Plasma diagnostics by near-resonant raman scattering

In Tokamak-type fusion reactors, the core plasma is magnetically confined within a large toroidal vessel. In the confinement region of the vessel, the magnetic field lines form closed loops, whereas outside the core plasma the field lines are arranged so as to strike the so called diverter plates. This is done in order to exhaust the energy outflow while simultaneously assuring a good core confinement. The diverter plates are under extremely large thermal load. In order to reduce this thermal load on the plates, buffer gas, e.g., argon (Ar), is pumped into the diverter region. Ar atoms absorb part of the energy of the escaped plasma particles and re-radiate it. That results in a more evenly distributed thermal energy load within the diverter region and, consequently, in a prolonged life-time of the diverter plates. In order to monitor the reactor operation, it is necessary to be able to monitor the concentration and temperature distributions of the partially ionized Ar gas. Such measurements have to be performed by non-perturbing remote sensing methods, e.g., by laser-based techniques. The conventional resonance-fluorescence method is not applicable, because its sensitivity is severely limited by the strong line emission of the hot Ar plasma. Moreover, the diverter plasma is expected to be optically thick, absorbing both the probe and the scattered radiation. The applicability of the near-resonant Rayleigh-scattering technique is limited by Thomson scattering from free electrons and by reflected stray light, originating from the probe beam, which both spectrally overlap the Rayleigh signal. Here, we study the applicability of the near-resonant Raman-scattering technique, free from the aforementioned difficulties, to plasma diagnostics. In preliminary test experiments, we studied scattering from an inductively coupled Ar plasma. The laser beam was focussed onto the plasma torch in a multipass geometry to enhance the Raman signal. The scattered radiation was collected at the angle of 90 deg C, filtered by a grating monochromator, and detected by a-photomultiplier tube, whose output signal was processed by a boxcar integrator. We demonstrate the observation of Raman scattering signal from the plasma at several different wavelengths with a fixed laser wavelength. We also present the saturation behaviour of the scattering signal. We discuss how Ar atom density and temperature are deducted from the magnitude of the scattering signal. Our experiments show that the Raman-scattering method is well-suited for plasma-diagnostics (author)