hi-pot tests of mqxf quench-heaters to coils

Hi-pot tests of MQXF

Quench-Heaters to coils

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

US HL-LHC Accelerator Upgrade Project

Hi-pot tests of MQXF Quench-Heaters to coils

M. Baldini

Abstract- The United States High Luminosity Large Hadron Collider Accelerator

Upgrade Project (US-HL-LHC AUP) is designing and fabricating 10 Q1/Q3 cold masses for the interaction regions of the LHC. Each cold mass contains two 4.2 m quadrupole magnets (MQXFA). The Nb3Sn quadrupole MQXFA magnets operate in superfluid He at 1.9 K with a nominal field gradient of 132.6 T/m. The MQXF protection system is based on CLIQ

(Coupling-Loss Induced Quench system) and outer layer quench heaters [1].

This note reports a summary of the quench heaters to coil high voltage (Hi-pot) tests performed on MQXF short and long coils. A total of 104 heaters were taken into consideration. All heaters passed qualification test after fabrication (at different thresholds because many of them were tested before threshold finalization). Some heaters failed after magnet cold powering test. In this note first voltage failures after cold powering have been identified. No first failures were observed below 1100 V in air at 300 K. Among all the heaters: 48 passed the high voltage tests above 2300 V and 6 failed below 2000 V in air.

This analysis helps to determine a range of voltage target values to perform high voltage tests without compromising the insulation integrity after exposure to liquid He.



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Revision History

Revision Date Section No.

Revision Description

v0 6/28/19 All Initial Release



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TABLE OF CONTENTS

1. INTRODUCTION ................................................................................................................................ 4

2. HIGH VOLTAGE TEST RESULTS AFTER COIL FABRICATION ........................................... 6

3. HI-POT PROCEDURES ..................................................................................................................... 7

4. HI-POT RESULTS IN AIR AFTER COLD TEST ........................................................................... 8

5. HI-POT RESULTS IN HE GAS ........................................................................................................12

6. ELECTRICAL INTEGRITY OF INSULATION (SECOND AND THIRD FAILURE) ..............13

7. CONCLUSIONS ..................................................................................................................................15

8. APPENDIX: DETAILED SUMMARY OF HEATER TO COIL HIGH VOLTAGE TESTS

PERFORMED FOR EACH MQXF MAGNET .......................................................................................16

9. REFERENCES ....................................................................................................................................25



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1. Introduction

Coupling-Loss Induced Quench system (CLIQ) and outer layer quench heaters are

used for protection of MQXF superconducting magnets for the Hi-Lumi Upgrade of the

LHC [1, 2]. The flexible quench heater circuits consist of heater strips glued on polyimide

layer (trace). The trace design for production magnet (see Fig. 1) is made of an insulating

layer of 50 µm thick polyimide followed by a 25 µm thick glue and a 25 µm thick resistive

layer of 316L stainless steel. The cable insulation consists of 145 m S2-fiber glass braided

on the cable and epoxy impregnated after coil heat treatment. The trace is glued to the coil

during the epoxy impregnation. No other insulation is present between the trace and the

coil.

Two heater strips are in the low-field region near the magnetic mid-plane and two are

in the high-field region near the magnetic pole, as displayed in Fig.1.

Electrical design criteria and high voltage test targets for MQXF magnet acceptance

are described in the EDMS doc-1963398 “Electrical design criteria for the HL-LHC Inner

triplet magnets” [3]. In Tab.1 a summary of the voltage test values is displayed. According

to Tab. 1 acceptance voltages are different for components after fabrication and after

magnet cold test. The maximum expected coil voltage at quench are reported in Table 2

for MQXFA and MQXFB magnet [4].

Maximum expected coil voltage at quench

MQXFA MQXFB

To ground 670 670

To quench heater 600 900

Table 2: Maximum expected coil voltage at quench for MQXFA and MQXFB magnets

Component HV test

@ warm

HV test

@ 1.9 K

HV test

@ warm

after He

Coil-Ground 3680 V 1840 V 368 V

Coil-Heater 3680 V 2300 V 460 V

Table 1: Target values for high voltage tests for MQXF magnets

Figure 1: Drawing of the quench heater polyimide trace used in MQXF magnets



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In Tab. 3, values of breakdown voltage for a 50 mm polyimide trace are reported at

selected temperature [2]. Holes are present on the polyimide trace to favor good epoxy

impregnation. The minimum distance between those holes and the heater is 4 mm.

Breakdown voltages for an He path of 4 mm are also shown in the same table [3].

Design 10 K 75 K 150 K 300 K

Polyimide BD Voltage (50

m)

11200 V

He BD (4 mm) 11000 V 2500 V 1500 V 900 V

Table 3: Breakdown (BD) voltages for 50 um polyimide layer and 4 mm He direct

path.

