Frequency Transfer Function Measurements during LS1 Emmanuele Ravaioli Thanks to Arjan Verweij, Zinur Charifoulline, Andrea Musso MPE-TM

Unbalanced voltage drop across 2 apertures of some main dipoles Emmanuele Ravaioli MPE-TM For more info: TE-Magnet-Seminary - Circuit simulations of the main LHC dipoles and the case of the 'unbalanced' dipoles – Ravaioli TE-Magnet-Seminary - Circuit simulations of the main LHC dipoles and the case of the 'unbalanced' dipoles – Ravaioli 5JO-3_Ravaioli_ Modeling of the voltage waves in the LHC main dipole circuits Modeling of the voltage waves in the LHC main dipole circuits 28 Hz Why is the impedance of the two apertures different? 28 Hz

Distribution of ΔU pp in S67 (ramp at 2 kA, 10 A/s) Emmanuele Ravaioli MPE-TM There is no correlation between ΔU pp and Electrical position Physical position Magnet manufacturer Cable manufacturer Mechanical parameters (checked with Andrea Musso) The phenomenon is more evident when the current in the magnets is about 2 kA. (less amplitude at 6 kA, less amplitude at 1 kA; only present when magnets are ramping) Why does the phenomenon peak at 2 kA? Will it be worse at 7 TeV? 28 Hz

Cumulative distribution of ΔU pp in all sectors Emmanuele Ravaioli MPE-TM About 50% of the LHC main dipole magnets have a ΔU pp larger than expected.

Some dipoles have even stranger signals – “Outlier dipoles” 5 Emmanuele Ravaioli MPE-TM Displayed dipoles B24L7 B10L7 C27R6 C17R6 FPA at 6 10 A/s (S67 20/05/ ) Some dipoles undergo a strange transient after the opening of the switches, lasting about a second Some magnets in every sector (5-10) present this behavior with different amplitude; in a few cases (a dozen in the whole LHC) the phenomenon was so significant that the oQPS threshold had to be changed to a higher value (see Zinur’s list). The behavior scales up with the current level. Not related to the DQQDL boards used for the QPS measurements. What is the origin of the behavior of the “outlier” dipoles?

Frequency Transfer Function – AC impedance of a system 6 Emmanuele Ravaioli MPE-TM Example:L = 2*L aperture = 98 mHC = 2*C ground = 300 nFR = R parallel = 100 Ω A fast and practical way to measure the behavior of a complex system at different frequencies.

Frequency Transfer Function – Setup 7 Emmanuele Ravaioli MPE-TM Equipment (borrowed from ElQA?) Gain Phase Analyzer Amplifier PC Remarks Time needed for a measurement (magnet+ 2 apertures + 4 poles) ≈ 10 min Max current in the magnets < 1 A If performed after the ‘removal’ of QPS no patches needed (connection to V-taps)

Frequency Transfer Function of dipole apertures in the tunnel 8 Emmanuele Ravaioli MPE-TM Magnet ΔU PP [mV] ΔZ A % % % % Procedure: EDMS Results: EDMS Nice correlation! 28 Hz >15% difference at 28 Hz!

Frequency Transfer Function – Tunnel vs SM18 Emmanuele Ravaioli MPE-TM Reddish → Tunnel Bluish → SM18 AC impedance measured in the tunnel above ~50 Hz about half of the impedance measured in SM18 Why are the FTF measured in the tunnel different from the ones in SM18?

Frequency Transfer Function – Tunnel vs SM18 vs Simulation Emmanuele Ravaioli MPE-TM Two apertures with different cross-contact resistance, i.e. with different inter-strand coupling losses, have a dramatically different AC impedance. In this example: Rc1=100 µΩ, Rc2=25 µΩ The model can simulate impedance differences between the two apertures in the correct frequency range, but it cannot simulate the high frequency behavior! Is there a correlation between AC losses and the measured FTF? Are inter-strand coupling losses the responsible for the unbalance?

