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Association
Euratom-Cea
TORE SUPRA
A few very simple technical elements
on the internal / external matching discussion
of ITER IC Array
S. BREMOND, G. BOSIA, B. BEAUMONT & TS ICRF Team
CCFW 34, Garching, June 24th
S. BRÉMOND
1/10
Association
Euratom-Cea
TORE SUPRA
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
CCFW 34, Garching, June 24th
S. BRÉMOND
2/10
Association
Euratom-Cea
RF Voltage stand-off 1/3
Line models
A
strap
'
a
'
a
Unmatched Vacuum Transmission Line (ext)
or lumped capacitor (int)
F
feeder
V, J
Z f , l f (min l f
'
a
R , L ,C ,l a
ZVTL , lVTL
due to geometry)
RF Voltage
Strap
2P '
VA 
wLA la
RA'
Feeder
VF  2 Z f P
VF
max
GA 
TORE SUPRA
Z  R  jX , Y  G  jB,
1
2
~  e j , P  G V
2
RF Electric field
Ra'
w L  l
'
a
to be minimized
by antenna front geometrical design
f ( l a ), f  1
2
a
(Low losses, short straps)
1   A2  2  A cos( A  2 lF )
1   A2
 2 Z f P vswr
Min vswr for Z f  Z A
X A  0 ( short strap) E  V and Z f  d (strip line)  VF
d
0 A  
 VF  VA
max

vswr
Zf
Emax depends above all on precise geometry
(strip line to coaxial line transition)
Un Z
1   F2  2  F cos( F  2lVTL ) X F  0 ( short feeder)
exp c 
matched VVTL  2 ZVTL P
V
 60  Min at Zc = 60 W
1   F2

0




E max( coaxial )  max
F
VTL
Zc
rext
60
 VVTL  VF
(external) VVTL

2
Z
P
vswr
VTL
max
CCFW 34, Garching, June 24th
S. BRÉMOND
3/10
Association
Euratom-Cea
F
RF Voltage stand-off 2/3
VJ
F
RF Voltage: Vmax (ext.) (~ 36
kV) > Vmax (int.) (~ 28 kV)
+ maximum RF voltage point
depending on frequency
F
VJ
RF Current : up to 1.8 kA
(ext.) > 1.1 kA (int.)
High RF current (and voltage)
at vacuum window
CCFW 34, Garching, June 24th
S. BRÉMOND
TORE SUPRA
VJ
RF electric field: more
freedom to adjust
geometry in internal
matching up to 1.4 (ext.)
> 1.1 kV/mm (int.)
4/10
Association
Euratom-Cea
RF Voltage stand-off 3/3
TORE SUPRA
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)
CCFW 34, Garching, June 24th
S. BRÉMOND
5/10
Association
Euratom-Cea
Matching 1/2
TORE SUPRA
1. External matching cannot apply conjuguate T principle; matching
algorithm becomes an issue
C1
End of
strap + feeder
End of
strap + feeder
T branche 1
T branche 2
G=0.5
G=0.5
RA
RA
l2
C2
T branche 2
T branche 1
Internal matching
l1
External matching
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
CCFW 34, Garching, June 24th
S. BRÉMOND
6/10
Association
Euratom-Cea
Matching 2/2
TORE SUPRA
2. Uncontrolled power sharing is very likely to reduce power handling capabilities
1st
VTL
2nd
VTL
1st
VTL
2nd
VTL
1st
VTL
2nd
VTL
PT1 ,2 ,3 
1
GT1 ,2 ,3 VT
2
2
- 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)
CCFW 34, Garching, June 24th
S. BRÉMOND
7/10
Association
Euratom-Cea
Components reliability
TORE SUPRA
External matching option
1. Unusual 4 ports passive junction to be developed an qualified
Developing 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
CCFW 34, Garching, June 24th
S. BRÉMOND
8/10
Association
Euratom-Cea
Miscellaneous
TORE SUPRA
- 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 simplicity
many 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 margins
Possible 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)
CCFW 34, Garching, June 24th
S. BRÉMOND
9/10
Association
Euratom-Cea
Tentative conclusion
TORE SUPRA
- 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.
CCFW 34, Garching, June 24th
S. BRÉMOND
10/10