DIP, a high-power IR dispersion interferometer using OpGaAs crystal for electron density measurement

Dispersion interferometers are installed on plasma reactors all around the globe to measure electronic plasma density. It is one of the key parameters to control the ITER fusion reactor, as such it needs to be measured with an accuracy better than 10¹⁷m^-3 at a kHz rate with a low latency. To do so, the interferometer measures the optical phase shift introduced by the plasma between the 1^st and 2^nd harmonic of a 9.6mm CO₂ laser which is proportional to the electron density.

Reaching desired performances requires that sufficient laser power returning to the reception system after travelling to the vacuum vessel, crossing the plasma and going back, a 120 m long travel. So, it’s mandatory to limit laser power loss in the system and generate sufficient power at the beginning.

Mirrors surfaces in the vacuum vessel will suffer from the very harsh environment. Since longer wavelength are less sensitive to surface deteriorations, this throughout loss is mitigated by operating the interferometer in the IR region.

Bertin is currently building an IR dispersion interferometer operating at unprecedented power levels in this spectral region. A commercially available CO₂ laser, is producing 30W CW of power at 9.6μm. One report the reliable generation of more than 100 mW CW at 4.8μm by frequency doubling, using a periodically poled GaAs (OpGaAs) crystal. This high pump power regime, will be on the emission side, before lasers are going through the plasma. After the plasma, on the reception side, a second doubling crystal needs to operate with 20 times less power. Bertin used the crystal in a different doubling geometry, in order to optimize 2^nd harmonic generation and get a high SNR on the interference signal observed at 4.8μm.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.