Chapter X. Synchrotron Radiation Sources
X.1 Scientific Motivation
Sources of synchrotron light: electron storage rings and Free Electron Lasers are today’s most exciting
research tools for a broad spectrum of scientists working at the frontier of modern technology. Suppose
we take a walk around an experimental zone surrounding a synchrotron light source. We would find
many groups of scientists, huddled their own cubicles taking data and with their latest conference posters
proudly adorning the doors of their enclosures. Let us imagine how some of these posters might start:
“The discovery of DNA and the mapping of the human genome have opened the way to
understanding our inherited characteristics as well as our propensities for certain diseases
and how these might be controlled. Few scientific discoveries have a greater potential to
change our quality of life in the future, yet there remains one missing link in our
understanding – how the biochemistry of complex protein molecules interprets the
genetic code and transmits this information to change the cells of our body. Synchrotron
radiation can provide images of these molecules and investigate their interactions. “
“Our concern for the future, if not the politics of the world, is increasingly dominated by
the need to find renewable sources of energy which do not contribute to global warming.
Synchrotron radiation is a powerful tool to study the chemistry of photosynthesis and
search for alternative means to convert sunlight into energy.”
“Technology eagerly awaits new materials – superconductors and crystals of familiar
materials like carbon formed as hollow spheres, tubes and planes with new and
exploitable properties.” Again it is synchrotron radiation that is most effective in
revealing these curious and exciting structures.
It is no wonder that there has been a surge in the number of sources of synchrotron radiation:
electron synchrotrons and latterly free electron lasers, to produce synchrotron radiation of ever
increasing brilliance and photon energy producing diffraction patterns which reveal detail with ever
increasing resolution. Synchrotron radiation sources, though they embody the most advanced features of
modern accelerators are relatively modest installations compared with the hadron colliders required by
particle physics. A single synchrotron radiation source can satisfy the research needs of several thousand
scientists and there are rich pickings for those who are eager to see research applied to everyday life. It is
no wonder then that these machines have become the accelerator of choice for the countries of the
developing world as they look for a foothold in rapidly changing landscape of modern technology.
Much of the broad electromagnetic spectrum emitted from circulating electrons in a synchrotron
can be made in other and often simpler ways. To take an example: furnaces, or hot elements of electric
fires, are copious sources of infra-red radiation; while lasers have been highly developed to work in the
visible, or near-visible, range. However the photons of synchrotron radiation produced in the x-ray range
are unique and it is this part of the spectrum that is particularly important in research, as the intensity of
this radiation is orders of magnitude greater than can be obtained from any other source.
In biology and medicine there have been studies of the structure of DNA and of protein, studies of
osteoporosis, of diseased human hearts, and studies of the fatal activity of anthrax and cholera toxins as
they attack a cell. The use of synchrotron radiation in the design of drugs is of particular importance, as
is the configuration of enzymes for various industrial processes. A picture of a protein strand showing
the detail obtained with synchrotron radiation is seen in Fig. 10.1.
In environmental science and geo-sciences there have been studies of the motion of toxic and
radioactive wastes, addressing the danger of contamination of water supplies and the design of
appropriate filters for removing toxic substances from run-off. There are studies of photosynthesis in the
hope of finding alternative methods of energy capture. In quite a different domain, that of art and
archeology, synchrotron radiation is applied to the restoration and identification of works of art. (See
XV.7)
Beams of x-rays from synchrotron radiation sources can be used in physics, to reveal the
structure of matter such as the magnetic properties of the material for hard discs for storing digital
information, and the study the origins of high temperature superconductivity. Surface phenomena studies
include the wear of aircraft turbines as well as the behavior of catalysts for industrial chemical processes.
The x-rays from synchrotrons are being used by more than 100,000 researchers at about 100
dedicated facilities in something like 20 different countries. There are ever more facilities under
construction, and even more in the design and planning stage. Through the years, the intensity of x-ray
beams has grown. Between 1960 and 2000 it first increased by a factor of a billion and then by a further
factor of a million more. In fact, this rate of growth beats the previous record set by computer
performance.
