A plasma is created by adding energy to a gas so that electrons are removed from atoms, producing free electrons and ions. Electric and magnetic fields interact strongly with the charged electrons and ions in plasmas (unlike solids, liquids and gases) and, consequently, plasmas behave differently to imposed electric and magnetic fields and modify electromagnetic waves in different ways to solids, liquids and gases. The different behaviour of plasmas has caused them to be regarded as a fourth fundamental state of matter in addition to solids, liquids and gases.

More than 99% of the observable universe is plasma. For example, the Sun is a plasma and has mass comprising 99.85% of the solar system, so the fraction of plasma in the solar system is slightly higher once interplanetary plasma is included. Present understanding of the universe has been enabled by the detection of electromagnetic radiation emitted by or passing through plasma material. To understand the universe, we need to understand plasmas, and, in particular, we need to understand the processes of light emission and propagation in plasmas.

Plasmas have many realised and potential applications. The fusion of isotopes of hydrogen in plasmas confined using magnetic fields or confined by inertia before a dense plasma can expand should provide a new source of energy production to replace the burning of fossil fuels, though the exact physics and many technical issues are not yet resolved [35]. The fuel for a fusion reactor (the deuterium isotope of hydrogen) is abundant in seawater (at concentration 33mg/litre). Large-scale experiments are under way to make fusion reactors because of the enormous potential impact of the development of a fusion power plant [79, 67].

Plasmas are used in many technological applications, including semiconductor etching and thin-film coating [15]. Plasma is created during the welding of solid material and is under study for biological and medical applications such as bacterial sterilisation. The emission of light from plasmas has many applications, ranging from fluorescent tubes to the use of extreme ultra-violet light emitted from laser plasmas for the lithography of semiconductors [105]. Many different lasers utilising plasmas have been developed, including argon ion lasers and an extensive array of plasma lasers designed to operate at short wavelengths [108, 91], with the record for saturated lasing achieved at wavelength 5.9 nm [125, 100].

Radiation can change the energy state of matter via three optical processes: spontaneous emission, absorption and stimulated emission. In spontaneous emission, an excited atomic or molecular state decays to a lower energy state with the emission of a photon of energy equal to the difference in the quantum state energies. Photon absorption is the reverse process, with a photon causing excitation from a lower to an excited quantum state. Stimulated emission was first proposed by Einstein in a paper published in 1917. He attempted to balance spontaneous emission and photon absorption, but could not get the correct balance without the postulated process of stimulated emission (see Section 4.2). With stimulated emission, an excited quantum state is stimulated to decay to a lower quantum state by an impinging photon. The existing photon survives with another identical photon being created (so that energy is conserved).

The rates at which radiative transitions occur can be calculated using quantum mechanics. The time-dependent Schrodinger equation is used with the system wavefunction comprising time-varying linear combinations of the two wavefunctions for the lower and upper quantum states. In semi-classical treatments, an atom is treated quantum mechanically with the radiation field treated classically. There is a perturbation in energy associated with, say, the electric field of the radiation interacting with the electric dipole of the atom, though other interactions can be significant if the electric dipole interaction energy is small. In absorption, for example, a differential equation representing the change in ‘weighting’ of the wavefunction from the lower to upper state is obtained. A ‘time constant’ for the speed of change represents the rate of the radiative process. We examine the quantum mechanical nature of radiation absorption in Section 10.1 before discussing (Section 10.3) another important parameter arising with radiative transitions between discrete bound quantum states: the lineshape function.

Quantum Theory of the Atom–Radiation Interaction

An expression for the Einstein B-coefficient for photo-absorption is derived using quantum mechanics in this section. Einstein determined the detailed balance relationship between the rates of spontaneous emission, photo-absorption and stimulated emission when light interacts with an atom by considering an atomic system of two energy levels which are in equilibrium with a black-body radiation field. We use a semi-classical treatment where the atom is treated quantum mechanically and the radiation field is treated as a classical oscillating electric field.

In this chapter we initially concentrate on radiation in thermal equilibrium. We derive the Planck radiation law for an equilibrium radiation field by considering the density of photons in a black-body cavity. As well as being applicable to many astrophysical and some laboratory plasmas, the concept of a radiation field in thermal equilibrium is useful in deducing the relationships between inverse processes interacting with a radiation field. The three possible radiative processes (spontaneous emission, photo-excitation and stimulated emission) between two quantum states in an atom or ion in the presence of an equilibrium radiation field are examined. It is shown that a simple thought experiment, where the rates of the three radiative processes are in balance with an equilibrium radiation field, leads to universal relationships between the radiative rates.

