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**BTeV C0 IR Quadrupole Magnet Outer Strand Specification**[^1]

1.  SCOPE

    This specification establishes the minimum requirements for the
    manufacture, inspection, test, identification, and delivery of
    superconducting NbTi composite wire. This material will be used in
    the manufacture of the outer cable for the BTeV C0 IR magnets.

    1.  [Definitions]{.underline}

        a\) [Production unit]{.underline}: Material from a single
        multifilament billet, which undergoes identical mechanical and
        thermal processing, and shall be identified as such. A
        production unit may be less than one full billet. All material
        from a production unit shall be thermally cycled together in the
        same furnace, for each and every heat-treatment.

        b\) [Foreign Object]{.underline}: A substance or article alien
        to the product or assembly that has been allowed to invade the
        product.

2.  APPLICABLE DOCUMENTS

    1.  [Applicability]{.underline}

        The following documents of the issue in effect on the date of
        invitation for bids or request for proposal form a part of this
        specification to the extent specified herein.

        1.  Industry and Society Documents

            a\) ASTM E 243; Eddy Current Testing

            b\) ASTM F 68; Oxygen-free Copper in Wrought Forms for
            Electron Devices

        2.  Fermilab Documents

            a\) 5500-ES-371015, BTeV C0 IR Quadrupole Magnet / Niobium
            Titanium Alloy Bars and Rods

            b\) 5500-ES-371016, BTeV C0 IR Quadrupole Magnet / Niobium
            Barrier Material (Reactor Grade I)

            c\) Appendix I - Critical Current

            d\) Appendix II - Determination of Filament Size and Spacing

            e\) Appendix III - Determination of Copper to Superconductor
            Area Ratio

            f\) Appendix IV - Sharp Bend Test

            g\) Appendix V - Determination of Springback Properties

            h\) Appendix VI - Determination of Normal State Resistance
            of NbTi Superconducting Wire

    2.  [Source of Documents]{.underline}

        Any difficulty in obtaining the applicable documents should be
        referred to the Fermilab Subcontract Administrator.

    3.  [Precedence]{.underline}

        In the event of conflict between the requirements of this
        specification and the above applicable documents, this
        specification shall take precedence. Any such conflict shall be
        brought to the attention of the Fermilab Subcontract
        Administrator.

    4.  [Deviations and/or Waivers]{.underline}

        The Vendor shall provide written request for any deviations or
        waivers to the technical requirements of this specification
        prior to proceeding with implementation. Such implementation
        shall require written authorization by the Fermilab Subcontract
        Administrator.

3.  REQUIREMENTS

    1.  [Wire Material]{.underline}

        Raw material used in the manufacture of NbTi composite wire
        shall be procured to the requirements of references 2.1.2a,
        2.1.2b and 2.1.1b, and shall be inspected/tested by the Vendor
        for conformance to those requirements before release for
        production use.

        1.  Non-conforming Raw Material

            Material found to deviate from specification requirements,
            and which is unable to be brought into a state of full and
            complete conformance by rework, shall be removed from the
            raw materials inventory.

        2.  Raw Material Identification

            Each lot of wire raw material shall be uniquely identified
            to allow all Vendor manufacturing, test, and inspection
            operation records for finished wire to be traceable to the
            affected material lot.

    2.  [Technical Properties]{.underline}

        1.  Conductor Type

            The conductor shall be a composite of NbTi filaments in an
            oxygen-free copper matrix. The NbTi composition shall be
            Nb47+1wt.%Ti, and shall be high homogeneity grade or
            equivalent, meeting the requirements of reference 2.1.2a.

        2.  Copper-to-Superconductor Ratio (Cu/SC)

            The conductor shall have a copper-to-superconductor area
            ratio of (1.80 *±* 0.10):1.

        3.  Niobium Barrier

            At monofilament assembly size, the Nb barrier shall consist
            of an area equal to a minimum of 4% of the NbTi area.

        4.  Critical Current

            The conductor shall have a minimum critical current of 185 A
            measured at 7 T and 4.22 K, with a critical current
            criterion of ρ = 1x10^-14^ Ω m, based on the wire
            cross-section area and with the applied magnetic field
            perpendicular to the wire axis (see reference 2.1.2c).

        5.  Wire Diameter

            The conductor shall have a diameter of 0.648 ± .0025 mm.

