CM-18 June 2007 1

The Measurement and Monitoring of the Field from

the MICE Magnets

Michael A Green

Lawrence Berkeley Laboratory, Berkeley, USA

Frank Filthaut

NIKHEF, Amsterdam, Netherlands

CM-18 June 2007 2

Magnetic Measurement and Monitoring

• Some comments about the MICE magnetic field.

• What is important to know from the MICE magnetic measurements?

• How should one measure the magnetic field in the MICE magnets?

• Should the field within the spectrometer solenoid be monitored?

• Should one monitor the MICE magnetic field while operating the magnet?

• Methods of magnetic field monitoring

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Tracker 1

AFC1

AFC2

Tracker 2

AFC3

Coupler 1

Coupler 2

MICE Magnet Coils

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The Characteristics of MICE Magnet

• The MICE magnets have a total of 18 coils. Various coils are connected in series with a total of 6 circuits.

• The MICE magnets ideally share a common axis.

• Since the MICE magnets are axially symmetric the fields should have only r and z component. There should be no component of the field.

• A component of field means that there are field errors. These errors are due to error in coil placement, lumps of ferromagnetic material, and other sources of magnetic field.

• The field should be proportional to the coil currents to better than 0.1 percent, except for the shield end of the detector.

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Sources of Field Errors in MICE

• The MICE magnets do not share a common axis. The axis error for a coil should be less than ±2 mm.

• The coil axis is tilted with respect to the MICE channel axis. The axis tilt should be less than ±3 mrad.

• The MICE coils are not in the correct z position. The z position should be correct to about ±5 mm.

• Lumps of ferromagnetic material, and other sources of magnetic fields are present near the coils. Calculations suggest that this effect is small for most of the channel.

• Circulating currents in the superconducting filaments can cause field errors. For the operating fields in the MICE channel. This error is small (<0.03 percent).

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Why Measure the MICE Magnets?

• It is important to know the location of the magnetic axis for each of the magnets in the MICE channel.

• The magnetic measurements should confirm whether the actual magnetic field is the same as the field calculated by the by the magnetic field codes.

• The magnetic field measured can be used to analyze how the muons will behave in the MICE channel. The actual field measurements can also be put into MICE simulation codes.

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The Expected Magnetic Field in the MICE Channel

Tracker Magnet

End AFC Magnet

Coupling Magnet

Center AFC Magnet

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MICE Magnet Magnetic Measurements

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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.

• The MICE magnets will use self-centering 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 error should be <0.8 mrad for the RFCC magnet, <0.5 mrad for the AFC magnet and <0.2 mrad for the tracker solenoid.

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Magnetic Measurement process 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 AFC and tracker magnets the beam diameter is 0.3 m in diameter. For the coupling magnet this diameter is 0.42 m. The tracker magnet these measurements will be made at FNAL. There is no decision as to where the other magnets will be mapped.

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A Magnetic Measurement Technique

• Measurements can be made using temperature compensated Hall probes. At fields from 0.5 T to 2 T the error is <0.03 percent. Most Hall probes are not calibrated above 2 T.

• Three axis Hall probes (to measure the r, z and components of the field) can be mounted on a rotating disc at different radii. The probe mounting errors are significant. Rotating the discs will allow one to determine whether a measured error is a probe mounting error or a real field error.

• The field should be measured at a number of z locations along the physical (room temperature) axis of the magnet.

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The Field within the Tracker Magnet

The radial component of field is small through much of the range. The peak radial field is~1.4 T at the ends of the magnet.

Magnet alone from 2.5 m < z < 7.0 mand from 0.0 m < r < 0.2 m

Note: In the good field region, the field is about 3.961 T. There is no field from the other MICE magnets.

CM-18 June 2007 13

The Field outside the Tracker Magnet

The 300 G line is about 1 meterfrom the ends of the trackermagnet. The radial position ofthe 300 G line is at a radius ofabout 1 meter. This is a stayclear line at Fermilab.

Magnet alone from 2.5 m < z < 7.0 mand from 0.0 m < r < 1.545 m

Note: There is no iron ring at the end of the spectrometermagnet in this case.

Tracker Magnet Coil

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Tracker Magnet Measurements

• The external field is important because of safety concerns, especially at fields >0.03 T.

