A self-triggered dust trajectory sensor

A self-triggered dust trajectory sensor not only determines the trajectories of charged dust particles but also acts as a highly sensitive dust detector. The objective of a dust trajectory sensor (DTS) is to measure charges Q≥100 aC and trajectories of submicron-sized grains with accuracies of ∼1° in direction and ∼1% in speed in the range v=1–100 km/s. It determines the trajectories from the signals induced in an array of wire electrodes. Most signals are weak and distorted by amplifier noise. Transient recorders continuously store them until a trigger signal indicates the presence of a dust particle. A trigger signal was previously available only after the particle had passed through the DTS and was detected externally by impact ionization. In certain important applications, however, an external trigger is not available or too late. In this paper we discuss a method of generating a trigger from noisy DTS signals before the dust grain exits the sensor. Specifically, we use seven parallel, double-triangle-shaped velocity filters on each sensor electrode to cover the velocity range v=1–128 km/s. Each velocity filter feeds into four parallel amplitude filters, two for single large signals and two for pairs of smaller but coincident signals from adjacent wires. We demonstrate by computer simulation that the probability of generating at least one trigger is high, typically pt≥0.99 at v=20 km/s, charge-to-noise ratio QNR≥6.25, and angle of incidence from 0° to 27°. The minimum QNR required to derive a trigger varies with dust velocity as ∼v1/2. The false event rate, caused by amplifier noise, is estimated to be on the order of one per year.