        6.  Filaments

            The filament size shall be fixed by design geometry such
            that at final size the filament diameter is approximately
            6.0 μm and the spacing is approximately 1.0 μm. The number
            of filaments shall not vary by more than 20 filaments from
            the design value.

        7.  Filament Array

            a\) The multifilament billet shall be assembled using a
            single stacking procedure. The sub-elements incorporated in
            the stack shall be hexagonal in shape.

            b\) The multifilament strand shall have a copper core, which
            approximates a circular cross section. The core shall be
            designed for a maximum of 10 % of the total cross sectional
            area in the final size strand.

            c\) The outside circumference of the filament array shall
            approximate a circular pattern.

        8.  Resistance at Room and Transition Temperatures

            The conductor shall have a maximum normal state resistance
            (R~295~) of 82.0 mΩ/m and a maximum transition state
            resistance (R~10~) of 1.17 mΩ/m in samples which have been
            annealed for 2 hours at 270 ^o^C ± 5 ^o^C.

        9.  Copper Residual Resistivity Ratio

            The RRR for wire at final size, equal to R~295~/R~10~, is
            defined by the values of R~295~ and R~10~ given in 3.2.8.
            The RRR shall be greater than 70.

        10. \"n\" Value

            The wire shall have a minimum \"n\" value (see reference
            2.1.2c) of 35 at 5.6 T.

        11. Twist Pitch

            All wire shall be twisted so the filaments follow the same
            rotation as a [right]{.underline} hand screw thread. The
            wire twist pitch for wire at final size shall be 13 ± 1.5
            mm.

        12. Final Temper

            The wire shall not receive any furnace anneals below 3.6 mm
            in diameter.

        13. Surface Condition

            The wire surface shall be free of all surface defects,
            slivers, folds, laminations, dirt, copper fines, or
            inclusions. No NbTi filaments shall be visible at 1x under a
            lighting level of at least 1076 lux.

        14. Mechanical Properties

            a\) The wire shall survive a sharp bend test, performed in
            accordance with 2.1.2f, without any sign of cracking at the
            outside surface of the sharp bend visible at 5x under a
            lighting level of at least 1076 lux. Filament damage in
            excess of 1% of design total number of filaments is cause
            for rejection.

            b\) The wire shall have a spring back value, measured in
            accordance with 2.1.2g, of less than 1090 degrees.

    3.  [Process Controls]{.underline}

        1.  Production Unit

            All superconductor strand produced to this specification
            shall be processed in production units, as defined in 1.1.

            Work-In-Process material from a production unit shall remain
            physically grouped together throughout all phases of the
            manufacturing process. Any portion of a billet, which
            becomes separated from its production unit, shall be
            considered non-conforming and shall be addressed as such.

        2.  End Effects

            Processing end effects shall be removed prior to yield
            determination. These defects include areas of distorted
            cross section due to wire pointing by swaging, foreign
            material attached as temporary leader, unsteady-state twist
            material, or areas of distorted filaments that occur at the
            start and end of an extrusion.

        3.  Minimum Lengths

            The minimum deliverable piece length shall be equal to the
            minimum piece length usable to make a minimum cable length
            as specified in **Spec \# 5500-ES-371014** "BTeV C0 IR
            Quadrupole Magnet Inner and Outer Cable Manufacturing
            Specification", after cutting all samples required for
            tests.

        4.  Product Identification

            Wire lot identification shall conform to the requirements of
            section 5.1 of this specification.

        5.  Manufacturing Plan

            A manufacturing plan shall be established by the Vendor,
            defined by flow chart, diagram, or narrative; and shall be
            submitted for review and approval by Fermilab Subcontract
            Administrator. After production start, any changes to the
            approved manufacturing plan must be brought to the attention
            of the Fermilab Subcontract Administrator. No changes shall
            be implemented without written authorization from the
            Fermilab Subcontract Administrator.

        6.  Control of Manufacturing Machines and Methods

            The machines and equipment used to process all
            superconductor made to this specification shall be
            identified and documented as part of the Vendor\'s Quality
            Plan. No modifications to machines, methods or processes
            shall be permitted without prior written approval.

