In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Cross sectional collision energy
Date: Mar 07, 10:27 am
Poster: Richard Hull
On Mar 07, 10:27 am, Richard Hull wrote:
This subject has been hashed out before, but a quick summary review will be good, and put it higher in the list here of things to wade through.
I put the question to the Princeton mad scientist network and the answer was just what we thought it might be. For those interested in the question, as asked, and the answer, which ultimately came from Stockholm University, check out the following URL.
Should the above URL go away, here is a summary:
All collisional cross section charts for d-d fusion are based on a STATIONARY deuteron (as in a target) being impacted by a MOVING deuteron (as from an accelerator.)
The fusor collides deuterons, therefore, every deuteron in the fusor is in motion!
Under ideal situations the deuteron is accelerated through the full fusor applied potential to collide head on with a like deuteron accelerated from the opposite side of the fusor.
Thus, if we have a 30kv accelearation potential, we have two 30kev deuterons collide. This is NOT a total of 60kev to be looked up on the collisonal cross section graph!
We must view the energy of collision from the reference frame of any one of the deuterons as these are what the chart is about. (deuterons)
For any deuteron at any energy (Ek- in electron volts), there is a specific velocity V. Therefore, relative to any given deuteron, it sees itself as being hit at 2V. Since this is squared in Ek=1/2MV^2, we have an effective energy of four times and not two times in the head on collision of moving particles.
It can be seen from the above example that with a 30kv applied potential, we should be looking up our cross section for IDEAL COLLISIONS at the 120 kev point on the target/projectile cross section graph.
This is a real plus for the fusor as a collider since lower voltages will push us way up the curve by a factor of four.
Th' Bad news:
In a simple fusor, ideal collisions are so rare that to do statistically significant fusion, we need high gas density (higher pressures). This allows much more ionization to take place. (more deuteron production). This is traded off against reduced mean free path, which at this pressure level (~1micron), sets a maximum on the fusor size of about 8 inches in diameter with 6 inches being the best. The simple fusor effectively "bullheads" fusion with massive power waste being the order of the day. Such a simple fusor also has virtually no recirculation. Ions that miss colliding are lost (recombine) and must be replenished with more input energy.
Still, we are doing fusion!!
In better fusors of lower pressure, with confined deuteron production near the outer walls or with ion guns, the power losses fall and more and more deuterons have a chance to fuse, arriving at the central focus at fusion energy. Those deuterons that miss colliding can be thriftily recirculated for more future chances at fusion in repeated passes.
Higher voltages not only offer better cross sections for collision, but allow fusion energy deuteron production over larger and larger volumes of the chamber.
Use the 4 X KEV applied in future for you ideal cross section lookups.
- Re: Cross sectional collision energy - Jim Lux Mar 07, 8:35 pm