The Academy of Sciences of the Czech rebublic - ASCR


 

Measured quantities:: Poloidal magnetic field perturbation, edge plasma current profile
Temporal resolution: ~ 1 μs
Responsible person: V. Weinzettl, P. Háček
Collaboration: Association EURATOM/HAS, Hungary

 

 

Diagnostic description:

 

Atomic Beam Probe (ABP) system is a new approach for measurement of the poloidal magnetic field fluctuations in the edge plasma. It is a an extension of a Beam Emission Spectroscopy (BES) system using neutral lithium diagnostic beam injected into the plasma (see Fig. 1).

 

 

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Fig. 1: Set-up of the diagnostic beam and beam diagnostics (BES and ABP).

 

When injected into plasma, the lithium atoms are collisionally excited and ionized. Lithium ions are deflected due to the magnetic field and collected by the ABP detector located in the vertical diagnostic port of the same poloidal cross section.

For calculation of the edge plasma current perturbations from the ABP detector signal, the equilibrium magnetic field has to be reconstructed by EFIT code. The edge current perturbations modify the poloidal component of the equilibrium magnetic field. This effect can be modeled by a sum of additional magnetic fields generated by “elementary” toroidal currents (Fig. 2). These perturbation current filaments are assumed to be distributed along the separatrix. The trace of lithium ions on the ABP detector shifts toroidally due to the j x B force caused by poloidal magnetic field. Using numerical optimalization techniques, the perturbative poloidal magnetic field and, therefore, also the change in the edge plasma currents can be determined.

 

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Fig. 2: Left: Trajectory of lithium ions in the COMPASS magnetic field, energy of the beam 100 keV, place of ionization r = 0.71 m. Right: Perturbation currents distributed along the separatrix.