Transcript Optical Diagnostics of High- Brightness Electron Beams Victor A. Verzilov Synchrotrone Trieste

ICFA AABD Workshop, Chia Laguna, Sardenia
Optical Diagnostics of HighBrightness Electron Beams
Victor A. Verzilov
Synchrotrone Trieste
Introduction
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“ID” of a high-brightness beam
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high charge per bunch (1 nC and more)
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small transverse and longitudinal beam dimensions
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extremely small normalized emittances
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high peak current
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space-charge effects in the beam dynamics
Two missions of beam diagnostics
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Provide instruments for study of the physics
Assist in delivering high quality beams for applications
Every machine is as good as its diagnostics
Introduction (continue)
For high-brightness beams control of following parameters is essential
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Vertical and horizontal emittances
Transverse beam profile
Beam trajectory
Energy and energy spread
Bunch length
Longitudinal bunch shape
Charge per bunch
Current (peak and average)
Bunch-to-bunch jitter
Some of the parameters are measured by traditional methods,
others require specific techniques and instrumentations
Specific requirements
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Take into account space charge forces
Resolution from several millimeters to few tens of
micrometers in both longitudinal and transverse plane
Large dynamic range both in terms of beam intensity and
measuring interval
Non-invasive
Single-shot
Real time
Jitter-free and synchronized
Usual (stability, reliability ,etc)
Optical diagnostics and others
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Optical diagnostics are based on analysis of photons generated by
a beam in related processes or make use of other optical methods
(lasers, etc.)
This talk reports the current status of optical diagnostics of highbrightness beams
Reasons
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significant progress
make an essential part of available tools
impossible to cover everything
Other techniques
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wire scanners
zero phasing
transverse rf deflection cavity
high-order BPM
Outline
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Transverse and longitudinal profile measurements give the
largest amount of information about beam parameters
Transverse plane
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Spatial resolution is a key issue
Survival problem for intercepting monitors
Non-invasive methods
Emittance measurement issues
Longitudinal plane
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Coherent radiation is a primary tool
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Direct spectral measurements
Fourier transform
CDR vs CTR
Electro-optical sampling
Transverse plane
OTR vs inorganic scintillators at a glance
Scintillators (YAG:Ce,
OTR
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instantaneous emission ~  / c
linearity (no saturation effects)
high resolution
surface effect: thickness doesn’t
matter
small perturbation to the beam
(small thickness)
small radiation background (small
thickness)
can be used in a wide range of
g
relatively low photon yield
(limitation in pepper-pot
measurements)
YAP:Ce, oth.)
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high sensitivity
no grain structure
time response ~ 100ns
conformance to HV
radiation resistance
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bulk effect
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TR spatial resolution
OTR resolution is determined
by the angular acceptance
 J1 x  
 x 
2
F 2 x 
g 100

 x 
K1 
g  J 0 x 
Fx  

g
x
 1/ g , M  1
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FWHM resolution is 2-3 times of the
classical PSF
scales as ~ /
tails problem; mask can help
high-resolution is experimentally confirmed
[CEBAF(4 GeV) SLAC (30 GeV)]
A.Murokh et al. BNL-ATF
Scintillator resolution
Recent experiment at BNL
expressed concerns about
micrometer-level resolution.
Strong discrepancy in the
beam size compared to OTR
and wire scans was observed.
Q=0.5nC
Confirmed at ANL
220 MeV @ 0.8 nC
30-40% discrepancy
N.Golubeva, V.Balandin TTF
Instantaneous heating. TR case
Si: 1GeV @ 300um. For Al values ten times smaller
T 
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 dE 


dm


Temperature limits
Si
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1
cp
Melting - 1683 °
Thermal stress –
1200°
Al
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Melting - 933 °
Thermal stress –
140-400°
N.Golubeva, V.Balandin TTF
Heating by a bunch train
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Two cooling processes contribute to the temperature balance
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Radiation cooling ~ temperature to the power of 4
Heat conduction depends on the thermal conductivity and
temperature gradient
Si@9MHZ
1nC
9MHz
20um
1MHz
50um
Si @ 20 um
1nC
W.P.Leemans et al. LBNL
90° Thompson scattering
66m FWHM
2g 2 0
 
