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 »

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

Post by Rich Feldman »

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

Post by Patrick Lindecker »

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

Post by Patrick Lindecker »

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

Post by Arun Luthra »

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

Post by Patrick Lindecker »

>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 »

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
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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 »

Richard Hull wrote: Sun Dec 01, 2019 6:38 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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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by Rich Feldman »

Arun Luthra wrote: Sun Dec 01, 2019 2:04 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 »

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.

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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 »

Here is a first pass at a chamber design:
jar.jpg
The dome is a glass bell jar. It's on a silicone rubber gasket ring and a 1/4" thick steel disk baseplate.

I send two high negative voltage lines into the chamber because they differ by the filament voltage. What can I use to supply to filament voltage? I need to add a few volts (preferably an ajustable voltage) to a line that is at -1000V. Maybe a battery and a potentiometer?

What's a good way to shape the field to focus the electrons onto the anode target? The targets will be insulators... could ring magnets beneath the target be useful?

The ground attached to the 0 V line of the high voltage seems slightly odd to me, but I saw that choice elsewhere. Is this totally wrong? In this picture, the entire baseplate is connect to the 0V on the high voltage supply.
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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 »

Your HV supply must be either totally isolated or either a positive ground supply. If isolated you will need to ground the positive line as you show in the diagram. You cannot use a normal negative grounded supply here. Most normal power supplies are negative ground. You may have to wind up making your own HV supply.

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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 »

My HV supply provides two wires, one is negative high voltage up to -1000V, and the return line. So, this is termed a "positive ground supply"? Good to go then.

For the filament voltage, I have a DC low voltage power supply with negative, ground, and positive terminals. Should I attach the -1000 V cathode wire to the low voltage DC supply ground pin? Then, then + terminal on the low voltage DC supply will be at -997 V? Or will this cause arcing and explosions...
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

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This is the norm. One side of the HV supply always goes to the filament. In this case your filament supply must no have a ground and be capable of handling 1 kilovolt without arcing within itself. A well insulated AC filament transformer driven by a variac is ideal here. There is no need for a DC filament supply at all. Why risk your DC low voltage supply when an AC filament transformer would do fine?

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

Post by John Futter »

Arun
With a 1000 volt supply you will get electrons with 1000eV and maybe a bit less arriving on your target.
You say you want 50 to 100eV electrons arriving then that is what the voltage should be
@ Work I use an electron microscope filament running white hot (as seen through viewport) with a 0 -60 volt supply to bathe a nonconducting sputter target with electrons so that the target charge is taken away so that the positive argon ion beam hitting the target is not repelled off the target

you cannot use vacuum tube filaments or magnetron filaments effectivly for an electron source as the coatings that promote electrons to leave get poisoned once in air and the effieciency / life drops.
SEM filaments use different coatings that are compatible with air but you have to run them hotter than vacuum tube filaments to get the same number of electrons leaving. If you do try a vacuum tube filament you will have to run better than yellow hot to get appreciable electrons
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

Post by ian_krase »

Ion beam sputtering? Sounds interesting -- and like something to try if I ever get a bigger chamber.

If I was doing this, I would put the filament at ground potential and drive the target positive, assuming I have three feedthroughs. Makes the filament heater supply easier.


I don't use SEM filaments -- I just weld my own filaments from tungsten wire. Far cheaper. Yes, they need to be white hot. You may be able to coat filaments with yttria or the like, but that's additional complexity.
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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 »

John Futter wrote: Thu Dec 12, 2019 7:44 pm Arun
With a 1000 volt supply you will get electrons with 1000eV and maybe a bit less arriving on your target.
You say you want 50 to 100eV electrons arriving then that is what the voltage should be
@ Work I use an electron microscope filament running white hot (as seen through viewport) with a 0 -60 volt supply to bathe a nonconducting sputter target with electrons so that the target charge is taken away so that the positive argon ion beam hitting the target is not repelled off the target
The supply (acquired after starting the thread) is adjustable 0 to -1000V. 50-100 eV is just the main area of interest. I would like the system to be robust enough to withstand -1000V.
you cannot use vacuum tube filaments or magnetron filaments effectivly for an electron source as the coatings that promote electrons to leave get poisoned once in air and the effieciency / life drops.
SEM filaments use different coatings that are compatible with air but you have to run them hotter than vacuum tube filaments to get the same number of electrons leaving. If you do try a vacuum tube filament you will have to run better than yellow hot to get appreciable electrons
I will only put current through the filament under vacuum (e.g. 1x10^-4 Torr). Will the filament be poisoned at atmospheric pressure in air even if there is no current through it?
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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 »

Richard Hull wrote: Thu Dec 12, 2019 6:28 pm This is the norm. One side of the HV supply always goes to the filament. In this case your filament supply must no have a ground and be capable of handling 1 kilovolt without arcing within itself. A well insulated AC filament transformer driven by a variac is ideal here. There is no need for a DC filament supply at all. Why risk your DC low voltage supply when an AC filament transformer would do fine?