Development of blisters in the polyimide trace have been observed in some MQXF

coils after magnet training. This may be a signature of thinning of heater to coil insulation.

The issue is under investigation and it will not be discussed in detail in this note. However,

it is useful to show the voltage breakdown values for a polyimide failure. In this case, the

word failure refers to the formation of a direct path between the coil and the quench heater

(200 um length). The case of He and Air path is reported in Table 4 [5].

Failure 10 K 75 K 150 K 300 K

He Breakdown Voltage (0.2

mm, 1 bar)

>1000 V 350 V 270 V 200 V

Air Breakdown voltage (0.2

mm, 1 bar)

1030 V

Table 4: He and air breakdown voltage for a 200 m direct path between the coil and

the quench heaters.

The goal of this note is to present a complete summary of all the heater to coil high

voltage tests performed on MQXF coils. Tests have been conducted on both short and long

MQXF coils. To verify the integrity of the insulation and voltage margins, those

measurements have been carried on multiple times at several high voltage limits over

different stages of MQXF magnet life. This survey includes measurements performed on

short coils (used in MQXFS1, MQXFS3, MQXFS4 and MQXFS5 magnets) and in long

coils (used in MQXFA and MQXFB prototypes) that have been fabricated and tested both

by US-AUP and CERN. The data are discussed in light of the history of each magnet.

No voltage breakdown is observed below 1100 V in any of the heaters. In cause of

failure, repetition of voltage tests to very high target limits after a first voltage breakdown

results in a further degradation of the insulation. This result indicates that electrical test

values have to be carefully chosen in order to guarantee the integrity of the insulation.



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2. High voltage test results after coil fabrication

Electrical quality assurance for MQXF coil fabrication have been approved in April

2018. Test lists and procedure are presented in docDB- 521 for US-AUP [6] and in a series

of documents on EDMS for CERN [7]. The minimum heater voltage to withstand by a coil

in warm air after fabrication is 3680 V. Several MQXF short and long coils have been

fabricated prior the approval date and, heater to coil tests have been therefore performed

over a range of voltage limits that goes between 2000 and 5000 V. In tab. 5 a summary of

the coil to heater voltage targets is shown.

* assembled and tested in a magnet.

A total of 55 coils (220 heaters) has been fabricated and tested over time. Among those

coils, 26 have been assembled in short or long MQXF magnets (* in table 5). Twenty-five

coils were tested up to 2500 V and twenty-nine up to 3000 V. No failures were observed,

and each coil passed the hi-pot tests indicating the integrity of the heater to coil insulation

after fabrication.

Coil names, number of quenches and thermocycles are summarized for each

assembled magnet in Tab. 6. The remaining sections of this note focus on hi-pot tests

performed after coils having been exposed to superfluid He during magnet training and

testing.

#coil 2000 V 2500 V 3000

V

3200 V 3680 V 3700 V 5000 V

Short

model

29 1 23 (17*) 3 1

1

MQXFA 18

2 (2*)

11 (7*) 5

MQXFB 8

4

4

Table 5: Voltage targets used in high voltage heater to coil tests prior to Electrical QA

criteria approval.

Magnets LONG MIRROR

US-HiLumi

MQXFS1

US-HiLumi

MQXFS3

CERN

MQXFS4

CERN

MQXFS5

CERN

MQXFAP1

US-HiLumi

MQXFAP2

US-HiLumi

MQXFAP1b US-HiLumi

coils P1 103, 104,

3, 5

105,

106 107, 7,

8

108,

109 110,

111

203,

204 205,

206

P2, P3,

P4, P5

A102,

A104 A105,

A106

P2, P3,

P4, P6

# of training 52 349 100 8 50 18 29 37

# of thermocycles

1 8 5 3 3 3 1 1

Table 6: Summary of coil labels and corresponding magnet history.



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3. Hi-pot procedures

Before presenting the high voltage measurements conducted after He exposure, it is

important to list and describe high voltage test procedures. Tests have been executed in

three different configurations:

1. with heaters connected in series when the magnet sits inside the cold test facility.

2. with heaters tested separately inside an assembled magnet

3. with heaters tested separately after magnet disassemble (coil sitting on the table)

Even in the case of tests performed on heater circuits, a voltage breakdown is always

associated to a failure of only one of the heaters in the circuit. This has been demonstrated

by supplementary tests performed after the first voltage breakdown was observed. Details

are found in the appendix in sec. 8.

