Photon emission in crystalline undulators
Producing a powerful laser beam by using antimatter and 'sequeezing' exotic crystals is becoming a scientific fact with the successful
completion of the PECU project. The science focuses on the properties of a new class of light sources called crystaline undulators (CU).
Such a device will be considerably more powerful than existing light sources in hard-X and gamma ray frequency range.
In the figure, the peack brilliance of existing and being constructed light sources (Gürtler, 2002) is compared to the one of CU and of the
crystalline undulator based laser (CUL) (A. Kostyuk, A. Korol, A. Solov’yov, W.Greiner, 2008). Currently, the most powerful type of laser
operating in the so X-ray regime is the free electron laser (FEL). A CUL could produce photons with much higher energies. In fact, its
photon energy range starts where FEL devices tail-off.
The feasibility of constructing CU is a very recent concept (A. Korol, A. Solov’yov, W. Greiner, 1998) and the European researchers
from Germany, Russia, Denmark and the UK, involved in PECU, are among the world leaders in the field. Several technologies for
manufacturing periodically bent crystals have been developed and tested. The method utilizes a powerful short-pulse laser to produce
equally-spaced trenches on the surface of the crystal resulting into periodic deformations in the bulk of the crystal. Another novel
method is based on the slight difference in lattice constants in silicon and germanium crystals, such that varying a small content x of
Ge to a Si crystal in e.g. a molecular beam epitaxy process (MBE), one obtains a strain in the crystal resulting in a bending of the crystal
lattice. In the MBE laboratory of Aarhus a Si1-x superlattice crystal with 4 undulator periods has been grown.
No matter how the crystal is constructed, the result is a short-wave undulator that will emit high-intensity, highly monochromatic
radiation when pulses of relativistic particles (particles traveling very close to the speed of light) are passed through its ‘bent’ channels.
The most suitable are high-energy positrons - the antimatter equivalent of electrons. Electrons are less favorable, because they tend
to move near the crystallographic planes and therefore, in contrast to positrons, suffer stronger random scattering. Therefore, it was
initially planned to use electron beams, which are more easily available than positrons, only for testing the crystal samples. However,
it was realized in the course of the project that an electron-based CU is also feasible. It was demonstrated that the technologies
at present are sufficient to achieve the necessary conditions to construct the electron-based CU and to create on its basis powerful
radiation sources in the gamma region of the spectrum.
Axial and planar channeling of electrons has been studied for silicon single crystals at the Mainz Microtron MAMI. The radiation emission
from a 4-period Si1-x CU has been measured in the experiments held as MAMI for electron beams with energies 855 MeV (1 MeV =
106 electron-Volt) and 1508 MeV. The most pronounced peak of the undulator radiation (around 1 MeV) was found for the 1508
MeV beam, and this was the first ever direct experimental observation of the radiation from CU. To determine the parameters of the
crystal sample that ensure the conditions at which CU becomes a powerful radiation source in the gamma and X-ray region of the
spectrum, a comprehensive numerical analysis has been carried out in collaboration between theoretical and experimental groups.
In part this analysis was done using a complex of the developed computer codes which allows one to analyze the motion of particles
in periodically bent crystals and to calculate the spectral and angular distribution of the radiation from CU. The emission of the gamma
rays will become even more powerful if the density of particles in the beam is modulated in the longitudinal direction with the period
equal to the wavelength of the emitted radiation. In this case, the electromagnetic waves emitted in the forward direction by different
particles have approximately the same phase. Therefore, the intensity of the radiation becomes proportional to the beam density
squared (in contrast to the linear proportionality for an unmodulated beam).
© European Union, 2011. This document should not be considered as representative of the Commission’s official position.
The idea of CU is based on the channeling phenomenon. Channeling takes place if charged particles enter a single crystal at a small
angle with respect to crystallographic planes or axes. The particles get confined by the electrostatic potential of crystallographic
planes or axes. It is remarkable that, even for bent planes or axes, the particles can follow their shape. A single crystal with periodically
bent crystallographic planes can force channeling particles to move along nearly sinusoidal trajectories and radiate in hard X and
gamma ray frequency range. The process of emission is very similar to that in the periodic magnetic field of a conventional magnetic
undulator. However, the electrostatic fields inside a crystal are so strong that they are able to steer the particles much more effectively
than even the most advanced superconductive magnets. Due to this fact, the period of crystal bending can be made two to four
orders of magnitude smaller than the period of a conventional undulator. Therefore the particle in a periodically bent crystal will emit
electromagnetic radiation with much shorter wavelength, i.e. in the hard X ray and the gamma ray range.
Peak Bri l l i ance [Phot./(sec · mrad 2 · mm 2 · 0.1% bandw .)]
1035
TESLA
SASE FELs
33
10
31
DESY
TTF-FEL
(seeded)
LCLS
10
1029
Spontaneous Spectrum
SASE FEL 1
DESY
TTF-FEL
20 GeV
Spontaneous Spectru m
SASE FEL 2
10 GeV
1027
TESLA
spontaneous
Undulator
CUL
SPring8
Undulator
(30m in vacuum)
1025
TTF-FEL
spontan
1023
CU
BESSY-II
U-49
1021
BESSY-II
U -125
PETRA
Undulator
1019
101
102
103
104
105
10
06
107
PhotonEnergy [eV]
This increases the photon flux by orders of magnitude relative to the radiation of an unmodulated beam of the same density.
The radiation of a modulated beam in an undulator is a keystone of the physics of free-electron lasers (FEL). It can be considered as
a classical counterpart of the stimulated emission in quantum physics. Therefore, if a similar phenomenon takes place in CU, it can
be referred to as the lasing regime of the crystalline undulator. Theoretical and numerical analysis of the lasing effect in positronbased CU has been carried out. The role of the bunch modulation has been studied. The process of demodulation in the presence
of the dechanneling has been analyzed. It has been shown that the demodulation length is sufficiently large to make a CUL feasible.
This opens the prospects for creating intense monochromatic radiation sources in the frequency range which is unattainable
for conventional free electron lasers.
CUL can find its application as laboratory equipment, for example, in material science, nuclear or plasma physics and molecular biology.
Potentially, it will have medical applications such as selective irradiation (or surgery) of individual cells. A tuneable gamma laser can
be used to produce useful isotops for medical applications or act as a source of positrons for imaging techniques known as positron
emission tomography.
Contract
PECU
Coordinator
Johann Wolfgang Goethe Universitaet of Frankfurt Am Main (JWGU-ITP-FIAS),
Germany
Partners
UAAR-DPA - Aarhus Universitet, Denmark
IMPERIAL - Imperial College of Science, Technology and Medecine, United Kingdom
UNI-MAINZ - Johannes Gutenberg Universitat Mainz, Germany
EC-contribution
1.098.000,00 €
Full partner and project information available on http://cordis.europa.eu/fp6/projects.htm
The coordinator provided text and pictures for the factsheet and his copyright is acknowledged
http://ec.europa.eu/research