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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:
ICRF system and diagnosticson ASDEX Upgrade
0
2
1PNBI = 5 MW
012
#22795
shunt on limiter
PICRF [MW] P12 P34
IDC [A]
RF power, E|| fields
loss of electrons
high rectified voltages on field lines connected to antenna
ICRF system and diagnosticson ASDEX Upgrade
0
2
1PNBI = 5 MW
012
0100
200
#22795
Characterization of the problem using diagnostics in AUG:
Vfl [V]
shunt on limiter
Langmuir probe
PICRF [MW] P12 P34
IDC [A]
RF power, E|| fields
loss of electrons
high rectified voltages on field lines connected to antenna
stronger erosion on PFCs (limiters)
ICRF system and diagnosticson ASDEX Upgrade
0
2
1PNBI = 5 MW
012
0100
200
0.01
0.1
1
#22795
Characterization of the problem using diagnostics in AUG:
YW [10-4]
Vfl [V]
shunt on limiter
Langmuir probe
spectroscopy(local signal)
PICRF [MW] P12 P34
IDC [A]
RF power, E|| fields
loss of electrons
high rectified voltages on field lines connected to antenna
stronger erosion on PFCs (limiters)
increase of W concentration
ICRF system and diagnosticson ASDEX Upgrade
0
2
1PNBI = 5 MW
012
0100
200
10.5
5
#22795
Characterization of the problem using diagnostics in AUG:
0.01
0.1
1YW [10-4]
CW at Te=1 keV [10-5]
Vfl [V]
shunt on limiter
Langmuir probe
spectroscopy(local signal)
W spectroscopy(global signal)
PICRF [MW] P12 P34
YW [10-4]
Prad [MW]
CW at Te=1 keV [10-5]
Vfl [V]
IDC [A]
RF power, E|| fields
loss of electrons
high rectified voltages on field lines connected to antennas
stronger erosion on PFCs (limiters)
increase of radiationPrad close to PICRF
increase of W concentration
shunt on limiter
Langmuir probe
spectroscopy(local signal)
W spectroscopy(global signal)
bolometer
ICRF system and diagnosticson ASDEX Upgrade
0
2
1PNBI = 5 MW
PICRF [MW]
012
0100
200
10.5
5
2.53.5
4.5
P12 P34#22795
Characterization of the problem using diagnostics in AUG:
0.01
0.1
1
Introduction
ICRF system and diagnostics on ASDEX Upgrade
Operational possibilities to reduce W source
Experiment and near-fields calculations
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]
Operational possibilities to reduce W source during ICRF
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°
Operational possibilities to reduce W source during ICRF
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°
~
Operational possibilities to reduce W source during ICRF
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
Operational possibilities to reduce W source during ICRF
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
Operational possibilities to reduce W source during ICRF
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
ICRF system and diagnostics on ASDEX Upgrade
Operational possibilities to reduce W source
Experiment and near-fields calculations
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
Experiment and near-field calculations
“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
Experiment and near-field calculations
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
Antenna 4: Antenna 3:
Experiment and near-field calculations
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
Antenna 4: Antenna 3:
Experiment and near-field calculations
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
Experiment and near-field calculations
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
Experiment and near-field calculations
V||= E|| dl
Antenna 4: Antenna 3:
Fields line types have similar connection lengths
Antenna 3 diagnosticsused to characterize antenna 4
Only antenna 4 on
Experiment and near-field calculations
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
Experiment and near-field 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]