Hi-pot measurements at Fermilab and BNL were performed at 1 kV/min ramp rate with a

10 uA current threshold and a 30 s flattop waiting time. Hi-pot tests were performed

following several procedures at CERN:

• Measurements in bldg 927: increasing voltage at 1kV/min and waiting for 30 sec

at flattop

• Measurements in bldg 927: Increasing voltage at 1kV/s with 200 V steps and

waiting 30 sec each step

• Measurements on the magnet sitting in the test facility (bldg SM18); Increasing

voltage at 1kV/s and waiting for 2 min/30 sec at flattop. Breakdown voltage cannot

be determined in this way. It is a failure/no failure mode.

• Measurements in bldg 927 burn mode: Ramping up voltage very fast with 500 V

step up to a selected voltage target. If the insulation is still above 1 G at the

selected target the test is successful. If the resistance is found to be below 1 G

during the ramp, the test is interrupted and considered not successful. No

breakdown voltage is recorded in case of failure. It is also worth noticing that this

test is more disruptive.



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4. Hi-pot results in air after cold test

A total of 104 heaters have been tested after superfluid He exposure. Since no failure

were observed after fabrication, the remaining discussion is only dedicated to those hi-pot

tests performed after magnet cold tests. The electrical design criteria and high voltage

levels for acceptance were defined and approved in April 2018. Most of the data were

collected before this time and different maximum voltage values were used to verify the

integrity of the heater to coil insulation.

The results are summarized in three main figures.

• Fig. 2 displays the first identified voltage failure value as a function of the number

of quenches.

• Fig. 3 shows the maximum voltages seen by the heaters that passed the hi-pot tests

vs the number of quenches.

• Fig. 4 displays again the first voltage failure values distinguish between data

collected on assembled coils and on coils on the table.

A detailed summary of the hi-pot history for each set of data is presented in the sec.8.



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All the data points in Fig. 2 – if not specified otherwise - refer to voltage failures

observed in air at 300 K after magnet training/cold test. In these conditions no failure was

observed below 1400 V: 15 heaters fail between 1400 and 3000 V and 11 fail above 5000

V. Failures above 5000 V are not displayed in Fig. 2. A figure containing the entire set of

data can be found in the appendix.

0 50 100 150 200 250 300 350

0

1000

2000

3000

MQXFAP1 (coil P05)

MQXFAP2

MQXFS5

LONG MIRROR (coil P01)

MQXFS3 (coil 8)

MQXFS1

He

ate

r to

coil

firs

t volta

ge

fa

ilure

(V

)

# of quenches

He 150 K

He 300 K

Figure 2: Heater to Coil first voltage failure in air after cold test. The dashed red line

identifies the number of quenches that correspond to a magnet life time based on

MQXFA Functional Specification (EDMS# 1535430).

The error bar of the light blue triangle refers to the voltage range in which the first

failure of coil #8 has been observed. The magnet was sitting on the insert in the test facility

at CERN and the test procedure does not allow to determine the failure voltage value. Coil

8 passed a prior hi-pot up to 1020 V and failed the one up to 3000 V. Therefore, voltage

failure is reported at 2050 V with error range between 1100 and 3000 V. The purple starts

refer to the first failure of MQXFS1 heaters which has been observed performing the

measurements in He gas at selected temperature (detailed discussion in section 5 and the

appendix). Breakdown voltage values result to be independent from the number of

quenches.



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Maximum hi-pot test voltage limits without failure are displayed in Fig. 3. Voltage

values refer to high voltage test performed in air at 300 K if not specified otherwise. The

number of heaters reaching the target values are also listed in the plot. A total of 78

heaters passed the hi-pot test: 37 of them demonstrate electrical integrity above 2300 V,

whereas the rest of them were tested only up to 1000/1500 V and passed the test. It is

worth to notice that the first voltage failure was observed above 5000 V for 11 heaters

(see Fig. 5 in the appendix). This brings the number of heaters with electrical integrity

above 2300 V up to 48.

0 50 100 150 200 250 300 350

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

#4

#16

#2

#2#3

#1

#1

#12

#14

#12

MQXFAP1 (coil P2, P3,P4)

MQXFAP2

MQXFS5


MQXFS3 (coil 8)

MQXFS3 (coil 105 106 107)

MQXFS3 (coil 7)

MQXFS4

MQXFS1

Ma

xim

um

hip

ot te

st vo

lta

ge

with

ou

t fa

ilure

(V

)

# of quenches

#4 He 150 K

#7 He 300 K

Figure 3: Heaters passing the hi-pot test as a function of the number of quenches.

The dashed red line identifies the number of quenches that correspond to a magnet life

time based on MQXFA Functional Specification (EDMS# 1535430).