Studies with synchrotron radiation span much of science. There have been 18 Nobel Prizes
awarded for work related to x-rays: 8 in chemistry, 7 in physics and 3 in medicine. Surely there will be
more.
X.2 Principles and Early History
Lightweight particles such as electrons radiate “synchrotron radiation” when travelling very close to the
velocity of light. The radiation occurs when the electrons are bent in a circular path and is proportional
to their sideways acceleration. Although the phenomenon is just like the transmission of radio signals by
electrons oscillating in a dipole aerial, the intensity of the radiation and the fact that it peaks in the
forward direction can only be explained by Einstein’s special theory of relativity.
From circular storage rings the radiation emerges in an intense and narrow cone which is
tangential to their circular path as they are bent by the magnets of the synchrotron. A very broad
spectrum of frequencies is emitted simultaneously. It can extend from the long wavelengths of red light
through blue and ultraviolet into the x-ray range. The peak of the spectrum may be shifted to shorter
wavelength by bending higher-energy electrons in stronger magnetic fields. Newer rings are designed
with “insertion devices”: wigglers that accelerate the electrons sidewise and give a very intense beam of
photons in the forward direction. See the sidebar on Wavelength, Frequency and Photon Energy to
understand the significance of these quantities.
The first step in understanding synchrotron radiation was taken in 1865 when James Clerk Maxwell
produced a set of equations that unified electricity and magnetism and, amongst other things, predicted
electromagnetic radiation. Refined versions of these equations, by one of the giants of physics in the
19th century, are still in use today. They are used to describe all classical electromagnetic phenomena,
including the emission of synchrotron light.
The next step was in 1887 when Heinrich Hertz demonstrated that, as predicted by Maxwell’s
equations, fluctuating charge densities and fluctuating electric currents produced by a spark gap generated
electro-magnetic waves that, like visible light, propagated over large distances. That, of course, was the
start of radio communication, radar, TV, cell phones, the GPS system and most of all the technical
aspects of modern life, for electricity and magnetism are at the heart of almost all modern technological
devices.
One of the missing links at that time was the realization that electric currents were really streams
of electrons. As early as 1858 “cathode rays” had been discovered; that is, rays that emerged from the
negative electrode or cathode of electric discharge tubes and were then accelerated to the positive anode.
In 1883, Hertz showed that these rays could be bent by a magnetic field, but it was not until 1897 that
J.J. Thompson definitively demonstrated that these rays were made up of electrons. It was he who really
discovered the electron, and who determined both its charge (equal to that of a proton, but of opposite
sign) and its mass (2000 times smaller than that of a proton).
The first prediction of synchrotron radiation based on Maxwell’s equations appeared in an
obscure French journal appropriately called “Lumière et Eclairage”. One might be tempted to think that
to understand synchrotron radiation from an electron undergoing acceleration would require all the
paraphernalia of modern quantum mechanics and relativity, but the basic idea is very simple. Imagine
that you were able to attach an electron to the end of a stick and wave it around. It would send out
electromagnetic waves that would be very weak but no different in nature and no more mysterious than
the radio waves emitted by electrons flowing up and down in a transmitting aerial. In order to produce a
detectable signal one would need many electrons and have to wave the stick very vigorously and
preferably close to the velocity of light! An electron coasting along in free space emits no such radiation.
It is the force applied to an electron, causing it to slow down, accelerate or be deflected sideways, which
produces the radiation. It may be a help to some readers who have met Maxwell’s equation to remember
that electromagnetic fields are produced by “changing” currents, equivalent to accelerating charges.
Now imagine an intense bunch of electrons being bent sideways by one of the many dipole
magnets in an electron synchrotron. The bunch produces radiation as it passes through the magnet. The
radiation is not of a single wavelength but rather a broad spectrum of frequencies. The same behavior
occurs in sound waves. A pure note is a sine wave going on forever but if the sine wave only lasts a short
time it is heard as a spectrum of frequencies and the width of the spectrum increases, as the pulse is
made shorter.
We now must bring in Einstein’s principle of relativity, which has a very strong effect.
