BDS day, November 22, 2005

1
Beam size measurement in the BDS
Electron beam Energy 1.5 TeV
No. of particles / bunch 2.56 109
No. of bunches / pulse 220
Bunch separation 0.267 (8 periods) ns
Bunch train length 58.4 ns

• The different techniques
• Limit of intercepting monitors
• Dynamic range of Laser wire scanners
• Diffraction radiation and beam halo monitoring
• Using X-ray from SR and beamstrahlung

BDS day, November 22, 2005
T. Lefevre
2
Optical diffraction and Transition radiation
DR is generated when a charged particle passes
through an aperture or near an edge of dielectric
materials, if the distance between the electrons
to the material (impact parameter, h) satisfies
the condition
TR is generated when a charged particle passes
through the interface between two materials with
different permittivity (screen in vacuum)
Radiation wavelength
Beam energy
Number of OTR photons per e-
Beam energy
Radiation wavelength
3. 10 -2 ph/electrons in 400nm-600nm 8. 10
7 photons per bunch
for l ltlt lDR, the IDR 50 ITR
BDS day, November 22, 2005
T. Lefevre
3
Laser wire scanner
High power laser
Detector (scintillator, ionization chamber)
hv0 wavelength P power sw spot size
Scattered Photons
Bending magnet
Scattered electrons
Scanning system
By counting the number of the scattered
particles (or photons) as a function of the laser
position the bunch profile is reconstructed
e- bunch
Ne Number of Electron/ bunch sy Transverse
beam size

• Resolution is fixed by the laser wavelength
• 266nm is close to the limit

• Dynamic range of the measurements depends
  directly on the available laser power
• (10GW at 266nm are commercially available but
  expensive)

(Nevents /Ne 10-5, for solid wire scenner
Nevents /Ne 1)
BDS day, November 22, 2005
T. Lefevre
4
Already achieved performances
Device Optical Transition radiation Optical Diffraction radiation Solid Wire Scanner Laser Wire Scanner
Performance 5mm measured at KEK-ATF Under development (15mm) Few microns (KEK) 1mm measured at SLC 70nm measured at FFTB using shintake interferometer
Limitations Damage threshold No profile (just s) Cross calibration Damage threshold Need expensive laser Need precise timing (ps)
Intercepting Yes No No No
Degradable Yes No Yes No
Simplicity Yes Yes Yes Not really
Possible use on low charge high charge low charge Small beam size
T. Muto et al, PRL 90, 104801, 2003 P. Karataev
et al,PRL 93, 244802, 2004
S. Anderson et al, KEK-ATF-2001-08
R. Alley et al, NIM A 379 (1996) 363 P.
Tenenbaum et al, SLAC-PUB-8057, 1999
Using a high magnification telescope using a
backward OTR screen tilted at 10
BDS day, November 22, 2005
T. Lefevre
5
Intercepting devices
The thermal limit for best materials (C, Be) is
106 nC/cm2

• Solid wire scanners and OTR screens must be then
  limited to this regime
• Dynamic range can be very high (gt105)

BDS day, November 22, 2005
T. Lefevre
6
Laser wire scanner in the BDS
Scattering angle and Energy spectrum of scattered
electrons (using a 266nm wavelength laser)

• Scattering angles of the degraded electrons
  lt10mrad

• At high energy the photons steal most of the
  electron energy (electron recoil becomes
  extremely important)
• Most of the degraded electrons are left with an
  energy of 28GeV

BDS day, November 22, 2005
T. Lefevre
7
Laser wire scanner in the BDS
Geant 4 simulations to estimates the detector
efficiency and the Signal to Noise ratio
From G. Penn
So extracting degraded electrons based on their
energy is the way to go. As dipoles would disturb
the main beam (even at TeV energies) A better
option is to do energy selection using quadrupole
lattice, with unstable region at 50 GeV.
BDS day, November 22, 2005
T. Lefevre
8
Beam size measurement in the BDS
Example
Maximum signal to noise ratio of 81
LWS could provide in best cases a 102-103
dynamic range beam profile measurement
BDS day, November 22, 2005
T. Lefevre
9
Optical diffraction radiation
Threshold DR photon energy as a function of the
impact parameter
1.5TeV electron beam
In the visible (hv 1-3eV), the intensity of TR
and DR will be of the same of order of magnitude
BDS day, November 22, 2005
T. Lefevre
10
Use of Synchrotron radiation / Beamstrahlung
X-ray beam imaging

• The beam if deflected also produces X-ray
  radiations. They are then focused using Fresnel
  Zone Plates onto an X-ray CCD camera.
• In order to minimize aberrations in the FZPs, a
  monochromator is first used to select the desired
  X-ray energy

BDS day, November 22, 2005
T. Lefevre
11
Use of Synchrotron radiation / Beamstrahlung

• Beam imaging is just impossible or using MeV
  X-ray

Diffraction limits the measurement
BDS day, November 22, 2005
T. Lefevre
12
Use of Synchrotron radiation / Beamstrahlung

• Interferometry Method Van Citterut-Zernike
  theorem

• Beam size is given by the Fourier transform of
  the complex degree of spatial coherence, which
  can be obtained using double slit interferometer,
  measuring to the fringes visibility of the
  interference pattern.
• The interferometer function is obtained by
  measuring the fringe visibility as a function of
  the slit width

Mitsuhashi Interferometer
BDS day, November 22, 2005
T. Lefevre
13
Conclusions

• OTR and solid wire scanner could be used for
  beam commissioning and reduced beam charge with a
  resolution of few microns and high dynamic range
  (gt105)
• Renewable screens could be envisaged to overcome
  beam induced damage
• Laser wire scanners could provide profile
  measurement with a 102-103 dynamic range and a
  spatial resolution of gt300nm.
• Spatial resolution could go down to few nm if
  using the shintake interferometer.
• ODR from a slit could be used as a beam size
  monitor but need to be cross calibrated. The
  performances of ODR must be confirmed and the
  resolution limit studied carefully. In parallel,
  beam loss monitors could be used for beam halo
  monitoring
• X-ray optic seems feasible but challenging

BDS day, November 22, 2005
T. Lefevre