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Topical Group on Heating and Current Drive Coordinating Committee on ICRH (CCIC) April 9, 2008, Culham. Load-tolerant matching systems for A2 antennae at JET: 3dB splitters and External Conjugate-T. I. Monakhov Acknowledgements M.-L. Mayoral and RF systems section.

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slide1

Topical Group on Heating and Current Drive

Coordinating Committee on ICRH (CCIC)

April 9, 2008, Culham

Load-tolerant matching systems

for A2 antennae at JET:

3dB splitters and External Conjugate-T

I. Monakhov

Acknowledgements M.-L. Mayoral and RF systems section

See TF-H web pages for more details –

http://users.jet.efda.org/tfhwiki/index.php/3dBs_couplers

http://users.jet.efda.org/tfhwiki/index.php/External_Conjugate_T

slide2

3dB and ECT systems at JET

  • Conceptually different matching systems,
  • however, the following features are common:
  • general purposes – ensure trip-free performance during ELMs
  • general configuration – power splits between two antennas
  • equipment used for real-time matching – TRIMPs
  • biggest trouble – protection against arcs in ELMy plasmas
slide3

Antenna = 4 straps

Module =

4 amplifiers

Common source: frequency,

timing,

waveform

Matching circuit

Antenna strap

Antenna strap

Matching circuit

Antenna strap

Antenna strap

Matching circuit

Amplifier

Antenna strap

Antenna strap

Matching circuit

Antenna strap

Antenna strap

Antenna = 4 straps

3dB and ECT: implications of power splitting

Two antennas per one module

  • Same frequency, phasing, power, timing etc for both antennas
  • Demanding for amplifiers – full power in case of good coupling
  • Any fault (amplifier, matching, antenna etc) affects both antennas
slide4

3dBcoupler

STL B

MTL B

OTL

B

B

Stub

Antenna strap

TRIMP

Amplifier

30 Ohm load

MTL A

A

STL A

A

ILA

Antenna strap

Stub

TRIMP

3dB splitter system

Original purpose: release module A for ILA project by splitting the output power of module B between antennas A and B

Supplementary purpose: ensure trip-free performance during ELMy plasmas

Installation: 2004-2005 shutdown

Operations: 2006-2007 C15-C19

slide5

3dB splitter: demonstration of ELM-tolerance

Pulse #65947, f=42.56MHz, amplifier B1

D emission from plasma (a.u.)

ELMs

Coupling resistance (Ohm): A1 and B1

STL VSWR: A1 and B1

OTL VSWR

Power going to dummy-load (kW)

slide6

3dB splitter: performance during ELMy plasmas

Pulse #65947,

f=42.56MHz, 180 phasing

Trip-free power injection during ELMs - substantial improvement as compared with conventional matching

3dB splitter system –

power coupled by antennas A and B

Parasitic spikes after the ELMs –

software bug in real-time coupled power control; now fixed

Conventional matching system –

power coupled by antennas C and D

ELMs

slide7

3dB splitter: power injection in ELMy plasmas

Pulse #70360,

f=46.16MHz, +90 phasing

Best shot so far:

~2.5 MW coupled ‘trip-free’;

Slow progress with increasing the power levels due to a combination of amplifier hardware problems and power control software bugs

ELMs

slide8

VREF

VREF

VFOR

VFOR

3dB splitter: arc protection

Power trip if the VSWR gets above the threshold in OTL, STL ‘A’ or STL ‘B’

STL B

MTL B

OTL

B

B

Arc

VREF

VFOR

3dB coupler

Amplifier

STL A

A

MTL A

Arc

Antenna

Stub

TRIMP

Typical trip threshold:

OTL VSWR=2.4

  • STL high-VSWR protection:
  • good sensitivity to most types of arcs
  • poor compatibility with requirements of ELM –tolerance (‘don’t trip on ELMs’)
  • poor sensitivity to voltage-node arcs
  • OTL high-VSWR protection
  • poor sensitivity to arcs

