I. Monakhov, ICRH CT antennas matching, SOFT-23 satellite meeting, Venice, Italy, September 21, 2004, Slide 1 I. Monakhov, A. Walden, T. Blackman, D. Child,

I. Monakhov, ICRH CT antennas matching, SOFT-23 satellite meeting, Venice, Italy, September 21, 2004, Slide 1

I. Monakhov, A. Walden, T. Blackman, D. Child, M. Graham, W. Hardiman,

P.U. Lamalle1, M. Nightingale, A. Whitehurst and JET EFDA contributors*

Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK1 LPP-EPM/KMS, Association Euratom-Belgian State, Brussels, B-1000, Belgium

* Appendix of J.Pamela, et al., “Overview of JET Results”, Fusion Energy 2002, IAEA, Vienna (2002)

Tests of external conjugate-T matchingTests of external conjugate-T matching

system for A2 ICRF antenna at JETsystem for A2 ICRF antenna at JET

ICRH Conjugate-T antennas matching, SOFT-23 satellite meeting,

Venice, Italy, September 21, 2004


OutlineOutline

Main topics :

• Matching procedures and typical examples

• Automatic control algorithm

• Cross-talk influence studies

• Matching peculiarities (strap balance, matching option etc)

Outside the scope of this presentation:

• Test ELM-tolerance issues (!)

• Test power handling issues

• Full-scale proposal for JET - design, plans etc

• ITER-relevance

Poster P3T-B-152 tomorrow morning

Emphasis on experimental results of matching to quasi-stationary load


External conjugate-T matching of A2 antennas at JETExternal conjugate-T matching of A2 antennas at JET

Features:

Externally (outside the vacuum vessel) located coaxial T-junction

Coaxial line-stretches (trombones) for conjugate-T tuning

Variable trombone and stub tuner as impedance transformer

Advantages:

Reliance only on tried-and-tested coaxial line components and technology

Manageable accuracy of control of the matching elements

Separation of launching and matching sub-systems

Capability to conjugate remote antenna straps

Straightforward strap loading RF diagnostic (directional couplers)

2003-2004 prototype tests - quick, inexpensive and risk-free assessment of

the principal features of the proposed scheme under the ‘worse-case’ conditions


External conjugate-T tests at JET: External conjugate-T tests at JET: A2 antennaA2 antenna

C1C2

C3C4

Straps involved in conjugate-T tests

• Different design Loading asymmetry

• Same array, adjacent Strong cross-talk

Conventionally matched straps

• Normally not powered and deliberately

mistuned during the tests*

* Exception: cross-talk studies in L-mode plasma


One amplifier (‘C1’) feeds two adjacent straps of antenna array ‘C’

Switchable to usual configuration in ~10 min (between shots)

Extraneous elements - SLIMPs and ‘parasitic’ open-ended stub

External conjugate-T tests at JET: External conjugate-T tests at JET: schemescheme

Straps

C14m trombone

3m stubs

C2

Amplifiers

1.5m trombones

C1

C2

‘Parked’ SLIMPs

‘Parasitic stub’

Vref VforControl signals:

T- junction, RT=3-6

~ 80 m transmission line


External conjugate-T tests at JET: External conjugate-T tests at JET: matching proceduresmatching procedures

• Network analyser vacuum matching

• High voltage operations in vacuum

• L-mode plasma operations

• ELMy H-mode plasma operations

• Five frequencies in 32-51 MHz band

• Four RT values in 3-6 Ohm range

• Two matching options

At all stages matching was originally achieved by ‘manual’ adjustments and

later complemented and refined by automatic ‘real-time’ control system

• No really serious troubles and principal complications

• Predictable performance, adequate response to changes

• Time-consuming initially (network analyser in vacuum)

• Straightforward at later stages (first shot ‘direct hits’)


Conjugate-T matching: Conjugate-T matching: trombone setting accuracytrombone setting accuracy

