

Table 2 Parameters of the high voltage power
sup-ply for the heating systems.

diagnostic subsystems. Every system user will be able tosign on to various monitor signals for observation of theplasma behaviour.
Diagnostic systems with high data rates and a large

Heating
Sv\temECRH
ICRH

Voltage

Cunent

<65kV

<50A

< 32.5 kV

I 15 sNBI (r)<65kV< 130 kv

< 130 A< l00A

<l

2

<130A

< 15 se

NBt (il)

7l

Pulse

numberof users

l02

FEIST J. et aI., Technical Aspects of Steady State Operation of WENDELSTEIN 7-X

where regulation and protection is integrated into the 80

active characteristics for l0 s and inertial cool down for

single I kV modules.

the 5 min inter-rupt time.

5.3 The Magnet Power Supply

5.6 Radiological Aspects
A detailed study is given in [9]. Two limits have to
be considered: The restriction ofthe radiation outside of

For the supply of the seven groups of coils, seven
identical power supply modules will be used. The power
supplies have a rating of + 30 V, 20 kA, which is
sufficient voltage to allow a ramp rate of 30 A./s. During
operation a voltage of appr. 3 V is expected due to the
Ohmic losses in the busbars and the feedthroughs. The
total losses will be in the order of 500 kW. Since themagnet will be operated continuously during thedaytime this power represents a non negligible load forthe cooling of the experiment.

the building and of the activation of the structure of theexperiment. W7-X is not designed for D-T operation,

therefore only the D-D reaction is the source ofradiation. With the envisaged plasma parameters only a
NBI heated plasma can generate a considerable amountof neutrons. The total time for NBI DD-plasma
operation at high power will be restricted to 500 pulses
with l0 s per year, giving an averaged neutron rate of1012 s-I. With this value, access to the machine is always
possible within a few days and no remote handling tools

For the protection of the coil system during aquench seven nearly identical fast discharge system will

are necessary, To reduce the long-term activation,

be used which can dump the stored magnetic energy
into dump resistors with a time constant < 5 s. After aquench some hours are necessary to cool down these
resistors and to restore the functionality of the system.

special attention is paid to the Cobalt content of thesteel. 500 ppm are allowed in the various parts of W7X proper. With this value hands on disassembly of
W7-X without any radiological precautions is possible

5.4 The Refrigerator

10-15 years after shut down.

The cooling power will be supplied by arefrigerator. A total of 2500-3500 W at 3.3-4.2 K and6. Conclusions
WENDELSTEIN 7-X will be the next element in
the stellarator line of the IPP on the way to a fusion
power reactor. Besides the many physics aspects of
optimisation of the stellarator special attention has to be
paid to the inherent capability of steady state operation.
All elements of W7-X proper are designed to fulfil this

50.000 W at 6G-80 K will be necessary, the actual needs
depend on the design of the current leads. Conventional

copper current leads may require a power up to 2 kW,
for HTSC type current leads only half of this value may
be necessary.

requirement. Although some components of theperiphery are limited at present to short (10 s) or long
(30 min) pulse operation, they could be upgraded to

5.5 The Water-Cooling System
Depending on the efficiency of the gyrotrons a cwpower of up to 28 MW has to be cooled away. Either alarge cooling tower or a large amount of ground-watercontinuous operation at a later stage if necessary.

would be necessary. For economic and environmental
reasons it was decided to restrict the maximum pulse
length to < 30 min. Therefore it is possible to subdivide
the cooling into an inertial part, where the actual powerheats up a reservoir of 1200 m3 of water by < l5'C andan active part, where cooling towers with a capacity of
5.5 MW cool down the water in a second 1200 m3
reservoir to ambient temperature. The experiment canstart in the morning with a 30 min ECRH pulse together
with lOs NBI/ICRH pulses every 5 min. After threehours of cool down, pulsed operation (lOs / 5 min) is
possible. At.the end of the day another 30 min pulse cantllReferences
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[3] J. Sapper, Fusion Technology, 30,1234 (1996).
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t5l V. Erckmann et al.,Proc. of 20th Symp. of Fus.
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[6] L. Empacher et al., Proc. of the 24th Conf. on
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t7l O. Vollmer et al., Proc. of 20th Symp. of Fus.

The cooling system for the ECRH and the PFC will
be designed to allow cw-operation from the very

Technology, 449 (1998).
[8] Hartfuss et al.,Rlev. Sci. Instrum. 68, 1244 (1997).
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beginning, the coolilg for NBI and ICRH will have72