Ultrasonic vibratory and rotary valve particle feeders have been designed, constructed, and investigated for application to feeding reactant powder to a solar thermochemical drop-tube reactor. Zinc oxide and carbon particles are fed continuously to the drop-tube under vacuum pressures as low as 1 mbar. The particles are probed in situ by laser transmission measurements with the aim to characterize particle residence time, axial and radial dispersion as a function of operating pressure. The ultrasonic feeder disperses particles well and can be operated at mass flow rates in the range of 57-288 mg min(-1). The rotary valve feeder operates in the mass flow range of 3.46-41.96 g min(-1) and exhibits reduced particle dispersion due to discrete pulsing mass flow created from the rotating valve. The time resolved transmission signals reflect characteristic changes under different experimental vacuum conditions. Particles traveling through the measurement zone at 1 mbar exhibit residence and clearance times of 0.05 s and 0.52 s, respectively. At 960 mbar, residence and clearance times are increased to as much as 0.16 s and 3.98 s, respectively. Particles falling at 1 mbar show radial dispersion three times less than those falling under ambient pressure. A critical result of the functional characterization of powder feeding under vacuum is a potential reaction capacity limitation at low vacuum pressures due to short particle residence time and narrow axial dispersion. (C) 2016 Elsevier Ltd. All rights reserved.