Typical trip thresholds:

STL VSWR=3, 4, 6, 13

Challenge : trip on arcs but don’t trip on ELMs while both produce high STL VSWR

slide9

3dB splitter: example of undetected arcing

Commissioning of 3dB couplers in ELMy plasma; CAMAC fast data acquisition (10 kS/s)

 Intensity of D line emission from plasma:

 Power delivered to B2 line: tripped externally by an unrelated event (and due to good luck)

 STL VSWR: note sustained increased level on B2 after the ELM, however no protection trip generated as the threshold was set too high VSWR=13

 Coupling resistance:note abnormally high values after the ELM

Other indications (not shown): some pressure increase on B2 VTL Penning gauge and elevated OTL VSWR

slide10

3dB splitter: revised VSWR protection levels

Rules were introduced after observation of ‘parasitic’ low-VSWR activity

  • Original card VSWR=3
  • New card, VSWR=3 option
    • antenna conditioning, all frequencies
  • New card, VSWR=4 option
    • normal (ELM-free) ops, all frequencies
  • New card, VSWR=6 option
    • Small ELMs; frequency restrictions
      • 37 MHz: not allowed
      • 42 MHz: risk for outer lines, not advisable
      • 25 MHz, 33 MHz, 44 MHz, 47 MHz, 51 MHz: OK
  • New card, VSWR=13 option
    • Big ELMs; frequency restrictions
      • 37 MHz, 42 MHz: not allowed
      • 44 MHz: risk for outer lines, not advisable
      • 33 MHz: risk for inner lines, not advisable
      • 25 MHz, 47 MHz, 51 MHz: OK

The approach proved to be very restrictive for most common operational frequencies – revision is needed to make further progress

slide11

3dB splitter: conclusions

  • The system behaviour is predictable and well understood
    • main teething troubles over
    • ELM-tolerance demonstrated
    • valuable operational experience gained
  • Some extra effort is required to fix the remaining problems
    • power limitations
    • TRIMP-related matching stability
    • full potential not achieved yet
  • ELMy plasma operations require some care and expertise
    • increased risk of arcing
    • VSWR trip settings – difficult choice of protection vs performance
  • Biggest issue - arc protection and compatibility with ELMs
    • ideally a new VSWR-independent arc protection is needed
slide12

TRIMP

C1

amplifier

T-junction

TRIMP

Antenna

straps

D1

D2

D1

C1

C2

D2

D3

C3

D4

C4

Trombone

Stub

External Conjugate-T (ECT)

Purpose: ensure continuous RF power injection into ELMy plasma without compromising capabilities and performance of conventional matching system

Installation: 2007 shutdown

Commissioning: 2008 April-May C20

Expected operations: 2008 C21 ? (subject to delays)

ect generator hall j1h installation
ECT: generator hall (J1H) installation

All eight conjugated lines have equal lengths to ensure common operational frequencies

TRIMPS are installed in the Torus hall

C1, C2 lines going to antennas

Change-over switch

C3, C4, D1, D2, D3, D4 lines going to antennas

In-line switch (T-junction)

Lines coming from C1, C2 amplifiers

Stub

Trombone

C4

C3

D3

D4

D1

D2

ect operational frequencies
ECT: operational frequencies
  • Operational band = overlap of ‘Conjugate-T’ and ‘Impedance Transformer’ bands
  • Limited variable length of phase shifters (~1.5m)  discrete matching windows
  • Position of the windows optimised by choosing appropriate fixed line lengths
  • All common JET operational frequencies available except for ’37 MHz’*
    • * technically attainable by fitting extra sections of lines and sacrificing ’42MHz’ band

