Abstract: This is a Proof-of-Principal (POP) proposal to determine the
extent to which superconducting magnet training can be reduced by
reducing strand displacements associated with Lorentz-induced increases
in axial cable tension. The POP requirements and purposes are
identified, and several executive proposals are compared.

**Introduction**

Present day superconducting accelerator magnets frequently require
several "training" quenches before they can operate at their conductor
limit. Attention at the Lawrence Berkeley National Laboratory has been
focussed on reducing this training. Quenching can be expected when the
local conductor temperture rise after slippage (*T~slip~*) exceeds the
local operating margin (*T~margin~*) of the superconductor. Under
adiabatic conditions, the quench condition *T~slip~ =
F~trans~\*K~slip~\*D~slip~*/*C~heat~* \> *T~margin~* reveals that there
are four generic actions that can be taken to reduce stick-slip
quenching: a) reduce the transverse loading (*F~trans~*), b) reduce the
relevant friction coefficient (*K~slip~*), c) reduce the relative
stick-slip strain (*D~slip~*, i.e., increase the relative coil modulus),
and d) increase the local heat capacitance (*C~heat~*). The first test
of these ideas at LBL was an increase in the pole-turn modulus of a 1
meter, 2-layer, NbTi dipole (D19, 1992). It's first quench was 99% of it
expected plateau. This magnet geometry, with new conductor, but no
effort to increase the pole-turn modulus, trained very poorly (D19b,
1993). Unfortunately, the new conductor had a lower copper content, so
the negative poor trining is not conclusive. Recent magnets, with high
coil modulus (potted Nb~3~Sn race-track dipole), and reduced the
pole-turn friction (standard loading against a mica-paper pole-shim)
yielded perfect training (RD201, 1998). Halving the cable-edge loading
pressure (RD202, 1998), and cable-flat loading (RD203, 1998) maintained
the perfect training.

Recently, Caspi and Lietzke ( ) have discussed a conductor movement that
can result from Lorentz-induced increases in the axial cable tension
within magnets wound with Rutherford or braided cables. Rutherford cable
is made from a collection of conducting strands that are twisted and
deformed in a manner to create and maintain a flat ribbon shape over a
finite range of axial loading. It responds to small axial loads like a
relatively soft scissor-like spring, where small compressions separate
the strands, while tension compacts the strands. At a tension where all
the gaps between strands have closed, it becomes very stiff. Stick-slip
strand movements (Caspi and Lietzke) have been hypothesized to exist in
those regions of a coil where the following conditions are
simultaneously satisfied: 1) a low-modulus strain-state exists in the
cable, 2) changes in this strain-state are primarily impeded by friction
against nearby magnet elements, and 3) a Lorentz-induced axial tension
exceeds this friction during magnet excitation.

The pole turn end-transition regions are believed to best meet these
conditions: 1) these are the expected locations \[Caspi and Lietzke\] of
the lowest cable tension after a coil's winding island has been removed,
2) the low-modulus strain-state is normally trapped by azimuthal loading
against the pole structure, 3) the Lorentz-induced axial stress increase
is largest near the end, while 3) the Lorentz-induced decreases in
friction force are normally large enough to permit slippage.

**Basic POP Requirements**

A POP experiment is proposed to determine the extent to which this
mechanism may be slowing the training of recent LHC quadrupole prototype
magnets. The following action sequence is proposed to make the strongest
POP statement:

+--------------------------------+-------------------------------------+
| **Action Sequence**            | **Purpose**                         |
+--------------------------------+-------------------------------------+
| 1.  Choose a magnet that       | The POP is most strongly            |
|     trains very slowly. This   | established by observing a dramatic |
|     magnet should one that is  | improvement in a magnet's training  |
|     either incompletely        | rate, after making a minimum number |
|     trained, or one that       | of variable changes.                |
|     reliably detrains after    |                                     |
|     warming and/or             |                                     |
|     un-collaring.              |                                     |
+--------------------------------+-------------------------------------+
| 2.  Completely de-collar the   | Fully detrain the magnet as much as |
|     POP magnet.                | possible, in preparation for        |
|                                | uniform conditions in step #3.      |
+--------------------------------+-------------------------------------+
| 3.  Re-collar the test magnet  | Reduce the likelihood of            |
|     under conditions where the | Lorentz-induced axial strand        |
|     pole-turn tension is       | movements, by friction-locking the  |
|     between the following      | cable's interlocking cabling        |
|     limits: a) the original    | saddles somewhere between their     |
|     cabling tension, and b)    | "minimum-energy" (cabling)          |
|     the maximum axial          | positions and their                 |
|     Lorentz-induced tension    | "unsupported-Lorentz-end"           |
|     for an unsupported         | locations. This is also expected to |
|     pole-end-turn .            | increase the end-modulus, so that   |
|                                | any movement will release less      |
|                                | energy.                             |
+--------------------------------+-------------------------------------+
| 4.  Re-test the magnet under   | Minimize extraneous variable        |
|     similar conditions.        | changes.                            |
+--------------------------------+-------------------------------------+

**Implementation Details**

A POP experiment is proposed to determine the extent to which this
mechanism may be slowing the training of recent LHC quadrupole prototype
magnets. The following action sequence is proposed to make the strongest
POP statement:

This is essential for maximum effect, since any cable that has a tension
below its cabling tension

The optimum tension is believed to depend the degree to which the
pole-turn's end is axially supported against its Lorentz end-load. The
sensitivity of this optimum is believed to depend upon the extent of
azimuthal unloading of the pole turn.