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In Fig. 4, the entire set of first voltage failure data is displayed again, distinguishing

between data collected from coils assembled into a magnet (green symbols), and the ones

collected from coils on the table (purple symbols). The results indicate that there is no

direct correlation between voltage failure values and pressure load/ prestress on the coils.

0 50 100 150 200 250 300 350

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Coil in assembled magnet

Coil on the table

MQXFAP1 (coil 5)

MQXFAP2

MQXFS5

LONG MIRROR

MQXFS3 (coil 8)

MQXFS3 (coil 105 106 107)

MQXFS1

Fir

st vo

lta

ge

fa

ilure

(V

)

# quenches

Figure 4: First failure voltages vs number of quenches distinguishing data collected

on assembled coils from those collected on coils on the table



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5. Hi-pot results in He gas

Hi-pot measurements in He gas at selected temperatures (75 K, 150 K and 300 K) were

performed for quench heaters installed in MQXFS1 and MQXFS3 and on MQXFA1b.

Results are shown in Fig. 5. Details about high voltage test procedures are described in the

sec.8.

50 100 150 200 250 300

0

200

400

600

800

1000

1200

1400

1600

1800

#4

#16

#11

#3

#2

#14

#15

#31#16

MQXFS1 passed

MQXFS1 failed

MQXFS3 passed

MQXFAP1b passed

# number of heaters that passed if >1

He

ate

r to

coil

volta

ge

s (

V)

Temperature (K)

Figure 5: Heater to coil voltage values for tests performed in He gas at selected

temperature.

Green (dark and light) symbols identify heaters passing high voltage tests up to the

selected voltage values. Red symbols identify failures. The first breakdown was observed

at 1500 V at 150 K in MQXFS1. The same heater experienced a second failure at lower

voltage (970 V) at 300 K. The observed breakdown values are compatible with He

breakdown voltage for a path of 4 mm presented in Tab.3. A failure of a second heater was

observed at 1360 V at 300 K. The remaining heater of MQXFS1 (14) were assumed to

have passed at 1300 V. Overall, no failures have been observed below 1000 V at 150 K

and all heaters passed the test up to 800 V at 300 K. All the coils of MQXFAP1b passed

the hi-pot test performed at 150 K up to 690 V.



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6. Electrical integrity of insulation (second and third failure)

High voltage tests were repeated several times before and after magnet training and

test.

Degrading of the electrical integrity of the insulation has been observed after the first

failure. In Fig. 6, high voltage values for consecutive tests are shown for coil 3, 107, and

206. The heater of coil 3 passed the hi-pot test at 1000 V multiple times over the entire life

of magnet MQXFS1a-e. It also passed the test in He gas up to 800 V at 300 K. The first

failure took place at 1500 V in He gas at 150 K, the second failure at 960 V corresponds to

the value expected rescaling the voltages with temperature for a 4 mm He path. A coil to

heater short was found repeating the test in air at 300 K after the first two failures.

Heaters of 107 and 206 coil, show a behavior that is consistent with degrading of

electrical insulation. Voltage breakdown values decrease after the first failure in both coils.

0 1 2 3 4 5

0

200

400

600

800

1000

1200

1400

1600

0 1 2 3 40

1000

2000

3000

4000

5000

6000

7000

0 1 2 3 4 51000

1500

2000

2500

3000He 300 K

Vo

ltag

es (V

)

air 300 K

He 300 K

Vo

lta

ge

s (

V)

# of hi-pot test

passed

failed

air 300 K

He 150 KMQXFS1 coil 3

# 349 quenches

MQXFS3 107

#100 quenches

air 300 K

passed

failed

MQXFS5 206

#50 quenches

air 300 K

# of hi-pot test

passed

failed

# of hi-pot test

Figure 6: Second and subsequent voltage failures for coil 3, 107 and 206.



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Similar conclusions can be reached analyzing the behavior of the failing heater of coil

8 (Fig. 7). A first high voltage test was passed at 1020 V. The first failure took place

ramping up to 3000 V and a second one was observed ramping to a lower target value

(1000 V) in a following test. The heater passed the hi-pot test up to 500 V afterwards. This

result suggests that keep testing to high values (> 1000 V) after a breakdown, further

degrades the insulation. Indeed, the second successful hi-pot was achieved at 500 V that is

half of what the insulation was previously able to withstand.

Fig. 6 and 7 suggest a further degradation of electrical integrity if tests are repeatedly

performed to high values after a first failure.

1 2 3 4 5 6

0

1000

2000

3000

4000

5000

300 K

#50 quenches

300 K

#50 quenches

300 K

#24 quenches

Vo

lta

ge

va

lue

s (

V)

# of hi-pot test

1st and 2nd failure

300 K, #50 quenches

Failure in He gas

80, 150 and 300K

COIL 8 Hi-Pot history Assembled in MQXFS3c

Figure 7: Hi-pot history of the failing heater of coil 8.



