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NbTi Busbars of Sc Link IT

J. Fleiter, J. Hurte, A Ballarino

16 April 2020

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Outline

Overview of electrical circuits

Two types of NbTi bus bars: Plug and Sc Link

Requirements of bus bars

Sc Link bus bar

Topology: round vs. rectangular

Geometry of the bus bar of DFX

Routing of Sc Link bus bar in DFX

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Overview of electrical circuits

The cold powering system of Inner triplets consists of:

2 x 18 kA MQXF main circuit

3 x 7 kA MQXF trims

2 x 13 kA D1 circuit

12 X 2 kA MCBXF circuit

A total of 19 branches of circuits routed via DFX

The DFX located in the LHC tunnel will host the NbTi bus bars and their splices

Two geometries of bus bar will be considered 18 kA and 2 kA.

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Two types of NbTi bus bar in DFX In DFX the NbTi bus bar extension of the Sc Link (spliced to MgB2

cables at surface) will be interconnected to the NbTi bus bars of the

plug (mounted on plug at surface).

There is therefore two pieces of bus bars in the DFX:

NbTi bus bar of the plug

NbTi bus bar of the Sc Link

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The plug with the 19 bus barsSketch from Y. Leclercq

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Requirements for the NbTi bus bars of DFX

Requirements for bus bars in DFX: Transport nominal current without losses and with temp margin of at

least 2.4 K at peak field of 2 T

Incorporate sufficient Cu to not exceed 100 K during quench

Immersed in LHe

Have low splice resistance to other NbTi bus bar <1 nΩ

Fulfill the voltage insulation requirements of the different circuits

Be flexible enough to allow their routing (and formation of a lyra)

Be mechanically maintained to avoid movement during powering of the circuit and/or transient on other circuits

Equipped with voltage taps for electrical protection

Provide continuity of Cu stabilizer at interconnects

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DFX bus bars of Sc Link: geometry Two options were considered at the preliminary stage of the design

Rutherford cables: Single or pair of Rutherford cable stabilized with Sn-Ag.

Additional copper shunt provided if not enough in the cable

Cables available and well known

Quite Easy to bend out of the plane

Easy to splice

Very difficult to bend in the plane

Require Sn-Pb or Sn-Ag stabilization

Round bus bars Round cables made from LHC NbTi strands

Cables not existing, need development

Easy to bend in any direction

Easy to splice

Doesn’t require Sn-Ag stabilization

=>Round bus bars selected for their flexibility in all direction

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Minimum Sc and Cu cross section Assuming Jc = 1620 A/mm2 at 6.8 K and 2 T, the minimum cross

section of NbTi is 11 mm2 for 18 kA and 1.1 mm2 for 2 kA.

Assuming 11 (1.1) mm2 of Sc for 18 (1.8) kA with a detection time of 0.4 s we need 43 (7) mm2 of Cu to not exceed 100 K during quench of the bus bars.

These values are almost meeting the requirements of WP3 edms2029211:

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Type Current Peak field Bp Temperature

margin Tmarg

Current density

at Top+Tmarg, Bp

SC surface

(kA) (T) (K) (A/mm2) (mm2)

18 kA 18 2 5 1500 12

13 kA 13 1.5 5 1900 7

1.7 kA 1.7 0.5 5 3600 0.5

Type Current Quench load

from detection

Detection

time

Quench load

total (Qlt)

Hotspot

temperature

SC

surface

Asc

Cu

surface

ACu

(kA) (MIITs) (s) (MIITs) (K) (mm2) (mm2)

18 kA 18 30 0.4 160 150 12 33

13 kA 13 29 0.4 100 150 7 28

1.7 kA 1.7 2.6 0.4 3.7 150 0.5 6

11 mm2

1.1 mm2

43 mm2

7 mm2

WP6 min Sc

WP6 min Cu

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Proposed geometry DFX bus bars of Sc Link

Two geometries of Sc Link bus bar:

34 LHC type 01 strands (Ø=1.07 mm) around a 10.9 mm

diameter multi-filamentary cooper core for:

MQXF main (18 kA, 2 units)

MQXF trims (7 kA during transients, 3 units)

D1 (13 kA, 2 units)

21 LHC type 02 strands (Ø=0.825 mm) around a 4.9 mm

diameter multi-filamentary cooper core for MCBXF magnets

Electrical Insulation with multilayer Kapton wrapping

(60% overlap).

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Type of circuitOD of core

(mm)Type of strands Trans. Pitch Sc Surface (mm2)

Cu surface

(mm2)

MQXF, D1 10.9 34 LHC 01 150 mm 11.4 84

MCBXFA/B 4.9 21 LHC 02 75 mm 3.8 20.7

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Expected performances and Hot spot

Critical current @ 4.3 K 2 T :63 kA (3.5 Iop) and 22 kA

(11 Iop)

Tcs 6.8 K for 18 kA and 7.8 kA for 2 kA

Adiabatic calculation of hot spot temp during quench: Only Cu and NbTi were considered

Conservative approach, enthalpy of He help to reduce hot spot

Detection time of 0.4 s for all circuits, time constant of 0.17 s (MQXF,

MCBXFA), 0.35 s for D1 and 1.7 s for MCBXFB

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Type of circuitCurrent

(kA)

Detection

time (s)

Equivalent

time

constant (s)

Quench load

from

detection

(MIITs)

Quench

load total

(MIITs)

Sc Surface

(mm2)

Cu surface

(mm2)

Adiabatic Hot

spot

temperature (K)

(Cu RRR= 100)MQXF 18 0.4 0.17 28 157 11.4 84 43

D1 13 0.4 0.35 30 97 11.4 84 36

MCBXFA/B 1.7 0.4 1.7 0.25 1.4 3.8 20.7 15

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Sc Link NbTi bus bar characterization Proof of concept cables (hand made) characterized in FRESCA

test station at 4.3 K vs external field in the range 0-2 T

18 kA cables 35 LHC Type 01 pitch of 200mm => run to 24 kA @ 2.4 T

2 kA cables 21 LHC Type 02 pitch of 130 mm=> run to 10 kA @ 2.3 T

Splice resistance of 1 nOhm

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2 kA bus bar

of Sc Link

13-18 kA bus bar

of Sc Link

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Sc Link NbTi bus bar characterization

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Proof of concept cables demonstrated their potential Ic, splices and stability.

Trans. pitch reached was too high (hand made) that need to be reduced (considered in the proposed geometry).

Inter-strand electrical resistance of Sc Link bus bar will be measured at 4.3 K

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DFX bus bars of Sc Link: routing

The transition vertical to horizontal of the bus bar has a double

bend S shape for allowing He vessel sub modules integration

Space reservation for bus bars in the S shape : 150 mm in

diameter, minimum bending radius of 125 mm (median bending

radius of 200 mm)

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DFX bus bars of Sc Link: routing Several short unit lengths of the Sc-Link bus bar cable were

produced.

The insulated Sc Link bus bar proposed are very flexible in all

directions, allowing short bending radius

A mock up with the 10 cables produced was performed

The routing of he bus bar was performed easily within the

allocated space reserved

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R= 125 mm

R= 125 mm

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Summary

Two geometries of bus bars for Sc Link IT: 18 kA rated cable for MQXF and D1: Round 34 strands (OD=13 mm)

2 kA rated cable for MCBXF round 21 strands (OD=7 mm)

Hot spot temp less than 50 K on Sc Link bus bar

Several short proof of concept unit lengths of the Sc-Link bus bar cable were produced and use for electric and mechanical characterization: The cables performs well above operating point.

The produced bus bar are flexible in all directions, allowing routing within the allocated space reserved

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