Fluctuations of charged particle and light emission in vacuum arcs

The high-frequency fluctuations, superimposed on arc current, ion current, light intensity and arc voltage of a single cathode spot Cu-vapour vacuum discharge are investigated. For a current above approximately 40 A, fluctuations in arc current are smaller than 1%. In arc voltage, variations on a MHz frequency scale are observed in the order of 10% of the time averaged voltage. A temporary increase of arc voltage is correlated to an increase of up to 100% in the ion current, drawn by a grounded probe 3.6 cm away from the cathode spot. A delay between the two signals is interpreted as the time of flight of a quasi-neutral high-density front (at a velocity of 1.1*10 4 m s -1 ), resulting from a pulsed production of ionised mass in the cathode spot region of the discharge. A synchronous correlation is established between the HF component of arc voltage and light intensity, originating from the cathode spot. The time-averaged light intensity, however, is proportional to arc current. Analysis shows that the fluctuations in light intensity-reaching amplitudes up to 100% of the average level-are probably due to a varying cathode spot density, rather than to a varying electron temperature. In the frequency spectra of all the signals involved, no peaks are found.