Cross-field discharges are efficient plasma sources. These can be used in various
applications like Hall thrusters and surface coating with magnetron sputtering devices.
When we examine the idea of electrical cratering this is often given a context of huge
interplanetary discharges. What may be overlooked is the role the planetary
magnetic field plays in creating secondary effects through magnetic dipole discharges.
Before we discuss the effect a magnetic field has we need to first discuss the properties of
the cathode. Specifically a cold cathode, this is a cathode that is not electrically
heated to generate electrons. The surface of cold cathodes can emit secondary electrons at a ratio
greater than unity. An electron leaving the cathode will collide with neutral gas molecules.
This collision may just excite the molecule, but sometimes it will knock an electron free
to create a positive ion. The original electron and the freed electron continue toward the anode
and may create more positive ions. The result is for each electron that leaves the cathode,
several positive ions are generated that eventually crash onto the cathode.
Some crashing positive ions may generate secondary electrons. The discharge is self-sustaining when
for each electron that leaves the cathode, enough positive ions hit the cathode to free, on average,
another electron. Cold-cathode discharge lamps use higher voltages than hot-cathode ones. The
resulting strong electric field near the cathode accelerates ions to a sufficient velocity
to create free electrons from the cathode material. Although creating the initial space
charge and the first arc of current through the tube may require a very high voltage,
once the tube begins to heat up, the electrical resistance drops, this increases the electric
current through the lamp. This means less voltage is now required to maintain the same current.
A cross-field configuration is where the electric and magnetic fields are perpendicular
to each other. The simplest cross-field discharge consists of a permanent magnet
which functions as a cold cathode and a cylindrical chamber wall as the anode.
The crossed fields confine the electrons. In this example, we can see a probe is used to
emit electrons which become confined within the crossed field. It can be seen that the
first-order electrons travel along the dipolar field lines, perform small cyclotron orbits,
and drift due to the field gradients and curvature around the equator which causes azimuthal offset
of the bright flux tubes. Collisions scatter the electron orbits and produce a torus shape.
If we examine the potential of the plasma in the cross-field discharge
something interesting stands out. As we approach the magnet radially the potential drops strongly.
This implies a radial electric field of an ion-rich plasma region. Electrons perform an
E cross B drift and both ions and electrons are energised near the cathode. Since the magnet
potential is much lower than the measured potential there must also be a thin cathode
sheath. Ions created at the sides of the disk are accelerated into the disk towards the cathode.
Secondary electron emission is not only confined to the magnet
but can arise on any electrode which is biased sufficiently negative.
In this image, we can see a Langmuir probe to the right of the magnet which is biased at -500V.
The light in the flux tube of the magnet is due to secondary electron emission.
The emitted electrons are confined by the magnetic field and the negative magnetic
potential. They ionise efficiently and enhance the ion impact on the magnet. The green colour of the
plasma in this region is distinctly different from the blue colour of an Argon discharge.
The cathode coating material, which is Nickel in this case is sputtered and ionised.
These types of configuration are subject to various instabilities.
There are drifts between electrons and ions,
there is an unstable cathode sheath, relaxation instabilities caused by ionisation.
When an ion-rich sheath is deprived of electrons it oscillates at a frequency close to the ion
plasma frequency. In this image, we can see an instability as measured by a number of probes.
In the frequency spectrum, we can see many harmonic lines. The secondary harmonic is
larger than the fundamental mode which is commonly seen in ion sheath instabilities. The frequency
increases with discharge voltage which determines the discharge current and plasma density.
When the discharge voltage is increased a new phenomenon arises, the formation of cathode
spots or short-duration arcs. These melt the surface material, ionise it and produce dense
plasma with copious electrons which can enhance the cathode emission by an order of magnitude.
The spots are sub-millimetre size and occur at random locations and times.
The spot discharges change the properties of the magnetron discharge profoundly. The torus disk
is gone. Instead, we now have localised patches of high-density plasmas. Azimuthal uniformity
guarantees that the electron drift closes around the magnet. For a localised discharge
the Hall current has a divergence and sets up a space-charge field in the azimuthal direction
which produces a radial electron drift. This causes the electrons to be energised and cause
ionisation which forms a highly conducting plasma channel to the chamber wall which is the anode.
When a grounded probe is brought into the vicinity of the magnet a spot discharge
develops on the probe. Using a magnetic probe the probe shaft carries the same current as
the wire to the magnet. The measured anode spot current density was about 100A/mm^2 indicating
a very high plasma density. The electrode material is melted, evaporated and ionised.
The bright streak you see in the image is due to the sputtered electrode material.
The fast-rising current of spot discharges triggers high-frequency ringing in the
discharge current. Ringing oscillations up to several hundred MHz have been observed.
This is important as Andrew Hall has pointed out
that many of the surface features on Earth are likely created by an AC current. This may be
one mechanism for creating a short-duration AC current through a magnetron discharge.
It also shows that we need not consider the discharge to have
occurred repeatedly to the surface from a distant body but that instead,
it is possible that simply the presence of a body at a significantly different potential
could be enough to trigger the initial electron avalanche which then creates highly energised
plasma around the body which in turn created secondary discharges to the surface of the
cathode. When we examine the cathode spotting we can see many comparisons to scarring we see
on the Moon, Mars and other bodies. But what exactly is the difference between
anode spots and cathode spots. How are they created and what effects do they leave behind?
Is it possible to examine the scarring on other moons and planets and identify them as either
anode spotting or cathode spotting? This is something we will examine in the near future