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Dresden EBIT/EBIS - Functional principle
The Dresden EBIT/EBIS consist of a cathode generating the electron beam, three drift tubes forming the ion trap, an electron collector, and an ion extraction lens.
The electron beam is radially compressed in the drift tube region by a strong magnetic field to a beam diameter in the range of 100 to 200 micrometers.
After passing the drift tubes the electron beam is widened and reflected by the extraction lens. The electrons are thereupon collected at the electron collector which is water-cooled.
The ions successively ionized by electron impact are trapped radially by the potential minimum formed by the negative space charge of the high-density electron beam.
Axial confinement is accomplished by the application of additional electrostatic potentials at the outer drift tube sections which also control the ion trapping time.
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Schematic of the potential setting inside the Dresden EBIT
The electron beam energy that represents the ionization energy is determined by the potentials of both the cathode and the middle drift tube section.
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Animation of the potentials and of the ionization prozesses inside the Dresden EBIT
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The degree of ionization increases with the ion confinement time. This results in an evolution
of the ion charge state distribution with the trapping time. Thus, a certain ion charge
state is reached after a characteristic ionization time.
At the end of the preselected trapping time the ion trap is opened by lowering
one side of the axial trap potentials and the ionization process is starting again.
Hence, the pulse frequency at which the ions can be extracted depends on the atomic number of the ion and the required charge state. It can vary from less than 0.1 Hz up to several hundreds of Hz.
The ions can also be extracted in DC mode, that is, in leaky mode at which the ions can escape from the trap over a reduced axial trap potential. The charge state that can be reached is rather limited but the total ion output can be increased largely.
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Evolution of the charge state distribution of xenon
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Measurement of the evolution of the charge state distribution of argon ions
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