Research with Beams of Highly Charged Ions
Generation of highest charge states with ultra-high vacuum technology
In our environment, we encounter mainly low-charged ions, for example in the flame of a candle or in thunderstorm lightning. But there are also naturally occurring highly charged ions, i.e. ions with a high number of missing electrons in the atomic shell. We experience these, for example, in exotic states such as the solar corona or in supernova events.
For this reason, the study of highly charged ions in the laboratory plays a major role for astrophysics. However, highly charged ions produced in the laboratory are also extremely important in other fields. Spectroscopy of highly charged ions is used to study processes in fusion plasmas. Basic research on the interaction of highly charged ions with solid surfaces provides interesting perspectives, for example, for future quantum computer systems.
"To date, spectroscopic measurement of atomic radii has been performed only on hydrogen-like systems with a single electron, because only for these is the theory sufficiently accurate. Experimentally, however, these simple atomic systems have the disadvantage that the wavelengths to be used lie far in the ultraviolet range of the optical spectrum and are thus difficult to access with current laser systems," explains Prof. Dr. Wilfried Nörtershäuser, head of the LaserSpHERe (Laser Spectroscopy of Highly Charged Ions and Exotic Radioactive Nuclides) research group at the Institute of Nuclear Physics at TU Darmstadt. "Currently, however, there are promising efforts to achieve the required accuracy also for more complex, helium-like systems with two electrons. Their wavelengths are much more accessible with laser systems, and thus the radii of atomic nuclei from helium to nitrogen can be determined much more precisely in the future than is currently possible. By installing the ion beam facility with the EBIS-A ion source, the COALA apparatus offers the ideal conditions for this." Prof. Dr. Nörtershäuser and his team conduct precision experiments at the frontier of atomic, nuclear and particle physics using the Collinear Apparatus for Laser Spectroscopy and Applied Sciences (COALA). Its research focuses on laser spectroscopy of highly charged ions and exotic short-lived isotopes, with the goal of precisely determining the charge radii of atomic nuclei.
EBIS ion source
Technologies for the generation of highly charged ions
The Electron Beam Ion Source (EBIS) used in Darmstadt is only one of several technologies for the generation of highly charged ions. Like lasers and electron cyclotron resonance ion sources (ECRIS), EBIS is considered a direct source of highly charged ions. In addition, low-charged ions can be converted into highly charged ions using high-energy accelerators and gas or foil stripper targets.
The energy transfer required for ionization is realized by radiation in laser ion sources. All other technologies have in common that the driving process for ionization is based on electron collisions. In high-energy accelerators, the singly charged ions are fired at quasi-resting electron impact partners at high energies. In electron cyclotron resonance and electron beam ion sources, the process is reversed. The initially gaseous neutral molecules or atoms are at rest. For electron beam ionization, the electrons are accelerated and collide with the shell electrons of the atoms. The transfer of kinetic energy from the fast electrons to the shell electrons gives them sufficient energy to leave the bond of the atomic shell.
Of all the direct sources of highly charged ions, the highest charge states were produced with electron beam ion sources, making them the optimal choice for use at COALA in Darmstadt. The technology offers ideal conditions for achieving high charge states, as long as the vacuum technology used provides sufficient general conditions.
Operating principle of an electron beam ion source
In an electron beam ion source of the Dresden-EBIS-A type, as used at the TU Darmstadt, a highly emitting cathode is heated to about 2200 K in a vacuum. This generates a beam of free electrons which are accelerated from the electron gun towards the drift tube ensemble acting as an anode. During this process, the electron beam is compressed by a strong magnetic field, causing the electron current densities to reach values of several 10 amperes per cm². This high-density, high-speed electron beam encounters thermal gas atoms in the area of the drift tubes and collides with their shell electrons. The resulting ions are trapped by an electrostatic field in the area of the drift tubes, in what is known as the Electron Beam Ion Trap (EBIT).
Electron beam ion source
As long as the energy of the electron beam exceeds the binding energy, further shell electrons are removed by continuous electron impact ionization, bringing the ion to an ever higher charge state. This can continue until all shell electrons are removed and only the bare nucleus remains.
After passing through the drift tubes, the electrons are electrostatically directed by the repeller voltage to a cooled electron collector. The highly charged ions can leave the ion trap and are available for various applications.
This is only an excerpt.
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