Magnetized electrons rotate at a specific frequency, and resonantly emit and absorb electromagnetic waves at that same frequency. We use these waves –specifically: microwaves- to stabilize MHD instabilities as well as to start up, heat and diagnose stellarator and tokamak plasmas.
The plasma, however, is a very peculiar optical medium: non-uniform, birefringent, anisotropic, sometimes lossy, it can have index of refraction N<1, strongly decouple phase and group velocity, sustain "backward waves"... To deal with this physics richness, we are developing a new numerical method to reconstruct wave patterns in plasmas. The method is based on the old Huygens’ principle, that treats each wavefront as an array of point-sources and reconstructs the next waveform as the envelope of wavelets emitted by those points. Initial results were obtained in a non-uniform, non-magnetized plasma. As expected, the method is able to model diffraction phenomena such as the finite waist of a Gaussian beam, or diffraction around an obstacle. This capability sets it apart from geometric optics methods. At the same time, we hope to demonstrate that the method is as accurate but not as computationally demanding as state-of-the-art methods in the field, called full-wave codes.
On a different topic, we patented a metamaterial lens -a special array of metallic and dielectric elements- predicted to exhibit “reverse chromatic aberration”. That is to say, its focal length increases with frequency; no such property is encountered in a convergent lens made of natural materials. The motivation came from the Electron Cyclotron Emission (ECE) technique to measure the plasma temperature. The technique is based on spectral analysis of waves emitted by the plasma. Because spectral components (“colors”) of higher and higher frequency are emitted farther and farther from the ECE receiver, this special lens of reverse chromatic aberration will simultaneously have all frequencies in focus, resulting in maximum spatial resolution. We also ink-jet-printed such lens, using plastic as a dielectric and a special ink containing silver nanoparticles as conductor. We are now in the process of optically testing it in the microwave range. If extended to the visible range, our metamaterial lenses could have game-changing applications to microscopy and satellite imaging.