4.  QUALITY ASSURANCE

    1.  [Quality Assurance Plan]{.underline}

> Prior to the contract being signed, the Vendor shall submit to the
> Fermilab Subcontract Administrator, for his/her approval, a documented
> Quality Assurance Plan, which fulfills all requirements described in
> the present specification. The Vendor shall also submit to Fermilab
> Subcontract Administrator, for its approval, a detailed description of
> the test procedures and the model test certificates, which he proposes
> to use.

2.  [Responsibility]{.underline}

    The Vendor shall be responsible for the performance of all tests and
    inspections required prior to submission to the Buyer of any of the
    products for acceptance. The performance of such tests and
    inspections does not limit the right of the Buyer to conduct tests
    and inspections to verify conformance to all requirements of this
    specification. Such Buyer testing and inspection shall be confined
    to the scope of requirements defined in this specification or
    approved variations thereof.

3.  [Data Requirements]{.underline}

    Data deemed proprietary by the Vendor shall be protected under terms
    of the contract negotiated between Fermilab and the Vendor. Fermilab
    and the Vendor shall establish procedures for the control and
    limited distribution of proprietary data.

4.  [Inspection and Test]{.underline}

    Fermilab reserves the right to witness manufacturing steps, tests
    and inspections established under the Vendor\'s quality assurance
    system to demonstrate compliance with this specification.

    1.  Wire diameter

> The Vendor shall verify the wire diameter by means of a calibrated
> laser micrometer. All the wire that has been manufactured must be
> checked on line with a two-axis laser micrometer. The wire must be
> within tolerance over the full length. The ovality of the strand,
> defined as the difference between the diameters measured for the two
> axes, must be less than 3 μm. The Vendor must provide statistical
> analysis of the laser micrometer measurements, together with the data
> in electronic format, to the Fermilab Subcontract Administrator.

2.  Copper-to-Superconductor Ratio/Twist Pitch

    Test both ends of each continuous length of wire for
    copper-to-superconductor ratio in accordance with 2.1.2e. During
    each test verify the presence and the direction of the twist.

3.  Critical Current

    a\) Test a minimum of three (3) samples from separate continuous
    deliverable lengths of each billet production unit, in accordance
    with 2.1.2c. The samples selected for critical current testing shall
    be the three samples having a Copper-to-Superconductor Ratio closest
    to the measured production unit mean.

    b\) Acceptance of the billet production unit shall be contingent on
    all samples meeting or exceeding the specification minimum critical
    current. Any billet production unit not meeting this specification
    will be dispositioned as per section 2.4 of this document.

4.  \"*n*\" value

    Report the \"n\" value or quality index obtained from the three
    samples used in 4.4.3, measured in accordance with 2.1.2c.

5.  Resistance Values

    Test the billet production unit samples selected for critical
    current measurement (4.4.3), in accordance with 2.1.2h.

6.  Filaments

    Check each billet production unit for filament size and spacing in
    accordance with 2.1.2d.

7.  Mechanical Properties

    a\) Perform a sharp bend test on a sample from each continuous
    length of wire in accordance with 2.1.2f.

    b\) Perform a spring back test on a sample from each continuous
    length of wire in accordance with 2.1.2g.

8.  Eddy current test

> Each strand must be checked continuously on its overall length by an
> eddy current method, performed as specified in 2.1.1a, to detect
> inclusions, voids, cracks and filament defects. Each deliverable piece
> length shall be free of any inclusion, void, crack and/or filament
> defect.

9.  Samples for tests at Fermilab

> The Vendor shall deliver to Fermilab a 5-m long sample and a 2-m long
> sample cut at opposite ends of each deliverable piece length. The
> Vendor shall also deliver to Fermilab the samples mentioned in
> Appendix I.

5.  [Certificate of Compliance]{.underline}

    The vendor shall provide a written statement certifying compliance
    with the requirements of this specification with each product
    release for cabling, together with a completed copy of all
    documentation including all test results. All documentation is to be
    provided in an electronic form (format should be approved by the
    Fermilab Subcontract Administrator) also. This documentation shall
    be provided to the Fermilab Subcontract Administrator after the
    completion of all tests and inspections, before release for cabling.

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5.  PREPARATION FOR DELIVERY

    1.  [Marking/Identification]{.underline}

        Spools and exterior packaging shall be identified with the
        following information in the order shown:

        \"Superconducting Wire\"

        Specification BTeV C0 IR Magnets Inner

        Fermilab P.O. NO.

