Electron-Cloud Effect

The Electron-Cloud Effect (ECE) is a phenomenon associated with particle accelerators.


Electron clouds are created when accelerated charged particles disturb stray electrons already floating in the tube, and bounce or slingshot the electrons into the wall. These stray electrons can be photo-electrons from synchrotron radiation or electrons from ionized gas molecules. When an electron hits the wall, the wall emits more electrons due to secondary emission. These electrons in turn hit another wall, releasing more and more electrons into the accelerator chamber.

The electron cloud produces a few very undesirable effects including impeding the flow of the accelerated particles, and creating a wake field which throws the particles off course.

Exacerbating Factors

This effect is especially a problem in positron accelerations, where electrons are attracted and slingshot into the walls at variable incident angles. Negatively charged electrons liberated from the accelerator walls are attracted to the positively charged beam, and form a "cloud" around it.

The effect is most pronounced for electrons with around 300eV of kinetic energy - with a steep drop-off of the effect at less than that energy, and a gradual drop-off at higher energies, which occurs because electrons "bury" themselves deep inside the walls of the accelerator tube, making it difficult for secondary electrons to escape into the tube.

The effect is also more pronounced for higher incidence angles (angles farther from the normal).

Electron cloud growth can be a severe limitation in bunch currents and total beam currents if multipacting occurs. Multipacting can occur when the electron cloud dynamics can achieve a resonance with the bunch spacing of the accelerator beam. This can cause instabilities along a bunch train and even instabilities within a single bunch, which are known as head-tail instabilities.

Proposed remedies

A few remedies have been proposed to deal with this, such as putting ridges in the accelerator tube, adding antechambers to the tube, coating the tube to reduce the yield of electrons from the surface, or creating an electric field to pull in stray electrons. At the PEP-II accelerator at SLAC, the vacuum pipe which contains the positron ring has a wire coiled around its entire length. Running a current through this wire creates a solenoidal magnetic field which tends to contain the electrons liberated from the beam pipe walls.

Measurment Techniques

There are many different ways of measuring the electron cloud in a vacuum chamber. Each one gives insight into a different aspect of the electron cloud. Retarding field analyzers are local grids in the chamber wall that allow some of the cloud to escape. These electron can be filtered by an electric field and the resultant current can be measured. A limitation is that retarding field analyzers measure only local cloud, and because they measure current, there is inherently some time averaging involved. Witness bunch studies measure the tune shift along successive bunches in a train and in a witness bunch that is places at varying locations behind the train. Since tune shift is related to the ring-averaged central cloud density if the tune shift is known the central cloud density can be calculated. A great advantage of the witness bunch studies is because the tunes can be measured bunch by bunch the time evolution of the cloud can be measured.

External links

* [http://mafurman.lbl.gov/ECLOUD04_proceedings/bellodi-v4.pdf "Electron Cloud Buildup in the ISIS Proton Synchotron and Related Machines", by G Bellodi]
* [http://www.symmetrymagazine.org/cms/?pid=1000341 "Battling the Clouds" article in "symmetry" magazine]
* [https://wiki.lepp.cornell.edu/ilc/bin/view/Public/CesrTA/ Cornell CESRTA wiki]

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • electron tube — an electronic device that consists, typically, of a sealed glass bulb containing two or more electrodes: used to generate, amplify, and rectify electric oscillations and alternating currents. Also called electronic tube. Cf. gas tube, vacuum tube …   Universalium

  • Electron — For other uses, see Electron (disambiguation). Electron Experiments with a Crookes tube first demonstrated the particle nature of electrons. In this illustration, the profile of the cross shaped target is projected against the tube face at right… …   Wikipedia

  • Électron — Traduction à relire Electron → …   Wikipédia en Français

  • Inductive effect — In chemistry and physics, the inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction.[1] The net polar effect exerted by a substituent is a… …   Wikipedia

  • Nephelauxetic effect — The nephelauxetic effect is a term used in the physical chemistry of transition metals. It refers to a decrease in the Racah interelectronic repulsion parameter, given the symbol B, that occurs when a transition metal free ion forms a complex… …   Wikipedia

  • Free electron model — In three dimensions, the density of states of a gas of fermions is proportional to the square root of the kinetic energy of the particles. In solid state physics, the free electron model is a simple model for the behaviour of valence electrons in …   Wikipedia

  • Auger effect — The Auger effect (pronEng|ˈɔːʒɚ, or Oh jeh) is a phenomenon in physics in which the emission of an electron from an atom causes the emission of a second electron. [GoldBookRef|title=Auger effect|url=http://goldbook.iupac.org/A00520.html] When an… …   Wikipedia

  • Monte Carlo methods for electron transport — The Monte Carlo method for electron transport is a semiclassical Monte Carlo(MC) approach of modeling semiconductor transport. Assuming the carrier motion consists of free flights interrupted by scattering mechanisms, a computer is utilized to… …   Wikipedia

  • Greenhouse effect — The Greenhouse effect refers to the change in the thermal equilibrium temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs infrared radiation. [ [http://www.ipcc.ch/pdf/assessment report/ar4/syr/ar4 syr… …   Wikipedia

  • Biefeld–Brown effect — The Biefeld–Brown effect is an effect that was discovered by Thomas Townsend Brown (USA) and Dr. Paul Alfred Biefeld (CH). The effect is more widely referred to as electrohydrodynamics (EHD) or sometimes electro fluid dynamics , a counterpart to… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.