Simulation

Dependence of amplifier output line VSWR

on lengths of conjugate-T trombones

Network analyser measurement

Matching option 2

Matching option 1

Vacuum loading, F=42.1 MHz, RT=3 Ohm*

* before impedance transformer settings optimisation

C1 trombone length, mm

C2

trom

bone

leng

th, m

mC

2 tr

ombo

ne le

ngth

, mm

C1 trombone length, mm

Manageable setting accuracy:

~2-3 cm setting ‘target’ in vacuum (toughest case)

~3-5 mm trombone length control accuracy


Conjugate-T matching: Conjugate-T matching: high-voltage vacuum operationshigh-voltage vacuum operations

# 3969, f=50.3 MHz, RT=4

Generated power

Maximum voltages in transmission lines

Forward and reflected voltage wave amplitudes in amplifier output line

VSWR in amplifier output line

Effortless matching on the basis of the settings found with network analyser

‘Perfect’ matching

* Real-time tracking OFF


Maximum voltages in transmission lines

Conjugate-T matching: Conjugate-T matching: L-mode plasma operationsL-mode plasma operations

Generated power



# 61646, f=42.1 MHz, RT=4

• Straightforward matching on the basis

of extrapolation of vacuum settings

• Trouble-free L-mode plasma operations

VSWR<1.1

14 sec



Conjugate-T matching: Conjugate-T matching: ELMy H-mode operationsELMy H-mode operations

Horizontal mid-plane chord D signal

Generated power



# 60530, f=42.1 MHz, RT=3

Trip-free performance with VSWR1.5-3** depends on discharge scenario and circuit optimisation



Conjugate-T matching: Conjugate-T matching: real-time control algorithmreal-time control algorithm

Vref Vfor

Existing trombone and stub matching at JET

Conjugate-T matching of ITER-like antennas

• reflection phase at couplers depend on frequency hence

• error signals require frequency correction

External conjugate-T matching at JET• no need in frequency correction (for equidistant couplers)

• fully compatible with existing matching (no new electronics)

Vref Vfor

Vref Vfor

Im(Vref/Vfor)

Re(Vref/Vfor)

Im(Vref/Vfor)

Re(Vref/Vfor)

Im(Vref/Vfor)

Re(Vref/Vfor)

new

New control algorithm (compatible with the existing JET error signals)


Automatic matching control:Automatic matching control: theory and simulationstheory and simulations

Matching reactance 1

Mat

chin

g re

acta

nce

2

Generalised symmetric conjugate-T error signal E=Vref/Vfor in the vicinity of matching point


Re(E)=0 and Im(E)=0 contours Solid lines - proposed algorithm

Dotted lines - JET EP algorithm

RC=1, RT=5, matching option 2

)()(2

)Im(

)()(2

)()Re(

210

0

0

120

0

YYR

Z

ZRR

YE

YYR

Z

ZRR

RRE

TTC

TTC

CT

Y0 - matching reactance, RC - coupling resistance,

RT - T-junction reference resistance,

Z0 - line characteristic impedance

Good algorithm convergence


Automatic matching control: Automatic matching control: performanceperformance


• Reliable and stable (after some teething troubles)

• Routinely used for matching refinement

# 61863, f=42.1 MHz, RT=4

Generated power

Trombone length control error signal

Trombone length deviation from the matching value


Automatic matching control: Automatic matching control: troubleshootingtroubleshooting

Matching ‘run-away’ failure due to trombone

tracking speed asymmetry (firmware fault)


Generated power



# 3936, f=37.45 MHz, RT=4

Tracking speed symmetry between

both matching elements is essential



Algorithm instability due to tracking inertia

(trombones set to run at fast speed regardless of Verr)


Generated power



# 4012, f=42.1 MHz, RT=4

Inertia-free or variable-speed tracking

is required for fast and stable matching



Control system auto-excitation due to tracking inertia

(trombones set to run at fast speed regardless of Verr)


Generated power



# 4070, f=50.3 MHz, RT=4

Inertia-free or variable-speed tracking is

required to avoid system auto-excitation


Automatic matching control: Automatic matching control: strap current reactionstrap current reaction

Generated power


Strap C2 and C1 current amplitude ratio

Strap C2 and C1 current phase difference

Strap coupling resistance

Trombone length deviation from the vacuum matching value

Strap currents sensitive to matching

- potentially a source of troubles


Cross-talk influence: Cross-talk influence: characteristics of A2 antennacharacteristics of A2 antenna

