# What if you build a big vacuum sphere

1. Jan 17, 2006

### magi

Hello.

I was just wondering.
What if you build a big vacuum sphere (of glas?) about 1-3 m in diameter and took a BIG parabol to focus the sun in the middle of the sphere.

How big must you make it to be able to reach the temperature for fusion?

If the diameter is to hugh for one, would it be possible to use many hugh parabols and send the energy in to a system of mirrors and lead the sun beam to the vacuum sphere where the last step of focus is made in to the center, from all directions.

What sun area is needed?

Regards Magi

Last edited: Jan 18, 2006
2. Jan 17, 2006

### Mk

REALLLLLLLY BIG, WAY too big for anyone below an international superpower to make.
Some of the world's biggest:
http://www.imp.cnrs.fr/foursol/index_en.shtml [Broken]
http://www.psa.es/webeng/instalaciones/ [Broken]
http://www.energylan.sandia.gov/sunlab/snapshot/stfuture.htm [Broken]

One you can make at home:
http://www.solardeathray.com/

Last edited by a moderator: May 2, 2017
3. Jan 17, 2006

### Tide

Temperature is only one consideration. You must also have the heated fuel remain at sufficient density and "confined" for a sufficient interval of time (i.e., the product $$n\tau$$ has to be sufficiently large!) in order to achieve breakeven conditions. Materials heated to fusion temperatures tend to expand quite rapidly! :)

4. Jan 18, 2006

### Mk

Unless of course we're using laser fusion, or ion beam fusion structures, then it heats up faster than it explodes!

They use tiny deuterium-tritium pellets, the enormous energy influx evaporates the outer layer of the pellet, producing energetic collisions which drive part of the pellet inward. The inner core is increased a thousandfold in density and its temperature is driven upward to the ignition point for fusion. Accomplishing this in a time interval of 10^-11 to 10^-9 seconds does not allow the ions to move, because of their own inertia. Ha! Using loopholes in the laws of physics against those little buggers.

How is it up in Sweedun, Magi? When does daytime start?

Last edited: Jan 18, 2006
5. Jan 18, 2006

### magi

Hi Tide.
I guess that there will be an expansion when you put fuel in it but that is next step.
I am just curious on what size of sunparabol is needed for a fusion to be possible.
Hi Mk.
Surise is around 8.22 here.

Is it that difficult to calculate a size of a sun parabol for fusion? :)

Last edited: Jan 18, 2006
6. Jan 18, 2006

### Staff: Mentor

Hi magi, The parabola approach is almost certainly the wrong approach. It's a creative idea, but has many problems. The better approach is the inertial confinement fusion mentioned already, like with multiple laser beams converging on a single fuel pellet. With one parabola, you will only be heating one side of a fuel pellet, and it will just vaporize away in the other direction. Also, the light from the sun contains many colors, so it is difficult to get a very precise focul point for all of the energy. Plus, the bigger you make the mirror, the more focus errors will be introduced.

Getting fusion to ignite is a pesky problem, because of the difficulty in getting the temperature*time product high enough. It's a very interesting subject to study, however, so keep on thinking and reading about it.

7. Jan 18, 2006

### magi

What if you have many hugh parabols and lead it in to a mirror system and direct it in to the sphere from all direktions.

What if you in the sphere have a magnetic field with a plasma and beam in the frequence of light you need for fusion.

With a filter of BIG prism, cant you separate out the photons you want?

Regards Magi

8. Jan 18, 2006

### Staff: Mentor

Hi magi, You are touching on a very fun and interesting field of physics, and I encourage you to get more into it. The parabolas are a fun mind experiment, but will not get you to the level that you want for temperature*time. Adding a magnetic field and a plasma are good things to think about, but again, in the end you will see that parabolas and the sun's energy aren't quite what is needed.

