The defining trick of cold plasma is that two thermometers placed in the same glowing gas would read wildly different temperatures. The electrons can sit at tens of thousands of kelvin while the bulk gas can remain comparatively cool — sometimes near room temperature — under suitable operating conditions. This is not a paradox and not a measurement error — it is a direct consequence of how energy moves between very light and very heavy particles. Understanding the split between electron temperature and gas temperature is the single most useful idea in low-temperature plasma, and it explains why a plasma can etch a chip, activate a polymer, or be studied on living tissue while the bulk gas stays comparatively cool.
In one paragraph: a plasma has (at least) two temperatures — a high electron temperature and a much lower gas (ion and neutral) temperature. Energy enters through the light electrons, which an electric field accelerates easily; but because an electron is thousands of times lighter than any atom, each elastic collision passes on only a tiny fraction of that energy, so the heavy gas warms very slowly. When collisions are frequent enough — high power, high pressure — the two temperatures converge into a hot thermal plasma; when they are not, the gap persists and you have a non-thermal (cold) plasma. That gap is the entire basis of low-temperature plasma processing.
1. Two temperatures in one gas
Temperature, at bottom, is a measure of the average kinetic energy of random motion. In everyday matter every species shares one temperature because collisions are constant and efficient: air molecules, whatever their mass, all settle to the same average energy. A plasma breaks that rule. It contains three populations — free electrons, positive ions, and neutral atoms or molecules — and under many laboratory conditions they simply do not share a common temperature.1,2
The reason energy enters through the electrons is that an electric field does work on charge, and for a given field the tiny, nimble electron is accelerated far more readily than a heavy ion. The electrons therefore become the hot species, reaching an electron temperature often quoted as 1–10 electron-volts, while the ions and neutrals — the gas temperature — can remain close to ambient. A plasma with this separation is called non-equilibrium or non-thermal; one where collisions have erased the gap is a thermal or equilibrium plasma.3,4
2. What "electron temperature" means — and why it is quoted in eV
Plasma physicists usually give electron temperature in electron-volts (eV) rather than kelvin, which trips up newcomers. The two are simply related through the Boltzmann constant: the conversion is 1 eV ≈ 11,600 K. So an electron temperature of 2 eV — a very typical value in a cold discharge — corresponds to roughly 23,000 K, and the 1–10 eV range spans about 10,000 to 100,000 K.1,2 Quoting energy directly is convenient because the rates of the processes that matter — excitation, ionization, dissociation — are set by whether electrons carry enough energy (in eV) to cross a given threshold, not by the kelvin figure.
It is worth being careful about what the number describes. When we say the electron temperature is 2 eV, we mean the electrons carry an average random energy corresponding to that temperature; individual electrons range from nearly zero to many times the average. It is the energetic tail of that distribution — the relatively few electrons above an excitation or ionization threshold — that actually drives most plasma chemistry, a point we return to below.5
Concrete numbers help. In atmospheric-pressure plasma jets used for surface and biomedical work, electron temperatures of a few electron-volts over a near-room-temperature gas are typical, and the exact values depend strongly on the feed gas and the excitation scheme.6,7 In filamentary dielectric barrier discharges the electron energy is concentrated into nanosecond micro-discharges, so an instantaneous electron temperature coexists with a time-averaged gas that barely warms.8 The single "electron temperature" you see quoted for a device is always a representative figure for a particular operating point, not a fixed property of the gas.
3. Why the gas stays cool: the mass mismatch
If the field heats the electrons, why doesn't that heat quickly flow into the gas and warm everything to a common temperature? The answer is the enormous mass difference between an electron and any atom, and the physics of elastic collisions.1,2
When a light object bounces elastically off a much heavier one, it rebounds with almost all of its original energy — think of a ping-pong ball off a bowling ball. The fraction of energy an electron of mass m transfers to a heavy particle of mass M in an elastic collision peaks at ΔE/E = 4mM/(m + M)², which for m ≪ M is approximately 4m/M. Because an electron is thousands of times lighter than even a helium atom, this is a fraction of a percent per collision. An electron must therefore suffer many thousands of elastic collisions to hand its energy to the gas — and in a low-power or short-lived discharge it may never get the chance, so under suitable conditions the bulk gas stays comparatively cool while the electrons stay hot.3 The simulator below lets you pick a gas and watch this mass mismatch play out.
