ASDEX Upgrade

Max-Planck-Institutf

..ur Plasmaphysik

Active in-vessel saddle coils at ASDEX Upgradefor MHD control

Wolfgang SuttropO Gruber, D Hahn, A Herrmann, M Rott, U Seidel, B Streibl, T Vierle, D Yadikin, and

the ASDEX Upgrade Team

Max-Planck-Institut fur Plasmaphysik,

Assoziation IPP-EURATOM, D-85748 Garching, Germany

International Collaboration:B Unterberg, O Neubauer —- TEXTOR (FZ Julich)

P Brunsell —- EXTRAP-T2 (KTH Stockholm)

E Gaio, V Toigo —- RFX (Consorzio RFX Padova)

ITPA Pedestal & Edge Topical Group, San Diego 30 April 2008

The proposal

Install into ASDEX Upgrade

• Active in-vessel saddle coils

• Conducting wall elements

Goals:

1. Edge Localised Modes (ELM)suppression

2. Neoclassical Tearing Mode (NTM)rotation control(locked mode avoidance)

3. Resistive Wall Mode (RWM) stabilisation

Bu

A

Bl

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 2

New possibilities with ASDEX Upgrade saddle coils

In-vessel coils→ fast response (AC operation)→ low coil current needed (≤ 5 kAt)

Three poloidal coil sets→ flexible m spectrumtest importance of resonances for ELM suppression

Eight toroidal coils→ up to n = 4 (avoid core islands)→ quasi-continuous phase variation for n ≤ 3

Goal:Maximum flexibility to assessITER saddle coils and study physics

1.00 2.50-1.59

-1.06

-0.53

0.00

0.53

1.06

1.59

R[m]

z[m]

1.30 1.60 1.90 2.20

PSL

PSL

Avacuumport

upper divertor

lower divertor

Bu

A

Bl protection limitercontour

high fieldside

low fieldside

ASDEX Upgrade

0.70

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 3

Moderate coil current needed for edge ergodisation

Pedestal island “overlap” below 250 At (zero plasma rotation)

n=3, I = 0.24 kA

n=4, I = 0.2 kA, +/-

normalised poloidal flux1.00.8 0.85 0.9 0.95

3

0

1

2

Chi

rikov

par

amet

erσ

|q|=4 |q|=5 |q|=6

max

“Chirikov” parameter[Chirikov 1979]

σ =

ψmax−ψmin

ψqin −ψqout

Radial field lineexcursions:GOURDON code

σ threshold?Poincare plot, Lyapunovexponent: → σcrit = 1at onset of stochasticfield line diffusion

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 4

Core island size reduced with n = 4

n=4, I = 0.2 kA, +/-

normalised poloidal flux1.00.80.7 0.90.6

0.25

0

1

0.75

Fie

ld li

ne e

xcur

sion

[cm

]

|q|=4 |q|=5|q|=3

n=3, I = 0.24 kAmax

0.50.40.30.2

|q|=2

0.5

|q|=5/2

|q|=3/2

|q|=7/3|q|=8/3

|q|=11/4

|q|=5/3

|q|=9/4

Low m components:Parasitic core islands

Unwanted side effects:

• Confinementreduction

• NTM seeding

(Figure:GOURDONfield line tracing,no rotational shielding)

Core island width ( ≈ 2∆R) significantly reduced for high n

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Controlled resonance by coil phasing

For n = 4, four phasings (Bu-A / A-Bl): +/+ +/- -/+ -/-

m spectrum (fixed n = 4) as a function of radius (ψ) [Becoulet et al. 2005, Burrell et al. 2005]

Resonant

n = 4 plus-minus phasing

Not resonant

n = 4, minus-minus phasing

poloidal mode number m

norm

alis

ed p

oloi

dal f

lux

norm

alis

ed p

oloi

dal f

lux

log (Br)log (Br)

+/- phasing -/- phasingn=4

poloidal mode number m

+

+

+

-

-

+

+

+

-

-

n=4

q profile ofAUG 17151t=3.85 s

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 6

Locked mode disruption avoidance

Locked mode disruption:

• Saturated (3/2) NTM

• (2/1) NTM grows(coupled to 3/2) andslows down

• Mode rotation dropsbelow ≈ 1 kHz→ locking, fast growthand disruption

⇒ Rotating error field withf ≥ 1 kHz can avoiddisruption and give controlsystem time to react

0.0

0.5

1.0

1.5

2.0

MA

, a.u

.

