Neutron Flux Modulation (By power supply ripple)

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Starfire
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Starfire »

A resistive ballast with negative coefficient is best - the humble domestic incandescent lamp will work well as the resistance will change as the lamp heats ( or use a thermistor, but hard to get at these voltages and currents ) - build a ballast plane of lamps - the mains voltage here is 250v { on a good day } so I use four lamps per kilovolt ( 15 watt lamps will give 60ma at full voltage or 25 watt for 100ma - a kilowatt of light on 10kv :) I also use them for HV cap bleeders
Jon Rosenstiel
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel »

Wilfried-
Sorry, wasn't very clear on a couple of points. The 40K resistor is in the secondary circuit and the choke (actually a ballast choke) is in the primary (mains) circuit. I found the choke in a surplus store. I don’t know its original purpose, but the seller did tell me that it came from a decommissioned TV transmitter. Printed on the choke is: “15 MHY, 16 AMP”. What is really cool about it is that it’s fairly easy to unbolt the “I” core and change the gap, thereby changing system response. I suspect it would be quite the challenge controlling a fusor without a ballast choke somewhere in the circuit.

Jon Rosenstiel
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel »

Great post, Dave. Thanks.

About inductive ballast in the secondary.... Richard mentions needing inductance values in the 100's of Henrys, but I don't follow. Here’s why: Right now my system is fairly controllable with about 15 mHy of inductive ballast in the primary circuit. My seat of the pants reasoning is telling me that if 15 mHy works well in the primary, something considerably less would work equally well in the secondary circuit. Or am I thinking about this bass-ackwards?

Below comments added 11-2-06, 1310 hr.

Ok, I see the error in my thinking.... for the inductive ballast to function properly in the secondary circuit it will have to have a high inductive reactance, (~40 Kohm) which means lots of inductance. (Over 100 Henries)

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Re: Neutron Flux Modulation (By power supply ripple)

Post by JohnCuthbert »

Not really my field, but I'm pretty sure the HT side is also the high impedance side so to make a difference you nead a bigger reactance ie a bigger inductor. With the added fun of trying to insulate an HT inductor, I think the primary side inductive balast is a good idea.
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel »

Right you are, John. You posted as I was in the process of editing (correcting) my post. Thanks

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Richard Hull
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Richard Hull »

There is no set rule on which side the inductor should go on as it is taken on a case by case basis. There are a boat load of questions to be asked to decide. What is the load impedance of you transformer?... Is it magnetically shunted in any way? What are the dynamics of the load? Is this a current and voltage limited supply? How fast does the inductive protection have to act? What kind of slower transients do you wish covered, if any? Will a resistor be used in concert with the inductor? Are there any large energy storage capacitors or stray capacitances present? Primary inductance usually works best in mult-kilowatt pulse situations.

Scott Little needed high side inductance with his setup as it was the quickest fix at hand.

The only optimum solution is a fast HV switcher with super fast acting current and voltage limit sets. Quite a tall order for the experimenter.

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
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DaveC
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Re: Neutron Flux Modulation (By power supply ripple)

Post by DaveC »

Just to add a few more thoughts to the discussion. As we noted earlier, you can add the ballasting impedance almost anywhere in the drive circuit. The primary side will require a high(er) current, low(er) voltage component and the secondary side the reverse - high voltage, low current. In either case the energy storage requirement is more or less the same.... 1/2 L i^2.

There will be some particular volts/turn value for whatever you decide on for the inductor, that will not change whether it is in the primary or secondary circuit. Since inductance L is proportional to N^2, the increased inductance of the secondary side inductor just compensates for the reduced current (squared). So all remains the same.

At issue is how much voltage boost or buck you need to stabilize your particular plasma (or fusor). That will depend on current level, operating voltage, gas density (or fill pressure) and temperature...at least!.

These are all dynamic variables, with time constants that are will be in the microseconds or faster, out in the ion cloud itself, all the way down to 100's of milliseconds in the shell, and electrode temperature changes. And most ion gages struggle to respond in tenths of a second, usually. The actual voltage and current signals will probably be the fasted responding in the setup.

