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In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation. Commonly used secondary emissive materials include alkali antimonide Beryllium oxide (BeO) Magnesium oxide (MgO) Gallium phosphide (GaP) Gallium arsenide phosphide (GaAsP) Lead oxide (PbO) In a photomultiplier tube, one or more electrons are emitted from a photocathode and accelerated towards a polished metal electrode (called a dynode). They hit the electrode surface with sufficient energy to release a number of electrons through secondary emission. These new electrons are then accelerated towards another dynode, and the process is repeated several times, resulting in an overall gain ('electron multiplication') in the order of typically one million and thus generating an electronically detectable current pulse at the last dynodes. Similar electron multipliers can be used for detection of fast particles like electrons or ions. In the 1930s special amplifying tubes were developed which deliberately "folded" the electron beam, by having it strike a dynode to be reflected into the anode. This had the effect of increasing the plate-grid distance for a given tube size, increasing the transconductance of the tube and reducing its noise figure.

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Secondary emission

Ion

An ion (ˈaɪ.ɒn,_-ən) 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 convention. The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons. A cation is a positively charged ion with fewer electrons than protons while an anion is a negatively charged ion with more electrons than protons.

Particle accelerator

A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle physics. The largest accelerator currently active is the Large Hadron Collider (LHC) near Geneva, Switzerland, operated by the CERN. It is a collider accelerator, which can accelerate two beams of protons to an energy of 6.5 TeV and cause them to collide head-on, creating center-of-mass energies of 13 TeV.

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Influence of ion-induced secondary electron emission parameters in PICMC plasma simulations with experimental validation in DC cylindrical diode and magnetron discharges

Ivo Furno, Alban Sublet, Thibaut François Hugo Richard

Niobium thin lms are used at CERN (European Organization for Nuclear Research) for coatings of superconducting radio-frequency (SRF) accelerating cavities. Numerical simulations can help to better understand the physical processes involved in such coatings and provide predictions of thin lm properties. In this article, Particle-in-Cell Monte Carlo (PICMC) 3D plasma simulations are validated against experimental data in a coaxial cylindrical system allowing both DC diode and DC magnetron operation. A proper choice of ion induced secondary electron emission (IISEE) parameters enables to match experimental and simulated discharge currents and voltages, with argon as the process gas and niobium as the target element. Langmuir probe measurements are presented to further support simulation results. The choice of argon gas with a niobium target is driven by CERN applications, but the methodology described in this paper is applicable to other discharge gases and target elements. Validation of plasma simulations is the rst step towards developing an accurate methodology for predicting thin lm coatings characteristics in complex objects such as SRF cavities.

2020

Investigations of Quench Limits of the LHC Superconducting Magnets

Minh Quang Tran, Bernd Dehning, Agnieszka Priebe

NbTi-based Rutherford cables are used in the coils of the Large Hadron Collider (LHC) magnets. These cables are designed to operate with currents up to 13 kA at temperatures of 1.9 K. Beam losses can locally heat the superconducting cables above the critical temperature and cause a transition to the normal conducting state (quenching). The quench limit, i.e., the energy needed for this transition, is studied to determine the maximum beam intensities and luminosity reach of the LHC. The amount of energy deposited in the coil cannot be measured directly. Therefore, Geant4 simulations are used to correlate the deposited energy with the signal from secondary particles detected outside the magnet cryostat by ionization chambers. An orbital bump technique is used to induce controlled beam losses and provoke a quench. The energy deposition is analyzed in terms of various beam loss patterns and beam energies. The validation of the heat transfer code is presented. The development of the resistive zone is estimated and compared with the voltage measurements over the coils.

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Investigations of quench limits of the LHC superconducting magnets

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Ieee-Inst Electrical Electronics Engineers Inc2012

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