According to this, as the electron approaches the velocity of light, the intensity of the radiation increases
and it is emitted in a narrow cone along the path of the beam. This narrow beam shines out on a tangent
to the synchrotron ring and may be directed onto a sample, for example a crystal.
In 1908, using these newly developed tools, G.A. Schott was able to present a definitive
theoretical study of the radiation from a single electron.
X.3 The First Observation of Synchrotron Radiation
For a long time synchrotron radiation was just a theoretical curiosity. The observation of the effect of
radiation of an accelerating electron, later to be called synchrotron radiation, did not occur until 1947,
when it was detected in a betatron. The betatron idea was invented by Wideroe - he called it the “ray
transformer” - the first practical machine was built by Donald in 1941 (as described in Chapter IV), and
after World War II a 100 MeV betatron was built at the General Electric Laboratories. This machine had
a ceramic vacuum chamber, so radiation did not escape into the outside world, but in 1946 John Blewett
observed that the rate of decrease in the radius of circulating electrons was consistent with the electrons
radiating away energy. This discovery was soon confirmed when E. McMillan visually observed
radiation on a synchrotron.
Blewett’s outstanding discovery was not given the attention it deserved. Taylor and Hulse were
awarded a Nobel Prize for a similar piece of scientific detective work when they were the first to observe
gravity wave radiation through its effect in changing of the rate of rotation of binary stars.
In 1945, McMillan and Veksler invented phase focusing (as described in Chapter V) and
betatrons were soon abandoned by high-energy physicists in favor of synchrotrons. In 1947, the world’s
second operating synchrotron (the first was only at 8 MeV) was built at General Electric. This 70 MeV
machine had a glass donut, transparent to synchrotron radiation — which to everyone’s surprise was in
the visible range. At first it had been thought to be in the radio spectrum, but no amount of crawling
round the machine brandishing what then served as a portable radio revealed the expected signal. It was
only when they took a photo of the machine in operation that they saw a bright beam of light. The whole
Board of Directors were brought to see it (through a mirror), and in the next two years there were visits
from many scientists, including 6 Nobel prize-winners.
The definitive theory of radiation from arbitrarily moving relativistic electrons was published
three years later in 1949 by Julian Schwinger (himself a Nobel laureate for his contribution to the
development of quantum electrodynamics). This work, which we rely upon today, is more elegant and
complete than that of Schott.
X.4 First Generation Synchrotron Sources
At first, synchrotron radiation was obtained from electron synchrotrons built for other purposes: mainly
for the study of high-energy physics. These machines are called First Generation Sources and prior to
1973, about 10 synchrotrons had been used in this way. They included machines in Germany, Denmark,
the Soviet Union, Japan, and Sweden. There were also four storage rings, one in France, one in the
Soviet Union, and two in the United States. Storage rings had the advantage over synchrotrons in that
they provided very stable and continuous x-ray beams.
By far, the most energetic of the first generation sources was the Cambridge Electron Accelerator
(CEA) in Massachusetts, but the machine was shut down in 1973, shifting the activity to SLAC, where a
new storage ring fed by the two mile linac was available (See sidebar on the CEA). This machine,
SPEAR, was perhaps the most effective of the first generation machines and, later, it was upgraded to
become a third-generation machine.
The first generation facilities date from the early 1970s and at first there were just a few scientists
using the radiation. It was soon realized that optimized rings could produce radiation with many special
and desirable aspects, but this step had to await the development of a user community. However,
gradually, the interesting scientific results obtained from the first generation stimulated ever more
interest and, as the community of users grew, pressure developed to build a second generation of
dedicated facilities.
X.5 Second Generation Synchrotron Sources
The distinguishing feature of second generation machines is the use of “wigglers” and
“undulators.” These devices consist of an alternating sequence of dipole (bending) magnets, which
deflect the electron beam, first to the left and then to the right. The magnetic field of the bending
magnets in a wiggler is stronger than that employed in bending the beam in the circular path of the
synchrotron. Of course it is the beam that wiggles and not the magnets. Radiation intensity depends not
only on the energy of the electron but the curvature of its path. If the radius of the orbit in the wiggler is
tight the radiation emitted greatly enhanced. They are called insertion devices because they are inserted
into the regular pattern of the focusing and bending magnets, which form the lattice of the ring, without
too much perturbation to the ideal path of the beam.