Two refinements complete the picture. First, this elastic argument concerns how electrons share energy with heavy particles; electrons exchange energy relatively efficiently among themselves, which is why a well-defined electron temperature is often a good description — though, as the next section notes, a single temperature fully characterizes the electrons only when their energy distribution is close to Maxwellian. Second, elastic collisions are not the only channel: an electron can also lose a large chunk of energy in a single inelastic collision by exciting or ionizing a molecule. In molecular gases such as nitrogen, vibrational and rotational excitation are efficient energy sinks that can warm the gas faster than the elastic route alone would suggest.4 But the elastic mass mismatch remains the foundation on which the two-temperature character of cold plasma is built.
4. Beyond a single temperature: the electron energy distribution
Strictly, "electron temperature" is a convenient shorthand. It is exactly meaningful only when the electrons follow a Maxwell–Boltzmann energy distribution, and in many low-pressure and transient discharges they do not.5,9 The complete description is the electron energy distribution function (EEDF): the probability that an electron has a given energy. From it, every rate coefficient the plasma needs — for excitation, ionization, attachment — can be computed by folding the distribution against the relevant collision cross-section.
The shape of the EEDF matters because chemistry lives in its tail. Two plasmas can share the same average energy yet drive ionization at very different rates if one has a fatter high-energy tail. Predicting the EEDF from the applied field and the gas is the job of dedicated numerical tools that solve the electron Boltzmann equation; the widely used solver BOLSIG+ does exactly this, turning cross-section data into the transport and rate coefficients that fluid models of discharges depend on.10,3 When you see a single electron temperature quoted, read it as a useful summary of this richer distribution, not the whole story.
5. From cold to thermal: a spectrum of equilibrium
Non-thermal and thermal plasmas are two ends of a continuum, and what sets a plasma's position on it is how effectively electrons and heavy particles exchange energy — essentially, the collision rate.1 Raise the pressure and the power density and collisions become frequent enough to force electrons and heavy particles toward a common temperature: the gap closes and the plasma approaches local thermodynamic equilibrium, with everything hot together at several thousand to more than 20,000 K.2 Plasma torches and arcs live here.
Lower the pressure or drive the discharge in short bursts and the electrons decouple: they stay hot while the gas relaxes toward ambient. Atmospheric-pressure cold sources hold this separation by other means — the dielectric barrier discharge chops the current into nanosecond filaments before the gas can heat,11 and pulsed jets give the gas time to cool between pulses.12,13 A helpful way to hold the whole idea in mind: thermal plasma is defined by everything reaching one temperature; cold plasma is defined by electrons and gas refusing to.
Where a given technology sits on this spectrum is a design choice with real consequences. Plasma catalysis and gas conversion deliberately exploit a large temperature gap so that electrons activate stable molecules while the catalyst and gas stay cool enough to remain selective and stable.14,15 Cold-plasma treatment of food and agricultural products relies on the same low gas temperature to avoid thermal damage to heat-sensitive material while reactive species do the work.16 In each case the position on the equilibrium spectrum — how far the electron and gas temperatures are allowed to separate — is exactly what is being engineered.
6. How the two temperatures are measured
Because the two temperatures are physically distinct, they are measured by different means. Electron temperature (and the fuller EEDF) is obtained electrically, most classically with a Langmuir probe: a small electrode is biased through a range of voltages and the current–voltage curve analysed, with the second derivative yielding the electron energy distribution. Langmuir probes are widely used in low-pressure plasmas; at atmospheric pressure a physical probe can be intrusive or hard to interpret, so optical diagnostics, modelling, and more specialized methods are often preferred instead. Doing any of this reliably is a craft — probe geometry, surface condition, and the non-Maxwellian nature of real distributions all introduce well-documented pitfalls.9,3
Gas temperature is commonly obtained optically, without disturbing the plasma, from the light it emits. The rotational structure of a molecular emission band (species such as N2 or OH are common thermometers) reflects the rotational population, which in many collisional plasmas tracks the translational gas temperature. The catch, examined carefully in the diagnostics literature, is that a non-equilibrium plasma does not guarantee the rotational distribution is in equilibrium with the gas, so the reading must be interpreted with care.17,18
Electrical diagnostics complete the toolkit. While probes and optical emission address the temperatures directly, the charge–voltage behaviour of a discharge — classically the Lissajous-figure method — yields the power actually dissipated, which sets the energy budget available to heat electrons and gas in the first place.19,20 Together, electrical, probe, and optical methods let an experimenter pin down both how much energy goes in and how it is partitioned between the hot electrons and the cooler gas.
