How is the plasma in a compact fluorescent lamp (CFL) different from a plasma in say ITER or the sun? Why does ITER need 100MK and a CFL can work at practically room temperature (apart from the filament)? Or could ITER also create a plasma by charging the gas inside the reaction chamber but not have enough energy for the reaction, so they heat it directly (microwaves) and charging it would be of no use?

Or is it the degree of ionization the volume of gas has achieved? Like, a CFL has around $x$ ions and a sun plasma has only ions?

  • $\begingroup$ sorry, I thought those were "well known" abbreviations in the US and co^^ I'm from germany, so idk :P $\endgroup$
    – Pwnie2012
    May 19, 2015 at 17:47
  • $\begingroup$ I don't know about ITER, but the pressure inside a fluorescent lamp tube is somewhat less that the pressure inside some regions of the Sun. $\endgroup$ May 19, 2015 at 18:15
  • $\begingroup$ Also, don't confuse the temperature of the outside of the glass envelope with the temperature (average kinetic energy) of the ions inside the tube. I don't know specifics, but I'd guess that the gas/plasma in the tube is quite high, but there is not much heat output because it is so rarefied. $\endgroup$ May 19, 2015 at 18:18
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    $\begingroup$ The argon in a CFL is not nearly completely ionized, it is a gas discharge, but not a full plasma. Being fully ionized is not necessary for the CLF gas, which is merely there to hold the mercury that emits the UV radiation that excites the visible light emitting phosphors on the inside of the tube. The mercury efficiently emits much of the power that is deposited by the electric current, which keeps the gas cool. That's exactly the opposite of the ultra-clean fusion plasma in ITER, where we are trying to produce as little radiation as possible to keep the temperature high. $\endgroup$
    – CuriousOne
    May 19, 2015 at 18:27

2 Answers 2


ITER needs very high ion temperatures (100M K) so the deuterons and tritium nuclei are fast enough to overcome electrostatic repulsion and undergo thermonuclear fusion. A CFL only needs to have a conductive plasma in order to have an electron current exciting atoms in the gas.

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    $\begingroup$ I don't believe that black body radiation plays a significant role in the operation of a fluorescent lamp. $\endgroup$
    – garyp
    Jan 15, 2016 at 14:11

A compact fluorescent lamp belongs to the glow discharge plasmas. Usually you have electron densities on the order of $n_e \approx 10^{16}\,\mathrm{m}^{-3}$, electron temperatures on the order of $T_e\approx 1\,\mathrm{eV}$ and ion temperatures being at least an order of magnitude lower. The degree of ionization is $1\,\%$ or lower. The room temperature you were referring to, only applies to the gas temperature and, more or less, to the ion temperature. The electrons are much hotter.

ITER, in contrast, will have plasmas parameters of $n_e \approx 10^{20}\,\mathrm{m}^{-3}$, $T_e\approx T_i \approx 10\,\mathrm{keV}$.

ITER requires such a high temperature because it is meant to investigate nuclear fusion and to achieve fusion you need to overcome the electrostatic repulsion of the nuclei (remember, both are positively charged). A fluorescent lamp, on the other side, just needs enough energy/temperature to achieve breakdown, as @Rod Price has written.

ITER could of course also create lower temperature plasmas, and it will do so for cleaning the wall, but for fusion, those high temperatures are required.


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