A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 1

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Influence of Toroidal and Vertical Magnetic Fields on Ion Cyclotron Wall Conditioning

in TokamaksPresented by A.Lyssoivan LPP-ERM/KMS, Brussels

With contribution fromG.Sergienko, V.Rohde, V.Philipps, G.Van Wassenhove, M.Vervier, V.Bobkov, J.Harhausen, R.Koch, J.-M.Noterdaeme, D.Van Eester, M.Freisinger, H.-U.Fahrbach, H.Reimer, A.Kreter, D.A.Hartmann, J.Hu, R.Weynants, O.Gruber, A.Herrmann, D.Douai, Y.D.Bae, H.G.Esser, J.G.Kwak, E.Lerche, O.Marchuk, V.Mertens, R.Neu, U.Samm, A.Scarabosio, C.Schulz, S.J.Wang,

TEXTOR Team and ASDEX Upgrade Team

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 2

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Outline

• Motivation

• ICRF Plasma / Antenna Coupling Characterization

• ICWC in TEXTOR and ASDEX Upgrade

• ICWC Extrapolation to ITER

• Conclusions

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 3

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Motivation

ICRF discharge has a high potential for wall conditioning (tritium retention, surface isotope exchange, wall cleaning/coating) in the presence of permanent high magnetic field.

Ion Cyclotron Wall Conditioning (ICWC) was approved for integration into the ITER baseline using ITER ICRF heating system.

Further development of the ITER relevant ICWC scenarios with conventional ICRF antennas is an important and urgent task.

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 4

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Plasma Production with Standard ICRF Antennas

TEXTORICRF antennas

f=25-38 MHz, BT=0.25-2.5 T, p=(1-10 )10-2 Pa

AUGICRF antennas

f=30.0; 36.5 MHz, BT=1.0-2.4 T, p=(1-8 )10-2 Pa

RF Field/Wavesexcitation

RF Powere-collisional absorption

Neutral Gase-collisional ionization

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 5

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ICWC Optimization

ICRF Plasma Production Removal Mechanisms

AntennaCoupling

GRFplRF PP

PlasmaHomogeneity/ Extension

Fast IonsGeneration

1. High Ion Cyclotron Harmonics, =nci, n>>12. Mode Conversion, = ci

BT+BV, BV<<BTFundamental

Ion Cyclotron Resonance = ci

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 6

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TEXTOR: ICRF Plasma Characterization

ICRF plasma can be produced at any BT-field

=10cH+ (BT0.2 T): High coupling (0.8), density (>21017 m-

3) and homogeneity

=cH+ (BT2.3 T): improved coupling (0.5) and homogeneity

ne, Te and Ppl vs BT

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 7

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AUG: ICRF Plasma Characterization

Mode conversion scenario in (He+H2)-plasmas:

Higher antenna coupling (up to 3 times)

Better homogeneity and extension in radial direction

Better performance at two frequencies

(He+H2)-plasma vs He-plasma

BT+BV vs BT

Vertical magnetic field improves plasma homogeneity in poloidal dirction and extends it towards divertor

He, f=30 MHz He+H2, f=30 MHz He+H2, f1=30 MHz+f2=36.5 MHz

I II III

BT BT

BV

BT=2.4 T, BV=0 BT=2.4 T, BV0.02 T

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 8

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ICWC in TEXTOR(C-coated wall)

=10cH+ (BT0.2 T): Effective conditioning due to high antenna coupling and homogeneity possible in both, low and high the BT-fields

=cH+ (BT2.3 T): Mode conversion in (He+H2)-plasmas is the best scenario for ICWC (coupling + homogeneity + fast particles)

Applied BV-field (BV << BT) increased ICWC yield

Removal rate: spdtdpVtQRR )/(~)(

Measured removal rate for m=3 vs BT Calculated absorbed power vs BT

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60

1

2

3

4

5

6

7TEXTOR- (He+H2)-plasma +Bv

- (He+H2)-plasma

- He-plasma

HD

Rem

oval

Rat

e[1

0

mol

/(skW

)]16

BT [ T

0

20

40

60

80

100

cH+ cH+

PePH+

cH+

Mode Conversion

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6BT [ T ]

