indico.fnal.gov · web viewthe first hi-pot test, at room temperature, showed that the...

Analysis of MQXFAP1 Short-to-Ground

US-HiLumi-doc-897Other:Date: 4/13/18 of 21

US HL-LHC Accelerator Upgrade Project


Prepared by:Vittorio Marinozzi, FNALReviewed by: Giorgio Ambrosio, FNAL

Approved by:

Revision History

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



Revision Date Section No.

Revision Description

v0 4/13/18 All Draft




TABLE OF CONTENTS

1. ABSTRACT............................................................................................................................................4

2. EFFECT OF HEATER-TO-COIL SHORT DURING A QUENCH.................................................4

2.1. SINGLE SHORT MODEL......................................................................................................................42.2 DOUBLE SHORT MODEL.....................................................................................................................7

3. HEATER-TO-COIL SHORT FORMATION MECHANISM........................................................10

3.1. TEST PROCEDURE............................................................................................................................103.2. INSULATION ASSESSMENT...............................................................................................................12

4. CONCLUSIONS...................................................................................................................................18




1. Abstract

The test of MQXFAP1, the first 4-m-long prototype of MQXFA, was stopped during training because of a short-to-ground. The short is located in the outer layer of coil QXF105. The outer layer of this coil previously had a heater-to-coil short in the low-field outer layer heater. In this document, an explanation of the possible mechanism connecting these two events is offered. In addition, some possible scenarios that may lead to a heater-to-coil short are described. The purpose of this document is to understand the mechanism leading to this failure, and to identify possible weaknesses in the test procedure or the magnet design so that they can be corrected.

2. Effect of heater-to-coil short during a quench

Coil QXF105 was known to have a heater-to-coil short in the outer layer, low-field zone. The involved heater has therefore been disconnected; nevertheless, it was not sufficiently clear whether quenching the magnet under these conditions could present a hazard. Here, we attempt to construct a simple model of the problem in order to identify the possible risks.

2.1.Single short model

The first reasonable assumption that can be made is that the heater and the coil have a single short, which can be represented in terms of resistance. In this scenario, the heater acts as a capacitance during a quench. The heater voltage follows that of the turn that is shorted with it with a certain delay that is dependent on the short’s resistance and on the heater capacitance. Due to the voltage difference among involved turn and quench heater, a certain current will flow between the heater and the coil. We try to estimate this current in order to verify whether it could be dangerous for the magnet during a quench. It is important to point out that in this model, whether or not the coil is superconducting is not relevant.

Figure 1: A simple drawing describing the single short model

The unknown parameters are the short resistance and the heater capacitance. We can describe the range of the short resistance between a low and a high value:

R sh=0.01−100Ω

We can make two assumptions about the heater capacitance:




We can consider the heater-to-coil capacitance, a value that can be measured. However, it represents an overestimation of the actual capacitance, which is lowered by the presence of the short. A measurement made on the coil QXF07 shows that the heater-to-coil capacitance isC 10nF

Figure 2: Measurement of the heater-to-coil capacitance in QXF07, outer layer, low field.

Assuming that it was insulated from the system, we can compute the heater capacitance. This number is the result of the capacitance of a rectangular sheet of conductor. It can be computed as

C=4 π ε0hL

∫ r−1dS(1)

where h and L are the height and the length of the quench heater, respectively. This is an underestimation of the capacitance because we completely neglect the capacitance effect of the heater with respect to the other metallic components of the magnet, and with respect to the ground. With this assumption, the capacitance isC 100 pF

We know that the actual capacitance falls between these two values.We can now solve the circuit. The unknown variables are the heater voltage and the current through the short, while we assume that we know the turn voltage during a quench. The current through the short is given by

I (t )=V turn (t )−V qh(t)

R sh(2)

where V turn ( t ) is the turn voltage and V qh(t ) is the quench heater voltage. It is also true that the current is equal to

I ( t )=CqhdV qh(t )dt

(3)

where Cqh is the quench heater capacitance. Therefore, we should solve the following differential equation for V qh

