Ion cyclotron resonant heating (ICRH) for the Wendelstein 7-X stellarator is ready for use. This is the result of close cooperation between the Laboratory for Plasma Physics of the Royal Military Academy (LPP-ERM/KMS) in Brussels, the Forschungszentrum Jülich (FZJ) and the Max-Planck Institute for Plasma Physics in Greifswald (IPP), both in Germany.
"This long-proven technique will bring the new heating system for Wendelstein 7-X to life," explains Johannes-Peter Kallmeyer, who is responsible for ion cyclotron resonant heating at the IPP. Two transmitters, with the most powerful electron tubes in the world, generate radio waves in the short wave range between 25 and 38 MHz, with a combined power of up to 4 million watts. Via large copper transmission lines, they reach the antenna in the torus hall. There, the new and unique antenna will emit the radio waves into the plasma. This system will be commissioned in early 2022 for use in the next experimental campaign on Wendelstein 7-X.
The central component that is so unique for this system is the 380 kg antenna together with the 4.7 tonne supporting structure. This system arrived in Greifswald on 10 August last year. During installation at Wendelstein 7-X, the antenna was aligned with a precision of about 1 mm, to avoid collisions when moving in the entrance gate of the torus. "Working with excessive haste or huge forces was out of the question. Patience and precision were required here," describes Dr Jozef Ongena, research director of LPP-ERM/KMS and project leader of the ICRH antenna.
Then the 96-metre transmission lines, connecting the two transmitters with the antenna in the torus hall, had to be finished. In addition, various pieces of equipment, switchboards and control boxes were built to calibrate the transmission lines and to ensure the proper functioning of the ICRH system.
A revolutionary antenna concept designed in the KMS
The ICRH antenna consists of two 75cm long electrical conductors that can operate with arbitrary phase differences to radiate the waves into the plasma. These electrical conductors, 'straps' in the jargon, are coated with copper. Their surface is perfectly adapted to one of the possible plasma configurations in Wendelstein 7-X, and are therefore curved in three spatial dimensions. Their manufacture was only possible with the most modern digital tools. These 'straps' are housed in a metal casing and are retracted by one centimetre to prevent direct interaction with the hot plasma. For other magnetic configurations of smaller or larger dimensions, the antenna can be moved horizontally over 35 centimetres.
However, the electromagnetic waves emitted by the antenna cannot propagate in vacuum or a plasma that is too 'thin'. Therefore, a system for injecting hydrogen gas is installed along in the antenna box. If the distance between the antenna and the plasma is too great in certain positions, gas can be injected locally, which then ionises and creates a sufficiently dense plasma. In this way, propagation of the waves to the plasma can be ensured. This is very important to optimise the radio frequency power radiated by the antenna into the plasma.
The antenna was built and tested in the research centre in Jülich under the direction of Dr. Bernd Schweer. This centre has many years of experience and state-of-the-art equipment for the construction of such high-quality instruments. In addition, the centre works closely with first-class suppliers. Like everyone else involved, they too had to come up with innovations to enable the construction of the antenna.
The antenna, part of the transmission lines and the two transmitters originate from the Forschungszentrum Jülich, where they were installed in previous years by LPP/ERM-KMS for use on the famous tokamak TEXTOR. Until the end of 2013, wall materials for future fusion plants were tested in this device. The transmitters and transmission lines formerly used in TEXTOR are thus given a "second life" at Wendelstein 7-X. This has advantages and disadvantages, as Johannes-Peter Kallmeyer explains: "On the one hand, new technologies could open up entirely new possibilities, but on the other hand, the electron tube technology is robust and therefore very reliable."
The complicated shape of the Wendelstein 7-X machine in Greifswald and the various possible plasma configurations dictated the technical and mechanical properties of the antenna. The relatively small size of the entrance port, although one of the largest at Wendelstein 7-X, the rather different shapes of the many possible magnetic configurations, and the heat load of about 100kW/m2 required an extremely precise and flexible antenna system.
