Download Coherent light sources and optical techniques for Thomson

Coherent light sources and optical techniques for Thomson
scattering experiments
19th January 2015
PhD Project Scheme
Università di Roma - La Sapienza
PhD Candidate: Bisesto Fabrizio Giuseppe
PhD Supervisors: Prof. Mataloni Paolo (UNIROMA1)
Dott. Ghigo Andrea (LNF-INFN)
My PhD activity is in progress within the SPARC_LAB facility at the INFN National Laboratory of
Frascati dealing with the analysis of laser sources and optical tools for Thomson Scattering (TS) experiments,
also concerning the employment of the new electron sources by laser-plasma interactions. SPARC_LAB is a
particle accelerator test facility devoted to research in high brightness electron and photon sources. My work
is focused on future upgrades, encompassing present, under construction and possibly future TS sources. Some
of the effort is devoted to the Southern european Thomson source for Applied Research (STAR) project, still
in construction. A key point in the realization of this project is the expertise in accelerator physics and high
power laser systems given by the SPARC_LAB, which is one of the main features of this infrastructure.
TS is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical
2
2
χ
dσ
= ( 4π0eme c2 )2 1+cos
,
electromagnetism, whose cross section, without taking into account the polarization, reads dΩ
2
where χ is the angle between the incident radiation and the scattered one, independent of photon frequency. It is
just the low-energy limit of Compton scattering, as long as the photon energy is much less than the mass energy
of the particle: Eγ = hνγ me c2 . Moreover, if the electrons are ultra-relativistic the scattered radiation is
frequency upshifted and it is emitted forward with respect to the particles motion, with a small aperture cone,
proportional to the inverse of the Lorentz relativistic factor.
TS of light from fast moving electrons is a well-known and established source of X-rays and γ-rays. It
was in the 1960s, after the discovery of the laser, when the first TS X-ray sources were proposed [1, 2] and
demonstrated in experiments [3]. Since then many important results were obtained describing TS sources
[4, 5, 6, 7, 8], including the first demonstration of femtosecond X-ray pulses at the Accelerator Test Facility
of Lawrence Berkeley National Laboratory [5]. This kind of sources allow to have monochromatic, ultrashort,
polarized and high flux photons, that guarantees higher performances than other ones (e.g. Bremsstrahlung) to
be used in many areas of science, industry and medicine.
The STAR project [9], in progress at the University of Calabria (UNICAL) in Cosenza, aims at the construction of an advanced Thomson source of monochromatic tunable, ps-long, polarized X-ray beams, ranging
from 20 to 140 keV. The project is pursued thanks to the collaboration among different institutions: UNICAL,
CNISM, INFN and Sincrotrone Trieste. The X-rays will be devoted to experiments of science matter, cultural
heritage, advanced radiological imaging with micro-tomography capabilities.
One S-band RF Gun at 100 Hz will produce electron bunches boosted up to 60 MeV by a 3m long S-band TW
cavity. The peculiarity of the machine is the ability to produce high quality electron beams, with low emittance
and high stability, allowing to reach spot sizes around 15-20 microns, with a pointing jitter of the order of a
few microns. The collision laser will be based on a Yb:Yag 100 Hz high quality laser system, synchronized to
an external photo-cathode laser and to the RF system with a time jitter less than 1 ps.
First of all, I’m focusing my activities on the optimization of the setup for the new STAR facility. In detail,
I’m involved in:
A Design study of photocathode laser system and its transport line. I’m involved in the analysis
of modern laser sources to be employed at STAR. A good choice is the new technology based on Yb:YAG
crystals pumped by diodes. In fact, they exhibit significant improvements with respect to other technology,
first of all a low thermal load that allows to reach an higher average power. Moreover, the employ of diodes
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gets the overall structure smaller and easier to manage with low maintenance costs. The low thermal load
ensures also the possibility to reach high repetition rates and a very good stability and reliability. For
these reasons, this solution is perfect for an user facility.
My task is to study the way to obtain the desired laser parameters on the photocathode by tuning properly
the system and by realizing a suitable transport line, by minimizing any possible optical aberrations, by
using ZEMAX.
So far, i have fixed the scheme for the photocathode laser transport line and I’m waiting for the delivery
of the overall system to start with tuning and measures. In the next weeks I will start also to study the
geometry of the interaction chamber in order to optimize the process.
