| MadSci Network: Physics |
Hi,
Over the last few days, I have been pondering your question quite a bit. You see, if we are looking at Earth, a plasma is normally really a system, consisting of a machine generating and containing the plasma and the plasma itself. I suppose the question "What is the easiest substance to turn into a plasma" then really means something like "What is the simplest machine I could use to create a plasma". If I think about the problem this way, I still get several different answers. I don't think it's really possible to come to a precise answer here, as what you think an easy machine is, I might find pretty complex and the other way around. Anyway, what I will do is take you through several relatively easy ways to make a plasma.
I think that technically, the easiest material to turn into plasma is plutonium. Take two chunks of it, put them together, and the resulting nuclear explosion, while leveling part of the city, will produce a plasma. On the one hand, moving two chunks of metal together must be technically the easiest way of making a plasma, on the other hand, I am tempted to disqualify it because it would kill the person doing the experiment.
Another answer I could think of would be the radiation coming from radioactive elements, such as curium. These produce so much radioactivity that the air around them turns into a (thin) plasma. The issue with this is that it is a bit of a stretch to call this a plasma. You see, without going into too much technical details, a plasma is an ionized gas: the gas is partially split in free electrons and ions. It is not quite clear where the lower boundary is, but obviously, having only a handful of electrons in a decent amount of gas is not a plasma, as they do not influence the behavior. [1]
Anyway, if we want to stay near gases that can only barely be called plasmas, we can consider flames as well. There are few electrons and oxygen in the flame might scavenge electrons up (oxygen has the property that it can bind electrons to form negative ions, hence reducing the electron density). That said, a flame does offer probably the easiest way of making a plasma in the home environment. You see, if you were to put a candle in a microwave (do not do this with a microwave you plan to keep using!), the energy in the microwaves can excite the free electrons in the flame, boosting the plasma electron density.
Now, although I think that all these answers technically qualify, none really are "conventional", usable plasma. To make a plasma, we need to ionize a gas. The problem is that ionization is usually the last stage in matter "falling apart" when you heat it up - it starts solid, becomes a liquid, then a gas, then the molecules in the gas dissociate into atoms and then the atoms ionize (sometimes, molecules can ionize, but those very easily recombine). Now, the last two steps take very large amounts of energy compared to the first two steps - ionizing an amount of argon [2, 3] takes an amount of energy in the same ballpark as heating it up to 100,000 K! So, you might think that we should use elements that are easy to ionize. The so-called alkali metals [4] (lithium, sodium, potassium, rubidium and caesium) are good choices, with caesium [5]having the lowest energy.
One thing we did not consider above is that although ionizing all atoms takes a massive amount of energy, in practice, we only need to ionize a small fraction (one per thousand or even less). If we use a solid material, we would however need to evaporate all of the solid. From that perspective, gases also look like a good choice, in particular mono-atomic gases such as noble gases (helium, neon, argon, krypton, xenon), of which xenon [6] is the easiest to use. In practice, people often use argon for making plasmas, as it is by far the cheapest noble gas - although neon (neon tubes) is also popular.
Another way of approaching this would find a compromise, namely a material that is both easy to turn into a gas and has a low ionization energy. Mercury [7] is such a metal; its ionization energy is 20% lower than that of xenon, and, being a liquid at room temperature, it readily evaporates. This would work especially well in a noble gas atmosphere, as the noble gas does not absorb free electrons, like ordinary air does. This is, incidentally, the way that household energy saving and fluorescent bulbs are filled.
In conclusion, there are two ways of going around this problem. One way involves tapping into the massive energy available in nuclear reactions. The other involves taking a material that is easy to move from its room temperature state to an ionized state.
I hope this answers your question.
Bart
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