Matti Laan Gas Discharge Laboratory University of Tartu ESTONIA Outline 1. Ionisation 2. Plasma definition 3. Plasma properties 4. Plasma classification 5. Energy transfer in non-equilibrium plasma 6. i-u characteristic of DC discharge PlasTEP Summer School; Vilnius/Kaunas; 17.07.2012
High-voltage powerline 1 150 000 V = 1.15 MV Resistivity of air is 10 28 times higher than that of copper A good insulator (properties are governed by neutral particles) Lightning: current- 30000 A A good conductor (properties are governed by charged particles) Two different states of matter?
Neutral versus ionised gas 1 nm = 10-9 m = 0.000000001 m N N r = 0.2-10 nm - r + Short-range interaction Long-range interaction Atom 10-15 m Binding energy decreases with the orbital number Hydrogen: energy level plot ev 0-1.5 Electron is free -3.4 2 3 4 5 10-10 m M A > 2000 m e -13.6 Ground state
In the microworld instead of energy unit Joule (J) the unit electronvolt (ev) is used C 1 V A = 1 ev 1 ev = 1.6 10-19 J
Let E i = 10 ev It is possible to ionise atoms by short-wavelength radiation ( < 120 nm) by high-energy particles Cosmic rays, radioactive decay, hot gas How hot the gas should be to have charged particles? 1 ev 11 600 K W boiling temperature T W = 5550 K = 0.48 ev 1 0.8 fe( ) 0.6 0.4 N i /N n < 10-8 0.2 0 0 0 1 2 3 4 0 4
Effect of other charges r In vacuum the potential of charge Q changes as 1/r Because of other charges the screening of charge Q takes place In an ionised medium the Q potential decreases exponentially 1/r exp(-r/ D ) 1 0.8 vacuum ( r) ( r) 0.6 0.4 0.2 ionised medium D = 0.1 Debye length (radius) D gives is the characteristic screening distance 0 0 0 0.2 0.4 0.6 0.8 0.05 r 1
What is plasma? (πλασμα (Greek) jelly) L Plasma is an ionised medium which linear dimensions are considerably larger than Debye length D In plasma a charged particle loses its individual properties, collective effects are at the foreground D
Plasma: the most important characteristics Quasineutrality Considering only single-charged particles the concentration of positive ions N + equals to the sum of concentrations of electrons n e and negative ions N - N + n e + N - = n The quasineutrality means that inside plasma E 0 Debye length depends both on charged particles concentration and the mean energy k B Boltzmann constant, 0 dielectric constant, e- elementary charge T mean energy n plasma concentration When T (K)& n (m -3 ), then: D 69 n T 1/ 2 (m) When T(eV) & n cm -3, then: D 7434. T n e 1/ 2 (cm)
Plasma: the most important characteristrics Plasma frequency A distortion of quasineutrality leads to the rise of oscillations P -1 gives the time needed for elimination of distorsions Some figures n E SI: p ne m e 2 0 P 8, 97 n 10 6 Power supply 10 2 10 8 In the case of atmospheric-pressure plasmachemistry applications: n = 10 16-10 20 m -3, T = 1-5 ev, D 10 m (free path length: 0.1 m)
Classification of plasma Classical plasma Quantum plasma (e.g. metals) Distance between particles is less than de Broglie wavelength (n 1/3 << h/p) Ideal plasma (C. potential energy << kinetic energy temperature)) Plasma of low ionisation degree ( charge carriers collide mainly with neutral particles) Fully ionised plasma (fusion reactors) Low temperature plasma (Gas temperature does not exceed 5000 K) Non-equilibrium plasma: (non-thermal) High temperature plasma T > 10 ev, in the case of fusion reactors T > 10 kev (10 8 K) different plasma constituents have different mean energy (temperature)
Energy transfer Steady plasma In steady conditions the concentrations of plasma components as well as their energy do not change with time Plasma of electropositive gas: atoms/molecules in their ground and excited states positive ions electrons Losses 1. Diffusion 2. Collisions elastic and non-elastic Non-elastic collisions lead to excitation, ionisation, dissociation, recombination and different other plasma-chemical reactions
Energy transfer Losses in plasma could be compensated by an external electric field T v T v T v d E v d v T >> v d +e As M i M A >> m e, the gain of energy from electric field occurs via electrons -e M Most of collisions in steady plasma are elastic as the amount of highenergy particles is small In the case of elastic collisions: 2m m m 1 1 m 2 2 2 M A >> m e? Transferred energy: 2m / M
Energy transfer E Electron travel in the energy space =2m e /M A a chance to have T e >> T g!
Why the condition T e >> T g is so important? Example: ozone production * O O2 M O3 M O3 M For efficient production of ozone precursors by reactions e + O 2 2 O( 3 P) + e 6.0 ev e + O 2 O( 3 P) + O( 1 D) + e 8.4 ev the mean energy of electrons should be e = 6-9 ev (9 ev 104 000 K) At the same time high gas temperatures lead to ozone decomposition Thus: a high yield of O 3 production is achieved when T e >> T g
Voltage U Production of plasma: current- voltage, i-u, characteristics of direct current, DC, discharge R DC power supply Argoon Pressure: 100 Pa U i 1 cm C A U = U 0 - ir Low electric field Background ionisation (non-self-sustained discharge) 1. Only drift and recombination 2. The role of recombination is negligible 1 10-10 10-8 2. saturation Current i, A
i-u characteristics of DC discharge - ionisation coefficient Voltage U Non-self-sustained discharge Strong electric field: Excitation & ionisation 10-8 10-6 Current i, A C A C A 0 d x Electron avalanche: number of charge carriers increases exponentially As = f(e), 2 the current increases rapidly with voltage n x = n 0 e x
i-u characteristics of DC discharge Voltage U When the size of the secondary avalanche reaches that of the primary one the transition to self-sustained discharge takes place Non-self-sustained discharge Strong electric field: Excitation & ionisation 10-8 10-6 Current i, A n x = n 0 e x C A photon ion Positive feedback
Voltage U i-u characteristics of DC discharge Transition to self-sustained discharge breakdown U b Breakdown 10-8 10-6 Current i, A At certain gas mixture and cathode material the breakdown voltage U b depends on production pd Paschen s law A
Voltage U i-u characteristics of DC discharge Depending on conditions the breakdown could be diffuse or filamentary and it could last from 10-3 to 10-9 s. Dark discharge Breakdown Glow 10-8 10-6 1 Current i, A Cathode layers Field E Anode layers Glow discharge emits intensive light Because of field distribution there is a number of characteristic luminous regions Light emitted by NG is caused by non-equilibrium (non-thermal) electrons PC- true plasma region
Voltage U i-u characteristics of DC discharge A growth of the current increases the power liberated in plasma and it finally leads to qualitative changes Dark discharge Breakdown Glow Arc discharge 10-8 10-6 1 100 Current i, A During each electron-atom elastic collision a energy fraction 2m e /M A is passed to the gas. A growth of the pressure increases the rate of collisions ea and the difference between T e and T g diminishes Growth of T g warms up the cathode material and electrons are emitted due to thermoionic emission for which lower applied voltage is needed. The glow transits to arc discharge.
Importance of DC discharge Qualitatively, the desribed regularities are independent of gas pressure (1 10 6 Pa) Atmospheric pressure The desribed regularities exist also in AC and HF discharges as well as in the case of pulsed nanosecond discharges AC glow, negative half-cycle Thank you for your attention!