Measuring plasma density with a OpGaAs based high-power IR dispersion interferometer.

Dispersion interferometers are installed on plasma reactors all around the globe to measure 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 9.6μm CO₂ laser.

For reaching the desired performance it is crucial to provide sufficient optical power for interference detection. This is achieved by limiting laser loss during the 120 meters of beam transport and generating sufficient power for the two laser harmonics at the beginning.

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

Bertin is building a dispersion interferometer operating at high power in the IR spectral region. From a CO₂ laser producing 30W CW at 9.6μm, Bertin already demonstrated second harmonic generation with a periodically poled GaAs (OpGaAs) crystal. However, the success of this first attempt revealed issues, triggering a redesign effort. Additionally, Bertin is exploring another geometry for 2^nd harmonic generation to get a high SNR on the interference signal observed at 4.8μm when operating at 20 times less power.

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