Leslie Rosenberg ljr@mitlns.mit.edu

The precision map points of the magnetic field in the spectrometer field region are taken with the mapping station of the AGS Magnetic Measurements Group (John Jackson, head, and Ed Hoey, main mapping technician). The lower precision field points come from a hand-held instrument belonging to the MIT-Axion group. The AGS mapping station consists of six parts:(1) The moving mapping stage (see sketch in Appendix 1); (2) The Gaussmeter itself (datasheet in Appendix 2); (3) A tilt-sensor to establish the vertical direction (datasheet in Appendix 3); (4) Three hall probes, one for each magnetic field component (datasheet in Appendix 4); (5) The probe truck containing the Hall probes and tilt sensor (sketched in Appendix 5); (6) The data acquisition computer.

The mapping stage can move along three Cartesian axes (see Appendix 1). There is 72 inches and 48 inches of travel in the horizontal direction, and 36 inches of travel in the vertical direction. Only travel in the 72 inch direction is automated. Notice that not all travel in the 72 inch direction is useful as the three Hall probes are significantly offset from each other (see Appendix 5).

The mapping stage must be surveyed into the magnet. The survey precision dominates the position accuracy of the map relative to the magnet. The survey group should be able to determine the relative position of the mapper and magnet to better than 100 micrometer. The experience of the mapping group is that the vertical mapper motion is precise to a fraction of a milliinch (a fraction of 25 micrometer), and that the horizontal motion is precise to a couple of milliinch (50 micrometer). I then take 100 micrometer as the vertical mapper positioning precision and 125 micrometer (quadrature sum of survey and horizontal motion precision) as the mapper positioning error of each horizontal component. The approximate line integral of the vertical magnetic field from the position of the IP to the last Si plane is about 1 Tesla-meter. A 100 micrometer precision in position translates into a maximum contribution to the magnetic field line integral of 2 Tesla-100 micrometer (the maximum field times the displacement). This is a fractional error of two parts in 10,000.

The mapping station Gaussmeter datasheet is shown in Appendix 2. It can measure fields to 3 Tesla with resolution of 0.5 Gauss (worst case) and absolute precision of a few parts in 10,000 (0.01 percent of reading plus 0.1 milliTesla: this, e.g., works out to 3.3 Gauss precision on the 3 Tesla range in a 1.8 Tesla field). The probe and Gaussmeter combination is cross-checked by the AGS mapping group against a NMR probe before mapping and after mapping to ensure the precision is unchanged.

A tilt sensor establishes the vertical direction (see Appendix 3). The sensor resolution is 30 arcseconds, and the precision is 0.03 degrees. The tilt precision translates into a "sine" error in the transverse components of Sin(0.03 degrees) or several parts in 10,000. The tilt sensor needs about 30 sec to settle to give a reliable measurement of the tilt angle.

The Hall probe is shown in Appendix 4. It is designed to work with the mapping station Gaussmeter.

The probe truck containing the Hall probes and tilt sensor is sketched in Appendix 5. Notice there is a considerable distance between the vertical field Hall probe ("By" in the sketch) and the two transverse field Hall probes ("Bx" and "Bz"; the z-direction is along the motorized direction).

The data acquisition computer is an antique, a PC AT with data saved on real floppy disks. Field data is saved on the hard disk, transferred to the true floppies, then moved to another machine and transferred to 3.5 inch floppies that can be read by a modern machine.

Although this AGS mapping station has only 1 axis automation and a primitive DAQ interface, it nonetheless has better position and field accuracy than similar stations from Bates and SLAC.

The mapping activity occurs in several phases:

1. Preliminary Activities;
2. Spectrometer field and Beamline Gradient map.
3. TOF map.

The Preliminary activities include checking for acceptable power supply ripple and noise, establishing the 10 Gauss personnel safety line, hand-held measurements at various field settings in order to decide on the nominal operating current, hand-held measurements of the residual (zero-current hysteresis) magnetic field in order to decide whether a zero-current map is necessary, hand-help measurements of the effect of the alloy steel TOF rails. The Spectrometer field and Beamline Gradient map include mapping the inner (including position survey of the mapping station with respect to the magnet at mapping-current settings) and outer (including position survey of the mapping station with respect to the magnet at mapping-current settings) spectrometer regions. There is a full spectrometer map at the nominal operating current, and sparse maps at two alternate current settings. There could also be a sparse map at zero current (to measure the hysteresis zero-current field). The spectrometer map region overlaps the nominal beamline axis; the RHIC machine group has requested we measure the field in this region. The TOF map is a low spatial precision and low magnet field precision map from the edge of the spectrometer map to the TOF with the goal of giving TOF reconstruction the characteristic kick of particles for TOF hit and spectrometer track matching. This will use the handheld Gaussmeter.

The map of both gaps of the magnet and the field gradients along the beam axis requires two mapping station surveys. The first survey places the mapper into the "inner" of the two gaps with slight overlap with the beamline axis. We originally thought about placing the mapper at 45 degrees relative to the beamline axis, but this would not allow a complete map of a single gap due to the limited mapper extension. The vertical mapper travel should at least cover the range +/- 3cm about the mid-plane of the magnet gap; the transverse mapper travel should just overlap the beamline axis by several cm; the longitudinal mapper travel (the motorized direction) should go from just outside the magnet face to deep inside the magnet. The second survey places the mapper into the "outer" of the two gaps; this is a mirror flip of the first position. The full 48 inch transverse mapper motion is usable. However, less than the 72 inch (motorized) longitudinal motion is useful (there is around 5 cm of dead area at the end of the slide, and the Hall probes are separated by 11.2 inches); the useful longitudinal travel is therefore about 150 cm. A plan view of the useful mapper travel is shown in Figure 1.

