•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. 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−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.