Satellite maser engineering

Design of space-based maser

Our objective is to create an optically-pumped maser that exclusively uses focused sunlight as the pump source. This would require a large parabolic solar mirror to reflect and focus sunlight used for pumping the maser.

Possible future extension: plasma cathode for electron gun

It is an interesting idea to consider whether, instead of a thermocathode that is solid, we can instead use a plasma “cathode” as an electron source. A plasma has no upper limit on how hot it can be heated, unlike a solid thermocathode that will eventually be worn down especially at very high temperatures. In addition, certain gases have strong absorption cross-sections in the UV-visible range, like hydrogen and oxygen, and some in the IR range, like CO2 (and the other greenhouse gases). A mixture of several different gases in different proportions (some noble gases as well, perhaps?) might allow us to create a plasma that can be optimally super-heated by sunlight into a plasma (a phosphor might be useful to make the best use of the UV bands).

The difficulty, of course, is to (a) ionize the gases to form a plasma in the first place, (b) make sure that the superheated plasma can form into the electron beam by selectively “choosing” electrons rather than (positive) ions from the beam and using the magnetic lens to steer it, and (c) prevent the plasma from leaking with magnetic containment. Lastly, once the free electrons have passed through the maser, some type of energy recovery scheme should be used to slow the electrons down and re-route them back into the plasma to replenish it.

• For (a) you would likely want the gas to be stored in a sealed container and only discharged once the laser turns on. The concentrated beam of sunlight will then be used to superheat the gas to 10,000K+ and ionize it by a combination of extremely concentrated sunlight and UV radiation present within it
  • If we design the electron gun housing of the laser very carefully to also be a RF cavity, a reasonably weak RF source (i.e. antenna) can be amplified inside the cavity to help ionize the gas with minimal power; the resonant cavity allows the waves to be self-sustaining even once the RF source is powered off.
• For (b) electrostatic acceleration is sufficient in driving the positive ions away, where they can be collimated and used to bombard more gas atoms to ionize them in a self-sustaining cycle.
• For (c) this is a problem that is extremely difficult to solve and the solution is unknown, but solid containment within the laser cavity (e.g. glass to seal the cavity except for a small aperture) may allow it to be mitigated.

The nice thing, however, is that what was basically described is just the same thing as a magnetoplasmadynamic (MPD) thruster, which people have definitely built already, and which can reach reasonably high efficiencies. The only difference is that solar energy is (primarily) used to create the superheated plasma instead of an RF field. In any case, it seems like a sufficiently viable idea to merit a feasibility study of:

1. Whether this is possible at all;
2. Whether this can meet our efficiency requirements, both theoretically and practically;
3. How it compares to hot cathode-based traditional electron guns

For the second and third points, they are as important as the first, because the theoretical efficiency of our current design (with a hot cathode) is very high (see Free-electron maser efficiency calculations), but this may not be the case for a plasma cathode. The key metric to answer the third point is the emission efficacy of the plasma cathode (measured in $\text{A/W}$ or $\text{mA/W}$), which tells us how well solar energy is converted into current; from $P=I\Delta V$ the output power can then be computed. Comparing against our hot cathode (see Thermocathode design for electron gun) can then give us a figure of whether this approach is worth pursuing.

Note: This paper may also be useful reading; it describes the design of a cold plasma‐cathode electron gun and is similar to our concept. Also, this paper describes a MPD thruster with up to 61.99% efficiency, so indeed it appears that requirement (2) can be satisfied.

Possible promising idea: free electron maser (current research)

This is the most promising idea so far: a free electron maser that uses a relativistic electron beam (like the type produced by a vacuum tube, a type of electron gun¹) as its gain medium. Free-electron masers are especially nice because they can be tuned to arbitrary frequency by simply varying the geometric properties of the maser. Furthermore, changing a vacuum tube to using concentrated sunlight as the anode heating source shouldn’t be too hard, and the gyrotron is an existing free-electron maser design that is already used for high-power fusion research. The only requirement is that we need strong permanent magnets, but those aren’t too hard to buy.

For Project Elara, the main challenge is to make the laser purely optically-powered, that is, purely powered by the concentrated sunlight, and as high-efficiency as possible. That is something we need to do a lot of work on.

For more info see the following pages:

• Calculation of free electron maser parameters
• Free-electron maser physics
• RF cavity simulation with aperture
• Laser aperture engineering
• Combined system efficiency calculations
• A realistic space-based prototype

Tested idea: solid-state optically-pumped maser

There is already published research on creating an optically-pumped solid-state maser pumped by sunlight². If direct solar pumping is too difficult, we can also theoretically use a laser diode powered by solar panels or a NdCrYAG laser as a pump source.

Possible but un-verified idea: microwave diode pumped gas maser

One possible idea is to use a microwave diode pumped gas maser. This uses electricity from solar panels to power a microwave diode (e.g. tiny gunn diode), which produces the microwaves that excite atoms in the gain medium (e.g. water/ammonia/methanol), leading to stimulated emission. The main issue is that we need to create a population inversion for lasing to occur (it must be a three-level or four-level maser), and additionally needing solar panels is a source of inefficiency.

