Beam Emission Spectroscopy

Measured quantities:

Electron density profile, electron density fluctuations at the plasma edge

Spatial resolution:

1 cm

Temporal resolution:

1 ms (CCD camera), µs (avalanche photodiodes)

Responsible person:

V. Weinzettl

Collaboration:

Association EURATOM/HAS, Hungary


Diagnostic description:


Beam Emission Spectroscopy using accelerated neutral particle beams become a routine technique for the determination of electron density profiles in fusion plasmas, see Fig.1. The method is based on the fact that light emission from the neutral beam penetrating the plasma depends on plasma parameters. In the case of particular beam species, e.g. Li, Na, sensitivity on the electron temperature is small, therefore, the electron density profile primarily determines the beam light emission.


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).


In the diagnostic neutral beam system planned for COMPASS tokamak, 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. The excited neutral atoms return to the basic state by emitting radiation with characteristic wavelengths (670.8 nm for 2p – 2s transition), which is detected by detectors of standard BES system (CCD camera for slow and avalanche photodiodes for fast measurement).

The detection system measures the beam radiation along multiple lines of sight. The light detected in a measurement channel originates from the crossing point of the beam and the line of sight. If the beam is scanned up and down using deflection plates, the measurement points are also moving up and down along the lines of sight. Performing a series of measurements at a set of different beam positions, the whole two-dimensional density profile can be reconstructed, although not on a rectangular grid of points (see Fig. 2).


Experimental setup for two-dimensional measurement using a neutral Li beam

Fig. 2: Experimental setup for two-dimensional measurement using a neutral Li beam.