Elekta Neuromag NM21283A, Руководство пользователя

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6 Principles of SQUID operation
28 NM21283A-A
ring will maintain its superconducting properties, but only up to a certain
limit.
When operating the SQUID, a small current (bias current) is fed through
the SQUID ring and the voltage over the device is measured. When the
current through the SQUID is small, no voltage appears because the ring
is totally superconducting. Above a critical value (critical current) a volt-
age drop appears. The apparent critical current of the SQUID depends on
the magnetic flux threading the ring. Thus, maintaining the bias current at
a suitable level, a small change in the magnetic flux coupled from the
external source via the flux transformer will change the point where the
ring loses its superconductivity and voltage drop appears. This results in a
modulation of the voltage as a function of magnetic field.
Another way is to fix the voltage (bias voltage) over the SQUID and mea-
sure the current through the device. Vectorview electronics uses this volt-
age-bias mode of operation. Figure 6.2 depicts the current vs. voltage
characteristics of a typical SQUID sensor. The characteristics consist
actually of a family of curves for each value of magnetic flux threading
the ring. However, the dependence on the magnetic flux is periodic, and it
is customary to plot only the extremum curves. As the magnetic flux
through the ring changes, the shape of the current vs. voltage curve
changes continuously between the two extrema. The period with which
the behavior repeats itself is one flux quantum
.
The two extremal curves shown in Figure 6.2 thus correspond to magnetic
flux values separated by
.
Near the origin, the SQUID is in superconducting state and current can
flow through without a voltage loss. In Vectorview, there is a deliberately
added small series resistance which causes a finite slope near the origin as
in Figure 6.2.
Figure 6.2 Current-voltage characteristics of a SQUID
I
B
V
B
Φ
0
2.07 10
⋅ 15
–
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=
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