The Planck Radiation Law

If the radiation and electrons in a medium have many interactions through emission and absorption, the radiation field becomes thermalised and can be regarded as having a temperature equal to that of the electrons. In this section, we calculate the form of the thermalised radiation known as the Planck radiation intensity.

An equilibrium radiation field is a collection of photons which follow the rules of statistical mechanics for a collection of bosons. We need to calculate the density of modes for light in a steady-state system. We imagine a cube in space with perfectly conducting (and hence perfectly reflecting) walls and sides of length L. Such a volume with an assumed radiation energy in equilibrium with the walls is known as a black-body cavity (though in inertial fusion work the German word hohlraum is often used).

The Density of Modes

The wavelengths ƛ of light that can exist in a steady state inside a conducting cube of sides L are quantised by the condition that a half-integer number of wavelengths only can exist in directions perpendicular to the walls. Otherwise, the oscillating electric field for the light causes transient interference effects and a steady-state solution for the radiation field does not exist.

A molecule can be defined as a group of two or more atoms held together by sharing one or more electrons. Approximately equal electron sharing between atoms is said to produce a covalent bond between the atoms. The shared electron(s) in a covalent bond attract the positively charged atomic nuclei sufficiently to overcome the mutual electrostatic repulsion between the nuclei. When the atoms in a molecule are not the same element, the electron(s) may not be shared equally around the nuclei. Depending on the level of electron sharing, the bond or attraction between atoms can have a contribution due to a net negative charge near one atom (associated with that atom attracting more of the electron wavefunction) and a net positive charge near another, leading to an ionic bond.

We discussed the energy levels of atoms and ions comprising a single element in Chapters 7 and 8. In discussing the periodic table of the pure elements, we saw that, after placing electrons into sub-shells of increasing energy, the last un-filled subshell largely controls the chemical behaviour of the element as the wavefunction(s) of the electrons(s) in this sub-shell usually extend farther away from the nucleus so that these electrons are more likely to interact with neighbouring atoms.

Neutral and ionised molecules occur in plasmas if the particle temperatures are sufficiently low that vibrations and collisions do not have sufficient energy to cause the molecules to dissociate. In ‘plasma chemistry’, plasma ions react with surfaces or other plasma constituents to form molecules [36]. Temporary molecules are sometimes formed in plasmas. Charge exchange occurs when a neutral atom such as hydrogen interacts with a highly stripped ion (with no or few electrons) to form a temporary molecule. It has been shown that to achieve good accuracy the Rosseland mean opacity (discussed in Chapter 6) should include molecular transitions at temperatures below 5,000K [2].

We consider a simple molecular ion comprising two protons and an electron: the hydrogen ion molecule. The physics associated with this simple H2+ molecule illustrates the behaviour of more complex molecules and is relevant to plasma physics. Ionisation of neutral molecular hydrogen gas creates the hydrogen ion molecule.

Much of the physics determined for hydrogen and hydrogen-like ions is relevant to atoms and ions with many electrons. In a multi-electron atom or ion, the quantum state energies and wavefunctions are dominated by the central potential arising from the nuclear charge as is the case for hydrogen and hydrogen-like ions. Consequently, quantum wavefunctions similar in form to those found for hydrogen or hydrogen-like ions are produced. There are some corrections needed to the exact electron energies associated with the nuclear electric field experienced by each electron due to shielding effects by other electrons – and some perturbing energy effects due to electron–electron Coulomb repulsion. The electron wavefunctions have similar orbital or angular shapes as for hydrogen and hydrogen-like ions (see Figures 7.2 and 7.3). The same electron-configuration system (i.e. 1s, 2s, 2p, …) can be used and the same degeneracies are associated with different values of principal quantum number n (degeneracy 2n 2) and orbital angular momentum quantum number l (degeneracy 2×2l(l+1)) as found for hydrogen and hydrogen-like ions. As multi-electron atoms and ions have quantum states following the ‘rules’ established for hydrogen and hydrogen-like ions, the quantum state energy values are primarily determined by a principal quantum number n with values n=1, 2, 3, …. Angular quantum numbers l with l=0, 1, 2, … n − 1 are found to have a small effect on energy values due to shielding of the nuclear charge. As for hydrogen, the number of individual quantum states at a particular energy involves the different magnetic quantum numbers m with values m= − l,−l + 1, …, 0, 1 … + l and the two possible spin orientations (relative to the orbital angular momentum). The states which the electrons occupy are called the electronic configuration for the atom or ion.