2 2
1 g 
e-beam: 50 [email protected]
laser: [email protected]m; 50-200fs
photons:30keV@105 ph/bunch
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Noninvasive
Both transverse and
longitudinal profiles
Synchronization
Powerful laser
Limited applicability
Diffraction radiation
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Diffraction radiation is emitted
when a particle passes in the
proximity of optical
discontinuities (apertures )
DR characteristics depend on
the ratio of the aperture size to
the parameter g
DR intensity ~ e-a/g and is
strongly suppressed at
wavelengths <a/g
TR vs DR from a slit
Transition
radiation
Diffraction
radiation
Effect of the beam size
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Angular distribution depends
on the relative particle
position with respect to the
aperture and can be used to
measure the beam size
Strong limitation is a low
intensity in visible and near
infra-red
Energy and angular spread,
detector bandwidth are
interfering factors
Still has to be proven
experimentally
A.Cianchi PhD Thesis
S.G.Anderson et all PRSTAB 5,014201(2002)
Emittance measurement. Multislit vs quadscan
High-brightness beam
at “low energy”
Widely used techniques
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Pepper-pot (multislit)
Quadscan
3 screens
I 2
R
2
2 I 0g n
Measure of
spaces-charge
dominance
drift
Space-charge forces
LLNL 5MeV@50-300pC
Longitudinal plane
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Small longitudinal bunches are crucial for many
applications
Bunch lengths are on a sub-ps time scale
Conventional methods often do not work
Several new techniques have been developed
Coherent radiation has become a primary tool to
measure the bunch length and its shape in the
longitudinal plane
It is very powerful tool with nearly unlimited potential
towards ever shorter bunches
Radiation from a bunch
All particles in a bunch
are assumed identical. No
angular and energy spread.
F ( )  FL ( ) FT ( , )
I tot    N I sp    N ( N  1)F ( )I sp  
N
N
  
1
i  / c nrk  r j 
F   
 e
N N  1 k j  k
Radiation zoo
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Any kind of radiation can be coherent and potentially valuable
for beam diagnostics
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Transition radiation
Diffraction radiation
Synchrotron radiation
Undulator radiation
Smith-Parcell radiation
Cherenkov radiation
Nevertheless, TR is mostly common
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Simple
Flat spectrum
Bunch form-factor and coherence
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F=0
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0 <F< 1
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F=1
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wavelength is much shorter than bunch dimensions
radiation is fully incoherent
particles emit independently
total intensity is proportional to N
wavelength is of the order of bunch dimensions
radiation is partially coherent
some particles emit in phase
increase in total intensity
wavelength is much longer than bunch dimensions
radiation is fully coherent
all particles emit in phase
total intensity is proportional to N2
Form-factor and bunch shape
F ( )  FL ( L /  ) FT ( T sin  /  )
Transverse
coherence comes
first. Unless the beam
is microbunched.
F ( x  0)  1
F ( )  FL ( ) ,   1
For the normalized longitudinal distribution of particles in the bunch (z)
F   

 dz ze
2
i / c z

By inverse Fourier transform
1
z 
c
 z 
 d F   cos c 
0

Symmetric bunch
Bunch shape and form-factor
Form-factors
Bunch shapes with the
same rms bunch lengths
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Although, in principle, the bunch
shape can be retrieved from a
measurement, be care, this could
be ambiguously.
The bunch size, however, is
recovered reliably.
R.Lai and A.J.Sievers NIM A397
Kramers-Kronig analysis
Both real and imaginary part of the
form-factor amplitude are to be
known to recover the asymmetry of
the bunch shape.

     dz  z e
i  / c  z
 f  e
0
Real part is the observable
F  *   f 2 
i  
If F() is determined over the
entire frequency interval, the
Kramers-Kronig relation can be
used to find the phase.
ln  f x / f  
 m    
dx

