1)A S M A L L S U R V E Y O N
BEAM DIAGNOSTICS FOR
SMALL SIZE BEAM
2) SHORT PRESENTATION ON
ACCELERATOR R&D AT LAL(ORSAY)
Robert CHEHABLAL-Orsayand KEKAbstract
The problem of measuring and controlling small beam sizes (much lessthan the mm) in linacs and transfer lines is particularly crucial for themain linacs of a linear collider, where the 6-D emittance has to bestrongly reduced. The problem becomes more complicated when thebeam intensity considered is very large. Some invasive and noninvasive diagnostics, which could be of interest for such application,
will be presented and compared between them.
Since many years, the Accelerator group in LAL-Orsay is being
involved in the study and realization of small and medium energy
linacs. Some recent illustrations of this activity, related to electron
injectors or to some linac components will be presented.

DIAGNOSTICS FOR SMALL SIZE BEAMS
Introduction
In order to assess the good behaviour of a beam in an accelerator, theknowledge of the 6-D emittance is necessary. This knowledge can beacquired during dedicated measurements or checked during control
operation => invasive/non-invasive
For the particular case of a linear collider, we need small sizes at IP, inorder to increase L. Thus, beam emittance is damped in PDR and DR.
Moreover, high intensities are required. The conjugation of small sizes
with large intensities with, hence,a high density of particles, makes:- the sensitivity, easier- the resolution and the problem of damages (invasive), harderfor beam profile monitors

SOME BEAM PROFILE MONITORS FOR
SMALL SIZE BEAMS
The following tools will be considered:- wire scanners- gas ionization detectors- laser wires- laser interferometer- optical radiation detectors:# Optical Transition Radiation# Optical Diffraction Radiation

WIRE SCANNER
Device with a thin fiber (tungsten or carbon) moving
across the transverse section of the beam.
Interception of the beam produces bremsstrahlung photons
and secondary emission electrons. Either γ rays orsecondary e- can be used for the information on beamprofiles (H & V).
The W.S. is attached to a precision stepped device moving
with µm steps.
The rms resolution of a W.S. is d/4, where d, is the wire
diameter. A 5 µm rms beam measurement would bewidened by only 6% , with a 7 µm diameter W.S.

Some details on Wire Scanners
Usually, two wires (H&V) are
mounted on a fork which isdisplaced at 45 degrees to thehorizontal plane.
In some cases, the γ rays areconverted into pairs which
create Cherenkov Radiation in a
medium (ex. SLAC/FFTB).
Minimum diameter size
actually realized is of 4 µm,
leading to a minimum
resolution of 1 µm.

GAS-IONIZATION BEAM SIZE DETECTOR
PRINCIPLE : the information on the transverse dimensions is obtained
through the kick given to the ions by the passage of the electron bunch.
At high energy, the space charge field of the bunch is mainlytransverse. The image of a “cloud” of quasi-real photons can be used.
(W-W method).
The ions are produced by the electrons of the bunch in a gas. Heliumas Argon are used.
If we consider Ar, for instance, the ions move only slightly during thepassage of the beam and their maximum velocity is proportional to themaximum electric field; which is inversely proportional to the largest
beam dimension (horizontal, for flat beams) The measurement of thetime of flight of the ions to the detector provides the horizontal
dimension
Measurement of the angular distribution provides bunch aspect ratio

Some informations on gas-ionization detector
The ion detector consists of
microchannel plates put around thebeam tube at some cm distance ofthe beam axis. Before reaching theMCP’s, the ions pass through anarrow slit (W); potential
difference slit-MCP [2-3 kV]
allows final acceleration of the ions.
Gas is injected by a small inlet in
the tubeVertical sizes as small as 60 nm
have been measured at FFTB(Orsay team)Considered at CERN for LHC

LASER WIRE MONITOR
Principle : Compton scattering of the electron beam bya laser light sent at right angle to the beam. The
emitted (hard) photons are measured in the forward
direction.
The laser beam waist is realized in the center of a
Fabry-Perot cavity
The counting rates of scattered photons are measured
as a function of the laser wire position
The actual electron beam profile is obtained after
deconvolution with the known laser distributionLaser Wire Monitor: technical details
Scattered γ are detected in theforward direction after having
swept off the electrons.
To detect the γ’s (some MeV, for aGeV e- beam and a visible laserlight), use of scintillators [CsI,NaI,..]. Photomultipliers areassociated with the scintillators.
Scan with the laser beam can berealized, moving the optical table(method at ATF)
A 10 µm e- beam has beenmeasured at ATF; σ=1 µm/SLC
Figure: typical scheme (ATF)
Limits: ~ λ ,wavelength:diffraction

Laser Interferometer TechniqueThis technique, used by T.Shintake at FFTB, uses
interferometry technique with a YAG-laser beam split into
two beams which collide, perpendicularly, with an electronbeam. The Compton back scattered γ, in the forward
direction (e-) are directed to a γ-detector (Conv. + gas
Cerenkov). As the laser light is stored in a cavity (SW), theγ’s present periodic variations with λlaser along laser axis,
when scanning the beam. Modulation depth=> size
This technique allows high accuracy; a 60 nm spot (V) wasmeasured at FFTB.

