Transcript Some Thoughts on Magnetic Measurements for MICE Michael A. Green Lawrence Berkeley Laboratory Berkeley CA 94720, USA.

Some Thoughts on Magnetic
Measurements for MICE
Michael A. Green
Lawrence Berkeley Laboratory
Berkeley CA 94720, USA
Steps in the MICE Magnetic
Measurement Process
• Magnetic alignment begins during construction.
Low current magnetic measurements at 300 K can
be used to verify that the magnetic axis is in the
correct x-y position and angle, at a time when one
can adjust the position of the cold mass.
• It is planed that the MICE magnets will use selfcentering supports. This means that the center
between the supports is in the same position at 4 K
as it is at 300 K, within ±0.2 mm. The axis angle
should not change more that ±0.5 mrad.
Magnetic Measurement Steps cont.
• Measurements at the vendor will verify that the
magnet will reach it full design field . The on-axis
field will be measured as a function of z. It should
be possible to determine the solenoid magnetic axis
angle with respect to the magnet bore axis.
• A field map will not be made at the vendor’s plant.
It is recommended that a magnetic field map be
made in the regions of the magnet that will see the
beam. For the tracker magnet these measurements
will be made at FNAL. There is no decision as to
where the other magnets will be mapped.
Magnetic Measurement Steps cont.
• Once the MICE channel has been assembled,
magnetic measurements of the channel magnets is
longer be possible. One cannot measure the effect
of current in another MICE magnet on the field
within a particular MICE magnet. One must use
the calculated to field to determined the effect of
one magnet on another magnet in MICE.
• Jean Michel Rey of Saclay proposed that the field
be monitored by three axis magnetic measurement
probes (Hall probes) in at least three places around
each of the coils in the MICE channel. This method
is used to monitor the field in ATLAS. Is this a
useful technique for the MICE cooling channel?
Is External Field Monitoring Necessary for
the MICE Channel Magnets?
• The field outside of the magnets is relatively low for
the tracker magnet and the AFC magnet in the nonflip mode (~0.05 T). Measurement sensitivity may
be a problem. The field outside the coupling coil is
high (1 to 2 T) and the field outside the AFC magnet
in the flip mode is medium (0.2 to 0.35 T).
• External field monitoring makes sense if there is no
iron around the magnets. There will be lumps of
iron (compressors, pumps and some other items)
that may affect the field outside of MICE, but the
field in the MICE channel will not be affected
much.
External Field Measurements cont.
• It is not clear that external field measurements are
useful for the tracker solenoid. Monitoring of the
field in the bore around the tracker is proposed.
• Lumps of iron outside of MICE can affect the field
outside of the coupling coil because the magnetic
moment generated by the coil is higher and the coil
is closer to the iron. The field outside of the AFC
magnet may also be affected by iron.
• Measuring the external field is useful to determine
the channel operating mode and an individual
magnet’s polarity. Other than this, field monitoring
outside of MICE magnets may not be very useful.
External Field Measurements cont.
• For the tracker module, three axis Hall probes could
be installed 120 degrees apart on the outside of the
tracker magnet cryostat over the center of each
match coil and the center of the long center coil in
the spectrometer solenoid. This is in addition to the
probes installed on the tracker frame.
• For the AFC module, a single set of three axis Hall
probes could be installed 120 degrees apart on the
outside of the AFC cryostat at the center.
• For the RFCC module, a single set of three axis Hall
probes could be installed 120 degree apart on the
outside of the magnet cryostat at the center.
Concluding Comments
• Magnetic measurements (both warm and cold) are
done at the vendor to determine that the magnet
meets the basic specification.
• Field mapping on all individual magnets should be
done in the region where the MICE beam will be.
The angle of the magnetic axis with respect to the
magnet physical axis must be determined.
• External field probes are useful for determining the
operating mode and the magnet polarity. Knowing
the field at the tracker frame is useful.