The development of these devices was pioneered at the CEA, where they were first employed,
not to give enhanced synchrotron radiation, but rather to control the motion of the stored electrons.
The first wiggler to enhance radiation was designed in Berkeley in 1978 for use at SPEAR. It
proved to be very effective and most of the dedicated second generation machines were built with very
long straight sections that would accommodate such insertion devices. Herman Winick (See sidebar for
Winick) was the person largely responsible for promoting these devices. The use of permanent magnets
for this purpose was pioneered and developed by Klaus Halbach (See sidebar for Halbach).
Undulators, the first of which was built in 1980 at SPEAR, are simply wigglers where the
deflection is not as severe but the period of the pattern of dipoles is much shorter. The radiation from a
number of bending sections interferes to produce an almost monochromatic and very intense spectrum
useful for many experiments.
At about the same time advances were made throughout the world. Two people very much
involved with advancing the science were Leonard Rifkin in Switzerland and Nikolay Vinokorov in
Russia. (See sidebars for Rivkin and Vinokorov)
X.6 Third Generation Synchrotron Sources
Given the great success of the large number of second generation facilities, enthusiasm developed to go
to even shorter wavelengths; i.e., to even more energetic (“harder”) x-rays which reveal even more
detail. This meant that the facilities had to store even higher energy electrons and thus occupied a larger
“footprint.” These facilities exploited the very latest in electron optics design and the latest insertion
devices to produce very intense x-ray beams. The first of this new generation was the European
Synchrotron Radiation Facility built at Grenoble (France), which stores 6 GeV electrons (see Fig. 10.2).
This synchrotron storage ring is higher in energy than second generation sources. A higher
energy moves the spectrum of radiation towards shorter wavelengths from UV, through VUV towards xrays. (See the sidebar on Wavelength frequency and photon energy). At the same time the total power of
the radiation increases, and to optimize the capability of the RF system to replace the electron energy
converted into radiation, the machine has a larger radius. One of the side benefits of the larger radius is
that more tangential beam lines and more users can be accommodated around the circumference, and
another one is that the electron beam size is smaller, thus the photon beam more brilliant. We see from
Figure 10.2 that the machine is on an island at the confluence of two rivers sharing the site with a
neutron diffraction facility which can complement the synchrotron radiation patters of electron
distribution with its own studies of the nuclear structure of crystals.
ESRF was so successful that it was quickly followed by other rather similar facilities: the Advanced
Photon Source (APS) at Argonne, USA with 7 GeV electrons and then Spring8 (Japan) with 8 GeV
electrons (see Fig. 10.3).
As third generation synchrotron radiation sources were designed, other smaller sources were built
such as the Advanced Light Source in Berkeley (ALS) and SPEAR III at Stanford (See Fig. 10.4). These
were based upon second generation sources that employed lower energy electrons (in the range of 1–3
GeV) but took advantage of the design improvements in the larger Third Generation Sources.
The ALS is a good example of these improved smaller sources. The machine has a circumference
of 197 m and produces electrons of energy up to 1.9 GeV. It is much smaller than the large third
generation machines but it is very powerful within the energy range available. Additional
superconducting magnets bending (“super bends”) were added to enhance the spectrum and continuous
injection was installed. It has 12 long straight sections with 11 insertion devices (wiggler magnets hat
produce very intense x-ray pulses). The x-rays are more than a million times brighter than a dental x-ray.
The machine runs 24-7 and services more than 2000 users per year from all over the world. It has as
many as 320 bunches, so it produces pulses of x-rays every 2 ns. With these it is possible to observe
rapid molecular reactions such as efficient energy conversion in combustion, catalysis and effects in
material science such as melting. The ALS has been used for biological studies and drug design, to study
dust particles (another environmental application), and for chip manufacture. It has also been used to
produce images of bones showing the effect of aging.
Another case of an aging second generation source being improved is the SESAME project (see
sidebar for SESAME in Chapter XVI and Fig. 16.1) where a source originally used in Berlin was
dismantled and shipped to Jordan where it is the focus of a truly remarkable initiative in bringing nations
together in a common venture.