V qh (t )=V turn (t )−R shCqhdV qh(t)dt

(4)




considering that V qh (0 )=0. The current can then be computed using equation 2. The equation has been solved using the Eulero method, with a Matlab macro. The current is proportional to the voltage derivative. Therefore, we consider the case in which the voltage derivative is at its maximum during a quench; that is, at the switch opening. We can assume that the voltage derivative is about 4000V /ms for a short time. The results of the simulations follow, with a description of the main assumptions for each:

R sh=0.01Ω Cqh=100 pF d V turn/dt=4000V /ms

Figure 3: Current and power dissipated into the short

R sh=100Ω Cqh=100 pF d V turn/dt=4000V /ms





R sh=100Ω Cqh=10nF d V turn/dt=4000V /ms


It can be concluded that current and dissipated power never reach risky values even in the worst-case scenario. So, we can assume that this situation is safe for the magnet.

2.2 Double short model

A second assumption could be that there are two shorts between heater and coil, present in two different, adjacent turns. This case is completely different from the previous one: indeed, the current can flow through two paths in parallel: following the usual path across the coil or following the path through the two shorts and the quench heater. Figure 6 shows this model.




Figure 6: A simple drawing describing the double short model

In order to estimate the current flowing in the secondary circuit, one should evaluate the short resistance, the quench heater resistance, the resistance in the turns, the self-inductance of the involved turns, and the mutual inductance between the involved turns and the magnet, while capacitance effects can be neglected. However, in the model presented here, the inductive voltages are neglected, making the results provided here preliminary. Moreover, the current change in the turns involved in the short is neglected. A more advanced tool to account for these two effects is under development. It is important to point out that, according to this model, whether the coil is superconducting or not is relevant: indeed, in the case of a superconducting coil, the current will never flow through the short because the resistance of the path through the coil is zero; instead, the presence of a given resistance through the coil can induce a given current into the short.Neglecting the inductive voltages, the current through the short is simply given by

I short (t )=I coil ( t )

1+Rqh+R sh1+Rsh 2

Rturn ( t )

(5)

The short resistance can be assumed to range between 0.01Ω and 100Ω, while the quench heater resistance can be computed using the resistance per unit length, that is, 0.725Ω /m. Considering a path of 1mm through the heater, the heater resistance can be estimated as 1mΩ.The coil current and turn resistance can be computed using a quench simulation tool. In this case we used LEDET. The main parameters used for the quench simulation and for the short current computation are summarized below:

Quench current: 18000 A Protection System: 37.5 mΩ dump resistor, CLIQ, IL heaters, OL heaters (shorted heater

not fired) Detection time: 11.5 ms Heater charging voltage: ±300 V CLIQ charging voltage: 500 V CLIQ unit capacitance: 40 mF R sh=0.01−100Ω




Rqh=1mΩ

Figure 7 shows the simulated current decay during a quench

Figure 7: Current decay after a quench detection

Figure 8 shows the current flowing through the short and the power dissipation assuming a short resistance of 0.01Ω

Figure 8: Current and power dissipation through the short with R sh=0.01Ω

In this case, a total energy of 1.2 kJ is deposited in the short zone. In principle, one should estimate the energy per volume unit in order to completely understand whether this amount of energy can actually damage the coil, but is difficult to evaluate the volume involved in the process; nevertheless, it can be concluded that this scenario presents a potential risk for the safety of the magnet. Indeed, the insulation to ground could be damaged, leading to a short-to-ground.Figure 9 shows the results assuming a short resistance of 100Ω.




Figure 9: Current and power dissipation through the short, with R sh=100Ω

In this case, the deposited energy is just 0.12 kJ, and therefore this scenario could be considered safe.

In these two scenarios, the resistance in the turns is due to CLIQ, which efficiently induces the quench in the whole magnet. For a fair comparison, one should simulate also the case with no CLIQ, considering quench propagation or quench-back, both of which can also induce a quench in the zone affected by the short, where instead the quench heater is assumed to not work. This analysis is a work in progress.

3. Heater-to-coil short formation mechanism

Since a heater-to-coil short can pose a risk for the magnet, it is important to understand the mechanisms that can lead to its formation, and if it is possible that it could cause a short-to-ground also. Here we present two hypotheses to explain the formation of such a short: the first finds its origin in the test procedure, the second in a possible design weakness in the insulation assessment.

