French term (edited): un plasma

Either, actually, but "a plasma" is much better in this case.
Two examples from the big Oxford English dictionary:

1976 T. Beer Aerospace Environment i. 16 The solar wind is *a plasma* of hydrogen ions (protons) and electrons travelling at speeds that range from 300 km s-1 to 1000 km s-1, depending on solar activity.
1974 Nature 5 Apr. 494/2 In a cold plasma (which is a good approximation for most of the magnetosphere away from the equatorial region) there are two wave modes.
Another OED example with the plural:
1971 E. Nasser Fundamentals of Gaseous Ionization & Plasma Electronics xiv. 427 Liquid plasmas exist in salt solutions where the positive and negative ions move separately.

And this one from the Britannica:
The observed radius of the Sun equals 6.95 10{sup 10} cm and is understood to be the result of a balance of forces between the Sun's self-gravity and the pressure of its hot gases, which exist in a nearly fully ionized state (a plasma of positive ions and free electrons) in the deep interior.

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Note added at 19 hrs 36 mins (2005-08-28 21:02:36 GMT) Post-grading
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Some post-grading notes:
My answer was that either would be correct (“plasma” or “a plasma”), but "a plasma" is much better in this case.
Let me explain this a little more:
Plasma is both countable (“a plasma”) and uncountable (“plasma”).
Plasma can signify a state of matter. According to Le Petit Robert, “État de la matière portée à très haute température, où les atomes sont en majorité ionisés. La matière des étoiles est à l'état de plasma. Transformer un gaz en plasma.”
In this case, plasma is uncountable, like air and water.

Plasma can also be a countable entity, like a gas, a liquid, a fluid, or a solid.
The OED’s definition makes this obvious:
plasma: 6. Physics. *A gas* in which there are positive ions and free negative electrons, usu. in approximately equal numbers throughout and therefore electrically neutral; esp. *one* exhibiting phenomena due to the collective interaction of the charges.
See also: Wikipedia, [http://en.wikipedia.org/wiki/Plasma] “In physics and chemistry, *a plasma* is an ionized gas…”

Apart from the previous examples, there are hundreds of other examples from *reliable* sources, such as:

Boston University Astronomy Department: GRS AS 703 -- Introduction to Space Physics: Survey of physical phenomena in the sun, solar wind, and magnetospheres, ionospheres, and upper atmospheres of objects in the solar system. Introduction to the physical processes governing *space plasmas*, solar-terrestrial interactions, and ionized and neutral media surrounding the Earth and other solar system bodies.

From a NASA glossary:
Plasma: A plasma is a gas consisting of ions and electrons. Unlike a gas of neutral atoms and molecules, such as the air at sea level, a plasma is capable of conducting an electric current because its ions and electrons are electrically charged. Furthermore, the motion of a plasma is affected by magnetic and electric fields (again because its constituent particles possess electric charges).
http://observe.arc.nasa.gov/nasa/ootw/1999/ootw_990428/ob990...
Lots of “a plasma” in the NASA pages.

The asker’s source says: “créer un plasma, gaz ionisé très chaud”. It does not say “de plasma”.
Therefore, it is absolutely right to translate it as “create a plasma, a very hot ionized gas”. On the other hand, there is nothing wrong with saying “create plasma...”.