There will be a full map of the inside gap at the nominal operating current (as previously established). The motorized direction travels into the gap (the 149 cm direction). A full map records all three components of the field every 1 cm of motorized travel. Each point requires 10 seconds or less for travel and for the reading to settle, therefore each 150 point trajectory takes approximately 1/2 hour. The 70 cm transverse distance between the beamline axis and the final Si layer will be mapped every 2 cm (that is, 2 cm between trajectories); this motion is not under motor control and is done by-hand. The spacing between trajectories from 70 cm transverse distance from the beamline axis to the end of the travel is 4 cm.

In order to establish any change in the vertical direction due to mechanical errors of the mapper, we will measure the tilt every 4^th trajectory. It takes of the order of 30 sec for the fluid in the tilt sensor to settle is, so the measurements on this special trajectory will be sparse, with a measurement every 10 cm.

The total number of trajectories in each vertical layer is 50 trajectories; this layer takes about 1 day. The gap will be mapped with 5 layers spaced 3 cm apart (the middle layer in the gap mid-plane). This one gap will take about 5 days to map.

There will be a sparse map at the first alternate operating current. There will be a field point measured every 4 cm of (motorized) longitudinal trajectory (37 points), there will be 10 cm of spacing between trajectories (12 trajectories) and 5 mapping planes as for the full map. These 2220 points will take approximately 6 hours. This procedure will be called the ‘sparse mapping procedure’ in the following.

There will be a sparse map at the second alternate operating current.

Depending on the hand-held measurements of the residual hysteresis field, there could be a sparse map at zero current.

With the current at the operating point (if not 3600 Amperes, the reduced operating point should be reached from 3600 Amperes to always follow the same hysteresis line), field points through a trajectory through the center of the gap should be recorded and checked against earlier data. They should agree to a few parts in 1000.

If used in the inside gap map, TOF rails will be installed near the outside gap. The second survey places the mapper into the "outer" of the two gaps with slight overlap with the beamline axis. There could be a full map of the outside gap at the nominal operating current (as previously established). To establish the necessity of this map, field points along a trajectory in the gap mid-plane and through the pole axis will be measured and compared against the map from the inside gap. Should a full map be required, the point spacing is the same as the inside gap. This one gap will take about 5 days to map.

If the outside gap field measurement require a full map, then there will be a sparse map at the first alternate operating current. Should a sparse map be required, the point spacing is the same as the inside gap. These 2220 points will take approximately 6 hours.

If the outside gap field measurement require a full map, then there will be a sparse map at the second alternate operating current. The point spacing will be the same as the inside gap.

Should the hand-held measurements of the residual hysteresis field require mapping and if the outside gap field measurement require a full map, then there will be a sparse map at zero current. The point spacing will be the same as the inside gap.

With the current at the operating point (if not 3600 Amperes, the reduced operating point should be reached from 3600 Amperes to always follow the same hysteresis line), field points through a trajectory through the center of the gap should be recorded and checked against earlier data. They should agree to a few parts in 1000.

Frank Wolfs requested a hand-held measurement of the field between the spectrometer map and the TOF wall at the nominal operating current. He is supplying a student to help with these measurements. The student will generate a template with a sparse number of points (say 20 to 40) and will use the handheld meter for measurements. The student needs to be back in classes by mid-January. He will come to BNL the first week in January and fabricate the measuring template, including fixing points and mapping points. He could perhaps come to BNL for a weekend later in January or February for the TOF measurements.

At the end of the mapping period, we will call the AGS control room and ask them to power-down the magnet and secure the magnet power supply. We will also inform Joe Scaduto that we are finished mapping; he might decide to shut-down and secure the cooling system. We will then ask the AGS Magnetic Measurements Group to cross check their Hall probes against their NMR standard. We are then finished with the mapping at RHIC.

6 MAPPING TIME

0.0 days, initial calibration of Hall probes (does not incur deadtime, happens when power supply group commissions supply)

1.0 days, power supply group commissions and tweeks supply

0.0 days, move mapping station into approximate position (does not incur deadtime, happens when power supply group commissions supply)

1.0 days, measure ripple and noise

1.0 days, possible extension of noise studies (might involve iterating with power supply group, modifying averaging time in Gaussmeter, or  affixing aluminum plates to pole faces)

1.0 days, measure 10 Gauss line

2.0 days, establish operating current: 1 day for measurements and 1 day for discussions with Mark Baker

1.0 days, measure residual hysteresis field

0.5 days, find effect of TOF rails

0.5 days, affix TOF rails if necessary

1.0 days, survey mapping station into "inner" gap

5.0 days, full map at nominal operating current

0.5 days, sparse map at alternate current 1

0.5 days, sparse map at alternate current 2

0.5 days, sparse map at zero (hysteresis) current if needed

0.5 days, recheck trajectory on full map

0.5 days, affix TOF rails if necessary

1.0 days, survey mapping station into "outer" gap

5.0 days, full map at nominal operating current if needed

0.5 days, sparse map at alternate current 1 if needed

0.5 days, sparse map at alternate current 2 if needed

0.5 days, sparse map at zero (hysteresis) current if needed

0.5 days, recheck trajectory on full map if needed

1.0 days, install template and take measurements at nominal operating current

1.0 days, closeout: secure magnet and check probe calibration

Appendix 3. The Tilt-Sensor Datasheet.