3.1.Test procedure

Figure 10 shows the test procedure adopted for MQXFAP1 during the cold vertical test [1], together with the quench history, focusing on the Hi-Pot tests performed on the coil QXF105. The Hi-Pot tests were performed according to the procedures used for cold tests of MQXFS (short models) by LARP and CERN.




Figure 10: History of quench and hi-pot tests on the coil QXF105, low-field zone

The first hi-pot test, at room temperature, showed that the heater-to-coil insulation was robust. However, after just one quench and one thermal cycle, the second room temperature hi-pot test showed a 2.38 kV breakdown, the first evidence of a heater-to-coil short. After cooling the magnet without any quench, the hi-pot showed a 0 V breakdown, clearly indicating a heater-to-coil short. After two quenches, the magnet was warmed up again, and the coil 5 outer layer, low-field quench heater was insulated. Then the magnet was cooled down. After 14 more quenches, it showed a short-to-ground, and the test campaign was stopped. The CLIQ system was inserted into the protection from quench 14 to quench 18.It is evident that the heater-to-coil short occurred immediately after quench 1. We hypothesize that it was caused by the second hi-pot test at room temperature. Indeed, after the first quench and thermal cycle, cracks in the epoxy resin can occur due to thermal contractions and electromagnetic forces; superfluid helium can infiltrate into these cracks and become trapped within the resin. At room temperature, helium gas is a “good” conductor, at least compared with air. Figure 11 shows the breakdown voltage of air and helium gas at different temperatures and pressures. It can be seen that the breakdown voltage is compatible with a ~10 mm path from the coil to the quench heater, considering that the hi-pot test was performed at room temperature (>275 K), but that there was not only helium gas, but some air too.Figure 12 shows the outer-layer quench heater trace. Since the quench heater trace contains holes that are about 5 mm away from the heater strips, it is possible to foresee a path from the coil to the quench heater through a crack in the epoxy filled with helium gas, and then through one of these holes. As a result, the electric arc can damage the polyimide insulation, creating a direct path from the coil to the quench heater that is just 200 µm long. This may explain the 0 V breakdown after the cool-down. It is not clear whether, in the specific case of QXF105, the heater-to-coil short had been in multiple turns since the beginning. the insulation-to-ground was exposed to heat deposition in 17 quenches; in particular, after quench 14, when CLIQ was inserted into the protection system, the zone affected by the short began to quench efficiently, emphasizing the effect described in the previous section, and possibly leading to the short-to-ground observed after quench 18.




Figure 11: Breakdown voltages of air and helium gas at various temperatures and pressures

Figure 12: Outer layer quench heater trace

In summary, we stress that performing hi-pot tests at room temperature, after the magnet has been in contact with superfluid helium, can pose a hazard. This procedure can cause a multiple heater-to-coil short, which, as we have seen, can pose risks to the safety of the magnet. After the magnet has been exposed to helium, the document “Electrical Design Criteria for the HL-LHC Inner Triplet Magnets” [2] (not available at the time of MQXFAP1 first and second cooldown) recommends decreasing warm Hi-pot to 1/5 of the minimum design to withstand the voltage at


Breakdown voltage



nominal operating conditions. We recommend following these guidelines also for short models and prototypes.

3.2. Insulation assessment

Due to thermal contractions or electromagnetic forces, it is possible that cuts in the polyimide insulation of the magnet can occur. These cuts, together with epoxy cracks, can create a direct path from the coil to the quench heater that is filled with helium. This short 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 resist the voltage difference between the quench heater and the coil during a quench. In this work, we have assumed that the helium is at the same temperature as the turn that is considered in computing the voltage difference, and that the pressure is 1 bar. These are conservative assumptions. The computation has been performed only for the outer layer quench heaters.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 SQXF (1 m long), MQXFA (4.2 m long), and MQXFB (7.15 m long). For each magnet, various protection systems have been considered:

CLIQ, 37.5 mΩ dump resistor, IL quench heaters, OL quench heaters CLIQ, IL quench heaters, OL quench heaters 37.5 mΩ dump resistor, IL quench heaters, OL quench heaters IL quench heaters, OL quench heaters CLIQ, OL quench heaters (nominal protection system)

In Table 1, the protection system parameters are reported for each magnet. These values are taken from the MQXF quench protection Technical Report [3].

