ITC12, Toki, Japan, December 2001

WENDELSTEIN 7-AS

First island divertor experiments on theW7-AS stellarator
P. Grigull for the W7-AS Team

Max-Planck-Institut für Plasmaphysik, EURATOM Ass., D85748 Garching, Germany

Outline: 1. Motivation
2. Divertor geometry in W7-AS
3. Major aims of the programme
4. Plasma performance at high density
5. Divertor regimes
6. Operational ranges
7. Summary

Motivation- Stellarators will need divertors for plasma exhaust.
- Solutions have to be compatible with stellarator configurations.
- Utilize flux diversion by magnetic islands at the edge----> Island divertor- W7-X: study of the reactor potential of this concept.
- W7-AS can be operated with similar edge structure,
divertor studies can provide preparatory information.
Major studies have been started in March 2001

Divertor geometry in 7-AS: boundary islands
R = 2 m, a ≤ 0.16 m, B ≤ 2.5 T, non-planar coils, five field periods- can be operated with large magnetic islands at the edge
- rad. position can be varied by adjusting the rot. transform- rad. width can be varied by special control coils
one of five magnetic field periods

Divertor geometry in W7-AS
Ten divertor modules

Bottom divertor
Probe arrays

Targets•••
Baffles••Titanium
evaporators

TargetBaffles

Major aims of the programme
To answer the questions:First experiments- Effects of the new divertors on the plasma performance?
- Cold divertor plasmas without too strong cooling of thecore ?
- Controlled? Compatible with active particle pumping?
- Compatible with improved confinement scenarios(e.g. ELMy H-mode)?
Data shall be used to validate the EMC3-EIRENE code.Summary of main results
Access to a new regime with NBI at very high density(up to ne ≈ 3.5x 1020 m-3) with improved confinement:- τE steeply increases with density,- τp and τimp decrease with increasing density(τimp ≈ τE at highest density)
Density control already without Ti-gettering,
quasi-steady state operation also including partial
detachment,
radiatiation always peaked at the edge.
Record value of <ß> ≈ 3.1% achieved (at B = 1.25 T)
Plasma heating by HF (EBW 140 GHz) successfully
demonstrated.

ne /1020 m-3220

P NIabs
1

PradW dia

01

10

W dia /kJ

power /MW

Plasma performance at high density: trans. to stationarity

0ne0.1nes- stored energy < 10 kJ,

0.01

ne /1020 m-3

0.2

0.4
time /s

2

Wdia

1

0.6

0.8
20

P NIa b s

10P rad

00

1

ne0.1

ne s
00.2

0.4
time /s

0.6

0.8

Wdia /kJ

power /MW0ne < 1.8 1020 m-3
normal confinement (NC)unstationary- radiation increasing
with time,- extremely small edge
densityne /1020 m-3

Wdia

220

P NBI

P NBIa b s

P rad1000

ne1.0

ne > 1.8 1020 m-3
improved confinement (IC)quasi steady-state- stored energy ≈ 20 kJ

ne s- Te(0) ≈ 500 eV, ν* ≈ 0.3

0.1- edge density ≈ 5 1019 m-3
Te (0)
100- edge temperature ≈100 eV

Tes
54
32
1

MWm-3

128

reff /cm

Te /eV

4

Wdia /kJ

power /MW

Plasma performance at high density: access to stationarity

separatrix

radiated power

4000.2

0.4

0.6

time /s

0.8

1.0- radiation edge-dominatedPlasma performance at high density: improved confinement20- IC: steep increase(´jump´) of τE,partialdetachment

τE /ms16- low to moderate τEdegradation atpartial detachment

12NBI, 1 MW
NBI, 2 MW

8- steep drop of τimp412
ne /1020 m-3

34

Plasma performance at high density: improved confinement
H_W7-95 ≈ 1.5
H_ISS95 ≈ 2
H_NLHD2 ≈ 2
H_NLHD1 ≈ 2.8
202 MW

1 MW

τE /ms15ISS95
W7_95
LGSNLHD1
NLHD2exper.

3.5 MW12105001230123
ne /1020 m-3034Plasma performance at high density: plasma radiation- radiated power fraction low tomoderate in attached regimes,- NC: core-dominated radiation,
unstationary- up to 90% at detachment- IC: edge-dominated radiation,
stationary

0.8

NBI
1 MW
2MW
3.5 MW

0.6
0.4

0.2
00.5

#51050, 51883, 51889

3
radiated power density /MWm-3

radiated power fraction fr

1

1

1.5

2
2.5
3
ne /1020 m-3

3.5

40.8 s0.7 snormal confinement,
unstationary

20.6 s
0.5 s

IC, stationary
part. detached10.4 sattached0.3 s
0.2 s

0024681012effective minor radius r /cm14Plasma performance at high density: core parameters
Te, ne profiles for NC, IC
500#51881, 51890

IC400

3.0NC ---> IC:- Te profiles unchanged,- ne profiles broadened.

2.5

IC

300
1.5200

NC
100
001.0

NC
0.5

4812048effective minor radius reff /cm0.0
12ne /1020 m-3

T e /eV

2.0

SITAR analysis of Alemission:- marginal change ofdiffusion coefficients,- reduction of vin at theedge by a factor of 4 - 5.