Please check out this site about inertial confinement fusion (ICF) at LLNL's website. It's a good starting place for you to do some more reading and research.

http://www.llnl.gov/nif/icf/icf.html

I took a class in fusion from a person at LLNL (Chip Smith) many years ago, and it was a very fun class. We covered ICF and magnetic confinement (like Tokamaks), and the math and physics behind it all is really amazing and fun. Plus, it's a pretty important research area, since there is the possibility of getting some net energy out of fusion reactors. There are still pretty vexing safety issues with fusion nuclear power, though. It's not as bad as with fission nuclear power, since a fusion reactor meltdown spoils the reaction, instead of just letting it burrow underground.... There are also radioactive waste disposal issues with fusion reactors -- the easiest fusion reactions to ingnite and sustain also produce some of the most waste. Again, not as bad as with fission reactors, but still nowhere near the "clean" fusion energy source that we would like to invent.

It's a great area for work and contribution. I encourage you to keep studying and brainstorming in this area. -Mike-

9. Jan 18, 2006

### magi

Just thinking loud to see what happens. :)

Nice of you, not to be agressive.... :)

10. Jan 18, 2006

### fargoth

hmm.. i thought the larger the optical equipment is, the better resolution it has... the airy discs radius is inverse proportional to the diameter of the lense\mirror as far as i can remember....
anyway, the problem with getting the energy from the sun is that:
the flux you got from it is about 1380 watts/meter^2. (because the face temprature is only around 5500 kelvins, and its so far away)
the temprature in which the core of the sun operates is 10-15 million kelvins.
on earth however, temperatures greater than 100 million Kelvin are required
now, using hisse's law (q = mc(delta T)) you could calculate the time you need to heat if you used all of the energy you collected from the sun on the sample...
but i guess you dont want to maintane constant pressure, because this means very rapid expansion of the container..
the constant volume heat capacity c_v is dependant on temprature... and i couldnt find it...
try looking around...
anyways, you could try to evaluate it going along these lines... but remember youre assuming that all the heat is tranfered to the sample and no heat is lost to enviorement, which is very far from reality... the hotter your sample gets, the faster it'll loose heat to the enviorement...
found some near-topic info (not really, but i think it'll be interesting anyway :tongue2: )
intersting cold fusion:
http://jlnlabs.imars.com/cfr/html/cfrtiny.htm [Broken]

Last edited by a moderator: May 2, 2017
11. Jan 18, 2006

### Astronuc

Staff Emeritus
In addition to what Tide and berkeman mentioned, the difficulty in fusion is confining the plamsa in a stable form under more-or-less steady state conditions.

The pressure is limited to B2/2$\mu$, where B is magnetic field in Tesla (T), and $\mu$ is permeability. This is the major technical limitation in addition to the fact that fusion requires high temperatures (kinetic energy) of the nuclei.

ICF is a transient confinement phenomenon.

As for size, the earth receives about 1.4 W/m2, and one might wish to build a fusion system requiring 3 GW of power input. That means approx 2 billion square meter mirror. Not very practical. It would be more practical to use PV (solar cells) and convert light directly into electricity.

Last edited: Jan 18, 2006
12. Jan 18, 2006

### Staff: Mentor

Good point, but I was thinking more in terms of the gravitational distortions on such large parabolic mirrors. Remember what they had to do with the Hubble's big telecsope mirror changing gravitational environments between calibration and use.... :uhh:

13. Jan 18, 2006

### Tide

That is the ideal! Those loopholes you mentioned are mitigated by other hurdles. such as those to which Astronuc alluded. For example, while the ablation of the outer material drives the core inward, you are accelerating material of higher density with material of lower density. In effect, you are supporting a heavy fluid with a light fluid which is unstable (the well-known Rayleigh-Taylor instability).

Moreover, the laser intensities required to produce the compression are sufficient to create nonlinear or "stimulated" interactions between the light and the plasma. The intense beams can interact with ion-acoustic oscillations (Stimulated Brillouion scattering) or electron plasma waves (Stimulated Raman scattering). Both of these will cause a significant portion (up to 100%) of the incident beams to be reflected thereby limiting the amount of electromagnetic energy that can be absorbed.

Similar problems are also present in the various charged particle beam approaches.

While these are very serious problems, they are by no means the last word since there are (or may be) ways of mitigating or reducing the effect of all these processes. The point, of course, is that the technical problems are nontrivial and we need dedicated, creative and capable people to address and resolve the issues.

14. Jan 19, 2006