Effect of plasma fluctuations on in-flight particle parameters

The influence of arc root fluctuations in direct current (DC) plasma spraying on the physical state of the particle jet is investigated by correlating individual in-flight particle temperature and velocity measurements with the instantaneous voltage difference between the electrodes. In-flight diagnostics with the DPV-2000 sensing device involve two-color pyrometry and time-of-flight technique for the determination of temperature and velocity. Synchronization of particle diagnostics with the torch voltage fluctuations are performed using an electronic circuit that generates a pulse when the voltage reaches some specific level; this pulse, which can be shifted by an arbitrary period of time, is used to trigger the acquisition of the pyrometric signals. Contrary to predictions obtained by numerical modeling, time-dependent variations in particle temperature and velocity due to power fluctuations induced by the are movement can be very large. Periodic variations of the mean particle temperature and velocity, up to DeltaT = 600 degreesC and Deltav = 200 m/s, are recorded in the middle of the particle jet during a voltage cycle. To our knowledge, this is the first time that large time-dependent effects of the arc root fluctuations on the particle state (temperature and velocity) are experimentally demonstrated. Moreover, large fluctuations in the number of detected particles are observed throughout a voltage cycle; very few particles are detected during parts of the cycle. The existence of quiet periods suggests that particles injected at some specific moments in the plasma are not heated sufficiently to be detected.