 0
x2 2
2

By inverse Fourier transform

1
z 





 z  
d

f

cos



 m
 c 0
c 
TESLA TDR
Kramers-Kronig analysis.Experiment
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Spectral intensity has to be defined
over a significant spectral range.
Errors are produced when
asymptotic limit are attached to the
data to complete the spectral range.
Front-tail uncertainty.
Analytical properties of the bunch
shape function have to be taken into
account.
Confirmed by recent SASE results!
T.Watanabe et al. NIM A480(2002)315
Tokio University
Polychromator
900fs
Results are consistent with
streak camera and interferometer
measurements
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1.6ps
Single-shot capable
Narrow bandwidth
Discreteness
M.Getz et al., EPAC98 TTF
Hilbert -Transform spectrometer
 t  (1.2  0.2) ps
Josephson junction
T= 4-78K
f= 100-1000GHz
2
S  d s
0  s2  02
2 2 
c
 2e  R I
I   
   4I
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Wide bandwidth
More R&D is necessary
Fourier spectroscopy
Measurement in the time domain
is a measurement of the autocorrelation
of the radiation pulse.
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I   


2
Et   E t   / c  dt
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Coupled to a frequency domain.
  
I     I  cos
d
 c 

Precise
Established
Time consuming
Low-frequency cut-off
• All experimental data suffer to a
different extent from the low
frequency cut-off.
• There is a number of reasons
which cause the cut-off: detector
band, EM waves transmittance,
target size etc.
• Data analysis usually consists in
assuming a certain bunch shape
and varying the size parameter for
the best fit to undisturbed data.
Analysis in the time domain (TR case)
A.Murokh,J.B.Rosenzweig et al
Filter function
g ( )  1  e
( / c ) 2
Model bunch shape
 ( z )   u ( z  y )e
 y 2 / 2 2
dy
1 /  , z   / 2
u( z)  
 0, z   / 2
Coherent spectrum
I ( ) 
e
 ( / c ) 2
sin 2 ( / 2c)
 2 2
(1  e
 ( / c ) 2 2
)
Autocorrelation curve

 s 
I s    I  cos
d

 c 
TR. Finite-size screen
The effect comes into play when
the screen size is comparable or
smaller than g
r=20 mm
d=0.05 rad
r
g
2mm
1mm
screen
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r  g
I ,  I inf  , 1  J 0 k rsin  
2
The TR spectrum from a finite size
target is a complex function of the
beam energy, target extensions,
frequency and angle of emission.
M.Castellano et al. PRE 63, 056501
TTF
Coherent diffraction radiation
Bunch length was measured
for slit widths 0 to 10 mm.
Effect of the target finite
size was proved.
M.Castellano et al. PRE 63, 056501
TTF
Coherent diffraction radiation.Result
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225MeV @ 1nC
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DR and TR results are
consistent in a wide range
of slit widths .
CDR can be successfully
used for bunch length
measurements.
Very promising for ultrahigh power beams,
because non-invasive.
Electro-optic sampling (EOS)
Modulation of the polarization
of light traveling through a crystal
is proportional to the applied
electric field
  (l /  ) E
Collective Coulomb field
at R is nearly transverse
E
i
gq
( R  ri )
2
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Noninvasive
Fast response ~40 THz
Linearity&dynamic range
Jitter dependent
EOS Single-shot option
Make use of a long
pulse with a linear
frequency chirp
Bunch time profile
is linearly encoded
onto the wavelength
spectrum
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Single shot
On-line
Nearly jitter-free
I.Wilke et al., PRL, v.88, is.2,2002 FELIX
EOS Single-shot option.First prove
e-beam: 46MeV@200pC
0.5x4x4mm3 ZnTe crystal
laser: 30 fs@800nm,chirp up to 20ps
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1.72 ps
Resolution ≈
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Pulse width 0
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~300fs
~70 fs achievable
(   1 ps,  0  5 fs )
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Chirp 
 0
Conclusions
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Beam diagnostics has significantly advanced to meet specific
requirements of high-brightness beams
Wide choice of available techniques from which one can
select
Lack of suitable (simple and reliable) non-invasive methods for
measurements in the transverse plane (near-future projects)
In the longitudinal plane CDR is likely OK
Difficulties with measurements at μm and sub-μm level in the
transverse plane