Possible but un-verified idea: plasma-based gas discharge maser

While our calculation indicated the high improbability of constructing a maser that replicated the conditions of astrophysical masers in the lab (considering the extremely long optical path lengths required), it may be possible to create a loosely-inspired variant. Buleyko et. al. (also this other paper by them) demonstrated the creation of a plasma-based maser (which replicates the extremely low gas densities and high temperatures under which astrophysical masers operate). Meanwhile, Obradović et. al. replicated similar conditions in a laboratory plasma. This may especially be promising given that astrophysical masers can output an incredible amount of power, so finding a way to partially replicate an astrophysical maser (e.g. hydroxyl maser, methanol maser, water maser, etc.) in the lab could be a promising idea.

Past idea: ammonia maser

Initially we thought we could use a pumpless ammonia maser. Since ammonia masers naturally work well in vacuum and cryogenic conditions (which naturally occurs in space) this is no issue. However, ammonia masers are incredibly weak - nanowatts would be the typical power output, which makes the idea very impractical. The issue is that ammonia masers don’t actually use any form of pumping; rather, an electric field separates ammonia molecules in a higher energy state from those in a lower energy state³. As soon as the higher-energy ammonia molecules decay into the lower energy state, however, the upper state population must be restored. The photons emitted through the decays do help re-populate the upper state population, but this also takes away from the output power of the maser. Thus, ammonia masers are far, far too weak to be used.

Past idea: radiative pumped astrophysically-inspred maser

Previously, we considered the idea of building the artificial equivalent of an astrophysical maser. Astrophysical masers are naturally-arising masers formed by gas clouds around planets and stars. Energy from the central astronomical body acts as a pump source for surrounding high-temperature, ultra-low-density gases, typically requiring temperatures of several hundred to over a thousand kelvin⁴. Larger astrophysical lasers tend to be radiatively pumped⁵ whereas the smaller ones tend to be pumped via collisional excitation - that is, by molecular collisions, which itself arises from the high kinetic energy of the heated gas itself. Both mechanisms, are, of course, related; masing within circumstellar clouds receives both heat and light from its central star. This causes lasing⁵, although only with gases spread over extremely long distances.

A few known natural masers and the necessary conditions for masing to take place (not including the density requirements) are listed below⁴:

Gas molecule	Emission frequencies	Required temperature
Silicon monoxide ($\mathrm{SiO}$)	43, 86, 344 GHz	$\ge 1500\mathrm{K}$
Water vapor ($\mathrm{H}{}_2\mathrm{O}$)	22, 183, 321, 325, 658 GHz	$400-1000\mathrm{K}$
Methanol ($\mathrm{CH}{}_3\mathrm{OH}$)	6.7, 12.2, 95 GHz	$\ge 10000\mathrm{K}$ based on source
Hydroxyl radical ($\mathrm{OH}$)	1612, 1665, 1667, 1720 MHz	$\approx 1500\mathrm{K}$ based on source

Note: For the appropriate density and temperatures conditions see the solar atmospheric data on Stanford solar center information page. We may presume that it is a good model for circumstellar conditions.

Note that these wavelengths are typically associated with rotational transitions in the gas molecules which commonly produce microwaves. In all cases, high temperatures are responsible for exciting the gain medium to allow lasing to occur. While this can be modelled using the effect of a pump EM field (after all, the light and heat emitted by stars does take the form of EM waves), we think it may be better to model this using a thermodynamical model by assuming molecules of high kinetic energy that undergo frequent elastic collisions, similar to the HeNe laser. Calculation of the rotational transitions can then identify suitable gaseous species for lasing - menthanol is particularly promising, but the very high required temperatures required for it to lase makes it incredibly impractical. Again, mixtures of gases may achieve better performance than a single gas, so we should simulate the effects of multiple interacting gases as well.

Note: for a model of high temperature-pressure quantum gases, there is this guide, which (while not being very comprehensible) seems to introduce some of the theory/models behind this branch of molecular physics.

Highly-concentrated sunlight does indeed hold potential for heating gases to those high temperatures required for lasing; indeed, the solar furnace of Uzbekistan⁶ can reach temperatures of up to 3200 K using focused sunlight alone. But to use highly-concentrated sunlight, as would be needed for a solar maser, efficient transfer of heat to the gaseous gain medium is very crucial. Purely requiring the sunlight to heat the gas is possible, but we’ve come up with an intriguing idea to make better use of the available wavelengths in sunlight, particularly in the UV range. The concentrated sunlight can be split with some more advanced version of a prism into its component wavelengths, with the short visible and UV wavelengths diverted to be incident on a photocathode (you would then need another beam collimator + prism to re-focus and recombine the beams of light). Here, the high-energy photons in the short visible/UV range would lead to the emission of photoelectrons that further collide with the gas molecules to transfer kinetic energy to them, leading to the overall effect of heating the gas. This is because UV is not really so good at heating as much as it is good for ionizing, but we don’t necessarily want ionizing (otherwise it chemically changes the composition of the gain medium) so this may be a better way.

Unfortunately, we found through some calculations and the literature that this design was impractical. See Calculation of a hypothetical astrophysical-like artificial maser for more details.

References

Footnotes

1. https://spark.iop.org/electron-guns ↩

2. https://www.nature.com/articles/nature11339 ↩

3. https://www.ehu.eus/documents/43252989/43253686/Ferez+PLANCKS+2022+solution.pdf/4a4d88c2-976e-5261-8a3d-5a47511620e2?t=1677789781459&download=true ↩

4. https://arxiv.org/abs/1104.2306 ↩ ↩²

5. https://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_946 ↩ ↩²

6. https://en.wikipedia.org/wiki/Solar_furnace_of_Uzbekistan ↩