3.6 3.7 3.8 3.9

time [s]

0

a.u.

m=2, n=1 m=3 n=2Bθ

4.0 4.1 4.2

D α

Ip

3/2

2/11 kHz

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 7

With conducting wall: RWM stabilisation

Vacuum vessel far from plasma onlow field side

Passive stabilising loop (PSL)reduces vertical growth rate (n = 0)(opposite current direction in upper and

lower branches → radial field)

Extend with conducting wall elementsbetween PSL branches→ allow helical currents

Holes for diagnostics and heating:3D structure

Electromagnetic surface model

˜ 9000 nodes, ˜ 19000 elements

ICRH

ICRH

ICRHICRH

upper PSL

lower PSL

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 8

Physics requirements

Reference point: 10 cm in front of coil (≈ plasma boundary)

Ergodisation by resonant magnetic perturbation: σ = 3 for Bn = 0.6 mTAllow for factor 10 to accommodate shielding by plasma rotation.

RWM control requires bandwidth (thumb rule: f3dB ≥ 40× γ) and phase margin.Field amplitude depends on noise level - RWM signal must exceed background.

Mode rotation ( fmax = 3 kHz) will be done by midplane (A-) coils

Physics parametersQuantity Symbol Value Units Conditions

DC normal field min. Bn 6 mT f = 0, n = 2,3,4AC normal field min. Bn 1 mT f = 500 HzPhase lag of field max. ΦBn−V 0 . . . -150 degrees

→ Verify coil performance in presence of passive conductors (coil housing, PSL)

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 9

Technical requirements

Operational limits (individual coil)Quantity Symbol Value Units Conditions

Number of turns 5Peak coil voltage max. Vcoil 500 V f = 1 kHzDC coil current max. Icoil,DC 1 kA f = 0Operation pulse duration min. tpulse 6 sCool down time max. tpause 15 min.

Isolation voltage min. Vi 3 kV 100% testedHousing temperature max. Th,max 180 0C Icoil = 0Housing temperature max. Th,max 90 0C Icoil 6= 0

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 10

B-coil design - similar to proven W7-X control coils

• 5 turns, copper with cooling channel

• glass fabric wound around coils

• isolation: epoxy cast

• vacuum tight enclosure:1.2 mm Inconel sheet with stiffening ribs

Bu coilupperPSL

A-coil

Conductingwallcontour

Bl-coillowerPSL

outer divertor module

torusaxis

midplane

Mounted on upper and lower PSL

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 11

Electromagnetic model verifies AC capability of coils

“B”-coils : close to PSL, metal casing→ shielding by eddy currents

|Bn| at 10 cm distance to coil (QuickField)

PSL

B-coil

normaldirection

Bn,peak at Icoil,peak = 1 kA

1

10

1 10 100 1000 10000

frequency [Hz]

mag

netic

indu

ctio

n [m

T]

Bu-coilsd=10 mm

Bl-coilsd=30 mm

Phase Bn wrt. Icoil

-45

0

phas

eΦ

Bn-

I[d

egre

es]

1 10 100 1000 10000frequency [Hz]

Bu-coilsd=10 mm

Bl-coilsd=30 mm

W Suttrop et al In-vessel saddle coils at ASDEX Upgrade for MHD control 12

Time schedule

Stage Hardware component Experiments Installation

1 4 upper + 4 lower coils ELM control , n = 2 2009+ 4 upper + 4 lower coils n = 4 2010

2 + 8 midplane coils Four n = 4 2011configurations

3 12 AC power supplies Mode rotation 2012(odd or even n) n = 3 ELM control

4 Conducting wall RWM stabilisation 2012sensors, controller

5 + 12 AC full voltage 2013(option) power supplies max. frequency

simultaneous oddand even n

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Summary: In-vessel coils for MHD control

• A set of 3×8 in-vessel coils is proposed for ASDEX Upgrade

• Most flexible field configuration

⊲ Three coils poloidally to improve m resolution

⊲ n = 4: Core island avoidance

⊲ n = 3: Quasi-continuous phase variation

• ELM suppression experiments

⊲ Investigate physics, in particular resonance condition

⊲ New diagnostics possibilities with rotating error field

⊲ Prepare for ITER, e.g., configuration of ferritic inserts

• Mode rotation control (with AC power supplies)Resistive wall mode feedback stabilisation (with additional conducting shell)

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References

[Chirikov 1979] CHIRIKOV, B. V., Phys. Rep. 52 (1979) 263.

[Becoulet et al. 2005] BECOULET, M. et al., Nucl. Fusion 45 (2005) 1284.

[Burrell et al. 2005] BURRELL, K. H. et al., Plasma Phys. Controlled Fusion 47 (2005) B37.

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