In contrast, an ionization ratio of close to 100% is already achieved in vacuum arc
deposition . While a superficial similarity to the self-sputtering mode in HIPIMS is present, the underlying physical process and scaling lengths are completely different. A cathodic vacuum arc is characterized by plasma production at micrometer-size cathode spots which are rapidly moving across
the cathode .
A small, dense plasma cloud with plasma and power density of up to 1026 m-3 and 1013
Wm-3 respectively is generated by evaporation processes at the spot surface. Within a small space charge layer of 5–10 nm, a high electric field is accelerating ions towards the surface and emitting electrons—either by thermionic emission or field emission—towards the plasma ball, despite a rather low voltage drop of only about 50 V. Within the plasma ball, the electron beam is stopped by collisions and the vapor is heated and ionized, with thermal ionization dominating over direct impact ionization [29]. This central main plasma of the arc spot has a typical diameter of about 5–20 µm. Here,
a local thermodynamic equilibrium is reached—albeit with the ion temperature lower than the electron temperature. Beyond this spot plasma, expansion of the plasma in the direction towards the anode is favored by the influence of the anode potential. While the ions are accelerated, the plasma density is rapidly decreasing, reducing the collision rate and freezing the plasma at a distance of less than 100 µm from the spot plasma. As a result, a supersonic ion flux with velocities of 1–2 × 104m/s and average charge states of 2+–3+, depending on the cathode material is observed [61]. It has to
be noted that a parallel transport of ions and electrons towards the anode is occurring, with typically 10% of the total current as ions and 110% as electrons. A minimum current of about 50–75 A is necessary to allow these collective processes to occur, with an upper limit—in the pulsed mode—of several kA only due to the power supplies.