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.

Signal extraction

The Compton photon beam exits the dipole 2cm from the centre of the electron beam line at an angle of 50 mrad. The Compton photons spatial distribution at this point has a RMS size of ~9mm. 70% of the LW Compton scattered photons make it to this point, after the beam has passed through the various apertures along the machine.

TO DO

• Check position of window.
• Check passage of Compton beam past the flange upstream of QD6 as this could block the beam.

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. First we find the peak signal by putting a 10 mum diameter tungsten wire into the beam. An 81 micron sigma x electron beam was generated (this is the same width as expected at wire scanner MW0X). This produced a beam of photons above the cherenkov threshold with a RMS sigma at the detector of about 2.5 cm. The photon output from the wire was 1.24 E-3 per incident electron. This is 7.44 E6 photons per 0.6 E10 electron bunch. To produce 1E4 photons (the same output as the laser wire) the flux should be reduced to 1e4/7.44e6 = 1.34e-3 of this value. Assuming the electron beam horizontal distribution is Gaussian, this occurs where x = SQRT(-2 ln(1.34e-3)) sigma = 3.64 sigma. Therefore the wire should be moved to 3.64 sigma from the centre of the distribution to approximate the laser wire signal. With the wire scanner at this location, the signal after passing through 20cm of steel is scattered enough that the number of photons hitting the front of the detector reduced by 80%. Therefore, we should be able to tell if the laser wire signal is being blocked by the magnet QD6 or not. If the photons pass through 5cm of steel the number reaching the detector is reduced by only 3%. Therefore we will not be able to tell using the wire scanner if our laser wire signal is being blocked by the flange at the front of QD6 or not.