As described in [[Free-electron maser efficiency calculations]], the theoretical efficiency of a free-electron maser can be very high, and indeed far higher than conventional masers (e.g. hydrogen and ammonia masers). We discussed that the theoretical efficiency is [only limited by the Carnot limit](https://stanford.edu/~vossj/project/thermionic-emission/), which sets the maximum theoretical efficiency according to the equation: $ \Gamma_{ac} = 1 - \dfrac{T_c}{T_h} $ Where $\Gamma_{ac}$ is the efficiency of the cathode-anode combination, $T_h$ is the temperature of the thermocathode, and $T_c$ is the temperature of the anode at the far end of the electron gun. In geostationary orbit, the cold side of a satellite can reach incredibly-low temperatures of as low as $-196^\circ\text{ C}$ ($\text{77 K}$)[^1], while hot cathode temperatures can reach $\text{2,000-2,500 K}$, although in practice the most efficient thermocathode designs today operate at closer to $\text{1000-1300 K}$. Even with this lower cathode temperature, the theoretical Carnot efficiency is very high (greater than 90%). Moreover, the anode temperature can be made even cooler by cryogenically cooling it before launch, since space is a near-perfect insulator, meaning that if the anode is cooled to near absolute zero (e.g. around $\text{3-4 K}$) prior to launch and kept at such temperatures throughout launch, it will maintain that temperature in space (even without active cryogenics). If we take this approach, we can reach a whopping 99.7% Carnot efficiency, making a free-electron maser extremely efficient - at least in theory. In practice, electron guns cannot reach anywhere close to efficiencies that high; typical modern designs have a efficiency of only around 10%, with projections of 35% efficiency or more possible only in theory[^2]. In our case, efficiency can be enhanced by photon-enhanced thermionic emission (PETE), although reaching 50% efficiency is already an enormous undertaking. (More on this is covered in [[Free-electron maser efficiency calculations]]). However, all of this ignores **one major issue**: even if we are able to overcome all the practical issues, the Carnot limit still requires that the anode be kept at a very cold temperature. Unfortunately, while space itself is very cold, the Sun-facing side of any satellite can be up to $128^\circ \text{ C}$ ($\text{401 K}$), at which the Carnot efficiency drops to only around 60%. Thus, it is imperative to shield the anode to ensure it is as cold as possible. ## Shielding materials Different materials have historically been used to shield satellites. **Mylar** is a popular choice because it is highly-reflective; metallized Mylar has up to 99% efficiency.[^3] The [Parker Solar Probe](https://en.wikipedia.org/wiki/Parker_Solar_Probe), designed to operate at distances of $\text{0.28 AU}$ from the Sun, uses a reflective alumina layer with a reinforced carbon-carbon heat shield[^4]. Satellites can be cooled in different ways as well. Radiators can be used to remove (and in some cases recycle) waste heat. Note that passive cooling is far preferred over active cooling since it avoids consuming precious energy. Ultimately, it will take time to decide on the most suitable design, but it will likely involve a highly-reflective coating for the entire exterior of the spacecraft, thermal isolation of the cold anode, and at least some cooling to carry away any heat from the proximity of the anode. If we are able to do so correctly (together with the pre-launch cryogenic cooling technique we mentioned at the start), we can maintain the anode at near absolute zero while ensuring the hot cathode is kept at a very high temperature, ensuring the maser is as efficient as possible. [^1]: See [this physics SE article](https://space.stackexchange.com/questions/63871/maximum-and-minimum-surface-temperatures-of-a-satellite-in-geo#:~:text=I%20found%20the%20document%20Environmental%20Conditions%20for,where%20an%20optimal%20microclimate%20can%20be%20maintained.) for more details. [^2]: See Campbell et. al., _Progress Towards High Power Output in Thermionic Energy Conversion_ (DOI: https://doi.org/10.1002/advs.202003812). [^3]: Figure is taken from the [Wikipedia article on Mylar](https://en.wikipedia.org/wiki/Mylar#Manufacture_and_properties) [^4]: Information taken from the [associated Wikipedia article](https://en.wikipedia.org/wiki/Parker_Solar_Probe#Spacecraft)