operation of icrf antennas in a full tungsten environment in asdex upgrade

Operation of ICRF antennas in a full tungsten environment

in ASDEX Upgrade

18th International Conference on Plasma-Surface Interactions, Toledo, Spain, May 26-30, 2008

V. Bobkov, F. Braun, R. Dux, L. Giannone, A. Herrmann, A. Kallenbach, H.-W. Müller, R. Neu, J.-M. Noterdaeme, Th. Pütterich, V. Rohde and ASDEX Upgrade team

Outline

Introduction

ICRF system and diagnostics on ASDEX Upgrade

Operational possibilities to reduce W source

Experiment and near-fields calculations

Conclusions

Introduction

RF E|| (|| to magnetic field) fields are involved

ICRF in full-metal machine is problematic

tolerable concentration of high Z is low (< 510-5)

high sputtering rates at PFCs during ICRF (up to 10-3)

near-fields: antenna and structures connected to antenna along field lines

far-fields: mainly due to bad fast wave central absorption

[S. Wukitch et al., PSI-17]

ASDEX Upgrade: full-W machine, no boronozation 2007 and first half of 2008

electrons react fast and follow E|| fields along field lines

rectified sheath voltages accelerate light impurity ions W sputtering

V||= E|| dl along field lines defines sheath potential drop

electrons are lost plasma charges positively, sheaths form

Potential of a magnetic field line:

V|| in vacuum

plasma (simple, for pi>0)

plasma time-averaged

Introduction

Simplified picture of sputtering due to E||

[Perkins F., Nucl. Fusion 29 4 (1989), 583]

[R. Dux, I-6]

ICRF system and diagnosticson ASDEX Upgrade

Antenna 1

Antenna 2

Antenna 3

Antenna 4

4 antennas with 2 straps each, usually operated in pairs (12) and (34)

Local diagnostics:

shunts on limiters,DC current

Langmuir probes,floating potential

Spectroscopy,sputtering yield YW=W / D,

a measure of V||

IDC [A]

RF power, E|| fields

loss of electrons

Characterization of the problem using diagnostics in AUG:


0

2

1PNBI = 5 MW

012

#22795

shunt on limiter

PICRF [MW] P12 P34

IDC [A]


loss of electrons

high rectified voltages on field lines connected to antenna


0

2

1PNBI = 5 MW

012

0100

200

#22795


Vfl [V]

shunt on limiter

Langmuir probe

PICRF [MW] P12 P34

IDC [A]


loss of electrons


stronger erosion on PFCs (limiters)


0

2

1PNBI = 5 MW

012

0100

200

0.01

0.1

1

#22795


YW [10-4]

Vfl [V]

shunt on limiter

Langmuir probe

spectroscopy(local signal)

PICRF [MW] P12 P34

IDC [A]


loss of electrons



increase of W concentration


0

2

1PNBI = 5 MW

012

0100

200

10.5

5

#22795


0.01

0.1

1YW [10-4]

CW at Te=1 keV [10-5]

Vfl [V]

shunt on limiter

Langmuir probe


W spectroscopy(global signal)

PICRF [MW] P12 P34

YW [10-4]

Prad [MW]

CW at Te=1 keV [10-5]

Vfl [V]

IDC [A]


loss of electrons

high rectified voltages on field lines connected to antennas


increase of radiationPrad close to PICRF

increase of W concentration

shunt on limiter

Langmuir probe


W spectroscopy(global signal)

bolometer


0

2

1PNBI = 5 MW

PICRF [MW]

012

0100

200

10.5

5

2.53.5

4.5

P12 P34#22795


0.01

0.1

1

Introduction




Conclusions

Operational possibilities to reduce W source during ICRF

100

10

1

10

1

2.12

2.16

2.2 2.4 2.6 2.8Time [s]

Rout [m] # 22100# 22098

PICRF=2.0 MW

Rout

Shifting plasma away from antenna:

Constant gas puff rate

edge CW [10-5] at 1 keV

ne E|| Te

At antennas and PFCs connected to antennas along field lines:

YW [10-4]


8

4

1

2.6 2.8 3.0 3.2Time [s]

YW [10-4]

Gas puff rate [1021 s-1] # 22099

PICRF=2.0 MW

Increasing gas puff:

Rout = 2.12 m

10

100

edge CW [10-5] at 1 keV

1

10

ne E|| Te

At antennas and PFCs connected to antennas along field lines:

is likely the important player

50

50

~

~ 3dB

spl

itte

r

3dB

cou

ple

r

Standard 3dB hybrids connections:

