Magnetohydrodynamics

Magnetohydrodynamics

From the point of view of magnetohydrodynamics (MHD), electromagnetic and hydrodynamic properties of the plasma are combined in order to obtain a comprehensive picture of both static states and dynamic processes.

 

Static equilibria between magnetic field pressure from the external field coils and magnetic field pressure from currents in the plasma are reconstructed on the basis of measurement data and provide detailed information about plasma parameters, such as pressure profiles or plasma current distributions. The latter are particularly valuable because current density profiles cannot be measured directly in the Wendelstein 7-X stellarator. The reconstructed magnetic field geometry plays an important role in the investigation of plasma energy and particle transport, especially in the case of fusion-relevant high-energy plasmas. It is also an important ingredient in complex simulation codes such as those used to investigate various magnetohydrodynamic phenomena.

In addition to profile diagnostics for plasma temperature and density, the necessary measurement data are provided by the magnetic equilibrium diagnostics. They measure the plasma energy (diamagnetic loops) and the proportions of the plasma current (Rogowski coils, saddle coils). Furthermore, the shift of the plasma axis can be derived from the measurements of the Soft X-ray tomography.

In addition to the states of equilibrium, MHD also deals with the stability of these states and with dynamic processes, such as waves and modes in the plasma. Such mode activity can be observed, among other things, with the Mirnov coils and with Soft X-ray tomography.

Diagnostics

Diamagnetic loops

The diamagnetic loops are located in the plasma vessel and surround the plasma in a poloidal manner. A total of three loops are built in, two of which are in a region with a bean-shaped plasma cut (bean-shaped plane) and one in a triangular plane. The latter is equipped with four compensation coils, with the help of which the measurement signal is directly compensated against external interference, such as current fluctuations in the main field coils of the Wendelstein 7-X. The measurement of the variable magnetic flux is proportional to the plasma energy.

Rogowski coils

An uninterrupted Rogowski coil surrounds the plasma poloidally, so that the measured change in magnetic flux is directly proportional to the toroidal plasma current. A coil of this type is installed on the Wendelstein 7-X, as well as a number of shorter Rogowski coil segments that do not completely enclose the plasma and which in turn provide information about poloidal current distributions. Almost all Rogowski coils are positioned both inside and outside the plasma vessel (in the cryostat). In this way, currents in the plasma vessel itself can also be characterized.

Saddle coils

The large-area saddle coils are located in the cryostat directly on the plasma vessel. They measure the proportion of the change in magnetic flux that is generated by vertical field components. It is proportional to the so-called "Pfirsch-Schlüter" currents, which contribute to the horizontal position and deformation of the plasma. They are also proportional to the plasma energy.

Mirnov coils

The total of 125 Mirnov coils are arranged in three toroidal positions in the plasma vessel, each in a poloidal ring of 21 or 41 individual probes. The remaining probes are distributed toroidally. The size and number of turns of these coils are designed to measure magnetic field fluctuations in the range of 1-1000 kHz and thus provide information about dynamic processes such as waves and modes in the plasma.

Soft X-ray tomography

A poloidal arrangement in the plasma vessel consisting of 20 cameras measures radiation emissions in the soft X-ray range over 360 individual rays of vision. A tomographic inversion enables the investigation of density fluctuations in the plasma, e.g. caused by instabilities or the transport of impurities. The measurements of the soft X-ray tomography system also allow conclusions to be drawn about the shift of the plasma axis due to changes in density and temperature ("Shafranov-Schift") and thus contribute significantly to the characterization of magnetic equilibria.

Pulse height analysis

The Pulse height analysis diagnostic provides measurements in the energy spectrum of soft X-rays, which are emitted along three lines of sight through the plasma. The diagnostics are designed to identify impurities in the plasma as well as to estimate electron temperature and charge numbers, as well as to detect super-thermal electrons.

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