[ Identification | Description | Input parameters | Benchmarking data | Links | Installation]

Information and benchmarking website for the McStas ESS_butterfly Component

Identification

• Author: Peter Willendrup and Esben Klinkby
• Origin: DTU
• Date: August-September 2016
• Version: September 15th 2016

Description

 COMPONENT Source = ESS_butterfly(sector="N",beamline=5,Lmin=0.1,Lmax=20,dist=2,
                                  cold_frac=0.5, yheight=0.03,focus_xw=0.1, focus_yh=0.1)
 

• A simple ZIP file, including codes usable for McStas 1.12c and McStas 2.x
• An updated component library for McStas 2.3 (also includes other improved components)

3D visualisation

Geometry

Brilliances, geometry adapted from earlier BF2 design

The following features are worth highlighting:

• The brightness data are still based on last years MCNP calculations, based on the Butterfly 2 geometry. As a result, the spatial variation of the brightness across the moderator face should be considered to have an uncertainty of the order of 10%. Detailed information on the reasoning behind the change to the Butterfly 1 geometry can be found in [1] and detailed information on horizontal spatial brightness variation can be found in [2]. The spectral shape has been checked and has not changed significantly.
• A scaling factor has been introduced to in order to account for the decrease in brightness since 2015. To accommodate the influence of the changed geometry, this scaling factor has been applied independently for the cold and thermal contributions and is beamline dependent. It is adjusted to agree with the spectrally-integrated 6cm width data shown in [1],Figure 3.
• To allow future user adjustments of brilliance, the scalar parameters c_performance and t_performance have been implemented. For now, we recommend to keep these at their default value of 1.0.
• The geometry has been updated to correspond within about 2 mm to the geometry described in [1]. This has been done by ensuring that the position and apparent width of the moderators correspond to [1],Figure 2, which has been derived from current MCNP butterfly 1 model.
• The beamport is now defined directly by its sector and number (e.g. 'W' and '5'), rather than giving the angle, as before. [1],Figure 5 shows the geometry of the moderator2, beamport insert and beamline axis for beamline W5. Since the underlying data is still from last years MCNP run, when the brightness was calculated at 10-degree intervals, this means that the spectral curve for the nearest beamport on the grid 5,15,25,35,45,55 degrees is used. The use of this grid has no effect on the accuracy of the geometry or brilliance because of the above- mentioned beamline-dependent adjustments to the brilliance and geometry. See the website [3] for details.

1. DRAFT Release document "Update to ESS Moderators, version 19/9/2016"
2. Release document "Description and performance of the new baseline ESS moderators, CHESS ID ESS-0068256"
3. http://essbutterfly.mcstas.org/ benchmarking website with comparative McStas-MCNP figures
4. html-based, interactive 3D model of moderators and monolith, as seen from beamline N4.
5. Source code for ESS_butterfly.comp at GitHub.

Benchmarking figures (all in EPS graphics format)

• Comparison between MCNP and McStas based
  • Average peak brightneses in sectors N + W
  • Average peak brightneses in sectors E + S
  • Apparent moderator widths in sectors N + W (and related error)
  • Apparent moderator widths in sectors E + S (and related error)
  • Brillance dependence on beamport position in sectors N + W (and related error)
  • Brillance dependence on beamport position in sectors E + S (and related error)
• Thermal brillance variation in each of the sectors
  • North
  • West
  • South
  • East
• Cold brillance variation in each of the sectors
  • North
  • West
  • South
  • East
• Further, one can find plots relevant for each individual beamline, such as
  • Layout of the beamline moderators and monolith (e.g. for E4)
  • Interactive html-based 3D visualisation of beamline moderators and monolith (e.g. for E4).
  • Plot of the moderator emission profiles (e.g. for E4). Dashed lines indicate the position of the "collimators" for measuring brilliance over a certain width. Solid horizontal lines indicate where the moderator width was measured. For a selected list of beamports ( E1, W2, W3, W4, W5, W6, W7, W8, S11 - and a high-statistics version of the same: E1 1e8, W2 1e8 , W3 1e8, W4 1e8, W5 1e8, W6 1e8, W7 1e8, W8 1e8, S11 1e8 ) data have been overlayed with MCNP output, demonstrating a geometrical precision of ~2mm.
  • Plot of a +/- 2cm scan of the collimators (e.g. for E4).
• The individual beamline data are given in the below folders, each for the sectors:
  • North
  • West
  • South
  • East

• Geometrically within ~1 mm (see plot of error in apparent moderator width as fct. of beamline)
• Brilliance wise within a few percent (see plot of error in brightness as fct. of beamline)

We expect to release an MCNP-event-based source model later in 2016, and possibly also new set of brilliance functions for ESS_butterfly.comp. These are expected to include more realistic brilliances in terms of variation across sectors and potentially also performance losses due to engineering reality.

Engineering reality

Input parameters

Links

• Source code for ESS_butterfly.comp at GitHub.
• html-based, interactive 3D model of moderators and monolith, as seen from beamline N4.
• DRAFT Release document "Update to ESS Moderators, version 19/9/2016"
• Release document "Description and performance of the new baseline ESS moderators, CHESS ID ESS-0068256"