X-rays are emitted when accelerated electrons are decelerated upon striking a target.
In an evacuated microfocus X-ray tube, a tungsten filament is heated up (heating current/filament current UH) until releasing electrons. Due to a difference in potential between filament (cathode) and anode, the electrons travel from cathode to anode, where they strike at one third the speed of light. The filament current is controlled by means of the Wehnelt grid, which is held at a negative potential (voltage UG).The beam passes through a hole in the anode and is then directed onto an electromagnetic lens by a series of deflecting magnets where it is then collimated and focussed onto the target.
The diameter of the focal spot dictates the size of the X-ray source and is only a few microns in size.
The transmission target consists of a thin layer of tungsten on a plate of light metal. This plate doubles as radiation exit window.
In case of transmission targets, the X-ray source is located very close to the outer wall of the microfocus X-ray tube allowing the user to bring samples very close to the source ensuring highest magnifications.