The development of the electron density distribution in a CO2-laser irradiated underdense N2 gas jet is investigated interferometrically and is correlated to the observation of fast electrons generated by the two-plasmon decay and stimulated-Raman-scattering instabilities. During the 2 nsec laser pulse the quarter critical density scale length initially increases but soon ponderomotive forces steepen the profile resulting in a quenching of the fast electron emission.