Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, 2011 - Newport 1 Mitigation of

Plasma FacingPFAAntenna Group

POLITECNICO DI TORINO

19 Topical Conference on Radio Frequency Power in Plasmas

June 1-3, 2011 - Newport

1

Mitigation of parallel RF potentials by an

appropriate antenna design using TOPICA

R. Maggiora and D. Milanesio

Special thanks to ENEA. ASDEX Upgrade and CYCLE teams





1. Motivations and background

2. ITER-like IC antenna

3. ASDEX Upgrade IC antenna

4. Plasma sensitivity

5. Conclusions

2

Outline





The successful design of an IC antenna is based not only on the capability to deliver enough power to the plasma (feature that has been extensively investigated in recent years), but also on the reduction of those unwanted phenomena, such as rectification discharges or hot spots, that are naturally associated with the required power levels.

In these conditions, an accurate design tool such as TOPICA, i.e. able to account for a realistic 3D antenna geometry and an accurate plasma model, is essential!

3

Motivations and background





4

Outline





5. Conclusions





5

TOPICA modeling: reference geometry

• Constant working frequency: 53MHz

• Full horizontal septa

• Constant plasma profile: Sc2short (~17cm SOL), Bangle=15°

• “V-shaped” curvature in the poloidal direction only

• Electric field computed at the plasma edge, located at constant 5mm distance from the FS

• 2cm gap all around the antenna box to account for the grounding, located 1m in the back

• Constant 4cm distance between the straps and the plasma edge

• Realistic ITER-like antenna geometry based on 2010 TOPICA model, not tilted FS





6

TOPICA modeling: retracted horizontal septa

• Constant working frequency: 53MHz





• Fully retracted (4cm) top and bottom horizontal septa

• Realistic ITER-like antenna geometry based on 2010 TOPICA model, not tilted FS






7

Top and bottom horizontal septa have been retracted of 4cm respect to the their original position in the reference geometry

The horizontal central septum has not been moved at this stage Reference

geometry

TOPICA modeling: retracted horizontal septa





8

RF potentials are shown 5mm in front of FS bars

Poloidal phasing: 0π

20MW coupled to plasmaTilted magnetic field lines

(15°)dlE //

RF potentials: retracted horizontal septa

Lower horizontal septum

RF potentials strongly depend on the input

phasing. Visible reduction localized on the magnetic lines crossing the upper

and lower horizontal septa





9• Constant working frequency: 53MHz

• Full horizontal septa





• Realistic ITER-like antenna geometry based on 2010 TOPICA model, tilted FS according to magnetic field lines

TOPICA modeling: tilted FS bars






10




(15°)dlE //

RF potentials: tilted FS bars

Remarkable reduction of RF potentials on the

entire poloidal section

Power coupled to plasma is 15% lower





11




(15°)dlE //

RF potentials: tilted FS bars & fields misalignment

Field misalignment could determine, depending on the poloidal position, a

not negligible increase of the RF potentials





12




(15°)dlE //

RF potentials: tilted antenna

The alignment of the whole antenna to B

(instead of the FS alone) remarkably reduces the RF potentials for 0ππ0 toroidal phasing, i.e. in

case of symmetrical feeding





13

Outline





5. Conclusions





14

TOPICA modeling: reference geometry

• Constant working frequency: 30MHz• Two plasma profiles: a small gap and a large gap measured in 2010 campains

• Electric field computed 3mm in front of the limiters

• Constant 4.5cm distance between the straps and the plasma edge

• Approximated flat model based on the “reference” ASDEX Upgrade IC antenna recently installed





15

The “broader limiter” approach

Radialplates

Return currents are induced on the two lateral radial plates and then screened by

the FS barsTapered straps

Broader

limiters





16

Reference vs. Broader Limiter

Reference antennaBL antenna

Plasma1 (small gap)

Plasma2 (large gap)


(11°)

Average reduction (global)Plasma1 : 1.85Plasma2 : 1.59

Average reduction (local)Plasma1 : 2.09Plasma2 : 1.88

Peak reductionPlasma1 : 1.77Plasma2 : 2.56

dlE //





17

An alternative approach

The basic concept is to have a two-layers recess to protect the back connections to the straps and to exploit all the possible

symmetries in the plasma facing components.

Two-layers concept

Broader limiter

Recessed horizontal limiter (to FS level)

Radial plate

Tapered simpler straps





An alternative approach

Reference antennaBL antennaAlternative design

Plasma1 (small gap)

Plasma2 (large gap)

Substantial RF potential reduction despite slightly higher absolute electric

field.

