X-rays are produced when energetic electrons are rapidly decelerated, typically by striking a target made from a high atomic number material.
A simple X-ray tube consists of two electrodes – an electron emitter and a target – spaced apart within a vacuum vessel, and with a high voltage difference between them. A heated thermionic emitter is often employed as the source of the electrons; cold emitters, such as carbon nanotubes, are also used, in which case emission relies on the presence of a high electric field to strip the electrons from the emitter surface.
The electric field produced by the applied voltage accelerates the electrons towards the positively biased target, where they are rapidly decelerated. A proportion of their energy is converted to X-rays (bremsstrahlung radiation), while the majority produces heat.
As part of the Opera suite, the Charged Particle module provides a comprehensive capability for modelling-ray tubes. The module includes an extensive set of emitter models, including thermionic and field effect emission from surfaces, secondary emission from surfaces and from within volumes (used to model gas ionization). With this Opera module, users can also include multiple species of charged and neutral particles, each having user-defined charge and mass.
As well as modelling the generation and propagation of the electrons through any combination of electrostatic and magnetostatic electron-optic components, such as those used for acceleration, focussing deflecting, the Charged Particle module enables analysis of those phenomena that can lead to device performance degradation or failure.
Clearly, one of these is the thermal issue. Most of the power contained in the electron beam is dissipated as heat in the target, and the design must make provision for this, and for the stress and deformation that an inevitable temperature rise will cause. Opera’s multi-physics facility make this a simple task, allowing Charged Particle, thermal and stress simulations to be ‘chained’ – all data from one stage is passed to the next automatically, without user intervention.
Depending on the material of the target and the energy of the electrons, some of the target material might be ejected (sputtered) as a result of the deposition of energy in the target. The sputtered material then coats the inside of the tube and can result in a reduction of the service life of the device. This process may simulated by the Charged Particle module – the sputtering may be characterized by secondary emitters that specify the generation of each sputtered species. Once generated, these are then tracked to their incidence with other surfaces in the model.
Applications of the Charged Particle module not only include the design of X-ray tubes, but also electron guns, ion-beam devices, electron microscopes and magnetron sputter coaters.
Please feel free to contact us if you would like a demonstration of how to simulate x-ray devices in Opera.