Plasma performance at high density: H-mode?
H-mode signatures duringtransition to IC,

Hαno ELM-likecharacteristics

IC: - only small spin-up,- no evidence for ETB# 51801

12ne /1020 m-3

stored energy /kJ#51791, 51793, 51794, 51801

singleELMs

84

dithers

NBI, 1 MW

02# 51794
grassy10# 51793

NBI, 1 MW
density scan
00.2

0.4
0.6
time /s# 517910.8

Plasma performance at high density: edge parameters
NC ---> IC---> detachment:- edge temperatures Tes and,hence, upstream pressures
drop already prior to detachment- edge density nes increases
steeply- drops at detachment

0.7NBI0.61 MW
2 MW
3.5 MW

140NBI2 MW

120

0.5

nes /1020 m-3

0.6

0.4

100

0.4

Tes

80

0.3

60

nes

0.2

40

0.2
0.1

20
000
1

2
3
ne /1020 m-3

401

23ne /1020 m-3

4T es, Tis /eV

nes /1020 m-3

0.8

Divertor plasma regimes
The plasma-target interaction
concentrates mainly at helical stripes
Hα traces
Region D:
rollover and detachmentof energy and particle flux
at high density

Watershed
Probe arrays

Region D

Region A

Region A:
attached spots even athighest density

Divertor plasma regimes: downstream parameters
Peak densities and temperatures
decreasingfrom probe
array
upstream pressure
5Probe arrays70

4

ned topned bott.

60

Ted topTed bott.

50

Region D
Region ANBI2NBIMW

3402 MW

30

220
1
10
0
0.5

1

1.5

2

2.5 3
ne /1020 m-3

3.5

40T ed /eV

ned /1019 m-3

Watershed

Region D:-rollover up to detachmentof ned, Γpd(confirmed by 2D Hα data)- Ted > 2 eV at detachment(from Hα/Hγ)Divertor regimes: downstream parameters
Watershed

Peak densities and temperaturesfrom probe arrayProbe arrays

70npeak

Tpeak

ed

3top target

Region D60

ed

19/10edn1

ned

30/eV40

peak

peak

50

ed

NBI
2 MW

2Region ATm-3

Ted

Region A:- Downstream peak Tedstays above 20 eV---> attached spot

200012ne

320/10-3m

4- inconsistent with low Tes---> inhomogeneous Tes?

Divertor regimes: stable partial detachment#510504Wdia20PNBI

2Pa b sPrad10008

separatrix
position4c)

Q tpeak /MWm-2

030

0.2

5432
1

radiation profile(bolometer)
0.4

0.6

0.8

time /souter peak region Apeaks region D2
1
0
0

0,20,40,6
time /s

0,8

MWm-3

0.1
12- ne = 3 1020 m-3- Te(0) = 400 eV,- Tes = 20 eV (?)- radiated power fraction up
to 80%- radiation peaked at the
separatrix position

ne sne1.0reff /cm

W dia /kJne /1020 m-3 power /MW

Example for stable partial detachment- outer peak in region AattachedDivertor regimes: controlled transition to part. detachment#51321410030624

ne1

ne s

2082
radiation profile(bolometer)

40- edge radiation dominated,- Tes ≈ 15 eV, nes drops- peak of C II radiation shifts incontrolled way towards thex-points.

imaged byspectrometer

31

MWm-3

separatrix
positionnes /1019 m-3

reff /cm

20

P NBI Prad Pabs2012∆z /cm

Wdia- radiated power fraction ≈ 85%

W dia /kJne /1020 m-3 power /MW

Controlled transition to ‚stronger‘ partialdetachment••

4800.2C II
5140 A
0.40.6time /s

0.8

Divertor regimes: reduction of thermal load

Thermal load patterns on target, discharge #51321(3D calculation from thermography)t = 0.35 s, attachedt = 0.65 s,partially detached
Region A
attached

watershed

atershed

Region D
detached

Divertor regimes: asymmetries
Neutral pressures inside divertor
subvolume show strong up/down
asymmetry.

- Asymmetry may even invert,- radiated power fraction is anordering parameterptop

3nee

00.2

1-3bott1pm

p202/1020NBI, 2 MW
NBI, 1 MW

1.4
pbott/ptop

3nneutral pressure p /10

1.8#51321

-3

mbar

4

10.6

00.2

torus

0.4
0.6
time /s

0.8

00.2

0.4
0.6
Prad/PNBIabs

0.8

1Operational ranges and density limit
Variation of a pl and ∆x bycontrol coil currents

separation ∆x between x-point and target /cm
3.9

3.2

2.4

ne /1020 m-3

part. detachment

RO

1.5

0.3

radiative collapse

NBI, 2 MW:- Access to IC independentof ∆x,- partial detachment at∆x ≥ 2.4 cm

improved confinement 20.14
stationary
17.

15.1

610.04normal confinement
unstationary- partial detachment extends
accessible density range

7.55

density limit SUDO et al.
plasma minor radius /cm

Operational ranges and density limit

toroidally averaged surfaces
and fit by κ and δκ = 2.0
δ = 0.3

2aRIeq1= 2 2 /[1 + κ 2 (1 + 2δ 2 − 1.2δ 3 )]qc
5a B

nG = κ J = Ieq/πa2

Summary- Access to a new NBI, high-density regime (up to ne ≈ 3.5x 1020 m-3)
with high τE and low τp and τimp
- Full density control already without Ti-gettering,- Quasi-steady state operation also including partial detachment,- Edge-dominated radiation, radiated power fractions are low tomoderate in attached and high (up to 90%) in detached regimes- Detachment is partial: it does not extend over the full target area,
and the particle and energy fluxes stay finite (Ted > 2 eV)- Improved confinement in all separatrix-bounded configurations,- Stable part. detachment restricts to configurations with larger
distance between x-points and targets (divertor configurations).