Elekta Neuromag NM21283A, Руководство пользователя

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Basic tuning operations 3
NM21283A-A 13
Warning: Tuning for excessively long times (>5 min.) does not make the
tuning better. Instead, if the noise level is limited by something else than
SQUID noise, the tuning settings may drift to unstable values.
3.3 Resetting channels
The SQUID sensors are operated in so called flux locked mode. See
“SQUID electronics of Vectorview” on page 29. In some situations the
electronics is not capable following the signal and the output gets satu-
rated. This can be due to very strong signals (e.g. environmental low fre-
quency drift) or sharp transients. To get the sensor working again, the
integrator in the feed back loop and the zeroing mechanism (high pass fil-
ter), need to be reset. Saturated channels that need to be reset are repre-
sented as small negative bars in the noise display.
To reset all channels:
1. Select Commands > Reset electronics from the menus.
3.4 Heating sensors
The coils and the SQUIDs in a sensor unit contain very thin superconduct-
ing film. Sometimes when the sensor is exposed to strong (relative to the
signals measured) magnetic field, these films “trap” magnetic flux. Strong
magnetic field can penetrate the film destroying the superconductivity
from a small patch of the film so that the field can go through it. Typically
the impurities in the films cause these “holes” to stick at some fixed posi-
tion, thus this phenomenon is often called “flux trapping”.
The flux trapping in the sensor coils is not problematic if the flux really
stays in place. Unfortunately it is quite common that the trapped flux
moves between e.g. two points. Since the flux pinned into the film is
always strong, even a very small movement cause severe jumping in the
sensor signal.
If the flux trapping occurs so that the flux penetrates the Josephson junc-
tions in the SQUID, the sensor ceases to work. If the pinning point is in
near neighborhood of the junctions, the extra field leaking through the
junctions cause the modulation depth of the SQUID to diminish. This can
manifest itself as increased noise or, if the effect is reasonably small, as
moving of the optimal operating point (bias, offset, and gate voltages) to
somewhat different values.
The magnetic flux in the thin films of the sensor can be removed by heat-
ing the sensor above the critical temperature of the films, and then letting
it cool down again. The sensors in Vectorview are equipped with small