An advanced diagnostic technique to measure the ion temperature and fast ions in open magnetic field systems is described. The method for the determination of deuterium to tritium ratio and the helium ash density in DT burning plasmas is also proposed. The measurement is made by small angle collective Thomson scattering (CTS) using a high power pulsed CO2 laser and heterodyne receiver system. The axial profiles can be measured by an axially injected beamline. Attenuation and refraction in the plasma are unimportant even for large devices. Scattered spectrum of the expected DT fusion plasma with currently developed laser and receiver system is presented. The component of scattered laser power nearly perpendicular to the magnetic field giving rise to ion cyclotron modulation of the scattered spectrum. Spectrum from pure deuterium and from D-T mixed plasma show the possibility of fuel ratio measurement. The scattered spectrum of D-T plasmas with thermalized helium ash is also calculated. Experimental arrangement to improve the S/N ratio in low density open magnetic systems are also discussed. The possibility of ion temperature and modulation measurement using a homodyne receiver system in GAMMA 10 are also evaluated.