Nuclear fusion reactions produce most of the energy irradiated by stars and are responsible for the synthesis of the elements in the Universe. In fact, through a complex network of nuclear reactions, stars forge most of the elements observed in the Universe starting from primordial hydrogen and helium left from the Big Bang. In addition, nuclear reactions determine stellar evolution and the flux of stellar neutrinos.
At typical astrophysical temperatures, nuclear fusion reactions can only take place in a relatively narrow energy window, called the Gamow peak. In most cases, such energy region lies far below the Coulomb repulsion barrier. As a consequence, fusion cross sections drop exponentially with decreasing energy, to the point that it takes millions or even billions particle collisions to give rise to a single fusion reaction. These extremely low cross sections are often inaccessible in laboratories at the Earth's surface, because the expected counting rate is much smaller than the background in a detector.
In 1991, the LUNA collaboration proposed a new, pioneering approach to nuclear astrophysics, that allowed to study a number of crucial nuclear reactions of astrophysical interest thanks to the extremely low background at LNGS.
The experiment started with a homemade 50 kV accelerator, installed in the underground facilities of LNGS in 1993 and operating until 2003. The LUNA-50 kV accelerator has been the working horse for the LUNA measurements on 3He(3He,2p)4He and the D(p,γ)3He experiments.
Since 2001, the LUNA experiment is equipped with a 400 kV accelerator and in the future a new 3.5 MV machine will be installed.