X.7 Various Approaches towards a Fourth Generation Radiation Source
The path of development for the first three generations of synchrotron radiation sources was straight
forward and involved almost exclusively circular electron storage rings. There was always a push to
reduce the wavelength of the radiation to be comparable to, or if possible even smaller than, the objects
under study. Brilliance was also at a premium since it governed the intensity and definition of the
diffracted image and minimized the analysis and reconstruction time required to determine the shape of
the scattering object.
There was considerable discussion when it came to designing a fourth generation facility. Some
users wanted pulses to be very short in time in the attosecond range (10-18 seconds) in order to study the
formation and breaking of chemical bonds and intramolecular machinery such as those involved in
photosynthesis and other energy harvesting. Others wanted very well defined frequency (or energy) of
the x-rays (E/E of the order of 10-4). Some wanted the x-rays to be coherent (all march in step), some
wanted large peak power (a very intense burst). For others the peak power had to be limited (so as not to
destroy the sample irradiated) but a large average power was nevertheless required implying many
pulses per second), Some wanted high frequency X-rays (many kilovolts) others preferred much longer
wavelength (Terahertz) radiation, and so on.
In particular there had been considerable excitement when sources of an earlier generation had
revealed the detailed structure of proteins, enzymes and other “molecules of life”. Methods had been
found to persuade these large and complicated molecules to line up in crystal form. Such crystalline
arrays act like three dimensional diffraction gratings and enhance the intensity of the diffraction pattern
maxima. Each maximum becomes narrower and more intense according to the number of molecules in
the crystal. The other advantage of crystallization is that the molecules do not move or rotate within the
exposure time. This is in contrast to molecules in solution or in vivo that can rotate on a time scale of
150 ps or less. Diffraction patterns from pulses of radiation that are longer than this merely produce a
rotationally averaged shape for the molecule. However the great drawback of the crystallization
technique is that only a small fraction (of the order of 10%) of these fascinating bio-molecules may be
crystallized.
For this class of user, fourth generation sources should be able to image single molecules in their
native – non-crystalline - environment and produce pulses of synchrotron light which are short in time
compared with the rotation and diffusion time (<150 ps.) of molecules in solution. The X-ray intensity
would have to be much higher and produces diffraction patterns clear enough for molecular structure to
be reproduced even without the help of the enhancement form the diffraction grating effect of a
crystalline structure.
A number of options were studied to satisfy the various user groups. One was to further refine
third generation design to produce smaller emittance but more intense and steady x-ray beams. This was
the path taken by the NSLS II at Brookhaven. Their $800M storage ring has a circumference of 780m
and the electron energy, 3.0 GeV. It had 15 straight sections of 8m and 15 of 5 m length. Its circulating
current is 500 mA spread out over 1040 bunches, each of 2.9 mm, or 10 ps in length. X-rays in energy
up to 20 keV are produced at with a flux exceeding that of all operating sources. The Swiss Light Source
has an electron energy of 2.4 GeV and a 288m circumference. It covers the spectrum from infrared to
hard X-rays. The experimental program started in June 2001 and in 2009 had grown to 18 experimental
stations. Yet another possibility is making special accelerators or special x-ray beam lines on existing
accelerators.
Another option studied was that of a recirculating linac that employs energy recovery and
produces a low intensity, very well-defined pulse at high repetition rate but doesn’t supply the large
average power of a storage ring. Fig 10.5 shows the concept of a recirculating superconducting proposed
by Cornell. The electrons make a single pass once around the ring. They are injected at low energy,
accelerated by the RF in the SC linac and then when they come back around, they give up their energy
before being discarded. Their great advantage compared to storage rings is that ERLs can generate
pulses some 3 orders of magnitude shorter.
