What are particle accelerators used for?
A particle accelerator is a device that uses electric fields to propel electrically charged particles in a vacuum, which approach the speed of light, towards each other and collide. The result hopefully being that the kinetic energy in the particles and other energy converted into subatomic particles or various types of nuclear radiation.
There is more than one type of particle accelerator; they come in two basic types: •
The linear accelerator, or linac, uses microwave technology to accelerate electrons in a part of the accelerator called the "wave guide". Particles pass through a line of hollow metal tubes enclosed in an evacuated cylinder. Within a hollow conductor there is no electric field so a charged particle travels at constant speed inside each of the tubes. Between one tube and the next there is a potential difference which varies in size and direction as an AC voltage is applied to the series of tubes. Bunches of charged particles are accelerated from tube to tube, moving with the voltage wave as it travels along the linac.
The largest linac in the world, at Stanford University, is 3.2km long. It is capable of accelerating electrons to an energy of 50 GeV. Stanford’s linac is designed to collide two beams of particles, accelerated in turn by the linac and temporarily kept in storage rings.
The two most important problems in the linac design are the accelerator cell voltage flatness and the transverse mode impedance of the cell.
The device length limits the locations where one may be placed. •
A great number of driver devices and their associated power supplies is required, increasing the construction and maintenance expense of this portion. •
If the walls of the accelerating cavities are made of normally conducting material and the accelerating fields are large, the wall resistivity converts electric energy into heat quickly. On the other hand superconductors have various limits and are too expensive for very large accelerators.
The cyclotron was the first circular accelerator. A cyclotron is somewhat like a linac wrapped into a tight spiral. Instead of many tubes, the machine has only two hollow vacuum chambers, called dees, that are shaped like capital letter Ds back to back. A magnetic field, produced by a powerful electromagnet, keeps the particles moving in a curved path. The potential difference between the dees constantly alternates in direction, so that every time the particles reach the gap they experience a forward acceleration. Within each dee the particles travel at constant speed during each half-revolution. As the particles gain energy, they spiral out towards the edge of the accelerator, where finally they exit.
Advantages of the Cyclotron
Cyclotrons have a single electrical driver, which saves both money and power, since more expense may be allocated to increasing efficiency. •
Cyclotrons produce a continuous stream of particle pulses at the target, so the average power is relatively high. •
The compactness of the device reduces other costs, such as its foundations, radiation shielding, and the enclosing building. The world’s most powerful cyclotron, the K1200, is capable of accelerating nuclei to an energy approaching 8 GeV.
The synchrotron is the most recent and most powerful member of the accelerator family. It consists of a tube in the shape of a large ring through which the particles travel; the tube is surrounded by magnets that keep the particles moving along the centre of the tube. The particles enter the tube after already having been accelerated to several million electron volts. They are accelerated at one or more points on the ring each time they make a complete circle around the accelerator. To keep the particles in a rigid orbit, the strengths of the magnets in the ring are increased as the particles gain energy. In a...
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