Since the inductor's induced voltage depends on the rate of change of the current, then both instantaneous current level and it's rate of change will set the instantaneous voltage the inductor will produce.in response to a current change at that point in time.

Thus for small rapid changes, you won't need a huge inductor, but you will need to guess about how stabilizing voltage change is needed. I would guess about 50% variation, minimum, of whatever voltage level you plan to operate. The required inductance then goes up with operating voltage and down, with operating current. ( More current, more Ldi/dt generated voltage... and etc.)

It IS a case by case anyalysis, as Richard notes, and the best arrangement for one may not be ideal for another.

It's a great exercise in understanding the flow of energy around the fusor circuit, which will give a lot of insight to the appropriate measurement techniques.

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Re: Neutron Flux Modulation (By power supply ripple)

Post by MCL »

A lot of useful information here. I use a long array of high-voltage
resistors to current-limit the output of my power-supply. This
seems to smooth things out, and is an absolutely necessity for
my config, as I have an old nitrogen laser DC powersupply that is
used to energize the fusor. The DC seems very smooth, but I
can't prevent the fusor from sometimes just pinning my
current-meter, as the whole fusor device will sometimes just
"short-out", when the voltage-levels are pushed up as needed.
As Richard points out, one must adjust and balance the gas flow
level, the applied voltage, the vacuum system operation and the
device radiated output. But perhaps you can make the
power-supply ripple work to your advantage. As you now
have modulated output, if you can devise another method that
also modulates the system neutron output (a mass-flow controller
to modulate the fuel gas flow?, or a magnitron or ion-gun or
RF-field applied to the grid?) - maybe it might be possible to
adjust the 2nd modulated parameter to constructively interfere
with the modulated activity that you currently show - the idea
being to boost momentarily to a higher, but unstainable, level of
neutron production.
But thanx for posting the data, and stimulating the discussion.
- Mark Langdon
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel »

Mark- Hmmm... I like the idea of modulating the gas flow, or maybe the chamber pressure.
While operating my fusor I've often noticed that a small, quick, increase in pressure is sometimes accompanied by a spike in the neutron count. More to think about.

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Richard Hull
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Richard Hull »

Interesting, indeed. I, too, have noted a pressure change to result in a neutron burst. Abrupt current changes usually have an opposite effect! (reduced neutrons)

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Frank Sanns »

Current is to electrons as neutrons are to ice.

Frank S.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
Starfire
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Starfire »

Make a ToDo list! Jon - lest a gem is forgotten and not explored.
As usual, work and observation of the highest calibre. Much respected.
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Re: Neutron Flux Modulation (By power supply ripple)

Post by Jon Rosenstiel »

Attached pic: Power supply ripple vs. neutron output. Upper (channel 1) trace is my fusor's hv input waveform. (From capacitive pickup on hv cable). Lower (channel 2) trace are NIM positive logic pulses from a Tennelec 440 single channel analyzer. The Tennelec sca outputs one logic pulse for each neutron detected by a paraffin moderated He3 detector. Note the absence of logic pulses (indicating absence of neutrons) as the power supply ripple goes positive.

I tried to calibrate the capacitive pickup, but my system is very non-linear, particularly above 30 kV. A rough estimate (plus / minus 10%) of the input waveform voltages are as follows. DC level = negative 43 kV. Positive peak = negative 20 kV. Negative peak (at reticule centerline) = negative 50 kV.

Additional info:

Fusor operated at 40 kV, 20 mA.
He3 detector was positioned as close as possible to fusor, and was clicking off ~ 5000 n/s.
The scope’s persistence was set to 1 second in order to better display the logic pulses.
It was somewhat difficult to capture a clean trace; instability of the fusor’s plasma creates huge, ugly, nasty, 50+ V peak to peak oscillations of around 40 mHz at the scope’s input.

Jon Rosenstiel
Attachments
40 kV, 20 mA 001a.jpg
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