• The field tracker magnet is above 2 T. The field from the center of the uniform field region to the end of the magnet is not linear with current because the iron shield can affect the field within the tracker volume up to 2 percent. This is why the two end coils have tuning power supplies.

• The tracker magnet room temperature Hall probes must be calibrated to 4.5 T. Can one differentiate between errors caused by the iron shield and probe calibration errors? YES

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The Field within the AFC Magnet

The radial component of field is large in the flip mode as shown. The peak radial field is ~2.4 T at the ends of the magnet.

Magnet alone from 0 m < z < 1.32 mand from 0.0 m < r < 0.2 m

Note: There is no field from theother MICE magnets.

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The Field outside the AFC Magnet

In the flip mode, the 300 G line is ~1.5 metersfrom the ends of the focusing magnet. The radial position of the 300 G line is at a radius of ~1.3 meters.

Magnet alone from 0 m < z < 2.0 mand from 0.0 m < r < 1.5 m

In the non-flip mode, the 300 G line is ~1.5 mfrom the ends of the focusing magnet. The radial position of the 300 G line is at a radius of ~1.3 meters. The line shape is different.

CM-18 June 2007 17

AFC Magnet Measurements

• The external field is important because of safety concerns, especially at fields >0.03 T. In the radial direction the 0.03 T line is further from the axis than in the tracker magnet.

• The field within AFC magnet bore is above 2 T. The field within the AFC solenoid is linear with current (to ±0.1 percent) over a broad range of currents. One must check to see if the cooler motor shields affect the field within the magnet bore more than ±0.1 percent.

• If the field is linear with current to ±0.1 percent, the room temperature Hall probes do not have to be calibrated above 2 T.

CM-18 June 2007 18

The Field within the Coupling Magnet

The radial component of field is small through much of the range. The peak radial field is~0.287 T at the ends of the magnet.

Magnet alone from 1.375 m < z < 2.518 m0.0 m < r < 0.2 m

Note: There is no field from theother MICE magnets.

CM-18 June 2007 19

The Field outside the Coupling Magnet

The 300 G line is ~3.35 metersfrom the ends of the trackermagnet. The radial position ofthe 300 G line is at a radius of~2.9 meters. This is a stayclear line at Fermilab.

Magnet alone from 1.375 m < z < 4.831 m0.0 m < r < 2.950 m

Coupling Coil

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Coupling Magnet Measurements

• The external field is important because of safety concerns, especially at fields >0.03 T. The 0.03 line is further from the center than in the other magnets.

• The bore coupling magnet field is just above 2 T. The field within the solenoid is linear with current (to ±0.1 percent) over a broad range of currents. One must check to see if the cooler motor shields affect the field within the magnet bore more than ±0.1 percent.

• If the field is linear with current to ±0.1 percent, the room temperature Hall probes do not have to be calibrated above 2 T.

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Monitoring the field within theSpectrometer Magnet

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The Blue rings are the tracker scintillating fiber planes. The spacing between planes is :

150mm, 180mm, 200mm and 470mm. All fiber planes are in the

magnet good field region.

The MICE Scintillating Fiber Tracker Module

Field Sensor

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Field Monitoring on the Tracker Frame

• The field in this region is about 4 T (depending on the muon momentum). The sensors should be calibrated up to 4.5 T.

• Since the tracker field is uniform (to ± 0.3 percent), one can use single axis (in a direction parallel to the magnet axis) Hall sensors to monitor the field.

• The z axis Hall sensors should be mounted on the tracker frame near the scintillating fiber planes at a radius greater than 150 mm. Three planes of sensors should be within the uniform field volume. There can be three or four z axis sensors in each sensor plane.

CM-18 June 2007 24

Monitoring on the Tracker Frame cont.

• The sensors on the tracker frame can be used for tuning the magnetic field in which the tracker is located.

• The average magnetic field (in the center of the tracker) is controlled using the 300 A spectrometer solenoid power supply.

• The field uniformity can be monitored using the end plane sensors. Field uniformity is controlled using the 60 A power supplies across the two end coils of the spectrometer solenoid.

• The tracker sensors are critical for tuning the tracker solenoids for MICE.

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MICE Magnet Monitoring usingExternal Probes outside of the Cryostat

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Is External Field Monitoring Necessary for the MICE Channel Magnets?

• 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 other MICE magnets 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?

CM-18 June 2007 27

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 two sets of three axis Hall probes could be installed 120 degrees apart on the outside of the AFC cryostat over the coils.

