Beam Line for Hall D

background material for meeting between Hall D collaborators and JLab staff April 15, 1999
Richard Jones

1. Basic Design Requirements
   1. Photon Energy
      • 8-12 GeV photons
      • eliminates laser backscatter source in 12GeV era
      • bremsstrahlung only viable photon source
   2. Photon Polarization
      • circular polarization may be useful
      • best physics information from linear polarization
      • requires coherent bremsstrahlung for photon source
   3. Photon Flux
      • limited by tagging to about 10 tagged photons per second.
      • eliminates laser backscatter source in 24GeV era
2. Design for Photon Beam Line
   1. Photon energy resolution
      • 0.1% r.m.s. required for optimum missing-mass resolution
      • eliminates collimation as a means to obtain monochromatic photons from a coherent bremsstrahlung source
      • requires photon tagging
   2. Circular photon polarization
   • comes for free if electrons are longitudinally polarized
   • approaching 100% transfer from electrons to photons at the endpoint
   • its presence does not decrease the linear polarization of the coherent bremsstrahlung
   3. Linear photon polarization
   • up to 30% is possible at 8GeV by severe collimation
   • polarization goes smoothly to zero at the endpoint
   • important factor in determining length of photon beam line
   • design is 80m from bremsstrahlung target to 2mm collimator
   • polarization present over limited photon energy range, determined by crystal orientation
   4. Beam flux and power
      • photon beam power in experimental hall is 1W max
      • photon beam power in tunnel can be up to 100W
      • most of the power is dissipated on the collimator
   5. Shielding the photon beam
      • most important in the tunnel
      • shielded "hut" at the end of the tunnel before hall to house collimators and sweeps

3. Tagger Design
   1. Magnet design based on Hall B tagger
      • same amount of iron, larger bending radius (main beam bends 9.4 inside spectrometer)
      • runs at low field for 12GeV, designed for full field at 24GeV
      • requires extra dipoles to bend electron beam into dump
   2. Special focal plane for coherent bremsstrahlung
      • electron rates as high as GHz per GeV
      • very narrow counters to keep rates down to 2MHz per tube
      • covers only the region of the coherent peak where tagging efficiency is high
      • focal plane counters can be moved to follow coherent peak
4. Beam Dump Design
   1. Horizontal beam dump
      • enables parasitic beam-dump experiments
      • facilitates access for repairs or shielding modifications
      • avoids some environmental concerns
   2. Design for operation at 24GeV
5. Design for Electron Beam Line
   1. Requirements for electron beam current
      • operation with optimum polarization foreseen at 3µA
      • design should include some safety factor
   2. Requirements on electron beam energy
      • tagging energy resolution requires beam energy spread less than 5MeV r.m.s.
      • similar value required for the energy stability over time
   3. Requirements on electron beam emittance
      • coherent bremsstrahlung calculations include beam emittance effects
      • collimation works best if the beam has cylindrical symmetry
      • beam emittance effects become important above about 1mm.µr (1-sigma)
      • stability of emittance is important in terms of an upper limit
      • emittance a major concern for this design
   4. Requirements on electron beam position control
      • photon beam must be centered on 2mm collimator 80m away
      • requires active feedback
      • uses active collimator (SLAC idea) as sensor for feedback control
      • feedback primarily concerned with low frequency (60Hz + harmonics)
      • stability required at the ±200µm r.m.s. level at the collimator
   5. Requirements on electron beam optics
      • virtual electron beam spot projected on front face of collimator
      • virtual spot should be circular, near focus in x (horizontal)
      • beam spot is large and elliptical on crystal radiator, but converging in both x and y
      • only raster foreseen is slow vertical motion of beam spot on radiator ±2mm