Choices of ITER antenna concepts

1
Choices of ITER antenna concepts
D. Swain, R. Goulding Oak Ridge National
Laboratory US/Japan/Europe RF Technology
Workshop Amsterdam October 4-5, 2004
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Outline

• Recent changes in the ITER antenna design
• Plasma moving closer to outer wall (gt 12 cm)
• Antenna recessed in port
• Effects on loading of the changes
• Pros and cons of different ELM-tolerant
  antenna/matching concepts
• Load-tolerant antenna
• Hybrid-combiner/splitter

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ITER Loading with new antenna geometry

• New separatrix/limiter/first wall/antenna
  geometry is being considered
• Eliminate port-mounted limiters
• Move separatrix so closest approach to the first
  wall is 120 mm ( gap)
• Recess antenna module completely in port, so
  antenna is protected by first wall around port
• Contour of antenna is the same as first wall
  (planar geometry)
• Need to calculate rf antenna loading with this
  geometry.
• Desire R 4 W/m to keep voltages and electric
  fields within tolerable range (from 2001 DDD).

Note that gap varies with poloidal position
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Calculation procedure first calculate loading
with uniform gap

• Use GLOSI/RANT3D
• Rectilinear antenna with slab plasma
• 3D antenna geometry
• Density profiles n(x) in scrapeoff
• RANT3D antenna model
• 4 current straps
• Front of septa are variable distance behind the
  Faraday shield
• full (0 mm from rear of FS)
• recessed (70 mm from rear of FS)
• Impose uniform current in straps
• Calculate 4 x 4 impedance matrix Z
• Specify antenna strap phasing, then use Z-matrix
  to calculate R
• Use R in transmission-line model to calculate
  voltages and currents needed to deliver required
  power

RANT3D antenna model
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Density profiles were obtained from ITER
International Team

• Two official ITER scenarios were studied
• Scenario 2
• High density
• Burning plasma, Q 10
• Ip 15 MA
• Scenario 4
• Medium density
• Weak negative shear
• Ip 9 MA
• Long-pulse operation
• Density profiles in scrapeoff region calculated
  by Kukushkin (ITER IT)
• Considerable uncertainty in density profile in
  far-scrapeoff region
• Two models
• Straight exponential decay(steep decay)
• Long-tail decay for x gt 0.04 m (gradual decay)

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R depends strongly on antenna phasing

• Four current straps in toroidal direction,
  phasing can be controlled
• Considered three possible antenna phasings
• 0-180-0-180 (0p0p)
• 0-0-180-180 (00pp)
• 0-90-180-270 (90)

Power spectrum vs. nz ( ckz/w) for Scenario 2
plasmas with straight exponential decay
R is proportional to area under the curve for
each phasing
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Results for uniform gap are promising

• R(gap) exhibits exponential decay
• For 00pp and 90 phasing
• Loading may be OK
• 0p0p phasing has very poor loading

Steep density decay, full septum
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Recessing the septa increases the loading
significantly

• Septa moved 70 mm back from rear of Faraday
  shield
• Loading increases significantly for 90 and 00pp
  phasings
• Inter-strap coupling also increases significantly

Steep density decay, recessed septum
R for 10-cm gap
No Phase Recess Recess Factor 90 6.35 8.26 1.30
00pp 6.81 9.30 1.37 0p0p 5.10 5.30 1.04
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Gradual density decay increases loading compared
to steep decay

• R for 00pp and 90 phasing increases by 22
• R for 0p0p phasing increases by 12

• Gradual density decay, recessed septum

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Calculation of loading with varying gap

• Use local loading approximation to calculate
  effective R with variable gap

Separatrix to first wall distance vs. vertical
position
Avg. gap 136 mm over antenna
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Results are adequate at 120 mm minimum gap

• Reff calculated for nominal gap (120 mm minimum)
  and plasma contour
• OK for 00pp phasing
• Marginally OK for 90 phasing (OK with recessed
  septa)
• Not OK for 0p0p phasing
• Critically dependent on density profile - loading
  for gradual decay profile much better

R for different antenna phasings, geometries,
and density profiles for Scenario 4
Gradual decay, recessed septum Steep decay,
recessed septum Steep density decay, full septum
Min. value needed
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What happens when minimum gap is changed?

• With 120 mm minimum gap, can meet criteria for
  00pp and (usually) 90
• 90 (for current drive) has lower loading than
  00pp
• Recessed septa increases loading
• Density profile has a large effect
• Not clear if septa can be recessed
• May have to extend to Faraday shield for
  mechanical strength

Reff vs. minimum gap for 90 phasing
Reff vs. minimum gap for 00pp phasing
Gradual decay, recessed septum Steep decay,
recessed septum Steep density decay, full septum
Gradual decay, recessed septum Steep decay,
recessed septum Steep density decay, full septum
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Questions on antenna design

• Is new first wall/separatrix configuration OK for
  ICRF system?
• Looks very promising for ICRF antenna
• Loading adequate, except for 0p0p (is this a
  problem?)
• Antenna protected by first wall
• Should we use recessed antenna septa?
• No recess for septa
• Loading (Reff) marginally adequate in some cases
• Can provide more mechanical support for Faraday
  shield
• Reduces inter-strap coupling so may be better for
  load-tolerant circuits
• Recessed septa
• Increases Reff by 30,
• Increases inter-strap coupling so may be hard on
  load-tolerant concept
• Mechanical support of FS may be needed

14
Decisions and options for handling varying plasma
loads

• How to deal with varying plasma loads?
• Conjugate-T load-tolerant design?
• Keeps full power going to plasma (if ELM doesnt
  cause breakdown)
• Phase shift between top and bottom strap segments
  may degrade power deposition (needs calculation,
  maybe experiment?)
• Implementation requires lots of hardware inside
  or near machine
• Internal concept lowers voltage in vacuum
  transmission line
• Protects transmitter well
• Hybrid splitter (ELM dump)?
• Power to plasma decreases during ELMs
• No phase shift during ELMs
• Simple antenna near machine, but high voltage in
  vacuum trans. line
• Protects transmitter well
• Other?
• Always looking for good ideas