• 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.

CM-18 June 2007 28

External Field Measurements cont.

• The field outside the tracker magnet and the AFC magnet in the non-flip mode is quite low (~0.05 T). Measurement sensitivity will be a problem. Earth’s field and iron near the magnet will cause errors. The field outside the coupling coil is high (~1 T) and the field outside the AFC magnet in the flip mode is at a medium level (0.2 to 0.35 T).

• External field monitoring makes sense if there is no iron around the magnets. There will be iron (pumps, compressors, shields, and other items) that may affect the field outside of MICE, but the MICE channel field errors should be less than ±0.1 percent.

CM-18 June 2007 29

External Field Measurements cont.

• It is not clear that external field measurements are useful for the tracker solenoid. The fields are low and the errors due to earths field and magnetization in the room are considerable. However, monitoring of the field within the magnet bore around the tracker is can help tune the magnet for different spectrometer field levels. This should be done.

• Lumps of iron outside of MICE (cooler motor shields and vacuum pump shields) can affect the field outside of the coupling coil because the magnetic moment generated by the iron is higher and the coil is closer to the iron. The field outside of the AFC magnet may also be affected by iron. It is not clear that three axis field monitoring is useful.

CM-18 June 2007 30

External Field Measurements cont.

• Measuring the external field may be useful to determine the MICE channel operating mode and an individual magnet coil’s polarity.

• Measurement of magnet polarity can be done with a single one z axis probe that is mounted outside of the magnet cryostat at the coil center. Such probes can be at room temperature or 4 K.

• Other than measuring the field direction and approximate magnitude to determine the channel operating mode, field monitoring outside of MICE magnets does not appear to be very useful.

CM-18 June 2007 31

Tracker Magnet Vacuum Vessel Sensor Location

M1 Sensor

M2 SensorSpectrometer Sensor

Redundant Hall sensors on other side are not shown.

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Focusing Magnet Vacuum Vessel Sensor Location

Coil 2 Sensor

Coil 1 Sensor

Redundant Hall sensors on other side are not shown.

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Coupling Magnet Sensor

Coupling Magnet Vacuum Vessel Sensor Location

Redundant Hall sensor on other side is not shown.

CM-18 June 2007 34

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.

• Single axis external field probes are useful for determining the operating mode and the magnet and its polarity. Knowing the field on three planes on the tracker frame is very useful.

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B-field sensor status

Frank FilthautRadboud University / NIKHEF

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The B-field sensor56

mm

56 mm

3 Hall probes (orthogonally mounted) 3D measurement

Temperature sensor(needed for calibration)

Obtain 2×10-4 relativeaccuracy through precise(s/w) calibration

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Sensor production• Order: ~ 36 probes suitable for B < 4T field

– Suitable for use within solenoid bore– 34 will be calibrated up to B < 4.5 T (Grenoble)

• 2 used for irradiation test

– 40 additional cards ordered (B < 1 T)

• Status:– All produced, but not yet calibrated– Calibrator tool vanished! F. Bergsma (CERN) re-

building• Aim is to be done by July 1• Measurement time (at GHMFL): July 6-21 (facility then closed

until September

CM-18 June 2007 38

Readout

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

• From experiment’s point of view: one CAN bus

• So far, used only stand-alone (Windows+Kvaser) applications– Apart from use in

LHC: PVSS (EPICS analogue)

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Readout (cont’d)

• Given experiment’s choice of EPICS: TEWS TIP810 Industry Pack card– Not yet ordered– EPICS support: under VxWorks only

• CAN or CANopen ?

– Little experience with / time for driver s/w etc.

• Continuing to read-out stand-alone would greatly facilitate this task!– If not: need help! (Even testing: availability IOC?)

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Mapping solenoid fields• Such sensors have been used on previous

occasions for field map purposes (ex: LHCb)

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

CM-18 June 2007 41

Mapping fields (cont’d)

• A priori, no reason why these sensors could not be used for field map purposes

• Requirements:– Fixture (rotating or not?), movement control– Upon sensor calibration, should be able to

provide h/w, s/w

CM-18 June 2007 42

Radiation test

• Radiation hardness a possible issue– RF emission: only as of 2009 (does not prevent

us from installing - and using - sensors earlier)

• Still would like to test in detail– Alan Bross: test in FNAL Lab-G setup– Being prepared (after CM?)