1. Powering Interlocks Quench back of corrector magnets vs revised ‘Global Powering Subsector OFF’ functionality M.Zerlauth, W.Venturini, R.Wolf, G.Kirby and many others

2. The problem: When and where was quench-back observed?
Glyn, Walter and other MPP colleagues observed numerous quench backs of 600A correctors during HWC test steps at nominal (ie 550A) and PGC tests (later on replaced by optics values)
Happens typically when correctors are powered at ‘nominal’ current of 550A and a FastPA occurs
Did NOT happen for 1st beam induced quench as mentioned in circulated
Quench back of magnet is a result of the high -∂B/ ∂t during the FastPA and is a function of remaining magnet energy, parallel resistor vs energy extraction and crowbar, current,…
In general, quench back in magnet less ‘harmful’ than natural quench as uniform energy distribution in coil
Question to be answered:
Should one modify the ‘Global Powering Subsector OFF’ to perform a SlowPA rather than a FastPA?

3. Current functionality ‘Global Powering Subsector OFF’
Functionality anticipates a shut-down of all circuits in the same powering subsector / cryogenic volume in case of main magnet quenches and consequent risk of quench propagation
Main idea: Avoid stress due to ‘predictable’ quenches in close-by circuits
Main magnets : MAIN DIPOLE, MAIN QUADRUPOLE, IPQ, IPD, IT
Risk of quench propagation not only within circuit, but also to (mainly busbars of) corrector circuits powered through busbars in the quenching cold-mass
Decay current and extract energy as fast as possible to minimise the risk of natural quenches in adjacent magnets or busbars -> Perform a Fast Power Abort

4. Current functionality
Configurable Parameter / circuit
If ‘Pow Sub OFF’ parameter is set, a quench in this circuit will trigger a Fast Power Abort in ALL other circuits of the powering subsector (exception A circuits which do not have the FastPA possibility)

5. Current functionality
Corresponding reaction of circuits depends on their INTERLOCK TYPE, no circuit by circuit treatment possible
Interlock Type A1 = Circuit Type (MAIN DIPOLE + QUADRUPOLES, IT)
Opening of EE for RB and RQD/F, FastPA in converter for IT
Interlock Type B2 = Circuit Type IPQ
FastPA in converter for IPQ
Interlock Type B1 = Circuit Type (600A with or without EE, 600A crowbar + IPD)
FastPA in converter for IPD and 600A + opening of EE switches if applicable
Interlock Type C = Circuit Type A
Slow Power Abort (only possibility)
Problem of quench back of many 600A correctors during FPA

6. Problem – Solution ?
Potentially very high number of quenches in corrector magnets (every time any of the main magnets quenches), contradiction to initial aim
Circuits could be driven down at a lower rate that will not result in a quench-back straight away
Correctors that actually quench due to high helium temperatures due to the proximity to quenching magnets or due to beam impact will be protected in the normal way
Should minimize number of overall quenches and better longevity?
Proposal: Replace FPA with Slow Power Abort for circuits of interlock Type B1
Note!
Concerns all 600A circuit but also IPD (8 * RD2, 4 * RD1, 2* RD3, 2* RD4)
No issue for RD1 and RD2 as separated by DFBX, RD3 in independent cryostat
RD4 + Q5 in IR4 in same cryostat: Q5 could risk to quench D4 (if at high current) as only decaying with some A/s during SPA

7. Does this really do the trick ?
SW change was prepared and tested in the lab; functionality worked well, ECR written,…
Still, when preparing 1st version of slides, tried to find some quantitative numbers on whether this solution is really improving the situation…
No data on dedicated quench propagation to correctors found from String 2, etc…
What we did:
Investigate FPA vs SPA, how does it affect the risk of having ‘natural’ quenches and what do we gain (or loose)?
With loads of help from Walter and Rob, tried to quantify and understand in detail the extent and reasons for quench back as observed during HWC and early operation
Propose what to do

8. FastPA vs SlowPA for 1Q converter (RD4.R4)
Slow power Abort
(assuming –di/dt_max)
25s
Fast power Abort
Examples:
RPHF.UA27.RD1.R E+01
RPHF.UA27.RD2.R E+01
RPHFD.UA47.RD3.R E+01
RPHFD.UA47.RD4.R E+01
Note: FGC will use same DIDT.TO_STANDBY[0] for Slow PA (=max di/dt as from Layout DB)
For 1 Q converters as RD%, the controlled decay will eventually become a natural decay