B Analysis of Nd:YAG pump laser cavities to improve their performances. Pump lasers are
usually constituted by an oscillator and one ore more amplification stages. The oscillator is an unstable
cavity based on a combination of two mirrors with radii R1 and R2 , separated by a distance d, such that
g1 · g2 < 0 ∪ g1 · g2 > 1, where gi = 1 + Rdi . In this configuration the beam size increases at each pass
in the cavity by filling all the active medium inside the resonator, extracting in this way all the available
energy. Moreover, the output mirror usually has a variable reflectivity profile to avoid any diffraction
effects due to the finite aperture. Sometimes, if the cavity is designed in such a way that the beam is too
big with respect to the active medium rod, diffraction ripples appear and can be the cause of damage on
rod surface of oscillator or, more probably, of amplification stages. In fact, their intensity can reach values
above the surface coating damage threshold. This kind of effects has been observed on the commercial
pump lasers of FLAME main amplifier.
To analyze this problem and study a possible solution, i have model the laser cavity with ZEMAX by
reconstructing the cavity gaussian mode. For this purpose, i have measured the mirror focii, the rod
diameter and the gaussian reflectivity profile of the output mirror. In this way all the geometrical cavity
parameters have been defined. From simulation, the diffraction ripples are confirmed and qualitatively in
accord with experimental measures. To completely model the cavity, I’m studying the full laser process by
using a laser cavity design software, LASCAD, taking into account diffraction effects as well as different
amplification effects (e.g. saturation regime, thermal lensing) that could change the transversal beam
profile.
For the future, once completed this analysis, the next step will be to find a new scheme for the oscillator
cavity to overcome this problem and to safely manage high energy levels as demanded by such lasers.
Hopefully, this scheme could be adapted to other systems.
At the same time, I’m studying possible future upgrades to increase the TS process efficiency and new
single shot optical-based diagnostics measurements on electrons. In detail, my activities in this direction can
be summarized as follows:
1. Study of laser beam propagation in a capillary. A capillary is a structure characterized by a core
with a refractive index ncore = 1 and a cladding around it with ncladding > 1. Typically, the cladding
is made with borate silicate glass (ncladding ∼ 1.5). Inside the core, quasi-modes of electromagnetic field
appear. In particular, by solving the Maxwell equations in cylindrical coordinates [10], it is possible to see
that the quasi-mode of order m of the e.m. field inside the capillary reads EH1m (r, z) ∼ J0 (k⊥ r)e−kz z ,
where J0 is the Bessel function of order 0, k⊥ and kz are, respectively, the radial and the longitudinal
component of the wavevector, r is the radial coordinate and z the longitudinal one.
This kind of structure allows to increase the Rayleigh length of a focused laser beam propagating inside.
This feature could be very useful in TS experiments to extend the interaction region between photons and
electrons, maintaining the laser with its highest intensity, in advanced sources like the plasma-based ones.
Several analysis for gaussian laser beam have been done [11]. At the moment, I’m working on simulation
to find the best conditions for the propagation of a Terawatt laser with a Flat-Top profile, that implies
different diffraction effects. In particular, I have found the ratio between capillary radius and beam size
to maximize the energy coupling and the transmission efficiency of the first mode, EH11 , for which we
get the maximum energy, by minimizing the contributions of the other modes. In the next future, an
experimental validation of found matching conditions will be performed.
Moreover, the capillary is helpful in laser plasma experiments, in order to extend the accelerating length
of particles. In fact, I will study the possibility to implement it in the next laser plasma experiments at
SPARC_LAB, EXIN. In this experiment, scheduled for the end of this year, electrons from a conventional
LINAC will be accelerated in a plasma waveguide, created from the interaction between a gas jet and an
W
intense (∼ 1018 cm
2 ) laser beam into a capillary. In this case, the laser parameters as well as the plasma
ones (e.g. plasma wavelength) will be taken into account. Since the luminosity is proportional to the
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inverse of electron and laser beam transversal sizes, a good choice could be also guiding the electrons in
a plasma channel waveguide [12].
2. Advanced diagnostics for ultrashort and ultraintense accelerated electron beams. The diagnostics of charged particles is fundamental in the field of accelerator physics. In particular, I’m focusing
on two kind of diagnostics to measure two main beam characteristics: longitudinal (i.e. temporal) length
by means of Electro-Optic Sampling (EOS) and transverse profile and emittance by exploiting Transition
Radiation (TR).
EOS is an optoelectronic technique of optical sampling, which exploits the linear electro-optic effect (also
called Pockels effect), inducing some birefringence in a non-linear crystal. This effect can be also inducted
by the transverse electric field of a ultra relativistic electron beam. If a polarized probe laser is used, by
measuring its polarization variation, It’s possible to derive the electron beam longitudinal profile without
intercepting and destroying the beam itself.