        Wire Identification No.

        Length (meters)

        Weight (kg)

        Date of Manufacture

        Name of Manufacturer

    2.  [Wire Identification]{.underline}

        Each deliverable length of wire shall be assigned a unique
        identification.

    3.  [Winding]{.underline}

        Wire shall be level wound so that the wire can be unspooled at a
        minimum of 30 m/sec without crossovers or kinks. There shall be
        only one length of wire per spool.

    4.  [Packaging]{.underline}

        The wire shall be packed in a manner that will protect it from
        any kind of damage that could occur in shipping from
        mishandling, exposure to the elements, and/or foreign object
        contamination. As a minimum, the following is required:

        1.  The wire shall be wrapped with a layer of new, clean
            polyethylene.

        2.  The spool shall be bagged in new 0.13 mm (5 mil.) clean
            polyethylene to protect the wire from foreign object
            contamination.

> 5.4.3. The spools shall be secured in the shipping container with the
> spool flanges maintained in a vertical orientation (axis horizontal)
> in order to prevent the wire from settling on the spool.

# Appendix I - Critical Current

Short sample test method for critical current determination of twisted
multifilamentary wire.

1\. [General Outline: Definition of Critical Current]{.underline}

The V-I curve is determined as a function of increasing current until an
irreversible transition or quench occurs. This measurement is carried
out in specified external fields, 5.6 T and 7 T for Outer Wire, or 7T
and 8T for Inner Wire; applied normal to the wire axis, and in a
temperature bath of liquid helium at 4.22 K. For current less than the
quench current, the V-I curve is reversible.

The critical current is defined as that at which the resistance per unit
length, R/L, is:

R/L = (4/πD2)10-8 ohms/m

where D is the wire diameter in millimeters. The effective resistivity
of the wire, at Ic, is 10-14 Ωm.

2\. [Sample Testing]{.underline}

The vendor shall measure the critical current of samples selected
according to 4.4.3a at B=5.6 T and 7 T, or 7T and 8T, and T=4.22 K. If a
temperature of 4.22 K is not possible, measurements may be made at
another temperature and a conversion constant must be supplied. The
conversion constant must be approved by Fermilab. A 2 m sample of wire
adjacent to each length used by the Vendor for critical current
measurements shall be sent to the Fermilab Subcontract Administrator.
These samples shall be identified by billet number, spool number,
original continuous wire length, and purchase order number. Samples may
be checked by Fermilab to insure that they conform to all aspects of the
specification, both mechanical and electrical.

3\. [Sample Mounting]{.underline}

The sample wire is most conveniently mounted on a cylindrical former so
that it fits in a solenoid magnet (see Section 4 below). Either bifilar
or monofilar mounting arrangement may be used, if the procedures
outlined below are followed. A non-inductive (bifilar) form will provide
adequate length, reduce inductive voltage signals, and provide for ease
of connection. Shorter, monofilar mounts may be used if adequately
sensitive signal detectors are available; voltage taps and wires should
be arranged to minimize the loop. Means must be provided for constraint
of mechanical motion without interfering with coolant contact; for
example, a G-10 former with grooved location of wire and careful
tensioning during mounting, could be used. Care must be taken to ensure
that a temperature gradient is not introduced into the region of
measurement (gauge length). Care must also be taken in bending the
samples, especially at the end of a bifilar sample.

4\. [Procedure]{.underline}

The sample length (between voltage taps) should be ≥250 mm. This
corresponds, typically, to a voltage drop of several microvolts. This is
readily measured with the aid of a suitable preamplifier or digital
volt-meter. Samples of shorter length may be used if a well functioning
nanovolt detection system is available. Equipment must be capable of
determining the effective resistivity to a precision of 10%, i.e. 10-15
Ωm.

The amplifier signal should be recorded on an X-Y recorder (or if
desired in a digital memory device). The V-I curve may be taken either
point-by-point (current constant for each measurement) or continuously
if induced signals due to ramping are not too large or noisy. Typically,
current is supplied by a stable, well-filtered power supply. The current
should be measured to a precision of 0.5% and an accuracy of 1%. Use of
a low resistance normal metal shunt connected across the sample is
permitted provided the resulting correction for shunt current is
accurately known and is \<0.1%. Electronic circuitry for quench
protection is preferable. The quality index, n, is estimated using the
equation V = constant x In and is evaluated in the resistivity range of
10-14 to 5 x 10-14 Ωm (see Section 7).

