1/10CCFW 34, Garching, June 24th TORE SUPRA Association Euratom-Cea S. BRÉMOND A few very simple...

1/10CCFW 34, Garching, June 24th

TORE SUPRAAssociationEuratom-Cea

S. BRÉMOND

A few very simple technical elementson the internal / external matching discussion

of ITER IC Array

S. BREMOND, G. BOSIA, B. BEAUMONT & TS ICRF Team



S. BRÉMOND

Main ITER issues for IC antenna system*

- High power density (~ 10 MW / m2 required)RF voltage stand-off margins

- Matching + load tolerance (ELMs)

- Components reliability

- Overall technical performance and margins

* we need a grid to compare solutions



S. BRÉMOND

RF Voltage stand-off 1/3

2

2

1,~

,,

VGPe

jBGYjXRZ

j

RF Voltage RF Electric field

Strap to be minimized by antenna front geometrical design

Feeder

Min vswr for

(strip line)

Emax depends above all on precise geometry (strip line to coaxial line transition)

Un -matched VTL

(external)

Min at Zc = 60

1f),l(f

lLw

RG a

a

2'a

'a

A aA

AA lwL

R

PV '

'

2

2

2

1

)2cos(212

A

FAAAfF

lPZV

vswrPZV fF 2max

Line models

(Low losses, short straps)

AF

A

A

VV

strapshortX

0

)(0

2

2

1

)2cos(212

F

VTLFFFVTLVTL

lPZV

strap

feeder

Unmatched Vacuum Transmission Line (ext)or lumped capacitor (int)

AF

a'a

'a

'a l,C,L,R )

(min,

geometrytodue

llZ fffVTLVTL lZ ,

V, J

FVTL

F

F

VV

feedershortX

0

)(0

vswrPZV VTLVTL 2max

60Z

60Zexp

r

VE

c

c

ext

max

)coaxialmax(

fFf Z

vswrVdZand

d

VE

max

Af ZZ



S. BRÉMOND

RF electric field: more freedom to adjust geometry in internal matching up to 1.4 (ext.) > 1.1 kV/mm (int.)


RF Voltage: Vmax (ext.) (~ 36 kV) > Vmax (int.) (~ 28 kV) + maximum RF voltage point depending on frequency

RF Current : up to 1.8 kA (ext.) > 1.1 kA (int.) High RF current (and voltage) at vacuum window

FV J V JV J FF



S. BRÉMOND

Impedance jumps and non TEM modes (external matching)

Non TEM modes at impedance jump (while evanescent, still increase local RF field) by about 40% (from a first estimate)




S. BRÉMOND

1. External matching cannot apply conjuguate T principle; matching algorithm becomes an issue

Matching 1/2

External matching: no more perfect cancellation of admittance imaginary part (which is the bigger) at junction when starting from matching at base loading (without Elms)

- theoretical issue: optimisation to be performed to find best trade-off (no more perfect match at base loading) and thus complicated tuning procedure required- operational issue: no more possible to find matching experimentally at base loading

G=0.5

C1

C2

RA

End of strap + feeder

T branche 1

T branche 2

G=0.5 RA

l1

l2

End of strap + feeder

T branche 2

T branche 1

Internal matching External matching



S. BRÉMOND

Matching 2/2

2. Uncontrolled power sharing is very likely to reduce power handling capabilities

1st VTL

2nd VTL

1st VTL

2nd VTL

1st VTL

2nd VTL

1st VTL

2nd VTL

1st VTL

2nd VTL

1st VTL

2nd VTL

2

TTT VG2

1P

3,2,13,2,1

- Any asymmetry between branches (plasma loading, poloïdal mutual coupling between straps, VTLs, 4-port passive junction, etc…) will lead to power unbalance with no means of correcting it- Effect of unavoidable asymmetry can not be minimise because depending on frequency (junction to be made at voltage anti-node)- Effect on tuning procedure to be documented (high sensitivity expected)



S. BRÉMOND

Components reliability

External matching option

1. Unusual 4 ports passive junction to be developed an qualifiedDeveloping a compact broadband three way power divider compatible

with ITER environment would certainly required much effort, design work, mock-up validation, let alone manufacturing difficulties for making the real thing.

2. Vacuum window (primary safety barrier) is in an unmatched section- it has to withstand higher RF current / voltage (typically 35 kV / 1.6 kA

depending on frequency) than in the internal option (typically 8 kV / 0.4 kA) - The risk that an arc in the VTL drift to the window is enhanced

3. CW high power phase shifter are to be developed

Internal matching option

Enhancement of existing commercial units to be performed in order to make them compatible with tokamak and ITER environment



S. BRÉMOND

Miscellaneous

- power efficiency (RF losses)From strap to T junction, typically twice as high in the external matching option (~170 kW instead of 85 kW)

- overall simplicitymany components in the unmatched sections and many optimisations required for the external matching option

- absence of in-vessel remotely operated component in external matching option: no significant gain in time for access and repair with external adjustable components (2 weeks cooled down required)

- overall technical marginsPossible hazard in the course of ITER experience (e.g. lower loading than expected) would hit:- many components technical margins in the external matching option (due to the very large resonant area, e.g. vacuum window which is a critical safety component)- mainly lumped capacitors technical margins in the internal matching option (large existing technical margins on upstream components)



S. BRÉMOND

Tentative conclusion

- Internal matching option with lumped capacitors is a well known scheme for which the issues are and will be even better documented (after JET-EP and TS ITER-like experience). Efforts have to be made on the enhancement of one particular component (capacitors) starting from existing commercial units to be adapted to ITER environment.

- External matching proposed option seems to have lower technical margins (higher RF Voltage), raises new and tricky issues, requires many component development and qualification, without being grounded on existing experiences.

1/10CCFW 34, Garching, June 24th TORE SUPRA Association Euratom-Cea S. BRÉMOND A few very simple...

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