Study of Longitudinal Phase Space
Distribution Measurement
via a Linear Focal Cherenkov Ring Camera
A. Lueangaramwong*, F. Hinode, S. Kashiwagi, T. Muto, I. Nagasawa,
K. Nanbu, Y. Shibasaki, S. Takahashi, K. Yanagi, H. Hama
Electron Light Science Centre, Tohoku University

PASJ105 August 2013 10:30 - 10:50

Outline• Introduction• Method for longitudinal phase space
distribution measurement– Cherenkov radiation– Reflective optics• Extracting beam from vacuum formeasurement– By studying Multiple scattering of electron beam

• Conclusion
2Introduction• A test accelerator for the coherent terahertz source(t-ACTS) at Tohoku University has been constructed– to generate intense coherent terahertz (THz) radiationfrom sub-picosecond electron bunches– an advanced independently tunable cells (ITC) thermionicRF gun consisting of two uncoupled cavities was proposed

Layout of t-ACTS project at Tohoku University3

Introduction• electron beam will be introduced from theRF-gun into the bunch compression system• To obtain extreme short electron bunch
production– proper longitudinal phase space distribution by theITC RF-gun adjusted relative RF phases and fieldlongitudinal phase space (phase dependence)
strengths of the two cavities

Cross-section view of the ITC RF gun

Injector part for t-ACTS

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Cherenkov Radiation• one of diagnostic tools to measure electron energy(electron velocity corresponds to opening angle of Cherenkov light)• Cherenkov angle contains information of the particle energy• aerogel (refractive index n = 1.05) = radiator• number of the Cherenkov photons can be enough to detect5Linear Focal Cherenkov Ring Camera
novel method for longitudinal phase space distribution measurement
12345• e- with same Energy -> photon with same Cherenkov angle• Special Mirror : “Turtle-back” mirror• Focus “same-Cherenkov-Angle photon” onto one certain Position• “different-Cherenkov-Angle photon” gives Linear Position (focal line)• Streak Camera• directly observe longitudinal phase space distribution
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Special Mirror : “Turtle-back” mirror• geometry of “turtle-back”
mirroryyx

parabolic curve• Parabolic curve : reflect the Turtle-back mirror
photons having the same
Cherenkov angle on acertain positions• Spherical curve : designedfor symmetry due toCherenkov cone (to focus
Cherenkov light on thebeam axis)• Surface Eq:

spherical curve

A = 2x(focal length of parabolic curve)– e.g. A = 60 cm (this number relates to energy dependence at focal position);
mirror azimuthal size = 36 deg (corresponds to number of photon that7
can be observed)

Optics for Measurement• “turtle-back” mirror(e.g. A = 60 cm)– focus the photons having the same
Cherenkov angle on a certainposition– gives a focal line on the s-axis• 2 off-axis parabolic cylinder mirrors(e.g. focal length = 10 cm)
e- beam– transport photons outside theradiator chamber and confine again– focal line of 1st parabolic cylinder mirror = 1st focal line
– focal line of 2nd parabolic cylinder mirror = 2nd focal line

• focal position on the focal line

correspond to
E+dE E E-dE

– energy dependence at focal position ~ 22.8 keV/mm around electron kinetic
energy of 1.870 MeV• If entrance slit size of the streak camera ~ 3 mm
: electron kinetic energy range of 1.870 ± 0.034 MeV can be observed at once
8Energy Resolution Factors•Transverse emittance– Beam size -> radiation area -> Cherenkov ring
– Beam divergence -> change direction of Cherenkov rad.

•radiation area & directionof Cherenkov radThickness of radiator-> Cherenkov ring

including small refraction
between 2 mediums

1. “turtle-back” mirror cannot
focus Cherenkov Ring fromsame electron energy to one
point
2. Direction of each electron
dictates direction of Cherenkov
cone which now contains
information of the particle
energy9Beam Transverse Emittance
Detector screen

energy dependence at
focal position ~ 22.8 keV/mm

Numerical ray tracing

Its size is proportionalto energy resolution byenergy dependence
Consider 1x(standard deviation)

Energy resolution ~ 3.78 keV
focal line

-about 18 corresponds
to entrance slit size of thestreak camera (3 mm)seems satisfied

Detector screen

Numerical ray tracing

Energy resolution ~ 5.20 keV- not much different fromabove caseTo focus beam properly canenhance energy resolution
focal line10Extracting Beam from Vacuum for Measurement
electronvacuum

electron

Radiator electron(aerogel)Beam Window

OpticsphotonDetector(streakcamera)• The radiator was intended to be placed outside thevacuum chamber• beryllium thin film was proposed as a beam window• Electron beam will suffer from multiple scatterings• root-mean-square deflection angle• if minimum thickness of the beryllium (Be) thin film is
50 microns, (minimum) rms deflection angle is 0.105
rad or about 6 deg (kinetic energy = 1.87 MeV)
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Geant4 Monte Carlo Simulation••

Geant4 can simulate the passage of particles through matter by using Monte Carlomethods
To investigate multiple scatterings of electron beam through Be window
50-micron Be window
point-like electron beam (Ek = 1.87MeV)

50 microns• significantly high deflection angle of an injected electron (through the 50-micron Be window)
 because of Cherenkov angle (that contains information of particle energy)• pretty high energy distribution
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Multiple Scatterings of Electron Beam
Geant4 -> Numerical ray tracing

50-micron Be window
point-like beam (Ek = 1.87MeV)= 24.3 mm

Its large size degradesenergy resolution
Consider 1x(standard deviation)

Energy resolution ~ 0.55 MeV

Detector screen

Spatial profile• multiple scatterings of the electron beam in the beryllium
window degrades energy resolution– (position on the focal line corresponds the electron energy)

13Discussion• With the 50-micron Be window– Energy resolution ~ 0.55 MeV (cannot be accepted)
for point-like e- beam

• Without the Beam Window– Energy resolution ~ 3.78 keV (can be satisfied)for e- beam with normalized emittance of 0.25 mm mrad

• To extract electron beam from vacuum chambers
cannot be applied for this measurement method.
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Conclusion and Prospect• Longitudinal phase space distribution measurement via
a linear focal Cherenkov ring camera has been studied• Numerical ray tracing combining multiple scatterings
effect of Geant4 results– to extract the electron beam from vacuum degradesenergy resolution of measurement• In vacuum setup was proposed– the radiator and the reflective optics should be placed
inside the vacuum chamber,• Concerning aerogel in vacuum which is dangerous due tovaporization– Cherenkov light transported through a quartz window outof the vacuum to the detector• Concerning refraction through a quartz window and its roughness
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Acknowledgment
We would like to thank• Mr. R. Yamazaki, Drs. M. Miyabe, and H. Kikunaga for technicalassistance (Geant4)

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Thank you for your kindattention

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