Particle Accelerators
EMMA and ALICE are developmental particle accelerators at Daresbury, Cheshire.The electrons travel through a pipe containing an ultra high vacuum, stronger than moon's. This vacuum is necessary because if there were any air, the electrons would scatter off it, disrupting the beam. Quad pole magnets surround the pipe at intervals to focus the beam.
EMMA is formed as a ring. EMMA stands for electron machine of many applications. The
beam into EMMA comes from ALICE. They are trying to make an accelerator which can accelerate particles but cheaper so that they can be used in, for instance, hospital treatments or power generator
ALICE
350 000 electron volts when come out of the electron gun but they want it up to relativistic speeds, that is 35 million electron volts. It is accelerated in super conducting niobium cavities. Niobium is a rare metal which at 2 Kelvin becomes super conducting which means there is no electrical resistance. If there were resistance, it would heat up and melt.
The electron bunch gets shorter, that is the back end catches up with the front end, but how short is short? 1 pico second long, that is 10-12 seconds to pass any point! So ALICE produces intense light pulses. These short pulses of light are like a video camera, so you shine very short pulses of light lots of times at a sample and you get info about the structure at a given time, so you can watch chemical bonds breaking and chemical reactions occurring.
Backstage Science: Making Particle Accelerators
Electron Guns
If the electron bunches are bad, they can not be improved down the a line in the accelerator.A photocathode is the electron source. It consists of a gallium arsenide wafer soldered, using indium, onto a molybdenum substrate, or puck. It draws the heat away. On the surface of the photocathode is a layer of caesium then oxidant, then caesium, then oxidant and so on. This is part of the activation process to make it efficient.
Behind the cathode is a power supply with 250-350 thousand volts. A few centimetres in front of the cathode is an anode plate with a hole in the middle. Everything is highly polished to allow for high voltage. The anode is at 0 volts. A laser is pointed at the cathode. The laser interacts with the photocathode surface. The electrons, at very high voltage, dammed behind the cathode come out through the surface of photocathode out into free space. They follow the electric field lines between the cathode and anode. They are accelerated very rapidly through the anode and out of the electron gun.
Backstage Science - Electron Gun
Diamond Light Source
(update 11th April)The Diamond Light Source is located in Didcot, Oxfordshire.
There are 3 different types of magnet around the circle: quadpoles, sextapoles and dipoles. The dipoles are set up so that they kick the electrons around the storage ring at 7.5 degrees.
Diamond is used to produce light from infrared to x-rays. The points where light is generated are called insertion devices. These are rows of permanent magnets N-S S-N N-S on either side of the beam. They cause the beam to wiggle as they pass through the dipole fields. Each of these wiggles produces light which is separated off for use in experiments.
Backstage Science - Inside Diamond
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