Dust Particle Analyzer

There are three basic components of the Dust Particle Analyzer: 1) a dual beam, frequency-biased laser Doppler velocimeter (LDV), 2) a relaxation cell, and 3) an electronic signal and data processing system. In the E-SPART analyzer, the LDV measures in a noninvasive way the particle velocity in its sensing volume. The sensing volume is located at the center of a relaxation chamber. The direction of the Mars dust particles flow is vertically downwards through the sensing volume. As the particles move downward, through the sensing volume, in the direction normal to the plane containing the two converging laser beams, it experiences AC electric excitation that causes the particles to oscillate in a horizontal direction parallel to the direction of LDV velocity measurements. The AC electric excitation is generated by using a pair of electrodes located inside the relaxation cell and positioned symmetrically across the LDV sensing volume.

The electronic signal and data processor, analyze the phase lag of the particle motion with respect to the AC electric field driving the particle. The gas dynamic diameter is derived from the phase lag value. The measurements of diameter and the direction and amplitude of the electrical migration velocity of the particles with respect to the electric field provide the polarity and magnitude of electrical charge of a particle.

An example of particle sampling of the JSC Mars-1 Martian Regolith Simulant tribocharged against stainless steel beads for 10 minutes at room temperature with 44% relative humidity is shown in the figure below. The simulant was ground down using a ball mill and measured with the E-SPART. The average diameter was 6.63 mm with a standard deviation of 1.91 mm. The total particle count was 5039 with a total mass of 3901.3 nanograms and total charge -311.02 femtoCoulombs. This gives a charge-to-mass ratio of -0.13 mC/g. The results are given (a) as a three-dimensional view showing the number of particles versus the diameter and the percent of maximum charge, and in (b), the charge-to-mass ratio (Q/M) of positively and negatively charged particles as function of particle size.

While the net charge-to-mass ratio (positive plus negative) for the Martian simulant dust in the size range 1 to 30 mm is -0.13 mC/g against stainless steel, the individual particles were found to be highly charged. For example, 1.06 mm diameter particles showed Q/M of -27.83 mC/g for negatively charged particles and +20.16 mC/g for positively charged particles.