indico.fnal.gov€¦ · Web viewHelium gas above atmospheric pressure The previous analysis was done under the assumption that the helium gas pressure is 1 bar. This assumption is

Failure Analysis of MQXF Heater-Coil Insulation

US-HiLumi-doc-921Other:Date: 1/31/19Page 1 of 52

US HL-LHC Accelerator Upgrade Project


Prepared by:Vittorio Marinozzi, FNAL

Reviewed by: Giorgio Ambrosio, FNAL

Approved by:

This document is uncontrolled when printed. The current version is maintained on http://us-hilumi-docdb.fnal.gov



Revision History

Revision Date Section No.

Revision Description

v0 4/13/18 All Initial draftV1 12/3/18 5, 6 Added section 5 and updated conclusionsV2 1/31/19 5,6,7,8 Revised section 5 and 8; added sections 6 and 7.




TABLE OF CONTENTS

1. ABSTRACT............................................................................................................................................5

2. INTRODUCTION..................................................................................................................................5

3. HEATER TO COIL VOLTAGE COMPUTATION..........................................................................5

4. HELIUM GAS AT ATMOSPHERIC PRESSURE...........................................................................10

5. HELIUM GAS ABOVE ATMOSPHERIC PRESSURE..................................................................11

6. QUENCH HEATER TEMPERATURE.............................................................................................12

7. NOTE ON THE QUENCH HEATERS CONNECTIONS...............................................................15

8. CONCLUSIONS...................................................................................................................................17

REFERENCES..............................................................................................................................................18




1. Abstract

The MQXF protection system is based on CLIQ (Coupling-Loss Induced Quench system) and outer layer quench heaters. The quench heater strips are electrically insulated from the coil by a 50 µm thick polyimide sheet. Small Cracks in the cable insulation (made of braided S2-glass filled with epoxy resin) may be present after coil impregnation. In the worst-case scenario the location of some cracks may coincide with a damage to polyimide insulation causing a direct path between heater and coil. In this document we study this worst-case failure scenario during a quench, in order to verify the electrical robustness of the MQXF magnet design.

2. Introduction

The purpose of this document is providing a complete analysis of a failure of the insulation between quench heaters and coil in MQXF magnet. The analysis is performed for both version of the magnet, MQXFA (4.2 m) and MQXFB (7.15 m). We compute the peak voltages between coils and quench heaters during a quench, in order to verify if they are still sustainable by the magnet when a damage in the insulation is present. We first present the simulations performed to compute the peak voltage between coils and quench heaters during a quench. Then, we discuss the analysis of a failure, assuming that the insulation between quench heaters and coil is cracked and filled with helium at atmospheric pressure, and at temperature equal to the coil temperature. Then, we investigate the case of helium pressure increase due to heating process. Finally, we consider the effect of quench heater temperature on the helium temperature as well.We aim at understanding if the electrical design of MQXF is robust against small defects and weakness of the heater-to-coil insulation.

3. Heater to Coil Voltage Computation

The MQXF cable insulation is made of S2-glass, braided on the cable, and filled with epoxy resin. Cracks in the cable insulation may occur, during MQXF test and operation, because of thermal contractions and electromagnetic forces. Polyimide cuts, together with epoxy cracks, may create a direct path from coil to quench heater. In the very worst case the locations of these failures may coincide, and the heater may be separated from the conductor only by a 200 μm deep crack filled with helium. The minimum distance is 200 μm because the minimum cable insulation is 140 μm thick, the polyimide insulation is 50 μm thick, and there is a ~10 μm layer of glue between the heater and the polyimide.This insulation failure is difficult to detect at cold (1.9 K), since superfluid helium is a good insulator. However, during a quench, the temperature of the coil, and therefore of the helium, can easily reach > 100 K. It is therefore important to check whether the helium gas can withstand the voltage difference between the quench heater and the coil during a quench without electric discharge. The quench simulations have been performed using LEDET. The voltage of each turn of the magnet has been computed, and the voltage difference between each turn and the corresponding quench heater has been evaluated. The simulations have been performed for the short models MQXFS (1 m long), MQXFA (4.2 m long), and MQXFB (7.15 m long).




