Neutron Flux Modulation (By power supply ripple)

For posts specifically relating to fusor design, construction, and operation.
Jon Rosenstiel
Posts: 1461
Joined: Thu Jun 28, 2001 5:30 am
Real name: Jon Rosenstiel
Location: Southern California

Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel » Thu Oct 26, 2006 6:20 pm

I’ve been eager to try this experiment since first hearing it mentioned years back in a post by Richard Hull. Now I have the tools; (good running fusor, He3 detector, etc), and the inspiration; (a recent post by Carl Willis “Another update: Carl’s Jr.” in which Carl mentions the differences between filtered and unfiltered power supplies, and my recent post, “Fusor plasma measurements” which included scope pics in which my power supply’s 120 Hz ripple was quite evident).

The setup…
Paraffin moderated He3 detector, (detector centerline to poissor = 20.7 cm).
He3 detector’s output fed into Canberra Series 35+ mca set to mcs (multi-channel scaling) mode with a dwell time of 1 ms per channel.
Fusor neutron output was around 3.5E+06 n/s, and the He3 detector was racking up just over 11,000 cps. (Whew, that’s a lot of counts)!

In the graph below one can see that the neutron output peaks every 7 to 9 ms, which coincides nicely with the period (8.333 ms) of my power supply’s 120 Hz ripple.

So, unless I have made some grave error in my test procedure it appears (as expected) that our fusor’s neutron output does indeed mimic the power supply’s input.

Questions & comments welcome.

Jon Rosenstiel
Attachments
1 ms dwell.gif
1 ms dwell.gif (15.56 KiB) Viewed 2255 times

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: Neutron Flux Modulation (By power supply ripple)

Post by Carl Willis » Thu Oct 26, 2006 7:11 pm

Hi Jon,

That was a good experiment and I will look at the data more carefully later to see what it tells us quantitatively about voltage and relative cross-section. You know what the peak-to-average ratio is for the power supply, and the peak-to-average ratio for neutron flux can be calculated pretty easily from the data in Excel. By the way, what was your grid voltage for this experiment?

Thanks for posting that.

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

Jon Rosenstiel
Posts: 1461
Joined: Thu Jun 28, 2001 5:30 am
Real name: Jon Rosenstiel
Location: Southern California

Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel » Thu Oct 26, 2006 8:04 pm

Carl-

Grid voltage around 46~47 kV. I should probably explain a little about how I calibrated my meter. A year (or two) back I placed my SiLi x-ray detector up to the viewport (using Pb collimator with tiny a hole). At that time my meter was indicating 30 kV, but the x-ray spectrum continued on out to 32.7 keV before it dropped down to zero. At that point I decided I should recalibrate my meter so it would match what the x-ray detector was telling me. (X-ray spectrum falls to zero at 32.7 keV, meter indicates 32.7 kV). So now my meter reading should match the supply’s peak voltage. (At 30 kV, anyway)

If you (or anyone else for that matter) want the Excel file just let me know and I’ll e-mail it off.

Jon Rosenstiel

User avatar
Steven Sesselmann
Posts: 2111
Joined: Thu Aug 11, 2005 1:50 am
Real name: Steven Sesselmann
Location: Sydney - Australia
Contact:

Re: Neutron Flux Modulation (By power supply ripple)

Post by Steven Sesselmann » Thu Oct 26, 2006 11:24 pm

Jon,

Nice work as always, you are helping all of us to get a better picture of what is going on.

Thanks..

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: Neutron Flux Modulation (By power supply ripple)

Post by DaveC » Fri Oct 27, 2006 8:03 am

Jon - Very interesting. Since the neutron flux drops to zero, your power supply ripple must be fairly large. Are there filter caps in the HV output? At 60 Hz, unfortunately you need some rather large capacitances to support the voltage, even with a full wave supply. A large series inductor, or best a pi section C-L-C filter would smooth things out.

Dave Cooper

User avatar
Adam Szendrey
Posts: 1333
Joined: Fri Mar 29, 2002 10:36 pm
Real name: Adam Szendrey
Location: Budapest, Hungary

Re: Neutron Flux Modulation (By power supply ripple)

Post by Adam Szendrey » Fri Oct 27, 2006 8:45 am

Hmm...a funny idea just popped into my mind...you know singing tesla coils..they are built by using a PWM modulated supply, usually. Now this can be a "singing fusion reactor" (though not audible) with an ampl. modulated supply ;).Imagine it, neutron communication...neat. Too bad the range would probably be...umm...not so good...And i'm not sure what the upper frequency limit would be. I suppose the x-ray flux ripples similarly?

User avatar
Steven Sesselmann
Posts: 2111
Joined: Thu Aug 11, 2005 1:50 am
Real name: Steven Sesselmann
Location: Sydney - Australia
Contact:

Re: Neutron Flux Modulation (By power supply ripple)

Post by Steven Sesselmann » Fri Oct 27, 2006 9:14 am

Jon,

What sheilding if any do you have around your Fusor when you are putting out an isotropic emission of 3.5 million neutrons per second?

I make this a flux of around 30 neutrons per cm^2/sec @ one meter.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

Starfire
Posts: 1482
Joined: Wed Oct 24, 2001 6:14 pm
Real name:

Re: Neutron Flux Modulation (By power supply ripple)

Post by Starfire » Fri Oct 27, 2006 4:49 pm

Jon - Is there a second component in the modulation? The primary is pseudo sine but I wonder if a sub-harmonic is also present - would be good to see the input waveform as a second graph - the synchronism and phase could also be seen.

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: Neutron Flux Modulation (By power supply ripple)

Post by DaveC » Fri Oct 27, 2006 10:13 pm

Good point, John -

There may actually be two (or more) other modulations.
It's pretty clear there is another lower frequency modulating the peaks of Neutron counts. If the primary modulation is at the mains frequency, this one is several times slower. Some type of pressure -thermal cycle is suggestive, here.

The other seen in several of the peaks, and might just be a sporadic current burst, which pulls the output voltage down. Or it might be a higher frequency ( couple or more times the basic power supply frequency.)

Nice to have some interesting data to scrutinize. Thanks, Jon!..

Dave Cooper

Verp
Posts: 171
Joined: Fri May 02, 2003 7:27 pm
Real name:

Re: Neutron Flux Modulation (By power supply ripple)

Post by Verp » Sat Oct 28, 2006 1:31 am

Is the flux strong enough to measure the higher frequencies that fusors might resonate at that we have talked about in other threads? Is there a reasonable way to build a power supply that would be quiet enough to actually see such things?

Rod

Post Reply

Return to “Fusor Construction & Operation (& FAQs)”