FOCUS COIL STATUS RAL DARESBURY V Blackmore V
FOCUS COIL STATUS RAL DARESBURY V. Blackmore V. Bayliss T. Hartnett J. Cobb T. Bradshaw S. Griffiths W. Lau M. Courthold I. Mullacrane R. Preece P. Owens M. Tucker C. White S. Watson Apologies if I have omitted anyone FC Status 6 Nov 2013 1
FC 1 TRAINING 240 Current (Amps) 220 200 180 160 140 120 100 0 2 4 6 8 10 12 14 16 18 20 Run Reached 188. 05 Amps – gnat’s whisker above 200 Me. V/c operating current of 187. 7 A FC Status 6 Nov 2013 2
COMMENTS • 16 training runs – stopped to disconnect two weeks before FC 2 due to arrive • Seems to have reached its limit • Baseline design in specification was for 200 Me. V/c – – All design calculations – forces &c – for 240 Me. V/c Barely reaching 188 A is not good enough no overhead and not guaranteed 170 A (+ a bit) might be safe • Quenches do not originate in HTS leads or LTS tails • 15/16 times Coil 1 quenched first FC Status 6 Nov 2013 3
EXPECTED TEMPERATURE MARGINS • Recently re-calculated by Holger Witte in connection with PRY • 1. 3 K @ 200 Me. V/c • 0. 5 K @ 240 Me. V/c As expected Coils should operate at 200 Me. V/c Tc maps of coils Hope (but not expectation) can get to 240 Me. V/c Lowest Tc = 5. 85 K @ 188 A 5. 05 K @ 225 A • It was always like this FC Status 6 Nov 2013 4
FC 1 CRITICAL TEMPERATURES (AC losses) Cold mass 4. 57 K Current He Channel 4. 37 K Heater on/off Temperatures stable until quench FC Status 6 Nov 2013 5
COIL 1 – COIL 2 DIFFERENCES ? Coil 1 quenched first 15/16 times Any evidence of difference? e. g. thermal contact? R(coil), Ohms 20. 00 Coil 1 Coil 2 15. 00 10. 00 5. 00 Monitor S/C transition during cool down after quench 0. 00 0. 20 0. 40 0. 60 0. 80 1. 00 1. 20 1. 40 Days after quench Both coils become superconducting within 1 minute ~1. 5 days after quench No obvious evidence of bad thermal properties FC Status 6 Nov 2013 6
FIELD MEASUREMENTS Basic ( ~ zero-cost) jig + Hall probe to measure field (almost) on axis Tape measure survey – good to ~ 1 mm wrt flanges Hall probe agreed with TWB’s calibration magnet to < 0. 2% at 5. 6 k. G FC Status 6 Nov 2013 7
FIELD MEASUREMENTS I = 50 Amps Measurements @ 50, 100, 150 Amps (some probe non-linearity above 50 A) Measured field ~1. 5% greater than calculated field at 50 A ~0. 5% agreement between coils at coil z-centres excludes short-circuited layers in one coil FC Status 6 Nov 2013 8
VOLTAGE MONITORING • Q. P. system uses voltage differences to avoid noise triggers • Voltage differences recorded with 16 -channel ‘Picologger’ – Commercial slow ADC system – 10 m. Sec samples – Channels sampled sequentially • Voltage differences somewhat difficult to interpret… Nevertheless… FC Status 6 Nov 2013 9
PRECURSORS Precursors 15 minutes Quench Difference voltages (after amplification of ~ 6) Precursors to quench Picologger can’t see coincidences of short transients + has dead time because of sequential sampling Average of 7 precursors / quench – true rate a few times higher FC Status 6 Nov 2013 10
PERCURSORS – 2 600 (Largest precursor) 200 0 -70 -60 -50 -40 -30 -20 -10 -200 -400 0 Quench Amplitude (m. V) 400 10 -600 Time before quench (min) Frequency & amplitude of precursors increase towards time of quench Last 10 minutes ~ last 5 Amps of current May not cause quench but suggests ‘stick-slip’ Coil movement change of flux induced EMF in coils FC Status 6 Nov 2013 11
STICK – SLIP Quench Fh COIL Quench F Current Bobbin Coils move against small irregularities – stick then slip Enough mechanical energy may be released in a slip to cause quench Irregularities slowly ironed out Hoop stresses also give radial motion FC Status 6 Nov 2013 12
NEW VOLTAGE LOGGING • Picologger records show something interesting – But difficult to interpret & too slow • New voltage logging system built by DL – 16 channels of National Instruments ADCs – Records voltages between adjacent voltage taps – 1 m. Sec sampling – Stores ~ 8 minute window of data around time of quench • Operational for last run of FC #1 FC Status 6 Nov 2013 13
LAST QUENCH OF FC #1 Contactor opens Detector threshold ~0. 6 V reached Coil voltages run away over 10’s of m. Sec Growth of normal (resistive) zone in coils – as expected FC Status 6 Nov 2013 14
PRECURSOR 100 m. Sec One precursor ~ 2. 5 minutes before quench Voltage spike of ~650 m. V for ~5 m. Sec on each coil; I = 188 Amps DU = Energy = Integral [ I V dt ] = 2 * 0. 65 * 0. 005 * 188 / 2 = 0. 61 J >> Estimated MQE ~ 1 – 3 milli. Joules Didn’t propagate as a quench not resistive movement of coils (? ) FC Status 6 Nov 2013 15
HOW LARGE IS MOVEMENT? F + F Up = I 2 M z + Coils move apart by Dz and mutual inductance M changes DV DU = 0. 61 Joules = change in mutual potential energy of coils = F Dz F = 221 Tons = 2. 17 MN (calculated) Dz = DU / F = 0. 61 J / 2. 17 MN = 0. 28 microns Heat capacity 1 coil ~ 140 J/K so 0. 61 J DT ~ 4. 3 m. K Whether movement causes quench depends where / how mechanical energy converted to heat FC Status 6 Nov 2013 16
IT LOOKS LIKE “TRAINING”… • American Magnetics (www. americanmagnetics. com/magnetp. php): “Premature quenching can also occur if the large forces between coil sections result in the motion of one coil with respect to another. This is most likely to occur in magnets having coils that are wound in opposition. ” • Whole business is a Dark Art • Obviously sensitive to things that are difficult to control – Surface properties • Would have been interesting to instrument the cold masses with microphones to pick up movements – but too late • External coils may pick up coil movements – Being considered FC Status 6 Nov 2013 17
FC 2 STATUS • FC #2 delivered to R 9 on 4 October • Swapped with FC #1 and pumped down • Started cooldown 4 November (i. e. two days ago) • Anticipate starting to train at end of November – Initially solenoid mode • Anticipate faster turnaround – Cryocoolers have proper insulation between Stages 1 & 2 – But have to co-exist in R 9 with SS #2 and Tracker • The fate of FC #1 will depend on experience with FC #2 – Please don’t advertise that it will be returned to manufacturer – it may not be – or company’s name • Commercial & legally sensitive issues FC Status 6 Nov 2013 18
THE END FC Status 6 Nov 2013 19
1000 800 Amplitude (m. V) No obvious correlation between positions of precursor and quench currents Precursor Quench current 600 400 200 0 150 160 170 180 190 20 18 16 14 12 0 6 0 18 4. 6 18 3. 9 18 6. 5 18 7. 8 17 7. 18 1. 17 9. 0 6 17 8. 8 17 0. 16 1. 15 9. 15 7. 7 10 8 6 4 2 0 8 Number of precursors may increase with quench current Number of precursors Current (Amps) Quench current FC Status 6 Nov 2013 20
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