In the simulations, the protection system is assumed to be the nominal one, which consists of CLIQ and OL quench heaters, with no dump resistor. The analysis has been performed at nominal current (16470 A). The quench is assumed to occur in the inner layer pole turn.In Table 1, the protection system parameters are reported for each magnet. These values are taken from the MQXF quench protection Technical Report [1].

Table 1: Protection System ParametersMQXFS MQXFA MQXFB

Heaters charging voltage [V] ±300 ±300 ±450CLIQ charging voltage [V] 200 500 1000

CLIQ capacitance [mF] 80 40 40

The connection scheme of the coils is the nominal one, and we assume that the quench heater polarities are opposite on the two sides of the same coil, that is the worst scenario for heater-to-coil voltages. Figure 1 summarizes polarities and connections.

Figure 1: Connection scheme of the magnet, and the polarities of CLIQ and quench heaters.




The results of the simulations are summarized in table 2. For each magnet, voltages are reported for Low-Field (LF) and High-Field (HF) blocks, and assuming the heater polarity positive (side 1) or negative (side 2). For each case, values are reported at two different temperatures to identify the worst cases for each heater. The worst cases for each magnet are highlighted in red. Worst cases are identified considering the worst combination of voltage and temperature, respect to the helium breakdown voltage at the same temperature.

