A two colour interferometer for PROTO-SPHERA experiment

Highlights: • PROTO-SPHERA is a new concept of spherical torus with closed flux surfaces and a force-free Screw Pinch with open flux surfaces. • A two colour interferometer is the diagnostic system to measure the line density and to characterize this uncommon plasma. • For the high densities and its rate variation which can provoke a beam refraction a CO2 laser is the wavelength choice. • A two-colour vibration compensation system is foreseen by using a He-Ne laser. • The interferometer was assembled in a laboratory, a test interferometric signal has been observed. -- Abstract: PROTO-SPHERA (Spherical Plasma for Helicity Relaxation Assessment) is a new concept of torus that aims to produce a spherical torus with closed flux surfaces and a force-free Screw Pinch with open flux surfaces, fed by electrodes. In order to verify the stability of the configuration and the technical components of the PROTO-SPHERA initial arc, the initial Phase-1 experiment was built, and its plasma, in the year 2108, has reached both the total current (10 kA) and total duration (1 s) targets. A two colour interferometer is one of the diagnostic system proposed to provide PROTO-SPHERA with the measurement of its electron density, which is currently of the order of ~10²⁰ m^âˆ'³. It will allow to obtain line density, necessary to characterize this uncommon plasma, as integrate measurement along the equatorial plane. The design and the interferometer characteristics are described in this paper.