1.2 how does the cls synch rot ron work
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8/7/2019 1.2 How Does the CLS Synch Rot Ron Work
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How does the CLS synchrotron work?
1. Electron Gun and Linear Accelerator
Electron Gun:
The process begins in the basement where high voltage electricity through a heated cathode
produces pulses of electrons. Heating the cathode to incandescence gives some electrons enoughenergy to leave the surface (essentially boils them off). The high voltage (approximately 200,000
volts – a car battery has only 12 volts!) repels the electrons, accelerating them toward the Linear
Accelerator or LINAC.
The source of the electrons, the cathode, isa tungsten-oxide disk (tungsten is thesame material as light bulb filaments). As
electricity flows through the disk, it will
heat it until electrons are emitted (about
1000oC). A screen nearby is given a
short, strong positive charge (125 times
per second) which pulls the electrons
away from the disk. The system is similarto that found in a television picture tube.
Electron gun in CLS basement.
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LINAC:
The electron gun supplies electrons to the Linear Accelerator (LINAC). Microwave radio
frequency fields in the 2856 megahertz LINAC provide energy to the electrons that are
accelerated to an energy of 250 million electron volts or MeV. At this energy the electrons aretravelling at 99.9998% of the speed of light (3.0 x 108 m/s).
The electrons are pushed by the microwaves much the same way a surfer is pushed by waterwaves. The LINAC produces pulses of electrons from 2 nanoseconds up to 140 ns for injectioninto the storage ring. The short pulses can be used to fill a single "bunch" in the storage ring for
use in time-sensitive measurement studies. The long pulses are used to produce a (3x140=) 420
ns pulse train in the storage ring. Electrons are supplied once per second by the LINAC. After
several minutes of operation sufficient current is accumulated in the storage ring and the LINACis turned off until it is required to refill the ring several hours later.
Vacuum Chambers:
The electrons (and later the photons) must travel in a vacuum to avoid colliding into atoms or
molecules and disappearing. The ultimate vacuum chamber pressure is lower than 10-11 torr (1
atm. Pressure is 760 torr). This means that there are fewer molecules present in our vacuumsystem than there are in space around the International Space Station.
2. Booster Ring:
In particle physics, the standard unit to measure energy is MeV or million electron volts. One eV(electron Volt) is the amount of energy that an electron gains when it moves through a potential
difference of 1 Volt (in a vacuum). As they circulate, electrons receive a boost in energy from
250 million electron volts (MeV) to 2900 MeV (energy equivalent to about 2 billion flashlightbatteries!) from microwave fields generated in the Radio Frequency Cavity at 2856 MHz. For
comparison, the energy of charged particles in a nuclear explosion range from 0.3 to 3 MeV. Thetypical atmospheric molecule has an energy of about 0.03 eV.
The electrons travel around the 103m ring approximately 1.5 million
times in .6 tenths of a second. Each
of 68 bunches contains 50 pC (3.1 x10 8 electrons) with a total energy of
9.92 J at 2900 MeV and 10 mA
circulating current.
The Booster Ring cannot increase the
speed of the electrons to, or beyond,
the speed of light, but the electronstravel at about 99.999998% of light
speed (4.7 m/s slower than the speed
of light).
Magnets in the booster ring.
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Magnets:
There are two types of electro-magnets in the booster ring. The blue dipole magnets weigh over
3000 kg. The magnetic field created by the magnets is used to direct the electrons around the
booster ring. The field of the green quadrupole magnets is used to force the bunches of electronsinto a fine beam within the vacuum chamber.
Radio Frequency (RF) Cavities:
There are two cavities that use microwaves to boost the energy carried by the electrons. A
cylindrical cavity in the booster ring delivers a high energy kick to the electron bunches during
each turn around the ring. It
operates with an RF frequency of 500 MHz.
The purpose of the cavity in thestorage ring is to replace the energy
lost by the electrons to light
production. Superconductivity is the
flow of electric current withoutresistance in certain metals and
alloys at temperatures near absolutezero. The operating temperature is -
2700C (-2730C is 0 K or absolute
zero). Operating at such cold
temperatures eliminates most of thepower loss, while the RF field
provides energy. Superconducting RF cavity in the storage ring.
3. Storage Ring:
When the electrons reach 2900 MeV, an injection system transfers them from the booster ring tothe 171m storage ring, where they will circulate for four to twelve hours producing photons of light at every turn. The process repeats once per second up to 600 cycles (about 10 minutes), as
required, to reach an average circulating current of 500 mA.
Once in the storage ring, theelectrons will circulate for four to twelve
hours producing photons every time the
6800 kg dipole magnets change thedirection of the flow of electrons. While
the ring looks circular, it is really a series
of 12 straight sections each with 2 dipolemagnets, and a series of
quadrupole and sextupole magnets to
narrow the beam.
Storage ring straight sections.
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Some straight sections also include space for special magnets called Insertion Devices. Aftereach turn there is a photon port to allow the light to travel down the beamlines.
Over time, the number of electrons stored in the ring will decline. This is inevitable because thevacuum isn’t perfect. Electrons collide with the few particles that are present and are lost. As a
result, CLS must either empty the ring and re-inject electrons, or add more electrons to maintainthe necessary current.
Insertion Devices:
The CLS is one of the brightest
synchrotrons in the world despite thefact that it is roughly 1/10 the size of
the other bright synchrotrons. One of
the ways that we achieve this is throughinsertion devices. While dipole
magnets change the direction of the
electrons, thus producing light, multi-magnet insertion devices called
undulators and wigglers move the
electrons back and forth many times
creating a narrow beam of much moreintense light.
Superconducting wiggler for
Biomedical Imaging & Therapy
Beamline
A wiggler or undulator consists of a periodic series of magnets, placed in a ring section where
the electron path would otherwise be straight. The magnetic fields force the electrons to ‘wiggle’around the straight path. The result is a very high flux of photons along the beamline. A wiggler
produces a wide range of high energy X-rays. An undulator produces even higher intensity X-
rays with a narrower range of energies.
4.
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One of the important pieces of equipment synchrotron light passes through on its way to thesample is a monochromator. Researchers use the monochromator to choose the wavelength of
light best suited to the experiment they are conducting. The monochromator is the device that
separates the wavelengths (much like a prism). This is done using either the phenomenon of optical dispersion (as in a prism), or of diffraction using a grating which spatially separates the
wavelengths of light and filters out the lightthat isn’t required. Each of the beamlines atCLS is unique and will have markedly
different monochromators specific to their
design.
Double crystal monochromator selecting X-
Rays for the CMCF beamline.
Endstations
The selected wavelengths of synchrotron
light are then focused by the mirrors onto the sample in the experimental endstation. Each
endstation is designed specifically for the types of experiments conducted on that beamline. Ingeneral, each one consists of a sample holder and a detection system, unique to the technique
employed by the scientist, as well as a bank of computers through which the researchers control
the mechanisms involved in the experiments and view the data as it is recorded.