Table 2 Nominal case: CLIQ, OL heaters

SQXF MQXFA MQXFBPole 1, OL-LF, side

150 V, 50 K / 10 V, 130 K 220 V, 50 K / 220 V, 80 K 100 V, 50 K / 320 V, 80 K

Pole 1, OL-LF, side 2

60 V, 50 K / 40 V, 120 K 100 V, 50 K / 200 V, 80 K 390 V, 50 K / 410 V, 80 K

Pole 1, OL-HF, side 1

50 V, 60 K / 70 V, 90 K 300 V, 45 K / 210 V, 100 K

180 V, 60 K / 400 V, 100 K


40 V, 60 K / 50 V, 90 K 270 V, 60 K / 250 V, 100 K

510 V, 60 K / 450 V, 100 K


70 V, 50 K / 50 V, 120 K 300 V, 50 K / 280 V, 100 K

600 V, 40 K / 380 V, 100 K


80 V, 50 K / 70 V, 120 K 200 V, 50 K / 250 V, 100 K

210 V, 40 K / 320 V, 100 K


40 V, 80 K / 45 V, 120 K 300 V, 50 K / 250 V, 80 K 610 V, 40 K / 330 V, 75 K


70 V, 80 K / 45 V, 120 K 150 V, 50 K / 220 V, 80 K 500 V, 40 K / 280 V, 75 K


120 V, 60 K / 80 V, 110 K 190 V, 40 K / 220 V, 70 K 600 V, 30 K / 600 V, 70 K


30 V, 60 K / 70 V, 110 K 400 V, 40 K / 320 V, 70 K 600 V, 40 K / 740 V, 70 K


110 V, 50 K / 90 V, 110 K 250 V, 75 K / 180 V, 120 K

600 V, 60 K / 600 V, 70 K


30 V, 50 K / 80 V, 110 K 330 V, 75 K / 190 V, 120 K

750 V, 60 K / 750 V, 70 K


100 V, 50 K / 60 V, 110 K 250 V, 75 K / 200 V, 120 K

400 V, 50 K / 500 V, 75 K


100 V, 50 K / 70 V, 110 K 200 V, 75 K / 190 V, 120 K

400 V, 50 K / 350 V, 75 K


70 V, 50 K / 50 V, 110 K 120 V, 50 K / 190 V, 85 K 220 V, 50 K / 300 V, 80 K


80 V, 50 K / 60 V, 110 K 80 V, 50 K / 150 V, 85 K 80 V, 50 K / 220 V, 80 K

Figures 2 and 3 show the worst case (highlighted in red in Table II) of the heater-to-coil voltage evolution during a quench, as a function of the corresponding temperatures, for MQXFA and MQXFB respectively. Appendix B and C of this document show similar plots for all the poles of the magnet, for MQXFA and MQXFB respectively, both in logarithmic and linear scales. Appendix D shows heater-coil voltages at 18kA in case of a quench in the pole turn of the outer layerThe values reported in Figures 2 and 3 are compared with several curves: the blue curves show the helium BreakDown (BD) voltage as a function of the temperature, at 1 bar (solid) and 10 bar (dashed), for a path of 0.2 mm. These curves are relevant for the discussion carried in the next section.




Black curves show the helium BD voltage as a function of the temperature, for a path of 4 mm. The quench heater traces have holes in the Kapton trace, made in order to get a better impregnation. The minimum distance between holes and heater trace is 4 mm. The black curves are useful to show the margin for this 4 mm path.Red curve is the 50 μm Kapton BD voltage, reported in order to show the margin of the Kapton, in case it is not damaged. It is evident that in this case, a good margin is present, and electrical design looks consistent.

Figure 2: Heater-to-coil voltages VS temperatures for MQXFA, pole 3, low-field zone, nominal protection, nominal current..




Figure 3: Heater-to-coil voltages VS temperatures for MQXFA, pole 3, low-field zone, nominal protection, nominal current.

Figure 4: Heater-to-coil voltages VS temperatures for MQXFA, pole 3, low-field zone, nominal protection, 18 kA




Figure 5: Heater-to-coil voltages VS temperatures for MQXFB, pole 3, low-field zone, nominal protection, 18 kA

The helium curves have been obtained by interpolations of the data reported in plots from A-1 to A-4 shown in Appendix A [2][6]. Figures 4 and 5 show the same plots, at 18 kA (slightly more than ultimate current, 17890 A). In average, voltages are ~25% lower at nominal current than at 18 kA.

4. Helium gas at atmospheric pressure

Looking at figures 2 and 3, it can be seen that for MQXFA, the maximum expected voltage between coil and heater is comparable with the helium BD voltage (blue solid curve), in case of a 200 μm path filled with helium at 1 bar (assuming the helium temperature is equal to the coil temperature); for MQXFB, the voltage is considerably higher than the helium BD voltage (blue solid curve).Under these assumptions, in case of damage of the Kapton in correspondence of an epoxy crack, MQXFA could hardly sustain the peak voltage, while MQXFB would not be able to sustain it. Both magnets overcome the helium BD at ultimate current. The same conclusion can be reached for the results presented in appendix B, C and D .As it can be seen in Table 2, for the short magnet MQXFS the peak voltage values are always lower than the He BD values, so it could be considered safe in any case, even if at the limit. We can therefore conclude that the fact that the short magnets did not detect any issues does not prove that the longest magnets will not do so.

5. Helium gas above atmospheric pressure




The previous analysis was done under the assumption that the helium gas pressure is 1 bar. This assumption is very strong and conservative, since helium gas tends to expand significantly at cryogenics temperature when subject to temperature increase, and being trapped inside the cracks, its pressure should likely increase. Making the simple assumption that the helium gas is trapped within the epoxy resin, and it preserves volume and mass during quench, the resulting pressure can be easily computed. For instance, starting from 1 atm pressure at 1.9 K, the helium pressure grows up to ~530 atm at 100 K [3]. In these conditions some helium gas is expected to flow out of the crack when subjected to such high pressure. Modeling this scenario is not trivial. Nonetheless, we should keep in mind that the outer coil surface is pushed against the structure by very strong magnetic forces, and the polyimide ground-insulation should act as seal. In particular, the contact pressure between coil (outer layer) and structure is always above 10 MPa, or 100 bar as it can be seen in figure 6 [4],Since the roughness of the two surfaces in contact is low, it is reasonable to assume that they act as a good seal, able to sustain helium pressure of ~ 100 bar. Moreover, assuming that just a fraction of this pressure (10% equivalent to ~10 atm ) can be actually sustained by the “seal”, the helium breakdown voltage would be > 1 kV at 75 K (see Figure A-2).

