Before this mini-tour, some good links to keep in mind are as follows: The principle source of information on the ALS is their web page: www-als.lbl.gov. A very nice web page that is very similar to this one is "ALS Components", and it is part of the award winning MicroWorlds site. For many of the scientific terms and ideas, please see HyperPhysics project, which is a remarkable endeavor. This is one site out of hundreds on the web that gives useful information about the world around us. A quick search is bound to find many more.

But before we see why we need such a large facility, we should introduce some background material; something should be said about the units we use to describe things at the ALS. Typically, in the accelerator community, we don't talk much about a particles speed - when particles are going nearly the speed of light, they don't so much seem to get faster, as they seem to get heavier, a consequence of the theory of relativity. Because of this, we prefer to talk about a particle's kinetic energy, which includes both its mass and its speed. Because of the particles' very high speed, it is important we use the full relativistic kinetic energy, and not the more familiar expression, which can be used when the particles are going much slower than the speed light. The units we use for energy is the electron volt (eV). This is the amount of kinetic energy a single electron would have if it was accelerated by an electric potential of 1 Volt. For example, if we had a 9 volt battery, connected it to two metal plates, and released a single electron right at the negative plate, by the time it reached the positive plate it would have a kinetic energy of 9 eV.

This would have to be done with no air between the plates, otherwise the electron would bump into the the nitrogen and oxygen molecules in the air and lose energy through these collisions. One eV is a very small amount of energy. Lets compare it to a more familiar unit, the Calorie. Note the big "C", this is because what most people think of when the say calorie, as it relates to their diet, is really 1000 calories, the scientific unit of energy. 1 (diet) Calorie = 1000 calories = 1 kilocalorie. To avoid thoughts of food, I'll use kilocalories ... so let's compare eVs to kilocalories...

1 eV = .000000000000000000000383 kilocalories or more succinctly 3.83 x 10^-23 kilocalories

This makes eV's look pretty feeble compared to the energy units we commonly use. In everyday use though, again for example, with food, or an air conditioner, and so on, we are used to dealing with large objects composed of many. many small particles. For a single very small particle to have 1 eV, that is actually a pretty large amount of energy. We perceive objects as warm or cold because of the kinetic energy of their constituent particles. When you heat something up, you are usually giving the atoms and molecules that make the object up more kinetic energy. Under normal conditions, like in a room or an office, the individual gas molecules that make up the air are each zipping about with 0.04 eV of kinetic energy. If each particle had 1 eV of kinetic energy the room would feel about 10,000 degrees! ALS electrons don't have 1 eV, they each have 1,900,000,000 eV (1.9 GeV) of energy.

It is important to us for the electrons to have this much energy. Charged particles, when accelerated, emit radiation. Under certain conditions, the particles generate synchrotron radiation. It is this radiation (light) that we use for experiments. In the ALS, a large fraction of the space is taken by the components to generate high energy electrons, which are then accelerated in a controlled fashion to give us the light we need. Why do we use electrons? The radiation emitted from an accelerating particle depends on how "relativistic" the particle is. The amount of power radiated by an electron is proportional to γ⁴. γ is the ratio of a particle's relativistic mass to its rest mass. For a given kinetic energy, an electron's γ is much larger then a proton's γ, which means it will emit much more radiation. (γ is lower for a proton because a proton's rest mass is much larger then an electron's)

The hand points to the electron gun and Linac - this is where the electrons start their journey...

On the left, in the silver box, is the electron gun. Inside, there is a barium aluminate cathode, which emits electrons when heated, a control grid, and an anode. The applied voltage on the grid is pulsed open for about 25 nanoseconds (25 billionths of a second) during which about 1.5x10¹¹ electrons pass through the grid (if the gun ran continuously instead, 6x10¹⁸ electrons would come out in one second, corresponding to 1 amp of current). These electrons, which will become 3 bunches, are then accelerated by the main anode to about 120,000 eV (120 keV), and enter the Buncher and Linac (linear accelerator), which are also seen in the above picture. The ALS linac is a traveling wave design, where the electrons can be thought of as "surfing" the electric field down the accelerator. The tan "ribs" seen on the linac are electromagnets. Electrons repel one another, and because we have many electrons in one space, they went to spread out in space to increase the distance between them, we call this distorting effect space-charge. The magnets provide a constricting force to keep the beam nice and tight and counteract the effect of space-charge. By the time they reach the end of the linac, seen at the extreme right in the picture, they have a kinetic energy of 50,000,000 volts (50 MeV), and are going nearly the speed of light. This is still less the 3% of the final energy of the electrons, so they still have alot of energy to gain. They get most of this energy in the booster ring, which is contained inside the structure you see below...

