Harnessing the Light of a Millon Suns
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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KEY COMPONENTS in Canada's new synchrotron, are stainless steel vacuum chambers, such as this. Electrons
are produced and stored in the vacuum chambers before they enter a linear accelerator.
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THIS IS THE TYPE of image that scientists will be able to see once the synchrotron is fully operational.
Pictured here is a living algae cell.
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THE BEAM OF ELECTRONS enters a booster ring 100 metres in circumference, fabricated from S30403 stainless
steel. Elliptical in cross section, this metal tube is 82.8 millimetres (mm) wide and 32 mm high, with a wall
thickness of 1.6 mm.
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HOW THE SYNCHROTRON WORKS is explained in this diagram. Click on the icon below to see a larger
view:
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Stainless steel contributes to the realization of the perfect vacuum. By Carroll McCormick
Nickel Magazine, March 2005 -- Microscopes need light, and one of the most powerful sources of
light in the world recently became operational.
Housed at the University of Saskatchewan, Canada, the synchrotron, as it is called, produces electrons that
give off light millions of times brighter than the Earth’s sun. Researchers use the light for various design
and manufacturing projects.
The synchrotron is no desktop microscope: its complex system of ultra-high vacuum chambers occupies 6,800 square metres in a 12,700-square-metre Canadian Light Source (CLS) facility on campus.
Stainless steel, of which S30400, S30403 and S31603 are the most common types, is used extensively in the vacuum chambers, as well as in such specialised applications as the K93600 supports that provide dimensional stability in some of the optic systems.
The need to achieve and maintain a vacuum of 10 [superscript]ֿ11 Torr (a million, million times less than atmospheric pressure at sea level) determines the choice of materials, and the target is challenging: the CLS began operating in October 2004, but it will be a full year before an ultra-high vacuum target can be achieved.
Achieving a vacuum requires the removal of as many molecules as possible. Impurities not only slow the electron beam; they diffract the electrons, much like fog scatters the beam from a car’s headlights. Some synchrotrons have been made of copper or aluminum, but stainless steel is more routine from a fabrication point of view, says Mark de Jong, CLS’s director of operations.
The vacuum chamber components must be cooked in huge bake ovens for as long as 40 hours at temperatures as high as 250ºC. Aluminum begins to lose its strength at 150ºC, but stainless steel does not -- a critical attribute considering that the components are baked under vacuum. "Stainless doesn’t lose strength at the typical pressures of our bake-out," confirms Mark de Jong.
Baking expels gases absorbed during manufacturing, such as water vapour, argon, oxygen, helium, nitrogen, hydrogen and carbon monoxide. Also, the metal components are washed as part of the degreasing process. Says Mark de Jong: "We want to avoid having any hydrocarbons inside. An absolute no-no is sulfur-based cutting oil, which can remain for an eternity."
Ontario, Canada-based Johnsen Ultravac uses S30400 in some of the vacuum chambers it manufacturers. The cost of S30400 is low, compared with other metals. It is also easy to machine and weld, and sufficiently hard that it can cut into the copper gaskets. The synchrotron’s many fittings, flanges, ion pumps and valves are always stainless steel, so mating them to like-metals simplifies the engineering.
How the synchrotron works
The CLS produces light at wavelengths ranging from infra-red to X-ray. One researcher might need to use light of a wavelength suitable for illuminating specimens at 100 nanometer spatial resolution. Another will want light that interacts with samples, say, for determining the chemistry of environmental samples in a sample size of 2 by 2 microns. Yet another will use X-ray lithography to etch circuit boards or cut micron-size gear pitches.
The electrons are created and stored in a series of vacuum chambers. The first, an electron gun (essentially a huge capacitor) produces the electrons and sends them into a linear accelerator. It accelerates them to 250 million electron volts and 99.9% of the speed of light.
The electron beam then enters a booster ring (BR) 100 metres in circumference, fabricated from S30403. Elliptical in cross section, it is 82.8 millimetres (mm) wide and 32 mm high, with a wall thickness of 1.6 mm. The BR is made by gently rolling pipe with a 60.3-mm diameter until the correct shape is reached. Powerful radio waves ramp up the energy of the electron beam circulating inside BR to 2.9 billion electron volts, and accelerate it to 99.99999% of the speed of light.
The electron beam is then directed into a storage ring (SR) 171 metres in circumference. Also elliptical in cross section, it is just 100 mm wide and 30 mm high, with a wall thickness of 3 mm. It is bent and formed from sheet S31603, with a TIG-welded seam along its length.
Positioned every few metres along the BR and the SR are clusters of electromagnets, which focus the electron beam and keep it in the centre of the rings. Vacuum pumps (28 along the BR and 70 along the SR) constantly remove unwanted molecules from the rings. Each has an S30400 cylinder with a wall thickness of 2 - 3 mm, which attaches to the ring.
A half-kilometre of 20.32-centimetre stainless pipe, and another half-kilometre of 7.62-centimetre stainless pipe carry de-ionized water, which cools the electromagnets and X-ray-absorbing copper blocks in the stainless steel pump cylinders. S31600 and S31603 were chosen because the corrosive de-ionized water would dissolve carbon steel.
When the target vacuum purity is eventually reached, the electron beam will be able to circulate in the storage ring for eight to 10 hours before the operators need to produce a fresh batch of electrons.
The final step entails collecting the photons emitted from the electron beam. The photons fly off the beam in the SR at a tangent, through gaps in the SR and into straight pipes called "beam lines," of which the CLS has eight so far, with capacity for 30 in total. Each beam line also includes optic-filled vacuum chambers, which select specific wavelengths of light and send them on the final leg of their journey to research stations.
The CLS will draw thousands of researchers from around the world for hundreds of uses, from understanding how proteins function, to medical imaging of tumors, to geochemical research.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Canadin Light Source Inc.
Mark De Jong |