Under these more realistic assumptions, the peak voltage in the coil is clearly lower than the helium breakdown, considering a 200 μm path (dashed blue curves in Fig. 2 and 3 and 4-5 for ultimate current). A complete analysis of all the poles of the magnet for MQXFA and MQXFB is reported in Appendix B and C respectively, confirming this conclusion. Therefore, in this more realistic case, we can conclude that there is enough margin to prevent shorts between the coil and the outer layer heater of the longest coils, although it should be assessed by more accurate modelling.




Figure 6: Average contact pressure between MQXF coil (outer layer) and structure on the midplane block (path 1) and on the pole block (path 2). Green curves show the contact pressure after cool-down (1.9 K), purple curves show the contact pressure after energization at nominal current (16.47 kA). The pressure is always larger than 10 MPa in all considered scenarios.

This model assumes that the initial pressure of the helium is 1 bar, which means that cracks are completely filled with superfluid helium before a quench. It is possible that this pressure is reached after few quenches. Cracks have a limited communication with the external environment, and the initial atmosphere is hardly pumped out during cool-down; then, when temperature is low enough, the initial atmosphere condenses, and superfluid helium takes its place. After a quench, the condensed elements return gaseous, and are pushed out of the crack by the pressurized helium. According to this mechanism, a larger fraction of superfluid helium fills the cracks quench by quench. So, initial helium pressure could be slightly smaller than 1 bar before the first quench, and then tend to 1 bar quench by quench [7]. We point out that this situation affects just the very beginning of the process, while, during the heating process, the pressure increase is consistent, and being the initial pressure 1 bar or slightly less becomes almost negligible.

6. Quench heater temperature

The previous analysis has been performed considering helium at the same temperature of the turns covered by the quench heaters. However, we should also consider that the quench heaters quickly heat up during a quench, reaching high temperature (≤ 200 K) in ~10 ms. Figure 8 [5] shows the temperature evolution of the quench heaters. In case of a failure of the polyimide insulation under a quench heater, the heater is expected to fail too, and to be disconnected, since it is printed exactly on the player. Under the assumption that polyimide fails without affecting the corresponding quench heater, we can imagine that helium close to the quench heater reaches similar temperature. At higher temperature, helium is a “poor insulator” (see figures in appendix A) and it could cause a short between coil and quench heater. Moreover, peak coil-to-heater voltage at the beginning of the discharge represents the worst case scenario if we assume helium to be immediately heated up. At quench start the insulation must be able to withstand up to ~950 V or ~600 V [1], for MQXFB (7.15 m) and MQXFA (4.2 m) respectively.




Figure 8: Temperature of the MQXF quench heaters after firing

Since the quench heater temperature increases rapidly, a temperature gradient in the 50 μm polyimide layer occurs; the top of the polyimide (close to the quench heaters) will be at a temperature similar to the heaters one whereas the bottom of the polyimide layer (close to the coil) will be at lower temperature. The temperatures of the top (blue curve) and bottom (grey curve polyimide layers are shown in fig. 9 [5]and compared to quench heater temperatures (orange curve) for MQXFA (left plot) and MQXFB (right plot). Average temperature of the polyimide after 10 ms is ~130 K for MQXFA, and ~150 K for MQXFB.

Figure 9: Temperature of the MQXFA (4.2 m, left) and MQXFB (7.15 m, right) quench heaters and 50 m polyimide layer after firing.

