Fluctuation characteristics of arc voltage and jet flow in a non-transferred dc plasma generated at reduced pressure

Abstract

A torch with a set of inter-electrode inserts between the cathode and the anode/nozzle with a wide nozzle exit was designed to generate plasma jets at chamber pressures of 500-10 000 Pa. The variation of the arc voltage was examined with the change in working parameters such as gas flow rate and chamber pressure. The fluctuation in the arc voltage was recorded with an oscilloscope, and the plasma jet fluctuation near the torch exit was observed with a high-speed video camera and detected with a double-electrostatic probe. Results show that the 300 Hz wave originated from the tri-phase rectified power supply was always detected under all generating conditions. Helmholtz oscillations over 3000 Hz was detected superposed on the 300 Hz wave at gas flow rates higher than 8.8 slm with a peak to valley amplitude lower than 5% of the average voltage value. No appreciable voltage fluctuation caused by the irregular arc root movement is detected, and mechanisms for the arc voltage and jet flow fluctuations are discussed.

Temperature measurements in a laminar plasma jet generated at reduced pressure

“…When the arc current is increased from 80 A to 100 A, the electron temperature increases from 9600 K to 13000 K. The variations with the arc current of the excitation temperature T ex and the gas temperature T p are not as appreciable as the electron temperature T e , but they also increase with the arc current. With the increase of the arc current, the input power and thus the jet temperature increase, which is consistent with the observation from the photos of the laminar plasma jet [8] mentioned above. From the measured results presented in Figs.…”

“…Plasma jets are generated using argon as the working gas with flow rate of 1.25 Â 10 À4 kg/s, arc current of 80 Ae110 A and vacuum chamber pressure of 170 Pae2000 Pa, where the laminar flow regime is achieved. Photos of the laminar plasma jets for different arc currents and chamber pressures [8] show that the volume of the luminous region of the plasma jet does not increase obviously with increasing arc current, but the jet center region looks somewhat brighter at higher arc current. At higher chamber pressure, the size of luminous region of the plasma jet is appreciably reduced but the jet central region becomes much brighter, indicating the notable decrease of the plasma volume and the appreciable increase of the plasma temperature and its radial gradient at higher gas pressure.…”

Energy Fluctuations in a Direct Current Plasma Torch with Inter-Electrode Inserts Operated at Reduced Pressure

“…[24], a 150 Hz arc voltage and current fluctuation inherited from the power supply have been identified. In our previous work, a similar 300 Hz fluctuation also exists [20][21][22][23]. In these cases, the amplitude of energy fluctuations in plasma might be larger than the arc voltage fluctuations.…”

“…Our previous research shows that with the addition of IEI, the fluctuation behavior of the arc is different from that of the selfsetting arc. The dependence of arc voltage pulsation on the arc root movement seems to be much less both in atmospheric and reduced pressure conditions [21][22][23]. This is explained as such: although the arc root movements are not fully restricted by the IEI and the larger diameter anode, especially in the circumferential direction, the arc length change caused by arc root movements is relatively small and the percentage of arc voltage fluctuations in voltage spectra caused by arc root movements can be negligible when the IEI channel is long enough.…”

“…With a long IEI channel, the arc length may be extended much longer than that in self-setting arc length torches, which leads to higher arc voltage for a given input power. Our previous research shows that in a non-transferred dc plasma torch with IEI, the arc voltage of argon plasma can reach about 50 V at atmospheric pressure [20], while in a reduced pressure chamber, the arc voltage can reach 75 V [21]. When a certain power level is needed for an arc, such a combination of higher voltage with lower current intensities is preferred in some applications because with lower current, there might be less heat loss and electrode erosion [10].…”