List of changes since the 2006 HallD Tagging Spectrometer and Photon Beamline Review

In January 2006 the lab held an external review of the Hall D Tagging Spectrometer and Beam Line. The list below represents an attempt to collect in one place all of the changes to the tagger and photon beam line that have taken place since then, up until the PAC report in summer 2010.

1. electron beam dump -- in 2006 the electron beam dump was located in an out-building that was connected to the tagger hall by a buried beam pipe. The present civil design now has the electron beam dump located in the tagger hall, which has been extended in the south-east direction to accommodate it.
2. electron beam emittance -- in 2006 the horizontal emittance of the 12 GeV electron beam predicted by ray-tracing simulations was 1 x 10^-8 m-r. Since then, a bug in the simulation was corrected, leading to a reduced estimate of 5 x 10^-9 m-r. The emittance requirement for GlueX has remained fixed at the 2006 value of 10^-8 m-r, but the improved outlook has caused some other requirements to be relaxed -- see next item.
3. virtual beam spot size at the collimator -- in 2006 this requirement was an upper bound of 500 microns. Based on interactions with CASA and further simulations, it was determined that this requirement could be relaxed to 600 microns without substantial impact on the photon beam characteristics.
4. radiator crystal size requirement -- in 2006 the requirement on the transverse dimension of the diamond crystal radiator was set at 8 mm minimum in order to contain the electron beam spot out to 2 standard deviations. Since then a new configuration has been found of the electron beam lattice that reduces the horizontal beam spot size to 1 mm rms. Based on this, the minimum transverse diamond radiator size requirement has been reduced to 4 mm.
5. electron beam halo requirement -- in 2006 we set a conservative requirement of 10^-6 on the fraction of the current that resides in the tails (outside the central Gaussian peak) of the electron beam. Considerable work has been done since that time on refining this specification and simulating the impact of halo on tagger performance. The new halo specification requires that no more than 5 x 10^-5 of the electron beam current lie outside a cylinder of radius 5 mm centered on the nominal beam axis.
6. minimum deliverable electron beam current -- in 2006 this requirement was set at 100 pA. After further simulation work, it was determined that this requirement could be relaxed to 1 nA.
7. change to a one-dipole tagging spectrometer -- in 2006 the tagger design called for two identical 3m magnets in series to disperse the post-bremsstrahlung electrons and divert the electron beam into the dump. Since then the decision has been made to build a single 6m dipole instead of the 3m pair.
8. change in the width and shape of the tagger magnet poles -- in 2006 the tagger dipoles had poles of dimensions 310 x 45 cm² with a simple 45° chamfer of width 1.5 cm around the edges facing the gap. The dimensions of the modified single-magnet pole are 622.7 x 41 cm² with a two-step chamfer approximating a Rogowski contour.
9. reduction in outer dimensions of the tagger dipole -- in 2006 the outer dimensions of the tagger dipole magnets were 109.0 cm x 141.0 cm in transverse cross section. In the present design the outer dimensions of the magnet in the transverse plane are 99.8 cm x 129.0 cm.
10. location and length of the focal plane -- in 2006 the position of the exit window of the vacuum chamber was configured to be a straight-line approximation to the focal surface of the two-dipole spectrometer. In the present design with a reduced pole width, the focal surface has shortened and moved closer to the magnet. The vacuum chamber dimensions have been reduced accordingly, with the vacuum exit edge located 10 cm from the straight-line approximation to the focal surface where the tagging detectors are placed.
11. coverage of the fixed-array focal plane counters -- in 2006 the array of tagging detectors covered the range 0.6 - 9.0 electron energies (3.0 - 11.4 GeV photon energies). The counters were spaced every 60 MeV and covered a range of 30 MeV, so that the spectrum was sampled at the 50% level. In the new design the high-energy region above 9.0 GeV photon energy has been augmented to provide 100% sampling at a spacing of 30 MeV, and the limit in photon energy has been extended to 11.7 GeV (300 MeV electron energy). To accommodate these changes, the number of fixed array counters has increased from 140 to 190.
12. design of the primary collimator -- in 2006 the primary collimator was a solid cylinder of tungsten of radius 10 cm, length 30 cm, with a 3.4 mm diameter cylindrical hole in the middle. The present design for the primary collimator is a rectangular block of tungsten, 12 cm (width) x 6.4 cm (height) x 20 cm (length), with two apertures drilled down the long axis, one of diameter 3.4 mm for use with polarized beam, and the other of diamond 5.0 mm for use with unpolarized beam. The holes are situated about 5 cm apart from each other. The tungsten block is surrounded by lead (blocks) with the outer dimensions of 30 cm x 20 cm x 20 cm. The primary collimator is placed on a horizontal translation stage that allows either aperture to be aligned with the photon beam, or removed entirely for alignment of downstream beamline elements.
13. design of the secondary collimator -- in 2006 the secondary collimator was a large nickel cylinder with an aperture of 5 mm diameter. This collimator has been replaced with a stainless steel cylinder with a stainless steel replaceable insert with a nominal aperture of 6 mm diameter.
14. shielding in the collimator cave -- in 2006 there was only a rough concept for cleanup of the photon beam after the two collimators, and shielding to absorb the radiation produced in the collimators. This background has been quantified with simulations for a variety of cleanup and shielding configurations, and the new design reflects an optimization in amount and placement of shielding components based on the simulation.
15. pair spectrometer and detector system -- in 2006 there was only a conceptual design for a pair spectrometer. Since then a concrete design has been chosen based upon an unused magnet at Brookhaven National Lab. A reference design for pair spectrometer detectors has been adopted based on an array of 40 scintillators. An alternative design based upon silicon strip detectors is also under consideration.
16. interface document -- in 2006 we had only informal discussions between the GlueX team designing the photon beam and the Accelerator Division who are responsible for the electron beam on the interfaces between these two systems. There is now a draft document that has been vetted by both groups that describes all of the major interfaces between these two systems and clarifies how responsibilities are distributed between the two teams.