Simulation of the ECT frequency coverage

ect traditional high vswr arc detection method

Arc

Arc

Arc

ECT: traditional high-VSWR arc detection method

C

CTL

C

Normally high VSWR ~10-60

Normally low VSWR~1

Normally high VSWR~6-10

OTL

ITL

DTL

D

VREF

VFOR

D

ZT<<30 

  • Applicable to matched line (OTL) only
  • Efficient against arcs in the impedance transformer (MTL)
  • Insufficient against arcs in conjugated lines (CTL or DTL)
ect new arc detection technique
Advanced Wave Amplitude Comparison System - AWACS :

compares reflected and forward wave amplitudes (similar to the VSWR method)

but the respective waves are measured in different points of the circuit

technically simple and inexpensive; no major development

based on available RF hardware (directional couplers)

uses familiar electronics (same VSWR trip cards - only different inputs)

complements the standard high-VSWR method

ECT: new arc detection technique
ect awacs arc detection

C

CTL

C

VFOR

Arc

Arc

MTL

DTL

OTL

D

VFOR

VREF

D

Nominator increases during arcs and ELMs

Denominator decreases during arc in the line

ECT: AWACS arc detection

Trip on high ratio of OTL reflected and CTL (DTL) forward voltage amplitude

Why VFOR drops in the arcing line only ?

- the parallel impedance of the line seen at the T-junction changes from normally small value to quite high value during arc causing the current going to the arcing branch to drop

The ratios are highly sensitive to the CTL (DTL) arcing and no so to ELMs

ect project status
ECT project status
  • Transmission line installation is done
    • Consistency verified during low power measurements; looks good
  • TRIMPs installed in the torus hall
    • Some issues emerged related to TRIMPs speed and accuracy
  • Control electronics upgrade nearing completion
    • Relatively low priority of the project as compared with ILA and normal ops
  • Tentative settings for vacuum matching obtained
    • All operational frequencies
    • Individual pairs of antenna straps only (no cross-talk)
  • First plasma operations expected in May (?)
ect commissioning general targets
ECT commissioning: general targets
  • Obtain matching settings for reference L-mode coupling (Rc~2-3 Ohm) at
    • frequencies: 42MHz, 47MHz, 51MHz, 33MHz and 44MHz (in order of priority)
    • phasings: dipole, +90, -90, other (in order of priority)
    • T-junction impedances: 3±j1 Ohm, 4±j1.5 Ohm, 5±j2 Ohm (in order of priority)
  • Test and optimize real-time matching control in L-mode plasma
    • during antenna loading scan by plasma position variation
    • during antenna strap current phase and amplitude modulation
  • Maximize coupled power in L-mode plasma
    • assess possible limitations and arcing problems
  • Optimize the ECT performance in ELMy H-mode plasmas
    • refine matching settings for reference H-mode coupling (Rc~1-2 Ohm)
    • optimise VSWR and AWACS arc protection thresholds compatible with ELMs
    • select optimum T-junction impedances for best ELM-tolerance
  • Demonstrate high power (max voltage) trip-free operations in H-mode
slide21

3dB and ECT: troubles with arc protection

The main method of protection against arcs in antenna and transmission lines is monitoring the VSWR levels and tripping the amplifiers if it gets too high – essentially the same method as used for amplifier protection during ELMs!

Conventional systems:

  • Strong VSWR change on amplifier output during both ELMs and arcs
    • relatively easy to detect
  • Arc protection is consistent with amplifier protection during ELMs
    • OK to trip on high VSWR regardless of the cause

Load-tolerant systems (3dB and ECT):

  • Weak VSWR change on amplifier output during both ELMs and arcs
    • more difficult to detect
  • Contradiction between VSWR requirements for ELM-tolerance and arc protection
    • No need to trip during ELMs as the VSWR is acceptable for amplifiers, but
    • Need to trip during arcs (even if the VSWR is not too high)
    • Difficult to set optimum ‘performance-vs-protection’ trip thresholds
  • VSWR-independent arc detection method is highly desirable (but not available)
slide22

