Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

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Arun Luthra
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Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Arun Luthra » Sat Nov 30, 2019 10:10 pm

My goal is to bombard an anode target with electrons in the 50-100 eV range. Assume I have a vacuum bell jar, backing pump, turbomolecular pump, tunable microwave oven transformer with full wave bridge rectifier and capacitor filter powered by a variac, and high voltage feedthroughs. I can get more components as needed.

The paschen curve has a minimum around 350 V. If use electrical breakdown as the ionization source, is there an equation that will tell me the approximate kinetic energy distribution of the electrons by the time they reach the target, for a given potential difference and pressure?

Another way to ionize the gas is a filament. Since this does not require electrical breakdown, I assume that lower electron energies are possible?

I know that ion beams may be an option, but according to my knowledge (which is limited), these are expensive and have low current. I would like a relatively high current - tens to hundreds of milliamps if possible.

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Rich Feldman
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Rich Feldman » Sun Dec 01, 2019 7:04 am

Filament in vacuum sounds like a good way to go. No complication from positive ions, gas molecules, sputtering, etc.
For example, look up vacuum tube rectifier type 5U4-GB. With nominal filament voltage,
44 volt drop across tube is enough for 450 mA of plate current.
There's similar emission current in microwave oven magnetrons, of which a million or more are discarded every week.

Other people here know better than I about special filament preparation,
necessary to get lots of electrons out at moderate temperatures.
I don't know if they survive exposure to air.
Higher filament temperature might make up for less than ideal surface chemistry.

How about removing the glass from a 12 volt lamp,
and putting that in your vacuum chamber not too far away from your target?
Measure the current in target circuit at, say, 100 volts, as you ramp up the filament current.
Not a thing I have done, but I bet there are stories not hard to find on the internet.
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Patrick Lindecker
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Patrick Lindecker » Sun Dec 01, 2019 9:38 am

Hello Arun and all,

For about filament and for information, I wrote a paper in French for a Ham magazine, with an analysis of the filament and its ability to supply electrons. The link is:
http://f6cte.free.fr/Etude%20d'un%20rec ... v.%20B.pdf

Note 1: the speed distribution of electrons follows a Maxwell-Boltzmann distribution. The electrons leave the cathode in any direction around a main direction, with:
* the colatitude according to the Lambert's cosine law, cosine measured by comparison with the direction perpendicular to the cathode surface,
* the longitude according to a uniform distribution.

Note 2: below a certain temperature for which I have forgotten the exact value (about 300 or 400 °C) the filament will not burn immediatly in a standard atmospheric pressure, but the flow of electrons will be very weak...

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Patrick Lindecker
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Patrick Lindecker » Sun Dec 01, 2019 10:35 am

Erratum:

At standard pressure, the flow of electrons would be almost nil, due to collision with atoms and molecules (neutrals). You must have a level of vacuum so that electrons have a very low probabibility to collide neutrals. See https://en.wikipedia.org/wiki/Mean_free_path

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Arun Luthra
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Arun Luthra » Sun Dec 01, 2019 7:04 pm

Is this simplified drawing correct?

In a perfect vacuum, if I generate free electrons near the cathode then they will arrive at the target anode with the energy specified by the potential difference (e.g. 100 eV), with a narrow energy distribution. In reality, the mean free path means they will collide with background gas molecules that are nearly at rest, and on the order of 50% of the energy of the electrons will be lost with each of these collisions. The accelerated electrons will hit the electrons in the neutrals, and all electrons have the same mass, so that is a lot of energy loss in each elastic collision...

At e.g. 10^-4 mbar, the mean free path is around 100 cm, so if the gap between electrodes is 10 cm, then most of the electrons will be collisionless and arrive with 100 eV energy. Great.

Something I don't understand: Suppose the potential difference across the filament wire needs to be 15V, then does the the "positive" terminal of the filament need to be at -100V and the "negative" terminal at -85V?
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filament.png

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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Patrick Lindecker » Sun Dec 01, 2019 9:44 pm

>does the the "positive" terminal of the filament need to be at -100V and the "negative" terminal at >-85V?
In my opinion, strictly speaking yes it will better to have around -100 V (so to not disturb the electric field), but if the filament is very small compared to the anode and the cathode, the electric field will be only very few modified by the filament, so...

Note: in principle, cathodes are covered by a layer of electric insulating material and then by a layer of strontium or baryum oxyde. Moreover if the wires are isolated, the electric field will not be modified and you will simply use a 15 V supply.

