In
accelerator physics
Accelerator physics is a branch of applied physics, concerned with designing, building and operating particle accelerators. As such, it can be described as the study of motion, manipulation and observation of relativistic charged particle beams ...
, ionization cooling is a physical process for reducing the
beam emittance
In accelerator physics, emittance is a property of a charged particle beam. It refers to the area occupied by the beam in a position-and-momentum phase space.
Each particle in a beam can be described by its position and momentum along each of t ...
of a
charged particle beam
A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. The kinetic energies of the particles are much lar ...
("cooling") by passing the particles through some material, reducing their momentum as they ionize atomic electrons in the material.
Thus the normalised beam emittance is reduced. By re-accelerating the beam, for example in an RF cavity, the longitudinal momentum may be restored without replacing transverse momentum. Thus overall the angular spread and hence the geometric emittance in the beam will be reduced.
Ionization cooling can be spoiled by stochastic physical processes. Multiple
Coulomb scattering
In particle physics, Rutherford scattering is the elastic scattering of charged particles by the Coulomb interaction. It is a physical phenomenon explained by Ernest Rutherford in 1911 that led to the development of the planetary Rutherford model ...
of
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
s as well as nuclear
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
of
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s and
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s can reduce the cooling or even lead to net heating transverse to the direction of beam motion. In addition, energy straggling can cause heating parallel to the direction of beam motion.
Muon cooling
The primary use of ionization cooling is envisaged to be for cooling of muon beams. This is because ionization cooling is the only technique that works on the timescale of the muon lifetime. Ionization cooling channels have been designed for use in a
neutrino factory The Neutrino Factory is a proposed particle accelerator complex intended to measure in detail the properties of neutrinos, which are extremely weakly interacting fundamental particles that can travel in straight lines through normal matter for thous ...
and a
muon collider
A Muon Collider is a proposed particle accelerator facility in its conceptual design stage that collides muon beams for precision studies of the Standard Model and for direct searches of new physics. Muons belong to the second generation of lept ...
. Muon ionization cooling has been demonstrated for the first time by the proof of principle
International Muon Ionization Cooling Experiment The International Muon Ionization Cooling Experiment (or MICE) is a high energy physics experiment at the Rutherford Appleton Laboratory. The experiment is a recognized CERN experiment (RE11).
MICE is designed to demonstrate ionization cooling of ...
(MICE).
Other PoP muon ionization cooling experiments have been devised.
Other particles
Ionization cooling has also been proposed for use in low energy ion beams and proton beams.
Longitudinal cooling
The technique can be adapted to provide longitudinal as well as transverse cooling by using a
dipole magnet
A dipole magnet is the simplest type of magnet. It has two poles, one north and one south. Its magnetic field lines form simple closed loops which emerge from the north pole, re-enter at the south pole, then pass through the body of the magnet. T ...
as a
dispersive prism
In optics, a dispersive prism is an optical prism that is used to disperse light, that is, to separate light into its spectral components (the colors of the rainbow). Different wavelengths (colors) of light will be deflected by the prism at dif ...
to divide the particles by energy, and then passing the resultant "rainbow" beam though a tapered wedge of cooling material. Thus, faster particles are cooled more and slower particles are cooled less. A simple way is to fill the dipole itself with cooling material, so that more energetic particles following a larger orbit pass are cooled more.
See also
*
Particle beam cooling Particle beam cooling is the process of improving the quality of particle beams produced by particle accelerators, by reducing the emittance. Techniques for particle beam cooling include:
* Stochastic cooling
* Electron coolingI. Meshkov, Electro ...
References
{{DEFAULTSORT:Ionization Cooling
Accelerator physics