Plasma and Ions - MIT Lincoln Laboratory - ll.mit.edu · PDF fileMIT Lincoln Laboratory Plasma...

MIT Lincoln Laboratory

Plasma and Ions-1A. Siegel 5/4/2007

Plasma and IonsAndy Siegel

MIT Lincoln LaboratoryPlasma and Ions-2A. Siegel 5/12/07

What is Plasma?

• Plasma is a unique form of matter which contains ions – Most of the matter around us consists of neutral atoms– Neutral atoms have an equal number of electrons (-) and protons (+), so their charge is balanced– Plasma is different - It’s like a gas consisting of ions and free electrons

• Ions are atoms which have either too few or too many electrons– This means that ions have a charge imbalance – a “net” charge– positively charged ions have too few electrons, and we call them cations– negatively charged ions have too many electrons, and we call them anions

• Fire, lightning, fluorescent lamps, and the Sun and stars all contain plasma– In fact, about 99% of the known universe is made up of plasma

HYDROGEN ION: +1 proton – 0 electrons = +1

CALCIUM ION: +20 protons – 18 electrons = +2

GOLD ION: +79 protons – 76 electrons = +3

CATIONS:

HYDROGEN: +1 proton – 1 electron = 0

HELIUM: +2 protons – 2 electrons = 0

LITHIUM: +3 protons – 3 electrons = 0

ATOMS:

CHLORINE ION: +17 protons – 18 electrons = –1

OXYGEN ION: +8 protons – 10 electrons = –2

ANIONS:


Plasma Exists in Many Different Forms

• Low-energy plasma– This weakly ionized plasma can exist only at very low pressure in sealed containers– Examples include fluorescent lamps, neon signs, plasma globes, plasma TVs– They easily form at room temperature, so they’re cool enough to touch, if you could . . .

• Medium-energy plasma– Partially ionized plasma is much hotter, so it can exist even at normal air pressure– Examples include fireworks, lightning, the surface of the Sun, metal cutting “plasma” torches – Their temperature is around ten thousand degrees (ouch!) – so way too hot to touch

• High-energy plasma– Super-mega hot (many millions of degrees!) and fully ionized– Very hard to create here on Earth, but it exists at the Sun’s core– Earthly examples include exploding bridgewires, particle acelerators, and nuclear explosions– At these pressures and temperatures, hydrogen nuclei can fuse, releasing a lot of energy

plasma globe brush fire ion implantation lightning carbon arc nuclear blast


How Does Plasma Behave?

• All plasma is electrically conductive– Even low-energy plasmas conduct electricity– Plasma can be “pushed” and “pulled” by electric fields – same as the free electrons in wires– Fluorescent lamps and plasma TVs use electricity to make the plasma which produces light – Fire sensors in gas furnaces quickly shut off the gas flow if the conductive flame goes out– Flame ionization detectors detect gas leaks and help researchers analyze chemical compounds– Ion engines for deep-space rockets and ion implanters for ICs use electric fields to acelerate ions

• Plasma can also be affected by magnetic fields– The aurora borealis is created by solar plasma trapped in the Earth’s magnetic field– Plasma is controlled in Tokamaks (a type of experimental fusion reactor) using magnetic fields

• Plasma is made of matter– Plasma has mass and inertia, just like everything else– The larger the ions are, the slower they move (this is how “ionization” smoke alarms work)– Ion-mobility spectroscopy (IMS) measures the speed of moving ions to detect explosives

fluorescent light ion propulsion aurora borealis inside a tokamak ion-mobility spectrometer


Medium and High-Energy Plasma

• Medium-energy plasmas produce intense light and extreme heat– The brightest lights on earth (carbon arc lamps) can illuminate the sky for many miles– Plasma torches are hot enough to cut through steel armor up to 12 inches thick!– Arc welders use plasma to weld iron and steel

• High-energy plasma can produce extremely high pressures– Inertial confinement fusion uses intense lasers to compress a small fuel pellet– The outer surface becomes so hot, it thrusts away like a rocket, forcing the pellet to compress– When it becomes dense enough, the hydrogen atoms fuse, releasing huge amounts of energy

• Plasmas can teach us a lot about our Universe – and about things closer to home– Plasma spectroscopy uses starlight to identify the chemical elements that stars are made of– A similar method is used here on earth to measure impurities in drinking water, and in people– It is so sensitive, it can detect as little as 1 part per trillion (1:1,000,000,000) of some chemicals– That’s like one crumb dissolved in an olympic-sized swimming pool!

carbon arc lamp RF plasma torcharc welding carbon arclights searching for aircraft


Ions and Conductive Liquids

• Molten metals and ionic solutions are both conductive liquids – Both liquids conduct electricity, but they behave very differently when they freeze . . .