It is also true that the coil is still cold after this short time. Indeed, quench heaters increase coil temperature slightly above the critical temperature (~ 10 K) with an average delay of ~15 ms.




Therefore, we can assume that helium close to the coil is still cold and being a better insulator than at room temperature (see figure 10) can sustain higher breakdown voltage values.A simple model to describe this complex scenario is assuming that half of the helium (100 μm) is “cold” (10 K) and at atmospheric pressure (1 bar), and half of the helium (100 μm) is “hot” (150 K, average peak temperature of MQXFB polyimide layer during a quench) and at 10 % of the seal pressure (10 bar). The corresponding helium breakdown voltages can be obtained interpolating data from plots in Appendix A for a 0.1 mm path, such as done for the 0.2 mm and 4 mm path curves shown in Figures 2-5. Under this basis, a damaged insulation can withstand up to 1.5 kV during a quench, at the very beginning of the discharge. This number should be compared with the peak heater-coil voltage, that is 950 V for MQXFB, and 600 V for MQXFA. A good margin is guaranteed.

7. Note on the quench heaters connections

Heaters connections, as stated at the beginning of this document, are assumed not being optimized to reduce heater-coil voltage (see fig. 1). In principle, it is possible to optimize the connections, reducing considerably the peak heater-coil voltage.Figure 10 shows the voltage distribution of MQXFB, as an example, immediately after CLIQ discharge. In order to reduce heater-to-coil voltage after CLIQ discharge, it is recommended to set a positive voltage on the heaters on pole 1 and pole 4, and a negative voltage on the heaters on pole 2 and 3. However, as shown in Appendix B and C (that show all the heater-to-coil voltages as a function of the temperature, compared with helium breakdown voltages) this generates a worst situation over the temperature range in which the coil is heated-up and the heater is already discharged; in these cases, heater connection will have slight impact on the coil-to-heater voltage, being the heater already at < 20 V. Moreover, in some cases this heaters connection would increase the coil-to-heater voltage immediately after heaters firing and CLIQ discharge . therefore a choice has to be made to optimize coil-to-heater voltage. In conclusion, it is recommended to explore the possibility of optimizing heaters connections to reduce peak heater-coil voltage and increase margins when heaters and CLIQ are fired and heaters are hot, reducing the helium breakdown voltage.




Figure 10: Voltage to ground distribution in MQXFB immediately after CLIQ discharge

Figure 11: Helium breakdown voltage for a 0.2 mm path, at 1 bar (blue line) and at 10 bar (orange line), as function of

temperature, compared with worst cases in MQXFA and MQXFB at nominal current

8. ConclusionsIn this report we have presented the analysis of the failure of the polyimide insulation between MQXF quench heaters and coils.




In this failure analysis we made the following assumptions: (i) the polyimide insulation under an MQXF quench heater has some damages or cuts, (ii) the heater is not disconnected from the Heater Firing Unit, (iii) the location of a cut coincides with the location of a crack in the cable insulation (made of fiber-glass filled epoxy), (iv) they generate the worst-case scenario of a 200 um direct path cable-to-heater filled with helium, and (v) this path is located where there is the maximum coil-to-heater voltage during a quench. Under these assumptions the coil-to-heater voltages at intermediate temperatures (~100 K) during quench appear to be comparable (MQXFA magnets) or higher (MQXFB magnets) then the helium gas breakdown voltage at 1 bar pressure. Nonetheless we should consider that the pressure of helium gas in the crack is expected to increase significantly with temperature, and that helium voltage breakdown increases with pressure. A significant helium pressure increase is expected in coil outer layer cracks where the polyimide ground-insulation, compressed between coil and structure, is expected to act as seal. In MQXF magnets the contact pressure between coil and structure is always larger than 100 bar. Therefore, assuming that the seal can sustain at least 10% of this pressure, the pressure of the helium in the crack should be equal or higher than 10 bar. The helium voltage breakdown at 100 K and 10 bar is higher than 1 kV for a 0.2 mm path. Therefore, it provides ≥ 700 V margin in MQXFA magnets, and a smaller value in MQXFB magnets (Figure 11). The same reasoning may not be applied to inner layer heaters, confirming that removing them from MQXF design was a prudent decision.A possible objection to this analysis is that it does not consider that helium close to the heaters warms up immediately after quench heaters firing. Since heaters warm-up and cool-down very rapidly with respect to coils, the scenario to be considered is the one immediately (~20 ms) after heaters firing. At this time the helium close to the heaters is warm and the coil-to-heater voltages are at the highest values. However, the coil is still cold at this time. Assuming that half of the helium is “cold” (10 K), and half of the helium is “warm” (150 K), then helium alone can withstand up to 1.5 kV, that is larger than the expected worst peak coil-to-heater voltage (~600 V in MQXFA, ~950 V in MQXFB) immediately after heater discharge. Moreover, these peak values can be reduced to ~300 V in MQXFA, and ~400 V in MQXFB, by properly connecting the quench heaters. However, this optimization in some cases slightly increases the peak heater-to-coil voltage at intermediate temperatures.

