New Guides for the NCNR Expansion

The new guides will view the existing liquid hydrogen cold source through the the “CryogenicTube West” (CTW) beam port currently occupied for the development of the MACS spectrometer. MACS will move to beam port BT-9 containing the new cold source.

New guides (colored) viewing the present cold source (blue).

General Features:

Perspective view of new guides, from the beam ends looking back towards the source.

Viewed in the horizontal plane (top view), the guides have a curved section near the source end followed by a straight section. The curved sections permit five beams to be extracted from the restricted angle of the CTW opening and also reduce or eliminate the need for crystal filters upstream usually required in straight guides to remove unwanted short wavelength neutrons and gamma rays. The straight portions help to minimize beam asymmetries after transmission through the curved sections.

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.

 

A conceptual design for guide NG-C is shown above in which a random selection of 4 Å 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.  In the top view the curved guide sections near the source contain elements approximating to a parabola along a curve, increasing the beam width to 11 cm. The vertical profile is quasi-elliptical with lower m supermirror coatings used near the center where beam divergence is minimal.

General Layout: 

Four initial guides splitting into five:

•  NG-A, is designed for optimal intensity within about a 4 cm x 4 cm region at the sample, for the neutron spin-echo spctrometer relocated from its current position at NG-5. The guide also allows the existing velocity selector to be used.

• NG-B which will be split vertically into two guides: the lower one is directed towards the 10 m SANS and the upper one is directed towards the 30 m SANS. The final straight sections of these guides will use natural Ni coatings and will have a 5 cm x 5 cm cross-section.

• NG-C, being developed for the Physics Station , is designed to have an 11 cm x 11 cm beam at its exit.

• NG-D directed towards the vertical sample reflectometer relocated from NG-1 and MAGIK . The guide then converges to provide small area, high flux beams for PGAA and CNDP.

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last modified 04 April 2008