Above is a section of the booster ring. You can see big gray girders holding things up, and chains connecting some things. This is not only to keep the ring in the proper position for smooth operations, it is also for safety - the ALS is located on a hillside in Berkeley, about half a mile from the Hayward fault. If an earthquake occurred, we want everything to stay in place, not slide down the hill! The yellow-orange structures are quadrupole magnets, used the keep the electron beam focused. The blue structures are dipole magnets, used to turn the electrons. As noted before, an accelerating charged particle gives of radiation (energy), so if we just had these components, the electron beam would lose energy, which is the opposite of what we want. Below is the device that remedies this...

This is the booster ring rf (radio frequency) cavity. The duct-work on top, with the DANGER sign, is actually a waveguide, channeling the power from a UHF transmitter providing about 30 kilowatts of rf power. It runs at 500 MHz and its peak effective voltage is ~250,000 volts (250 keV). Though this voltage is much lower then that needed to accelerate the electrons to their final energy (1.5 GeV) in one shot, we have a ring. A simple analogy would be a playground merry-go-round. To make it spin quickly, you could give it one tremendous push, or you can give it lots of appropriately timed little pushes, each time it comes around. In the booster ring (and storage ring) the latter method is used. This device provides millions of little pushes a second. Each push replaces the energy lost by synchrotron radiation, and gives a little extra boost as well. In the ALS booster, the radiation losses make the particle lose about 100 keV per turn once it has 1.5 GeV of energy, so the 250 keV that the rf cavity can provide is more the enough to get the electron moving. Once they reach the energy of 1.5 GeV, which takes less than one second, the electrons have gone around the booster 1.3 million times, and are going 99.999994% the speed of light. The electrons are then injected into the storage ring, the largest component of the ALS with a diameter of 63 meters...

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Storage Ring

This is a picture of a section of the storage ring. This looks very similar to the booster ring - not surprisingly - they are both synchrotrons, built by the same people, with the same paint scheme! In this ring, the electrons are accelerated to 1.9 GeV. The electrons spend much more time in the storage ring however. The electrons go around over one and a half million times in one second, and stay in the ring for about 6 hours. By this time the electrons have traveled over 4 billion miles, or the equivalent of about 22 round trips to the sun. To keep speedy electrons for such a long time, you must try very hard to minimize the number of things they can bump up against. In the storage ring, booster, and linac, the electrons travel within a metal chamber, which has almost all the air pumped out of it. The pressure of gas in the chamber only 0.0000000000001 times that of the atmosphere, so there aren't many gas molecules to bump into. In fact, in our ring, the electrons have a bigger chance of bumping against one another, a process called Touschek scattering, than hitting a stray gas molecule. This effect dominates the lifetime of the electrons in the ring, because if they bump one another hard enough, they can bounce so they don't make it around the next turn...

In the cartoon above are three bending magnets - this is no accident. You should know the storage ring is not really a circle but a dodecagon (12 sided polygon). It has 12 straight sections, where things called insertion devices can sit, punctuated by twelve bending sectors. Each "bend section" has 3 bending magnets, the blue ones, which you can see below...

The technical term describing this particular layout is "Triple Bend Achromat". The triple bend part is clear, with the three blue bend magnets easily visible. The term achromat means the bends treat electrons of (slightly) differing energies the same way - they all get turned to the same spot. There are special multi-pole magnets between the blue bend magnets that help accomplish this. Like in the booster ring, the electrons are losing energy as they turn, and this has to be replenished somehow. At 1.9 GeV, the electrons lose up to 300keV per revolution from radiative losses. In the storage ring we use a pair of rf cavities, with a peak voltage of 1.5 Megavolts, and which operate at 500 MHz. One of which is shown below...

You need to provide lots of rf power for these things. This power comes from a large klystron, which looks like this...

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Time Structure and Bunching

The RF acceleration causes the electrons to be distributed in bunches. This happens because there is a particular part of the RF wave where the electrons like to be. If they come earlier, the wave slows them down, if the come later, the wave speeds them up. With a cartoon, you end up with something like this...

Because the rf wave goes up and down 500 million times a second, we have a ring that has 500 million bunches of electrons going by in a second. In time the bunches are then spaced by 1 / 5x10⁸, or about 2 nanoseconds (2 x10^-9s) apart. Because we know the electrons' speed (nearly the speed of light) we can calculate the average spacing between the bunches, which is a little under 2 feet. The main part of each electron bunch goes by in about 40 picoseconds (1x10^-12s) which corresponds to half an inch long. The number of bunches we could fit in the storage ring is about 328. These bunches just go around the ring many times a second - about 1.5 million times for each bunch. If you do the math, you will see that this many bunches going around this many time in a second is about 500 million, the same number as the RF wave frequency. For operational purposes we usually have little less than 300 bunches filled. When each bunch goes by, it gives off light. When there are no bunches, there is no light (dark gap). Thus the light comes in a pattern like below:

Some people completely ignore this time structure, and do experiments as if the light was shining continuously. Some people rely on the fact that the dark gap comes around 1.5 million times a second. Others use this time structure to their advantage and do experiments where they want to record how soon after they irradiate a sample a process occurs. Because the ALS operators have good control over the electrons in the ring, they can even do things like fill one bunch with extra electrons, and use this extra bright bunch to trigger some event (camshaft mode), or we can fill only one or two bunches, and use the occasional blips of light to initiate a process at a well defined time (two-bunch mode).

