Faraday Cup

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George Schmermund
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Faraday Cup

Post by George Schmermund »

Sooner or later someone here is going to get a complete beam-on-target system up and running. When that happens it will be instructive to characterize what's coming out of the beam tube and the amount of current that is available for doing experiments. One could just ground an insulated plate through an ammeter, but that can be a deceptive measurement if the secondary electrons are not accounted for. This now leads me to another one of my diversionary projects. Building a useful faraday cup for beam measurements will have to be done by someone, so I might as well get started.

As an aside, I think that it's important to use the proper terminology when referring to the machines that will most likely be used here to produce beam-on-target experiments. A linear accelerator (alias LINAC) is a machine that accelerates charged particles using AC. Cyclotrons also use AC. All electrostatic accelerators use DC and are often referred to as 'potential drop' accelerators. Cockroft - Walton (alias CW) machines use DC voltage multipliers fed by transformers. They are also considered potential drop accelerators. There are many more ways to do beam-on-target experiments, but they're not likely to show up here in these forums. But, It will be interesting if they do!

Anyhow, assuming that the currents will be low (nano to microamps), the use of a picoammeter and/or electrometer will be most useful. Keithley makes some of the best instruments out there for these measurements and I've given shelf space to many of their fine products over the years.

Short of having an actual accelerator beam to play with, (for the time being, of course) I'll substitute it with a small Am source that has been liberated from a smoke detector. The current from this source should be low enough to really test the design. More to come!
Anything obvious in high vacuum is probably wrong.
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Richard Hull
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Re: Faraday Cup

Post by Richard Hull »

I am somewhat of a collector of Keithley instruments and have about 20 of their electrometers, both normal and their rare vibrating reed instruments as well as picoammeters, nano volmeters, current sources, HV supplies, etc. They have meter readouts, nixie readouts and LED 7 segment readouts. My oldest goes back to their first portable instrument that used a gaggle of batteries 6 - 22.5 volt, 2 - 30 volt, 4 D cells and one C cell! Needless to say, that one will never get powered up again. Through the model 610 series, they used special matched 5886 vacuum tubes as the electrometer tube input handlers. Beyond that, they used FETs which were easily destroyed. The vacuum tubes could take a lightning hit and still be good!

New electrometers are priced in the the thousands. Most all of mine are functional and were purchased surplus at hamfests for under $50.00 each over the last 25 years. My most super purchase was $10.00 for a working 610C. In another great buy, I found a box of 100 5886 raytheon tubes. I've been selling them over the years to keithley owners and repair points.

I did a lot of electrostatics work back in the 80's and 90's. Tom Ligon who worked for Bussard met me in 1996 to purchase one of my Keithley 610's and introduced me to Bussard's work on the fusor and it is all history from there.

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
George Schmermund
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Re: Faraday Cup

Post by George Schmermund »

It was really easy to start this experiment. As usual, the Home Depot plumbing dept. was the first choice for parts. The basic cup consists of a 3/8" Cu cap with a banana plug soldered to it. This cup can be inserted directly into the high impedance input of a Keithley 610C electrometer for starters. The offset current on the electrometer is about 7 x 10^-14 amps with the input cap on. This is about a decade higher than when it left the factory (which was quite a while ago) and can probably be brought back to the original spec of <5 x 10^-15 amps if the input insulator was meticulously cleaned, but that chore can be delayed for the time being.

By moving the Am source into and out-of the FC, the meter peaked at about 1x10^-11 amps (10 picoamps). That's at least a couple of decades above the instruments noise and offset. To make sure that I wasn't fooling myself about the readings, I covered the business end of the source with a small piece of foil. Repeating the same test revealed no current. Using a dissipative work area and a wrist grounding strap are essential.

These measurements can be fraught with potholes and misdirections all along the way. Things such as secondary electrons and other gremlins will need to be addressed as the FC project evolves. I'll try to sort them out as best I can and report the findings.

The photos show the first primitive steps taken to try and accomplish the task.
Attachments
1-P1030927.JPG
2-P1030929.JPG
Anything obvious in high vacuum is probably wrong.
prestonbarrows
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Re: Faraday Cup

Post by prestonbarrows »

George Schmermund wrote:One could just ground an insulated plate through an ammeter, but that can be a deceptive measurement if the secondary electrons are not accounted for.
The standard way to deal with this is a tube/plate located directly upstream of the collector floated at ~-100V. Secondaries coming off your collector will have around a few eV of energy and get turned around by the E-field back into the collector and thus not contributing current. The floating plate will ideally not draw any current and not effect the ion beam since its voltage is much much less than the ions' energy.
George Schmermund
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Re: Faraday Cup

Post by George Schmermund »

prestonbarrows - Your preaching to the choir again. Of COURSE a suppressor plate will be needed in the final version. The test was quick and dirty to see if the Am source could be used as an ersatz beam when fired directly into the electrometer. Maybe you could have guessed what the numbers would be without doing any measurements at all. Then I'd be impressed!
Anything obvious in high vacuum is probably wrong.
Jon Rosenstiel
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Re: Faraday Cup

Post by Jon Rosenstiel »

George,

I tried this with my 610C, very cool indeed! Thanks for sharing.

Jon Rosenstiel
prestonbarrows
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Re: Faraday Cup

Post by prestonbarrows »

George Schmermund wrote: Maybe you could have guessed what the numbers would be without doing any measurements at all. Then I'd be impressed!
It's actually pretty simple in this case to make ballpark guesses. And after a few lines of algebra, it looks like you are either getting some measurement errors or you have a scarily huge Am source.

