1. Transverse emittance measurements
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Transverse emittance measurements
Enrico Bravin BDI-PM
7 October 2003
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2. Content What is the transverse emittance
How do we measure the transverse emittance
Devices used in the CPS complex
Devices being developed for future accelerators
7 October 2003
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3. What is the transverse emittance
A beam is made of many many particles,
each one of these particles is moving with
a given velocity. Most of the velocity
vector of a single particle is parallel to the
direction of the beam as a whole (s).
There is however a smaller component of
the particles velocity which is
perpendicular to it (x or y).
7 October 2003
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4. The emittance  is defined as the area occupied by the particles.
If we create an XY plot with the particles position (x or y) on the horizontal ax and the corresponding component of the velocity on the other ax (vx or vy), we obtain something like this (Usually vx and vy are replaced by the angles x’ and y’.)
The emittance  is defined as the area occupied by the particles.
The definition of the contour is ambiguous so one defines a statistical emittance (rms emittance) based on the statistical distribution of the particles.
7 October 2003
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5. Along the path of a storage ring the shape of this surface changes, but the area (emittance) is conserved.
Many factors can cause the emittance to increase. These factors are normally unwanted and can be reduced by optimizing the machine. This is why we need to measure the emittance.
7 October 2003
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6. If the beam is accelerated the emittance is not conserved
If the beam is accelerated the emittance is not conserved. In this case the longitudinal momentum increases while the transverse momentum is not affected and the angles x’ and y’ become smaller. What is conserved in this case is the normalized emittance *
7 October 2003
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7. How do we measure the emittance
There are three main ways of measuring the emittance
Direct measurement of the spatial and angular distributions of the particles (pepper pot)
Measurement of the spatial distribution of the particles and use the optics parameters of the machine (most)
Measurement of the phase space density (amplitude density) using movable scrapers
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8. The pepper pot (almost)
The integral of each peak is proportional to the particles density at the corresponding location.
Each peak corresponds to the angular distribution for a given location.
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9. Emittance from beam profile and optics
Measuring a beam profile (particle density vs. position) is much easier and some time can be done without perturbing the beam.
If one knows the optic parameters of the machine it is possible to calculate the emittance.
is the width of the profile and  the betatron function. In reality things are a bit more complicated by the presence of the momentum spread and the orbit dispersion.
The difficult part here is the measurement of the  parameter.
7 October 2003
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10. Emittance from scraping the beam
By measuring the beam current decrease as function of the scraper position, one can reconstruct the phase space density (amplitude density) and thus obtain the emittance. The optics parameters need to be known.
7 October 2003
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11. The CPS complex
The CPS complex includes a number of machines; two linacs (protons and ions), the Booster, the PS, AD ISOLDE and LEIR.
There are large differences in the characteristics of the machines and the beams they provide. This requires a large number of different equipments for measuring the emittances.
7 October 2003
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12. The LINACS The main tool used in the linacs is the slit + semgrid
This is based on the pepper pot technique, the beam is scanned through a small slit using deflection kickers.
A SEM grid measures the angular distribution of the portion of beam that managed to pass trough the hole.
Two measurement lines featuring such devices exist at the end of the Linacs.
There is also an Ion Pencil Monitor (R&D)
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15. The Ion Pencil Monitor
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16. The Ion Curtain Monitor
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17. The Booster
The booster is a circular machine consisting of 4 staked rings and emittance measurements are required on both planes of each ring. Several instruments are available.
Beam scope (4x2)
Fast Wire scanners (4x2)
Fast Blade Scanner (1)
SEM grids on the transfer line to the PS (3x2)
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18. The Beam Scope
The beam is kicked on a collimator and the current is recorded as function of the kick amplitude. This gives the phase space density function (density of particles vs. maximum oscillation amplitude) from which the emittance can be computed (knowing the value of .)
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20. Fast Wire Scanners
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23. SEM Grids – Measurement Line
There is a measurement line between the booster and the PS equipped with 3x2 SEM Grids. This allows the measurement of the vertical and horizontal emittance.
If one knows the transport matrix between the three grids and the optics is chosen properly, it is possible to calculate the emittance and the twiss parameters.
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25. The PS Fast Wire Scanner
In the PS the main tool for the measurement of the emittance is the wire scanner. There are 4 (2V + 2H) Fast Wire Scanners identical to those of the Booster.
Measurement Target (flip target)
This is a scraper that can be pre positioned and then swung into the beam at any point in the cycle (~20ms)
Fast SEM grids for matching
This grids (1V+1H both 24 wires) allow the measurement of the profiles turn by turn after injection. They are used for matching.
Quadrupolar Pick Up
This is a magnetic pick up which allows the measurement of a RMS beam size. Two such devices are installed.
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26. The Flip Target
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28. Fast SEM Grids for matching
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29. The 2 kB buffers allow the acquisition of ~30 consecutive turns.
The signals of the 24 wires are sampled at 40 MHz and stored into 24 buffers, 2 kB each.
The single passage signal is then reconstructed and the beam profiles calculated.
The 2 kB buffers allow the acquisition of ~30 consecutive turns.
Fast charge amplifier
40MHz
8 bits
ADC
2k buffer
2 X 24 wires
SEM grids
Insertion
mechanism
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31. The Quadrupolar Pick Up
Instrument comparison
Correction for beam position
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32. ISOLDE
Isolde produces (almost) continuous low energy radioactive beams. The tools used for measurement of the emittance are SEM Grids and Scanners.
Due to the low energy of the beams the ions stick to the wires and needles, eventually causing problem.
There is also an emittance meter (movable slits and SEM grids) bought in the industry.
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33. In this case what is detected is the Secondary Emission current plus the direct charge deposition, the ions stick to the wire or needle. The induced “coating” can change the SE yield.
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35. AD
The emittance of the decelerated antiprotons of AD is measured using SEM grids on the extraction line.
There is also an Ion Profile Monitor installed in the AD ring for the continuous monitoring of the beam emittance during the cooling process (stochastic + electron).
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37. CLIC R&D
The CLIC project has very special demands in terms of diagnostics. It is part of a new generation of colliders being developed around the world and the most likely successors of LHC if any
In fact CLIC consists of two completely different accelerators
The Drive beam accelerator, for RF production
The Main beam accelerator, the real one
The main difference between the two is that the first has very intense, low energy beams, while the second has low current high energy beams (and thus small beam sizes)
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38. The strategy for coping with it develops in two ways
Both beams however pose the same kind of problem, the extreme thermal load on any intercepting device.
The strategy for coping with it develops in two ways
Find better material than those used today
Develop non intercepting devices
Electron linear machines make great use of radiators (OTR and Cherenkov) for beam diagnostics. Investigation of replacing aluminum with thin carbon foils is one possibility.
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39. OTR Light View port e- OTR foil Beam profile Calibration :
150mm/pixel – 40mm total
OTR screen :
100mm thick carbon foil (26% reflectivity)
Size : Ø3cm
ph/el (on camera) for 20MeV electrons
‘ photons ’
Beam profile
sx = 7.6mm
sy = 2.1 mm
sx = 4.7 mm
sy = 3.3 mm
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40. Optical Transition radiation
140keV
20MeV
150MeV
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41. Angular properties of OTR
Focused Image
Defocused Image
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