Dynamics of bipolar HiPIMS discharges by plasma potential probe measurements

The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3 – 2 Pa), HiPIMS peak current (10 – 70 A corresponding to 0.5 – 3.5 A/cm2), HiPIMS pulse length (5 – 60 μs) and the amplitude of the positive voltage U+ applied during the positive pulse (0 – 150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall.

Ion energy distribution and non-linear ion dynamics in BP-HiPIMS and ACBP-HiPIMS discharge

Investigating the ion dynamics in the emerging bipolar pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge is necessary and important for broadening its industrial applications. Recently, an optimized plasma source operating the BP-HiPIMS with an auxiliary anode and a solenoidal coil is proposed to enhance the plasma flux and energy, named as ACBP-HiPIMS (‘A’－anode, ‘C’－coil). In the present work, the temporal evolutions of the ion velocity distribution functions (IVDF) in BP-HiPIMS and ACBP-HiPIMS discharges are measured using a retarding field energy analyser (RFEA). For the BP-HiPIMS discharge, operated at various positive pulse voltages U+, the temporal evolutions of IVDFs illustrate that there are two high-energy peaks, E1 and E2, which are both lower than the applied U+. The ratio of the mean ion energy Ei,mean to the applied U+ is around 0.55－0.6 at various U+. In ACBP-HiPIMS discharge, the IVDF evolution shows three distinguishable stages which has the similar evolution trend with the floating potential Vf on the RFEA frontplate: (i) the stable stage with two high-energy peaks (E2 and E3 with energy respectively lower and higher than the applied U+ amplitude) when the floating potential Vf is close to the applied positive pulse voltage; (ii) the transition stage with low-energy populations when the Vf drops by ~20 V within ~10 μs; and (iii) the oscillation stage with alternating E2 and E3 populations and ever-present E1 population when the Vf slighly descreases unitl to the end of positive pulse. The comparison of IVDFs in BP-HiPIMS and ACBP-HiPIMS suggests that both the mean ion energy and high-energy ion flux have been effectively improved in ACBP-HiPIMS discharge. The formation of floating potential drop is explored using the Langmuir probe which may be attributed to the establishment of anode double layer structure.

Distribution and evolution of surface charge in surface dielectric barrier discharge driven by AC and pulse dual power supply

The evolution of surface charge in surface dielectric barrier discharge (SDBD) is observed by using Pockels effect. SDBD is driven by sine AC and pulse dual-power supply voltage. The filamentary discharge and glow-like discharge are enhanced by superimposing positive pulse on sine trough and negative pulse on sine crest, respectively. The interval of enhanced discharge is adjusted by pulse repetition rate (PRF). The formation and decay of surface charges are analyzed at low PRF, and the accumulation effect is analyzed at high PRF. The results showed that the decay rates of charges decrease with increasing distance from the exposed electrode. When a positive pulse is superimposed on sine trough, the traces of positive charges are filaments with long extending lengths, which are the footprints of discharge channels. The lifetime of positive charges is hundreds of AC cycles (tens of milliseconds). Under certain conditions, subsequent glow-like discharge evolves as “flying” above the dielectric surface (3D propagation). Most of the negative charges are neutralized in subsequent filamentary discharge. Some negative charges accumulate downstream and exist longer than positive charges. In the case of negative pulses superimposed on sine crest, the enhanced glow-like discharge appears 3D propagation. The propagation distance is much smaller than that of positive pulse. Most of the negative charges are uniformly distributed near the exposed electrodes with a short lifetime (a few hundred microseconds) and are quickly neutralized in subsequent discharges. The occurrence of 3D propagation requires certain conditions and the mechanism needs further research.

The effect of process current parameters on the properties of oxide layers under plasma electrolytic oxidation of AMg6 alloy

The effect of current density and current ratio in the cathodic and anodic half-cycles during prolonged (180 minutes) plasma electrolytic oxidation (PEO) of AMg6 wrought alloy on the oxide layer wear and corrosion resistance were studied. It was established that the best wear resistance is achieved in the oxide layers obtained in the “soft sparking” mode (negative-to-positive pulse ratios of 1.15–1.30) at current densities of 9–15 A dm−2, and the best set of wear resistance and corrosion resistance – in the oxide layers obtained in “symmetrical” mode (negative-to-positive pulse ratio of 1.00).

Theoretical Comparison of Test Performance of Different Pulse Decay Methods for Unconventional Cores

Various pulse decay methods are proposed to test tight cores. These methods can be divided into three types. This study compares the performance of these methods to test the permeability of unconventional cores in terms of homogeneous cores, dual-medium cores, and gas adsorption, including the pressure equilibrium time, possible errors caused by conventional analysis methods, and reflections on the characteristics of dual-media. Studies shows that the two test methods with an antisymmetric relationship in the boundary conditions have basically identical test performance. When testing homogeneous cores, regardless of whether the gas is adsorptive or not, the pressure equilibrium time of the first type of method is approximately half of that of the second type of method. The dual-medium parameters seriously affect the pressure equilibrium time of different methods, which may cause the difference of order of magnitude. For homogeneous cores, the permeability errors of the first and second types of methods caused by porosity errors are similar and larger than that of the third type of method. For dual media, the fracture permeability obtained by the third type of method using the conventional analysis method may differ from the actual value by tens of times. No method can significantly eliminate the sorption effect. When the core is a dual-medium, only the pressure curves of the upstream positive-pulse method, downstream negative-pulse method and one-chamber method can reflect the characteristics of dual media. The pressure derivative of the one-chamber method cannot reflect the characteristics of dual media at the early time. The pressure derivative of the second type and the upstream positive-pulse downstream negative-pulse method can reflect the complete characteristics of dual media, but their pressure derivative of the constant-slope segment is small, and the interporosity flow parameter may not be identified.