This diagnostics has been successfully implemented at SPARC_LAB in the single shot spatial encoding
configuration. In particular, the probe laser crosses the crystal with an angle of 30◦ : one side of the
laser pulse arrives earlier on the electro-optic crystal than the other by a time difference ∆t. In this way,
Coulomb field inducing birefringence is encoded in the spatial profile of laser pulse. This setup doesn’t
require high laser energy, but the crystal surface quality is very important to avoid any other spatial effect
(e.g. diffraction). So far, some important results have been achieved [13], but the temporal length of the
laser system employed limits the time resolution. To improve the latter, I’m studying the possibility to
increase the laser frequency bandwidth and consequently compress the beam by exploiting some non-linear
effects, as for example supercontinuum generation in optical fibers [14], to lower the time pulse duration
to about 50f s RMS from the current 96f s RMS. For this purpose, optical simulations and a design study
of experimental setup will be needed.
Optical TR is produced by relativistic charged particles when they cross the interface of two media of
different dielectric constants. It is emitted both in the forward direction and reflected by the interface
surface. I’m studying a new scheme for single shot transverse emittance measurements, suitable also for
not high quality electron beams like those from plasma driven acceleration. This kind of beams, in fact,
differs shot-to-shot and a statistical measure might be misleading. The key point concerns the study
of incoherent optical TR, which gives us informations about correlation between position and angular
divergence. Essentially, the optical setup consists in two lenses and a mask. One lens is used to perform
an imaging of the source, while the second one propagate the image to its focus plane, where visualizing
the intensity angular distribution [15]. The mask, constituted by a series of symmetrically placed vertical
slits, is placed in the image plane of the first lens and used to measure the beam emittance. Usually, this
measure is performed directly on electrons, known with the name of "Pepper Pot". This idea, instead, is
a completely new scheme, more flexible since it allows to work with light in air, not with electrons in the
accelerator beam pipe.
To understand if some aberrations could affect the measure, I’m developing a complete toolbox with
ZEMAX, an optical and illumination design software. In particular, once calculated the single electron
TR electric field with MatLab for using it with ZEMAX, I’m studying its propagation along the system.
Moreover, in order to simulate the incoherent radiation, I have written a code in the ZEMAX Programming
Language (ZPL). In the detail, It can propagate single electron e.m. field starting from different point of
the bunch and sum all the contributes incoherently.
I’m finishing the first part of the study, by considering single electron e.m. fields propagating in the
same direction. Once taken into account all the aberration effects, next step will consist in introducing
e.m. fields with other wavevectors and the effect of electron energy spread, that causes chromaticism
aberrations in the optical system.
Acronyms
TS Thomson Scattering
STAR Southern european Thomson source for Applied Research
EOS Electro-Optic Sampling
TR Transition Radiation
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References
[1] F. Arutyunyan and V. Tumanyan, Sov. J. Exp. Theor. Phys. 17, 1417 (1963),
[2] R. Milburni, Phys. Rev. Lett. 10, 1 (1963),
[3] O. Kulikov, Y. Telnov, E. Filippov and M. Yakimenko, Phys. Lett. 13, 344 (1964),
[4] E. Esarey, S. K. Ride and P. Sprangle, Phys. Rev. E 48 (1993),
[5] W. Leemans, R. Schoenlein, P. Volfbeyn, A. Chin, T. Glover, P. Balling, M. Zolotorev, K. Kim, S. Chattopadhyay, and C. Shank, Phys. Rev. Lett. 77, 4182 (1996),
[6] W. P. Leemans, E. Esarey, J. van Tilborg, P. A. Michel, C. B. Schroeder, C. Toth, C. G. Geddes, and B. A.
Shadwick, IEEE Trans. Plasma Sci. 33, 8 (2005),
[7] H. Ohgaki, T. Noguchi, and S. Sugiyama, Nucl. Instr. Meth. Phys. Res. A 353, 384 (1994),
[8] G. Matone et al., Photonuclear Reactions II - Lecture Notes in Physics, 62 (1977),
[9] A. Bacci et al., Proceedings of IPAC2014 Dresden, WEPRO111, (2014),
[10] A. Yariv, P. Yeh, Photonics Optical electronics in modern communications, p. 797-811 (2007),
[11] B. Cros et al, Phys. Rev. E, 65 (2002),
[12] S.G. Rykovanov et al., arXiv 1406.1832v1 (2014),
[13] R. Pompili et al., Nucl. Instr. Meth. Phys. Res. A, 740 (2014),
[14] J. Dudley and R. Taylor, Supercontinuum Generation in Optical Fibers, Cambridge University Press,
(2010),
[15] M. Castellano et al., Nucl. Instr. Meth. Phys. Res. A, 435 (1999)
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