The center column contains plots of evaluated along virtual magneticfield lines which lie on the flux surface as shown.• Typically, ICRF antennas are not operated in monopole phasing due to

deleterious plasma edge effects.• All four phasings show a reduction in for the rotated antenna, and• Monopole phasing provides the largest reduction.

ICRF Antenna Optimization for Alcator C-modM. Garrett, S.J. Wukitch, W. Beck, P. Koert, Y. Lin, R. Parker

*Plasma Science and Fusion Center, MIT, Cambridge, MA, USA

Alcator C-mod rotated design

0 1 2 3 4 5 6 7-0.3

-0.2

-0.1

0

0.1

0.2

0.3

abs(Vll) [0 0]

Potential [kV]

Ave

rag

e y

po

sitio

n [

m]

New 4-strap

Existing 4-strap

0 1 2 3 4 5 6 7-0.3

-0.2

-0.1

0

0.1

0.2

0.3

abs(Vll) [0000]

Potential [kV]

Avera

ge

y p

ositio

n [m

]

New 4-strap

Existing 4-strap

0 1 2 3 4 5 6 7-0.3

-0.2

-0.1

0

0.1

0.2

0.3

abs(Vll) [0 0 ]

Potential [kV]

Ave

rag

e y

po

sitio

n [

m]

New 4-strap

Existing 4-strap

0 1 2 3 4 5 6 7-0.3

-0.2

-0.1

0

0.1

0.2

0.3

abs(Vll) [00 ]

Potential [kV]

Ave

rag

e y

po

sitio

n [

m]

New 4-strap

Existing 4-strap

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

0

0.5

1

1.5

2

[kA

/m]

Tangential |H| Field Tangential |H| FieldEll on flux surface Ell on flux surface

FEM simulationFinite element simulations were conducted for simplified toroidal and slab geometriesfor both four strap and two strap antennas. A new four strap antenna was designed andoptimized based on the existing C-mod four strap antenna to be rotated so that it isaligned with the average magnetic field at the plasma edge. Simulations were carriedout using a lossy isotropic dielectric chosen to mimic observed antenna loading.

Key Results:• Rotating the ICRF antenna such that it is aligned with the equilibrium magnetic

field reduces in front of the antenna by a factor of 2-3.• Monopole phasing [0,0,0,0], which provides optimal magnetic flux coupling to

the plasma, has the lowest contribution of all likely phase combinations.• In simplified slab geometry, removal or slotting of antenna box top and bottom

decreases .

ldE

||

ldE

||

IntroductionA major challenge for ICRF utilization is reducing impurity generation. In Alcator C-Mod,we are designing a new 4-strap antenna where we seek to lower E-parallel. ICRF sheathsare thought to contribute to impurity production in tokamak plasmas [1,2]. Bydecreasing the parallel E-field within and around the antenna box, impurity productionis anticipated to decrease.

• Simulations were conducted to analyze the effect of antenna orientation on theparallel electric field.

• In addition, simulations were carried out on antenna box and limiter structureswith particular emphasis on parallel electric field reduction through imagecurrent modification.

• Antenna septum and limiter thickness, position, and separation were varied. Theeffect of slotting antenna limiters and antenna box walls was also investigated.

Assumptions and simplificationsThe FEM simulations presented here are a simplified model. These simulations canprovide a qualitative description of the relevant electric fields, as long as thesimplifications are considered.

• Antenna is loaded with a graduated lossy dielectric• Model is simplified to include only fundamental geometry to capture relevant

physics• Ell is calculated in vacuum regions• No faraday screen• Radiative boundary condition is used to prevent a standing wave condition

New 4-strap ICRF antenna Existing 4-strap ICRF antenna

0 0.1 0.2 0.3 0.4-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

Vll 0 0- [slice 1]

Potential [kV]

Positio

n [m

]

230-30

230-90

230-150

230-all

360-30

0 0.2 0.4 0.6 0.8 1-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

Vll 0 0- [slice 2]

Potential [kV]

Positi

on [m

]

230-30

230-90

230-150

230-all

360-30

Toriodal rotated antenna study

Largest Ell mitigation for the most efficient flux coupling phasing: [0,0,0,0]

Side limiter: goodTop / Bottom limiter: bad[slice 2, x = (lim + 0.5 cm)]

0 0.5 1 1.5 2-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

Vll 0 0- [slice 2]

Potential [kV]

Po

siti

on

[m

]

nominal

lip

lip sideslot

lip sidecut

sidecut

no TB

none

nominal

sidecutlip

lip sideslot no TB

lip sidecut

Slab limiter slotting: OK[slice 2, x = (lim + 0.5 cm)]

lip

lip sideslot

0 0.5 1 1.5 2-0.35

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

Vll 0 0- [slice 2]

Potential [kV]

Po

sitio

n [m

]

sideslot

lip

topslot 1cm

allslot

topslot

none

allslot

topslot 1cm

topslot

none

230-30

230-90

230-150

230-all

360-30

Tangential magnetic field magnitude [kA/m] is plotted in the outertwo columns above.• ICRF antennas function as magnetic flux coupling structures.• At the antenna midplane [shown above], the H field is similar for both

the existing antenna design and the new rotated antenna.• The plots illustrate the preference for monopole phasing [0,0,0,0]

for optimal flux coupling to the plasma.

