What is a Synchrotron Anyway?

1. What is a Synchrotron?
   1. Introduction
   2. How does the CLS synchrotron work?
   3. Synchrotron Light
   4. Salute to Safety!
   5. Engineering Highlights

You can download a pdf version of the information here.

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Introduction

A synchrotron is a source of brilliant light that scientists can use to gather information about the structural and chemical properties of materials at the molecular level.

A synchrotron produces the light by using powerful electro-magnets and radio frequency waves to accelerate electrons to nearly the speed of light. Energy is added to the electrons as they accelerate so that, when the magnets alter their course, they naturally emit a very brilliant, highly focused light. Different spectra of light, such as Infrared, Ultraviolet, and X-rays, are directed down beamlines where researchers choose the desired wavelength to study their samples. The researchers observe the interaction between the light and the matter in their sample at the endstations (small laboratories).

This tool can be used to probe the matter and analyze a host of physical, chemical, geological, and biological processes. Information obtained by scientists can be used to help design new drugs, examine the structure of surfaces to develop more effective motor oils, build smaller, more powerful computer chips, develop new materials for safer medical implants, and help with clean-up of mining wastes, to name just a few applications.

Quick Facts:

• More than 40 synchrotron light sources have been built around the world. The Canadian synchrotron is competitive with the brightest facilities in Japan, the U.S. and Europe.

• As of 2009, more than 2000 scientists have used the CLS.

• More than 3,000 academic, industrial, and government researchers a year from across Canada and from other countries are expected to use the facility once the full complement of beamlines is developed. Beamlines carry the synchrotron light to scientific work stations capable of operating 24 hours per day, 7 days per week, approximately 42 weeks of the year.

• Initially, the CLS will focus on research in three key areas:

• mining, natural resources and the environment

• advanced materials, information technologies and micro systems

• biotechnology, pharmaceuticals and medicine

• The first synchrotrons were additions to facilities built to study subatomic physics. Synchrotron light was an annoyance to the researchers because it meant their electron beams lost energy every time they went through a bending magnet. However, the remarkable qualities of this light were soon recognized, and researchers began to come up with ways to use it.

Currently, CLSI has more than 130 employees. The work force of scientists, engineers, technicians, and administrators is growing to match additional CLSI users. Located in the midst of a research cluster on the north end of the University of Saskatchewan, next to Innovation Place, one of Canada’s leading high-tech industrial parks, CLSI strengthens Saskatoon’s reputation as “Science City” as a much-needed national R&D facility.

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How does a 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 enough energy 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.

Electron gun in CLS basement.

The source of the electrons, the cathode, is a tungsten-oxide disk (tungsten is the same material as light bulb filaments). As electricity flows through the disk, it will heat it until electrons are emitted (about 1000 °C). 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 similar to that found in a television picture tube.

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 are travelling 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 water waves. The LINAC produces pulses of electrons from 2 nanoseconds up to 140 ns for injection into 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 LINAC is 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 vacuum system 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 flashlight batteries!) 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. The typical atmospheric molecule has an energy of about 0.03 eV.

The electrons travel around the 103 m ring approximately 1.5 million times in .6 tenths of a second. Each of 68 bunches contains 50 pC (3.1 x 10 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 electrons travel at about 99.999998%  of light speed (4.7 m/s slower than the speed of light).

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 electrons into 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 the storage ring is to replace the energy lost by the electrons to light production. Superconductivity is the flow of electric current without resistance in certain metals and alloys at temperatures near absolute zero. The operating temperature is -270°C (-273°C is 0 K or absolute zero). Operating at such cold temperatures eliminates most of the power loss, while the RF field provides energy.

3. Storage Ring

When the electrons reach 2900 MeV, an injection system transfers them from the booster ring to
the 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.

Storage ring straight sections.

Once in the storage ring, the electrons will circulate for four to twelve hours producing photons every time the 6800 kg dipole magnets change the direction of the flow of electrons. While the ring looks circular, it is really a series of 12 straight sections each with 2 dipole magnets, and a series of quadrupole and sextupole magnets to narrow the beam.

Some straight sections also include space for special magnets called Insertion Devices. After each 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 the vacuum 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 maintain the necessary current.

Insertion Devices:

Superconducting wiggler for Biomedical Imaging & Therapy Beamline

The CLS is one of the brightest synchrotrons in the world despite the fact that it is roughly 1/10 the size of the other bright synchrotrons. One of the ways that we achieve this is through insertion 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 more intense light.

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. Beamlines Monochromator

One of the important pieces of equipment synchrotron light passes through on its way to the sample 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 light that isn’t required. Each of the beamlines at CLS is unique and will have markedly different monochromators specific to their design.

 

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. In general, 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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Synchrotron Light:

A synchrotron is certainly not the only way to generate IR, UV, or X-Ray light and so many other techniques can also be used at other types of facilities. There are four general advantages to using synchrotron generated light for these techniques and there are some techniques that can only be successful using synchrotron light.

