  Charge-state-resolved ion energy-time-distributions of pulsed Cu arc plasma
were obtained by using direct (time dependent) acquisition of the ion detection
signal from a commercial ion mass-per-charge and energy-per-charge analyzer. We
find a shift of energies of Cu2+, Cu3+ and Cu4+ ions to lower values during the
first few hundred microseconds after arc ignition, which is evidence for
particle collisions in the plasma. The generation of Cu1+ ions in the later
part of the pulse, measured by the increase of Cu1+ signal intensity and an
associated slight reduction of the mean charge state point to charge exchange
reactions between ions and neutrals. At the very beginning of the pulse, when
the plasma expands into vacuum and the plasma potential strongly fluctuates,
ions with much higher energy (over 200 eV) were observed. Early in the pulse,
the ion energies observed are approximately proportional to the ion charge
state, and we conclude that the acceleration mechanism is primarily based on
acceleration in an electric field. This field is directed away from the
cathode, indicative for a potential hump. Measurements by a floating probe
suggest that potential structures travel and ions moving in the traveling field
can gain high energies up to a few hundred electron-volt. Later in the pulse,
the approximate proportionality is lost, which is either related to increased
smearing out of different energies due to collisions with neutrals, and/or a
change of the acceleration character from electrostatic to "gas-dynamic", i.e.,
dominated by pressure gradient.

Plasma Physics