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, or Dave Dowell at X 2494, .

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.)

AB

  1. 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.)
  1. 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.
  1. 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.)
  1. 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.)

Figure 1. Drawing showing definitions of the beam-in and beam-out centerlines.


Figure 2. drawing showing the locations of the magnetic field measurement grid at the entrance and exit pole edges.

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