# Solar heating the hot cathode

• JoJo

#### JoJo

I know typically the hot cathode of an electron gun or vacuum tube is heated either directly or indirectly by electric resistance heating. What if the tungsten (cathode) were heated by another means, say focused sunlight? Electrons would still boil off through thermionic emission. If a small accelerating current where applied to motivate the free electrons to the anode would and the anode were connected in a closed loop circuit across a load could useful power be generated? Look forward to your input.

Welcome to the PF.
What if the tungsten (cathode) were heated by another means, say focused sunlight?
Heating the cathode for thermionic emission is heating, as far as I know for this application. Heating from focused sunlight should be equivalent given the same power absorbed.
If a small accelerating current where applied to motivate the free electrons to the anode would and the anode were connected in a closed loop circuit across a load could useful power be generated? Look forward to your input.
So "accelerating current" should be "accelerating voltage" or "potential", but your question is a good one. I interpret it as "is there a way to extract energy from sunlight with this different technique?"

Any accelerating voltage that you would apply would supply the power that was in the electron beam, once the electrons were liberated from the cathode, so no, you wouldn't be able to extract extra energy beyond what you are investing in the accelerating voltage.

JoJo
I know typically the hot cathode of an electron gun or vacuum tube is heated either directly or indirectly by electric resistance heating. What if the tungsten (cathode) were heated by another means, say focused sunlight? Electrons would still boil off through thermionic emission. If a small accelerating current where applied to motivate the free electrons to the anode would and the anode were connected in a closed loop circuit across a load could useful power be generated? Look forward to your input.

Why don't you calculate the amount of power needed to cause the temperature to be raised high enough to cause such "thermionic emission" on a tungsten, and then compare that to melting point of tungsten. And BTW, you need to overestimate the power, because metals have poor absorbance of light in the first place when compare to semiconductors/insulators in general.

You can't look at this for just one property without considering others that might be expected. I've done a few of these "electron emission" simply by heating the cathode using lasers, but these were done accidentally, and we called this "explosive emission", which is not desirable because we are basically ablating the surface!

So sure, you may get electrons out of such a process, if that's all you care about, but you are also melting the material and causing the emission of other "crap", not just the electrons. Think of the stuff that will start coating other components within the vacuum vessel when this occurs.

Zz.

JoJo and berkeman
Thank you both for your input, I really appreciate it!

Berkeman, I completely understand what you are saying but it is difficult for me to believe that the process would not capture at least some part of the energy required to free the electrons (work function). For tungsten this is about 4.5 eV. (I think of this similar to the PV junction of a photovoltaic cell.) But even if the process does capture some of this work function energy, the question is whether the energy output is greater than the energy input? What are your thoughts?

Great points ZZ, what I found is thermionic emission is prevalent in tungsten around 2200 C, it melts around 3991 C. In my mind, the process is not much different to that used in a vacuum tube or CRT. You make a good point about all of the other crap. I think this is a problem in vacuum tubes and they equipped them with a "getter" to try to capture the contaminants. What do you think?

JoJo

Great points ZZ, what I found is thermionic emission is prevalent in tungsten around 2200 C, it melts around 3991 C. In my mind, the process is not much different to that used in a vacuum tube or CRT. You make a good point about all of the other crap. I think this is a problem in vacuum tubes and they equipped them with a "getter" to try to capture the contaminants. What do you think?

JoJo

You are missing my point, especially when you did not calculate the amount of power needed to cause thermionic emission. By the time you have enough light to get to that regime, you will have melted the surface of tungsten. This is what I meant by ablation. You are causing the material to evaporate!

The "getter" in vacuum tubes do not handle such things. In fact, these getters will soon be rendered useless if a constant ablation takes place and coats the getter material. There are no ablation in ordinary vacuum tubes!

Zz.

JoJo
Got it, I think I need to do a little more research. Thanks again for the input. I will read up on it and report back. I would certainly appreciate any references or links you could forward.

JoJo

Sorry Zz, but I think I am missing your point and I really want to understand what you are saying. How is heating tungsten to 2200 C with concentrated sunlight different than heating it to 2200 C with electric current.

Berkeman, I completely understand what you are saying but it is difficult for me to believe that the process would not capture at least some part of the energy required to free the electrons (work function). For tungsten this is about 4.5 eV. (I think of this similar to the PV junction of a photovoltaic cell.) But even if the process does capture some of this work function energy, the question is whether the energy output is greater than the energy input? What are your thoughts?
The thermal energy supplied to the cathode overcomes the work function and electrons are emitted.
By whatever means, that energy is necessary for thermionic emission.
If you then add a battery between the cathode/anode, that circuit does not care how the electrons were freed from the metal.
You can also look up photoelectric effect, and perhaps field electron emission, both of which where one does not have to heat the cathode to release electrons. Thermionic emission, though, releases electrons in greater numbers after a certain temperature is reached for the cathode. ( As long as we don't have arcing, which is undesirable is particular circumstances, but necessary in others ).

