Ultra-sensitive HALO monitor

N. Vinogradov, A. Dychkant, P. Piot

Motivation (courtesy to Daniel Mihalcea)

Beam requirements (original design):

Charge/pulse: 133pC (Iavg = 100 mA)

Transverse emittance < 3m

Longitudinal emittance < 100 ps-keV

Energy 7 MeV

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 wideUseful 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

5·10-12

3·10-9

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