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The new guides will view the existing liquid hydrogen cold source
through the "Cryogenic Tube West" (CTW) beam port that is currently
occupied for the development of the
MACS
spectrometer. MACS will move to beam port BT-9 which will contain
the new cold source.
Again, viewed from the top, in order to promote total reflection from the outer curve for the minimum wavelength required by the instruments served, a greater critical angle is needed compared to the inner curve. Thus the supermirrors there have a higher m value (the effective supermirror critical wavevector transfer to that of natural Ni). In cases where the beam divergence must be restricted (e.g., for the SANS or NSE instruments), lower critical angle, high reflectivity coatings are used on the straight portions and inner radii of the guides (guides NG-A and NG-B, see "General Layout" below), For high flux guides (NG-C and NG-D), high m supermirrors are used. Because the reflectivity of these supermirrors usually decreases with increasing angle and m value, focusing and defocusing sections have been used to increase the transmission efficiency of these guides as well as for beam reshaping. One extreme example is the use of a quasi-elliptical vertical guide profile for guide NG-C shown in the figure below. FIGURE 3. A conceptual design for guide NG-C. A conceptual design for guide NG-C is shown above in which a random selection of 4 Angstrom wavelength trajectories make at least one reflection from the top or bottom surface. (Straight-through trajectories are omitted for clarity.) The colored portions represent supermirror regions having different "m" values (reflectivity cutoffs). In the top view the curved guide sections near the source contain elements approximating a parabolic curve, increasing the beam width to 11 cm. The vertical profile is quasi-elliptical with smaller "m" supermirror coatings used near the center, where beam divergence is minimal. Four initial guides become five:
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Expansion home | Overview | Views | BT-9 cold source | Guides | Instrumentation | User services | Time lines | Occasional diary | Photos | FAQs | Contacts |