**A Compendium of Conductor Damage Mechanisms in High-Field (13T) NB3Sn
Magnets and Associated Mitigation Measures**

**in the D20 Design**

**Purposes:**

1)  Raise consciousness of design assumptions and the magnet
    construction and assembly practices that might affect 13 Tesla
    operation.

2)  Provide a comprehensive list of Nb3Sn conductor damage mechanisms.

3)  Provide a comprehensive list of proposed damage-mitigation
    alternatives.

4)  Produce a defacto Nb3Sn magnet failure analysis, in the hope of
    identifying and addressing all failure mechanisms which might
    interfere with reliable 13^+^ Tesla operation.

**Basic Problems:**

1)  Nb3Sn\'s critical current is strain-sensitive.

2)  Some of Nb3Sn\'s strain-sensitivity is irreversible.

3)  Two-layer wind/react construction (internal ramp) complicates
    interactions.

4)  Careless attention to design or assembly details can cause damaging
    conductor strains prior to initial excitation.

5)  Solutions to one failure mechanism might aggravate other conditions.

6)  All failure-modes need to be identified while options still exist.

**Analytical Method:**

1)  Examine all conductor handling operations (bare wire to final
    magnet).

2)  Identify all imagined sources of conductor damage in each operation.

3)  Specify all associated damage-mitigating features.

**Magnet Production & Test Operations:**

1)  Cabling.

2)  Winding.

3)  Reaction.

4)  Potting

5)  Azimuthal preload.

6)  Axial preload.

7)  Thermal cycling of the magnet.

8)  Magnet excitation.

9)  Magnet quenching

**[Degradation Source Mitigation Features]{.underline}**

**[1) Cabling:]{.underline}**

a\) Tin sausaging - Rectangular cable.

\- Low compaction.

b\) RRR degradation of copper. - Rectangular cable.

\- Low compaction.

\- Strong enough tin barrier.

**[2) Winding:]{.underline}**

a\) Shorts - Continuous monitoring during winding.

\- Al~2~O~3~ sprayed pole/wedge/end pieces.

**[3) Reaction:]{.underline}**

a\) Conductor growth during reaction. - Unconstrained ends.

b\) Differential thermal expansions. - Bronze wedges, pole/end pieces.

c\) Inter-layer movement @ ramp. - Well supported ramp section.

\- Pole-pieces pinned close to ramp.

\- No twisting permitted.

d\) Inter-layer movement @ \"bumps\" - Smooth irregularities.

\- Wind cable onto smooth surfaces only.

e\) RRR degradation of copper. - Minimize oxygen during reaction.

\- Careful control of impurities.

\- Measure "control" short-sample.

f\) Shorts. - Al2O3 sprayed metal pieces.

\- Resistive monitoring during insertion.

**[4) Potting:]{.underline}**

a\) Cond. movement during handling. - Cradle-supported coil transfers.

\- Gently and gingerly. No force-fits.

b\) Inter-layer twisting or separation. - Pole-pieces bolted/pinned
together.

b\) Splicing operation. - Minimum-handling solder technique.

\- Complete cable support.

c\) Voltage-tap attachment. - Flexible attachment lead.

\- Thin SS conductor-protection clips.

d\) Bubble voids - Warm vacuum impregnation.

**[5) Azimuthal pre-load:]{.underline}**

a\) Coil sausaging. - Wire-wind or full-coil press.

\- Constrain end-shoe axial movement.

b\) Differential Poisson expansion - Constrain end-shoe axial movement.

c\) Excessive pre-load. - Monitor pole pressures during trial loadings.

\- Use best-guess shim sizes from test loadings.

\- Apply final preload during cool-down.

d\) Plastic-flow stress concentrations. - Dhole \< .1 x Wcable.

\- Apply final preload during cool-down.

e\) \"Metal-to-metal\" deformation. - Inner: No solution, except
minimize peaks.

\- Outer: Maintain \"hydraulic pressure\".

f\) Discontinuity intersects wide-edge. - Feather edges (Redge = 2 x
Dstrand).

\- Bridge-shims provided.

g\) Discontinuity intersects narrow edge. - Feather edges (Redge= 2 x
Dstrand).

\- Inter-layer shim-sheet.

**[6) Axial pre-load:]{.underline}**

a\) Axial gap-closure (azimuthal load). - Close gaps before potting.

\- Pot gaps with stiff material.

\- Keep gaps closed while azimuthal preloading.

b\) Axial gap-opening (azimuthal load). - Apply axial load simultaneous
to azimuthal load.

\- Constrain end-shoe axial movement.

\- No axial hydraulic leaks.

c\) Inter-layer movement stress. - Smooth inter-layer irregularities, or

\- Lock layers together.

d\) Inter-layer crack propagation. - Double inter-layer reinforcement
glass cloth.

\- Plug hydraulic leaks adequately.

**[7) Thermal cycling of the magnet:]{.underline}**

a\) Differential expansion (materials) - Bronze wedges, pole &
end-pieces.

\- Pack voids with enough glass.

b\) Differential expansion (gradient). - Slow cool-downs & warm-ups.

\- Monitor temperature differences.

**[8) Magnet excitation:]{.underline}**

a\) Pole-turn unloading. - Apply \"adequate\" azimuthal preload.

b\) Mid-plane overloading. - Use \"minimum\" azimuthal preload.

\- Uniformly stiff midplane shim.

c\) Axial gap-opening. - Apply an \"adequate\" axial preload.

\- Make ends stiff enough to keep gaps closed.

d\) Inter-layer transfer non-uniformities. - Strong, smooth interface
surface.

e\) Excessive ramp-rate sensitivity - Slow ramping of magnet.

\- Install copper foil between layers.

**[9) Magnet quenching:]{.underline}**

a\) Excessive heating. - Early quench detection

\- Maximize energy extraction.

\- Efficient quench heaters on largest inductance.

B\) Excessive voltage. - Minimum energy extraction.

\- Simultaneous quenching of entire magnet.

**Areas of Present Concern:**

-   2a) Unconstrained ends during reaction is currently believed to be
    responsible for the looseness of the end-regions after reaction,
    thereby aggravating conditions 5d, 5g, 6a, & 8c.

-   8e) No copper foils presently in D20.