An important feature of multi-electron atoms is established by the Pauli exclusion principle which requires that only one electron can occupy an individual quantum state. In the ground state of an atom or ion, the required number of electrons effectively fills the lowest energy quantum states designated by the n, l, m and s values according to the hydrogen degeneracies.

The definition of the necessary density to have a high-density plasma probably depends on the research area of the person seeking the definition. Many regard departure from coronal equilbrium (see Section 12.5) a suitable definition of high density. However, we will consider plasma material at sufficiently high densities to be ‘high density’ if LTE or near-LTE occurs between the ground states of different ionisation stages (see Section 12.7). With LTE conditions, new concepts (not, for example, considered in Chapter 12) often need to be considered. Even with comparatively low temperatures (e.g. a few eV), plasmas formed at high density have a high energy content per unit volume and form a subset of the research field of high-energy density physics [20, 28]. A high-energy density is defined as an energy exceeding 1011 Jm−3 by most authors working in the field (e.g. [20]). Such energy densities are found in materials at solid density and above when temperatures exceed a few eV.

The physics of plasmas at high density requires an understanding of equilibrium relationships. Equilibrium relations are valid at sufficiently high densities which often makes for simpler calculations of plasma ionisation and radiation emission. In plasmas at high density, many interactions can lead to statistical distributions, so that equilibrium ionisation populations and equilibrium radiation distributions as introduced in Sections 1.4.1 and 4.1, respectively, are present. The equilibrium relationships for ionisation and radiation distribution were used to deduce rates of inverse processes by invoking detailed balance (see Chapter 4 for radiative processes and Chapter 12 for collisional processes).

At very high densities and low temperatures, we need to modify the equilibrium ionisation relation (the Saha-Boltzmann equation) as it is necessary to allow for free-electron quantum states becoming fully occupied so that Fermi-Dirac rather than Maxwellian electron energy distribution is required (see Section 13.4). At high density, photons are more likely to interact with particles. However, the Planck black-body radiation distribution does not require modification at high-photon or particle density as photons are bosons and any number can occupy an excitation state.

If the density is high and the temperature of the plasma is not high, the chemical potential and Fermi energy associated with the near-full occupancy of free-electron quantum states are important.

To achieve a plasma with free electrons requires elevated temperatures and hence light emission, propagation and absorption can be important. The propagation of light, unlike many other familiar waves, does not need a medium in which to oscillate. Light propagates in plasma and the free electrons are driven to oscillate, but the electrons generally impede the wave oscillation rather than aid the process. It becomes impossible for light to travel through an unmagnetised plasma if the frequency of the radiation is less than the plasma's natural oscillation frequency: the plasma frequency is discussed in Section 1.1. At low frequencies, the electron oscillations relative to the ions dampen the electromagnetic oscillation of light. Light of all frequencies passes with no absorption or alteration in phase in vacuum, though the intensity from any finite-sized source ultimately falls proportionally to the inverse of the square of distance from the source.

In this chapter, we show how Maxwell's equations describing the relationships between electric and magnetic fields (and electric current and electric charge) are consistent with oscillating electric and magnetic fields propagating in vacuum at the velocity of light c. The oscillating fields are solutions of Maxwell's equations. We treat the electric currents generated in a plasma by light to show how the currents affect electromagnetic waves. The acceleration of any charge is shown to produce transverse electric field oscillations, thus providing a mechanism for the production of electromagnetic waves.

The electromagnetic spectrum of interest in plasma physics ranges from radio waves to X-rays and gamma rays (see Figure 2.1). The propagation of the different components of the electromagnetic spectrum involves identical physics with variations only occurring when the radiation interacts with matter. The highestfrequency, highest-energy gamma rays at photon energies above, say, 100 keV are not created typically by thermal processes, but can be important when fast ‘superthermal’ particles are created in plasmas. Radio-wave propagation can be important in low-density plasmas, such as the ionosphere.

Electromagnetic Waves in Plasmas

The propagation of radiation through a medium can be examined using Maxwell's equations.

The emission of radiation is one of the most important tools for diagnosing conditions in plasmas and can play a significant role in moving energy. For plasmas at all but the highest densities, we can assume that electrons are either in bound quantum states with energies dominated by the central potential of ionic nuclei, or they are unbound, occupying a continuum of free-electron states. We have seen that free electrons do not really occupy a true energy continuum (see Section 1.3), but a free electron means that the density per unit energy of quantum states is high, so that we can often consider the free-electron energies as continuous.