In conclusion, this failure analysis has shown that MQXF magnets are expected not to suffer coil-to-heater voltage breakdown even in the worst-case scenario of a direct coil-to-heater path.The peak coil-to-heater voltages immediately after heater firing can be reduced by optimizing the heaters connections respect to the CLIQ polarities, which is highly recommended.It is also recommended to perform simulations of helium behavior in epoxy cracks during quench in order to obtain more realistic temperature and pressure estimates. In addition, it is advisable to check the robustness of the heater-to-coil insulation in helium gas at intermediate temperatures (or at warm temperature) after cold test with an experimental activity.

References[1] E. Ravaioli, “Quench protection studies for the High luminosity LHC inner triplet circuit” CERN

EDMS No. 1760496.




[2] P. Fessia, “Guidelines for the insulation design and electrical test of superconducting accelerator magnets during design assembly and test phase”, CERN-EDMS-1264529, 2018.

[3] Roger Rabehl, private communication.[4] H. Pan, private communication.[5] T. Salmi, private communication.[6] F. Rodriguez Mateos, “ELECTRICAL DESIGN CRITERIA FOR THE HL-LHC 11T DIPOLE”,

CERN-EDMS-1995595, 2018.[7] Rob Van Weelderen, private communication

APPENDIX AHELIUM BREAKDOWN VOLTAGE PLOTS




Figure A-1: Breakdown voltages of helium gas at 10 K and different pressures






Electrode distance [mm]0.001 0.01 0.1 1 10 100

Brea

kdow

n Vo

ltage

[V]

100

1000

10000275 K, 1 bar ( Air data)300 K, 1 bar (He)200 K, 1 bar (He)150 K, 1 bar (He)80 K, 1 bar (He)

Figure A-4: Breakdown voltages of helium gas at different temperatures and 1 bar pressure

APPENDIX B




MQXFA PEAK HEATER-TO-COIL (OL) VOLTAGE DURING A QUENCH, NOMINAL PROTECTION (CLIQ, OL-QH, NO DUMP, NOMINAL CURRENT), ALL COILS

























SAME PLOTS, NO LOGARITMIC SCALE

























APPENDIX CMQXFB PEAK HEATER-TO-COIL (OL) VOLTAGE DURING A QUENCH, NOMINAL PROTECTION (CLIQ, OL-QH, NO DUMP, NOMINAL CURRENT), ALL COILS.




























SAME PLOTS, NO LOGARITMIC SCALE

























APPENDIX DWORST CASES FOR MQXFA AND MQXFB IN CASE OF A QUENCH ON THE POLE TURN IN THE OUTER LAYER AT 18 kA


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indico.fnal.gov€¦ · Web viewHelium gas above atmospheric pressure The previous analysis was done under the assumption that the helium gas pressure is 1 bar. This assumption is