OPTICAL RADIATION DETECTORS
Optical radiation detectors can use Cherenkov radiation,Transition radiation, Diffraction radiation and of course,
fluorescent screens. But due to the perturbation introduced
by a physical medium (multiple scattering, energy
deposition,.) we shall concentrate on two particularradiations: Transition radiation (with a very thin foil) andDiffraction radiation.
Before describing these two possibilities, we shall get
familiar with a useful method to describe them: the virtualor pseudo photon method (Weiszacker-Williams)

The Virtual Photon Method (Fermi, WeiszackerWilliams)
The electric field created by aparticle of charge Ze:(ero/4πεoro3).(1-u2/c2)
E= ----------------------------[1-(u2/c2).sin2η]3/2
depends on the angle η between
the direction of motion and theradius vector ro. At high u (~c),
the transverse component ismuch stronger than thelongitudinal one. We have apancake field shape. It is like aplane wave.=> photon cloudVirtual Photon Method

The photon cloud has a transverse
dimension: γ.λ/2π
When the electron beam propagating in vacuum with the associatedCoulomb field in the form of the photon cloud meets:- an inclined surface=> Transition radiation (backward andforward). The backward radiation! reflection of the photon cloud on
the surface- a hole or an edge, smaller or closer to the axis than the photon cloudtransverse dimension ! diffraction=>diffraction radiation

TRANSITION RADIATION
Transition radiation is created
when a charged particle crosses theinterface between two media ofdifferent optical properties.
• The radiation is emitted in two
directions:• -Forward: in direction of beam
propagation• -Backward: in direction ofspecular reflection.

TRANSITION RADIATION : possible measurements
Bunch length: due to the rapid formation of the radiation (~10-13 s), it ispossible to measure very short bunches:* With a Streak-Camera (SC): the limitation is brought by the SC
resolution (~ps)* With coherent TR using wavelengths comparable to the bunch size:
resolution < psTransverse phase space:# transverse dimensions are measured with CCD cameras at the image
plane of the lens# angular distributions are measured with a CCD camera at the focal
plane of the lens.
Beam energy:# determined by the angular position of the maxima (1/γ). Moreprecision is obtained with interferometer (Wartski)

Transition Radiation: some activities at LAL-Orsay
The more “recent” activities
with OTR in our laboratory,
concerned:* a test on the OTR resolution
on the 2 GeV Orsay e- linac
On figure H profile,
σ=0.3 mm
* a series of measurements on
the Tesla Test Facility (TTF) atDESY. In that case, beamemittance has been determined
and observations on beam
behaviour inside the pulse, also.
-> PhD report of A.Variola

Measurement of transverse dimensions with OTR:
the problem of resolution
OTR angular distribution! peaked into a cone of half aperture 1/γ. Doubt
about obtaining good geometrical resolution at high energy due to “selfdiffraction” of OTR spot. The self diffraction, expressed with Heisenberg
relation:∆x∆px >h/2π
Using the relations p=h.k/2π and ∆kx=k∆αx, we get in the case of separate
measurements for angle and dimension:∆x∆αx >λ/4π
Referring to such relation and taking as ∆αx, 1/γ, certain authors have abusively
concluded that the spatial resolution will be worse than λγ (ex, 5 cm at 50 GeVfor optical λ) . But:-Heisenberg relation concerns rms values (and rms >>1/γ)- A large part of OTR intensity, emitted in angles >> 1/γ=> Many works to determine theoretically and experimentally OTR resolution

Study of OTR resolution
Theoretical & Experimental
studies to determine resolution
Theoretical: Rule&Fiorito,
X.Artru et al, M.Castellano
PhD thesis (K.Honkavaara: Orsay){Figure from PhD report of KH}• Experiments at LAL-Orsay (2GeV), CEBAF (4 GeV) andCERN (22 GeV), showed anOTR resolution << λγ• => practical resolution: FWHM
• of the diffraction function• => OTR measurements of smallbeams at high energy: possible

Diffraction Radiation
Small recall:
As for Cherenkov Radiation and Transition Radiation, theDiffraction Radiation was studied by Russian theoreticians
Papers from 1958 (Y.Dnestrovskii and D.Kostomarov),
analysed the radiation of a beam of particles passing
through a circular aperture. Later, in 1962, A.Kazantsev
and G.Surdovich studied the radiation of a particle beam,close to a metal screen. They considered this radiation as
resulting from the diffraction of the electromagnetic fieldof the particle by the metal screen.

