• Revision:Particle Accelerators

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Particle Accelerators

There are 2 main type of particle accelerators used:

- The Cyclotron

- The Linear Accelerator

Cyclotron as its name suggests, it circular and particles accelerate in a circular motion. This accelerator consists of 2 semi-circle flat metal boxes, open at their diameter, referred as Dees. The Dees are connected to a very high frequency alternating voltage and are situated in strong vertical magnetic field. A source of protons is placed in the centre of the 2 Dees. Following steps outline the functionality of the cyclotron:

1) Some protons that emerge from the central source move in a horizontal plane enter Dee 1 (D1)

2) They now follow a semi-circular path under the action of the magnetic field

3) When they arrive at the centre of the 2 Dees, they "know" that D2 is at a negative potential and so are accelerated acroos the gap by the electric field

4) They enter D2 with extra energy eV

5) Now they continue in their semi-circular path at a greater speed and arrive back at the gap and follow steps (3) and (4) but in this case, D1 instead of D2.

The Linear Accelerator (or linac) as its name suggests, is a particle accelerator in a linear motion. The structure consists of a vacuum chamber, an electron gun to fire the electrons, drift tubes, a.c supply connected to tubes and electron beam. Following steps outline how the linac works:

1) The electrons are given energy which comes from the electric field as they pass between the tubes which are charged.

2) A few electrons are fired through the electron gun and are attracted to the 1st tube.

3) While it is in the 1st tube, the charge on the next tube changes from negative to positive

4) So this 2nd tube attracts the bunch of electrons leaving the 1st tube and the process continues

5) The lengths of the tubes increase as the speed of electrons within the linac increase. Due to the increasing velocity, these electrons drift as they are in the electric field within the linac.

The Basis of all Particle Accelerators

Particle accelerators consist of 4 main parts:

  1. Particle production
  2. Particle acceleration
  3. Particle guidance
  4. Particle containment

These parts are common to all systems – only the design and form of the system changes.

Particle Production

  • Particle production depends on the particles used in the accelerator.
  • At its simplest, this could be a hot wire producing electrons via thermionic emission.
  • More complicated systems involve the use of radioactive sources or particle collisions from other particle accelerators to produce the particles.
  • One feature common is the use of charged particles.

Particle Acceleration

  • The section of the accelerator which increases the energy of the charged particles.
  • This usually uses potential difference to accelerate the charged particle.
  • The energy increase is equal to eV (where e is the charge of the particle, and V is the accelerating potential difference) and results in an increase in the kinetic energy of the particles.
  • Usually the particles are accelerated multiple times.
  • but the speed canot be increased indefinitely, because when the speed increases mass of the particle also increases.

Particle Guidance

  • Depending on the design of the accelerator, the charged particles may need to travel in a straight or curved path. If a curved path is required, the particles are deflected by a magnetic field – magnetic fields are chosen because they cause the particles to travel in a predictable circular path, which is easily modelled.

Particle Containment

  • Groups of similarly charged particles tend to separate from each other and hence need to be contained into a tight packed or beam.
  • To do this, magnetic fields are often used as a containment field.
  • This also helps to keep the particles away from the walls of the containment vessel and so stop them interacting with their surroundings.
  • The containment vessel is evacuated in order to stop interactions with air.
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