Part I Object of Plasma Physics BACK. I. Object of Plasma Physics 1. Characterization of the Plasma State 2. Plasmas in Nature 3. Plasmas in the Laboratory.

Apresentação em tema: "Part I Object of Plasma Physics BACK. I. Object of Plasma Physics 1. Characterization of the Plasma State 2. Plasmas in Nature 3. Plasmas in the Laboratory."— Transcrição da apresentação:

1 Part I Object of Plasma Physics BACK

2 I. Object of Plasma Physics 1. Characterization of the Plasma State 2. Plasmas in Nature 3. Plasmas in the Laboratory

3 1. Characterization of the Plasma State 1.1 Definition of the Plasma State 1.2 Historical Perspective 1.3 Transition to the Plasma State 1.4 Examples BACK

4 1.1 Definition of the Plasma State 1.1.1 Atomic Physics Brush-Up 1.1.2 Thermodynamics Brush-Up 1.1.2 Ionized Gases 1.1.3 From Ionized Gas to Plasma 1.1.4 The “Fourth State” of the Matter BACK

5 1.1.1 Atomic Physics Brush-Up How do atoms really look like? Atoms in a Silicon crystal as seen through a Scanning Tunnel Microscope

6 Looking at an Atom An “electron” cloud…

7 Looking inside an Atom Inside the “electron cloud”: Electrons, Protons and Neutrons

8 Ionization Process Energetic electron causes ionization

9 Nucleus Atomic Structure The real proportions inside an atom 8 miles Electron

10 A velocity distribution function represents how many particles have a certain velocity Example 1: a stream of particles, with (one-dimensional) velocities u 1 =0.5 (m/s): BACK 1.1.2 Thermodynamics Brush-up u f(u) 1 0.5 14

11 Example 2: counter-streaming particles, half with (one- dimensional) velocities u 1 =0.5 (m/s) and half with u 2 =- 0.5 (m/s): Thermodynamics Brush-up (II) u f(u) 1 0.5 -0.5 7

12 Example 3: a system with a velocity spread and density n (m -3 ). In general the distribution is normalized to the density: For a discrete distribution: BACK Thermodynamics Brush-up (III) f(u) u 1 0.5 uu -0.5

13 Thermal equilibrium: all the components of the system have the same temperature or average kinetic energy At thermal equilibrium the velocity distribution function becomes a Maxwellian: The constant A is found by imposing BACK Thermodynamics Brush-up (IV)

14 An ionized gas is characterized, in general, by a mixture of neutrals, (positive) ions and electrons. For a gas in thermal equilibrium the Saha equation gives the expected amount of ionization: The Saha equation describes an equilibrium situation between ionization and (ion-electron) recombination rates. BACK 1.1.3 Ionized Gases

15 Solving Saha equation BACK Example: Saha Equation

16 Example: Saha Equation (II)

17 Backup: The Boltzmann Equation The ratio of the number density (in atoms per m^3) of atoms in energy state B to those in energy state A is given by N B / N A = ( g B / g A ) exp[ -(E B -E A )/kT ] where the g's are the statistical weights of each level (the number of states of that energy). Note for the energy levels of hydrogen g n = 2 n 2 which is just the number of different spin and angular momentum states that have energy E n.

18 BACK 1.1.4 From Ionized Gas to Plasma An ionized gas is not necessarily a plasma An ionized gas can exhibit a “collective behavior” in the interaction among charged particles when when long-range forces prevail over short-range forces An ionized gas could appear quasineutral if the charge density fluctuations are contained in a limited region of space A plasma is an ionized gas that presents a collective behavior and is quasineutral

19 From Ionized Gas to Plasma (II) (Long range) Coulomb force between two charged particles q 1 and q 2 at distance r: r q2q2 q1q1

20 From Ionized Gas to Plasma (III) (Short range) force between two neutral atoms (e.g. from Lenard-Jones interatomic potential model) attractive repulsive r

21 1.1.5 The “Fourth State” of the Matter The matter in “ordinary” conditions presents itself in three fundamental states of aggregation: solid, liquid and gas. These different states are characterized by different levels of bonding among the molecules. In general, by increasing the temperature (=average molecular kinetic energy) a phase transition occurs, from solid, to liquid, to gas. A further increase of temperature increases the collisional rate and then the degree of ionization of the gas. BACK

22 The “Fourth State” of the Matter (II) The ionized gas could then become a plasma if the proper conditions for density, temperature and characteristic length are met (quasineutrality, collective behavior). The plasma state does not exhibit a different state of aggregation but it is characterized by a different behavior when subject to electromagnetic fields. BACK

23 The “Fourth State” of the Matter (III) BACK