In comparison to neutral fluids, electromagnetic interaction supports a wealth of different instabilities that can drive a fusion plasma into a turbulent state due to nonlinear interaction. The instabilities are driven by gradients in thermodynamic quantities such as the particle density or the electron and ion temperature. The associated spatial and temporal scales spread over a wide range. One of the most fascinating properties of turbulence is its ability to exchange energy between different scales allowing the smallest scales (millimeter range) to determine the properties of the largest scales (several meter range). In fusion plasmas, large-scale flows can be driven by micro turbulence, considerably influencing the associated transport.
The major goals of the investigations are:
- Characterization of plasma turbulence in three-dimensional magnetic geometry and identification of the underlying dominant instabilities
- Studies of turbulent transport and assessment of its effect on confinement
- Comparison of the experimental results to nonlinear numerical simulations
The experimental characterization of turbulence is a challenge since it requires high spatial and temporal resolution and involves a wide range of diagnostic methods:
The Multi-Purpose Manipulator is a universal carrier system which can introduce various kinds of probes into the plasma boundary. Besides diverse use cases such as gas and impurity injection or material exposition studies, the manipulator is mostly employed for electric and magnetic probe measurements in the plasma boundary. In particular, arrays of electric probes simultaneously provide profiles of electron temperature and density as well as plasma turbulence and plasma flows. While this probe operation is invasive to the plasma and cannot be performed continuously due to the high heat loads from the plasma, it offer the clear advantage (over e.g. line of sight-integrated diagnostics) of a highly localized measurement with high temporal resolution.
Similar to a radar system, reflectometry radiates a weak microwave signal into the plasma and measures the reflected response. The frequency of the microwaves is typically 50-100 GHz. Since the microwave propagation in the plasma depends sensitively on the plasma parameters, the wave is reflected if the plasma density is sufficiently high, producing an echo. From the measured signal, the local plasma density and its fluctuations at the reflection layer can be characterized. Spatial resolution is achieved by tuning the microwave frequency. Using multiple antennas and detectors, the spatial structure of the fluctuations and its propagation can be analyzed using correlation methods. At oblique incidence, the density fluctuations cause scattering of the microwave beam similar to a diffraction grating. This allows for the measurement of the wavelength and phase velocity of the fluctuation.
A different method is based on the passive microwave emission due to the cyclotron motion of the electrons in the local magnetic field (typically between 126 and 162 GHz). The emission spectrum is dominated by thermal noise but also contains the temperature fluctuations of the electrons. To detect these small emission intensity, several radiometric detectors are used to extract the correlated part of the emission.
Phase contrast imaging:
The diagnostic uses an infrared laser beam, which is radiated through the plasma and is partly scattered on the fluctuations of the plasma density. The intensity of the scattered light depends on the amplitude of the density fluctuation, and its angle on the wavelength of the fluctuation. To detect the scattered light the phase contrast method is used, which originates from optical microscopy. Here, a phase shift between the scattered light and the laser beam is introduced and the resulting intensity after interference is proportional to the plasma density fluctuation. This non-invasive method allows turbulence measurements in the hot core of fusion plasmas.
Gas puff Imaging:
The Gas Puff Imaging diagnostic measures plasma turbulence in the scrape-off layer of the plasma, which is the transition region between the magnetically confined volume and the walls of the experiment. The diagnostic consists of two main components: A gas injector puffs a controlled amount of gas (hydrogen or helium) from specially shaped nozzles. The gas cloud is excited by the plasma electrons and emits visible light. The intensity fluctuations of the emitted light can be related to fluctuations in the plasma density.
The measurement of the emitted light is done by an optical system with a specialized ultra-sensitive fast camera that records at several hundred thousand images per second to capture the dynamics of turbulent structures as they move through the scrape-off layer
Comparison with numerical simulations:
To interpret the experimental results, we use simplified models in fluid-simulations or gyro-kinetic simulations with experimental values to compare the trends. This qualitative analysis helps us to understand the turbulent transport behavior and to identify the key parameters of the dynamics. The characterization of the dominant micro-instabilities, their range of validity and how the plasma parameters influence them are the main objective of these simulations.