9. FastPA vs SlowPA for 4Q converter (ROD.A23B2 + EE)
Examples:
RPLB.UA23.RCO.A12B E+00
RPMBA.RR13.RQS.A81B E+00
RPMBA.RR13.RQT12.L1B E+00
50s
Slow power Abort
Fast power Abort
(+ EE)

10. FastPA vs SlowPA?
When performing a SlowPA in correctors after a quench in the cold-mass, current decays only with di_dt_max (typically 1-5A/s)
Instead of few seconds, energy will remain up to several minutes in circuit
Almost sure to quench busbars in the quenching cold-mass after few seconds only as still at considerable current level
Detected quench will provoke a FastPA in the circuit, open (possible) EE systems and in turn very likely a quench back of all magnets in the circuit
Feeling (but no quantitative confirmation yet) that we’ll quench-back almost same amount of magnets as if doing the FastPA straight away (as current in circuits in case of SlowPA will not change significantly within the first 10s of seconds)
Wit the exception of Line N circuits not passing through this CM
Remember: Problem is mainly the busbars running through the cold-mass (ie affecting ALL circuits), not only the few correctors at the extremities the cold-mass

11. Understanding the observed quench back in the LHC
Walter was digging into HWC data of several sectors, identifying corrector families where quench back happens
Mainly used data from 1st commissioning of sector 45 beg 2008
Reason for this is ‘old’ test plan where we still had PLI2.c* and PNO.c* tests done
PLI2/PNO.c* is a test where the PIC triggered a FastPA in the circuit
Done at 200A and 550A (= close to real optics & ‘nominal’)
Quench back happens for 3 magnet types (MQT, MS and MQS), ie circuit families RQTD/F, RSD/F, RSS and 550A
Quench back does NOT happen for ANY circuit 200A
Courtesy of W.Venturini