RF transmitters

matching

0°

90°

Work with paired antennas, good load tolerance, but with fixed = 90°


antenna 3

antenna 4

50

50

3dB

spl

itte

r

3dB

cou

ple

r

To operate at any , bad load tolerance L-mode discharges

Bypassing 3dB hybrid connections:

~

0°

~


Used also for the experiments with one antenna powered

[°]

90°0

200

400#22925

1.5 2.0 3.0Time [s]

2.5 3.5

270°

90°

1

2

3

1

2

3

208

6


More experiments with moreflexible 3dB-hybrid configurationneeded

Minimum in CW close to 270° (-90 °)is likely due to changes in V||

Optimizing phase between antennas:

CW correlates with YW at antenna 3

At positive netto effect there are locations with high YW (antenna 4)

CW [10-5] at 1 keV

YW [10-4] antenna 3 at Z=0.2

YW [10-4] antenna 4 at Z=0.2


Limits and drawbacks:

1) Shifting plasma away from antenna

2) Increasing gas puff rate

3) Optimizing phase between antennas

All methods are limited and limit operation themselves:antenna design with reduced near-fields needed!

low antenna resistance, voltage stand-off issues, E|| penetrate further away from antennas

high density and worse confinement

visible only in low density discharges

Validation of computational tools for E|| needed

[V. Bobkov et al., AIP Conference Proc. AIP Press Melville NY 933 (2007) 83]

Introduction




Conclusions

Experiment and near-field calculations

0

IRF IRF

“Simple approach”on considering V|| :

V||= E

|| dl

V|| due to RF flux from straps, uncompensatedcontributions in the corners

Approach based on recent codecalculations for E||:

first by: [L. Colas et al., PPCF 49 (2007) B35]

Contribution from box currents to V|| can be significantly larger

Re E||>6<-6 3.6-3.6 -1.2 1.2 [kV/m]

1 MW matched

0

IRF IRF


“Simple approach”on considering V|| :

V||= E

|| dl

V|| due to RF flux from straps, uncompensatedcontributions in the corners

HFSScode

0

IRF IRF

spectroscopic observations

shields to cover corners

Antenna 4: Antenna 3:

From “simple approach” reduction of YW at shields is expected


spectroscopic observations

shields to cover corners

Reduction of YW at shields is expected from “simple approach”

Antenna 3,HFSS:

Antenna 4,HFSS:

Re E||Re E||

B at 11°B at 11°

HFSS code shows no significant difference



No large difference at shields + high YW on antenna 3 edge contributions of box currents important

Antenna comparison:

antenna 3,only antenna 3 on

antenna 4,only antenna 4 on

YW/YWmax

antenna0.0

1.0

0.2

0.4

0.6

0.8

0.4-0.4 -0.2 0.20.0vertical position Z in AUG [m]

normalized to YWmax

to compensate smalltoroidal asymmetry

#22926



Z

ZSpectroscopy LOS spot is broad: various field lines need accounting

Antenna 3:

V||= E|| dl

Relative contribution to V|| varies along Z ( )varying connection lengths and limiter shape

Only antenna 3 on


Z

No conclusive statement possible

vertical position Z in AUG [m]0.4-0.4 -0.2 0.20.0

antenna 3

YW [10-4]

Spectroscopy LOS Spot is broad: various field lines need accounting

Relative contribution to V|| varies along Z ( )varying field line connection length and limiter shape

V||= E|| dl [V]

0200

400600

0.0

1.0

2.0#22926

Antenna 3:

Only antenna 3 on experiment

HFSS calculations


V||= E|| dl


Fields line types have similar connection lengths

Antenna 3 diagnosticsused to characterize antenna 4

Only antenna 4 on


Reasonable agreement between the shapes of YW and V||

0.4-0.4 -0.2 0.20.0

YW [10-4]

antenna 4

Antenna 4:

vertical position Z in AUG [m]

V||= E|| dl [V]

0200

400600

0.0

1.0

2.0#22926

Antenna 3:

Fields line types have similar connection lengths and V|| profiles

Antenna 3 diagnosticsused to characterize antenna 4

Only antenna 4 on experiment

HFSS calculations


Z

Conclusions

Shifting plasma away from PFCs at low field side

During ICRF operation W source can be reduced by:

Increasing gas puff

Improvements can be useful, but better antenna design needed

Optimizing the phase between antennas

Calculations (HFSS code) were validated:

Dominant influence of box currents on E|| confirmed for AUG

Reasonable agreement achieved between shapes of YW profile in experiment and calculated V|| profile

Antenna designs with reduced E|| are in progress [EPS 2008]

operation of icrf antennas in a full tungsten environment in asdex upgrade

Documents

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