How can we regulate fusion reactions?

[arman]

This is my first post here, so I'm not entirely sure whether or not this topic warrants a new thread. I've just had a simple idea that I'd like to get some feedback on, from people who have a little more experience in nuclear chemistry than I do.

I was wondering if it would be possible to regulate the exothermic nuclear fusion of hydrogen by simple adding an element that is endothermic under fusion to the mixture? This energy would be wasted, but wouldn't it serve to reduce the volatility of the hydrogen reactions?

 

[russ_watters]

Trouble is, the reaction won't occur without extreme heat and pressure, so absorbing some of the energy would just make the reaction stop.

 

[Astronuc]

Fusion occurs in plasmas which are very hot by human standards. For DT plasma, the optimal temperature is approximately 80 keV, and one eV is equivalent to 11605 K, so that's a temperature of 928 000 000 K.

So I am not sure what one means by volatility.

Adding other heavier elements increases the loss of energy from the plasm by cyclotron radiation, bremsstrahlung and recombination.

At present time, the problem has been to generate sufficient excess of energy and minimize losses in order to produce a self-sustaining reaction for a long period of time while producing useful energy, e.g. electrical power.

 

[sid_galt]

Don't they want to control the plasma (in magnetic fusion) rather than regulating the fusion reactions?

 

[Astronuc]

In Tokamak's like ITER, the objective is to confine a plasma for as long as possible while heating it to temperatures at which the fusion process occurs. Ideally the fusion process produces more energy than is put into the plasma, i.e. it heats itself, and the excess energy is sold as electrical power or useful thermal energy.

The higher the plasma temperature the higher the pressure for a given particle density. The confining magnetic has to generate a magnetic pressure to maintain confinement. Magnetic flux density, B, is on the order of 5 T.

 

[arman]

I am not sure what one means by volatility

I guess this was inappropriate analogy. I am referring to the 'volatile' (exothermic) nature of the H plasma at 1 billion K, in contrast to some heavier elements which might be endothermic under fusion.

I'd like to ask what temperatures we can control. I am assuming we can control the initiation energy for a fusion reaction (i.e. 1 billion degrees), and the problem lies in the fact that this temperature quickly increases to something we CANNOT control. if so, couldn't we simply add heavy elements to keep the temperature down near 1 billion?

 

[Astronuc]

There are two problems with the plasma -

1. retaining the energy - electrons moving somewhat randomly will radiate energy in a magnetic field, e.g. cyclotron radiation, and interaction with ions and other electrons (bremsstrahlung and recombination).

2. confining the plasma - ions tend to diffuse (with the help of collisions) through the magnetic field and out of the hot plasma, and then there are the magneto-hydrodynamic instabilities.

Heavier ions add more electrons to the plasma (Z electrons per ion) for a given ion density, and this produces more energy loss.

The cross-section for the fusion reaction decreases with Z for a given temperature.

 

[Astronuc]

Fusion occurs in plasmas which are very hot by human standards. For DT plasma, the optimal temperature is approximately 80 keV, and one eV is equivalent to 11605 K, so that's a temperature of 928 000 000 K.

I should clarify something about the temperature I used here.

This is the optimum temperature in terms of realizing the maximum cross section and reaction rate. However, as temperature increases, so does energy loss from the plasma by virture of cyclotron radiation.

http://www.pppl.gov/projects/pages/tftr.html achieved a plasma temperature 510 million K (equiv. to 44 keV), and the site states that is well above 100 million K (8.6 keV) "required for commercial fusion". The lower energy will have significantly less radiation losses, but will have to make up for the lower cross-section by maintaining a relatively high plasma density - and that increases energy loss by particle diffusion out of the magnetic confinement.

Reaction rate in a plasma is given by RR = n₁ n₂ <\sigma v>, where <\sigma v> is the reaction rate coefficient (m³/s), and n₁ and n₂ are the particle densities for species 1 and 2 (1/m³), respectively.

This site may be of help regarding some of the physics of fusion reactions.
http://fusedweb.pppl.gov/CPEP/Chart_Pages/3.HowFusionWorks.html