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Plasma Physics 2
Adam Obrusník, adamobrusnik@physics.muni.cz
Tomáš Hoder, hoder@physics.muni.cz

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Lecture series contents
1.Townsend breakdown theory, Paschen‘s law
2.Glow discharge
3.Electric arc at low and high pressures
4.Magnetized low-pressure plasmas and their role in material deposition methods.
5.Brief introduction to high-frequency discharges
6.Streamer breakdown theory, corona discharge, spark discharge
7.Barrier discharges
8.Leader discharge mechanism, ionization and discharges in planetary atmospherres
9.Discharges in liquids, complex and quantum plasmas
10.Thermonuclear fusion, Lawson criterion, magnetic confinement systems, plasma heating and
intertial confinement fusion.

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Discharges – what this Lesson covers?

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Contents of this lesson
̶Transition between Townsend and Glow discharge modes – what we observe and what is the physics
behind it.
̶
̶Typical glow discharge structure and how it changes with conditions.
̶
̶Glow discharge scaling laws
̶
̶Glow discharge applications

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Observation of the transition between Townsend and Glow Dischage

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Observation: Townsend > Glow transition
̶The relevant phenomena take place at the time scale of single ns.
̶Light or electrical signals travel ca 30 cm in 1 ns => very high demands on instrumentation and
triggering.
̶ns-plasmas and processes are in focus of fundamental plasma reaseach in the past decade.
̶
̶
[M. S. Simeni, Y. Zheng, E. V. Barnat, and P. J. Bruggeman, “Townsend to glow discharge transition
for a nanosecond pulse plasma in helium: space charge formation and resulting electric field
dynamics,” Plasma Sources Science and Technology, vol. 30, no. 5. IOP Publishing, p. 055004, May
01, 2021. doi: 10.1088/1361-6595/abf320]

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Observation: Townsend > Glow transition
̶The relevant phenomena take place at the time scale of single ns.
̶Light or electrical signals travel ca 30 cm in 1 ns => very high demands on instrumentation and
triggering.
̶ns-plasmas and processes are in focus of fundamental plasma reaseach in the past decade.
̶
̶
[Reference see previous slide]

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Mechanics behind the transition

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Townsend > Glow transition mechanics

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Townsend > Glow transition mechanics

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Townsend > Glow transition mechanics

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Townsend > Glow transition mechanics


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Glow discharge structure, typical conditions and similarity laws

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Townsend > Glow transition mechanics
The structures below have been observed.
Depending on the layout, many of them may not form.
•d is the thickness of the cathode voltage drop
•VC is the cathode potential
•
•Aston space immediately close to the cathode, the electrons do not have enough energy to ionize or
excite.
•
•Cathode light – accelerated electrons start to excite neutral species, which de-excite while
emitting a photon.
•
•Cathode dark space – the mean energy exceeds a certain threshold, beyond which they can ionize.
This leads to a decrease in the absolute count of excitations.
•
•Negative glow – Electrons have lost some energy due to ionizations, light emission may increase
again.
•
•Faraday dark space – electrons have now passed the crazy sheath region and entered the region with
a rather low energy, so the emission is reduced again.
•
•Positive column – always exists, this is the bulk plasma with small but consistent E field.
Electrons are in equilibrium with the field and continuously excite.
•
•Anode light – slight acceleration of electrons by the anode voltage bump.
1.
1.

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Townsend > Glow transition mechanics
The structures below have been observed.
Depending on the layout, many of them may not form.

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Townsend > Glow transition mechanics
The structures in the previous slide have been observed. Depending on the conditions, many of them
may not form.
[V. Gorokhovsky Modeling of DC Discharges in Argon at Low Pressures. COMSOL Conference 2012]
[T. Hoder and team]

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Pressure/geometry vs discharge strucure
̶If we are decreasing the pressure at fixed other conditions OR increase voltage at fixed pressure:
The negative glow and Faraday space start to increase, positive column shortens, until it
disappears completely. https://www.youtube.com/shorts/nn8796OnTsk
̶
̶If we reduce the cathode-anode distance: Positive glow again starts to shorten while the negative
glow and Faraday space remain the same. Ultimately, there is a minimum distance at which the plasma
will still operate.

