PWI interactions in tokamaks

Plasma-Wall interactions are a very important area of ​​international effort to master energy based on thermonuclear fusion. It is clearly marked on the road map of these studies, as evidenced by the establishment of the WP PFC work package dedicated to PWI in the EUROfusion organization (Work Package: Plasma Facing Components) and their also present in other programs such as MST (Medium Size Tokamaks).

The purpose of the research is to develop materials, components, diagnostic and maintenance techniques that will ensure the efficient operation of the new generation of tokamaks in terms of durability, safety and minimization of the impact of the internal wall of these devices on the plasma. The scope of research is very wide, because it considers not only the impact of destructive factors of plasma (high energy ions, electrons, radiation and neutrons), but also the complex phenomena that occur under their influence. These include physical and chemical erosion, re- and co-depositing, recycling, arcing, fuel retention, cracks, bubbles and micropores, and material activation.

Diagnostics are needed

In the face of the complexity of these processes, it is very important to develop appropriate diagnostics that would allow the characterization of significant parameters of occurring phenomena, such as the scale of fuel retention, material migration from other areas of the tokamak or the predominance of erosion or deposition in a given area of ​​the device. Such information could be provided by a method that allows chemical composition measurements, but one should be aware that in the case of tokamak, a number of difficult-to-meet requirements are imposed on the method.
In order not to get into extensive considerations: the method must be non-contact, resistant to destructive environmental factors such as temperature and electromagnetic radiation, but also may not cause such interference itself. According to current knowledge, only the LIBS method can meet these requirements.

Operation and benefits of LIBS
LIBS (Laser induced Breakdown Spectroscopy), consists in stimulating the examined material with an intense laser pulse (but with low energy) and observing the spectrum resulting from this plasma. Based on this measurement, information about the chemical composition of the sample can be obtained, but this is not trivial, because the spectra of various elements sometimes consist of many lines located close to each other and thus difficult to clear identification. Despite this, in tokamak applications this method is irreplaceable and thanks to the work and experience of numerous teams it works great to identify the chemical composition of the so-called calibration samples simulating the composition of layers resulting from the transport of materials in new generation tokamaks, elements dismantled from closed and currently operating devices, as well as even inside the tokamak itself through the use of a remote control system. The last of the achievements became possible thanks to the cooperation of the LLPS team with scientists from the Italian ENEA Frascati.

Tasks and challenges for LIBS in fusion energy
The task in the context of which LIBS appeared in fusion research, was fuel retention, initially in the carbon co-deposit, and later in the so-called mixed materials. Originally LIBS was proposed as a diagnostic accompanying the fuel removal (by the team from IFPiLM), but due to the growing role of mixed materials, that may also contain impurities, LIBS was considered a universal method. In the current research on fusion technology materials, the method has demonstrated the ability to detect not only hydrogen isotopes, but also carbon, tungsten, molybdenum, beryllium, aluminum, iron, copper, tantalum and other elements considered as both construction materials and potential contaminants.
To prove its effectiveness, the method had to meet several important challenges. First, the need for quantitative characterization of hydrogen isotopes can be mentioned, which requires not only high signal quality and low measurement error, but also the appropriate conversion of optical signal parameters into the concentration of atoms of the isotope in question. This is a very difficult task due to the scattering of parameters of layers containing fuel particles, which can be composed of different matrixes of materials, have different ablation coefficients and different layer removal dynamics. All these parameters affect the optical signal, which becomes difficult to interpret. In such conditions, universal calibration becomes practically impossible ... therefore, the calibration free method is used.

The LIBS calibration-free method, with respect to the hydrogen isotope characterization, must meet two contradictory requirements: have a high resolution to distinguish between closely located hydrogen isotope lines and have a wide bandwidth to observe lines of other elements for the correct determination of plasma parameters (such as temperature and electron density) necessary for the calculations on which it is based. This problem was solved with the cooperation of Polish and Italian teams by the simultaneous use of two spectrometers: an ENEA spectrometer enabling isotope separation and an IFPiLM spectrometer enabling the detection of other elements and determining plasma parameters.

Another problem was previously indicated: difficulties in separating the lines of individual isotopes of hydrogen, located very close to each other. Even when measuring with a very high resolution spectrometer, their separation may not be possible due to the Stark broadening phenomenon, which causes the lines to completely overlap. To deal with this problem, a special measuring head was used during the ENEA / IFPiLM experiment to allow pressure reduction and measurement in an argon atmosphere. Thanks to this, it became possible to reduce the Stark effect and separate the hydrogen and deuterium lines.

There are various ways to deal with the problem of improving the signal-to-noise ratio needed to minimize the measurement error. A popular procedure is to average a large number of measurements taken in nearby spots of the sample. Another method is to use an additional electrical discharge synchronized with the laser generated plasma. In LLPS, on the other hand, the two-pulse variant (Dual-Pulse LIBS) is used. This is a very elegant solution because it avoids averaging, thus shortens the measurement time and reduces damage to the surface, and at the same time does not require the use of additional equipment, as in the case of the method with assisted discharge. It is only necessary to use a dual-pulse laser (which is not much more expensive than the standard one) and a lot of work related to the optimization of the two-pulse interval, which is largely influenced by the conditions of the experiment. However, the team's experience allows them to deal with such works quite quickly.

Still another problem is taking measurements inside a tokamak, i.e. a device in which access to the inner wall is a complicated task and requires advanced opto-mechanical systems. Teams from IFPiLM and ENEA conducted a successful experiment; first on the mock-up, then on the FTU tokamak itself, in which effective measurements of the internal wall composition and calibrated samples installed inside the device were made using a remotely controlled arm equipped with a measuring head.

The continuation of this experiment is planned for 2021, when we will try to build an analogous system for a larger tokamak: JET or WEST. Until then, we will certainly not be idle. After taking measurements on calibration samples, we are prepared for the characterization of samples that we receive from currently operating devices, including AUG tokamak and W-7X stellarator. Other tasks await us, such as the automation of measurements and data analysis, and the use of modern machine learning methods. You can read more about it here.