Measurement of the force exerted on the surface of an object immersed in a plasma

An experiment is described in which the force exerted by a low-pressure rf plasma on one side of a test surface is measured. The test surface is part of a cubic object immersed in the plasma and is mounted on a pendulum whose small deviations are recorded by a digital camera. The force balance is calibrated by means of an electrostatic method. The measured forces on the surface (4.8 cm(2)) are in the micronewton range and increase with the plasma density, while an increased collisionality reduces the force. The measured forces are discussed on the basis of a simple model taking into account the momentum fluxes across the sheath edge. It is concluded that ion-neutral collisions in the presheath enhance the force caused