JoJo
How is heating tungsten to 2200 C with concentrated sunlight different than heating it to 2200 C with electric current.
If you heat it with a DC current, heat is generated all through the 3D material. If you heat via external light, the surface becomes much hotter than the interior of the material.

JoJo
Sorry Zz, but I think I am missing your point and I really want to understand what you are saying. How is heating tungsten to 2200 C with concentrated sunlight different than heating it to 2200 C with electric current.

If you heat it with a DC current, heat is generated all through the 3D material. If you heat via external light, the surface becomes much hotter than the interior of the material.

In addition to what anorlunda said, the surface of your tungsten is not perfectly flat. Areas of protrusions, etc. will be unevenly heated, and these will often be the centers for ablation. Your light source will have to provide significantly higher power than what you think is necessary, because of the nature of metals being highly reflective. So already you are providing way more power per unit area, which triggers surface ablation even more.

Look, this is a highly silly and inefficient method to generate electrons. I work with electron sources, both for particle accelerators and photodetectors. We can a combination of heating and photoemission (to lower the work function), forward bias and photoemission (Schottky effect), etc.. but simply heating using light is not in the cards because it is highly inefficient. If I have a light source with enough power to heat something to cause it to ablate, I can get more electrons simply by changing the light's frequency to tune it above the work function of the cathode, and I'm done!

Zz.

JoJo, berkeman, Vanadium 50 and 1 other person
If I have a light source with enough power to heat something
Making the externally heated cathode tube cumbersome and technologically challenged.
An indirectly heated cathode could be considered as being heated by a "light source", although here the filament is internal to the vacuum tube and heats the 360 degree surrounding cathode via radiation. With an external radiation heat source, the glass envelope of the tube and its melting becomes another limiting factor.
As said, more power has to be provided to an external heat source due to the fact of how radiation works - one has two extreme choices ( or a configuration in between ) to heat the internal cathode:
1. the external heat source has to surround the internal cathode, in which case radiation is also emitted outwards, so we are losing at 50% of it to the ambient, which as to be taken care of someway, or
2. the external heat source is a directed beam of radiation to the internal cathode.

In the first case, the air will be heated by the external source, and if close to the glass envelope, will affect the conductively heated envelope's integrity, We may need a second cooling system for the glass envelope. We could move the external heat source farther away, with more outwards radiation from a larger surface area heating the ambient surroundings.
In the second case, ( a possibly for the first also ), the beam has to have radiation removed that is not glass transparent ( wording ? ) for the glass to be not spot heated. In which case, we lose some of the external energy.

While sunlight could be considered free, using it to heat the cathode of a tube, presents a heat problem and removal of its own, IMO.

JoJo
I've done a few of these "electron emission" simply by heating the cathode using lasers, but these were done accidentally, and we called this "explosive emission", which is not desirable because we are basically ablating the surface!
LOL, did you say any bad words when this happened?

Berkeman, I completely understand what you are saying but it is difficult for me to believe that the process would not capture at least some part of the energy required to free the electrons (work function). For tungsten this is about 4.5 eV. (I think of this similar to the PV junction of a photovoltaic cell.) But even if the process does capture some of this work function energy, the question is whether the energy output is greater than the energy input? What are your thoughts?
It's good to ask questions about how energy conversion mechanisms can work, especially when trying to increase the efficiency and practicality of things like solar energy conversion. I think I've seen a few patents for solar energy conversion using the photoelectric effect (at the IC level), but they didn't seem very practical and since they haven't been commercialized, they probably were like many patents that are granted -- not practical in reality.

But, it's good for you to do more reading about various creative ways that folks are looking at to do solar energy conversion. I'd recommend looking through this Google hit list for a search I did just now. There look to be some pretty interesting articles in the hit list. Enjoy, and let us know if you have any questions about that reading.

JoJo, tech99 and anorlunda
When thermionic electrons are emitted, they have momentum, so a small current will flow to an anode when it is near cathode potential. So we obtain electrical energy from cathode heating. If you apply a positive potential to the anode, a current flows, but there is no gain of energy as far as I can see.

JoJo
All great points. It will take some time for me to read and digest this information but I will do it and report back.

I am really blown away by your kindness! It is so nice of you all to take the time to read, think about and respond to my post. Thank you!

berkeman
When thermionic electrons are emitted, they have momentum, so a small current will flow to an anode when it is near cathode potential. So we obtain electrical energy from cathode heating. If you apply a positive potential to the anode, a current flows, but there is no gain of energy as far as I can see.
Any thermionic electrons that hit the (colder) Anode will stay there or could be passed through a circuit back to the Cathode, transferring some energy. We are describing a diode, a load and a source of DC.
I would say that the difference between melting temperature and thermionic temperature would be large enough to protect the Cathode by simple control of the light flux. (If the system were designed appropriately - it wouldn't need to be as catastrophic as @ZapperZ describes in his experiments? What's the work function associated with ablation?

However, this idea is sure to have been tried before and, if it actually worked, I'm pretty sure a Google search with the appropriate terms would tell us about it. So it's probably not a possible method for PV.