The second and fourth columns display E|| [kV/m], plotted on a projectionof a toroidal flux surface shown above.• Rotated antenna has better cancellation of Ell along field lines.• Fields along top and bottom of antenna box lower for rotated antenna.

ldE

||

Field lines

Integrated Ell

Slab limiter studyA simple slab model of the C-mod two strap antennas was used toinvestigate the relative effect of limiter topology on Ell.

• The antenna box was aligned with the magnetic field• Antenna phased [0, ] for all simulations.• was calculated on two y-z planes [slice 1 and slice 2]. Slice 1 is located 0.5 cm

inside the antenna limiter edge [away from the plasma] and slice 2 is located 0.5 cmoutside the antenna limiter edge.

The nomenclature for each configuration is as follows:• “lip” - addition of a 6 cm lip at the top and bottom of the antenna box.• “sidecut” - 6 cm deep cut through the antenna limiters.• “sideslot” - 6 cm deep, 1 cm wide slotting of the antenna limiters.• “no TB” - top and bottom of the antenna box have been removed.• “none” - [shown in next section] the antenna limiters have been removed completely.

Results• increases with antenna side limiter removal [strongly] and,• Also increases with the addition of a lip [weakly] .• However, the removal of the top and bottom of the antenna box

results in a large reduction in integrated Ell.

[1] Bobkov, V., et al, EPS Conf. 32D (2008).

[2] D’ippolito, D.A., et al, Nucl. Fus. 38 10 (1998).

[3] Wukitch, S. J., et al, Plasma Phys. and Cont. Fus. 46, 1479 (2004).

Design is based upon the existing 4-strap antenna• Targeting performance 25% higher in

voltage than the 2-strap antennascurrently in use.

• Reduce boronization erosion and impurity production.

• Improve voltage and power handling.

Antenna general specifications• Operate at 2 MW and 50 kV,• Have pulse length up to 5 seconds at repetition time of 1800 s,• Have thermal loads at plasma limiter of 12 MW/m2 with a 3 mm scrape-off length,• Withstand a disruption load of 1 T/msec at 9 T, • Utilize single horizontal port, and • Have 50-80 MHz range.

Manufacturing has begun• Shown to the right is one of the new rotated straps to

test the manufacturing process.

SummaryFEM toroidal simulations of two and four strap ICRF antennas• Results suggest that a rotated topology (aligned with B) will decrease .• Monopole [0,0] phasing is preferable to couple more magnetic flux to the plasma.• Simulations of the rotated antenna indicate that monopole phasing is the optimal

operating point for both criteria above.

FEM slab simulations of two strap ICRF antenna• Wider antenna limiters reduce integrated Ell.• Removal of material from antenna side limiters increases integrated Ell.• Removal of material from antenna box top and bottom reduces integrated Ell.• Simulations are being done to better quantify Ell, with particular emphasis on

SOL effects at limiter surface.

X

Z

XZ

Y

XZ

Y

The nomenclature for each configuration is the same as in the previous section withthe following additions:• “topslot” - 6 cm deep, 1 cm wide slotting of the antenna box top and bottom.• “allslot” - 6 cm deep, 1 cm wide slotting of all antenna walls.

Results• increases [strongly] with side limiter slotting, and• Scales inversely [strongly] with top and bottom slotting.• Depth of the slotting also makes a difference.

ldE

||ldE

||

ldE

||

ldE

||

The first number of each configuration above represents the +/- z position of the inside wallof the antenna limiter [mm]. The second number represents the limiter thickness [mm].

Results• scales inversely with limiter thickness [strongly], and• With limiter position [weakly].

ldE

||

ldE

||

ldE

||

-50-40-30-20-1001020304050

[kV

/m]

-30

-20

-10

0

10

20

30

[kV

/m]

-30

-20

-10

0

10

20

30

[kV

/m]

-30

-20

-10

0

10

20

30

[kV

/m]

-50-40-30-20-1001020304050

[kV

/m]

-60

-40

-20

0

20

40

60

[kV

/m]

-70

-40

-20

0

20

40

70

[kV

/m]

-100-80-60-40-20020406080100

[kV

/m]

Flux surface for evaluation of Ell lies 0.5 cm inside[toward plasma] the edge of the toroidal antennalimiters.

ldE

||

Model• 3.0 x 106 degrees of freedom• 6.0 x 105 quadratic elements• Run on quad core 3GHz CPU w/ 84GB RAM• >100 elements / wavelength• PIN = 1MW / strap for all simulations

For all toroidal simulations, the models aredeveloped based upon Alcator C-mod geometry• The following shaped

plasma parameters were used:r = 0.22,R0 = 0.68,

= 1.6,= 0.5.

Evaluation flux surface: R = 0.903 m

Antenna strap: R = 0.938

Plasma edge: R = 0.900 m

Antenna limiter: R = 0.908 m

Limiter thickness: wider is better [slice 1, x = (lim - 0.5 cm)][slice 2, x = (lim + 0.5 cm)]

*this work is supported by U.S. DOE, OFES