12. ‘quench back’ phenomenon
Courtesy of W.Venturini

13. Calculating the not-extracted energy and -∂B/ ∂t_max during the FPA
CIRCUIT NAME
DESCRIPTION
Number of magnets
Inductance (linear)
total inductance
Parallel resistor
cables
R dump
Crowbar
magnet internal time constant for FPA
Nominal energy
Extracted energy
Deposited energy in Rpar
Extraction
nom B coil
DB/Dt max
Dep En/Mag
Rpar power/mag
Quench on FPA at 550?
Quench on FPA at 200? # H Ω s J % T
T/s W
RCD.A45B1
Decapole spool piece circuit, all magnets in MBA dipoles in series per sector 77
4.00E-04
3.08E-02
1.00E+09
0.008
0.7
0.05
0.0406
4.66E+03
4.59E-08
100.00
0.47
11.51
5.96E-10
1.47E-08 No
RCD.A45B2
RCS.A45B1
Sextupole spool piece corrector circuit, all magnets in MBA and MBB dipoles in series per sector
154
8.00E-04
1.23E-01
0.08
0.07
0.1602
1.86E+04
1.75E+04
1.16E+03
93.76
1.02
6.36
7.55E+00
4.71E+01
RCS.A45B2
ROD.A45B1
Defocusing Octupole circuit, all magnets in series per sector 8
1.50E-03
0.012
0.0034
0.0159
1.82E+03
1.81E+03
1.71E-07
0.92
57.90
2.14E-08
1.34E-06
ROD.A45B2 13
0.0195
0.0033
0.0259
2.95E+03
35.63
1.31E-08
5.08E-07
ROF.A45B1
Focusing Octupole circuit, all magnets in series per sector
ROF.A45B2
RQS.A45B1
Skew quadrupole circuit 4
0.031
0.124
0.25
0.0031
0.2887
1.88E+04
1.07E+04
8.06E+03
57.04
3.00
10.39
2.01E+03
6.98E+03
Yes
RQTD.A45B1
Defocusing tune shift quadrupole circuit
0.248
0.007
0.4516
3.75E+04
2.72E+04
1.03E+04
72.54
6.64
1.29E+03
2.85E+03
RQTD.A45B2
RQTF.A45B1
Focusing tune shift quadrupole circuit
RQTF.A45B2
RSD1.A45B1
Defocusing chromaticity sextupole circuit. magnets on beam 1 in series per sector 12
0.036
0.432
0.15
0.8107
6.53E+04
4.60E+04
1.93E+04
70.39
2.77
3.41
1.61E+03
1.99E+03
RSD1.A45B2 11
0.396
0.7631
5.99E+04
4.11E+04
68.55
3.63
1.71E+03
2.24E+03
RSD2.A45B1
Defocusing chromaticity sextupole circuit, magnets on beam 2 in series per sector
RSD2.A45B2
RSF1.A45B1
Focusing chromaticity sextupole circuit, magnets on beam 1 in series per sector 9
0.324
0.6680
4.90E+04
3.14E+04
1.76E+04
64.07
4.14
1.96E+03
2.93E+03
RSF1.A45B2 10
0.36
0.7156
5.45E+04
3.62E+04
1.83E+04
66.46
3.87
1.83E+03
2.55E+03
RSF2.A45B1
Focusing chromaticity sextupole circuit, magnets on beam 2 in series per sector
RSF2.A45B2
RSS.A45B1
Skew sextupole circuit
0.144
0.4312
2.18E+04
9.66E+03
1.21E+04
44.34
6.42
3.03E+03
7.03E+03
RSS.A45B2
RQS.L5B2 2
0.062
0.0026
1.3027
9.38E+03
8.48E+03
8.93E+02
90.48
2.30
4.46E+02
3.43E+02
RQS.R4B2
0.0066
1.2194
8.42E+03
9.54E+02
89.83
2.46
4.77E+02
3.91E+02
RQT12.L5B1
Tuning trim quadrupole circuit in the extended dispersion suppressor region, next to Q12 1
0.0025
0.7145
4.69E+03
3.88E+03
8.14E+02
82.64
4.20
1.14E+03
RQT12.L5B2
0.0024
8.12E+02
82.67
4.19
RQT12.R4B1
0.0067
0.6707
3.82E+03
8.67E+02
81.51
4.47
RQT12.R4B2
0.0052
0.6856
3.84E+03
8.48E+02
81.91
4.38
1.24E+03
RQT13.L5B1
Tuning trim quadrupole circuit in the extended dispersion suppressor region, next to Q13
RQT13.L5B2
RQT13.R4B1
0.6717
8.66E+02
81.54
RQT13.R4B2
RQTL11.L5B1
Dispersion suppressor trim quadrupole circuit, next to Q11
0.12
0.2
2.8814
1.82E+04
1.44E+04
3.78E+03
79.18
1.04
1.31E+03
RQTL11.L5B2
RQTL11.R4B1
2.7164
1.41E+04
4.01E+03
77.91
1.10
1.48E+03
RQTL11.R4B2
2.7053
4.03E+03
77.82
1.11
1.49E+03
Courtesy of W.Venturini

14. Calculating the not-extracted energy and -∂B/ ∂t_max during the FPA
Quench-back
Courtesy of W.Venturini

15. Calculating the not-extracted energy and -∂B/ ∂t_max during the FPA
Quench-back
Courtesy of W.Venturini

16. Conclusions
More detailed investigations and analysis of quench back phenomena performed, revealing a number of circuit types that will consistently quench back at 550A
No circuit type will quench back at 200A, which is a value the concerned circuits will most likely NEVER attain during nominal optics of the LHC
SlowPA instead of FastPA will not (or only insignificantly) decrease the number of magnets quenching back + increase the risk of natural quenches
Our conclusion: Quench back during few HWC nominal current will happen and are unavoidable, during nominal operation very limited risk of quench back
Stay as is with the FastPA for B1 type circuits
Further measures that could be taken
Activate Global Powering Subsector OFF as late as possible & decrease the number of circuits that activate it (to RB, RQD and RQF) to further minimize it’s occurrence
SlowPA modification documented and available and if investigations still show wrong during operation could be implemented (rather) quickly

17. THANKS A LOT FOR YOUR ATTENTION

18. Quench back of numerous 600 corrector circuits during PGC

19. Details of Change

20. Conclusions
Possibility to change the FastPA (provoked by PIC in case of main magnet quenches) of B1 type circuits to a Slow PA has been implemented and tested in lab
Avoids quench-back of corrector mechanisms, will increase number of natural quenches due to slower current decay , relation to DI_DT_PNO?! (quantitative numbers?!)
Change requires a modification of the generic SW package of all 36 installations + update of SW repositories (CVS/SVN)
For us sufficient to perform type tests after SW change in the field (no changes in protection of individual circuits)
If change approved today, ECR is ready for formal circulation
Change will be applied before re-starting HWC end April