Increasing particle energy
For particles initially at rest or moving slowing, the velocity of the particle increases rapidly as its kinetic energy increases.
Once the particle speed gets near lightspeed, although the kinetic energy may still be increasing at a reasonable rate, the change
in particle velocity is slow. In many accelerators the charged particles are moving close enough to the speed of light that relativity
theory is needed for an accurate description of the particle motion.
The energy of the charged particles in an accelerator is increased using applied electric fields. Some accelerators employ constant
electric fields (not varying in time). In these machines the charged particles 'fall' through the electric field gaining kinetic energy,
in much the same way that an apple gains kinetic energy when it falls from a tree toward the earth. For more information about this sort
of acceleration, see (on this site):
- DC Acceleration, electrostatics
Many accelerators use electric fields that vary with time. For this type of acceleration the arrival of a particle in the region with the
electric field must be synchronized with the temporal variation of the field so that the particle is accelerated properly. If not timed
correctly, a particle might even be decelerated instead of accelerated! For more information on this type of acceleration, see (on this site):
- Acceleration with alternating electric and magnetic fields
Acceleration with time-varying fields allows for bigger peak electric fields without having electrical breakdown of the air (sparking,
arcing) around the structure providing the electric field. With any technology, there is still a maximum achievable electric field, so to
give the particles more energy boost, accelerating structures must be strung together, or made longer. If particles travel in a straight
line though such a structure or set of structures, the machine accelerating the particles is called a Linac, or Linear Accelerator.
Controlling the direction of particle motion
Straight line motion for particle accelerations to very high energies is not an economical use of land; more energy means more kilometers of accelerating
structures. If once the particles exit an accelerating structure they are brought back around to the entrance of the accelerator
for another pass through the machine, then the same accelerator may be used over again. In this way the length of the accelerator does not have
to be so long. So, many accelerators have a circular shape; the particles in the beam are brought back to the same accelerating structures time
after time. Magnets may be used to control the direction of a beam. Magnetic fields exert a force on charged particles, as long as they are not
standing still. The magnetic force goes up as the particle velocity goes up, and always pushes particles at right angles to the direction of their
motion. Magnetic fields can turn moving particles, and so allow beams to be steered around in a desired direction. Circular accelerators have magnets
distributed all around the circumference; these provide the centripetal force responsible for guiding the beam around in a circle. For more information
about magnets and magnetic fields, see (on this site):
- Magnets and magnetic fields, magnetostatics