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7. Conclusions

A summary of heater to coil high voltage tests performed on each coil used in short

and long MQXF magnet is presented in this note. A total of 220 heaters have been tested

up to several voltage target values. Voltage breakdown values appear not to be affected by

the number of quenches and by pressure load or prestress.

No failures have been observed below 1400 V in air and below 800 V in He gas at

ambient temperature. The lowest value for a breakdown voltage at 300 K was observed in

one heater of MQXFAP2 in air, and of MQXFS1 in He gas. The second lowest high voltage

failure in air was observed at 1500 V in one of the Long Mirror heaters, and in He gas at

1360 V in MQXFS1.

Tables 7, 8 and 9 in the appendix suggest that repeating hi-pot measurements up to

really high voltage targets (>5 kV) may represent a risk for the integrity of the insulation.

Indeed, the percentage of failures increases performing the test in burning mode.

Considering the potential weakness of the insulation due to the presence of polyimide

blistering, this survey indicates that a high voltage test at 850 V in He gas at 100 K can

guarantee insulation integrity after liquid He exposure for the entire lifetime of the MQXF

magnets.



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8. Appendix: detailed summary of heater to coil high voltage tests

performed for each MQXF magnet

The entire set of data for the first failure voltages is displayed in Fig. 8.

0 50 100 150 200 250 300 350

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

MQXFAP1 (coil P05)

MQXFAP2

MQXFS5


MQXFS3 (coil 8)

MQXFS3 (coil 105 106 107)

MQXFS1

He

ate

r to

co

il firs

t vo

lta

ge

fa

ilure

(V

)

# of quenches

He 150 K

He 300 K

Figure 8: first voltage failure vs number of quenches. The dashed red line identifies

the number of quenches that correspond to a magnet life time based on MQXFA

Functional Specification (EDMS# 1535430).



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A detailed history of the electrical tests performed in each coil is presented here.

High voltage tests performed on MQXFAP1 (coils: P2, P3, P4, P5):

• Hi-pot at room T in wiring (vertical) standing @BNL: high voltage tests up to

3000 V was passed

• A thermocycle was performed after the first quench of magnet MQXFP1 because

of the rupture of the burn disc. The hi-pot test was repeated at room T in wiring

standing: a voltage breakdown was observed at 2380 V for outer layer low field

(OL-LF) heater of coil P5 (first failure: blue square in Fig. 2 and Fig.8)

• Hi-pot in liquid He (4.5 K): breakdown of coil P5 OL-LF heater at 0 V

• Hi-pot in liquid He (4.5 K): breakdown of coil P5 OL-LF heater at 20 V.

Formation of a coil to heater short

• Coil P5 OL-LF heater was isolated and not charged because of the short.

• The test was resumed and then interrupted after 18 quenches because of the

development of a heater to ground short on coil P5.

• Coil P5 was removed from the magnet and high voltage tests were performed on

each single heater @FNAL. One of the OL-HF heater was shorted too so it was

not possible to determine the first failure voltage value. However, a second hi-pot

test was performed after short removal and the result (2770 V) was used in Fig.2

(blue square). Voltage breakdowns were observed at 2290 V and 2820 V for the

two remaining heaters (first failures: blue squares in Fig. 2).

• The remaining 12 heaters (coil P2, P3 and P4) were tested again at 460 V. The

magnet was reassembled with a new coil: P6. P2, P3 and P4 heaters were assumed

to have passed the high voltage test up to 2340 V (blue square in Fig. 3).

High voltage tests performed on MQXFAP1b (coils: P2, P3, P4, P6):

Coil P5 of MQXFAP1 was substituted with coil P6 and the magnet was reassembled.

MQXFAP1b reached 17672 A and underwent 36 quenches.

• Each coil has been hi-potted, according to electrical requirements displayed in

Tab. 1 up to 460 V before testing.

• At the end of the training, all the coils were tested in He gas at 150 K up to 690 V,

passing the test successfully.

High voltage tests performed on MQXFAP2 (coils: A102, A104, A105, A106)

• Hi-pot up to 1660 V at room T in air with magnet in the cryostat @ BNL

• The magnet training was interrupted after 22 quenches because of the breaking of

one of the magnet shells.

• Hi-pot at room T was performed up to 460 V after the thermocycle @ BNL



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• High voltage tests were then repeated at FNAL on MQXFAP2. Two heater strips

were connected in series. There is a total of 8 heater circuits. The test was

performed up to 1500 V. Two circuits failed the test: Circuit 3 failed at 1400 V

and circuit 4 failed at 1500 V. Those results were identified as first failure voltage

values for two heaters (light green triangle in Fig. 2). The remaining heater all

passed at 1500 V (light green triangle in Fig. 3)

• All heaters passed the test up 460 V afterwards.

