Splice Joint Design and Analysis John Escallier Brookhaven National Lab bnl - fnal- lbnl - slac US LHC Accelerator Research Program

2 Splice Joint Design overview (1) Impregnated coil structures have compromised heat paths Epoxy has limited thermal conductivity Conductor to epoxy bonds are less thermally conductive than the epoxy or the metals TCE mismatches fracture epoxy to metal bonds Splice joints generate heat from IR loss Splice design affects generated heat Total overlap area Solder thickness Joint topology

3 Splice Joint Design overview(2) Splice design affects heat removal to helium Total length and cross sectional area of the NbTi leads within the impregnated structure Current sharing details Splice topology Lead topology and stabilization

Full heat flux pathway 4 Heat to helium Heat to coils Peak source temperature Spread to turns Spread to adjacent coil Dissipation source Solder (green) Niobium Titanium (orange) Niobium3 Tin (maroon)

Steady state condition 5

Splice joint temperature profile at 11 kAmps 6 Assumptions: 11 Kiloamp current (input variable) Joint resistances of 1 nano-ohm (input variable) Uniform joint cross section NbTi cable effective thermal conductivity 70% room temperature copper (input variable) No thermal path provided by epoxy Liquid helium temperature of 4.5 kelvin (input variable) Linear material properties in the 4 to 10K range (equations may be used)

Impact of effective joint resistance on final temperature at 11 kAmps 7

Overview of possible splice geometries 8

Interconnect type A 9 Soldering the two leads will current share and reduce dissipation by 20 percent (5.3 Kelvin if the leads are soldered together their length)

Interconnect type B Kelvin final temperature

Interconnect type C Kelvin final temperature

12 Splice Joint Summary Vacuum impregnation: removes direct helium contact cooling of all conductors and connections creates higher temperatures internally given internal dissipations Splice design requires configuring conductor layers in the splice for reduced dissipation Splice design needs to provide adequate thermal conductive paths to helium for generated heat I squared R dissipated heat