Ultra-sensitive HALO monitor N. Vinogradov, A. Dychkant, P. Piot

Motivation (courtesy to Daniel Mihalcea) Beam requirements (original design): l Charge/pulse: 133pC (I avg = 100 mA) l Transverse emittance < 3  m l Longitudinal emittance < 100 ps-keV l Energy  7 MeV l Energy spread < 1% Keep bunch charge at 1nC and decrease the radius at cathode from 3mm to 2mm => Halo formation downstream of SRF cavities (rings in the transverse plane separated from the beam core). No Halo 16% of particles 19% of particles

Schematic layout of the HALO monitor Primary beam Scanning actuator Plate with narrow slit Beamlet “Cleaning” dipole “Cleaned” signal from HALO Scintillator with array of fibers Long flexible shielded lightguide Photo Multiplier Tube in magnetic shielding High voltage to PMT Signal from PMT

Can we clean the beamlet good enough? Spatial distribution of beam particles right after they passed the slit Momentum distribution (p/mc) in collimated beam right after it passed the slit Momentum distribution (p/mc) of collimated beam at the location of detector head Spatial distribution (m) of beam particles right after they passed the slit (green) and at the location of detector head (red) Initial 10 MeV electron beam: Gaussian distribution for coordinates and momentums X, Y RMS = 2 mm Tungsten plate of 2 mm thickness Slit is 1 mm wide Useful signal Slit at 1 mm from beam axis Computer model: SHOWER is used to simulate the scattered particles passed the collimator along with the true beamlet ELEGANT is used to track the signal from the collimator to the detector location through the cleaning dipole Slit at 7 mm from beam axis

Who is who? Integrated signal from PMT (what we actually see) True HALO signal (signal picked up by scintillator) Background signal picked up by scintillator: should be small or repeatable Background signal picked up by PMT: can be suppressed by shielding the PMT 1. “Good” scenario: the true HALO signal is easy to distinguish 2. “Worse” scenario: still can measure HALO repeatable 3. “Worst” scenario: background is large and not repeatable ←beam is not stable ← there is no stable HALO anyway I I r r

Background test at AWA (Argonne) Shielded scintillating head in fixture Fiber Kuraray Y-11 outer diameter 1.2mm Scintillating material BC-408 with grooves for the fibers (no glue) PMT Hamamatsu R580Lightguide 5· ·10 -9 Beam on scintillator was not measured but calibration on lab source shows signal at the level of 5·10 -9 for 10 single electrons! 6 MeV electron beam; 15 psec pulse; 50 nC bunch charge