Plasma Source “PlaQ”
In order to investigate plasma-material interaction issues in the laboratory, material samples are exposed to various plasmas under as well-defined conditions as possible.
In the plasma source “PlaQ”, low pressure plasmas at typical gas pressures around 1 Pa and with electron temperatures of a few eV are generated. The working gases are typically hydrogen (H2) and deuterium (D2), but also helium, nitrogen, oxygen and other gases, as well as gas mixtures.
In the “PlaQ” set-up, an ECR plasma is generated with the help of microwaves and an external magnetic field inside a “cage” within the vacuum vessel. The cage confines the largest part the microwave radiation and thus also the plasma. The plasma streams through an aperture at the bottom of the cage and irradiates the sample holder. An electrical bias voltage accelerates ions from the plasma in a thin boundary layer. Due to the spatial separation between sample holder and plasma generation, the source plasma is barely influenced by the bias voltage. Therefore, the ion energy can be adjusted more or less independently of the ion flux.
Some plasma-wall interaction processes depend strongly on temperature, e.g., the uptake and retention of hydrogen isotopes (i.e. fusion fuel). Therefore, the “PlaQ” set-up is equipped with a system for precise temperature control, which uses two liquid-medium thermostats for different temperature ranges. Thus, a temperature range from about 200-600 K is accessible.
“PlaQ” is furthermore equipped with a moveable Langmuir probe, which is able to measure the profile of the ion flux onto the sample holder, as well as plasma parameters.
A high-performance camera system allows observing sample surfaces in real time while they are being exposed to plasma. It is able to resolve details as small as about 5 µm. The live observations make it possible to not only investigate the final state after the end of the plasma exposure, but also the dynamics of sample surfaces in real time.
The scientific focus at “PlaQ” is the uptake and retention of hydrogen isotopes in materials for the first wall and the divertor of fusion devices. After plasma exposure, this is measured by nuclear reaction analysis at the tandem accelerator as well as by thermal desorption spectroscopy. The composition and energy distribution of the ion flux for standard deuterium plasmas at PlaQ was calibrated with a substantial effort in the past and subsequently published.
Apart from tungsten (divertor material), also steels developed specifically for the use in fusion reactors are investigated. Other materials of interest are novel tungsten-based materials such as fibre-reinforced tungsten, as well as tungsten alloys and liquid metal (tin) as possible alternative divertor materials.
Electron cyclotron resonance heating
For heating the plasma, electron cyclotron resonance heating (ECRH) is used. This method makes use of the fact that charged particles follow circular, respectively spiral, trajectories around the magnetic field lines. Their gyration frequency is proportional to the charge and the magnetic flux density, and inversely proportional to the mass of the particles. By using electromagnetic waves with exactly this frequency, the particles resonantly absorb energy from the waves. ECRH is common for generating low-temperature plasmas in laboratories as well as in industrial processes, and is furthermore used for heating fusion plasmas. For the former, microwave transmitters with the frequency of 2.45 GHz are used, which is widely available used for industrial, scientific and medical applications. The resonance conditions for this microwave frequency is reached at a magnetic flux density of 87.5 mT. In fusion experiments, much stronger magnetic fields are necessary to confine the plasma, which makes transmitters with correspondingly higher frequencies necessary.
ECR heating allows running plasmas efficiently, with a high electron density and in a wide range of pressures. Because this kind of electromagnetic wave heating does not require any electrodes or antennas in direct contact to the plasma, the plasma typically contains only a very small concentration of impurities. This makes ECR plasmas particularly suitable for laboratory applications and issues of fundamental research. The plasmas at PlaQ can furthermore be sustained for extended periods of time, which allows long-term expositions (record up to now: 2 weeks of continuous plasma irradiation).
A Langmuir probe is a tool to measure plasma parameters in low-temperature plasmas as well as in the boundary layer of fusion plasmas. It usually consists of a, more or less, thin wire that is electrically isolated from the vacuum vessel and can thus be biased. Measuring current-voltage characteristics allows deriving the electron temperature and density. By moving the probe through the plasma at a fixed voltage, density or ion flux profiles can be measured.
In situ camera system:
Samples are observed during plasma exposition using various telecentric objective lenses. These special purpose lenses produce an image without perspective distortion, i.e., their image magnification does not depend on the distance to the observed object, at least while it is within the focused working distance. Because the plasma would damage the sensitive glass parts of the lenses, they are placed outside the plasma vessel. Due to space restrictions, they observe the samples at a shallow angle. Accordingly, only a small part of the sample actually appears in focus. By moving the objective lens along a motorized optical rail, all parts of the sample can be consecutively imaged in focus. Thanks to the telecentric lenses, a fully focused, distortion-free image can be created by digital post-processing.
Images are captured using a high-resolution color CMOS camera with 4096x3000 pixels. The best possible spatial resolution of 5 µm that can be obtained with the available objective lenses corresponds to about 3 pixels on the camera chip.