Radiation arising from free electrons dominates in low atomic number plasmas. Radiation transition probabilities scale rapidly with increasing atomic number: for example, as Z 4 for hydrogen-like ions, while radiation for free-electron transitions scales at a lesser rate proportional to Z 2 (see Section 5.2). In complete thermal equilibrium with the radiation field in equilibrium with particle temperatures, the emission of radiation is given by the Planck black-body formulas derived in Chapter 4. However, complete thermal equilibrium is rare in laboratory plasmas and the more tenuous astrophysical plasmas as radiation absorption within the dimensions of the plasmas is small.

We consider the radiation processes involving free electrons in this chapter. A full quantum mechanics understanding is generally not required to model emission from free electrons, so we discuss the emission of radiation in this chapter using largely classical non-quantum treatments. The Bohr model for bound energy states is utilised in considering emission from an electron making a transition from a free to bound state.

Cyclotron Radiation

Astrophysical plasmas such as those near a neutron star have strong embedded magnetic fields and even in interstellar space there is a weak magnetic field (≈ 10−10 Tesla). The plasmas studied in magnetic fusion research have magnetic fields in the range 0.1–10 Tesla confining the plasma. Confinement works as charged particles orbit with helical-shaped trajectories around an imposed magnetic field B0.

An important type of emission from plasmas consists of spectral lines originating from transitions between bound quantum states. Quantum mechanics gives information on the energies of the quantum states and can give information on the intensity of emission. The absorption of radiation resulting in transitions between bound quantum states can dominate the calculation of absorption coefficients, particularly for higher atomic number ions where the spectral density of absorption and emission lines is large and the radiative-transition probabilities are high (for example, hydrogen-like ion transition probabilities scale proportionally to Z 4).

The Bohr model of the atom where electrons are said to orbit the nucleus like planets orbiting the Sun was introduced in Section 1.5. The Bohr model gives a good approximation of the energies of hydrogen and hydrogen-like ions. It is also a reasonable model for the energies of excited states of higher-atomic-number atoms and ions. Excited electrons of multi-electron atoms and ions have orbits some distance from the nucleus and other electrons, so their kinetic and potential energy is determined by the near point-like net charge near the nucleus as occurs with hydrogen-like ions. The Bohr model, however, fails to predict the correct angular momentum of orbiting electrons and fails to predict the fine structure of the energy levels. Fine structure causes, for example, the energy states to split and produce two or more closely spaced spectral lines, rather than a single line. The Bohr model also does not enable a satisfactory method of evaluating the rates of radiative absorption and emission (which can be done with time-dependent quantum mechanics; see Section 10.1).

We consider the quantum mechanics of hydrogen and hydrogen-like ions in this chapter. Hydrogen and hydrogen-like ions only differ in the charge Z of the nucleus, so it is possible to treat both almost identically. We commence with solutions of the Schrodinger equation, treat the effects leading to the fine structure and mention hyperfine energy splitting arising from electron interaction with the nuclear spin. There are many books dealing with the details of the quantum mechanics of hydrogen. We refer the reader to Haken and Wolf [43] for further reading.

A Quantum Mechanical Treatment of Atoms and Ions

A quantum mechanical treatment of atoms and ions can start with the timedependent Schrodinger equation.

This book provides an introduction to the physics of emission, absorption and interaction of light in astrophysics and in laboratory plasmas. Such study necessarily requires a wide range of modern physics understanding involving electricity and magnetism, relativity, atomic structure, quantum mechanics, particle collision theory, statistical physics and more. Indeed, the analysis of light emission and collisional processes relevant to plasmas has provided much of the experimental evidence for quantum mechanics. The atomic and radiation physics of plasmas is, consequently, an ideal subject for study as an extension to material taught to physics undergraduates. The book combines undergraduate-level studies of the quantum mechanics of ions/atoms with the atomic and radiation physics of plasmas, though non-quantum models are used extensively. Atomic and radiation physics is presented at a level aimed at undergraduates in their final two years through to graduate students and researchers. Material needed for research in plasma physics and astrophysics is derived.

Plasma physicists working in a range of areas from astrophysics, magnetic fusion and inertial fusion to low-temperature plasmas of technological significance utilise atomic and radiation physics to interpret measurements. Plasma physics is a growing research area with the construction of the ITER tokamak, new laser-plasma facilities and the development of new methods of creating plasma, such as with free-electron lasers. Atomic and radiation physics is also an essential component in the theoretical development and simulation of astrophysical and laboratory plasmas. One aim of this book is to emphasise the overlap of atomic/radiation physics between astrophysical and laboratory plasmas, an imbrication exploited in the expanding field of laboratory astrophysics where physical scenarios relevant to astrophysics are simulated in the laboratory.