Still another possibility that emerged are free electron lasers (FEL) that produce coherent
radiation and intense short bunches in normal operation (the SASE mode, explained below). These can
provide a beam of very high brightness and well-defined photon energy (frequency or wavelength). (See
sidebar on FEL)
One such proposal for an FEL was at the Argonne National Laboratory., pioneered by Kwang-Je
Kim. (See sidebar for Kim) An overview of the concept is shown in Fig 10.6. The device produces
radiation of as short a wavelength as one Angstrom, with peak spectral brightness comparable to an
average spectral brightness much higher than is available from SASE. In the end the most popular
solution and one which seems now to have taken root in a number of laboratories is that of the FEL
driven by an intense electron linac. As well as producing X-ray pulses which are orders of magnitude
more intense and derive their pulse length and repetition rate from the electron pulses accelerated in a
linac. Here, one has more control of the structure than in an electron storage ring. It is even possible with
some FEL’s to produce a sequence of single pulses at 10 – 100 KHz or faster to make a “cine film” of
such chemical processes as they occur during nano-particle synthesis, and catalysis. Many chemical
bonding processes for example in photosynthesis and energy capture occur rapidly and benefit from this
technique, as do the processes that go on when molecules or viruses interact with the walls of living
cells. The same cine technique can give tomographic views of single molecules as they are dropped one
at a time into through the x-ray beam.
X.8 Free Electron Laser Facilities for Sub-nanometer X-Rays
The FEL concept was invented by John Madey (See sidebar for Madey) In an FEL, a beam of tightly
bunched electrons, accelerated in a linac, passes through a wiggler (similar to an insertion device for the
synchrotrons described above). The radiation emitted in the bends remains in phase with the bunches
and, reacting back on the bunches, causes them to bunch even more sharply, enhancing the brilliance of
the radiation. The device is called a Free Electron Laser because the effect is very similar to how the
light from a laser stimulates the emission of further light in the laser medium: light that is coherent,
adding up in amplitude so that double the amplitude produces four times the power.
The pioneering theory by Motz (1950) and Phillips (1960) as well as experimental demonstration
by Madey was based on an optical cavity with little radiation gain per electron bunch as it passed the
undulator, but mirrors for optical cavities were not available for the interesting ends of the spectrum
below the visible and into the X-ray regime. The key to reach these wavelengths was the high-gain
concept devised by Kondratenko and Saldin in 1978. They realized that, when a sufficiently bright
electron beam passes only once through an undulator which is factor of 20 longer, the gain in radiation
power is, not larger just by a factor 20 but by more than a million, due to an exponentially growing
interaction of the radiation and the electrons. The development of X-Ray FELs required large advances
in electron sources. This was achieved by a laser-driven photo-cathode. This was developed by Richard
Sheffield following a Russian invention, (See sidebar for Sheffield)
Claudio Pellegrini (See sidebar for Pellegrini) pioneered the use of an FEL for a Fourth
Generation Light source. It is effectively an amplifier for x-rays but one cannot feed x radiation into the
wiggler; there simply are no sources powerful enough. Instead, the first few wiggles are enough to
produce some x-ray “noise,” enough for the rest of the wiggler to amplify. The idea “Self Amplified
Spontaneous Emission” (SASE) was first published by Kondratenko and Saldin, and later worked out
quantitatively by Bonifacio et al. SASE was first produced in the micro wave region at Livermore in the
80s; and then, at ever shorter wavelengths, at UCLA and at ANL. While these were proof-of-principle
experiments, in 2000 it was first demonstrated at DESY for wavelengths below 100 nm where optical
resonators don’t exist. It was subsequently turned into the first FEL user facility for wavelengths down
to the few nm range, now called FLASH.
The Jefferson Laboratory is a leader in the development of FELs with a very high average power.
High peak power is not unusual for an FEL, but high average power required the superconducting
technology that Jefferson Laboratory had pioneered for CEBAF. In 1998 they exceeded previous records
for FELs by a factor of 30 and soon, after considerable effort, they had reached 14 kW. For this they
used a recirculating configuration containing, for the first time, an energy recovering linac. We describe
in the chapter on National Security that efforts are underway to reach an even higher power and it has
been decided to move this facility to LANL (who will collaborate with Boeing) where the initial goal is
100 kW.
