To: Zach Wolf – Magnetic Measurement Group
From: Dave Dowell – SLAC and John Schmerge – SSRL
Subject: Gun Test Facility Spectrometer Magnetic Field Measurements
Recent beam experiments at the Gun Test Facility (GTF) use the dipole spectrometer to
measure not only the energy spectrum but also to determine the time-resolved emittance
of the beam. These experiments require accurate knowledge of the spectrometer’s
transport characteristics. Unfortunately the spectrometer was not magnetically
characterized before its installation in 1997. Beam-based measurements demonstrated
significant pole-edge rotations and fringe field effects for this magnet. In addition, the
linac klystron upgrade will allow operation at beam energies up to 80 MeV, making
measurements of the interior field and fringe field at high excitation current (~90 amps)
essential to verify the magnet iron is not saturating.
The spectrometer measurements will require a power supply capable of delivering 90
amps at 40 volts (approx. 0.4 coil resistance). The magnet polarity is marked. The
magnet cooling water supply pressure is 110 PSI with a flow rate of 5 GPM. The inlet
temperature is 30 degrees C.
The locations of the beam-in and beam-out centerlines, as defined and shown in the
drawing below, should be transferred to references on the magnet. This allows future
magnetic measurements and installation alignment to be performed independent of the
vacuum chamber. Magnetic field measurements can be done with the vacuum chamber
removed.
The magnet will be delivered to the magnetic measurement lab during the week of
August 26, 2002 such that the field measurements can be made during the following
week, beginning September 3, 2002. We would like to review the results after each
measurement, in order to verify the data make sense and are complete.
The charge number for magnetic measurements is 95-7945-0. If you have any questions
please contact John Schmerge at X 2320, pager 846-9979, Schmerge@slac.stanford.edu
or Dave Dowell at X 2494, dowell@slac.stanford.edu.
Desired Measurements:
1. Interior magnetic field vs. current for four standardization cycles from 0 to
90 A and back to 0A. This should be performed with the probe located in an
interior uniform field region, along the AB centerline. (See Figure 1.)
2. Transverse field in the magnetic median plane along the AB centerline in 3
mm increments, at different currents of 0, 15, 30, 60 and 90 A. The probe
should be aligned to enter the magnet at the beam-in flange (Point A in drawing)
and pass completely though the magnetic field region, stopping in a zero field
region near Point B. (See Figure 1.)
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3. Transverse magnetic field map across the entrance pole edge region. With
the magnet current set to 60 amperes, measure the field in steps of 5 mm across a
grid pattern centered on the beam-in centerline and the pole edge, as shown in
Figure 2. The longitudinal extent of the measurement grid should include both
the fringe field and constant interior field regions. The measurement grid should
be 30 mm wide.
4. Transverse field in the magnetic median plane along the beam-out flange
centerline (Point C and 60 degree angle to AB). Similar to Measurement #2
above, measure the transverse field vs. longitudinal position in 3 mm increments
along the beam-out centerline. Measurements should be made at 0, 15, 30, 60 and
90 A, and extend from Point C to where the beam-out centerline crosses the
beam-in centerline. (Please see Figure 1.)
5. Transverse magnetic field map across the exit pole edge region. Rotate the
magnet 60 degrees to align the x-y measurement grid at the exit pole edge and to
center it on the intersection of the beam-out centerline and the exit pole edge.
With the magnet current set at 60 amperes, measure the transverse field in the
magnetic median plane across the x-y grid in steps of 5 mm. Similar to
Measurement #3, the longitudinal extent of the grid should include both fringe
field and constant interior field regions. (See Figure 2.)
Point A
Beam-In
Centerline*
Point B
~91 cm
60
Measure magnetic field
along line AB
Measure magnetic field
along line passing through
Point C at 60 degree angle to line AB
*Transfer these centerlines to references
on the magnet.
Magnetic measurements are performed
with vacuum chamber removed.
60 degrees
Point C
Beam-Out
Centerline*
Figure 1. Drawing showing definitions of the beam-in and beam-out centerlines.
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Pole edge
Fringe field region
Measurement Grid
Interior magnetic
field region
5 mm
5 mm
15 mm
Beam-In
Centerline
Enlarged view of
entrance pole edge
30 mm
55 mm
Beam-In
Centerline*
Entrance pole edge
Exit pole edge
60
60 degrees
Rotate dipole 60
degrees to orient
measurement grid at
exit pole edge
*Transfer these centerlines
to references on the magnet.
Magnetic measurements are performed
with vacuum chamber removed.
Beam-Out
Centerline*
Figure 2. drawing showing the locations of the magnetic field measurement grid at the
entrance and exit pole edges.
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