First ideas for the antenna were outlined about ten years ago during a brainstorming session with Prof. Michael Van Schoor, director of the LPP-ERM/KMS, Dr. Jozef Ongena and the colleagues in Greifswald. Since LPP-ERM/KMS can draw on extensive experience with ICRH antennas for tokamaks, the start was quick. "Then came the difficult questions from Dr. Dirk Hartmann and Prof. Robert Wolf," recalls Dr. Jozef Ongena. What are the highest densities compatible with the use of ICRH? Is it possible to generate the required amount of fast particles with this system? How much power can be radiated into the plasma? Theoretical physicists in LPP/ERM-KMS studied these and other questions in detail and concluded that a well-planned ICRH system can meet the requirements of the Wendelstein7-X team. This finally resulted in a very flexible ICRH system.
From a "spare" heating system ...
Before the commissioning of Wendelstein 7-X, it was not clear whether the planned magnetic field strengths of 2.5T could actually be achieved on this complicated fusion machine. The main heating system of Wendelstein 7-X is the electron cyclotron resonant heating (ECRH) system that operates at a fixed frequency of 140GHz, in the microwave range, with a maximum power of 10 million watts. This system only works optimally at the intended high magnetic field strength. ICRH is much more flexible and allows, in principle, to heat the plasmas in Wendelstein 7-X even without ECRH.
Another problem with ECRH is that at high particle densities in the plasma, its operation becomes more difficult, as then specific details of the heating method have to be changed, thus reducing its efficiency. However, these high densities are necessary for crucial experiments on Wendelstein 7-X. For proper operation under such conditions, ICRH is a very important complement.
... to a versatile research tool ...
Apart from pure heating, ICRH offers further attractive possibilities. "For experimental research, it is important to have various options. With ICRH it becomes possible to modify the heating system and therefore to better understand the behaviour of the plasma" explains Dr. Dirk Hartmann. Furthermore, high-energy ions can be generated with ICRH that behave in the same way as fast helium particles. Such helium particles are produced during fusion reactions in future fusion power plants and will be needed to maintain the high plasma temperature. Therefore, it is important to understand how they are confined. Using the fast ions produced with the ICRH system, particle trajectories and interaction patterns can be studied. Conclusions can then be drawn about the behaviour of helium particles and their confinement in a future fusion reactor.
Studying the trajectories of fast ions is also a way to check the optimisation of Wendelstein 7-X. "Constructing a stellarator just on basic physical principles, it won’t work because the particles leave the plasma far too easily," explains Dr Jozef Ongena. An initial optimisation for better particle confinement was tested in the past with the small stellarator Wendelstein 7-AS at the Max Planck Institute for Plasma Physics in Garching bei München. Wendelstein 7-X has an even further optimized magnetic field. Its effect on the particle tracks can be studied in detail with ICRH.
Another advantage of the ICRH radio transmitters is that the frequency of the emiited waves can be changes rather easily. This is not the case for the ECRH system with its fixed frequency. Therefore, waves can be radiated into the plasma that resonate with ions other than hydrogen ions. As a result, impurity ions can also be heated and this can cause them to leave the plasma. "This could go so far," explains Dr. Dirk Hartmann, "that a homeopathic dose of ICRH is sufficient to positively influence the plasma to a very significant extent."
... for new insights in international nuclear fusion research
"These are new techniques that have never been tried on a stellarator before," summarises Dr Jozef Ongena. "We want to see whether these methods, which should work in theory, also fulfil their promise in practice. And, of course, we want to deposit the maximum heating power into the plasma, so that there are sufficient fast particles in the centre of the plasma, which is important for essential tests of the optimised magnetic field configuration of Wendelstein 7-X."
This also makes it clear where the hopes of the ICRH project lie. It is not only about the successful heating with ICRH, but also about positive effects on the magnetic confinement of the plasma particles. The participating partners IPP, Forschungszentrum Jülich and LPP/ERM-KMS want to advance fusion research as a whole. "Good cooperation between all involved always yields the best results," says Dr Jozef Ongena. As soon as the radio waves reach the antenna via the copper transmission lines, new scientific discoveries will not be long in coming.
Source: Article from IMPULSE No 4/21 - title: "Präzise geplante Antenne eröffnet neue Möglichkeiten" - Freely translated from German by Jef Ongena.
Main image: The assembly team after the successful installation of the antenna in Greifswald. From left to right: Matthias Stern and Peter Kallmeyer (IPP), Jef Ongena and Yevgen Kazakov (LPP-ERM/KMS), Noah Richter and David Castaño-Bardawill (FZJ), Bernd Schweer (LPP-ERM/KMS).
