How many protons?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Starfire
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How many protons?

Post by Starfire » Fri Jul 21, 2006 9:15 pm

Just curious – what is the formula to calculate the total number of protons { H+ }, per unit time, in an Ion beam of a particular current at a given pressure? ( say 10e-8 torr ) - I presume a uniform density for a fast transient square wave pulse from a PFN driving the beam.

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Frank Sanns
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Re: How many protons?

Post by Frank Sanns » Fri Jul 21, 2006 9:44 pm

Hi John,

The fundemental unit of charge is 1.602 x 10^-19 coulombs. That is the charge of one proton, one electron, or any other ion that has a +- 1 charge. Only the sign changes for + or - charges but the quantity stays the same.

One amp is one couloumb per second. A milliamp then is 0.001 coulombs/second. To find how many fundemental charge units (i.e. protons) per second take the current divided by the fundemental charge 0.001/1.602 x 10^-19 = 6.2 x 10^15 protons per second.

Pressure of the chamber has nothing to do with the calculation but it has a ton to do with what practical current can be established at a given pressure and gas fill. Currents go up as the pressure goes down to around the torr region and then fall off qucikly as the pressure drops to single micron and below pressures. Rules are a little different for molecular and ion guns which can maintain current to lower pressures since they themselves supply conductive species.

If an ion gun is supplying 6.2 x 10^15 ions per second then it can only produce a current of 1 ma and no more no matter how much current the power supply can produce. For more current, you need more ions.

Ions form very quickly compared to the discharge of a typical Pulse Forming Network. Unless you are into nanosecond discharges, the ions will be formed as the voltage rises.

Frank S.

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Re: How many protons?

Post by Starfire » Sat Jul 22, 2006 7:08 am

Thank you Frank – I am playing with a novel gun design and trying to determine the critical control requirements and to what degree the feed gas pressure/flow to the gun effects the ion production and density.

I have purchased a SF6 filled triggered spark gap switch on Ebay and it is quoted to be capable of nanosecond switching Kilovolts at Kiloamps, { should be better than my Thyratron } but my pulse requirement is much smaller than I had anticipated – I only need a tiny Alpha ( & Neutron ) spark, if it works – not a fire.

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Richard Hull
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Re: How many protons?

Post by Richard Hull » Tue Jul 25, 2006 6:58 pm

Alpha and neutron spark?? That made me curious. What is an alpha or neutron spark.

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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Re: How many protons?

Post by DaveC » Wed Jul 26, 2006 7:14 am

John - I am sure you will find the gap very fast.

I had a home built triggered spark gap, made from two brass doorknobs. They were mounted vertically, and the lower one was drilled for a small, insulated trigger wire, that was connected to the ouput of a standard 12V automotive spark coil. To fire it, the spark coil was simply flashed via a pushbotton. For big splashes, we had a fairly large 40kv pulse duty capacitor, 0.25 mfd. The capacitor and a pair of adjustable resistors and a second cap formed a standard impulse generator, with adjustable rise and fall times. That one could get down to 100 nanosecond rise times and up to a few hundred usec decay time.

For really steeply rising impulses, we used a coaxial high voltage transmission line we built using a 20 ft long, 3" dia. copper water pipe for the outer electrode, and 1" dia cu pipe for the center electrode. Some circular PMMA spacers kept the central conductor centered. The line could produce a 20 kV, 0.9nsecond risetime impulse when the gap fired. When the line was left open ended, you could see 20 or more reflections of the initial pulse.

We used a TEK 7912AD to record the data. Wide open that had a 100GS/sec data acquisition rate for 512 samples!. Not much memory depth (it was a scan convertor CRT that did the storage and), but great speed.

I'm not sure how much faster the gap could go, but I think strongly overvolted triggered gaps can get down to about 100 pS or a bit less. The issue eventually becomes "when" it will fire, since there is a time lag after the triggering event, with pico second variability..

The line impedance was around 62 ohms, so peak currents could get up to 20kV/62 or a bit over 300 amps. With about 60 kV you could get kA levels, if the rest of circuit was clean.

So, I am reasonably confident you will have good results, provided you have some wide bandwidth connections. Be interested to hear how well it works.

Dave Cooper

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Re: How many protons?