3dB and ECT at JET: key differences

  • ECT
  • RF power keeps going to plasma during ELMs
  • Adjustable (more control parameters)
  • 37MHz, 23MHz, 28MHz not available
  • Switchable to normal configuration
  • Never used before (similar concept with technically different implementation is also used in TEXTOR and for ILA at JET and Tore Supra)
  • 3dB splitters
  • RF power doesn’t go to plasma during ELMs (diverted to load)
  • Not adjustable
  • All operational frequencies
  • Permanent installation
  • Long experience of using on other tokamaks (ASDEX Upgrade, DIII-D)
slide23

1.0

Simulations of OTL VSWR dependence on the STL VSWR

(case of no additional phase shift between the reflected voltages on both STLs)

3dB splitter: load-tolerance and implications

  • Mismatch (high STL VSWR) -
  • Fast antenna loading change, e.g. ELMs
  • Wrong stub/TRIMP settings or tracking failure
  • Arcing in antenna or transmission line

Low sensitivity of OTL VSWR to mismatch

  • very good for amplifier protection - less likely crowbars, over-voltages, tube grid overheating, end-stage arcs etc
  • more difficult to detect arcs in antenna and transmission lines using traditional method of VSWR monitoring
jet icrh antenna and vacuum transmission line vtl
JET ICRH antenna and Vacuum Transmission Line (VTL)

Vacuum transmission line (VTL)

  • Vulnerable area:
  • Bellow (4 Bar abs)
  • Side port
  • ‘Cold’ inner VTL
troublesome frequencies for arcing at the vtl bellow
Troublesome frequencies for arcing at the VTL bellow

Simulations of VTL bellow vulnerability to ‘low-VSWR’ arcing at different frequencies -two extreme cases of strap loading: vacuum and big ELM

 Distance of the voltage node from the VTL

side port axis

  • VSWR in matched line during arcing at

the VTL bellow

In the frequency band of 35-43 MHz the voltage node is located around the VTL bellow and arcing in this area could remain undetected causing substantial damage

vtl damage by arcing at the voltage node in 2004
VTL damage by arcing at the voltage-node in 2004

Multiple streaks due to persistent but ‘non-catastrophic’ breakdowns

Ag-plating stripped (merged streaks)

‘Catastrophic’ arc spots

(bellow punctures)

to DCF 

Outer conductor

side port

All inner conductor bellow damage is localised in front of the side port on the outer conductor

Silver-plated Inconel

Thermal discoloration

Inner conductor bellow

slide27

TRIMP: what is it?

Essentially, the same thing as the ‘trombone’ – a phase shifter made as a piece of transmission line with variable length; used together with ‘stub’ for matching (including real-time control) in the 3dB and ECT systems.

slide28

SLIMP

TRIMP

~80 Ohm

~105 Ohm

~30 Ohm

~24 Ohm

TRIMP vs ‘trombone’: what is different?

Unlike the ‘trombones’ (manufactured by Spinner), the TRIMPs are ‘home-made’ refurbishments of the existing equipment - SLIMPs (SLiding IMPedance)

TRIMP vs trombone:

  • Longer variable length
    • TRIMP: ~1.7m
    • trombone: 1.5m
    • good for frequency coverage
  • Different motors
    • TRIMP: conventional DC motor
    • trombone: step motor
    • implications for control
  • Slower speed
    • TRIMP: <100mm/s
    • trombone: 165 mm/s
    • bad for real-time tracking
  • Slower acceleration
    • bad for control stability
slide29

Vinput

Vinput /2

‘input’

‘through’

through

throughVinput /2

Vinput (through- coupled)

‘isolated’

‘coupled’

Vinput ej/2 (through+ coupled)

Vinput ej/2 /2

coupledVinput ej/2 /2

coupled

3dB coupler: how does it work as a power splitter?

Expressions for complex forward and reflected voltages on all the ports of an idealized 3dB coupler with the ‘isolated’ port terminated by a perfect load and with different complex reflection coefficients through andcoupled on the ‘through’ and ‘coupled’ ports. Note, that the reflected voltage on the input port is zero if through =coupledregardless of the  magnitude!

slide30

3dB splitter: how to optimize the arc protection?