Table 1SQXF 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 have assumed that the quench heater polarities are opposite on the two sides of the same coil. Indeed, the quench heater connection scheme is still under discussion. Figure 13 summarizes polarities and connections.




Figure 13: Connection scheme of the magnet, and the polarities of CLIQ and quench heaters.

The results are summarized in the following tables. Each table refers to a different protection system. For each case, two values are reported at different temperatures in an attempt to identify the worst case. The worst cases are highlighted in red.




Table 2CLIQ, 37.5 mΩ dump, IL heaters, OL heaters

SQXF MQXFA MQXFBPole 1, OL-LF, side

1240 V, 50 K / 180 V,75 K 250 V, 65 K / 180 V, 120

K200 V, 65 K / 250 V, 130

KPole 1, OL-LF, side

2400 V, 50 K / 200 V, 75 K 400 V, 65 K / 200 V, 120

K450 V, 60 K / 230 V, 130

KPole 1, OL-HF, side

1200 V, 65 K / 130 V, 90 K 200 V, 75 K / 140 V, 110

K310 V, 60 K / 300 V, 140


2300 V, 65 K / 140 V, 90 K 230 V, 75 K / 110 V, 110

K200 V, 65 K / 300 V, 140


1180 V, 60 K / 60 V, 75 K 200 V, 75 K / 200 V, 110

K310 V, 75 K / 330 V, 120


2180 V, 50 K / 60 V, 65 K 300 V, 50 K / 210 V, 110

K480 V, 75 K / 330 V, 120


1220 V, 50 K / 110 V, 75 K 410 V, 75 K / 140 V, 110

K310 V, 50 K / 600 V, 95 K

Pole 2, OL-HF, side 2

190 V, 50 K / 100 V, 75 K 400 V, 75 K / 150 V, 110 K

500 V, 50 K / 600 V, 95 K

Pole 3, OL-LF, side 1

320 V, 50 K / 160 V, 75 K 600 V, 70 K / 370 V, 110 K

1000 V, 80 K / 390 V, 130 K


230 V, 50 K / 160 V, 75 K 550 V, 70 K / 360 V, 110 K

980 V, 80 K / 380 V, 130 K


320 V, 60 K / 170 V, 90 K 600 V, 80 K / 400 V, 110 K

1000 V, 90 K / 400 V, 150 K


240 V, 60 K / 160 V, 90 K 580 V, 85 K / 380 V, 110 K

990 V, 90 K / 400 V, 150 K


120 V, 70 K / 80 V, 100 K 340 V, 95 K / 280 V, 140 K

630 V, 110 K / 300 V, 160 K


200 V, 70 K / 80 V, 100 K 360 V, 95 K / 290 V, 140 K

620 V, 110 K / 290 V, 160 K


40 V, 60 K / 50 V, 80 K 200 V, 75 K / 210 V, 130 K

400 V, 100 K / 140 V, 140 K


140 V, 50 K / 30 V, 80 K 220 V, 75 K / 210 V, 130 K

410 V, 100 K / 140 V, 140 K

Table 3CLIQ, IL heaters, OL heaters


170 V, 80 K / 20 V, 140 K 180 V, 50 K / 310 V, 100

K450 V, 50 K / 470 V, 110


260 V, 60 K / 40 V, 120 K 50 V, 50 K / 300 V, 100 K 300 V, 50 K / 450 V, 110


175 V, 85 K / 50 V, 140 K 250 V, 50 K / 350 V, 110

K500 V, 80 K / 480 V, 110


270 V, 70 K / 50 V, 130 K 100 V, 50 K / 330 V, 110

K400 V, 50 K / 470 V, 110


160 V, 50 K / 60 V, 120 K 140 V, 50 K / 310 V, 130