A2 antenna strap cross-talk vs frequency

solid lines - network analyser measurements in vacuum

broken line - MWS simulation for vacuum loading

P.Lamalle, et al, EPS-30, 2003, P-1.193

dots - phase-ramp measurements under plasma loading

P.Lamalle, et al, EPS-22, 1995, II-329

Cross-talk amplitude

• Typically increase with frequency

• Does not exceed |Sij|~0.1, i.e. relatively low

• Is overestimated by MWS simulations

• |Sijplasma| ~ 1.5-2.5 |Sij

vacuum| for near neighbours

|Sijplasma| ~ 3-5 |Sij

vacuum| for far neighbours


Cross-talk influence:Cross-talk influence: experimental assessmentexperimental assessment

Simultaneous operations of two pairs of straps under plasma loading:

C12 (conjugate-T matching) and C34 (conventional matching)

• High frequency (the highest cross-talk)

• ~180 relative phase sweep

• Modulated current amplitude ratio

• 3-8 cm antenna-plasma gap sweep

• Real-time tracking ON and OFF

• No dramatic influence on matching

and strap current balance and phase

• No signs of matching algorithm

instability

Caveats: 1. JET A2 test layout doesn’t represent the case of ITER-like antenna toroidal cross-talk adequately

2. Initial settings were too close to ‘perfect’ matching to explore the ‘matching-from-scratch’ situation


Cross-talk influence studies:Cross-talk influence studies: phase sweepphase sweep

# 63133, F=50.3 MHz, RT=4


C12 (conjugate-T) + C34 (usual matching) with

~180 relative phase sweep

Generated power per each pair of straps

Conjugated straps current amplitude ratio

Conjugated straps current phase difference

VSWR in C1 amplifier output line

Small matching perturbation due to cross-talk

Strap pairs current phase difference


Cross-talk influence studies:Cross-talk influence studies: amplitude modulationamplitude modulation

# 63135, F=50.3 MHz, RT=4





C12 (conjugate-T) + C34 (usual matching) with

modulated current amplitude ratio (=-90)


Strap pairs current amplitude ratio

Small matching perturbation due to cross-talk


Cross-talk influence studies:Cross-talk influence studies: loading variationloading variation

* Real-time tracking ON

# 63130, F=42.1 MHz, RT=4 C12 (conjugate-T) + C34 (usual matching)

during antenna-plasma gap variation (=-90)





Algorithm stability in presence of cross-talk

Mid-plane antenna-plasma distance


Conjugate-T matching: Conjugate-T matching: strap current balance and phasestrap current balance and phase

Strap current amplitude balance:

• Anticipated asymmetry due to different strap design

• Observed imbalance in matched straps: IC2/IC1 ~ 0.8-1.0

• Observed imbalance during ELMs: IC2/IC1 ~ 0.5-2.5

Strap current phase difference:

• Sign defined by the matching option

• Difference reduces under loading, typically - vacuum: ~ 155-150

L-mode plasma: ~ 140-130

ELM: ~ 90-60

Both amplitude balance and phase difference

are sensitive to the matching option used


Conjugate-T: Conjugate-T: matching option choicematching option choice

Loading asymmetry and complex* loading perturbation cause noticeable

difference in circuit response between the two matching options

Both simulations and ELM observations during the tests show

that the matching option choice has a strong influence on

• magnitude and sign of strap current amplitude ratio variation

• magnitude and sign of strap current phase difference variation

• VSWR variation in matched line (load-tolerance)

Judicious choice of the matching option is important

* Plasma loading (including ELMs) = strap resistance increase AND inductance decrease


ELM

Strap C2 and C1 current amplitude ratio:

• option 1 - ratio decrease, small variation

• option 2 - ratio increase, large variation

Simulations of strap current ratio IC2/IC1 amplitude

during asymmetric loading (F=42 MHz, RT=4)

strap 2 impedance variation is 75% of strap1

Matching option 1 (capacitive matching reactance in C1)