There are already two successful FEL-driven fourth-generation sources running which produce
10 keV X-rays. One is the LCLS at SLAC which uses part of the SLAC two-mile long electron linac to
produce the electron beam (See Fig. 10.7). The other is at Spring8 The LCLS employs SASE and
12
provides x-ray pulses of 10 photons of wavelength 0.15 to 1.5 nm, with a pulse rate of 120 Hz and a
pulse duration of 230 femtoseconds down to 20 fs (they may be even shorter, but they are unable to
measure below this very brief interval of time). The radiation is transversely coherent. The average
brightness of the x-ray beam beats the present third generation light sources by a factor between 1,000
and 10,000 and is some 1022 (photons/(s, mm2, mrad2, 0.1%BW). The real sensation is however the
peak brightness available during the ultra-short radiation pulse: With its 1032 value it represents an
increase by 8 orders of magnitude and opens thus an entirely new world of science and applications. Its
construction, at a cost of 420M$, started in 2007, was led by John Galayda (See Sidebar for Galayda),
and was completed in only three years. The machine was then arguably the most advanced accelerator
project in the world. Its challenges included the need to compress the intense electron bunch from 0.8
mm down to 0.023 mm to reach a peak current of 1.2 kA, overcoming both space charge effects and
coherent radiation effects, and the construction of a high precision 122 m wiggler. Compressing the
beam required the novel use of a laser (called a laser heater) which increased the energy spread in the
beam so as to overcome the coherent radiation effect.
The Sping8 FEL came to life in 2011 and produces X-rays down to 0.15 nm. Its special feature is
a very short wavelength wiggler which permits operation at 8 GeV compared with the 15 GeV of the
LCLS (See sidebar for Shintake).
Another FEL facility that ran in 2011 is the FermiFEL in Trieste. This operates at longer
wavelengths than the LCLS.
The LCLS II project (Figure 10.8) will be an addition to LCLS. At the time of writing it has
been authorized at a cost of M$ 400. The basic idea is to use the second third of the SLAC 3 km linac to
provide a beam of lower energy than the top energy at LCLS. This, combined with the proper choice of
FEL, broadens the wavelength range of the x-rays. Clever methods are employed to use the FEL to
amplify injected beams of x-rays rather than rely on SASE which starts by amplifying noise.
The injected x-rays come from techniques known as self-seeding and external “echo-enabled”
seeding, a scheme, invented at SLAC. In the SASE mode of FEL operation the initial bunching of the
electron beam, due to noise on the beam, is amplified by the FEL as energy is transferred from the
electron beam to the wave. However the bandwidth of the amplifier is very wide and it is therefore not
longitudinally coherent. For many experiments coherence is required and thus great effort has gone into
devise a system that would have that feature.
Longitudinal coherence would result if one provided a seed that was the output of a laser at the
correct frequency, but unfortunately power sources in the hard X-ray region do not exist. The generation
of high harmonics (HHG) from a conventional laser is ever-improving, and at the present time extends
into the VUV. It has been employed, at the FERMI FEL to seed the FEL.
As energy is taken from the electron beam to generate an electromagnetic wave an FEL will
bunch the electron beam into packets that are even shorter than the fundamental wavelength. Thus an
FEL generates harmonics and these are again longitudinally coherent if the original seed is. Thus it is
possible to seed with HHG, and then generate harmonics. That can even be done twice (HGHG in Figure
10.9). In this approach the intensity of the harmonic decreases rapidly with increased harmonic number.
The dispersive chicanes translate the energy modulation that occurs in the electron beam at the end of
each wiggler into time modulation that enhances the higher harmonic content of the beam. An alternative
is the “echo scheme” (EEHG) in which the wave from a first wiggler, which already contains some of
the higher harmonic, is passed on to a second wiggler which also adds harmonics. The two successive
stages mix their harmonics together shifting the harmonic content of both the electron beam and with it
the X-ray beam, to much shorter wavelengths. The net effect of these schemes is to produce rather
intense beam bunches at the frequency which is a high harmonic of the initial laser frequency but in the
echo scheme the decrease of intensity with harmonic number is very much reduced compared to the
previous ones. The scheme has experimentally so far been shown to work for a few harmonics, but many
more will be needed for LCLS II (and the NGLS described below). The scheme is shown in Fig 10.9.
