16-09-2014, 12:37 PM
[u]Thermal Properties of Impregnating Materials
for Stable Superconducting Magnets [/u]
Thermal Properties o.pdf (Size: 493.79 KB / Downloads: 9)
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The minimum quench energies (MQEs) of supemndueting
(SC) vvires have been measured precisely by means of a 'Carbon Paste
Heater'. The thermal behavior just before the normal generation of
the heavy loaded SC wire was observed to determine the quenching.
We le;& that once a normal region is produced, it should spread
rapidly regardless of the surrounding conditions. The MQE, however,
would increase when a loeal temperature rise before quenching is
prevented by using certain materials around the SC wire. For
example, high thermal diffusivity in impregnat@ materials or low
thermal oondwtivity in wire insulation will be needed when
disturbaoees oecur outside of the SC wires. In the previous paper, we
showed the effects of enamel insulation for improving stabiity. Thus,
in this study, we examine the relation between MQE and the thermal
properties of the impregnating material. Several kinds of
imprepting materials were selected and the thermal properties are
compared with enlarging MQE in mind. We have searched for an
impregnating material that has the thermal property required for
i"Vd stabiity.
I. INTRODUCTION
A "Carbon Paste Heater" has been developed to measured
the m&um quench energy(MQE) of SC wires acCUratey[l].
It was c"ed that when we discuss the stabhty in the
supertmductivity, it will be important to consider both 1) the
quant~ity of energy which enters the SC wire and 2) the rate at
whch it occurs[l],[2].
Fig 1 shows an initiation of a quench. It consisks of2 phases
in the heat transfer mechanism. First, the external thermal
disturbance caused by something like an epo'xy debondmg or
a cracking and some of the energy will enter the SC
WirefJ'HASE 1). After that a normal conducting area occm
and if it spreads out, it will lead to a quench. If it is cooled
sxdkiently and vanishes, the SC wire will recover to a
supemmducting state (PHASE 2). E the SC magnet is
designed based on the concept of cryogenic stabhation, the
staJAty is determined by the phenomenon described in
PHASES. The SC wire is cooled by smundmg mate&
such as liquid helium (Me) when the normal zone occurs.
However, the SC magnet with a hgh current density does not
recover once the normal zone is produced and spreads. In
the previous papers, we reported that SC wire under a heavy
load quenches regardless of cooling condition[l]. To make tins
sort of magnet stable, we have to look at PHASE 1. If, just after
MQEMeamementfar the SC Wm Impmted w‘kh iii
MQEs were measured and the influence of impregnating
mate~ials on stability was examined F‘g. 7 shows a schematic
illustration of a sample. A heater was attached to the middle of
a bane NbTi SC wire about 160” long. After that, it was
impregnated with epoxy or ice. The specifications of the SC
wire are shown in Table I. A test disturbance was introduced
and the MQE was measured hm the signal of the voltage
taps, ‘fie contents offour samples are shown in Table II. For
sample 1 and 2, the carbon paste heaters were utilized, so that,
the energy of the test disturbance could enter the SC wire
&&u.ely. The cahn paste heater was pressed by the G10
plate and glued with epoxy, thedore, 4 mm stretch of the SC
wire vvas thermally jsolad With these two samples, we can
evaluate the efFects of the impregnating materids after the
energy has entered and a normal zone has spread assuming
PHASEB. Sample 3 and 4 use heating wires 0.l” i
CONCLUSION
It is ddkdt to improve stability through the use of
conventional organic impregnating materials. The
impregnating material whose thermal conductivity is as much
as 10 times larger than conventional one's is necessary to
kpme the stability. Ice impregnation sohies the problem and
makes the supercondudor stable