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The blackbody radiation shift, a manifestation of the Stark effect caused by blackbody radiation, contributes to the systematic uncertainty in Cs-based frequency standards at a level of 1 ?? 10-15. Few measurements of the third-order scalar electric polarizability of the Cs ground states, mainly responsible for the ac Stark shift in atomic clocks, have given better than 10% accuracy. We report progress in the development of a fully optical Ramsey pump-probe measurement in a thermal atomic beam, based on coherent population trapping (CPT), for the measurement of the third-order scalar and tensor polarizabilities of the Cs ground states. We give details of the apparatus and measurement techniques as well as our first 500 Hz half-period Ramsey fringes.