Plasma

Plasma is the fourth state of matter, after solid, liquid and gas. It is a super-hot, electrically charged "soup" of free electrons and positively charged atomic nuclei. Plasma is rare on the surface of Earth, but it makes up most of the visible matter in the universe. Stars (including the Sun) are giant balls of plasma. Lightning bolts are brief streaks of plasma. Glowing neon tubes, fluorescent lights and lava lamps all contain plasma at the heart of the glow. It is hot, bright, electric and powerful.

• What it isIonised gasAtoms split into ions + electrons
• Made byExtreme heating or strong fieldsStrips electrons from atoms
• On EarthLightning, flames, plasma globesPlus auroras and neon signs
• In spaceStars, nebulae, solar windOver 99 per cent of visible matter
• TemperatureOften thousands of degreesCan be cooler in low-pressure tubes
• Conducts electricityYes, very wellResponds to magnetic fields

How plasma forms

To make plasma you have to ionise a gas. That means giving the atoms so much energy that their electrons get knocked off. The result is a mix of positively charged ions (the atoms without their electrons) and negatively charged free electrons.

You can ionise a gas in two main ways:

• Extreme heating: at thousands of degrees, atoms collide so violently that electrons fly off. This is how stars produce their plasma.
• Strong electric fields: pushing electricity through gas can rip electrons off, even at lower temperatures. This is how fluorescent tubes and plasma globes work.

Once formed, plasma behaves very differently from ordinary gas. It conducts electricity, it responds to magnetic fields and it glows with characteristic colours depending on the gas.

Plasma in nature

• The Sun and other stars: huge balls of plasma. The Suns surface is around 5,500 degrees Celsius; its core reaches 15 million degrees.
• Lightning: a brief column of plasma in the sky, reaching temperatures of around 30,000 degrees Celsius (5x hotter than the surface of the Sun).
• Aurora (northern lights): thin plasma high in the atmosphere, glowing as solar wind particles excite air molecules. Best seen near the magnetic poles.
• Comets tails: a stream of plasma stretching millions of kilometres behind a comet, pushed away from the Sun by the solar wind.
• Solar wind: a constant stream of plasma flying off the Sun in all directions.
• Nebulae: enormous, glowing clouds of plasma where new stars are forming.
• Earths ionosphere: a thin layer of plasma in the upper atmosphere, important for reflecting some radio waves.

Fact Over 99 per cent of all the visible matter in the universe is plasma. Stars are plasma. Most of the gas in interstellar space is plasma. Even the dust clouds between stars are partly ionised. Earth and its rocky, watery, breathable world is the exception: we live on a tiny island of unionised matter in a plasma universe.

Plasma in everyday technology

• Fluorescent lights: a tube containing low-pressure gas (often mercury vapour). An electric current ionises the gas, producing ultraviolet light that excites a coating on the inside of the tube, which glows visible white.
• Neon signs: glass tubes filled with neon (red), argon (lavender), helium (orange) or other gases. Electricity ionises the gas inside; each gas glows a characteristic colour.
• Plasma globes: a sealed glass ball with low-pressure gas and a high-voltage electrode in the centre. The electricity ionises the gas, creating bright pink-purple streamers that follow your fingers when you touch the glass.
• Plasma TVs: an older flat-screen technology with millions of tiny cells each containing a small puff of gas. Electricity excites the gas, which glows briefly to produce the picture.
• Welding arcs: a powerful electric arc between two electrodes ionises the air between them, producing intense heat to melt metal.
• Plasma cutters: industrial tools that use a thin jet of high-temperature plasma to slice through thick metal sheets.
• Nuclear fusion reactors: experimental machines that hold super-hot plasma using powerful magnetic fields, trying to copy the power of stars.

Plasma colours

Different gases glow in different colours when made into plasma, because each kind of atom releases light of specific wavelengths when its electrons jump back to their normal positions:

• Neon: bright red-orange (the original "neon" sign colour)
• Argon: pale lavender or sky blue
• Helium: orange-pink
• Krypton: white-yellow
• Xenon: pale blue or violet
• Mercury vapour: bluish-green (combined with phosphors, white)
• Sodium vapour: bright orange-yellow (used in old street lamps)

Did you know? The aurora colours that dance over the Arctic and Antarctic skies come from different gases in Earths upper atmosphere. Green aurora is oxygen at around 100 to 300 km altitude. Red aurora is oxygen even higher up, above 300 km. Blue and purple are from nitrogen. The next time you see an aurora photo, you are looking at plasma glowing in its native colours.

Fusion: harnessing the power of stars

One of the biggest scientific dreams of the 20th and 21st centuries is to build a nuclear fusion reactor that produces electricity by copying what happens in the cores of stars. Inside the Sun, hydrogen nuclei fuse together (at temperatures over 15 million degrees Celsius) to form helium and release vast amounts of energy.

To recreate this on Earth, scientists must heat hydrogen plasma to over 100 million degrees Celsius (hotter than the core of the Sun) and hold it in place. No solid container can hold such a plasma, so reactors use powerful magnetic fields to suspend the plasma in mid-air. This kind of reactor is called a tokamak.

The international ITER project in France is the largest tokamak ever built, with first plasma planned later this decade. If it works, it will be a giant step towards limitless, clean energy from fusion.

Try this Look closely at a fluorescent light tube or an LED screen when it is on. Inside the fluorescent tube is gas being ionised into plasma, which gives off ultraviolet light that makes a special coating glow. If you have ever seen a plasma globe in a museum or shop, place your finger on it (it is safe) and watch the bright pink-purple plasma streamers follow your touch. You are looking directly at the fourth state of matter.

Deeper dive: the heart of the Sun

The Sun is the closest example of plasma to Earth, and the most important. Every photon of sunlight that warms your face has travelled from the Suns surface, and the energy in that light was originally released in the Suns plasma core.

The Sun is a giant ball of plasma about 1.4 million kilometres wide (109 Earths across). It has three main zones, all plasma:

• Core: the innermost 25 per cent of the Sun by radius. Temperature around 15 million degrees Celsius. Density about 150 times that of water. This is where nuclear fusion happens: hydrogen nuclei fuse into helium, releasing huge amounts of energy.
• Radiative zone: the middle layer. Energy slowly works its way outward as photons of light, bouncing off particles for around 100,000 years before reaching the surface.
• Convective zone: the outer layer, where hot plasma rises in giant columns to the surface, gives off its heat, then sinks back down. This is what makes the Suns surface look granular.

Above this, the visible surface (the photosphere) is at around 5,500 degrees Celsius. Strangely, the very thin atmosphere above the photosphere (the corona) is even hotter, reaching 1 to 2 million degrees Celsius. How that can be is still being researched: it seems to be powered by twisting magnetic fields.

The Sun has been a stable plasma ball for about 4.6 billion years and will continue for about another 5 billion. In its final stages it will swell into a red giant, then shed its outer layers as a planetary nebula, leaving behind a small dense remnant called a white dwarf. The whole life cycle is a story of plasma physics played out over billions of years.

For more, see gas and Bose-Einstein condensate.