At the ALS, an external trigger signal is provided that allows us to synchronize our experiments with the electron bunches. This trigger is movable in 2 nanosecond increments so we may trigger on whichever bunch we want. Later in this tour (Chemical Dynamics Beamline section) we will see why having time information is important.

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Insertion Devices

As mentioned earlier, the ring has 12 straight sections. One of the straight sections is occupied by the pair of RF cavities that provide the energy to keep the electrons speeding around the ring. Another straight section is used for injection, which is what we do to get the electrons from the booster ring into the storage ring. The other sections are used for insertion devices, the hallmark of third generation synchrotrons like the ALS. Insertion devices are large periodic magnetic structures that make the electrons move rapidly in a well defined way. Remember that an accelerating charge gives off radiation, so in these insertion devices, the electron changes directions many times (accelerates) and gives of large amounts of radiation.

In the ALS there are three types of insertion devices: undulators, wigglers, and EPUs.

First, we well talk about the undulator. The typical ALS undulator is a large structure about 14 feet long and weighing about 50,000 pounds. In it, are numerous permanent magnets, arranged like the adjacent picture. The spacing between the magnets, from N to N or S to S, is fixed for each undulator - there are ones with 5cm periods, 8 cm periods, and 10 cm periods. Changing the period length alters the spectral range the undulator can cover. On the Chemical Dynamics Beamline, we use a full length undulator, 10 cm period length, for 43 total periods. The properties of the light emitted, depends on a few things. First, the wavelength of the light is dependent on the period of the undulator. For us, stationary like the undulator, we see 10 cm as the period (λ_u). For the electron, which is moving the speed of light, it sees something very different (λ_e). To see what the electron "observes", we have to do a Lorentz Transform. The result of this is the electron "sees" a shortened period, given by λ_e=λ_u/γ Gamma (γ), which we saw earlier, is related to how relativistic a particle is. For ALS electrons γ is ~4000. Thus the period seen by the electron is 2.5 x 10^-3 cm, or 25 millionths of a meter. Light of this wavelength corresponds to the infrared region of the spectrum.

The fact that the electrons are moving nearly the speed of light has some interesting consequences. The light emitted in the forward direction is being "chased" by electrons going essentially the same speed. Many people are familiar with the experience that the pitch of an ambulances siren changes as the ambulance drives by. This happens because the sound waves bunch up in the front, raising the pitch, as it moves toward you. The sound waves behind get stretched out, lowering the pitch, as it drives away from you. This is called the Doppler effect and occurs in light as well as sound. Because the speed of light is typically much greater than the speed of anything we commonly encounter in our lives, we don't usually notice it. In the ring though, it has major consequences. For light emitted in the forward direction, the relativistic Doppler shift shortens the wavelength of the radiation by another factor of 1/2*γ, or another 2000 times. The light, which had a wavelength of 25 millionths of a meter, now has a wavelength of 12.5 billionths of a meter, which is firmly in the VUV part of the spectrum.

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Radiation Shielding, etc.

If you noticed in many of the pictures, on the walls there are numbers painted. These are the weights of the blocks that make up the wall. The electrons, as they go around the ring, are emitting radiation - some of this is the useful radiation we use for experiments. The rest isn't used. The radiation given off goes from low energy infra-red photons, to high energy x-rays. In addition, if the electrons bump into anything they slow down quickly, and give off Bremsstrahlung radiation in large amounts. This radiation would make short work of anyone standing nearby when the ring is running. As this wouldn't be acceptable to any of the parties involved, we use large heavy blocks containing concrete and lead, which prevent any of this stray radiation from escaping the storage ring in any places where we don't want it.

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Appendix

TV's and Electron Guns:

Electron guns are actually something many of us see and use everyday. The traditional television and computer monitors have a CRT or picture tube. A CRT is a Cathode Ray Tube, and at its core is an electron gun. The commonplace TV actually shares many of the same concepts with the ALS. Both have an electron gun, both use magnetic fields to steer the electron beam, and both are evacuated (all the air sucked out) to avoid collisions between the electrons and other particles. There are of course, very large differences in the level of control, intensity, and speed of the electron beam in your TV and our synchrotron, but that doesn't diminish from their similarities.

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