Matching option 2 (inductive matching reactance in C1)

Conjugate-T: Conjugate-T: current balance and matching optioncurrent balance and matching option

ELM


ELM

Strap C2 and C1 current phase difference:

• opposite signs for the matching options

• variation is smaller for matching option 2

Simulations of strap current ratio IC2/IC1 phase

during asymmetric loading (F=42 MHz, RT=4)




Conjugate-T: Conjugate-T: strap phasing and matching optionstrap phasing and matching option

ELM


trip

ELM

ELM

Conjugate-T: Conjugate-T: load-tolerance and matching optionload-tolerance and matching option



Simulations of VSWR in the matched line

during asymmetric strap loading (F=42 MHz, RT=4)


• Load-tolerance depends on the matching option

• Smaller VSWR values expected for option 2


Strap C2 and C1 current amplitude ratio

Matching option 1

# 62103, f=42.1MHz, RT=4

Matching option 2

# 62099, f=42.1MHz, RT=4

ELM ELM

System response to ELMs depends on the

matching option (as predicted by simulations)

Strap C2 and C1 current phase difference


Conjugate-T: Conjugate-T: response to ELMs and matching optionresponse to ELMs and matching option


small decrease large increase

positive sign, large reduction negative sign, small increase

large perturbation small perturbation


Advantageous factors of JET A2 conjugate-T configuration:

• Relatively high resistive loading (~2-3 Ohm compared with expected ~0.5-1 Ohm JET ITER-like strap)

• Relatively low cross-talk between the conjugated straps (~-20dB compared with ~-7dB Alc-C Mod)

• Manageable matching element control accuracy (~0.5 cm compared with ~0.1 mm)

• Familiar, well documented and well diagnosed strap load (privilege of external matching)

External conjugate-T tests at JET:External conjugate-T tests at JET: summarysummary

• Successful plasma loading matching on the basis of vacuum settings

• Adequate and predictable circuit response to changing conditions

• Reliable new automatic control algorithm for matching refinement

• No dramatic influence of strap cross-talk on circuit matching

• Importance of matching option choice in presence of loading asymmetry


The next slides are not a part The next slides are not a part

of the presentation and included of the presentation and included

to facilitate discussions onlyto facilitate discussions only


Involves ½ of the existing RF plant – modules C and D

currently matched by stub and trombone circuit

SLIMPS are installed, but currently not used

35kV voltage limit ~1MW per strap in L-mode plasma

Conjugate similar straps of different antenna arrays

symmetric conjugate-T loading better load tolerance

arbitrary phasing between straps within antenna array

same frequency and phasing for both arrays

Switchable to the existing configuration between JET pulses keep advantages of the existing system (different array frequency&phase)

fallback in case of installation delays or operational problems four spare 2MW amplifiers during conjugate-T operations

SLIMPs are modified into Z0=30 trombones used to match the T-junction to ZT=3-6 increase flexibility of the existing matching scheme reduce project costs

Stub-trombone wideband variable impedance transformer based on stub and trombone used for conventional matching additional 2m trombone for full frequency coverage

Straightforward low-power RF diagnostic and tuning change-over switches for easy access to measurement ports four-port network analyser for array vacuum matching

Same error signals for real-time matching of both configurations Re(Vref/Vfor) and Im(Vref/Vfor) Conventional configuration: adjust lengths of 1.5m trombone and stub Conjugate-T configuration: adjusts lengths of 1.7m trombones

D2

Antennastraps

1.5m trombone

stub

amplifierSLIMP: 24 & 105

External conjugate-T at JET: External conjugate-T at JET: the next stepthe next step

1.7m trombone

T-junction Network analyser

D1D2

D3D4

C1C2

C3C4

D1

C1

2m trombone

Vref Vfor


R0R0

- jX0+ jX0

Z0

T-junction impedanceRe(ZT)=RT<<Z0

Im(ZT)=0

Impedancetransformer

Matched lineLoad and matchingelement reactance

Load resistance

VSWR (for line with Z0 = RT )

Amplifier trip threshold

RR0

2RT-R0

RT/(R0(2RT-R0))1/2

Conventional matching

Resistive loading change in both branches of tuned parallel resonant circuit

(conjugate‑T) causes smaller variation of the resulting impedance than the equivalent

loading change in a single branch of the circuit (conventional schemes) comparatively

high load-tolerance

Conjugate-T: Conjugate-T: why is it load-tolerant ?why is it load-tolerant ?

