Proposal for FTF measurementson dipole magnets in the tunnel

during the Christmas break

Emmanuele Ravaioli

TE-MPE-TM08-12-2011

Emmanuele Ravaioli TE-MPE-TM 08-12-2011 2

Background

• It has been observed that after a Fast Power Abort the two apertures of about 50% of the dipoles in every RB circuit behave differently (unbalanced dipoles). This phenomenon caused problems at the beginning of 2010, when modifications to the QPS were required in order to avoid spurious QPS quenches.

• A PSpice electrical model of the RB circuit is available. It simulates accurately the behavior of any dipole in the 8 sectors, and it was used to successfully reproduce unusual events (AUG event, ungrounding of the RB chain during ElQA tests).

• Nevertheless, a few questions remain:

• How do the apertures of the unbalanced dipoles behave at different frequencies? The PSpice model is validated only for frequencies close to 28.5 Hz (frequency of the filter at the output of the power converter).

• What is the origin of such a phenomenon? Why does it peak at 2 kA?

• Spare dipoles are being tested in SM18 in order to investigate the phenomenon, but there is no prove that they are unbalanced.

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Summary of the results of the FTF in SM18

1. Frequency Transfer Function of MB1089 is now available in a wide frequency range (10 mHz – 50 kHz) and under different conditions. With these new data, it will be possible to extend the validity of the electrical model of a dipole to a wider frequency range.

2. The dipoles MB1089 and MB2431 are not unbalanced dipoles. No difference between the FTF of their two apertures have been observed in the relevant frequency range (10-200 Hz).

3. The influence of the current level is important only at low frequency (< 50 Hz). Nevertheless, a relevant current effect was to be expected only for the unbalanced dipoles, whose unbalance peaks at 2 kA.

4. The inductance of a dipole at low current (< 300-500 A) is about 80% of the nominal value. This is evident both from the FTF performed in SM18 and from PM Browser data referred to a typical LHC ramp.

More details here:Measurement of the Frequency Transfer Function of the MB1089 - Ravaioli

Results – FTF – Is MB1089 an unbalanced dipole? Gain

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0 A Without 100 Ω resistor in parallel to the dipole Apertures

Results – FTF – Is MB1089 an unbalanced dipole? Phase

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0 A Without 100 Ω resistor in parallel to the dipole Apertures

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Proposal for the Christmas break 2011-12Measuring the Frequency Transfer Function of 4-6 selected dipoles in the tunnel (at 1.9 K and at zero current) and of their apertures individually.

1. Verifying the existence of the so-called “unbalanced dipoles“, whose apertures have different AC behavior (in particular in the frequency range 10-200 Hz). The two dipoles tested so far in SM18 are perfectly balanced (MB1089, MB2431).

2. If they do exist, obtaining detailed information about their behavior in a wide range of frequency (10 mHz – 200 Hz). The present PSpice electrical model reproduces correctly the AC behavior of the apertures of balanced and unbalanced dipoles, but is validated only for transients with frequency of about 28.5 Hz (frequency of the filter at the output of the power converter).

3. In any case, acquiring more statistics about the different AC behavior of different dipoles. The available data are mainly referred to pre-series dipoles or to first dipoles of a series (X001).

Goals of the tests

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Details about the FTF measurements• Gain-Phase analyzer (Powertek GP102)• Current injected from the diode current-taps EE012

and EE013 (< 1 A)• Voltages measured through voltage-taps EE112,

EE219, EE113, EE211, EE219, and EE212 (patches needed)• Time required: 1 QPS 2 hours for installing the

patches. ER 8 hours for performing the tests. 1 QPS 2 hours for taking out the patches.

• Flexibility: There are interesting unbalanced dipoles to test in any sector.

Limitations• The huge capacitance to ground of the rest of the dipole chain (~46 μF) will

affect the results at high frequency.• The presence of the 100 Ω resistor in parallel to the dipoles will affect the

results for frequencies >~200 Hz.• The expected difference between the impedance at 28.5 Hz of a balanced and a

highly unbalanced aperture is about 10%. It is enough to be spotted in SM18, not sure in the tunnel.