K350 V, 65 K / 410 V, 130


2120 V, 50 K / 70 V, 110 K 310 V, 50 K / 310 V, 130

K500 V, 65 K / 410 V, 130


180 V, 50 K / 50 V, 130 K 200 V, 80 K / 260 V, 110

K400 V, 100 K / 300 V, 120

K Pole 2, OL-HF, side

260 V, 50 K / 50 V, 130 K 180 V, 70 K / 250 V, 110

K500 V, 50 K / 340 V, 120


1110 V, 50 K / 80 V, 110 K 400 V, 80 K / 340 V, 110

K870 V, 80 K / 400, 130 K





40 V, 50 K / 85 V, 110 K 350 V, 80 K / 320 V, 110 K

820 V, 80 K / 380 V, 130 K


110 V, 50 K / 90 V, 110 K 400 V, 80 K / 350 V, 120 K

890 V, 100 K / 410 V, 150 K


50 V, 50 K / 80 V, 110 K 380 V, 80 K / 350 V, 120 K

840 V, 100 K / 380 V, 150 K


10 V, 50 K / 70 V, 110 K 250 V, 80 K / 310 V, 140 K

600 V, 110 K / 400 V, 130 K


100 V, 50 K / 80 V, 110 K 280 V, 80 K / 320 V, 140 K

620 V, 110 K / 400 V, 130 K


50 V, 50 K / 50 V, 110 K 70 V, 70 K / 250 V, 130 K 410 V, 90 K / 200 V, 130 K


80 V, 50 K / 60 V, 110 K 120 V, 70 K / 250 V, 130 K

400 V, 90 K / 180 V, 130 K

Table 437.5 mΩ dump, IL heaters, OL heaters


1200 V, 50 K / 100 V, 80 K 250 V, 50 K / 80 V, 110 K 200 V, 50 K / 300 V, 100


2300 V, 50 K / 100 V, 80 K 250 V, 50 K / 60 V, 110 K 220 V, 50 K / 280 V, 100


1150 V, 60 K / 90 V, 90 K 100 V, 50 K / 100 V, 110

K50 V, 50 K / 350 V, 110 K


250 V, 50 K / 100 V, 90 K 220 V, 50 K / 100 V, 110 K

50 V, 50 K / 340 V, 110 K


150 V, 50 K / 20 V, 90 K 120 V, 50 K / 190 V, 110 K

60 V, 60 K / 410 V, 100 K


50 V, 50 K / 20 V, 90 K 50 V, 50 K / 190 V, 110 K 180 V, 60 K / 420 V, 100 K


200 V, 50 K / 50 V, 90 K 180 V, 50 K / 140 V, 120 K

200 V, 50 K / 310 V, 110 K


80 V, 60 K / 40V, 90 K 140 V, 60 K / 120 V, 120 K

120 V, 50 K / 310 V, 110 K


230 V, 50 K / 80 V, 90 K 300 V, 50 K / 200 V, 110 K

500 V, 50 K / 680 V, 90 K


150 V, 60 K / 50 V, 90 K 400 V, 65 K / 200 V, 110 K

300 V, 50 K / 620 V, 90 K


350 V, 50 K / 150 V, 90 K 400 V, 50 K / 440 V, 100 K

500 V, 50 K / 710 V, 100 K


170 V, 50 K / 80 V, 90 K 220 V, 50 K / 420 V, 100 K

200 V, 50 K / 700 V, 100 K


70 V, 60 K / 20 V, 100 K 50 V, 50 K / 290 V, 110 K 200 V, 50 K / 520 V, 110 K


100 V, 60 K / 20 V, 100 K 250 V, 50 K / 290 V, 110 K

310 V, 50 K / 540 V, 110 K


50 V, 50 K / 20 V, 90 K 50 V, 50 K / 230 V, 100 K 50 V, 50 K / 400 V, 120 K


60 V, 60 K / 20 V, 90 K 200 V, 50 K / 230 V, 110 K

250 V, 50 K / 420 V, 120 K

Table 5IL heaters, OL heaters


150 V, 50 K, 10 V, 130 K 150 V, 50 K / 300 V, 100

K280 V, 50 K / 580 V, 100

KPole 1, OL-LF, side 60 V, 50 K / 40 V, 110 K 100 V, 50 K / 280 V, 100 150 V, 50 K / 520 V, 100




