Charge and velocity distribution of ions emitted from two simultaneously operating and serially connected vacuum arcs

Ion velocity distribution (IVD) and ion charge state distribution (ICSD) of silver ions in a plasma beam emitted by two compact, serially connected, and simultaneously operating vacuum arcs was measured using a dynamic time of flight diagnostics. The anode and cathode of each arc were made of two strips of silver layers pasted on an alumina wafer and separated by a narrow 100 mum gap. The two arcs were laterally separated by 0.015 m and located on the same horizontal plane, emitting two plasma plums, which merged into a single one few cm away from the arcs. Each arc was ignited by high voltage breakdown on the alumina surface gap by 3mus 100 A current. The IVD of the merged plasma beam was analyzed 0.7 m away from the arcs, and compared with that of the ions emitted by a single arc. When only a single arc was active, the plasma beam consisted of more than 90 % of Ag ions with average velocity = (0.7-0.8)times10 4 m/s and charge state z=1. About 10% of the beam ion population consisted of oxygen and carbon. When the two arcs operated simultaneously, the form of the IVD was significantly different, containing a second peak with velocity of peak1 >=(1.15-1.25)times10 4 m/s, significantly higher than that observed with a single arc. However, the integrated ion charge in the merged beam was lower by 30% from the sum of the ion charge obtained from the plasma of each arc. It was found that the observed phenomena were correlated with a formation of conducting channel between the cathode of the first serially connected arc and the anode of the second arc, occurring when the plasma plums expanding from the two arcs merged. When a physical barrier was inserted between the two arcs to prevent a contact between the two expanding plums, the observed plasma shunting did not occur, although the two beams merged in space beyond the barrier, as before. The effect on the IVD due to arc transfer from the first cathode to the second arc anode is currently being investigated.