177 research outputs found
Increases in the Irreversibility Field and the Upper Critical Field of Bulk MgB2 by ZrB2 Addition
In a study of the influence of ZrB2 additions on the irreversibility field,
Birr and the upper critical field Bc2, bulk samples with 7.5 at. % ZrB2
additions were made by a powder milling and compaction technique. These samples
were then heated to 700-900C for 0.5 hours. Resistive transitions were measured
at 4.2 K and Birr and Bc2 values were determined. An increase in Bc2 from 20.5
T to 28.6 T and enhancement of Birr from 16 T to 24 T were observed in the ZrB2
doped sample as compared to the binary sample at 4.2 K. Critical field
increases similar to those found with SiC doping were seen at 4.2 K. At higher
temperatures, increases in Birr were also determined by M-H loop extrapolation
and closure. Values of Birr which were enhanced with ZrB2 doping (as compared
to the binary) were seen at temperatures up to 34 K, with Birr values larger
than those for SiC doped samples at higher temperatures. The transition
temperature, Tc, was then measured using DC susceptibility and a 2.5 K drop of
the midpoint of Tc was observed. The critical current density was determined
using magnetic measurements and was found to increase at all temperatures
between 4.2 K and 35 K with ZrB2 doping.Comment: 15 pages, 5 figs, 1 tabl
AC Loss and Contact Resistance In Copper-Stabilized Nb3Al Rutherford Cables with and without a Stainless Steel Core
Calorimetric measurements of AC loss and hence interstrand contact resistance
(ICR), were measured on three samples of Rutherford cable wound with
Cu-stabilized jelly-roll type unplated Nb3Al strand. One of the cable types was
furnished with a thin core of AISI 316L stainless steel and the other two were
both uncored but insulated in different ways. The cables were subjected to a
room-temperature-applied uniaxial pressure of 12 MPa that was maintained during
the reaction heat treatment (RHT), then vacuum impregnated with CTD 101 epoxy,
and repressurized to 100 MPa during AC-loss measurement. The measurements were
performed at 4.2 K in a sinusoidal field of amplitude 400 mT at frequencies of
1 to 90 mHz (no DC-bias field) that was applied both perpendicular and parallel
to the face of the cable (the face-on, FO, and edge-on, EO, directions,
respectively). For the cored cable the FO-measured effective ICR (FO-ICR), was
5.27 . Those for the uncored cables were less than 0.08
. As shown previously for NbTi- and Nb3Sn-based Rutherford
cables, the FO-ICR can be significantly increased by the insertion of a core,
although in this case it is still below the range recommended for
accelerator-magnet use. Post-measurement dissection of one of the cables showed
that the impregnating resin had permeated between the strands and coated the
core with a thin, insulating layer excepting for some sintered points of
contact. In the uncored cables the strands were coated with resin except for
the points of interstrand contact. It is suggested that in the latter case this
tendency for partial coating leads to a processing-sensitive FO-ICR.Comment: Four pages, with two figure
Transport and magnetic Jc of MgB2 strands and small helical coils
The critical current densities of MgB2 monofilamentary strands with and
without SiC additions were measured at 4.2 K. Additionally, magnetic Jc at B =
1 T was measured from 4.2 K to 40 K. Various heat treatment times and
temperatures were investigated for both short samples and small helical coils.
