Atomic Beam Probe (ABP)

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).


Set-up of the diagnostic beam and beam diagnostics (BES and ABP)

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


Lithium ions will be emitted constantly during the tokamak discharge ( ~ mA ) by a resistively heated solid ion emitter, accelerated to energies up to 100 keV and focused by ion optics. Deflection plates will be used to deflect the beam trajectory in the plasma or to target it outside into a Faraday cup, which will allow a background noise measurement. Lithium ions will be neutralized via charge exchange by passing through a chamber with sodium vapor.

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.


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

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.