Average reduction (global)Plasma1 : 2.89 (1.85)Plasma2 : 2.63 (1.59)

Average reduction (local)Plasma1 : 4.20 (2.09)Plasma2 : 3.65 (1.88)

Peak reductionPlasma1 : 18.39 (1.77)Plasma2 : 7.63 (2.56)

BL antenna


(11°)dlE //

18





To be continued …Are flat models a reliable approximation of the real

curved geometries, both in terms of coupled power and RF potentials?

Milanesio D. et Al. - “Analysis of the impact of antenna and plasma models on RF potentials evaluation”

poster session B31

Do the differences between flat models hold true also for curved geometries?

Krivska A. et Al. - “Density profile sensitivity study of ASDEX Upgrade ICRF Antennas with the

TOPICA code”poster session A55

Are 3 straps or 4 straps geometries better than the 2

straps solutions?

Come and check it at the poster session!









5. Conclusions

20

Outline





21

Plasma sensitivity

Antenna

position

Cutoff density

The plasma sensitivity is tested for a number of plasma profiles (varying the density gradient

and the antenna-cutoff distance) both in terms of power transferred to plasma and of RF

potentials

Antenna

position

Cutoff density





22

Plasma sensitivity

Antenna

position

Cutoff density



potentials

Antenna

position

Cutoff density

2cm vacuum

layer





23

Plasma sensitivity

Antenna

position

Cutoff density



potentials

Antenna

position

Cutoff density

4cm vacuum

layer





24

Plasma sensitivity

Antenna

position

Cutoff density



potentials

Antenna

position

Cutoff density

4cm vacuum

layer





25

Plasma sensitivity: density gradient

25

Provided the same antenna-cutoff distance, a negligible dependence on the density

gradient is observed

dlE //


(11°)

Test1a

Test2a

Test3a

No vacuum

2cm vacuum

4cm vacuum





26

Plasma sensitivity: antenna-cutoff distance

26

Provided the same density gradient, the RF potentials

notably increase with a vacuum layer insertion

dlE //


(11°)

Test1a

Test2a

Test3aNo vacuum

4cm vacuum





The RF potentials are considerably different if, instead of adding a

vacuum layer, a full plasma profile is loaded


(11°)

Vacuum vs. full plasma profile

Antenna

positionCutoff

density

dlE //

Effect of the slow wave…





Antenna

position

Cutoff density

28

Plasma sensitivity: edge density

28

dlE //


(11°)

Provided the same core-cutoff density profile, the increase of the

edge density determines an RF potentials reduction









5. Conclusions

29

Outline





• Localized modifications of the geometry can be rather effective in terms of RF potentials mitigation. As a consequence, the precise knowledge of the antenna geometry (in particular of the front part) is mandatory to carefully predict the electric field distribution in front of it and, therefore, to compute the RF potentials.

• At least two approaches can be pursued to reduce RF potentials, i.e. to lower the electric fields absolute value and to reduce geometrical asymmetries.

• A plasma loading should be adopted in order to be as realistic as possible. Moreover, a parametric study of the plasma profile (above all of the SOL) is extremely important to provide accurate predictions.

Is there a perfect antenna?

• Retracted horizontal septa • A Bangle tilted antenna and FS• A toroidally symmetric antenna• Additional feeding lines with phase and amplitude flexibility

30

Conclusions





31





32

Reference vs. Broader Limiter

Reference antennaBL antenna

ε=81+2400i

Plasma1 (small gap)

Plasma2 (large gap)


(11°)

Average reduction (global)Dielectric : 2.19Plasma1 : 1.85Plasma2 : 1.59

Average reduction (local)Dielectric : 2.89 Plasma1 : 2.09Plasma2 : 1.88

Peak reductionDielectric : 1.71Plasma1 : 1.77Plasma2 : 2.56

dlE //





An alternative approachdlE // dlEabs //


(11°)

Reference antennaBL antennaAlternative design

Plasma1 (small gap)

Plasma2 (large gap)

Plasma1 (small gap)

Plasma2 (large gap)

Substantial RF potential reduction despite the higher total

electric field.

Average reduction (global)Plasma1 : 2.89 (1.85)Plasma2 : 2.63 (1.59)

Peak reductionPlasma1 : 18.39 (1.77)Plasma2 : 7.63 (2.56)

BL antenna

Documents

Plasma Facing PFA Antenna Group POLITECNICO DI TORINO 19 Topical Conference on Radio Frequency Power in Plasmas June 1-3, 2011 - Newport 1 Mitigation of