Measurement of the metastable helium atomic rates important for astrophysical observations

We present an experimental setup for measurement of the collisional rates relevant for the population of the triplet level of metastable helium. To estimate and verify the theoretical values, the absorption in helium plasma in the 1083 nm line will be measured. We are going to use plasma consisting of helium ions generated by $\theta$-pinch, filling in a 6 m$^3$ volume of a vacuum chamber at concentrations of $10^{12}$–$10^{13}$ cm$^{-3}$. Based on the expected parameters, the optical depth $\tau = 1$ is going to be achieved with a path length $L = 1$ km. To increase the effective path length in the plasma, an optical scheme based on a concave resonator is proposed.