Post by Starfire » Wed Jul 26, 2006 9:50 am

Richard,
I can't detect the Alpha from a B11 reaction directly, don’t know how to detect Alpha in such a hostile environment, so I have included a piece of Beryllium with the Boron reagent to produce Neutrons from the Alpha, which I can detect. The ' spark ' or flash will occur if my pulse Ion system works and as I am using very tiny amounts of Boron/Beryllium I expect that if the thing reacts, it will light up with Alpha’s and Neutrons. It should be short duration, due to (I hope ) the good pulse control of the Ion beam.

My question related to trying to determine the effect of H2 feed pressure verses electric Ion production and wondered if the feed rate will have much effect on Ion current and if there was a standard formula which included working pressure as a factor.

After many attempts at Ion guns, I have designed a novel kw Ion gun to follow your criteria viewtopic.php?f=12&t=4969#p32277 and hope to post details when it is finished. I am attempting to establish fine H2 feed control by “ Bumping ‘ a MFC { as you suggested } into a LP reservoir and feeding the Ion gun via a long {coiled } Hypodermic SS tube to smooth out the flow. The reservoir { about 2 Ltr ) is initially evacuated to system pressure via a bypass valve, during system pump down.

Dave,
Thank you – very useful information as I don’t have experience using triggered spark gaps. I had designed with a large HV Hydrogen Thyratron to switch the beam, but when the gap appeared, I went for it. I have a older Tek. 7834 storage (400Mhz ), but need high bandwidth amps and time-base. Was thinking of a sampler to catch pico-sec pulses, as I can run the pulse generator and switch, repetitively off line into a matched dummy. In initial experiments I will selectively trigger – just in case

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Re: How many protons?

Post by Richard Hull » Wed Jul 26, 2006 1:48 pm

Thanks John for clearing my understanding a bit.

I thought you felt the spark gap, itself, was a source of neutrons and alphas.

You obviously have a formal ion sourced machine in mind and not just a gap system.

You mention H2 as your gas..... do you mean D2? Hydrogen would not make neutrons or alphas, of course, in any conceivable simple fusor machine.

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.

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Re: How many protons?

Post by DaveC » Wed Jul 26, 2006 6:48 pm

John -

Beware of a sampling scope for single shot very fast risetime transients. They may not get anything. For subnanosecond ristetimes, you would want at least 10 sample per nanosecond, to be able to see the risetime...(assuming you ARE in fact interested in it). That works out to 10 GSamples per sec, which is above almost all DSO's, except the very high end TEK, LeCroy, and Agilent, which are $$$$$.

I also have a 7834, which will record sub-ns (best with the fast amplifiers, of course) in the FAST Variable Persistence Mode. I think the mainframe risetime is 750 ps .... Not quite so easiy as with the faster machines, but fast enough for you to tell if you're getting there with the spark gap.

I bought a 7912AD, yrs ago from a surplus store where I found it in a pile of newly arrived stuff. The scanconverter CRTs are still available, but getting scarce. There is nothing like raw acquisition speed, when it comes to transients.

There's a bit more to the story of getting the signal into your scope. I used a capacitively coupled flat plate detector straight into coax to the scope. The coupler mounted on the grounded outer shell of the storage line. This gave adequate attenuation as a capacitive divider.

Strictly speaking, through a capacitive divider, the input signal becomes the derivative of the voltage pulse, since the signal into the scope develops across a 50 ohm resistance to ground, and is thus a current. Current through a capacitor is the time derivative of the voltage change.
q = cv i = dq/dt = c dv/dt
Vscope = iZin = C(dv/dt)Zin.

Dave Cooper

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Re: How many protons?

Post by Starfire » Wed Jul 26, 2006 8:13 pm

While speed is important for me - pulse shape is more so, thus the PFN requirement. For good meterology, I need to know the number of Protons reacting to measure the efficiency and need as near square as possible with super fast rise & fall edges. The beam current control should give me one control factor but pulse duration equally important.

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Re: How many protons?

Post by Starfire » Wed Jul 26, 2006 8:28 pm

Going to try H2 as feed, Richard. The B11 - H+ reaction is Aneutronic hence the Be. Wonder if using D2 Ions would give me the Neutrons ejected and at what energy? Or can you suggest how to varify the reaction { other than the Alpha generation }

It should be simple to swap D2 as the feed if desired.

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