Stochastic cooling

Stochastic cooling: As particles travel around a circular accelerator, a detector or "pick up" measures their motion and sends a signal across the ring to a corrector, the kicker, which adjusts their angles (Image: CERN)

The purpose of stochastic cooling, a technique invented by Simon van der Meer that was developed at CERN at the beginning of the 1970s, is to reduce the energy spread and angular divergence of a beam of charged particles. During this process, the particles are “compressed” into a finer beam with less energy spread and less angular divergence. By increasing the particle density to close to the required energy, this technique improved the beam quality and brought the discovery of the W and Z bosons within reach.

Organisation of particles and plunging thermometer

By analogy with the kinetic theory of gases, where heat is equivalent to disorder, the term “cooling” here denotes the reduction of disorder in the beam. The focus of stochastic cooling is on reducing the amplitudes in the motion of particles in a beam in order to contain them in a restricted space and maintain their energies close to a mean value. The particles are thereby better organised and the beam is cooled.

Feedback and corrections

Stochastic cooling is a feedback system comprising two components: a detector or pick up which measures the motion of the particles and a corrector, the kicker, which adjusts their angles. When the beam passes in front of the pick-up, the latter measures the deviation of the centre of gravity of a sample of particles in the beam with respect to the requisite orbit. It then sends this error signal to the kicker which, further down the line, applies an electric field to this same sample to correct the deviation measured (see diagram above).

Each particle in turn…

Stochastic means random. Unlike the majority of feedback systems used in accelerators the stochastic cooling system does not regard the beam as a single unit but as the sum of its separate particles with their own individual characteristics. The system cannot correct the deviation of all the particles in one go, but after a sufficient amount of time has elapsed and the beam has circulated several times around the accelerator, each particle will be concentrated around the chosen orbit. The beam becomes finer and denser and the “cooling” process is complete.

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