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Richard Hull
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Richard Hull » Sun Dec 01, 2019 11:38 pm

If you have an emitter (filament), it is the cathode and it is always connected to the negative potential applied to the system! The anode is always the other terminal...The positive voltage of the system. In a a perfect vacuum, 100 volts between the cathode and and the anode will have the anode being bombarded by 100ev electrons. it is that simple.

And now for my traditional Debbie downer...

There are no perfect vacuums on earth or in the finest of vacuum systems. Collisions with remnant gas molecules, though rare in ultra high vacuums, will ionize the gas atoms and some small or even tiny fraction of those electrons created will not bombard the anode at 100 ev. It is just not that simple if absolute purity to theory is to be sought. Chalk it up to real-world physics which always says "toughski-stuffski" to perfect replication of physics theory. There will forever be flies in the ointment. Lots of 'em....Standing in line to uncover as you kill them one by one in a working system based on theory.

In the final analysis, just how many flies can you let stay in your ointment without fouling your original goal? Usually you will find that after killing a few of the most offensive "horse" and "blow flies" and pulling them out of your ointment, you can live with all the other little "fruit flies or gnats" swimming around in it and, for your purposes, claim a "win".

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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Arun Luthra
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Arun Luthra » Mon Dec 02, 2019 4:53 am

Richard Hull wrote:
Sun Dec 01, 2019 11:38 pm
If you have an emitter (filament), it is the cathode and it is always connected to the negative potential applied to the system! The anode is always the other terminal...The positive voltage of the system. In a a perfect vacuum, 100 volts between the cathode and and the anode will have the anode being bombarded by 100ev electrons. it is that simple.
I've been trying to understand Paschen's law, it prevents emission below about 350 V, no matter what the pressure or gap size is. But, basically Paschen's law does not apply to thermionic emitters.
paschen.png


Anyway, I guess my cathode will be a thermionic emitter.

As for vacuum tubes, are you recommending a vacuum tube because they are good for high electron emission current? I just need to remove the glass, and remove the control grid and anode if they are in the way.

Here is someone explaining filaments and high voltage:
https://www.quora.com/Why-does-high-vol ... x-ray-tube
I think he's a little confused, because he says "The high voltage never polarizes the filament circuit.". In his diagram, the filament circuit is at high voltage, but the voltage drop across the filament is a small filament voltage, a few volts.

I don't fully understand why you would even need a separate high voltage cathode if there is a filament at high voltage. Maybe related to electric field shaping.

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Rich Feldman
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Rich Feldman » Mon Dec 02, 2019 5:49 am

Arun Luthra wrote:
Sun Dec 01, 2019 7:04 pm
Something I don't understand: Suppose the potential difference across the filament wire needs to be 15V, then does the the "positive" terminal of the filament need to be at -100V and the "negative" terminal at -85V?
Filaments are powered by AC more often than not.
It's common to have a dedicated filament winding on a transformer, sometimes with a center tap for the high voltage connection.
In the 5U4 example above, you could get 5 V RMS between the filament pins, each 2.5 V RMS away from the -100 V DC supply.

The center tap refinement is generally skipped in higher voltage applications, like x-ray tubes and microwave oven magnetrons (2 to 5 volts AC, with HV connected to one arbitrary side).

X-ray tubes are one example where, as you surmised, you can find a metal part at cathode potential for field shaping. See Wehnelt cylinder or focus cup.
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Richard Hull » Mon Dec 02, 2019 6:09 am

Starved circuit amplifiers with vacuum tubes work just great with 20 volts. When you say "breakdown" what do you mean??? If it is a gaseous break down, you are not in a significant vacuum at all. the NE-2 glow lamp breaks down at only 60-70 volts with a hysteresis arc drop to 50 volts to retain the discharge. A mercury vapor rectifier will break down in the 100 volt range and exhibit and arc drop of only 14 volts.

High voltage, high vacuum diodes of exceedingly low current capacity have been in common use up until the 1950's with just tungsten hairs as the cathodes relying on field emission to force them to incandese at their tips at very low current drain, supplying all the electron current needed.

In air or in vacuum, electric breakdown depends on a huge number of factors, Paschen's law is applied within a given set of conditions. read carefully....

https://en.wikipedia.org/wiki/Paschen%27s_law

This is for smooth "flat plates" in a gas, I assume with other tightly held conditions. Flat plates of various metals in a gas will break down outside of his law if flashed or bathed with continuous UV light or due to a large RF pulse or if the gas is radioactive. So many variables.

Breakdown is an arc in a gas Paschen's basic conditions must be met to the letter before it works.

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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