2 K KPole 1, OL-HF, side

160 V, 60 K / 20 V, 140 K 150 V, 50 K / 320 V, 100

K 270 V, 50 K / 600 V, 110


250 V, 50 K / 50 V, 110 K 50 V, 50 K / 310 V, 100 K 50 V, 50 K / 580 V, 110 K


20 V, 50 K / 60 V, 120 K 60 V, 60 K / 300 V, 110 K 60 V, 50 K / 520 V, 120 K


90 V, 50 K / 70 V, 120 K 160 V, 60 K / 310 V, 110 K

250 V, 50 K / 540 V, 120 K


70 V, 50 K / 50 V, 120 K 60 V, 60 K / 240 V, 120 K 60 V, 50 K / 410 V, 120 K


100 V, 50 K / 60 V, 120 K 140 V, 60 K / 240 V, 120 K

230 V, 50 K / 410 V, 120 K


50 V, 50 K, 10 V, 130 K 150 V, 50 K / 300 V, 100 K

280 V, 50 K / 580 V, 100 K


60 V, 50 K / 40 V, 110 K 100 V, 50 K / 280 V, 100 K

150 V, 50 K / 520 V, 100 K


60 V, 60 K / 20 V, 140 K 150 V, 50 K / 320 V, 100 K

270 V, 50 K / 600 V, 110 K


50 V, 50 K / 50 V, 110 K 50 V, 50 K / 310 V, 100 K 50 V, 50 K / 580 V, 110 K


20 V, 50 K / 60 V, 120 K 60 V, 60 K / 300 V, 110 K 60 V, 50 K / 520 V, 120 K


90 V, 50 K / 70 V, 120 K 160 V, 60 K / 310 V, 110 K

250 V, 50 K / 540 V, 120 K


70 V, 50 K / 50 V, 120 K 60 V, 60 K / 240 V, 120 K 60 V, 50 K / 410 V, 120 K


100 V, 50 K / 60 V, 120 K 140 V, 60 K / 240 V, 120 K

230 V, 50 K / 410 V, 120 K

Table 6Nominal case: CLIQ, OL heaters


150 V, 50 K / 10 V, 130 K 160 V, 50 K / 310 V, 100

K400 V, 60 K / 430 V, 110


260 V, 50 K / 40 V, 120 K 100 V, 50 K / 300 V, 100

K300 V, 60 K / 420 V, 110


150 V, 60 K / 70 V, 90 K 250 V, 50 K / 340 V, 110

K 450 V, 60 K / 480 V, 120


240 V, 60 K / 50 V, 90 K 80 V, 50 K / 320 V, 110 K 350 V, 60 K / 480 V, 120


170 V, 50 K / 50 V, 120 K 140 V, 50 K / 280 V, 120

K320 V, 60 K / 390 V, 120


280 V, 50 K / 70 V, 120 K 310 V, 50 K / 290 V, 120

K500 V, 60 K / 400 V, 120


140 V, 80 K / 45 V, 120 K 210 V, 60 K / 240 V, 110

K300 V, 60 K / 410 V, 100


270 V, 80 K / 45 V, 120 K 310 V, 45 K / 250 V, 110

K 500 V, 50 K / 410 V, 100


1120 V, 60 K / 80 V, 110 K 390 V, 50 K / 330 V, 120

K820 V, 70 K / 390 V, 110


230 V, 60 K / 70 V, 110 K 350 V, 50 K / 330 V, 120

K 810 V, 70 K / 390 V, 110


1110 V, 50 K / 90 V, 110 K 400 V, 65 K / 350 V, 130

K880 V, 80 K / 400 V, 120


230 V, 50 K / 80 V, 110 K 370 V, 65 K / 350 V, 130

K820 V, 80 K / 400 V, 120

K





100 V, 50 K / 60 V, 110 K 220 V, 60 K / 300 V, 110 K

200 V, 60 K / 560 V, 110 K


100 V, 50 K / 70 V, 110 K 250 V, 60 K / 300 V, 110 K

80 V, 60 K / 590 V, 110 K


70 V, 50 K / 50 V, 110 K 150 V, 65 K / 240 V, 130 K

90 V, 60 K / 400 V, 110 K


80 V, 50 K / 60 V, 110 K 100 V, 50 K / 230 V, 130 K

140 V, 60 K / 390 V, 110 K

Figures 14 and 15 show two examples of the heater-to-coil voltage during a quench, and corresponding temperatures.