Conjugate-T - a parallel connection of two loads with their impedances modified to present

the T-junction with complex conjugate values and to ensure purely real resulting impedance

(i.e. equivalent to a tuned parallel resonant circuit with losses in both branches).


Conjugate-T tests at JET: Conjugate-T tests at JET: frequency coveragefrequency coverage

• Discrete windows due to limited trombone length variation (1.5m)

• Adequate representation of typical JET ICRH frequencies

• Comparable with the existing stub/trombone matching at JET

The diagram is based on network analyser measurements in vacuum, RT=3

Test frequencies

Amplifier dead band Amplifier band edges

Allowed frequencies (conjugate-T matching possible)


‘‘Proof-of-principle’ test: Proof-of-principle’ test: installationinstallation

T-junctionChange-over switch ‘Big trombone’ (4m)

1.5 m trombone3m stub

Done during the experimental campaign without disruption to RF operations



Simulations: Simulations: load tolerance under load tolerance under symmetricsymmetric loading loading

F=42 MHz, RT=4, symmetric strap loading

Contours of amplifier output line

VSWR versus strap impedance

Strap resistance

Str

ap r

eact

ance

cha

nge

Measured ELM ‘footprint’

Load tolerance of symmetric pair:

• looks promising in general

• reactance perturbation narrows

the tolerance margins

• matching option doesn’t matter


Simulations: Simulations: load tolerance optimisationload tolerance optimisation

F=42 MHz, symmetric strap loading

RT=5 : good for big perturbations, but very

narrow margin at low coupling

RT=3 : good at low coupling, but limited

tolerance for big perturbations

T-junction reference impedance choice:

• optimum setting depends on coupling scenario

• no ‘magic’ setting covering all situations hence

• variable transformer is needed


‘ ‘Proof-of-principle’ test:Proof-of-principle’ test: summary of resultssummary of results

Vacuum matching over 32-51 MHz band (at typical JET ICRH frequencies)

High voltage (up to VATL~35 KV) vacuum operations over the frequency band

Real-time matching control on the basis of the existing error signals

‘Perfect’ matching and high power* (<1MW) long pulse (15sec) operations

at different frequencies during L-mode plasma discharges

Trip-free performance during strong sawtooth activity, L-H transitions and

ELMy (including Type-I) H-mode at generated power levels up to 0.8 MW *

Demonstration of ELM-tolerance optimisation and insensitivity to cross-talk

* 1. Power is from one amplifier (1/4 of a module or 1/16 of the plant)

2. Limit due to arcing in a makeshift rectangular section of ‘big trombone’


Conjugate-TConjugate-T load tolerance: load tolerance: sawteethsawteeth

RF module ‘A’ - conventional matching (superimposed traces from four amplifiers)

Amplifier ‘C1’ - conjugate-T matching

Forward and reflected (x5) voltage wave amplitudes in output lines of RF amplifiers

RF module ‘B’ - conventional matching

(superimposed traces from four amplifiers)

RF module ‘D’ - conventional matching

(superimposed traces from four amplifiers)

Soft X-ray emission intensity signal# 60184 f=42.1 MHz, RT=3

Comparatively small reflection perturbation


# 60531, f=42.1 MHz, RT=3




Antenna strap coupling resistance

Zoomed to show fast data for three ELMs

Trip-free performance with RC ~9


Conjugate-T load tolerance: Conjugate-T load tolerance: big ELMsbig ELMs


Conjugate-T load tolerance: Conjugate-T load tolerance: L-H mode transitionL-H mode transition


Antenna strap coupling resistance; note fast Rc reduction during L-H transition -

theoretically troublesome for Conjugate-T

Forward and reflected voltage wave amplitudes in amplifier C1 output line

VSWR in amplifier output lines A1,B1 D1 (conventional matching)

