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An Introduction to Synchrotron Radiation Techniques and Applications by Philip Willmott (z-lib org)

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calculations predicting the rate of energy loss due to radiating electrons [15].
Initially, synchrotron radiation was seen as an unwanted but unavoidable loss of energy
in accelerators designed (ironically) to produce intense beams of x-rays by directing
accelerated electrons onto a suitable target. The potential advantages of synchrotron
12 An Introduction to Synchrotron Radiation
Figure 1.12 Reproduction of Figures 11 and 12 in the Physical Review paper by Tomboulian
and Hartmann [16], showing the experimental setup and photographic plate of the spectra
produced by synchrotron radiation from monoenergetic electrons. Reprinted from [16] with
permission of the American Physical Society.
radiation for its own end were detailed by Diran Tomboulian and Paul Hartman from
Cornell University in their seminal paper ‘Spectral and Angular Distribution of Ultra-
violet Radiation from the 300 MeV Cornell Synchrotron’ in the Physical Review [16],
describing their findings after being granted two-week’s access there to investigate the
possible applications of synchrotron light. The experimental setup and photographic-plate
results reported in this paper, showing the broadband synchrotron radiation emitted by
monoenergetic electrons, are reproduced in Figure 1.12.
Five years later, in 1961, a pilot experimental programme exploiting synchrotron
radiation began when the National Bureau of Standards modified its 180-MeV electron
synchrotron in Washington, D. C. to allow access to the radiation via a tangent section
into the machine’s vacuum system. Thus was born the Synchrotron Ultraviolet Radiation
Facility (SURF I), the first facility catering for regular users of synchrotron radiation.
The first generation of synchrotron radiation sources were sometimes referred to as par-
asitic facilities (reflecting, perhaps, the perception of the primary users of these facilities
towards these interlopers), as the synchrotrons were primarily designed for high-energy
or nuclear physics experiments. In addition to SURF I, facilities in Frascati and in Japan
soon after began to attract a regular stream of physicists keen to explore the possibilities
of synchrotron radiation. More would soon follow suit. Because most of these early facil-
ities had storage-ring energies around or below 1 GeV, experiments were concentrated
in the ultraviolet and soft x-ray regimes. 1964 saw the first users of synchrotron radiation
from the 6 GeV Deutsches Elektronen-Synchrotron (DESY) in Hamburg – suddenly the
range of synchrotron radiation was extended to the hard x-ray region down to 0.1 Å
(about 125 keV). Many further facilities began to accommodate synchrotron-radiation
users. One of the first of these was the 240 MeV machine ‘Tantalus’ in Wisconsin.
Although not originally designed to provide synchrotron radiation, it became the first
Introduction 13
facility to be exclusively used for synchrotron experiments, providing radiation in the
ultraviolet up to a few tens of eV.
After the development of efficient electron storage rings for long-term operation, the
time was ripe to develop the first dedicated facilities designed specifically for synchrotron
radiation. The 2 GeV Synchrotron Radiation Source (SRS) at Daresbury, England, was
the first of these so-called ‘second-generation’ synchrotron sources. Experiments began
in 1981. Several more new facilities were soon built and commissioned, while some
first-generation sources were upgraded to second-generation status.
At approximately the same time it was becoming clear that increased x-ray beam
brilliance could be achieved by optimizing the property of the electron beam in the
storage ring called the ‘emittance’, described in detail in Chapter 3, by careful design
of the array of magnets (the ‘magnet lattice’) used to manipulate the electrons, and by
employing so-called ‘insertion devices’. These include ‘wigglers’ and ‘undulators’ and
are placed in straight sections in between the curved arcs of large storage rings. They
operate by perturbing the path of the electrons in an oscillatory manner, so that although
their average direction remains unchanged, synchrotron radiation is produced. Details
of undulators and wigglers are covered in Chapter 3.
The inclusion of insertion devices in storage rings defines the third generation of
synchrotron sources, which are designed for optimum brilliance, that is, the amount of
power per unit frequency, surface area and solid angle. The first third-generation facility
to be completed was the European Synchrotron Radiation Facility (ESRF, 6 GeV storage
ring) in Grenoble, France, which began experiments in 1994. At the time of writing,
the ESRF is undergoing a facility-wide upgrade which promises an enhancement of
performance by approximately an order of magnitude.
The most recent third-generation facilities are the National Synchrotron Light Source
II (NSLS II) at the Brookhaven National Laboratories in Upton, New York and MAX IV
in Lund, Sweden. Construction of NSLS II began in 2009, with MAX IV about one year
behind. The projected specifications for these medium-energy storage rings will redefine
the benchmark for synchrotrons – it is planned to provide spatial and energy resolutions
of the order of 1 nm and 0.1 meV, respectively, while the brilliance and emittance, at 1021
photons/mm2/mrad2/0.1% bandwidth and 0.5 nm rad, improve on previous performances
by an order of magnitude. These significant advances are largely made possible by recent
advances in the design of storage-ring magnets, but also (in the case of NSLS II) by the
fact that although the storage-ring energy is, at 3 GeV, fairly moderate, its circumference
of 800 m is more typical for high-energy facilities.
Beyond this, a fourth generation of synchrotron radiation facilities is coming of age in
the first decade of the twenty-first century, which will be defined by a greatly improved
performance, especially with regards to the coherence and brilliance of the x-rays, using
so-called energy recovery linacs (ERLs), and free electron lasers (FELs). There is, respec-
tively, a further 3 and 10 orders of magnitude in peak brilliance to be gained using ERLs
and FELs, and facilities such as the Linac Coherent Light Source (LCLS) at Stanford,
and the FLASH and European XFEL facilities at Hamburg, are presently striving to
achieve this goal. The first hard x-rays from an FEL were produced at the LCLS in
April 2009.
14 An Introduction to Synchrotron Radiation
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