Tests performed on MQXFAS1a-e (coils: 3, 5, 103, 104)

MQXFS1 has been trained and tested multiple times since 2016. The latest test was

conducted in October 2018. More info on MQXFS1 test history can be found in Ref. [8].

The outer layer heaters are connected in series with two circuits: CL3 for coil 3 and 103

and CL1 for coil 5 and 104, respectively. Therefore, there are 8 strips for each circuit.

• Quench heater to coil high voltage tests were performed up to 1000 V with a

1kV/min before and after each MQXFS1a-e magnet test. All heaters passed the

tests.

• High voltage tests in He gas at selected temperatures and voltage levels were then

conducted on MQXFS1e. Results are described in section 5 of the note. Some of

those data were also used in Fig. 2 and 3.

• Heater to coil voltage test up to 800 V: each circuit passed at 75K, 150 K and 300

K. A thermocycle was then performed and the test was repeated reaching higher

voltage levels.

• Circuit CL1 and CL3 passed the test up to 1500 V at 75 K. At 150 K, CL3

reached 1500 V but failed at flattop. This was identified as the first failure value

of one of the heaters (purple star at 150 K in Fig. 2). CL1 passed the hi-pot up to

1500 V at 150 K.

• CL1 passed the high voltage test up to 1200 V at 300 K. It failed at 1360 V at 300

K. This was identified as a first failure value of a second heater (purple star at 300

K in Fig. 2).

• The heater of CL3 failed the test a second time at 300 K at lower voltage level

(970 V).

• All heaters were tested again up to 460 V in air at 300 K after measurements in

He gas. The mid plane heater of coil 3 was found to be shorted to coil. This heater

was in the CL3 circuit that failed the hi-pot test in He gas at 150 K and 300 K.

• Each heaters of coil 3 were further hi-potted in air up to 6000 V ramping current

at 1kV/min. Two of them passed the test up to 6000 V (purple starts in Fig. 3).

One of them failed at 1840 V (first failure voltage value, purple star in Fig.2)

• The remaining heaters (11) were assumed to have passed the test up to 1500 V at

150 K (4 of them) and at 1300 V at 300 K (7 of them). Those are identified with

purple stars in Fig. 3



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Tests performed on MQXFS3a-c (coil 105, 106, 107, 7, 8)

Magnet MQXFS3a-c was trained and tested multiple times. The magnet was fully

disassembled and coil 7 was substituted with coil 8 for MQXFS3c since it was

responsible for the detraining observed in MQXFS3a and MQXFS3b. More info about

training and test history of MQXFS3 can be found in [9].

• Several high voltage tests in air were performed before MQXFS3a magnet

training. All heaters passed the tests. The maximum voltage levels used for those

tests were: 2500 V (3 times), 1000, 1018 and 3200 V.

• After the MQXFS3a training, all heaters were tested up to 1000 V and passed.

The test procedure does not allow to determine the voltage breakdown: it is a very

fast ramp to the selected voltage target. A failure was observed ramping up to

3000 V. MQXFS3b magnet was then retrained in liquid He. All the heaters passed

the hi-pot test conducted afterward up to 2500 V in air at 300 K. The observed

failure was therefore ascribed to instrumentation problems.

• MQXFS3c was assembled substituting coil 7 with coil 8. All heaters were tested

again in air up to 2500 and up to 3000 V and all of them passed the test. Those

tests were repeated 4 times for both voltage levels. This guarantees the integrity of

the insulation and demonstrates that the observed failure was due to

instrumentation issue.

• All heaters passed the hi-pot performed up to 1000 V with MQXFS3c inside the

cryostat.

• All MQXFS3c heaters passed the hi-pot test performed after re-training at 300 K

in air up to around 1020 V. A second magnet cold test was performed after the

thermocycle. The first failure was identified hi-potting the heaters in air at 300 K

ramping fast up to 3000 V. A second failure was observed repeating the test up to

1000 V. All the heaters were able to pass up to 500 V. High voltage measurement

of each single heaters were performed at this point. The failure was identified in

one of the high filed (HF) heaters of coil 8. This correspond to the light blue

triangle with error bar in Fig. 2. The remaining heaters reached 1000 V with no

issues.

