Plasma A Basic Introduction & Some Cool Stuff Matthew Wan PHYS 420.

PlasmaA Basic Introduction

&

Some Cool Stuff

Matthew WanPHYS 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

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

What Did That Mean?

• Because the particles are not neutral:– Plasma behaves differently then regular

gases– For instance, in the presence of

electromagnetic fields

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

Characteristics: Temperature

• Defines two kinds of plasma: Cold and Hot

• Refers to the electron temperature

• Ion temperature may be very different (lower)

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

CoolWhere Can I Find Cold Plasma?

• Fluorescent Lights

• Strobe Lights

• Experimental Fusion Research Devices

Hot Plasma

• Really hot

– 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

ScorchingWhere Can I Find Hot Plasma?

• In Nature, actually– The Sun and other stars– Lightning– The Aurora Borealis

Characteristics: Density

• Plasma (electron)

density

– The number of free

electrons per unit

volume

• Ion density

– Related to above by

the average charge

state:

en

ie nZn

on

Density

• Neutral Density– In hot plasmas, this

quantity is very small, but may still determine important physics

• The degree of ionization is given by: io

i

nn

n

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

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.