ATF2 LW Simulation

Electron beam

Laser beam

Compton

Compton scattered photons were generated using BDSIM from a 1.28 GeV electron beam colliding at 90 degrees to a 532 nm laser. The usual electromagnetic processes (Bremsstrahlung, pair production, multiple scattering etc.) were turned on, and the low energy cut off was at 1 keV. The Compton photons were tracked for 25 m and then through a steel plates of varying thicknesses. The results in the file below show the fraction of unscattered photons as a function of material thickness. The calculation shows that if the beam passes through flanges, which are a few cm long, it will be attenuated by ~50%. If the beam hits a quadrupole magnet (e.g. QD6), which are 20cm long, the signal will be be blocked.

• lwPhotonsThroughSteel.pdf: ATF laser wire Compton scattered photons through steel.

OTR Screen

A similar study to the above was done with the electrons beam incident on an OTR screen, which is modeled as a 300 micron thick silicon plate with a 1 micron aluminium coating. The rate of photon generation was 0.047 photons per incident electron. For a bunch charge of 0.6 E10 electrons this equates to 2.8 E8 photons generated. This is 2.8 E4 times larger than the estimated laser-wire Compton rate of 1E4. Also, the photons generated by the screen have an energy spectrum which goes up to 1.28 GeV, whereas the Compton photons have a high energy cutoff at ~30MeV.

The results show that if the OTR screen generated photons pass through 20cm of iron then 1.20 E-3 photons per incident electron at twice the Cerenkov threshold energy (Ec = 2.983 MeV) or above will hit the front the detector. For a bunch charge 0.6 E10 electron beam this equates to 7.20 E6 photons hitting the detector. Therefore, even if this signal is blocked by the magnet, it will still be 7.2E6/1e4 = 720 times stronger than the laser wire signal.

Wire Scanner

The laser wire signal could be approximated by moving a wire scanner into the edge of the beam to generate a small number of photons.