Measurement of ion drag force in the pre-sheath of single- and dual-frequency CCPs using optically trapped microparticles

The ion drag force, arising from momentum exchange between accelerated ions and charged microparticles, plays a crucial role in dusty plasma dynamics but remains difficult to measure due to its relatively small magnitude compared to dominant forces such as electrostatic and gravitational forces. In this study, we present a novel diagnostic approach using optically trapped microparticles to directly resolve the ion drag force within the pre-sheath region of low-pressure capacitively coupled radio frequency plasmas operated at single- (13.56 MHz) and dual-frequency (13.56 MHz + 27.12 MHz) excitation. By setting the laser trapping power to a low value, the optical trap becomes sensitive enough to resolve weak forces on the order of a few piconewtons. At reduced trapping power, a distinct dip in the measured force profile is observed just before the sheath edge, which is interpreted as a signature of the ion drag force. The measurement is repeated under varying discharge conditions, including different pressures, rf amplitudes, and excitation frequencies. Additionally, phase-resolved measurements in a dual-frequency discharge reveal a clear dependence of the ion drag force on the phase angle between harmonics, which correlates with changes in internal plasma parameters. The experimental results are compared to a collisionless theoretical model, and the observed force magnitudes and trends are found to be in good agreement. This work demonstrates a noninvasive method for quantifying weak forces in plasmas and provides new insights into ion dynamics in the pre-sheath region.