| Input parameters
| Benchmarking data
Information and benchmarking website for the McStas
ESS butterfly moderator, August-September 2016 revision
- Author: Peter Willendrup and Esben Klinkby
- Origin: DTU
- Date: August-September 2016
- Version: September 15th 2016
ESS butterfly moderator with automatic choice of coordinate system, with origin
placed at relevant "Moderator Focus Coordinate System" depending on sector location.
To select beamport N 5 simply use
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)
The example instrument used to generate the below benchmarking data is available here.
The component and all necessary bits will be released mid September 2016 as
As soon as possible we will further provide a full new release, McStas 2.4.
- 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)
For installation / instructions for updating your McStas installation, please consult the installation subpage.
Have a look here for a html-based, interactive 3D model of moderators and monolith, as seen from beamline N4.
The geometry corresponds correctly to the latest release of the butterfly moderator,
including changes warranted by the ESS CCB in July 2016, the so called BF1 type moderator.
A set of official release documents are available with this component, see the benchmarking
website mentioned below.
Brilliances, geometry adapted from earlier BF2 design
The geometry and brightness data implemented in the McStas ESS source component ESS_butterfly.comp,
are released as an updated component library for McStas 2.3, as well as a stand alone archive for
use with earlier versions of McStas.
The following features are worth highlighting:
As before, the beamports all originate at the focal point of the sector. The beamline will in almost all cases be
horizontally tilted in order to view the cold or thermal moderator, which should be done using an Arm component.
- 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  and detailed information on horizontal spatial brightness
variation can be found in . 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 ,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 . This
has been done by ensuring that the position and apparent width of the moderators correspond to ,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. ,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  for details.
- 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"
- http://ess_butterfly.mcstas.org/ benchmarking website with comparative McStas-MCNP figures
- html-based, interactive 3D model of moderators and monolith, as seen from beamline N4.
- Source code for
ESS_butterfly.comp at GitHub.
Benchmarking figures (all in EPS graphics format)
Based on the above data, we estimate that the current McStas model is in agreement with
the MCNP BF1 model within
- Comparison between MCNP and McStas based
- Thermal brillance variation in each of the sectors
- Cold brillance variation in each of the sectors
- 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
- and a high-statistics version of the same:
W2 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:
- 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.
An ad-hoc method for future implementation of "engineering reality" is included, use the
"performance" parameter to down-scale performance uniformly across all wavelengths.
Parameters in boldface are required;
the others are optional.
|Name ||Unit ||Description ||Default|
||Defines the 'sector' of your instrument position. Valid values are "N","S","E" and "W"
||Defines the 'beamline number' of your instrument position. Valid values are 1..10 or 1..11 depending on sector
||Defines the moderator height. Valid values are 0.03 m and 0.06 m
||Defines the statistical fraction of events emitted from the cold part of the moderator
||Relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically.
||Distance from origin to focusing rectangle; at (0,0,dist) - alternatively use target_index
||Width of focusing rectangle
||Height of focusing rectangle
||Cold brilliance scalar performance multiplicator c_performance > 0
||Thermal brilliance scalar performance multiplicator t_performance > 0
||Minimum wavelength simulated
||Maximum wavelength simulated
||Defined maximum emission time at moderator, tmax= tmax_multiplier * ESS_PULSE_DURATION.
||Number of pulses simulated. 0 and 1 creates one pulse.
||Accelerator power in MW
| Input parameters
| Benchmarking data
Generated automatically by McDoc, Peter Willendrup
Thu Sep 15 12:08:55 2016