C1 (conjugate-T matching)

L-H mode transition

# 61865, f=50.1 MHz , RT=4

No problems due to fast RC reduction


Conjugate-T load tolerance: Conjugate-T load tolerance: comparative performancecomparative performance

Instantaneous and moving average power generated by RF modules

module ‘B’ - conventional matching (total power from four amplifiers; 75 trips)

module ‘D’ - conventional matching (total power from four amplifiers; 33 trips)

amplifier ‘C1’ - conjugate-T matching (power from one amplifier; no trips)

# 62111, f=42.1 MHz, RT=3

module ‘A’ - conventional matching (total power from four amplifiers; 167 trips)

Higher average power, better waveform

control, lesser strain on end-stage tubes

D


Conjugate-T load tolerance: Conjugate-T load tolerance: limitationslimitations

Strong and variable coupling resistance asymmetry between two straps

RC1 > RC2 - between ELMs

RC1< RC2 - during ELMs

Strong and asymmetric strap electrical length perturbation during ELMs

LC1~30 cm (!) ; LC1> LC2

VSWR in amplifier output lines

Asymmetric strap loading andstrap reactance perturbation

# 62107, f=42.1 MHz, RT=5

* Also note high T-junction impedance setting, see next slide

Noticeable matching perturbation*during some types of ELMs

D


Conjugate-T optimisation: Conjugate-T optimisation: T-junction impedance choiceT-junction impedance choice

RT=4 # 62099 f=42.1 MHzRT=5 # 62107 f=42.1 MHz

Different RT settings Different VSWR response during similar ELMs

RT=3 # 62109 f=42.1 MHz

D

VSWR

RC1

RC2

better still better


Conjugate-T optimisation: optimisation: matching option choicematching option choice

Two matching options have different ELM-tolerance characteristics

(in presence of loading asymmetry and reactance perturbation - see slide 7)

Horizontal mid-plane

chord D signal


Matching option 2, # 62099, RT=4Ohm, f=42.1 MHz

Matching option 1, # 62103, RT=4Ohm, f=42.1 MHz

Better load-tolerance


Conjugate-T optimisation: Conjugate-T optimisation: fixed offset matchingfixed offset matching

Matching between ELMs can be intentionally fixed at (reasonable) VSWR > 1

VSWR decrease during ELMs a ‘safeguard’ against trips&arcs

Real-time matching, # 62099 f=42.1 MHz Fixed offset matching, # 62101 f=42.1 MHz

Strap C1 and C2coupling resistance

Horizontal mid-plane

chord D signal


VSWR decrease during ELMs


Conjugate-T at JET: Conjugate-T at JET: new research opportunitiesnew research opportunities (outside the scope of this presentation)(outside the scope of this presentation)

• Antenna coupling during big ELMs

no trips regardless of ‘severity’ of ELMs and applied power levels

disabled generator frequency fast feedback loop

• Antenna electrical strength deterioration during and after ELMs

no mismatch trips during ELMs regardless of strap voltage

• ICRH heating efficiency and power deposition during ELMs

RF power delivered to plasma not only between, but also during ELMs

Allows to investigate a number of important problems

previously inaccessible for experimental studies


External conjugate-T: External conjugate-T: ITER-relevant advantagesITER-relevant advantages

• Allows robust and simple launcher design

high reliability and longevity in hostile environment, easy maintenance

bigger non-RF design margins: stresses, cooling, pumping, diagnostics etc

• Possibility to conjugate remote straps

better phasing higher coupling and better load tolerance

lower cross-talk higher load tolerance and matching stability

• Independent launching and matching sub-systems

easy reparability or ‘inexpensive’ replacement

straightforward upgrades or new versions of either of the systems

• Well-established coaxial line matching technology

manageable tuning accuracy with simple drive systems

• Straightforward RF diagnostic of individual straps

better antenna and generator protection

transparent coupling interpretation and simplified matching

Documents

I. Monakhov, ICRH CT antennas matching, SOFT-23 satellite meeting, Venice, Italy, September 21, 2004, Slide 1 I. Monakhov, A. Walden, T. Blackman, D. Child,