SiC additions were seen to improve high field transport Jc at 4.2 K, but
improvements were not evident at 1 T at any temperature. Transport results were
relatively insensitive to heat treatment times and temperatures for both short
samples and coils in the 700C to 900C range.Comment: 8 text pages, 1 table, 4 fig
Stability mechanical considerations, and AC loss in HTSC monoliths, coils, and wires
For monolithic high-T(sub c) superconductors (HTSC's) calculations are presented of: (1) the initial flux jump field, H(sub fj), in melt-processed YBCO based on a field and temperature dependent J(sub c), and (2) the radial and circumferential stresses in solid and hollow cylinders containing trapped magnetic flux. For model multi filamentary (MF) HTSC/Ag strands calculations are presented of: (1) the limiting filament diameters for adiabatic and dynamic stability, and (2) the hysteretic and eddy current components of AC loss. Again for MF HTSC/Ag composite strands the need for filamentary subdivision and twisting is discussed
Solenoidal Coils Made from Monofilamentary and Multifilamentary MgB2 strands
Three solenoids have been wound and with MgB2 strand and tested for transport
properties. One of the coils was wound with Cu-sheathed monofilamentary strand
and the other two with a seven filament strand with Nb-reaction barriers, Cu
stabilization, and an outer monel sheath. The wires were first S-glass
insulated, then wound onto an OFHC Cu former. The coils were then heat treated
at 675C/30 min (monofilamentary strand) and 700C/20 min (multifilamentary
strand). Smaller (1 m) segments of representative strand were also wound into
barrel-form samples and HT along with the coils. After HT the coils were epoxy
impregnated. Transport Jc measurements were performed at various taps along the
coil lengths. Measurements were made initially in liquid helium, and then as a
function of temperature up to 30 K. Homogeneity of response along the coils was
investigated and a comparison to the short sample results was made. Each coil
contained more than 100 m of 0.84-1.01 mm OD strand. One of the 7 strand coils
reached 222 A at 4.2 K, self field, with a Jc of 300 kA/cm2 in the SC and a
winding pack Je of 23 kA/cm2. At 20 K these values were 175 kA/cm2 and 13.4
kA/cm2. Magnet bore fields of 1.5 T and 0.87 T were achieved at 4.2 K and 20 K,
respectively. The other multifilamentary coil gave similar results.Comment: 22 pages, 8 figures, 2 table
Effects of Core Type, Placement, and Width, on the Estimated Interstrand Coupling Properties of QXF-Type Nb3Sn Rutherford Cables
The coupling magnetization of a Rutherford cable
is inversely proportional to an effective interstrand contact
resistance, Reff, a function of the crossing-strand resistance, Rc,
and the adjacent strand resistance, Ra. In cored cables Reff varies
continuously with W, the core width expressed as percent
interstrand cover. For a series of un-heat-treated stabrite-coated
NbTi LHC-inner cables with stainless-steel (SS, insulating) cores
Reff(W) decreased smoothly as W decreased from 100% while for
a set of research-wound SS-cored Nb3Sn cables Reff plummeted
abruptly and remained low over most of the range. The
difference is due to the controlling influence of Rc – 2.5 μΩ for the
stabrite/NbTi and 0.26 μΩ for the Nb3Sn. The experimental
behavior was replicated in the Reff(W)s calculated by the program
CUDI© which (using the basic parameters of the QXF cable)
went on to show in terms of decreasing W that: (i) in QXF-type
Nb3Sn cables (Rc = 0.26 μΩ) Reff dropped even more suddenly
when the SS core, instead of being centered, was offset to one
edge of the cable, (ii) Reff decreased more gradually in cables with
higher Rcs, (iii) a suitable Reff for a Nb3Sn cable can be achieved
by inserting a suitably resistive core rather than an insulating
(SS) one.Funding was provided by the U.S. Dept. of Energy, Office of High Energy
Physics, under Grants No. DE-SC0010312 & DE-SC0011721 (OSU) and DEAC02-
05CH11231 (LBNL).The coupling magnetization of a Rutherford cable is
inversely proportional to an effective interstrand contact
resistance, Reff, defined as Reff = [1/Rc + 20/N3Ra]-1. In uncored
cables Reff is primarily controlled by Rc. The LHC magnet’s
uncored NbTi cables, wound with specially heat treated
stabrite-coated strands, evidently have acceptable Rcs. It has
been reported that the current ramping of LHC magnets
produces field errors: (i) in dipoles of about 1 unit of b1 and
less than 0.1 units of cn, consistent with Rc well above 50 μΩ,
(ii) in quadrupoles of about 2 units of b1 and less than 0.2 units
of cn, consistent with Rc between 100 and 150 μΩ. Evidently
such Rcs have contributed to the successful operation of the
LHC dipoles and quadrupoles to date and hence could be
thought of as new target values when designing the Nb3Sn
cables for the LHC upgrades. But with measured Rcs of
typically 0.3 μΩ bare Nb3Sn cables are unsuitable; the cables
need to be furnished with some kind of core to separate the
crossing strands. In cables with insulating cores Reff (now a
function of both Rc and Ra) increases continuously with W (%
core cover), with Ra eventually taking over as the controlling
ICR. In seeking an optimal core width a large assortment of
research cables were wound and measured over the years. The
results, assembled and compared here for the first time, show
Reff(W) reaching acceptable values only when W approached
~90% beyond which it increased very steeply. These
experimental values were compared to modelling results using
the program CUDI© choosing as our model cable a variablewidth-
core version of QXF. Further application of the program
demonstrated that core positioning was important, Reff
decreasing by about 2½ times as the cores shifted from the
center to one edge of the cable. As a result it is predicted that
irregularities in core placement could produce a large scatter
in Reff. The sensitivity of Reff to core width and position in the
optimal large-W range leads to the suggested inclusion of a
core, not of SS (which has a stable, insulating oxide surface
layer), but of a resistive composite such as Cr-plated SS or Crplated
Cu
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