5\. [Magnetic Field]{.underline}

The external field is most conveniently applied by means of a
superconducting solenoid. The field must be uniform over the sample
reference length to 0.5%. The direction between field and wire axis must
be 90° ± 6° everywhere between the voltage taps. This range of angles
corresponds to a variation in Ic of 0.5%. The system should provide a
field with a precision of 0.5% and an accuracy of 1%.

6\. [Temperature Bath Correction]{.underline}

The specification temperature is 4.22 K, that of boiling helium at
standard atmospheric pressure. The bath temperature must be recorded
with the aid of appropriate thermometry (cryogenic thermometer or vapor
pressure of bath) with a precision of ±0.010 K (10 mK). For larger
temperature excursions, the \"linear T\" type of correction should be
applied:

where Tc is the transition temperature at the specified magnetic field.
(Tc= 6.23 K at 7 T.) Im is the current measured at temperature Tm, and
Ic is the critical current at the specification temperature, T=4.22 K.

7\. [\"n\" Value Calculation Procedure]{.underline}

A quality index, n, is determined from the V vs. I data using the
relationship V=c x In. A minimum of 5 data point pairs from the
[reversible]{.underline} part of the V vs. I data are taken over the
resistivitiy range of 1x10-14 to 5x10-14 Ω-m . Data points taken should
cover the range as much as possible, however only data points on the
reversible part of the V vs. I curve are allowed. The points taken are
plotted on a log-log scale plot and a linear best fit is determined. The
slope of the best fit line is reported as \"n\".

# Appendix II - Determination of Filament Size and Spacing

[Purpose:]{.underline} To determine the acceptability of filament size
and spacing.

[Method:]{.underline} Mount a strand sample at the appropriate final
size (.648 mm and .808 mm for Outer and Inner respectively) in a
suitable mounting material and polish an acceptable cross section.
Photograph the sample at sufficient magnification to clearly view
individual filaments and the copper between them. Photographs need not
capture the whole strand cross section. Verify that the magnification of
the micrograph is known and has been calibrated. Measure the mean
diameter of 10 representative filaments on the photograph and average.
Calculate the actual filament diameter by dividing by the magnification.
This is the filament diameter.

Draw a line along 10 filaments which lie in a close packed direction.
Measure the distance from the outside to outside of the 10 filaments.
This measured distance is equal to 10 d + 9 s, where d is the filament
diameter above and s is the filament spacing. Solve for the value of s.

An alternate procedure is to use an Image Analysis technique, reviewed
and approved by the Fermilab Subcontract Administrator to determine area
fractions, diameters, and spacings.

# Appendix III - Determination of Copper to Superconductor Area Ratio

COPPER TO SUPERCONDUCTOR RATIO TEST PROCEDURE

1\. Materials List: Pyrex Container

Analytical Balance

Nitric Acid/Water Solution

Ethanol

Sodium Bicarbonate

2 gram minimum wire sample

Oven

Tweezers

2\. Label a clean Pyrex container with the spool number for each wire to
be tested.

3\. Wipe each 2 gram wire sample with alcohol, coil and place in its
assigned Pyrex container. Use tweezers to handle wire for the rest of
testing procedure. Weigh the wire sample to the nearest .0001 gram and
record its weight on the incoming strand inspection form.

4\. Place 1 tablespoon of sodium bicarbonate in a container and dissolve
in water. Use this to neutralize any acid spills.

[Danger]{.underline} - [Nitric acid can cause severe burns and may be
fatal if swallowed. Read all labels and use all safety precautions in
handling acids.]{.underline}

5\. Under fume hood, pour a Nitric Acid/water solution into each Pyrex
container to cover the wire samples. Place signs \'\'WARNING-NITRIC
ACID.\"

6\. Allow 30 minutes, then check each wire sample to see if all of the
copper is etched away. This is done visually and by using tweezers to
check for stiffness. Be careful not to cause damage to the sample.
Stiffness indicates presence of copper. If copper is present, but acid
appears to have stopped etching, pour the acid into a used acid
container and add fresh solution.

7\. After the copper is completely etched away, pour acid into a
well-labeled used acid container. The remains left in the Pyrex
container are the superconductor filaments.