• After a third cold test, high voltage measurements were performed in He gas. The

HF heater of coil 8 did not pass at 200 V at each selected temperature. Seven

heaters were tested at 80 K: all of them passed in a range that goes from 1529 V

to 1834 V (light green diamonds in Fig. 5). Eleven heaters were tested at 150 K

up to 1000 V: all of them passed. All the heaters were tested up to 800 V at 300

K. All of them passed the test with exception of HF coil 8 heater.

• Further high voltage measurements were performed on MQXFS3c heaters after

cold test. In this case voltage was increased up to 10000 V with 200 V steps and a

30 sec waiting time between steps. All the failures were observed above 5000 V.

Those values are reported as first failure in Fig. 8 (dark green diamonds).

• The remaining heaters (3 of coil 8 and 2 of coil 107) passed the hi-pot up to

10000 V (green (yellow) triangles and diamonds in Fig.3)



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• Further tests were performed afterward and are summarized in the table 7 at the

end of this document.

Tests performed on MQXFS5 (coils 203, 204, 205, 206)

Magnet MQXFS5 was trained up to ultimate current (17890 A), underwent a thermocycle

and demonstrated to have very good memory after that. The cold test was completed in

October 2017. More info about training and test history of MQXFS5 can be found in

[10].

• All outer layer quench heaters were tested multiple time before the training: 6

times up to 3000 V and 4 times up to 1000 V. All of them reached the selected

voltage levels with no breakdown.

• An hi-pot test in air at 300 K was executed after the training up to 1000 V: all the

heaters passed.

• Further high voltage measurements were performed after the cold test and all the

heaters reached 3000 V with no failure observed.

• In Jan/Feb 2019 measurements were repeated on disassembled magnet ramping at

1kV/min up to 3700 V. The first failure occurred in one of the low field (LF)

outer heater of coil 206 at 2000 V (orange triangle in Fig.2).

• Coil 206 heaters underwent further testing. Failures were observed in the two HF

heaters of coil 206 at 2500 and 5000 V (orange tringle in Fig.2).

• The 13 remaining heaters are displayed in Fig.3: 12 passed the test up to 3700 V

and one up to 10000 V. Further tests were performed on coil 206. A summary of

MQXFS5 heater to coil high voltage tests is displayed in tab. 8 and 9.

Tests performed on MQXFS4 (coils 108, 109, 110, 111)

Magnet MQXFS4a was trained up to ultimate current (17890 A) in six quenches. Cold

bore and beam screen were then installed after a thermocycle and MQXFS4b was tested

again. Details in Ref. [11]

• All heaters passed the hi-pot test up to 3000 V three times and twice up to 1000 V

before cold test.

• After the training, all heaters passed the high voltage test performed up to 1020 V

and the one up to 460 V performed after testing MQXFS4b. Data are displayed in

Fig. 3 (black diamonds)

Tests performed on MIRROR magnet

• All heaters passed the hi-pot test executed up to 2500 V before magnet training

• High voltage measurements were repeated up to 2500 V after magnet training at

300 K in air @BNL. Two heaters failed the hi-pot: one at 1500 V and the second

at 2070 V (brown triangles in Fig. 2).

• Further measurements were carried on up to 2500 V with magnet assembled and

magnet disassembled, and another failure was observed on a third heater at 2340

V (brown triangle in Fig.2).



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• The remaining heater passed the hi-pot up to 3700 V (brown triangle in Fig. 3)

High voltage tests performed on long LQS long quadrupoles

In Fig. 9, the first voltage failure of LQS long quadrupole quench heaters is displayed as

function of the number of quenches. Each coil has four heaters: two inner layer and two

outer layer heaters. The polyimide thickness is 25 m. Data are shown for all the heater

of LQS02 and LQS03 magnets and for 2 heaters of coil C08 of LQS01 magnet.

The lowest voltage breakdowns are observed in heaters of LQS02 magnets at 1088 and

1200 V. The remaining failure are observed above 1500 V. Those results are consistent

with those reported for MQXF coil and suggest that polyimide thickness has a negligible

effect on failure voltage values.

40 45 50 55 60 65 70 75

0

500

1000

1500

2000

2500

3000

3500

4000

LQS03

LQS02

LQS01

Co

il to

he

ate

r firs

t vo

lta

ge

fa

ilure

(V

)

# quenches

Outer layer quench heaters

Figure 9: Coil to heater first voltage failure for LQS heaters



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Table 7: Hi-pot tests for MQXFS3c coils. June 2018 results were used in Fig.2 and Fig.3.

