Moderator please remove this message thread

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Post Reply
George Dowell
Posts: 158
Joined: Fri May 09, 2003 2:30 pm
Real name:

Moderator please remove this message thread

Post by George Dowell » Fri May 15, 2009 3:27 pm

A source of energetic alpha particles should ionize about 10,000 atoms per particle. This would yield 1 He atom per 10,000 ionized Deuterium atoms.
Can someone do the math and figure the end point distance of 5.5 MeV alphas at typical fusor operating pressure?

Thanks.

Geo

User avatar
Frank Sanns
Site Admin
Posts: 1622
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Cold Ion Gun

Post by Frank Sanns » Fri May 15, 2009 4:13 pm

George,

It takes just under 14 eV to ionize deuterium so a 5,500,000 eV alpha has the potential to ionize ~400,000 atoms. Of course not all of those will be productive ionizations.

Mean free path scales inversly linearly linear with pressure. An alpha with a mean path of 1 cm at 760 torr will be 10 cm at 76 torr and 100 cm at 7.6 torr. At fusor pressures you can see that the mean path will be in the km range. Not much chance for ionization.

Somewhere on the fusor site was an experiment that I did with thoriated tungsten welding rods because I was hesitant to put a smoke detector source into the fusor. The result was ineffective.

I was about to end my post here and say that there would be no helium formed from this mechanism but that is not correct. Helium WILL be formed in very, very small amounts. This is due to the fact that if at least 15 kv of the 5.5 Mev energy were imparted to single atoms (it does), then there is sufficent energy for fusion with a neutral.

So the idea device would be of a normal fusor size (6" diameter) with hot alpha sources around the entire inner surface. The pressure of Deuterium inside would be around 25 torr and the rate of fusion would be on the order of 1 x 10E-12 times lower than the output of the sources combined.

Another bleak but interesting picture.

Frank Sanns

User avatar
Frank Sanns
Site Admin
Posts: 1622
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Cold Ion Gun

Post by Frank Sanns » Fri May 15, 2009 4:49 pm

Post above was revised.

George Dowell
Posts: 158
Joined: Fri May 09, 2003 2:30 pm
Real name:

Re: Cold Ion Gun

Post by George Dowell » Fri May 15, 2009 10:07 pm

Thanks for the heavy lifting on the theory Frank, your explanation makes good sense. It's hard for me to quite comprehend the conditions in the fusor, the heat, the low pressure etc.
Geo

Chris Trent
Posts: 291
Joined: Tue Jan 17, 2006 9:24 pm
Real name:

Re: Cold Ion Gun

Post by Chris Trent » Sat May 16, 2009 3:54 pm

Not quite,

The mean free path is the average distance that the particle will travel between collisions, or in this case how far it will travel before the first collision.

What Frank is pointing out in a nutshell is that 99%+ of the alphas will simply zing right through and hit the opposite wall at full energy, never having even grazed a neutral.


An alpha source for ionization might be really useful for jump starting a high pressure fusor, if anyone could figure out how to make one.

User avatar
Frank Sanns
Site Admin
Posts: 1622
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Cold Ion Gun

Post by Frank Sanns » Sat May 16, 2009 4:41 pm

George,

I think I answered two other questions, neither which were ones that you asked!

What you were asking was when does an alpha particle aquire one or two electrons and become a helium atom. This is an easy question and it is a difficult question. The easiest answer refers back to my first post and that is when the alpha particle has impacted enough deuteriums (ionizations or not) to lose enough energy to be below the 54.4 eV Helium ionization energy then it will hold on to one of its electron. It will still have enough energy to have the chance to ionize a couple more dueterium at 13.6 eV without loosing its first electron. After that collision it will then have lost enough energy to hold on to its second 24.6 eV electron and be neutral but still have enough energy to ionize one more deuterium.

What I have described is a perfect world but in reality, an alpha particle at any energy has a chance of picking up an electron or two and becoming a Helium atom. It will not stay around long as the electron can be stripped back away as fast as it is stripping deuterium electrons away as it slows down and loses translational energy. This means that there is a chance that as soon as an alpha comes near a deuterium atom, or any other atom or electron source for that matter, there is a chance that the alpha can pick up an electron or two and briefly become a 5.5 MeV speedy helium atom.

Unfortunately at fusor pressures, there are just not many atoms to collide with and the most likely scenario is that an alpha will traverse the fusor with very few collisions and hit the wall on the far side with nearly all of its original 5.5 MeV energy.

Frank Sanns

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: Cold Ion Gun

Post by Carl Willis » Mon May 18, 2009 4:51 am

>Can someone do the math and figure the end point distance of 5.5 MeV alphas

Hi George,

These kinds of problems are easily done with the aid of SRIM (freeware) and the ideal gas law. That suggestion should work for you. My own two-minute result is that an alpha particle of 5.5 MeV has a projected range in 20-mtorr D2 gas of 6.3 km, but the method of checking is straightforward enough that I recommend interested parties do it themselves and satisfy their own standards of accuracy. Anyway, the number's huge compared to usual dimensions of fusors.

As regards the utility of using common alpha sources for providing ionization in a fusor, consider this simple situation and reach your own conclusions: a household smoke detector incorporating 1 microcurie of Am-241 and a dense gas, air, in which the particles can travel almost all of their range and stop, producing copious ions at the Bragg peak, only manages to sustain an ion current of a few dozen picoamperes (without smoke present).

-Carl
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Richard Hull
Moderator
Posts: 11619
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Cold Ion Gun

Post by Richard Hull » Mon May 18, 2009 2:47 pm

I am glad Carl got to this issue of ion CURRENT. That is the key here and alpha sources needed to produce the required ion current are just not obtainable. And then there is always the mean free path which drives this issue, as well.

In a fusor, all alphas will wind up as helium atoms in the chamber or buried in the walls as all will hit the wall. Some will bury and later re-enter the gas volume as common He4 neutrals. Of course this will have zero impact on the contained D2 gas and the dilution and contamination of the D2 environment would be miniscule, atomically.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Post Reply