First experimental evidence for paser: particle acceleration by stimulated emission of radiation

Franck and Hetrz in 1914 were the first to demonstrate that free electrons can be decelerated by mercury atoms in discerete energy quanta. In 1930 Latyscheff and Leipunksy have demonstrated the inverse effect namely, free electrons can be accelerated by energy stored in the mercury atoms (collision of the second kind). It was only in 1958 that Schawlow and Townes have used multiple collisions between photons and excited atoms to amplify radiation (MASER and LASER). In 1995 Schachter suggested to use excited atoms for coherently accelerate particles. The results of a proof-of-principle experiment (2006) demonstrating the PASER scheme are reported here. Performed at the BNL-ATF, the essence of the experiment is to inject a 45 MeV density modulated electrons´ beam, bunched by its interaction with a high- power CO₂ laser pulse within a wiggler, into CO₂ excited molecules cell. The electrons experienced 0.15% relative change in their kinetic energy, in less than 40 cm long interaction region. The experimental results indicate that a fraction of these electrons have gained 200 keV each, implying that such an electron has undergone two-million collisions of the second kind. Hence, this is the first experimental demonstration of coherent collisions of the second kind.