8\. Gently rinse the filaments in the Pyrex container with water. Be
careful not to rinse away loose filaments. Repeat 4-5 times or until
thoroughly rinsed.

**DANGER: Fine filaments of NbTi are extremely flammable. Keep away from
open flame or static electricity sources.**

9\. Put the Pyrex container containing the sample in the oven at
60°C-70°C for approximately 15 minutes. Gently move the filaments to
prevent sticking to the Pyrex container.

10\. Dry the filaments for approximately 45 minutes. Weigh the sample to
the nearest .0001 gram and record weight on the incoming inspection
form. With these weights, you can compute the copper-to-superconductor
ratio.

11\. Repeat steps 5 through 10. The two measurements should agree to
within 5%, if not then continue repeating 5 through 10 until the
measurements agree to within 5%.

12\. Use the filament weight and copper weight in the following formula:

[Weight of copper (Kg)]{.underline}

8940 Kg/m3

Cu/SC Ratio=

[Weight of non-copper components (Kg)]{.underline}

Vol % Nb barrier (8570 Kg/m3) + Vol % NbTi (6000 Kg/m3)

13\. Dispose of filaments in a safe manner to prevent accidental fire.

ALTERNATE PROCEEDURE FOR CU/SC DETERMINATION

An electrical method can be used by the vendor providing that the
vendor\'s procedure is reviewed and approved by the Fermilab Subcontract
Administrator. In addition, provisions must be made, in the procedure,
for periodic calibration to the chemical technique.

# Appendix IV - Sharp Bend Test

The sharp bend procedure is to simulate the deformation of the strand
that may occur during cabling.

1.  Fabricate a test fixture consisting of a metal block with a slot and
    a slab that freely slides in the slot, as indicated in figure below.
    The slab width should be equal to the strand diameter. The slab
    should slide freely into the slot without gaps. A drawing will be
    provided upon request. The vendor can propose his version of the
    fixture that should be approved by the Fermilab Subcontract
    Administrator.

![](media/image2.wmf)

![](media/image3.wmf)

2.  Cut a length of strand sample approximately 20 cm long. Bend the
    strand sample in half over a rod approximately 2 mm in diameter as
    indicated below.

![](media/image4.wmf)

3.  Remove the rod and place the bent sample in the slot of the fixture
    as indicated in the top part of the figure. Slide the slab into the
    slot of the fixture to squeeze the bent sample to the value of 1.616
    mm (inner cable strand) or 1.296 mm (outer cable strand) to obtain a
    hairpin shape.

4.  Examine the bend region at a magnification of at least 10X under a
    lighting level of at least 1076 lux and verify that the surface of
    the copper is not cracked, split, or otherwise deformed to prevent
    successful cabling.

5.  Etch the bend region in dilute nitric acid and examine the filaments
    at a magnification of at least 10X and a lighting level of at least
    1076 lux and verify that no filaments have been broken.

# Appendix V - Determination of Springback Properties

1\. [Purpose]{.underline}

This test establishes a standardized method for testing superconducting
(S.C.) wire to determine its springback acceptability.

2\. [Materials Required]{.underline}

2.1 Cut three lengths, 1 m each, of S.C. wire to be tested.

Note: Do not bend wire unnecessarily.

3\. [Test Equipment]{.underline}

3.1 Springback Test Fixture approved by the Fermilab Subcontract
Administrator.

3.2 2 kg weight

4\. [Test Procedure]{.underline}

4.1 Prepare one end of wire sample with a 13 mm, 90° bend, and tie the
other end securely to a 2 kg weight.

4.2 Test the spring fixture to be sure it turns freely.

4.3 Thread the 90° bend through the test fixture and place in the hole
in the spring winder with the locking pin in place.

4.4 Tighten the wire.

4.5 Make sure the 90° bend is not affecting the \"Zero\" reading and the
wire is tangent to the spring winding shaft.

4.6 Set \"Zero\" on the degree wheel.

4.7 Hang the 2 kg weight over the end of the table. Release the clamp.
Hold the spring winder handle and pull the locking pin.

4.8 Wind 10 complete turns and replace locking pin. Then tighten wire
clamp.

4.9 Hold spring handle and remove locking pin. Gently let the spring
unwind and note the number of revolutions.

4.10 Once the spring has stopped, gently touch the spring handle to make
sure the spring is at equilibrium and has reached its full springback.
Do not unwind the spring.