Annex

8Emmanuele Ravaioli TE-MPE-TM 08-12-2011

Most slides from:

LHC-CM 2011-11-11 Ravaioli

Measurement of the Frequency Transfer Function of the MB1089 - Ravaioli

Dipole Voltage Taps

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Courtesy of Guy Deferne

Distribution of dipoles of all the 8 sectors of the LHC

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Max amplitude of QS_0 after PC switch-off: Difference between measurement and balanced case

Distribution of dipoles of all the 8 sectors of the LHC

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Max amplitude of QS_0 after PC switch-off: Difference between measurement and balanced case

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Setup of the measurement systemsCONFIGURATION 1 (C1)Direct measurement with Gain-Phase analyzer (Solartron 1260A or Powertek GP102). Power provided by a generator controlled by the GPA.

CONFIGURATION 2 (C2)13kA-200V power converter controlled by the Gain-Phase analyzer (GP102)

CONFIGURATION 3 (C3)13kA-200V power converter provides the DC current; 60A-08V power converter provides the AC voltage, controlled by the Gain-Phase analyzer (GP102). Current measured through the DCCT signal, filtered of its DC component and amplified by 1000 times.

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List of performed measurements

Date Current Frequency Range Notes

11 /10/2011 0 A 100 mHz – 20 kHzDirect measurement with Gain-Phase analyzer (Solartron 1260A). Power provided by a generator controlled by the GPA. With 100 Ω resistor in parallel to the dipole. (C1)

16-18/11/2011 0 A 50 mHz – 50 kHz (2 MHz*)Direct measurement with Gain-Phase analyzer (Powertek GP102Power provided by a generator controlled by the GPA. Without parallel resistor. (C1)

30/11-01/12 /2011 120 A – 2 kA 10 mHz – 30 Hz 13kA-200V power converter controlled by the Gain-Phase analyzer (GP102). Without parallel resistor. (C2)

4-9/11/2011 50 A – 6 kA 30 Hz – 200 Hz (2 kHz**)

13kA-200V power converter provides the DC current.60A-08V power converter provides the AC voltage, controlled by the Gain-Phase analyzer (GP102).Current measured through the DCCT signal, filtered of its DC component and amplified by 1000 times.With and without parallel resistor. (C3)

* Maximum measured frequency, but data not valid due to capacitive effects which may come from the measurement system** Maximum measured frequency, but data not valid due to inadequate response of the power converter (it cannot follow!)

28/11-01/12 /2011 0 A – 6 kA ---Measures of the voltage across the dipole and the current flowing through it during various ramps, in order to measure the real inductance of the dipole. (C2)

Results – Frequency Transfer Function of a dipole Gain

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Configuration 1 0 A With and without parallel resistor

Results – Frequency Transfer Function of a dipole Phase

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Configuration 1 0 A With and without parallel resistor

Results – FTF – Comparison with old SM18 data Gain

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Configuration 1 0 A Without parallel resistor

Results – FTF – Comparison with old SM18 data Phase

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Configuration 1 0 A Without parallel resistor

Results – FTF – Dependence on the current level Gain

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Configuration 2 120 A – 2 kA Without parallel resistor Low f

Results – FTF – Dependence on the current level Gain

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Configuration 3 1 kA – 6 kA Without parallel resistor High f

Results – FTF – Is MB1089 an unbalanced dipole? Gain

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Configuration 1 0 A Without parallel resistor Apertures

Results – FTF – Is MB1089 an unbalanced dipole? Phase

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Configuration 1 0 A Without parallel resistor Apertures

Results – Inductance of a dipole (from PM Browser!) -1

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V_meas, I_meas, dI_meas/dt, L during a typical LHC ramp to 2 kA

Results – Inductance of a dipole (from PM Browser!) -2

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L of a single dipole during a typical LHC ramp to 2 kA

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Short-term proposals• FTF of the two apertures at low frequency and high current (C2) Before 17/12

• FTF at high current (6 – 12 kA) (C2-C3) Before 17/12

• Series of tests on new spare dipoles (C1-C2-C3)

Ready: 1088. Available in the future: 3128, 3110 (Jan-12); 2194 (mid-12); 1109, 1235 (after).