So far, the main (and the only) trick is to select the VSWR trip thresholds

  • OTL VSWR trip thresholds
  • ‘flat’ dependence on forward power
  • one fixed setting VSWR=2.4; initially VSWR=2.0 was used (shown on the plot)
  • STL VSWR trip thresholds
  • ‘flat’ dependence on forward power
  • four options: VSWR = 3, 4, 6, 13
  • could be changed between pulses
ect why use non zero t junction reactance
ECT: why use non-zero T-junction reactance?

This helps to deal with ELM-induced perturbations of antenna equivalent length (inductance of antenna strap), which accompany the loading resistance changes

The influence of T-junction reference reactance setting on VSWR change during ELM

Trip

OK

ect mode of operation

‘Fixed’ transformer

C

D

ZT

30 Ohm

‘Real-time’ control

ZT const <<30 Ohm

Antenna straps

Amplifier

D

ECT: mode of operation

TRIMP

TRIMP

  • Real-time control of TRIMPS ensures that the T-junction impedance is kept constant and low during slow antenna loading variation:
    • Re(ZT)  3-6  << 30 Ohm – required for antenna resistance tolerance
    • Im(ZT)  -1-2   0 – required for antenna reactance tolerance
  • Stub and trombone (fixed lengths during the pulse) transform ZT to 30 Ohm
ect real time control algorithm

Vref

Vfor

Re(Err)

Im(Err)

Vref

Vfor

Re(Err)

Im(Err)

ECT: real-time control algorithm

Adjusts TRIMP lengths to keep ECT matched during slow antenna loading variations (i.e. between ELMs)

Conventional scheme

  • Error signal = Vref /Vfor i.e. complex ratio of voltages at the stub
  • Common signals for conventional configuration and ECT (!)
  • Re(Err) and Im(Err) independently control individual elements
  • Signal signs define the direction of elements movement
  • Depending on the sign allocation the tracking trajectory could
  • go to the matching point in co- or counter-clockwise manner

ECT

The algorithm is sensitive to strap loading asymmetry and T-junction reference reactance sign -

  • JET ECT control system will accommodate both tracking options
  • Possibility of remote selection between them will be available to RF pilot
ect summary and expectations
ECT: summary and expectations
  • Trip-free operations during ELMy plasmas (subject to arc detection restrictions)
  • Power coupled to plasma over the whole ELM duration
  • ~4 MW coupled to H-mode plasmas by both arrays (assuming Rc~1-1.5 Ohm)
  • Timing, waveform, modulation etc – same for both arrays
  • Antenna array phasing – arbitrary; similar for both arrays involved
  • Frequencies - same for both antenna arrays; ’37MHz’ band unavailable
  • Switchable to conventional matching with individual array control – no ELM-tolerance, but possibility of different timing, phasing, frequency etc

ECT configuration is mainly intended for RF power injection into ELMy H-mode plasmas; L-mode plasma operations are certainly possible but serve no purpose as switching to the conventional configuration could bring more benefits

ect original plans for commissioning and operations
ECT: original plans for commissioning and operations

1. Low power (network analyser) measurements  restart, Jan-Feb 

2. Module D high power dummy load tests  restart, Jan-Feb?

3. ECT high voltage vacuum tests  restart, Feb-March?

4. L-mode plasma tests restart, C20a March- April ?

5. ELMy H-mode plasma tests C20b, April – May ?

Ideally, starting from C21 the ECT should become available for ICRH operations (by an expert pilot and on special request)

6. Optimisation, gaining experience, pilot training  C21-C24 (June ….)

Key experiments relying on the ECT:

H-1.2.2 ‘Push LH and ICRH power on ELMs’  June 6 and July 15

H-1.2.1 ‘Efficient power coupling for different ELM-tolerant ICRH systems’

 December 1