I. Cycle averaged amplification and dampingConcerning the origin of equation ( 1) in the main text, and referring to figure 1, the respective scattering forces 1 are F c,a (v, i c,a ) = αi c,a f γ (∆ω c,a − kv) where i c,a is the cooling/amplification beam intensity at the ion position; ∆ω c,a ≡ ω c,a − ω o (ω o is the optical transition frequency); and α ≡ hkγ/2i sat (i sat is the saturation intensity of the dipole transition, and k = ω o /c). f γ (ω) is

Severe reductions in the greenhouse gas intensity of aviation fuels are required to get a growing aviation sector on a flightpath compliant with the Paris Climate Agreement. Introducing renewable energy into aviation is challenging. The power‐to‐liquid (PtL) pathway uses renewable electricity, CO2, and water to synthesize a sustainable alternative fuel that chemically resembles conventional jet fuel. The state‐of‐art of key technologies for PtL fuel production as well as the environmental and techno‐economic performance of the resulting fuel in comparison with fossil and biomass‐derived jet fuel are reviewed.

The European consortium SOLARJET has experimentally demonstrated the first ever production of jet fuel via a thermochemical H 2 O/CO 2 -splitting cycle using simulated concentrated solar radiation. The key component of the production process of sustainable "solar kerosene" is a high-temperature solar reactor containing a reticulated porous ceramic (RPC) foam structure made of pure CeO 2 undergoing a 2-step redox cyclic process. During the first endothermic reduction step at 1450− 1600 °C, the RPC was directly exposed to concentrated thermal radiation with power inputs ranging from 2.8 to 3.8 kW and mean solar flux concentration ratios of up to 3000 suns. In the subsequent exothermic oxidation step at 700−1200 °C, the reduced ceria was stoichiometrically reoxidized with CO 2 and/or H 2 O to generate CO and/or H 2 . The RPC featured dual-scale porosity: millimeter-size pores for volumetric radiation absorption during reduction and micrometer-size pores within its struts for enhanced oxidation rates. For a cycle duration of 25 min, mean reduction rates were 0.17 mL O2 min −1 g −1 CeO2 and mean oxidation rates were 0.60 mL CO min −1 g −1CeO2 . The solar-to-fuel energy conversion efficiency was 1.72%, without sensible heat recovery. A total of 291 stable redox cycles were performed, yielding 700 standard liters of syngas of composition 33.7% H 2 , 19.2% CO, 30.5% CO 2 , 0.06% O 2 , 0.09% CH 4 , and 16.5% Ar, which was compressed to 150 bar and further processed via Fischer−Tropsch synthesis to a mixture of naphtha, gasoil, and kerosene.

The 1S-2S two-photon transition in singly ionized helium is a highly interesting candidate for precision tests of bound-state quantum electrodynamics ͑QED͒. With the recent advent of extreme ultraviolet frequency combs, highly coherent quasi-continuous-wave light sources at 61 nm have become available, and precision spectroscopy of this transition now comes into reach for the first time. We discuss quantitatively the feasibility of such an experiment by analyzing excitation and ionization rates, propose an experimental scheme, and explore the potential for QED tests.