Richard Hull
Thanks for the tip. I have a microwave oven transformer. It has a filament winding (something like 1 or 2 turns) in addition to the HV winding. Maybe I can add some voltage adjustability somehow. I need to be able to limit the current. I can leave the high voltage secondary winding unused.

Then, my cathode would have DC high voltage (tens of volts up to 1000V) with a few volts AC on top of it.
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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 »

Still trying to understand Paschen's law.

Suppose I have a hypothetical vacuum chamber with a cathode and an anode. There is no hot filament, just cold flat plates at high voltage. At 10^-4 Torr and 1 kV between the plates, the breakdown gap distance is 2500 meters for Argon. Meanwhile, vendors sell vacuum high voltage feedthroughs that operate at a few thousand volts with gaps that range from a few mm to a few cm. So there is many orders of distance between the practical high voltage situation and Paschen's law. What is going on here? Is the Paschen "breakdown" describing 1 nano amp of current? 1 microamp? Something larger? Are there any tables/charts that answer this?
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Re: Efficient way to bombard target with electrons in the 50-100 eV range? (not for fusor)

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Read and take in the issues here

http://www-eng.lbl.gov/~shuman/XENON/RE ... report.pdf

There are constants and terms in the law that must be defined in its application. Breakdown is ionization of the contained gas. Plates are the key in the law as originally stated. It is extremely rare to have two identical flat plates in a gas at varying pressures of just air. (what the law was written for). Change just one or multiple conditions and breakdown occurs at a different potential.

It is an equation with unknowns that the user must define.

This is why you can't get a "feel" for this until you do some work with voltages and electrodes in a vacuum. Until that time you, yourself, are in an intellectual vacuum! This is why I spent 1 year of my life with a demo fusor and three books read from cover to cover as I experimented, observed and read. In the book "Ionized Gases", Von Engel is no prima donna, egg head... He gives, dutifully, all the relevant math to handle special cases, but never fails to warn the reader............change one thing outside of the equation's derivation and setup conditions and the equation fails. Fail to define the unknowns in the equation properly and the equation fails.... I learned a lot of things about electrostatics, far beyond my college days teaching.... Not by reading only, but by active hands-on experiment in a demo device.

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

Post by ian_krase »

I will only put current through the filament under vacuum (e.g. 1x10^-4 Torr). Will the filament be poisoned at atmospheric pressure in air even if there is no current through it?
If you're using pure tungsten filaments, they will not be poisoned by air at any time whatsoever, but they will *burn out* if they are heated to red hot or hotter in the presence of air. 1E-4 torr is kind of a minimum vacuum for this kind of thing, they won't last very long at that level but in this kind of experiment you'll probably physically break them first. (Tungsten that has ever been heated to glowing is extremely mechanically fragile.) You can tell that the filament burned out from air if there's yellowish-whitish oxide on the inside of your chamber near the filament. The higher the vacuum, the longer the filament will last -- go to 10E-6 torr if you can!

Pure tungsten filaments work JUST FINE. They just need to be white hot. They are resistant to poisoning unlike other filaments.

If you are using cathodes from vacuum tubes that are coated... who knows? There are different coatings. I am not sure of the details of all of them. Some will hate air, others will not. Definitely don't heat them in air, that will destroy them. I would not bother messing around with this.

The third option is the yttria-coated iridium filaments from ion gauges. These are designed to not be poisoned by air, and can operate at fairly high pressures (even 1E-3 torr!) without rapidly burning out, although they will still have a longer lifetime the better the vacuum. However, they can be poisoned by various chemicals -- this is why tungsten-filament ion gauges are still used, since they are poisoning resistant.


A big part of what is going on regarding feedthroughs is that much work happens at high to ultrahigh vacuum, 1E-5 torr up to "how many zeros do you want". (and how many days do you bake your all-Conflat chamber while ion-pumping it!) I have worked with special motors that use high voltage -- they have a label saying that you can use them at atmospheric, and you can use them at high vacuum, but you cannot use them at between 100 torr and 10 microns because they will have glow discharges inside. Lots of stuff is assumed to just be for high vacuum. You may have seen Doug Coulter's posts on making special feedthroughs for fusion -- in his view, most feedthroughs are not designed to work with medium vacuum and glow discharges in general.

On the other hand, in a complex setup where there are not just simple anode and cathode plates or spikes, but a bunch of weird shapes... things get weird, and often there will indeed not be discharge in places you don't want it.
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