source 11 causes air to ionize and creates a plasma when the ac voltage is applied a plasma discharge appears on the surface of the dielectric 9 above the covered electrode 7 and directed momentum is coupled into the surrounding air the momentum coupling alters the airflow over the actuator 1 the plasma in the presence of an electric field gradient e results in a body force vector f b that acts on the airflow as shown in fig1 the body force is a body force per volume of plasma that varies in time and space during the ac voltage cycle the body force vector f b is defined by f b u03c1 c e where u03c1 c is the charge density of the plasma and e is the electric field gradient using the boltzmann relationship and substituting for u03c1 c the body force vector can be rewritten as f b u2212 u03b5 0 u03bb d 2 u03c6e where u03b5 0 is the permittivity of the free space u03bb d is the debye length i e the characteristic length for electrostatic shielding in plasma and u03c6 is the electric potential from this equation it is clear that the body force from the plasma actuator 1 decreases with decreasing plasma density i e increasing debye length referring again to fig1 the exposed electrode 5 and the covered electrode 7 are overlapped by a small amount in order to produce a more uniform.