A Basic Introduction & Some Cool Stuff Plasma A Basic Introduction & Some Cool Stuff Only a basic explanation; the physics is upper level university. Who has ever heard of plasma? Matthew Wan PHYS 420

What Is It? A hot ionized gas The ‘fourth state of matter’ Unlike gases, solids, or liquids, plasma does not contain molecules Instead, it is a gas that is composed of ions Composes more than 99% of the known visible universe Hot is relative; explained later

So What’s In It? Some, or all, of the electrons in the outer orbitals have been stripped away The result is a collection of ions and electrons, which are no longer bound together Atoms have a nucleus, with electrons orbiting at different levels (acknowledge that this is greatly simplified) These particles are charged

What Did That Mean? Because the particles are not neutral: Plasma behaves differently then regular gases For instance, in the presence of electromagnetic fields i.e.: can use magnets to manipulate plasma, such as in a fusion reaction Magnetohydrodynamics

Who Found It? First discovered by Sir William Crookes, in 1879 But it wasn’t called ‘plasma’ until 1928, when Irving Langmuir coined the term Little bit of history: Crookes called it ‘radiant matter’

Characteristics: Temperature Defines two kinds of plasma: Cold and Hot Refers to the electron temperature Ion temperature may be very different (lower) Three major characteristics: temperature, density, potentials, in order of importance

Cold Plasma Isn’t really cold Typical electron temperatures for cold plasmas are in the thousands of degrees Only a small fraction of the gas molecules are ionized (degree of ionization) Usually on the order of 1% Often created using strong electric fields Relative temperatures, as stated on first slide Also known as technological plasmas

Cool Where Can I Find Cold Plasma? Fluorescent Lights Strobe Lights Experimental Fusion Research Devices Generally ‘man-made’

Hot Plasma Really hot The molecules are nearly if not fully ionized Like the Sun (15,000,000° C at the core) The molecules are nearly if not fully ionized Created by heating the molecules to extremely high temperatures Degree of ionization is close to 100% Usually natural processes as opposed to man-made

Scorching Where Can I Find Hot Plasma? In Nature, actually The Sun and other stars Lightning The Aurora Borealis Also in fire

Characteristics: Density Plasma (electron) density The number of free electrons per unit volume Ion density Related to above by the average charge state: <Z> is the average charge state of the ions

Density Neutral Density The degree of ionization is given by: In hot plasmas, this quantity is very small, but may still determine important physics The degree of ionization is given by: Because cold plasmas have a low degree of ionization, there are more neutral particles in the plasma, thus the neutral density is higher

Characteristics: Potentials Plasmas are excellent conductors Simple view: Due to the above, the electric fields in plasmas tend to be very small Quasineutrality: On the one hand, we can assume that densities of positive and negative charges are equal However, we can assume that electric fields exist as needed for the physics at hand So they allow us to make the needed approximations Navier-Stokes equations and Maxwell’s equations combine to describe electrically conducting fluids. Plasmas fit this description. These differential equations must be solved simultaneously; in all but the most trivial cases, symbolic (algebraic) solutions are impossible. Super computers have been used to find numerical solutions

An Application: Let’s Get Hands On (sort of) Plasma Globes An electrode sitting inside a vessel containing some kind of inert gas The electrode is energized by a high-voltage, high-frequency power supply This globe uses voltages around 10 000 volts, and frequencies ranging from a few kilohertz to a few 10’s of kilohertz

The Power Supply:

How It Works The IC sends out pulses which trigger a transistor This causes a pulse of voltage to pass through the coil This induces a large burst of voltage on the secondary of the coil

Which Does? The large voltage produced by the coil can be harnessed to produce a cold plasma The light bulb contains an inert gas that is relatively easy to ionize With any gas, a small fraction of the particles are always ionized

Cosmic Cosmic rays that are constantly bombarding the Earth ionize some of the particles The voltage and current from the power supply accelerate these particles As they move around the vessel, they ionize other particles

Exciting The other particles are excited into a higher energy state As they come back down to their ground state, they release the extra energy: Light, in the form of photons Because particles have unique wavelengths, the colour of the plasma depends on the gas inside the vessel

Pressure The gas vessel is at a reduced pressure As a result, the particles are spaced out This gives them time to accelerate If the particles were tightly packed, they would not build up enough energy to initiate a cascade effect

BE CAREFUL The currents and voltages in this project are extremely dangerous Touching the wrong part could seriously harm you I’ve already zapped myself. It hurt.