Laser-Produced Aluminum Plasmas Expanding in an Applied Field: Plasma Generation in Single Particle Aerosol Mass Spectrometers

Single particle aerosol mass spectrometers (SPAMS) are used to generate mass spectra of solid particles ranging in size from 10^-7-10^-5 m in diameter, which are critical in air pollution and climate science. In most SPAMS devices, one or more lasers are used to vaporize and ionize each particle.  A high electric field ~10⁵ V/m is also applied to the plasma to collect the positive and negative ions. In this presentation, we use computational modeling to investigate the effect of the applied electric field on the laser produced plasma as it expands into the surrounding vacuum. The model system of interest is a spherical aluminum particle which is ionized by a 248 nm laser with an 8 ns pulse. A zero-dimensional plasma chemistry model is used to investigate the dynamics. Immediately after the laser pulse, the high plasma density enables shielding of the applied field, and the influence of the field is relatively small. After expansion the decrease in plasma density results in the electrons being rapidly removed from the plasma by the applied electric field. The effect of the applied electric field on the total number of ions produced per particle is discussed in order improve future SPAMS devices.

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