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From this photo I assume the plasma is also moving in a loop through the reactor due to it being a charged particle.

I have been trying to find a speed at which it does this but couldn't and neither could I find the amount of time from entering the tokamak to fusing.

I want to know how many times a single hydrogen nucleus goes around the tokamak before fusing and distance it traveled from entering until it became helium?

I suspect this will be different for different reactors but if you could tell me the highest and lowest distance that would be great.


From this photo I assume the plasma is also moving in a loop through the reactor due to it being a charged particle.

Plasma is not a "charged particle", there are free electrons and nucleons within it but over all it is neutral.

Left to itself, a plasma - like a gas - will occupy all the geometrical space available, because of the collisions between the particles. Magnetic fields can confine a plasma, because the ions and electrons of which it consists will follow helical paths around the magnetic field lines.

One can define a charge neutral Δ(V) in the collective motion induced by imposed magnetic fields which has a fluid flow around the tokamak geometry.

The tokamak is one of several types of magnetic confinement devices, and is being developed to contain the hot plasma needed for producing controlled thermonuclear fusion power. It is the leading candidate for a practical fusion reactor. Magnetic fields are used for confinement since no solid material could withstand the extremely high temperature of the plasma

Thus the whole business of magnetic fields and collective motion of plasma is for containing the plasma at its high temperature ( 150 million degrees Celcius) so that fusion will happen statistically locally while going around. The collective rotations of the plasma have nothing to do with fusion. It is the thermodynamic statistical scattering of nuclei that will fuse, going every which way.

Lifetime of plasma has to do with materials used for the Tokamak and there are a number of studies if one searches.

This answer here is relevant :

In a tokamak reactor at kT = 15 keV, for electrons, this is really fast: something like 73 million m/s, or almost a quarter the speed of light. (This means that we actually have to modify the Maxwell–Boltzmann distribution to account for relativity; I won't go into that here.) For deuterium nuclei it's lower, about 850 km/s because they're heavier.


The bulk fluid speed is probably more interesting, like how the net motion of the water in a river is generally more interesting than the thermal speed of each water molecule. In a tokamak this fluid rotation might be some 10s of km/s in the toroidal direction, up to maybe 100 km/s in today's tokamaks (like Alcator C-Mod or DIII-D). ITER might rotate toroidally at several hundred km/s.

It gives a plot:

speed of tokamak plasma

  • $\begingroup$ I know this isn't to do with the mechanics of fusion however am looking into possible sizes and shape of reactors and still want to know the average distance travelled before the hydrogen nucleuses fuse. Thank you for your input anyway! $\endgroup$ May 14 '17 at 18:25
  • $\begingroup$ Lastly, how long does it take from plasma injection until the first nucleuses start to fuse, I tried looking up the lifetime but still couldn't find anything. Thanks! $\endgroup$ May 14 '17 at 19:00
  • $\begingroup$ Plasma is not injected , at least it is injected as plasma in the beigning after it is built up and the nuclei accelerated until the fusion temperature is reached. Large mean free paths are needed for acceleration so that energy is not lost in radiation. After fusion temperature is reached injection is done to replace burned fuel, in pelets for ITER iter.org/mach/FuelCycle $\endgroup$
    – anna v
    May 14 '17 at 19:28
  • $\begingroup$ How long does it take to get to fusion temperature from effectively, when you start the heating in the tokamak $\endgroup$ May 14 '17 at 19:32
  • 1
    $\begingroup$ You should make some corrections to your question. (1) Plasma is not a particle, it is made of particles. Pressumably you mean how many times a plasma ion makes a full loop before fusion. (2) Plasma is not itself making a loop. Plasma is formed of both electrons and ions (different species of them) which have distinct behaviors and generally move collectively in opposite directions. $\endgroup$
    – Germán
    May 15 '17 at 1:34

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