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

Information and benchmarking website for the McStas ESS_butterfly Component

ESS butterfly moderator, August-September 2016 revision



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.


For installation / instructions for updating your McStas installation, please consult the
installation subpage.

3D visualisation

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. References:
  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)

Based on the above data, we estimate that the current McStas model is in agreement with the MCNP BF1 model within

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

An ad-hoc method for future implementation of "engineering reality" is included, use the "performance" parameter to down-scale performance uniformly across all wavelengths.

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
sector str Defines the 'sector' of your instrument position. Valid values are "N","S","E" and "W" "N"
beamline 1 Defines the 'beamline number' of your instrument position. Valid values are 1..10 or 1..11 depending on sector 1
yheight m Defines the moderator height. Valid values are 0.03 m and 0.06 m 0.03
cold_frac 1 Defines the statistical fraction of events emitted from the cold part of the moderator 0.5
target_index 1 Relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically. 0
dist m Distance from origin to focusing rectangle; at (0,0,dist) - alternatively use target_index 0
focus_xw m Width of focusing rectangle 0
focus_yh m Height of focusing rectangle 0
c_performance 1 Cold brilliance scalar performance multiplicator c_performance > 0 1
t_performance 1 Thermal brilliance scalar performance multiplicator t_performance > 0 1
Lmin AA Minimum wavelength simulated  
Lmax AA Maximum wavelength simulated  
tmax_multiplier 1 Defined maximum emission time at moderator, tmax= tmax_multiplier * ESS_PULSE_DURATION. 3
n_pulses 1 Number of pulses simulated. 0 and 1 creates one pulse. 1
acc_power MW Accelerator power in MW 5


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

Generated automatically by McDoc, Peter Willendrup <pkwi@fysik.dtu.dk> / Thu Sep 15 12:08:55 2016