Evaluating advanced fuel candidates in surface cleaning of optics by plasma exposure (scope)

One of the leading issues leading to decreased mirror lifetime is that debris contamination and buildup of debris on the surface of the primary mirror optics that comes from the use of both Sn and Li in GDPP or LPP. This debris generation leads to a decreased reflectivity from the added material thickness and increased surface roughness that contributes to scattering. In order to overcome this issue, it is important to understand the implantation of debris material into the optic material matrix and the subsequent diffusion of this debris within the mirror matrix. This change in composition and surface morphology can dramatically affect reflectivity and mirror lifetime. A multifunction test stand was built to investigate this mirror lifetime issues that arise from the use of both Li and Sn. Through the use of both a Li+ ion beam, electron beam evaporator for Li and Sn deposition, and a quartz crystal microbalance to study both lithium implantation and sputtering at various incident ion energies, angles, quantity of debris, and mirror temperatures, the problem of Li generated debris was investigated. Subsequent surface analysis via SEM, XPS, AFM, and TOF-SIMS, were used to study the resultant mirror material properties of composition, diffusion, and surface roughness leading to a better overall understanding of the mirror contamination issue. Preferential Li diffusion through a ruthenium capping layer was seen depthwise into the mirror matrix with minimal radial diffusion while the mirror sample was at room temperature. However, at an elevated temperature, enhanced surface radial diffusion of lithium and segregation in the capping layer was seen with minimal diffusion into the underlying mirror matrix. In conjunction with study of the contamination mechanism, the use of a secondary plasma was investigated as a way to prevent degradation and clean the mirror optics so as to extend the lifetime with no interference to the source or other components. Through the use of a helium helicon plasma, it was possible to induce lithium evaporation and break up of clusters that form on the surface. The plasma parameters were measured using a RF-compensated Langmuir probe to yield electron temperature and density while the sample mirror material was at various temperatures and voltage bias. It was shown that in situ plasma cleaning of optics is a valid method towards achieving improved mirror lifetime. For Sn optics cleaning, the use of halide etching to readily transport Sn off the surface was successfully demonstrated. Ar and Cl plasma and other gas mixtures demonstrated selective etch rate for Sn versus mirror materials of SiO ₂, Mo, Si, and Si ₃N ₄ and it is possible to set a scheme such that Sn debris is preferentially sputtered while leaving the mirror material undisturbed and yielding a long lifetime for the optics.