TEXTOR

P abs/

spec

ies

/PR

F-t

see G.Sergienko, P2-45, 27/05/2008

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 9

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ICWC in ASDEX Upgrade (W-coated wall)

Benefit from mode conversion in (He+H2)-mixture with ICR (=cH+) location closer to the antenna ICWC output correlates with fast particles energy and power absorbed by protons BV-field improves the ICWC effect Major concern – ICWC homogeneity (efficient cleaning from ~25% of the AUG surface)

Measured removal rate for m=40 vs BT Fast particles energy/power vs BT

0

1.0

2.0

3.0

4.0

5.0

1.6 1.8 2.0 2.42.2

Ar

Re

mo

val

Ra

te [

a.u

.]

AUG- ICWC in (He+H2) + Bv- ICWC in (He+H2)- He-DC

BT [T ]1.6 1.8 2.0 2.2 2.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

BT [T ]

0.1

0.2

0.3

AUG

P /P [ r.u.]H+ RF-totH-atom averaged energy[ keV ]

< E > experimentH

P / P modelRF-totH

experiment

modeling

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 10

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ICWC Extrapolation to ITER:scenario for operation

0.32 m

row 1 & row 2: /3, f=40 MHzrow 3 & row 4: /6, f=48 MHz

12

34

TOMCAT modeling (rpl2.4 m, R0=6.2 m, BT=3.6 T, ne0=3x1017 m-3, Te0=5 eV):- Mode conversion in (He+H2)-plasmas at two frequencies

-2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.50

0.2

0.4

0.6

0.8

1.0

r [m]

Abs

orbe

d po

wer

-per

-spe

cies

[a

.u.] ITER

cHe4+ cH+

f=40 MHzf=48 MHz

eHe4+H+

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 11

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Modeling with 0-D plasma/transport code

1. 0-D Plasma/Transport code: ne(1-4)1017 m-3, Te~1.5 eV, ioniz=1-2%, p=(2-8)10-2 Pa PRF-pl (ITER)= 0.2-1.5 MW (coupl0.40) PRF-G (ITER)0.5-3.8 MW2. Extrapolation from TEXTOR data (assuming similar power density and

coupl0.40):PRF-pl (TEXTOR) 12-30 kW P RF-pl (ITER) 1.0-2.5 MW P RF-G (ITER) 2.5-6.0 MW

ICWC Extrapolation to ITER:power for operation

0

0.2

0.4

0.6

0.8

1.0

Ee-excitEe-ionizEe-rec

Ee-iEe-confEe-heat

ITER

Time [s]0 0.02 0.04 0.06 0.08 0.1

Ele

ctro

n E

nerg

y L

osse

s [

r.u.

]

excitE1234

2468

10

1012

1014

1016

1018

00.20.40.60.81.0

Time [s]0 0.02 0.04 0.06 0.08 0.1

TeTiTa

nei

0

ITER

nei

n [m ]e -3

P [MW]RF-e

i [%]

T [eV]

A.Lyssoivan – 18PSI, Toledo, Spain 27/05/2008 12

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Conclusions

Inter-machine (TEXTOR, ASDEX Upgrade) ICWC studies:

Wall conditioning in the mode conversion scenario in the presence of toroidal and vertical magnetic fields (BV<<BT) may be considered as the most promising candidate for application in ITER using the main ICRF antenna. Better radial/poloidal homogeneity of the ICRF plasma and its ability to accelerate ions at the fundamental ICR may contribute to improving the conditioning effect.

ICWC at high cyclotron harmonics appears also to be attractive mainly due to very high antenna-plasma coupling (80%) and plasma homogeneity. However, the scenario needs operating at high generator frequencies for the nominal magnetic fields and does not produce fast ions.

Modeling with the 1-D RF and 0-D plasma codes and extrapolation from the existing machines give a good evidence for the feasibility of using ICWC in ITER with the ICRF heating system.