Diffraction Radiation: a simple approach
We can consider the DR as
resulting from the diffraction ofthe virtual photon flux by theaperture (hole, slit, edge).
[Figure from Moran in NIMB]
The source of DR is a “disk” ofdiameter γλ/2π. Consider a slitof aperture a:∗ γλ<<2πa =>DR radiation ~0∗ γλ>>2πa =>DR similar to TR∗ γλ ∼ 2πa =>DR intensity<TR,but enough for measurementDiffraction Radiation: statusTheoretical works worked out since a while (Ter-Mikaelian,..)More recent calculations by Moran, Rule and Fiorito,
Castellano,Potylitsin, Artru
Measurements (few) are concerning :- bunch length measurements (coherent DR; λ~mm); Y.Shibata et al- transverse beam characteristics (slit 1mm, λ=1.6 µm) at ELETTRA(team Castellano) and DESY (?)
At ATF, with an inclined edge at some tens of µm of beamaxis.Detection of optical λ for 1.28 GeV e- beam (angular distrib.
Nota: as for TR, the DR resolution depends on the optical acceptanceDue to its non-invasive character, DR will have large developments

Summary: comparison of different methods
Ultimateresolution

Limit (µm)

Defined byIntense beamMethods

Invasive ?

OTR

yes

~10

CCD pixel
size +optics

noODR

no~10

CCD pixel
size +optics

yesWire scanner yes

~1

Wire
diameternoLaser wire

no~0.3

Laser (λ)

yes

Laser
interferom.

no~0.01

laser

yes

Gas ioniz.

no~0.01

Long.spacechargeyesSome R&D on Accelerators at LAL-Orsay
Some examples related to the R&D on Accelerators atLAL-Orsay, are presented. They concern:* studies and realizations related to linear collider projects:# injector for the drive beam of CTF3, for CLIC# RF couplers for the TESLA cavities* study and realization of a small energy ( <10 MeV) linac
for research on rapid reactions in chemistry

Injector for CTF3/CLICFor CLIC, the RF power is extracted from a drive beam of highintensity propagating in S-Band structures. The extracted power isfeeding accelerator structures at 30 GhZ.
A facility (CTF3), using large part of the LIL installation, is to be built
in order to validate the principle.
LAL is being involved in the study and construction of:* 1 gun CLIO-like at 90 kV, 2A max with 1 to 7 pulses of 20 ns and
1 A, separated by 450 ns (one ring revolution). Risetime < 1ns* 1 gun SLAC-like at 140 kV, 7A; pulse width 1.6 µs* 2 pre-bunchers:# 1 prebuncher 3 GhZ with SW cell (60 kV, max)
# 1 prebuncher (inox) with very low Q

Injector for CTF3: status
The CLIO-like gun has been delivered and works properly.
The SLAC-like gun: to be constructed. The associated electronics areunder development
The prebunchers: to be constructed
An agreement has just been signed, between CERN and IN2P3, for theprovision of the new gun and of the prebunchers.

RF couplers for TESLALAL is involved in the construction of RF couplers prototypes for
TESLA. These prototypes are based on the current TTF linac design.
But, they have a larger diameter: 80 mm. Because:- they have to carry more power [ 2 MW instead of 250 kW]. Suchdesign was conceived for the TESLA “superstructure” , alternative
cavity design for TESLA {4x7cells cavities} with one coupler.
- the choice of the diameter is such to push the multipactor limits
beyond the operating power. The coupler can also be DC biased tostop multipactor (cf LEP couplers)Infrastructure to clean the coupler in ultra-pure water is in preparation.
The couplers wil be “baked” in a 400 deg vacuum oven, before final
assembly with the cavities in a “class 10” clean room. They will betested with a 4.5 MW-1ms pulse Thomson KlystronShort e- pulses for rapid reaction chemistry
The project ELYSE consisting in a 9 MeV electron linac with an RF
gun is destinated to allow studies of rapid reactions in chemistry.
Two synchronized pulses are used: the first initiates the reaction andthe second interacts with the excited system. The informations derived
from the system, associated to the delay between the two pulses,
provide some knowledge on the reaction path during that time interval.
A laser (Sapphire-Titanium) is triggering a Cs2Te photocathode(QE~0.01) . The λ used is 266 nm.
Two possibilities are then offered to the chemists:-use of photon pulses (photolysis)- use of electron pulses (radiolysis)
Comparison between the two modes is foreseen.