Christmas break proposals• FTF of selected dipoles (mostly unbalanced). Using a Gain-Phase analyzer,

injecting current from the diode voltage-taps, and measuring the voltage across the two apertures through the dipole voltage-taps (patch needed). (C1) Feasible?

Long term proposals• Series of tests on the dipoles taken out from the tunnel during LS1 (C1-C2-C3)

Provisional list: 1007, 2007, 2138, 2214, 2413 (sure); 1263, 1372, 2252, 2336, 2353, 2357, 2372, 2373, 2377, 2387, 2395, 2413, 2438, 3708. (highly unbalanced)

• Writing a procedure for FTF tests to be performed on each new dipole that arrives in SM18 (wider range of frequency; different current levels; compare apertures).

Simulation of the electrical behavior of the LHC dipole circuits

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For more info:• TE-Magnet-Seminary - Circuit simulations of the main LHC dipoles and the case of the 'unbalanced' dipoles – Ravaioli• 5JO-3_Ravaioli_20110916• Modeling of the voltage waves in the LHC main dipole circuits

Model of a dipole

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For more info:• 5JO-3_Ravaioli_20110916• Modeling of the voltage waves in the LHC main dipole circuits

Eddy Currents in the coils

Magnetization Effects

Parasitic Coil-to-Ground Capacitance

Inhomogeneous AC behavior of the two

apertures of the dipole

Different frequency response

Phase-velocity of the wave changing along the

dipole chain

Each aperture shifts the wave of a different angle

L = Laperture = 49 mHC = Cground = 150 nFRp = Rparallel = 100 ΩCp = 1 pF (for the moment)

k = 0.757 Ω < R1,2 < 10 Ω

Parasitic Turn-to-Turn Capacitance

Frequency Transfer Function

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Example: L = 2*Laperture = 98 mH C = 2*Cground = 300 nFR = Rparallel = 100 Ω

Matlab applicationfor the study of the parameters

of the proposed modelof a dipole aperture

Impedance of a stand-alone aperture model: (C/2) // [ (1-k)*L + (k*L // R) ] (second Z/2 bypassed by a short-circuit)

Impedance of a series of aperture models: (Cp, Rp ignored here for simplicity)

(C/2) // ∑Nmodules [ (1-k)*L + (k*L // R) ] + C // [ (1-k)*L + (k*L // R) ]

Developing of a new electrical model of a dipole -1

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1. The measured Frequency Transfer Function (FTF) is not fitting with the expected one, and corresponds to parameters different from the nominal ones:

• L 49 mH → 35 mH • k 0.75 → 0.45-0.55• Rparallel 100 Ω → 80 Ω • R 10 Ω → 30 Ω

• The shape of the measured Frequency Transfer Function does not correspond to the expected curve calculated with the adopted electrical model.

Possible explanation: The model has been tailored on the measurements during Fast Power Aborts, when the main excitation frequency is ~28.5 Hz. Therefore it is possible that the model fits well the behavior of the dipoles, but only around 30 Hz. (see the qualitative example below: measurement, old model, new model)

Developing of a new electrical model of a dipole -2

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• Development of a new electrical model of the dipole apertures, fitting their behavior in a wider range of frequency, and test of its capability to simulate the actual behavior of the dipole circuit. Such a model could be developed by fitting the curve of the impedance of an aperture without Rparallel , and may include the splitting of the inductance in 3 parts (4 free parameters: k1, k2, R1, R2). Fitting already started with Matlab.