4.11 Note and record the total number of degrees of springback.

4.12 Cut the sample at the wire clamp and the 90° bend.

4.13 Carefully slide the spring winder out of its bearings and remove
the sample.

4.14 Measure and record the inside diameter, label the sample and store
in archives.

4.15 Three sections of each wire sample shall be measured and reported.

# Appendix VI - Determination of Normal State Resistance of NbTi Superconducting Wire

1\. [General Outline; Definition of Residual Resistance
Ratio]{.underline}

This method covers the measurement of electrical resistance of NbTi
multifilamentary composite wire, which is used to make high current
superconducting cables. The resistance per unit length is determined at
room temperature (295 K) and just above the transition temperature
(Tc\~9.5 K). These quantities are designated R295 and R10, respectively,
and are measured with an accuracy of 0.5%. The ratio R295/R10 is defined
to be the residual resistance ratio, RRR.

2\. [Apparatus Description]{.underline}

A four wire method is used to determine the resistance. The wire sample
is mounted on a probe which may be used for superconducting critical
current measurements. It has leads which are suitable for carrying the
required current 0.1 to 0.5 A from room temperature into a liquid helium
bath, and potential leads for measuring the voltage drop across a
measured length of the test specimen. The probe should be mounted so
that the test specimen can conveniently be raised and lowered through
the level of a helium bath.

Voltage drops are measured with a digital voltmeter of 0.5µV resolution.
It is helpful during the low temperature measurement to use an X-Y
recorder simultaneously with the digital voltmeter, with Y set to
voltage and X to time (see Section 4 below).

Current is provided by a well- regulated and filtered DC power supply.
It is measured by a shunt of 0.5% accuracy.

In the room temperature measurement, a temperature measuring device of
0.1°C accuracy is used to determine the ambient temperature.

3\. [Sample Mounting]{.underline}

The test specimen is attached to a holder. The ends are soldered to the
copper terminations of the current leads over a minimum length of 25 mm.
Voltage taps are soldered to the specimen at a distance of at least 25
mm from the current joint. Voltage taps are soldered to the specimen at
a separation distance of at least 25 mm from each current lead
connection. It is advisable that these taps be in the form of fixed pins
so that the test length be constant throughout a series of measurements.
In order to assure an accuracy of 0.2% the voltage tap length should be
.5 m or more. The voltage leads should follow the sample in a
non-inductive fashion so as to minimize noise pickup. Alternatively, the
sample may be wound non-inductively on the form.

4\. [Procedure]{.underline}

Room temperature measurements are made at currents which are a
compromise between the requirements of sensitivity and negligible ohmic
heating. Voltage readings are taken for forward and reversed current and
averaged.

Low temperature measurements are made in a helium dewar. The probe is
withdrawn from the bath so that the lowest point of the specimen is a
few centimeters above the liquid helium bath level while measuring
current is flowing. As the sample warms, the voltmeter reading will go
suddenly from zero to a finite value corresponding to its normal state
resistance. The latter is substantially independent of temperature from
the transition temperature, Tc, to 15 K, so that the voltage remains
constant long enough to be read and is recorded. In addition, it is
unnecessary to determine the exact temperature as long as it is assured
that the low temperature measurement is made while the voltage is nearly
constant. With a reasonably designed probe and former, it may take 1 or
2 seconds for the specimen to go normal. The resistance will remain in
the residual resistance region for several seconds. When the X-Y
recorder is used, a series of abrupt voltage changes are recorded as the
specimen is alternately raised and lowered through the helium bath
level. The height of these steps should be reproducible. At least two
cycles with similar residual resistances shall be performed to complete
the R10 measurement.

5\. [Room Temperature Correction]{.underline}

Normally occurring room temperature variations may produce significant
variations in the measured resistance. Designating this resistance as Rm
and the ambient temperature as Tm (°C), the resistance at the reference
temperature of 295 K is calculated as follows:

R295 = Rm/\[1 + .0039 (Tm - 22)\]

The effect of the NbTi is neglected for the purpose of this correction.

[^1]: ^\$^ This specification is similar to SSC inner strand
    specification, SSC-Mag-M-4146