ORH heater of coil 8 failed before. Colors indicate test results: successful test (green),

breakdown values above 5000 V (yellow), failure below 1000 V (red)

coil Jun 2018 (200 mV step, 30 s flat top, 10 kV target)

Jun 2018 (200 mV step, 30 s flat top, 10 kV target)

Jan 2019 (1kV/min, 5kV target)

Feb 2019 (1kV/min, 10 kV target)

Burn mode (Feb 2019)

105 OLL

9585 V 7589 V Fault in wires Fault in wires 10000 V

105 OLH


105 ORH

5887 V 6869 V Fault in wires Fault in wires Failed during ramp

105 ORL


106 OLL


106 OLH


106 ORH


106 ORL


107 OLL

10000 V N/A 5079 V 8720 V 10000 V

107 OLH

10000 V N/A 5075 V 10000 V 10000 V

107 ORH

6747V 136 V N/A N/A Failed during ramp

107 ORL

7211V 144 V 5081 V 7201 V Failed during ramp

8 OLL 10000 V N/A 5000 V 6879 V Failed during ramp

8 OLH 10000 V N/A 5000 V 10000 V 10000 V

8 ORH 370 V 859 V 659 V 737 V Failed during ramp

8 ORL 10000 V N/A 5000 V 10000 V 10000 V



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Table 8: Hi-pot test summary for MQXFS5 quench heaters. Colors indicate test results:

successful (green), breakdown above 2000 V (orange). January 2019 data were used for

Fig. 2 and Fig. 3.

coil Dec 2017 (30 s flat

top, 3 kV target)

Jan 2019 (1kV/min, 30 s

flat top, 10 kV target)

205 OLL 3000 V 3700 V

205 OLH 3000 V 3700 V

205 ORH 3000 V 3700 V

205 ORL 3000 V 3700 V

206 OLL 3000 V 3700 V

206 OLH 3000 V 2000 V

206 ORH 3000 V 3700 V

206 ORL 3000 V 3700 V

203 OLL 3000 V 3700 V

203 OLH 3000 V 3700 V

203 ORH 3000 V 3700 V

203 ORL 3000 V 3700 V

204 OLL 3000 V 3700 V

204 OLH 3000 V 3700 V

204 ORH 3000 V 3700 V

204 ORL 3000 V 3700 V



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Table 9: High voltage tests performed on heaters of coil 206. Results observed in

February 2019 were used in Fig.2 and Fig.3 Colors indicate test results: successful

(green), breakdown above 2000 V (orange), breakdown above 5000 V (yellow), failure

below 2000 V (red).

coil Dec 2017 (1kV/min, 30 s flat top, 3.7 kV target)

Jan 2019 (1kV/min, 30 s flat top, 3.7 kV target)

Feb 2019 (1kV/min, 30 s flat top, 3.7 kV target)

Feb 2019 (1kV/min, 30 s flat top, 5 kV target)

Feb 2019 (1kV/min, 30 s flat top, 10 kV target)

Burn mode (Jan/Feb 2019)

206 OLL

3000 V 3700 V 3700 V 2500 V N/A Failed during ramp

206 OLH

3000 V 2000 V 1800 V 1803 V N/A Failed during ramp

206 ORH

3000 V 3700 V 3700 V 5000 V 5282 V Failed during ramp

206 ORL

3000 V 3700 V 3700 V 5000 V 10000 V Failed during ramp



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Acknowledgements

This note summarizes the work performed by many people. I would like to thank

everyone who contributes to this work, collecting, providing and discussing the data with

me:

Ambrosio Giorgio, FNAL

Contat Pierre-Antoine, CERN

Feher, Sandor, FNAL

Ferracin Paolo, CERN

Krave Steve, FNAL

Mangiarotti Franco Julio, CERN

Marinozzi, Vittorio, FNAL

Muratore Joseph, BNL

Parker Marcellus, FNAL

Perez Juan Carlos, CERN

Petrik Jan, CERN

Sabbi Gianluca, LBNL

Santini Carlo, FNAL

Stoynev Stoyan Emilov, FNAL

9. References

[1] MQXFA Final Design Report, docDb-948

[2] Hl_LHC Technical Design Report, EDMS 1833445

[3] “Electrical design criteria for the HL-LHC Inner triplet magnets” EDMS 1963398,

US-Hilumi Docdb-879.

[4] MQXFA Electrical Design Criteria, Hi-Lumi Docdb-826.

[5] Guidelines for the insulation design and electrical test of superconducting accelerator

magnets during design assembly and test phase, EDMS doc-1264529.

[6] QXFA Coil Fabrication Electrical QA, US-Hilumi docdb-521

[7] EDMS 1773276 and EDMS 2119748

[8] MQXS1a, b and c test reports: Hi-Lumi DocDB 930, 933 and 936

[9] Report on the MQXFS3 test campaign, EDMS 2112519

[10] Test report MQXFS5, EDMS 2165441

[11] MQXFS4 test result, EDMS 2046123

hi-pot tests of mqxf quench-heaters to coils

Documents