Thomson, Rayleigh And Raman Scattering
Recent improvement of the Thomson scattering setup
Recently, our setup has been improved on two points. One of the problems is the extreme low scattering cross section for Thomson scattering. In order to improve the detection limit we used to reach (~1019 m-3) a very sensitive intensified CCD camera (Andor) with an extremely low noise level has been purchased. This should bring the detection limits down to about 1017 m-3.
Another important problem is stray light, that is scattering of laser radiation on the surroundings, is another important problem. Especially for plasmas contained in glass, like the QL lamp, this can be a serious limitation. Stray light photons undergo no significant wavelength shift (the surroundings are not moving) and therefore stray light is visible as a narrow peak in the center of the Thomson spectrum.
A single spectrograph is not sufficient for the suppression of the wings of this enormously intense signal, which thus distort the measured Thomson spectrum. Therefore a triple spectrograph system is used. The last of the three spectrographs retains its old function, and the two spectrographs in front of it serve as a very narrow notch filter that cut out the central part (~ 1 nm) of the spectrum. In the figure on the right this principle is sketched schematically. The first grating disperses the spectrum, a mask blocks the stray light and a second grating antidisperses the beam again. An intermediate slit blocks remaining stray light and the spectrum is finally created by the third grating and recorded by our iCCD camera.
The principle and a picture of the triple spectrograph for stray light rejection