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Plasma properties in glow discharge
Theoretically, you can construct arbitrarily large glow discharges, but the typical reasonable
conditions are as follows:
Quantity
Usual range
Comment
Gas pressure
0.1 – 1000 Pa
*APGD up to 1 atm
No ignition for too low pressures, development of plasma instabilities for high pressures.
APGD = atmpospheric-pressure glow discharge, requires tiny gaps and very high electrode precision.
Plasma density
1015 – 1019 m-3
At higher densities, conductivity is too high and plasma collapses into arc discharge.
Electron mean energy
1– 2.5 eV in positive column
In the positive column, electrons reach some type of equilibirum (especially at higher pressures).
In the near-electron regions, the EEDF is highly non-Maxwellian
Gas temperature
300 – 500 K
* up to 2000 K in hollow cathode configs
Input power
10 W – 1 kW
*up to 20 kW in hollow chathode configs

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Similarity laws
̶There have been various attempts to identify similarity laws for glow discharges – essentially
attempting to answer the question “at what two conditions do I get the same discharge structure?”
̶
̶This is quite important for practical applications of plasmas – in many cases, we want to make a
small-scale prototype for testing and then upscale it. But the plasma behaves in a highly
non-linear manner!

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Similarity laws

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Similarity laws

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Similarity laws
̶Example 2: Deriving a similarity law for normal glow discharge
(1)As we have seen, most of the voltage drop occurs in the cathode region and indeed, in most glow
discharge conditions, majority of energy is dissipated in the cathode fall.
(2)
(2)So let’s see if we can derive a useful expression for the current conducted through the cathode
fall.

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Similarity laws

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Similarity laws

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Similarity laws
Interpretation:
1.When we are increasing the pressure, the current density through the discharge tends to grow
accordingly.
2.If we increase the total current, the current density tends to remain the same, which means that
the diameter of the glow discharge is increasing or shrinking depending on the total current/power.
3.For a given combination of gas and electrode, the cathode fall thickness and the voltage in the
cathode drop are approx. constant – you can find tables for various gasses that tell you with which
parameters will the discharge stabilize.

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Subnormal / normal / abnormal glow discharge

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Subnormal / normal / abnormal glow discharge

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Subnormal / normal / abnormal glow discharge
̶Abnormal glow discharge = when the discharge has filled the entire electrode surface, the scaling
law stops to work.
̶
̶Simply put, the plasma starts to behave like a resistor – to force more current through it, you
need a higher voltage.
̶
̶In reality, it is a slightly non-linear resistor, conductivity changes due to the change in
voltage and current.
̶

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Applications of glow discharge
(today we cover only plain non-magnetized glow discharge in DC or slow AC < 1 kHz)

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Application: Fluorescent lamps
̶Nowadays an obsolete device, mostly surpassed by LEDs.
̶Plasma ignited in mercury vapors producing UV
̶UV causes fluorescence on a photofluorescent layer
̶Still the go-to method for producing large-scale UV e.g. for disinfection
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Application: Fluorescent lamps
̶Nowadays an obsolete device, mostly surpassed by LEDs.
̶Plasma ignited in mercury vapors producing UV
̶UV causes fluorescence on a photofluorescent layer
̶Still the go-to method for producing large-scale UV e.g. for disinfection
How Fluorescent Lamp Lights Work

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Application: GDMS
̶Glow discharge mass spectrometry = glow discharge acts as a source of ionization of samples which
cannot be ionized chemically or by photons
Q: Why are M atoms emitted from the surface?
A: This is due to the effect of “sputtering” – low temperature removal of solid material by
energetic ions (more on that next lecture)

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Application: Hollow cathode GD in satellite propulsion
̶In a very special arrangement called the “hollow cathode”, glow discharge can generage very dense
but stable plasma at 10-100 A of current.
Q: Why do you think a HC plasma can get much more dense than the typical glow discharge?
A: Due to secondary emission and thermoemission being confined in the volume – it is largely a
geometrical effect.
RF Acceleration - ThrustMe: Advanced In-Orbit Propulsion Systems

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Application: Hollow cathode GD in satellite propulsion
̶In a very special arrangement called the “hollow cathode”, glow discharge can generage very dense
but stable plasma at 10-100 A of current.
Research - Plasma Controls, LLC

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Main take-aways

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Take-aways from lesson 2
1.Mechanisms of Townsend to Glow transition
2.
2.Similarity laws
3.
3.Subnormal, normal and abnormal discharge
4.
4.Application overview
̶