Due to the range of understanding required for research in the atomic and radiation physics of plasmas, the underlying physics is often not developed in research publications in astrophysics and plasma spectroscopy. An aim of this book has been to start with the knowledge obtained by physics graduates before they begin to specialise and to develop formulae and explain techniques used in plasma spectroscopy. The areas of plasma research utilising aspects of atomic and radiation physics are briefly introduced before spectroscopic applications are covered, but this book concentrates on the underlying atomic and radiation physics.

In a plasma in complete thermodynamic equilibrium, the radiation field is given by the Planck black-body expression (see Section 4.1), the ionisation is determined by the Saha-Boltzmann equation (see Section 1.4.1) and the population ratios of bound quantum states are determined by the Boltzmann ratio. In local thermodynamic equilibrium (LTE), quantum state populations are given by the Saha-Boltzmann equation and Boltzmann ratio, but the radiation field is not in equilibrium with the particles. We discuss the plasma conditions needed to establish equilibrium later in this chapter, but it is worthwhile to note that LTE often occurs when collisional processes dominate the populating and de-populating of the quantum state populations and radiative processes are not significant.

Radiative rates of decays for bound quantum states were determined in Section 10.1 and between free and bound states in Section 5.4. The cross-sections for collisional processes were discussed in Chapter 11. The cross-sections depend on the energy of the incident colliding electron, but in a plasma we have a Maxwellian distribution of the energies of the free electrons. The cross-section values need to be averaged over the Maxwellian distribution to produce a rate coefficient which when multiplied by the density of free electrons and the initial quantum state density yields the rate of change of the quantum state. The radiative reactions involved were listed in Table 11.1. A list of collisional reactions affecting quantum state populations has been given in Table 11.2. Models calculating plasma quantum state densities and consequent radiation emission and absorption properties using rates of radiative and collisional processes are known as collisional radiative models [89].

Collisional Excitation and De-Excitation

Our investigation of cross-sections for excitation by inelastic electron collisions has shown a variation with the energy E of the incident electron approximately proportional to 1/E (see Section 11.4). The cross-section for collisional excitation can be written in terms of a collision strength Ωpq(E) such that

where πa 2 0 is a cross-section for the ground state of the hydrogen atom (taken as the area associated with the Bohr radius a 0) and gp is the degeneracy of the initial quantum state. ‘Effective collision strengths’ γpq are tabulated for different temperatures where the collision strength has been averaged over the electron distribution.

Free and bound electrons in a plasma are accelerated by electromagnetic radiation. The interaction with the electrons affects the propagation of the radiation by altering the phase of the oscillating electric and magnetic field of the electromagnetic wave and by absorption of the electromagnetic wave energy as discussed in Chapter 2. As well as affecting a propagating electromagnetic wave, the acceleration of the free and bound electrons in a medium also gives rise to radiation emission: a process referred to as ‘scattering’.

As the acceleration of electrons affects the propagation of electromagnetic waves while producing emission of radiation, scattering of light by electrons in a medium can be regarded as determining the optical properties of the medium. Resonances in the responses of free and bound electrons to oscillations from electromagnetic waves tend to have a dominant effect on light propagation. We determined the refractive index arising in plasmas from free electrons and the resonance at the plasma frequency (see Section 2.1). Other resonances associated, for example, with bound electrons also produce refractive index effects.

By determining the refractive index of the medium in which light propagates, scattering processes ultimately govern the reflection and refraction behaviour of light at the junction between materials with different refractive indices. For example, macroscopic particles such as dust in plasmas or water droplets in clouds in the atmosphere reflect light from surfaces (known as Mie scattering). Gradients of refractive index lead to refractive bending of the direction of light propagation.

The fraction of electromagnetic radiation scattered by free electrons is typically a small loss mechanism for radiation of frequency much greater than the plasma frequency, but it is useful for diagnosing conditions in plasmas. For diagnostic measurements of plasmas at optical (ultra-violet to infra-red) frequencies, a laser radiation source is usually employed so that light can be spatially located and the high laser power per unit area ensures that the scattered light is greater than the emission associated with the thermal energy of the plasma. With radio waves and ionospheric scattering, high-power radar systems are employed. In dense plasmas of relevance to inertial fusion, incoherent X-ray sources or free-electron laser sources are used [99].