Fusion reactor helium poisoning

[Crazymechanic]

Hi I have a question , now in fusion reactors like a tokamak design and also in others , if they use say the deuterium tritium mixture as fusion fuel which ends up in some released binding energy , alpha particles, a energetic neutron etc. Now the neutron and alpha particles an d all other stuff tends to either escape , hit the walls cause heat or heat the plasma etc , ok now let's assume a fusion reactor is up and running , now after some time of fusion helium as the end product of deuterium and tritium fusion builds up , and hence the fusion reactor is not capable of fusing helium it just builds up and because the plasma (atleast in tokamak configuration ) is confined with a magnetic field and as we know a magnetic field can trap particles with charge helium has protons and electrons so it is also trapped by the magnetic field ,

so the question is how do they get rid of the helium once it has started to build up ?
Now they could stop the fusion and refuel the reactor chamber but that wouldn't be practical I guess so what is the way they plan on doing it ?

 

[mfb]

As far as I know, you can select nuclei with multiple charges with a clever combination of magnetic fields, and direct them to the divertor. Don't ask me about details, I have no idea how this works.

 

[Crazymechanic]

ok so can we imagine we have a small negatively charged sphere in the center of such a fusion plasma that would attract not only the unwanted helium but also the reaction products tritium and deuterium , so we put a magnetic field around it with certain given strength , now could the magnetic field strength be the "deciding" factor which particles to let through and which not to ?
As before deuterium and tritium have fused they each weigh less than the helium atom , since it has more mass given it is in the same temperature and conditions as the other fusion particles it has more mass and a larger kinetic energy so could this form something like a magnetic door ? smaller unfused particles cannot go through the field (insufficient energy) larger fused particles or ions can go through and hit a electrode which then drains their charge etc?

I hope someone can tell me a bit more about how this could work or already works or is proposed to work?

 

[nikkkom]

google "tokamak divertor"

 

[the_wolfman]

Helium and higher Z impurities radiate energy more efficiently than hydrogen. This has a cooling effect on the plasma. Thus we want to keep the amount of helium ash and other impurities in a tokamak as small as possible.

Plasma particle transport is incredibly complex, but at the most basic level nature helps us out. The classical outward radial diffusion of particles in a magnetically confined plasma scales with the gyroradius which in turn scales with the mass of the particle. Therefore the massive a particle is, the quicker it diffuses out of the plasma.

ok so can we imagine we have a small negatively charged sphere

In the presence of an external electric field a plasma will reorganize itself to shield out and cancel that electric field. In will only take the plasma a few millimetres to completely cancel the electric field, and the bulk of the plasma will experience no effects from the applied electric field.

 

[Astronuc]

ok so can we imagine we have a small negatively charged sphere in the center of such a fusion plasma?

No. A negatively charged sphere would polarize the plasma, with positive particles being attracted toward the negative charge, and the outer surface of the plasma would be richer in electrons. Moreover, a negatively charged sphere would soon reach is boiling point in a hot plasma and basically the high Z atoms would quench the plasma.

Removal of He would need to take place at the periphery of the plasma, and it is intended to used 'divertors' for that purpose. Charged particles can be selected/separated by momentum/velocity, but meanwhile, the He particles (alpha particles, or nuclei of He atoms) impart their kinetic energy (initially several MeVs) to the plasma, which is at a cooler temperature in the low keV range.