Plasma, often referred to as the fourth state of matter, exists when gas is heated or energized sufficiently that its atoms become ionized, separating into a superheated, quasi-neutral mixture of negatively charged electrons and positively charged ions. Plasma streams are found in the cosmos (like the sun) and in everyday occurrences like lightning and numerous terrestrial applications. Accurately stating the approximate temperature of a plasma stream is complex because this state of matter encompasses an immense thermal range, from near room temperature to hundreds of millions of Kelvin. Understanding plasma temperature first requires understanding how temperature is defined within this ionized gas.
Defining Plasma and Temperature Challenges
Plasma is fundamentally an electrically charged gas where the number of positive and negative charges is roughly equal, maintaining an overall neutral state. Unlike a standard gas, where all particles share a single, uniform temperature, the concept of a single temperature often breaks down in plasma streams. This occurs because the plasma is frequently not in thermal equilibrium, meaning different particle species possess widely varying average kinetic energies.
Temperature is typically differentiated into two separate measurements: the electron temperature (\(T_e\)) and the ion or neutral gas temperature (\(T_i\) or \(T_g\)). Since electrons are significantly lighter than ions and neutral atoms, they are much more readily accelerated by electric fields, allowing them to gain energy rapidly. Consequently, the electron temperature can be orders of magnitude higher than the temperature of the heavier particles. This distinction is fundamental because highly energetic electrons drive the ionization and chemical processes, while the cooler ion or gas temperature dictates the macroscopic heat delivered to a surface.
Categorizing Plasma Stream Temperatures
The approximate temperature of a plasma stream depends heavily on whether it is classified as cold (non-thermal) or hot (thermal) plasma, representing two vastly different thermal regimes.
Cold (Non-Thermal) Plasma
Non-thermal plasma streams are utilized in industrial applications like material processing or sterilization. They are characterized by a profound temperature difference between particles. The electron temperature (\(T_e\)) can range from 1 eV (11,600 Kelvin) up to 10 eV (116,000 Kelvin). However, the neutral gas temperature (\(T_g\)) remains much lower, often near ambient room temperature, or up to about 1,500 Kelvin. This allows the plasma to modify surfaces chemically with highly energetic electrons without causing bulk thermal damage to heat-sensitive materials.
Hot (Thermal) Plasma
Hot, or thermal, plasma streams are those in which electrons and heavier particles are in approximate thermal equilibrium, sharing the same high temperature. These streams are used in applications requiring intense heat, such as plasma arc welding or cutting, where temperatures range from 7,000 Kelvin up to 30,000 Kelvin. Argon plasma arcs typically operate between 10,000 and 20,000 degrees Celsius. At the most extreme end are magnetically confined plasma streams pursued for fusion energy. To sustain nuclear reactions, the plasma in these experiments must reach temperatures exceeding \(10^8\) Kelvin (tens of keV), far hotter than the core of the sun.
Key Factors Influencing Stream Temperature
A plasma stream’s temperature is determined by several physical parameters that control the energy distribution within the ionized gas.
Power Input
The most direct factor is the power input, as temperature is directly related to the amount of electrical energy pumped into the gas to sustain ionization. Higher power densities lead to more frequent and energetic collisions, resulting in a hotter plasma stream.
Pressure and Density
The pressure and density of the gas play a significant role in determining how energy is partitioned among the particles. At low pressures, particles collide less frequently, allowing electrons to maintain high energy, which contributes to the large temperature difference characteristic of non-thermal plasma. Conversely, at high pressures, increased collision frequency forces the electrons and ions toward thermal equilibrium, resulting in the uniformly high temperatures observed in thermal plasmas like welding arcs.
Gas Composition and Confinement
The specific gas used, known as the plasma gas composition, influences the energy required for ionization and the final stream temperature. For instance, in plasma welding, using hydrogen gas can yield a stream temperature up to 25,000 degrees Celsius, while a nitrogen-based plasma may only reach 12,000 degrees Celsius under similar conditions. The method of confinement, such as the use of powerful magnetic fields in fusion experiments, allows for the stabilization of extremely low-density, high-temperature plasma.
Methods for Measuring Plasma Temperature
Measuring the temperature of a plasma stream is complicated by the extreme thermal environment and the often-remote nature of the discharge. Direct contact measurement is impossible because physical sensors would be instantly destroyed or would significantly perturb the delicate plasma conditions. Therefore, non-invasive diagnostic techniques are employed to infer the temperature from the plasma’s emitted radiation or its interaction with external probes.
Three primary methods are used for temperature measurement:
- Optical emission spectroscopy analyzes the light emitted by excited atoms and ions. By examining the spectrum and intensity of specific emission lines, scientists determine the electron temperature and particle density without touching the plasma.
- A Langmuir probe, a small metallic electrode inserted into the plasma, measures the current-voltage characteristics. This technique provides localized measurements of the electron energy distribution for low-temperature, low-density streams.
- Thomson scattering is a highly accurate, non-intrusive method for high-temperature plasmas. A powerful laser beam is directed into the plasma, and the faint scattered light is analyzed for its Doppler shift, which directly yields the electron temperature and density.