1. High resolution profile measurements
Enrico Bravin
CERN AB-BI
DIPAC 07
21-23 May 2007
Mestre, Italy

2. What is high resolution for?
Emittance preservation
Injector chains
Coasting
Luminosity maximization
Beam sizes at the IP
Beam loss reduction
High current “drivers”
Beam halo / beam blow-up
Development of new machines
Future linear colliders
Complex damping rings for nano beams
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

3. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Profile monitors
Two main families
Ring
Fast beam scanners
Laser wire scanners
Synchrotron light
Rest gas ionization
LINAC / transfer lines
Slow beam scanners
Imaging (scintillators / OTR)
Wire grids (harps)
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

4. Emittance preservation
Particularly important for hadron and linear colliders where there is no SR damping
Luminosity of colliders depend on beam emittance
Tight emittance budget for the acceleration chain
Emittance has to be monitored precisely at each stage
Measurements from ~100keV to 7TeV
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

5. LHC Acceleration chain
e* s
[p mm] [mm]
LINAC ~1.0
BOOSTER PS
SPS
LHC
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

6. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Beam Size at IP
In order to maximize the luminosity beams are strongly focused at the IP
Tevatron x,y  30m
LHC x,y  16m
PEP-II x  200m y  7m
SLC x  1.5m y  0.65m
ILC x  600nm y  6nm
CLIC x  43nm y  1nm
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

7. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Wire scanners
Principle
Scan a thin wire trough the beam
Detect secondary particles created in the interactions between the beam and the wire
SEM (low energy electrons evaporated from the wire) by measuring the electric current on the wire
High energy secondary from nuclear interactions between the atoms of the wire and the beam using a scintillator + PMT downstream of the wire
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

8. Flying wires in the CERN PS & PSB
Need fast scans to sample the profile at a precise time in the ramp and to avoid burning the wires.
scintillators + PMT and SEM
24m carbon fibres
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

9. Wire scanner studies @ SPS48
0.235 IN
0.921
Blow-up [nm]
0.283
Emitt. [mu]
1.010
Sigma [mm]
OUT
Scan
Beam blow-up due to the scans can be important
LHC pilot 26GeV
Blow up is ~20% per scan
Linear scanners res. ~10m
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

10. SLC Wire Scanners Step size 1mm 3 Scanners Horizontal + Vertical +45°
Second design
all 3 planes in one
device
2 Carbon wires
on each arm
7mm + 25mm
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

11. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
SLC Wire Scanners
There were over 30 scanners installed on SLC
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

12. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
SLC Wire scanners
7m wire
H=8.3m
V=6.5m
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

13. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Wire Scanners
Well known and reliable technology
Resolution down to a few microns, currently limited by the diameter of the wire
Signal is generally large and clean (secondary particles)
Need sufficient space downstream of scanner for secondary particle detectors
Wires can burn if not carefully handled
Can not be used near super conducting cavities due to risk of contamination (broken filament)
In most accelerators the scanners are used as reference for beam sizes and emittance measurements
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

14. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Synchrotron light
Principle
Charged particles emit synchrotron radiation (visible light or X Rays) when accelerated (curved) by the magnets
Use an optical system to acquire images of the beam
As the source is not well defined in case of bending magnets, angular filtering is needed (slits)
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

15. LEP synchrotron light telescopes
In LEP two different types of synchrotron radiation monitors existed
BEUV observing (images) in the visible (near UV) range with a complex optical system. Resolution ~ 100mm
BEXE observing (profiles) the X-Ray range without imaging (or pinhole imaging). Resolution of ~50mm
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

16. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
LHC telescopes
Light from D3+ undulator
Light pickoff
Long. diags
Optical
Attenuator
+ colour filters
Cameras with pulsed intensifiers
Video camera
Stepping motors
Slit, Corona,
At 1st image plane
Sph. focusing mirror f=4m
Sph. Relay mirror, f=750mm
90/10 beam
splitter cubes
All reflective optics, diffraction limited performance from 400nm-2.5µm
Optical path folded, need to keep angles low to minimise astigmatism
Silicon mirror in vacuum chamber
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

17. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Synchrotron light
Widely used in electron machines
Also used in high energy hadron machines (above 400 GeV)
Allows continuous monitoring of the beam
Ultimate limitation is diffraction
Profile monitors in the X-Ray range using Fresnel zone plates have been developed with resolutions of the order of microns (KEK-ATF)
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

18. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Laser Wire Scanners
Collide a high power, focused, pulsed laser with an electron beam
X-ray or g-ray are produced by Compton scattering
Detect the x-ray / g-ray or the degraded electrons downstream
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

19. PETRA Laser Wire Scanner
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

20. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
SLC Laser Wire near IP
In SLC the space available for the LWS was reduced. A different concept was used where the particle beam is scanned across the laser beam
Laser waist size 400nm
s= 1.14mm
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

21. KEK-ATF LWS (Damping Ring)
At KEK-ATF a low power CW laser and Fabry-Pérot optical cavities are used in place of the high power laser
Beam Sizes x100m y10m
Laser Waist size W05.6m
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

22. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
Laser wire scanners
Non interceptive
Can be tuned to very small laser sizes
Complex optical and laser system
Problem measuring flat beams due to the relation between laser spot size and Raleigh length
Can only be used with electrons and positrons as Compton x-session with protons is too small
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

23. Optical transition radiation
OTR is emitted when charged particles cross the surface of a metal foil
The backward emission can easily be observed by an optical system
The image of the beam can be acquired
BW OTR Light
View port
FW OTR Light e-
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

24. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
TTF2 OTR
Si radiators, 3 selectable optics + filters, 10m resolution
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

25. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
KEK-ATF
Mirror has 10° angle to reduce depth of field problem
Microscope optics used to achieve high resolution
Beams ~5mm measured
Problem with damage of the mirror surface
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

26. Matching in the SPS and LHC
OTR foil can be very thin (little blow-up), turn by turn image acquisition over a limited number of turns is thus possible
SPS has 12m Ti OTR + fast camera for matching studies
Oscillations are caused by optics mismatch
between the TT10 transfer line and the SPS ring
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

27. E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07
OTR
Very simple and reliable devices
Thin radiators reduce blow-up making possible to have multiple simultaneous measurements in LINAC or transfer lines
Very good spatial resolution (diffraction limited)
Very high time resolution
No radiators (yet) for beams densities above few 1015 e-/cm2
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07

28. Outlook and Acknowledgments
A lot of work has been done in the field of high resolution profile measurement
A lot more remains to be done and this will remain an active field for a while…
I would like to thank all of you for providing excellent documentation and making this kind of presentations possible
21 May 2007
E. Bravin - CERN - "High resolution profile measurements" - DIPAC 07