Anode and Cathode spots can lead to crater formation. The most obvious
difference is obviously on which electrode the craters form. Initially, an anode or cathode
spots form which might then create craters. These spots can be stationary or can move.
We have previously examined cathode spots in two separate videos so what
are the differences compared to anode spots and craters? Let's dive in and find out more.
An anode spot is a highly ionised plasma formed at the contact area of a plasma channel
to a positively biased electrode. Despite many studies examining the formation and properties
of anode spots, the mechanism of initiation for these plasma objects is still subject
to debate. By analyzing the behaviour of the anode surface and the nearby plasma,
the most probable explanation for the formation of anode spots can be given as a combination theory,
which considers magnetic constriction in the plasma together with the material
fluxes from that anode as well as the thermal, electrical and geometric effects of the anode.
When the current density of the anode is large the anode glow splits into a series of bright
rings or spots, arranged in a regular pattern, often stationary but sometimes moving rapidly.
The actual arrangement of spots depends upon the current density and the shape of the anode.
They are very sensitive to the presence of impurities in the has and on the electrode
surface. The formation of anode spots has shown that in the gas discharge between cold electrodes,
the cathode absorbs gas through positive bombardment and the anode emits gas at rates which
are functions of current density, gas pressure, the type of gas, and the electrode temperature.
There are at least three different modes.
Diffuse mode. This is low current mode, where the anode is inert
Footprint mode. Here the is an intermediate current and that anode stars to be an active
part of the discharge. The spots are luminous and cause the anode to melt.
Real anode spot mode. There is a high current and it is associated with a very high temperature
and the vaporization of the anode material.
Anode and Cathode cratering
The mechanism for the formation of craters in the anode and cathode are similar
but there are some distinctions.
In the cathode, there are a number of mechanisms that can cause electrons to be emitted.
This could occur through a cold cathode, where electrons are ripped from the surface due to the
electric field or a hot cathode where the cathode is heated so that it starts to emit electrons.
In the latter case, this could be caused by impacted ions.
The essential mechanism is that the electrons leaving and the ions impacting the surface
cause the material to start to vaporise. This will already be partly ionised but
the electric field will quickly ionise all of it. The ions will not only feel the force from
the strong electric field but will also be repelled by each other. This creates a cloud
of vaporised material above the cathode and leaves behind a crater. As this material falls
back down to the cathode secondary craters can be formed as new plasma channels are created.
For the anode, it is the incoming electrons that collide with the surface
which causes a heating effect and also causes ionisation. Again here it leaves
a crater behind but in the case of the anode, the vaporised material which is ionised does not fall
back but will journey towards the cathode eventually being implanted on its surface.
So are there any visible differences between anode and cathode craters?
There are several experiments which have attempted to look at exactly this problem.
In these experiments, they simply reversed the polarity to examine
the differences between the anode and the cathode.
What these showed was that the polarity influenced the arc plasma movement and the crater morphology.
The images show strikingly different crater morphology
with the anode one being much more precise compared to the cathode.
If we examine the anode crater we can see that at a higher current the craters became more irregular
and deeper. This can be explained by the higher energy input and arc plasma movement with a higher
average moving speed and a larger moving range. The cathode crater was deeper with lapped pits.
Around the edge of both cathode craters, a band of tiny craters can be observed.
One striking difference between the two is the speed of the moving arc. The arc plasma moving
speed with the cathode workpiece was always higher than that with an anode workpiece under the same
discharge conditions. The movement speed of the arc could weaken the stability of the arc plasma.
The experiment used a tool electrode which was
brought up to the sample surface and a discharge was initiated.
Here it can be seen that when the workpiece polarity was the same,
the diameter of the crater was larger in the case of the flat-end.
But when the workpiece was the anode, the crater shape generated with the taper-end electrode
was more regular than that generated with the flat-end. In comparison when it was the cathode,
the crater shape generated with the taper-end was a single big pit,
while in the case of the flat-end electrode, the crater shape was an overlap of small pits.
In the case of the taper-end tool electrode, when the arc plasma moved away from the tip
to the side, its diameter increased, which means that the energy density
decreased. Therefore, during discharge, only in the centre area of the molten pool
could the arc plasma provide the strongest energy density to the workpiece, so the crater morphology
obtained was more regular and symmetrical. In contrast, when the flat end electrode was used,
the arc plasma would not change obviously in diameter and it could move within a larger area,
which was considered to be one of the reasons that caused the irregular morphology.
