Optimizing the shape of broadband microwave absorbers for cosmic microwave background experiments

Primožič, Vid (avtor), Bradač, Maruša (mentor), Gudmundsson, Jón E. (komentor)

Observations of the cosmic microwave background polarization are an important field in cosmology. These observations, conducted both from Earth and space, deal with extremely weak signals that require very precise detectors. Microwave absorbers are used to cover the supporting structures of the telescope to minimize noise caused by unwanted reflections. These absorbers consist of plastic tiles with additives (carbon), featuring pyramids with square unit cells on their absorbing side. The pyramids reduce the reflections at the interface between the absorber and vacuum caused by a mismatch in refractive indices between the vacuum and the absorber. When the pyramids are small compared to the microwave wavelength, they can be modeled using effective medium theory as impedance tapers. If they are larger than the wavelength, the waves reflect multiple times between individual pyramids. An impedance taper is a function that gradually changes the impedance from the impedance of free space to that of the absorber, reducing the reflected radiation (reflectance). Five different impedance tapers were tested. For absorber fabrication, two 3D printing methods, stereolithography and fused filament fabrication, were evaluated, along with the use of composite resin. Fused filament fabrication using ESD HIPS filament was selected. The reflectance of the manufactured absorbers was modeled using the transfer matrix method and simulated using the finite element method. Reflectance was measured with the Toptica TeraScan 1550 THz spectrometer at 100--400~GHz. Because it was not possible to fabricate absorbers with a sufficiently small unit cell to satisfy the assumptions of the effective medium theory, the theoretically estimated reflectances deviated from the measurements. In the regime with a unit cell larger than the effective medium theory limit, non-specular scattering (scattering in angles other than incidence) in several discrete directions occurs. The fabricated absorbers achieved reflectance comparable to that of other state-of-the-art absorber designs. Significant differences in reflectance were observed between absorbers corresponding to different impedance tapers, highlighting the need for further optimization of the shape of surface structures.

• Jezik: Angleški jezik
• Ključne besede: microwave absorbers, cosmic microwave background, 3D-printing, surface structures, effective medium theory
• Vrsta gradiva: Magistrsko delo/naloga
• Tipologija: 2.09 - Magistrsko delo
• Organizacija: FMF - Fakulteta za matematiko in fiziko
• Leto izida: 2025
• PID: 20.500.12556/RUL-171164
• COBISS.SI-ID: 246918659
• Datum objave v RUL: 14.08.2025

Sekundarni jezik

• Jezik: Slovenski jezik
• Naslov: Optimizacija oblike površinskih struktur širokopasovnih mikrovalovnih absorberjev za uporabo v eksperimentih za opazovanje mikrovalovnega sevanja ozadja

Izvleček

Opazovanja polarizacije mikrovalovnega sevanja ozadja so pomembno področje v kozmologiji. Mogoča so z Zemljinega površja in z vesoljskimi teleskopi. V vseh primerih gre za izjemno šibke signale, ki zahtevajo detektorje s čim manj motnjami. Za zmanjševanje motenj zaradi neželenih odbojev mikrovalov se uporabljajo mikrovalovni absorberji, s katerimi so prekrite nosilne strukture v teleskopih. Absorberji so ploščice iz plastike z dodatkom ogljika, ki imajo na absorptivni strani piramide s kvadratno osnovno celico. Piramide preprečujejo močne odboje na meji med absorberjem in zrakom, ki bi nastali zaradi razlike med lomnim količnikom vakuma in absorberja. Če so piramide majhne v primerjavi z valovno dolžino mikrovalov, jih lahko modeliramo s teorijo efektivnega medija kot transformator impedance, če pa so večje, se valovi večkrat odbijejo med posameznimi piramidami. Transformator impedance je funkcija, ki postopno spremeni impedanco od impedance praznega prostora do impedance absorberja in tako zmanjša količino odbitega valovanja (odbojnost). V nalogi je bilo testiranih pet različnih transformatorjev impedance. Za izdelavo absorberjev sta bili testirani dve tehnologiji 3D tiska -- stereolitografija in tehnologija taljenega filamenta -- in izdelava absorberjev iz kompozitne smole. Za izdelavo absorberjev je bila izbrana tehnologija taljenega filamenta in ESD HIPS filament. Odbojnost izdelanih absorberjev sem modelirali z metodo prenosne matrike ter simuliral z metodo končnih elementov. Odbojnost izdelanih absorberjev smo izmerili s sistemom za THz spektroskopijo Toptica TeraScan 1550 na frekvencah 100--400~GHz. Ker ni bilo mogoče izdelati absorberjev z dovolj majhno osnovno celico, da bi zanje veljala teorija efektivnega medija, se teoretične odbojnosti niso skladale z izmerjenimi. V primeru osnovnih celic večjih od meje veljavnosti teorije efektivnega medija, se pojavlja sipanje mikrovalov v nekaj diskretnih smeri. Izdelani absorberji so dosegli odbojnost primerljivo z drugimi sodobnimi absorberji. Med absorberji, ki ustrezajo različnim transformatorjem impedance, so opazne znatne razlike v odbojnosti, kar opravičuje nadaljnjo optimizacijo oblike struktur na površini.

• Ključne besede: mikrovalovni absorberji, kozmično sevanje ozadja, 3D tisk, površinske strukture, teorija efektivnega medija