Step-wise excitation for the determination of the resonance frequency of a microparticle confined in a low pressure plasma

Abstract

We propose, test, and analyze a new step-wise excitation approach to quickly obtain, or determine with high accuracy, the resonance frequency of a microparticle confined in the space charge region of a low pressure plasma operated above a horizontal surface. The local value of this resonance frequency can directly be related to the charge of such a plasma-immersed microparticle and is, therefore, essential to be obtained with respect to the interpretation of many dusty and complex plasma laboratory experiments. The presented approach applies a step-wise change in plasma power, after which the damped harmonic oscillatory motion of the plasma-confined microparticle toward its “new” equilibrium confinement position is monitored temporally resolved and fitted with the theory describing this damped harmonic oscillator. Proof-of-principle experiments demonstrate that the results from this method match the results of the traditional frequency sweep resonance method within 5%. Comparing to existing—for instance, phase-resolved or single particle injection—methods to determine the resonance frequency, this approach offers the experimenter a unique possibility to choose the trade-off between reduced measurement time—essential when working with particles that are vulnerable to plasma etching—and measurement accuracy.