The EPA claims the average smoke detector contains ~1 uCi (~3.7E4 Bq) of Am-241, an alpha emitter.

3.7E4 [alpha/s] * 2 [elementary charge/alpha] * 1.602E-19 [Coulomb/elementary charge] = 1.185E-14 [A]

This would be an absolute upper limit to the alpha current possible to collect and it is a few orders of magnitude less than what you are seeing. Even neglecting alpha attenuation in air which is significant, and assuming full solid angle collection, a typical smoke detector Am source just can't create the currents you are measuring through alpha decays directly. Alphas cause secondary electron emission from surfaces, but only about 10 electrons per incident alpha. Alphas also cause all sorts of complex ionization trails through the air. This is a big factor for why their range is so short. The mean range for alphas in air for Am-241 is only a few cm.

AM-241 also emits photons, which would also contribute to secondary electron emission. I am not that familiar with secondary electron emission behavior outside of vacuum systems though.

Its late, so forgive me if I am missing something, but it looks like most of this current is coming from some secondary source, which is interesting. I don't think accurately measuring alphas is your goal here anyways of course. I've never seen this direct method done before.
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Richard Hull
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Re: Faraday Cup

Post by Richard Hull »

I showed this effect using Am 241 some years back at one of our local monthly HEAS gatherings on my 610C in the lab. One fellow got the idea he could make a nuclear battery. Naturally, his efforts failed.

The gammas from Am241 are ideally weak! Just enough oomph to whallop the daylights out of the "sea of electrons" at a metal's surface. It would be cool to polish up a number of different metal cups or even plates to see what metal would imact the electrometer's Am241 readings the most. It might give an insight to the amount of electron current due to gamma compton and scattering effects.

Any effect that demands an electrometer to detect and measure, is of no earthly use as a viable energy source to do any form of macroscopic work. However when one sees a meter needle deflect the ideas of harnessing energy and power come to an untrained mind.

At the intellectual level, it is totally cool and wows 'em every time.

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
George Schmermund
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Re: Faraday Cup

Post by George Schmermund »

pb - Lousy guess. I'm not impressed. As far as what you're missing, it's that you probably wouldn't have done the experiment based on your calculation! This brings me to reiterating my mantras:

Never calculate what you can measure.

One test is worth 100 expert opinions.
Anything obvious in high vacuum is probably wrong.
prestonbarrows
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Re: Faraday Cup

Post by prestonbarrows »

Richard Hull wrote: Just enough oomph to whallop the daylights out of the "sea of electrons" at a metal's surface.
Yup, secondary electron emission from alphas and gammas is well established. Knoll has a few good sections on it.

It would be interesting to run a positively biased collector above the electrometer plate to directly measure the secondaries and back out what the actual alpha current was. Doing this under vacuum would probably simplify the analysis a lot.

A similar fun experiment is an alpha source with a detector located further away than the alphas range in air. Put the setup in a vacuum chamber and ramp down the pressure and eventually alphas start to get be detected. This is one way to work out the stopping power of a gas.

George, it just means that most of your observed current is probably from secondary effects and not directly from alphas. Calculations add context to any measurement.

Another good mantra:
Never do an experiment without first doing a page of math.

What sort of current are you expecting to collect on the full beamline?
George Schmermund
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Re: Faraday Cup

Post by George Schmermund »

pb - When you talk about secondary electrons as if no one else here has given it any thought, I find it hard to believe that you're not some level of troll.

Why don't you introduce yourself the same as everyone else has. Then maybe I can take you seriously.
Anything obvious in high vacuum is probably wrong.
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Rich Feldman
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Re: Faraday Cup

Post by Rich Feldman »

Hey Preston, did you know you have two different user names here?
"prestonbarrows" joined Sat Jun 23, 2012 10:27 pm.
"preston barrows" joined Sat Mar 02, 2013 9:24 pm.

From both I see a number of helpful, knowledgeable posts about vacuum and high voltage technology.
But nothing in the "please introduce yourself" board.

-Rich
All models are wrong; some models are useful. -- George Box
prestonbarrows
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Re: Faraday Cup

Post by prestonbarrows »

viewtopic.php?f=8&t=9598
Cheers guys, didn't realize I hadn't already done that.

George, I have never intended any offence here. Feel free to check my numbers, there is always a good chance they are wrong.

I would be happy to discuss any other constructive theories about the apparent difference between the measurement and theory. So far, you have not offered anything besides unprovoked malice.
John Futter
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Re: Faraday Cup

Post by John Futter »

Preston
whoa up
Us old farts have been around the block a lap or two.
I know you mean no harm and neither does George
But telling us how to suck eggs sticks in the craw a bit----the outcome of this is how we feel on the day
and may include Dismay Dislike Derision Indifference.

I have plotted faraday cup current against electron suppression voltage many times and for the voltages that people use here ie 10 -70kV you will need to
bias this electrode up to around 400 volts negative to suppress all the secondaries
As I have listed here on the site you can do this without a seperate bias supply by using a tansil diode PKE400A or similar. I measured the reverse characteristics of a hundred of these to make sure that they posed no burden before breakover. Its somewhere here on the site.

One final note
This is Georges post on how he is doing it!!!!!
this includes me and you Preston on not diluting it
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