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Modern Developments in X-Ray and Neutron Optics Episode 4

Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 102 S. Lagomarsino et al. Fig. 6.5. a Intensity distribution in monomodal regime for the first mode in front of and along a front coupling WG b Intensity profile along 0Y of the same waveguide calculated with computer code full line and with 6.17 open circles The distribution of intensity for the first mode in free space close to the WG exit coincides with the diffraction pattern of a confined wave with amplitude given by 6.17 by a slit with size d corresponding to the WG gap. In the near field zone i.e. Ax xdif d2 4X 1.6 X 104 nm where Ax is the distance from the WG end the beam cross section is practically constant d . At Ax Xdif it is possible to note a focusing effect see Fig. 6.7a with a clear narrowing of the cross section. In the far field zone x xdif the beam diverges with the divergence ttX d. 6.3.2 Radiation from an Incoherent Source at Short Distance As mentioned before the WG accepts and transmits the coherent part of radiation. WGs are used mostly with synchrotron radiation sources but it has been demonstrated that they can provide useful intensity also with tabletop laboratory sources 15 44 . For the latter we have to investigate the effect of the illumination of the waveguide entrance with spatially incoherent radiation on the properties of the exiting beam. This may enable us to derive the geometrical condition for optimized coupling. For excitation of independent modes in the front coupling waveguide the next condition based on the definition of the transverse coherence length Lc should be fulfilled 2d Lc XX s 6.18 where X is the distance between source and waveguide d is the thickness of the guiding layer 2d is the spatial acceptance of the waveguide s is the size of the source and X is the wavelength of the incoming radiation. If the equality sign holds in 6.18 the distance X can be considered as Xmin the minimum distance for coherent illumination. If d 100 nm s 15 pm and X 0.154 nm then Xmin 20 mm. An incoherent source of radiation in computer .