Emmanuele Ravaioli MPE-TM Why is the impedance of the two apertures different? Why does the phenomenon peak at 2 kA? Why are the FTF measured in the tunnel different from the ones in SM18? What is the origin of the behavior of the “outlier” dipoles? Will it be worse at 7 TeV? What about quadrupoles? And what about other magnets? Is there a correlation between AC losses and the measured FTF? Are inter-strand coupling losses the responsible for the unbalance? Effects of currents to ground? Phenomenon realated to superconductivity effect or not? We need to know more about the AC impedance of the LHC magnets at different frequencies

Test Proposal – Frequency Transfer Function 12 Emmanuele Ravaioli MPE-TM #WhatWhereWhenTimeMotivationI need help! 1 FTF of 15 to-be-replaced dipoles (2372, 2373, 2377, 2395, 2387, 2413, 2336, 2353, 2357, 2438, 2252, 2138, 1007, 2007) S12, S45, S78, etc Start of LS13 h Compare FTF from tunnel and SM18 (1007↔2431, 2007↔3128, 2373↔2868) 2FTF of 30 more dipoles (60?)S67?Start of LS16 h - More statistics - Look for outliers 3 FTF of 10 “outlier” dipoles (22 non- conformities) Mostly S78 Start of LS12 hBetter understanding 4 FTF of 4 dipoles moving the grounding point of the chain S67?Start of LS12 h Asses influence of paths to ground ElQA 5FTF of 10 main quadrupolesAny SStart of LS12 hV-taps? 6FTF of 1 MQXA, MQXBAny SStart of LS11 hV-taps? 7FTF of 1 MQM, MQMC, MQMLAny SStart of LS11 hV-taps? Start of LS12 days? 8FTF of 4 dipoles at 20 KS23 During CSCM time-frame 1 h Assess impact of superconductivity effects 9 FTF of 15 (5?) to-be-replaced dipoles at warm After cooling down 2 h Assess impact of superconductivity effects 10 FTF of 2 to-be-replaced dipoles (+AC loss measurements) SM18 1 month? - Compare FTF from tunnel and SM18 - Relate FTF and AC loss SM18 team 11FTF of 15 replaced dipoles S12, S45, S78, etc End of LS13 h - Compare FTF from tunnel and SM18 New QPS?

Annex 13 Emmanuele Ravaioli MPE-TM

Distribution of ΔU pp in S12 (ramp at 2 kA and at 6 kA, 10 A/s) -1 Emmanuele Ravaioli MPE-TM Why does the phenomenon peak at 2 kA? Will it be worse at 7 TeV?

Distribution of ΔU pp in S12 (ramp at 2 kA and at 6 kA, 10 A/s) -2 Emmanuele Ravaioli MPE-TM Why does the phenomenon peak at 2 kA? Will it be worse at 7 TeV?

RB.A78: 30/06/ A 5000A 3000A 2000A Top 10 magnets: A16L8 – 3504 B17L8 – 2006 B30R7 – 1007 * B8R7 – 1031 C28R7 – 2011 B29L8 – 1009 A15R7 – 3020 B29R7 – 1013 A21R7 – 3014 A22R7 – 1023 Threshold changed to 264mV! Emmanuele Ravaioli Special TE-MPE-TM Outlier dipoles – Past observations -1 From MPP meeting (16/07/2008) Zinour Charifoulline Note: Data acquired when the value of the extraction resistor was halved

Emmanuele Ravaioli Special TE-MPE-TM Outlier dipoles – Past observations -2 From Zinour Charifoulline

Dipoles high ΔU pp for several seconds 18 Emmanuele Ravaioli MPE-TM Displayed dipoles B28L7 A27L7 A28L7 B32R6 FPA at 6 10 A/s (S67 20/05/ ) Some dipoles present a voltage difference larger than normal after the opening of the switches; such a behavior lasts for many seconds (during the whole discharge?). After the first switch opening, the voltage difference of these dipoles is about half of that measured after the second switch opening A few magnets in every sector (5-10) present this behavior This phenomenon was found to be related to electronics (Board A and B give different results!)

Frequency Transfer Function – Four poles of an unbalanced dipole Emmanuele Ravaioli MPE-TM

Frequency Transfer Function – Modeled AC impedance Emmanuele Ravaioli MPE-TM