SKEDSOFT

Quality Control Engineering

Thomson, Rayleigh and Raman scattering

In laser scattering diagnostic techniques, the radiation scattered by the plasma is studied. The probability of scattering can be as low as 10-17, which causes need for a strong light source such as a pulsed laser. Our lab is equipped with a 10 Hz pulsed, frequency doubled Nd:YAG laser (532 nm; 450 mJ/pulse). The pulse length is about 7 ns. If this laser beam is pointed at the plasma, the laser photons can be scattered on free electrons in the plasma (Thomson Scattering), atoms and molecules (Rayleigh scattering) and possibly even accompanied by a rotational or vibrational transition in the molecule (Raman Scattering).

Thomson scattering can be used to determine the electron density (through the number of scattered photons) and temperature (Doppler broadening of the Thomson spectrum). The experimental setup to perform Thomson Scattering measurements is sketched in the figure on the right (click for more detail). The laser system is shown on the left; the scattered photons are imaged onto the the entrance slit of a spectrograph system (on the right). The spectrum is recorded by very sensitive detector (in our case an Andor intensified CCD). Below a (slightly enhanced :-)) picture of the setup, with laser beam and Thomson scattered light, is shown

Schematical layout of the experimental setup for Thomson Scattering

 

In the same way as Thomson scattering can give information on free electrons in the plasma (density and temperature), Rayleigh scattering can reveal the density and temperature of atoms and ions (heavy particles). Rayleigh scattering provides a way to calibrate the setup by doing a measurement on a known gas density (at room temperature). This makes the atom and electron density measurements in plasmas absolute.

Thomson scattering can be used to determine the electron density (through the number of scattered photons) and temperature (Doppler broadening of the Thomson spectrum). The experimental setup to perform Thomson Scattering measurements is sketched in the figure on the right (click for more detail). The laser system is shown on the left; the scattered photons are imaged onto the the entrance slit of a spectrograph system (on the right). The spectrum is recorded by very sensitive detector (in our case an Andor intensified CCD). Below a (slightly enhanced :-)) picture of the setup, with laser beam and Thomson scattered light, is shown.

A sketch of a typical Rotational Raman spectrum on N2