A method is presented which allows a determination of electron density profiles in the plasma boundary of a fusion device up to some 1013cm−3within about 100 &mgr;s. For this purpose, the complete attenuation of an injected lithium beam is determined by measuring its optical emission profile. The beam is generated by a ruby laser, which ablates small portions of a LiF coating with a thickness of about 1000 A˚ from the rear side of a glass substrate. The produced lithium atoms have velocities of 1×106cm/s and can penetrate into the plasma untilne×l≊1×1013cm−2. For the measurement of the optical emission profile of the excited lithium atoms, a silicon photodiode array camera is used. The emission profile is then converted into an electron density profile with the help of the ionization rate for lithium atoms by electron impact.