Reply To: X-ray cooling?

[jamesr]

dash wrote:

Accelerator experiments under carefully controlled conditions, like you describe, are useful to determine the precise collision cross sections (reaction probabilities), but useless as a way of getting energy out.

I’m not necessarily convinced of this. First off, you want the target cold because you want to eliminate the thermal heat velocity noise. Maybe there is an exact energy the ions need to fuse.

Now as regards energy coming out, you’d charge up the target to a high or low voltage, whatever is correct to get the right speed ions hitting it. When an ion hits and fuses, does it change the net charge on the target? Maybe the charge stays the same.

Keeping it all cool instead of a general hot plasma seems easier to control. Hot plasma is difficult to contain, as everyone knows.

Just tossing out ideas. Thanks for your response.

-Dave

It’s good to toss ideas out there, but in this case its not going to work.

An accelerator of this energy would typically be only a few percent efficient at ionizing and accelerating the ions (this efficiency drops rapidly as the beam energy goes up). Even then, if an ion beam hit a solid target, the ions would interact (via coulomb force) with many atoms at once, leaving an ionized trail of atoms in their wake. Gradually loosing their energy and heating up the target material. The chance of hitting a nucleus head on and so getting close enough to fuse is tiny, so most will loose their energy and thermalise with the surroundings after a few dozen close-ish collisions.

Only a proportion of the fusion energy comes out as X-rays the bulk will be the kinetic energy of the Helium nuclei. These will come out at random angles – not related to the beam direction and quickly dissipate their energy in the target material as heat. If there were sufficient reactions to produce a meaningful amount of energy the target would vapourise and become a plasma anyway.

There is indeed some preferred energies due to resonances (excited states) in the resulting transient nucleus. There is a small spike in the absorption cross section at around 148keV which could be critical in igniting pB11 fusion plasmas, but the main broad peak is over 500keV. The difference between having a target at room temperature (300K = 0.025eV) and cooled to say 3K is insignificant compared to the width of the resonance peak which is around 10keV.

Nevins and Swain 2000 is a recent analysis of the pB11 reaction probabilites.

The reaction rate for thermal plasmas is a combination of the reaction cross section for each speed, the maxwell-boltzmann thermal distribution, and the losses due to radiation. This masks out the spike of the resonance, but it adds a significant contribution at low temperatures.