Starting point of the GSI accelerator facility: Ion sources

Two huge metal containers painted bright orange, each the size of a single-family house, and one smaller setup mark the starting point of the GSI accelerator facility: the ion sources. This is where the particles are generated before they are accelerated in UNILAC and SIS.

The scientists at GSI carry out research with electrically charged atoms, so called ions. The difference between a positively charged ion and an electrically neutral atom is to be found in the electron shells of the two atoms. In the case of the positive ion, fewer negatively charged electrons are present. This is what gives it a positive charge.

Unique facilities for researchers

Thanks to the variety of ion sources at its disposal, GSI is unique among research facilities worldwide in that it can generate positively charged ions of many different naturally occurring element. These range from “simple” hydrogen ions to ions of uranium, the heaviest of all the stable elements occurring on earth. GSI’s area of special expertise is heavy ions. The type of ions produced, and their charge state, depends on the requirements of the scientists who are currently conducting experiments. The scientists decide whether they require a large number of ions, highly charged ions or very fast ions.

Ions are generated in plasma

The elements used by the researchers are initially present as atoms—in a block of metal, for example, or a gas in a bottle. How are they converted into ions and ultimately an ion beam?

Inside the huge orange steel containers are high voltage cages. They contain the actual sources, which come in different types. The centerpiece of the source is a metallic chamber with a size between cubic centimeters and cubic decimeters. These contain atoms of the element from which the ion beam is to be generated as a solid or as a gas. Free electrons are produced by applying a large amount of energy to a wire filament or by means of an electric discharge—like a bolt of lightening in a storm. Whenever one of these free electrons collides with another electron in the shell of an atom and “knocks it out” of the shell, an ion is created. This process produces more and more free electrons, which then become available to ionize further atoms. The result is a chain reaction — a process known as electron ionization. The end result is known as a plasma—a mixture of free electrons and positive ions.

From zero to 2,000,000 km/h

The ionization process takes place in the plasma generator. Before the particles can be accelerated to create a beam, they must first be extracted from the source. This is achieved through the repeated application of extremely high positive voltages, depending on the type of ion between 20,000 and 130,000 Volts. Thus the ions are accelerated to 0.2 percent of the speed of light, i.e., to around two million km/h. They now have enough energy to be injected into the GSI accelerators.

Different ion sources for different beam properties

The accelerators at GSI have three injectors that can host six different types of ion sources. Two of the injectors are at the head of the UNILAC linear accelerator, one in the middle. This means that not only several experiments with different ions can be conducted in parallel but also different methods can be employed to vary beam properties.

Distinguished are ion sources that produce highly charged ions and high intensities, i.e. many ions. To gain highly charged ions many electrons are stripped out of the shell. The Pb27+ ion, for example, has 27 fewer electrons in its electron shell than a normal lead atom. As a rule the higher the ions are charged the higher is the speed that can be reached. An ion beam with high intensity is important for scientists to observe rare phenomena in the experiments at the end of the accelerator chain. They quickly want to see as many particle collisions as possible.

New installation for FAIR

Following completion of the new accelerator facility FAIR, researchers will have access to beams of a higher energy and higher intensity. As part of the project, members of the ion source department will install another injector and improve the existing systems.