On other factor that can affect the morphology of craters is the gap distance between the
electrodes. As the discharge gap was increased the crater went from an irregular profile
to a much more regular profile on the anode end. The size of the crater decreased and
the movement and speed of the arc plasma also decreased leading to the more regular shape.
When the discharge current was the same, a smaller arc plasma diameter meant higher energy
density. The difference in energy density is considered to be one of the reasons that
cause the difference in arc plasma movement and crater morphology under different gap distances.
On the surface of the anode workpiece, as the arc plasma on the anode is restricted by the molten
pool, higher energy density caused by smaller gap distance might intensity the fluctuation of the
molten pool during discharge and thus rapid arc plasma moving speed, generating a crater with a
more irregular shape and a larger diameter. By comparison on the cathode workpiece surface,
the arc plasma could move out of the molten pool. A higher energy density caused by the smaller gap
distance also means higher electrode emission density, which might require the arc plasma to
move with a larger amplitude and faster speed. Due to this quicker movement, a smaller volume
of material was removed and thus a crater with a larger diameter and smaller depth is created.
In a different experiment, they examined the micro-craters on the anode and cathode
created by different discharge energies.
Here it is important to point out that they did not use the same metal in the anode and cathode
so we need to be careful in comparing the size of the two based on these images.
In these types of experiments, they either use a vacuum arc discharge
or introduce a dielectric material between the electrode.
So what effect does the dielectric have on the shape and size of the craters?
In this image, we can see the difference between single discharges in air and oil
and the change in shape as the length over the duration of the discharge.
One important point to consider here is that in the formation of these craters in a dielectric
gas or liquid part of the formation process is governed by the formation and collapse
of a bubble in the dielectric just above the electrode.
Peaked craters
So far these craters are all hollows so how are peaked craters created?
One possible explanation is through helical currents in the discharge. In this experiment,
they used a gliding arc plasmatron to create anode spots. The most important difference here is the
non-melted plate, and therefore low current, in the middle of the eroded areas, where based on
conventional models, a maximum current density is expected. The most likely explanation for
this is that a toroidal heating of the surface by a helical current. The plasma plume created
by the gliding arc plasmatron has been shown to generate a helical current around the plume.
Sliding Arc spots
Several factors can affect the movement of the arc spots. We have previously examined these in
cathode spots. For prolonged discharges, the movement of the arc discharge or the movement
of the anode will cause dramatic changes to the shape of the crater created. Starting
with no movement and more spherical crater to a more teardrop crater with increased movement.
Energy Distribution
In electric discharges, electrons are seen as simply falling from the plasma into the anode
because of its higher electrical potential, whereas the cathode has to develop very
efficient emission mechanisms to extract electrons into the plasma.
The voltage jump is mostly localised near the electrodes, in the anode and cathode layer
So is there a difference between the energy distribution at the anode compared to the cathode?
In this graph, we can see a comparison of different experiments. It shows that for
short discharge durations, most of the energy is dissipated into the plasma and dielectric,
whereas for a longer discharge duration when the plasma becomes self-sustaining,
most of the energy is dissipated into the anode and cathode.
In general for discharge durations longer than 20 microseconds,
anode craters are smaller compared to the crater on the cathode.
Removal Mechanism
In order to understand the crater formation, Samela Nordlund and separately Timko et al
used particle-in-cell simulations. They were able to show that huge fluzes of energetic
ions can form craterlike damage on the surface and lead to sputtering of large atom clusters.
The craters have a complex shape that can be explained by the strong non-equilibrium
heating of material due to energetic ions accelerated in the plasma sheath potential.
From these simulations, if can be inferred that for lower power density,
material is removed by vaporisation, whereas for higher power densities,
bubble explosion plays the dominant role and the material is removed in the form of clusters
Erosions and Deposition
At the Cathode
The cathode will be emitting electrons this will heat the surface
causing it to be vaporised. Some of this material will fall back to the surface cool
and solidify. At the same time ions created from the emitted electrons crashing into neutral atoms
and ionising them will be attracted back to the cathode and deposited on the surface. Ions
ejected from the anode or ionised near the anode will have felt a much greater
acceleration before impacting on the cathode and will therefore be entrenched much deeper
and likely cause secondary emission of local material leading to further electron emission.
At the Anode
As the current density increases material will be removed from the anode. Material which is
vaporised and not ionised could fall back to the anode or could end up being ejected outwards.
Any ionised material will end up being attracted towards the cathode
and away from the anode. The longer the discharge the more material will be removed.
and that brings us to the end of this video.. I hope you have found this useful.
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