1.2 How Does the CLS Synch Rot Ron Work

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.



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.



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.



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.

Beamli

nes

Monoc

hroma

tor



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.

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1.2 How Does the CLS Synch Rot Ron Work