The practical lesson: quoting "the temperature" of a plasma is ambiguous unless you say which temperature and how it was found.
7. Why the split makes cold plasma useful
The two-temperature split is not an academic curiosity — it is the enabling feature of an entire technology. Because the energetic electrons drive chemistry while the gas stays cool, a cold plasma can deliver reactive species — radicals, excited states, ions, and (in air or with oxygen and water) reactive oxygen and nitrogen species — to a surface while keeping the thermal load low. How warm a substrate actually gets still depends on power density, exposure time, duty cycle, gas flow, nozzle distance, reactor geometry, and the substrate’s own thermal properties, so “cold” is a design target rather than an automatic guarantee.21,22 That is what lets cold plasma treat the heat-sensitive materials thermal processes cannot: activating polymer films and textiles for printing and bonding, and being studied on living tissue in plasma medicine.23,24,25
The same electron-driven, low-temperature chemistry underpins plasma-enabled gas conversion and catalysis, where electrons activate stable molecules such as CO2 and N2 at near-ambient bulk temperature, opening reaction routes that thermal processes reach only with intense heat.26,27,28 Everywhere cold plasma is used, the underlying reason traces back to this one fact: keep the electrons hot while holding the bulk gas comparatively cool, and you can do energetic chemistry on delicate things.29,30
ACS Material builds equipment for exactly this regime. The CTP-2000K low-temperature plasma power supply and its variants drive non-thermal discharges in dielectric barrier discharge devices and atmospheric plasma jets, keeping the gas cool while the electrons do the work. Just how cool the gas stays is something an operator tunes deliberately — through the frequency, duty cycle, and waveform of the drive. For the full picture of how discharges, power supplies, and applications fit together, see our plasma processing guide.
Knowledge hub
- Plasma Processing Guide — the pillar overview: what plasma is, discharge types, applications, and how to choose a power supply.
- Plasma Frequency, Duty Cycle & Waveform — how the drive is tuned to trade reactive-species output against gas heating.
- DBD Reactor Design — choosing a reactor geometry for your sample and process.
- DBD Reactors: Physics & Power — the physics of a dielectric barrier discharge and the supplies that drive it.
- CTP-2000K Power Supply — the low-temperature plasma supply behind these cold, non-thermal discharges.
FAQ
What is the difference between electron temperature and gas temperature?
Electron temperature measures the average random energy of the free electrons; gas temperature measures that of the ions and neutral atoms. In a cold plasma the electrons are much hotter (often 1–10 eV, i.e. tens of thousands of kelvin) than the gas, which can stay near room temperature.
Why is electron temperature given in electron-volts?
Because the energy directly determines whether electrons can cross the thresholds for excitation, ionization, and dissociation. The conversion is 1 eV ≈ 11,600 K, so a 2 eV electron temperature is about 23,000 K.
Why does the gas stay cool if the electrons are so hot?
An electron is thousands of times lighter than any atom, so in each elastic collision it transfers only about 4m/M of its energy — a fraction of a percent. It takes many thousands of collisions to warm the gas, which a low-power or short-lived discharge never supplies, so the gas stays cool.
What makes a plasma "thermal" instead of "cold"?
Collision rate. At high pressure and power density, electrons and heavy particles collide often enough to reach one common temperature (local thermodynamic equilibrium) — a hot thermal plasma. When collisions are too infrequent, the electron and gas temperatures stay separated — a cold, non-thermal plasma.
How is electron temperature measured?
Most often with a Langmuir probe: a biased electrode's current–voltage curve is analysed (its second derivative gives the electron energy distribution). Reliable measurements require careful probe design and interpretation, especially when the distribution is non-Maxwellian.
Is "electron temperature" always an exact quantity?
Only when the electrons follow a Maxwell–Boltzmann distribution. In many discharges they do not, and the fuller description is the electron energy distribution function (EEDF); a single temperature is then a useful summary rather than a complete description.