• Metals conduct electricity in both liquid and solid form– Only electrons can carry charge in solid metals, because the electrons are still free to move– Copper wires are made from “frozen” copper, but the electrons can move just fine

• Ionic solutions also conduct electricity– In these solutions, electrons cannot flow like in metal, but positive and negative ions can– So the positive ions (cations) and negative ions (anions) carry charge through these liquids– Ionically bonded chemicals (like acids, bases and salts) ionize when they dissolve in water– But covalently bonded chemicals (like sugar and oil) don’t ionize, so they conduct very poorly

molten mercury Svante Arrheniusmolten gold T-1000 Terminatormolten gallium


Ionic Solutions

Solution ConductivityPure water 0.055 µS/cmPower plant boiler water 1.0 µS/cmGood city water 50 µS/cmOcean water 53 mS/cm31.0% HNO3 865 mS/cm

• Ionic solutions can be very conductive– Hundreds of amps of current flow through sulfuric acid in the battery when you start your car!– The acid concentration in car batteries is about 30%, which provides maximum conductivity

• Ionic liquids behave very differently than metallic liquids, though– Metals become poorer conductors as they heat up (electrons must flow around moving atoms)– But ionic liquids become better conductors as they heat up– Because all atoms (including the ions) are moving faster when a solution is warmer

• And when ionic liquids freeze, they conduct very poorly– Because when a liquid freezes, the atoms and ions can hardly move at all– The same is true for ionic solids like table salt and baking soda– If you heat pure table salt until it melts, it too will become conductive!

Nitric acid is sixteen million times more conductive than pure water!


Electrochemistry

• Electrochemistry is the study of ions in solution– Analytical chemistry (cyclic voltammetry, anodic stripping voltammetry, chronoamperometry)– Medicine, brain research, batteries and fuel cells, integrated circuits, metal refining, chemical

synthesis, nanotechnology, water purification, electroplating . . .

• Electric batteries and fuel cells use ions to produce energy– A “battery” (a group of of individual cells) can be made by combining metal atoms which want to

become cations (like zinc) with anions which want to become atoms (like copper)– Electrons released by anode flow through the circuit to reach the cathode– Electrons carry charge through the metal electrodes (and the rest of the circuit) while ions carry

an identical amount of charge through the electrolyte– In some batteries (lithium-iodine cells), the ions flow through a solid electrolyte– Their current output is low, but they last a really long time

simple battery Daniell and Faraday ion-exchange water softener cyclic voltammogram fancy chrome


We Are Full of Moving Ions

• You are an ion generator!– Our nervous system uses ions to convey impulses throughout our brains and our bodies– Special pores in our nerves (called ion channels) open up, allowing certain ions to flow– This moving charge creates an electric field which travels down the nerve to the end– Special chemicals called neurotransmitters are released, which causes muscles to contract – Each ionic impulse creates a small voltage change that can be measured with electronics

• Monitoring electrical signals from the body– We often monitor the electrical activity of the heart (called ECG) to check on its health – We look at signals from the brain (EEG) during sleep studies and brain research– We also monitor muscles (EMG) to measure how well they’re working

baby wearing an EEG cap

MIT Lincoln LaboratoryPlasma and Ions-10

A. Siegel 5/12/07

Simple Conductivity Meter

• You can build this circuit to measure the conductance of different fluids– Make probes from solid wire with about ¼” of the insulation removed from the ends– If these probes are too sensitive, then clip off the bare ends, use thinner wire, or both– Try measuring distilled water, bottled spring water, and fresh rainwater– How could you use this to make a flood alarm? a tilt sensor? a rain gauge?

1KΩ(#271-1321)1 2 3 4

8 7 6 5

0.1uF(#272-1069)

piezo speaker(#273-073)

7555 IC timer(#276-1718)

9V Conn.(#270-324)

9V Battery

pushbutton sw.(#275-1566)

Radio Shack part numbers are shown in parentheses

solid wire(#278-1221)

to probes

Plasma and Ions - MIT Lincoln Laboratory - ll.mit.edu · PDF fileMIT Lincoln Laboratory Plasma...

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