Brightness or Flux

If you were to expose a 1mm² sample, similar to what a researcher might put under a regular light microscope, to a number of different light sources and measure the amount of energy the matter in that sample interacted with, you would find that the energy generated by a synchrotron using Insertion Devices is considerably higher than what is produced by other light sources. Generally speaking, synchrotron sources of light pack more photons into a smaller beam of light. This offers researchers more information about their sample and a greater variety of techniques to use to learn about their sample.

In situ experiments

Another advantage to some synchrotron techniques is the ability to conduct the experiments in situ, or as they are – without treatment. There are a number of techniques that have been used in research that require the scientist to treat their sample (crush it; make a solution; slice it; etc). While this is also required for some synchrotron techniques, there are also some that allow for the sample to be analyzed without treatment or with less treatment, which can be a significant advantage.

Tunability or ability to select specific light

By producing high flux light across a significant portion of the spectrum, a synchrotron offers many different techniques to researchers in one building. In order to gather information, the wavelength of the light has to be appropriate for the size of the matter of interest. Shorter wavelengths allow scientists to gather information about smaller things.

In addition, each element absorbs energy at a known level. Thus, being able to select a specific wavelength, or range of wavelengths, allows researchers the flexibility to direct their research towards specific questions.
 

Speed

Due to the extreme brightness of the light, it does not take as long to conduct the same experiment using a synchrotron source of light as it does with a ‘table top’ source for some techniques.

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Salute to safety!

CLSI is committed to provide a safe and healthful working environment for all staff and to protect the general public and the environment from unacceptable risks.

Control Room:

Control-room management is an important part of ensuring personnel safety throughout the facility.

From the Control Room we monitor

• Personnel Safety Systems (PSS) - includes Fire Alarms System, Accelerator Access Interlock Systems, Oxygen Monitoring System
• Equipment Protection Systems (EPS) – includes beam position, size and quality, cooling/heating systems, vacuum systems, power supplies and magnet settings, timing systems, and valve controls
• Accelerator operators disable/enable all or individual beamlines including start and shutdown processes and select mode of accelerator operation such as normal (beam available to users) versus accelerator studies

In the case of an emergency the synchrotron can be shut down automatically in less than 20 milliseconds from the control room and from several other locations manually.

Radiation Safety

When dealing with sources of energy, there are safety concerns that must be addressed. Radiation is energy that comes from a source and travels through some material or through space. Light, heat and sound are types of radiation. There are many natural sources of radiation including the sun and various elements in the earth. Since CLS is a source of light radiation, thermo-luminescent detectors (TLDs) are used to record any possible radiation that escapes the shielding surrounding the rings and end stations. These are located in various locations around the facility as well as carried by personnel. The national limit from natural sources (background radiation) is 3 milli-Sieverts (mSv) per year. CLS measures very little above what is detectable in the background and is well within the annual regulatory limit imposed by the CNSC (Canadian Nuclear Safety Commission) which is 50 mSv - equivalent to approximately 500 chest X-rays per year.

Experiments and Safety

CLS will authorize an experiment to be conducted only after the activities associated with the experiment have been defined, hazards have been identified, and adequate hazard controls have been implemented. Once the proposals have met all applicable requirements, a permit is issued identifying engineering, and administrative controls and training requirements.

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Engineering Highlights

The 84m x 83 m facility won the Canadian Council of Professional Engineers’ 2002 award for “exceptional engineering achievement.” The six-storey building required 1,300 tons of steel and enough concrete to build 160 1,200-square-foot homes.

A major challenge was to ensure stability of the light beams that travel along 30 m beamlines to the user end stations. There’s a risk that vibrations – from traffic, wind, crane, mechanical pumps, etc. – will affect the beam and invalidate data in scientific experiments. To address this, the facility is built on more than 700 piles, each 10 – 20 m deep and the floor is vibrationally isolated from the walls. Temperature must also be controlled. The booster and storage rings can only vary from 23°C by 0.1°. The beamline hall has the air circulate every seven minutes in order to keep its temperature within a degree of 23°C.

Accomplishments:

• Equipment Alignment: absolute position to 150 µm with 3σ confidence level over a diameter of approximately 50 m

• Vibration Stability: no vibrational noise above 1µ amplitude at less than 300 Hz.

• Ultra-High Vacuum Systems: ultimate chamber pressure less than 10-11 torr.

• Motion Requirements: sub-micron and sub-micro radian positioning of optical components.

• Controls System: twice as many control points as a Candu reactor.

• DC Magnet Power Supplies: stability better than 10 ppm with 24 bit resolution. Maximum magnet power supply output is 700V and 600A.

• Cryogenics: 280W at 4.5K liquid helium (LHe) liquefier/refrigerator and LHe distribution system.

• Superconducting RF cavity: the first light source in the world to use a superconducting RF cavity to power the electron beam from day one.

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Last modified: 2009-10-08 16:10:25