Figure 14: Heater-to-coil voltages and temperatures for MQXFB, pole 3, low-field zone, nominal scenario.




Figure 15: Heater-to-coil voltages and temperatures for MQXFA, pole 3, low-field zone, nominal scenario.

It is possible to understand from the tables that both the dump resistor and CLIQ tend to increase the peak heater-to-coil voltage. Moreover, the situation becomes worse when the length of the magnet is increased, even though each magnet has its own critical situation in each protection scenario.




Figure 16: Breakdown voltages of helium gas at 75 K and various pressures

The worst cases are comparable to or larger than the helium gas breakdown voltage (see figure 16) with a 200 µm path, which can be estimated to be ~330 V at 75 K. Nevertheless, it should be kept in mind that the helium gas pressure could be larger than 1 bar and its temperature could be lower than the coil’s; nevertheless, these two quantities are difficult to estimate, and for this reason we are presenting this conservative scenario. For the short magnet SQXF the peak voltages could be considered safe in any case, even if at the limit, while for the longest coils the peak voltages could be too high to be kept by the helium gas alone without generating a discharge. We could 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. An experimental activity to check the robustness of the coil insulation at intermediate temperatures is recommended. Nonetheless, making the simple assumption that the helium gas is trapped within the epoxy resin, and that 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 to ~530 atm at 100 K [4]. Of course, some helium gas is expected to flow out of the resin crack when subjected to such high pressure. Nonetheless, we should keep in mind that in the outer layer the coil surface close to the heaters is pushed against the structure by the magnetic forces. Therefore, even if the crack is open, some fraction of the helium gas will be trapped within the crack. Assuming that only a small fraction of the original helium gas remains trapped, reducing the pressure to just 2% of the computed value (~10 atm), the helium breakdown voltage would be > 1 kV (Figure 16). This simple computation shows that there is some margin also for the longest coils, although it should be assessed by more accurate modelling.

4. Conclusions

If a heater-to-coil short is present during a quench, some current may flow through the short. This current can be considered not worrying in the case of a single contact short; instead, if two or more shorts are suspected, the situation should be investigated with care. Indeed, large currents (~1 kA) can flow through the short, depositing a large amount of energy (> 1 kJ), possibly damaging the coil and insulation. In this situation it is important to evaluate whether the zone affected by the short could become largely resistive during a quench due to CLIQ, quench propagation, or quench- back. In this case, the decision to quench the magnet should be taken with care. It is possible that the heat created in the short zone during the quench damages enough of the ground insulation to cause a short-to-ground.

After helium exposure we recommend using reduced warm Hi-Pot values (following [2], for instance), and also using these values when testing short models and prototypes.In addition, the coil-to-heater voltages during a quench at intermediate temperatures (~100 K) appear to be high compared to the helium gas breakdown voltage computed using the very conservative assumptions of constant helium pressure during quench. Nonetheless, it could be acceptable under more realistic assumptions (increasing helium pressure during a quench), that should be modelled properly. It is therefore important to perform simulations of helium behavior in epoxy cracks during quench in order to obtain more realistic temperature and pressure estimates, and to check the robustness of the coil insulation at intermediate temperatures in order to prove that the voltages foreseen during MQXFA/B quench can be sustained by the magnet.




References

[1] J. Muratore et al., “Design and Fabrication of the 1.9 K Magnet Test Facility at BNL, and Test of the First 4 m-Long MQXF Coil”, IEEE Trans. on Appl. Supercond., Vol. 28, No. 3, 2018

[2] F. Menendez Camara, F. Rodriguez Mateos, “Electrical Design Criteria for the HL-LHC Inner Triplet Magnets” CERN EDMS No. 1963398; US-HiLumi-doc-879.

[3] E. Ravaioli,, “Quench protection studies for the High luminosity LHC inner triplet circuit” CERN EDMS No. 1760496.

[4] Roger Rabehl, private communication.


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