The schemes we have described produce beams with coherence in the longitudinal direction (and, of
course, also in the transverse direction). Polarization control is also possible.
Two-color variable delay schemes are proposed for LCLS II which will enable one x-ray
wavelength to excite a molecular energy level while a second wavelength produces the diffraction
pattern.
A device somewhat similar to LCLS is the European XFEL. It has the special feature that it is
based on a superconducting linac giving almost continuous bursts which deliver maximum average flux
when flux per pulse is limited for fear of the damage to the sample. The superconducting linac was
originally intended as DESY’s contribution to the development of the International Linear collider.
Prior to starting construction of the European XFEL (which will be 2.1 km long), DESY had
already constructed a VUV FEL — now named FLASH — operating in the vacuum ultra violet.
FLASH is really just a 260 meter version of the XFEL and until 2008 it was the only FEL operating in
the soft-x-ray range. This device uses electrons of 1.25 GeV and has a wiggler of 30 m length. (See Fig.
10.10)
The XFEL will accelerate electrons from 10 to 20 GeV into an undulator of 150 m length, and
produces hard x-rays from 0.085 to 6 nanometers in pulses as brief as 100 femtoseconds. Because it
employs a new superconducting linac, the cost is much higher (in the $1 billion range) than that of the
Linac Coherent Light Source (LCLS) (which uses the last 1/3 of the SLAC linac), but it will produce
30,000 pulses a second (unlike all the other FELs which employ a room temperature linac and have
pulse rates in the 300 Hz range). However detector development is required in order to take advantage of
this high pulse rate.
A list of future proposed angstrom wavelength FEL sources also includes the Shanghai FEL, the
Swiss FEL and the Pohang FEL. These are FELs which are driven by electron beams of more modest
energy and producing X-rays above 1 kV.
Another proposal different to the LCLS and the other XFEL projects above, is the NovoFEL at
Novosibirsk which claims it will be the most intense radiation source in the terahertz spectral range. The
terahertz spectrum though very different in wavelength from that produced by other X-ray FEL
proposals is used in backscatter scanners for whole body scans now being introduced at airports.
Tethers radiation unlike normal X-rays causes no ionization but can be used for non-destructive studies
of some biochemical processes such as the strand division of DNA. The high power available from
NovoFEL can be used for machining metal and for ablating stone. A schematic is shown in Fig10.11. It
operates in the continuous mode with a pulse repetition rate of up to 11.2 MHz and an average power of
up to 500 W in the range of 50 to 240 microns. The radiation wavelength can be precisely tuned from
120 to 240 mm with a relative line width of 0.3–1%, which corresponds to a pulse length of 40–100 ps.
The facility has beam energy of 11 MeV, a current of 30 mA and a bunch repetition rate of 22.5 MHz.
Following on from the LCLS and XFEL the Lawrence Berkeley group has now proposed a Next
Generation Light Source (NSLS) based upon the interesting possibility of a more modest facility than
the LCLS, but one with very short, femtoseconds, even 100 attosecond pulses of intense x-rays —
shorter than the timescale for the dynamic processes involved in the study of the dynamics of chemical
reactions or melting of materials. The large facilities can also do that. However a smaller facility
achieving the same short pulses, but at a longer x-ray wavelength, has interested potential users and
more favorable economics. An overview is shown in Fig 10.12. Such a device is based upon a super
conducting linac producing 1.8 GeV electrons, rather than a synchrotron, so as to obtain very intense
short pulses. The device has three FEL lines in stage one, one SASE, one two-color, and one with a laser
harmonic as the feed (also using the echo scheme). The beam lines are very similar to those in LCLS II,
but because of the SC linac (rather than a room temperature linac) the pulse repetition rate is in the MHz
range (rather than a few hundred Hz). Recent work has been focused upon a two-section undulator and a
careful study of performance in the fundamental and the third and fifth harmonic as a function of the
many variables available. It is likely that the energy will be chosen higher (not more than 20%) than the
first design of 1.8 GeV. A similar high repetition rate is also possible with the EU_FEL since that also
has a SC linac