Superconducting conductor system

Details Superconducting conductor system Superconducting conductor system

1996-10-31

2002-07-09

Barrera, Ramon M. (Department: 2832)

Electricity: magnetically operated switches, magnets, and electr

Magnets and electromagnets

Superconductive type

C505S879000, C336S175000, C361S019000

Reexamination Certificate

active

06417751

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates to a superconducting coil system and, more particularly, to a technique of suppressing and reducing generation of current imbalance in a superconducting strand.
BACKGROUND
FIG. 1
shows, in a cross-section, an illustrative cable-in conduit conductor (CICC) constituting a conventional superconducting coil system.
Referring to
FIG. 1
, the cable-in-conduit conductor is comprised of tens to hundreds of yarned (or twisted) superconducting strands packed in a stainless steel conduit in the form of a pipe.
In a cable-in-conduit conductor, the void ratio, representing the ratio of an area in a cross-section excluding an area occupied up by the strands, is usually set to approximately 35 to 37% (see Takahashi et al., “Chromium Plating Thickness Dependency of Cable-in-conduit Conductor on Coupling Loss”, . Extended Abstract to the 52nd Autumn Meeting of 1994 of Low-Temperature Superconducting Association [Teion Kogaku Chodendo-Gakkai], A3-6, page 225, ‘literature 1’).
The liquid helium (He) or supercritical He is allowed to flow between these strands to cool the strands to permit the current to flow therein in a superconducting state.
The conduit performs the role of securing a flow channel for He, in addition to the role of supporting the gigantic electro-motive force exerted on the conductor.
FIG. 2
shows an illustrative method for producing this sort of the CIC conductor.
Referring to
FIG. 2
, each strand is of a diameter of 0.76&PHgr; [mm] and has embedded in the mid portion thereof a superconducting filament formed of copper and NbTi, Nb
3
Sn or the like.
In the embodiment of
FIG. 2
, three such strands are twisted together to form a sole twisted yarn. Three such twisted yarns are twisted together to form a sole twisted wire. This operation is repeated twice resulting in a twisted cable, and ultimately six resultant cables are accommodated within a 23.0×27.6 mm sized conduit.
Thus, in the embodiment of
FIG. 2
, 3×3×3×3×6=486 strands are used.
There are several reasons of using a large number of twisted yarns.
One of such reasons is reducing AC loss. Eddy current flows on the surface of a conductor placed in an AC circuit or in a changing magnetic field as time lapses. This phenomenon is known as the skin effect.
The surface of the strand is formed of copper, as shown in FIG.
2
. Thus the eddy current is apt to flow on the strand surface such that heat is evolved by the resistance of copper to detract from stability of the superconducting coil. Therefore, a strand of small diameter is used for reducing the eddy current loss.
Meanwhile, if the characteristic depth (penetration depth) of the skin effect is &dgr; and the strand diameter is d, a designing standard is given by the following inequality:
d<&dgr;
  (1)
The strand of such fine diameter is satisfactorily compatible with machining NbTi or the like into a filament.
Another reason of twisting (or yarning) a number of strands is that, since the conductor is intended for fabricating a coil, a bending operation is required.
If the strand is not twisted, it is poor in bendability and might occasionally encouter fracture.
During coil production, a coil is bent in one direction, as a result of which the coil length as measured on its inner diameter side differs from that as measured on its outer diameter side.
If the strand would be not twisted, it would be stretched on its outer diameter side while being contracted on its inner diameter side.
The twisting operation is performed for preventing lowering of superconductor characteristics due to such non-symmetrical structure.
The CIC conductor, thus fabricated, is wound to a pre-determined shape for producing a coil.
If the coil is used for an AC circuit or the like, it is preferred that the strands be electrically insulated from one another for the above reason. The reason is that, if the surfaces of the strands are electrically connected to one another, the plural strands can be regarded as a conductor with a larger surface area and a larger volume, so that the eddy current loss W is increased. The eddy current loss is proportionate to the square of the characteristic size such that
W∝d
2
  (2)
In the above equation (2), W represents the eddy current loss and d represents the strand diameter.
Since there are, in effect, a large number of contact portions in a sole strand, the eddy current flows in a complex pattern.
For the above reason, in preparing an NbTi-30KA grade coil (DPC-U) by a CIC conductor in the demonstration poloidal coil (DPC) project of Japan Atomic Energy Research Institute, referred to hereinafter as JAERI, each strand is insulated by Formvar insulation.
That is, the surface of the strand shown in
FIG. 2
is coated with a Formvar insulating material to a thickness of several micrometers (see FIG.
3
). By coating the strand surface with the insulator, the individual strands are insulated satisfactorily from one another.
By employing such structure, a superconductor coil of high stability and low eddy current loss with only little AC loss have been expected to be producible. In the case of a superconductor coil used in an AC circuit, although the AC losses may be enumerated by hysteresis loss and proximity effect loss etc., in addition to the eddy current loss, the eddy current loss is predominant among these AC losses.
PROBLEM TO BE SOLVED BY THE INVENTION
However, experiments on DPC conducted by the JAERI was not successful, as will now be explained.
Before an experiment on passing the AC current, an experiment was conducted using a pulse-shaped current waveform (see FIG.
4
). Due to excitation of a sole coil, the waveform of the magnetic field generated by the coil is analogous to FIG.
4
.
Consequently, the rate of change of the magnetic field (flux density) dB/dt during a time period 0 to t
1
(time differential of magnetic field) is found. In an experiment, the time from 0 to t
1
is controlled by an external power source and the rate of change of the magnetic field dB/dt was varied for finding data such as stability of the superconducting coil.
Thus it was found that, in this superconducting coil, the value of the rate of change of the magnetic field dB/dt which permits stable operation was approximately one thousandth of the initial design value.
It was intended at the outset to achieve a world record in connection with the rate of change of magnetic field dB/dt. However, in fact, the coil quenching occurred at a value of the rate of change of magnetic field dB/dt which was significantly lower than that with the conventional coil.
The reason therefor was checked precisely by researchers of the JAERI, manufacturers and universities. It was found that the currents flowing in the individual strands are not the same but large current imbalance exists. The results of the analysis are substantially as follows:
FIG. 5
shows an equivalent circuit in case two strands are used.
Referring to
FIG. 5
, L
1
and r
1
denote self-inductance and resistance of strand 1, L
2
and r
2
denote self-inductance and resistance of strand 2, respectively, and M denotes mutual inductance.
The circuit network equation is given by the following equations (3) and (4):
 
V=r
1
·i
1
+j&ohgr;L
1
·i
1
+j&ohgr;M·i
2
  (3)
V=r
2
·i
2
+j&ohgr;L
2
·i
2
+j&ohgr;M·i
1
  (4)
In the above equations, &ohgr; and j denote the oscillation frequency of the circuit and an imaginary number of j
2
=−1, respectively.
By solving the equations (3) and (4) with respect to the currents i
1
, i
2
, the following equation (5) is derived:
i
1
i
2
=
r
2
+
jω
⁡
(
L
2
-
M
)
r
1
+
jω
⁡
(
L
1
-
M
)
(
5
)
Since the strand is in the superconducting state, r
1
=r
2
=0 in the equation (5), and thus the current ratio of the two strands is given by the equation (5′):
i
1
/i
2
=(
L
2
−M
)/(
L
1
−M
)  (5′)
As will be understood from the

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