Ishtar is the name of the Mesopotamian goddess of love and war. But, at the Max Planck Institute for Plasma Physics (IPP) in Garching, Ishtar is a testbed designed to help unravel the secrets of the plasma edge in fusion experiments.

Beneath the eyes of a lion

Whenever they can, the members of Ishtar’s international team show visitors the refurbished home of the Ion Cyclotron Sheath Test Arrangement, short Ishtar, at the IPP site. A large lion painted onto the back wall of the control room greets the guests. Back in Mesopotamia, this animal represented the antique deity Ishtar – and can still be seen today on the eponymously named Gate which is exhibited in the Berlin Pergamon Museum.

According to legend, Isthar had used all her wiles to access the secrets of the underworld. If we apply this tale to a fusion device, the underworld represents the region of the plasma edge. The section haunted by the electrical fields of the antennas which heat the plasma.

Part of the team in front of the Ishtar lion: (from left to right in the front row) Roman Ochoukov (developing the probe diagnostics and operating Ishtar), Rodolphe D’Inca (responsible for the machine); (in the back) Kristel Crombé (Ishtar project leader), PhD students Ana Kostic, Jonathan Jacquot (developing the theoretical models and operating Ishtar), Marii Usoltceva and Jean-Marie Noterdaeme (Head of the ICRF department). Picture: EUROfusion

Part of the team in front of the Ishtar lion: (from left to right in the front row) Roman Ochoukov (developing the probe diagnostics and operating Ishtar), Rodolphe D’Inca (responsible for the machine); (in the back) Kristel Crombé (Ishtar project leader), PhD students Ana Kostic, Jonathan Jacquot (developing the theoretical models and operating Ishtar), Marii Usoltceva and Jean-Marie Noterdaeme (Head of the ICRF department). Picture: EUROfusion

The wall and the energy

The experiment in Garching will deliver valuable data to enhance the quality of fusion plasmas. In a fusion experiment, the plasma edge may be imagined as a protective skin: its properties control the power and particle exchange between the plasma core and the vessel walls. There is a strong interplay between the plasma edge and the first wall. The processes that take place at the plasma edge are crucial when it comes to both protecting the vessel’s inner wall (“first wall) from the plasma and for confining the plasma energy, which is important for a future fusion power plant. Unlike a tokamak or a stellarator, Ishtar offers flexible operation which costs less and requires less preparation time.

Nevertheless, the outcome of research carried out using Ishtar is important for European fusion science and is therefore supported by EUROfusion.

Antennas under fusion conditions

Since 2013, the experiment investigates models of ICRF antennas in realistic plasma edge conditions. ICRF stands for Ion Cyclotron Range of Frequencies. These antennas inject waves into the plasma. In a fusion experiment, such as a tokamak, the waves resonate when their frequency matches the frequency at which ions gyrate around the magnetic field lines and subsequently the plasma is heated. More complicated effects may occur at the edge, which may also deteriorate the heating.

Ishtar is developing into an experimental platform for master and PhD students and for international collaborators.

Not a doughnut, but a cylinder

The Ishtar testbed delivers plasma in a cylindrical shape. Picture: EUROfusion

The Ishtar testbed delivers plasma in a cylindrical shape. Picture: EUROfusion

Ishtar forms an entirely different plasma shape than that of a tokamak. Instead of the famous doughnut, it provides a cylindrical plasma in a longitudinal magnetic field.

Therefore, the machine consists of a main vacuum vessel which is connected to a plasma source. Two magnetic field coils encircle the container. The installed ICRF antenna is powered by an ASDEX-Upgrade generator providing up to several hundred kilowatts. Attached to the side of the vacuum vessel is the large plasma source (1m long and 40 cm in diameter), a glass tube fed with argon, helium or hydrogen. It is equipped with a helicon antenna and magnetic coils.

Wave coupling

The design of Ishtar generates a plasma with the characteristics of a tokamak’s edge plasma in the main vessel, right in front of the ICRF antenna. Hence, it is a facility which enables the investigation of the ICRF wave coupling with the fourth matter.

Predicting big plasma and beyond

The device was set up specifically to deliver the data for the benchmarking of the ICRF-edge theories, but it already contributes to other areas related to plasma production and ICRF-plasma interactions.

After implementing additional diagnostics in 2017, the researchers are confident that they will be able to deliver experimental data to verify or falsify theories describing the edge of ICRF heated plasmas. The models will also allow extrapolations to larger plasmas such as those in JET or ITER.

Ishtar – an international experimental platform

Ishtar is developing into an experimental platform for master and PhD students and for international collaborators. It not only facilitates the investigations of phenomena between the ICRF antenna and the plasma. Thanks to the flexible conditions it creates, it also enables the validation of codes with data in simplified geometries or diagnostics, before implementing them on an operational tokamak.