Ray tracing and wave propagation in x-ray regime, primarily meant for modeling synchrotron beamlines and beamline elements
Package xrt (XRayTracer) is a python software library for ray tracing and wave propagation in x-ray regime. It is primarily meant for modeling synchrotron sources, beamlines and beamline elements.
Features of xrt
- Rays and waves. Classical ray tracing and wave propagation via Kirchhoff integrals, also freely intermixed. No further approximations, such as thin lens or paraxial. The optical surfaces may have figure errors, analytical or measured. In wave propagation, partially coherent radiation is treated by incoherent addition of coherently diffracted fields generated per electron.
- Publication quality graphics. 1D and 2D position histograms are simultaneously coded by hue and brightness. Typically, colors represent energy and brightness represents beam intensity. The user may select other quantities to be encoded by colors: angular and positional distributions, various polarization properties, beam categories, number of reflections, incidence angle etc. Brightness can also encode partial flux for a selected polarization and incident or absorbed power. Publication quality plots are provided by matplotlib with image formats PNG, PostScript, PDF and SVG.
- Unlimited number of rays. The colored histograms are cumulative. The accumulation can be stopped and resumed.
- Parallel execution. xrt can be run in parallel` in several threads or processes (can be opted), which accelerates the execution on multi-core computers. Alternatively, xrt can use the power of GPUs via OpenCL for running special tasks such as the calculation of an undulator source or performing wave propagation.
- Scripting in Python. xrt can be run within Python scripts to generate a series of images under changing geometrical or physical parameters. The image brightness and 1D histograms can be normalized to the global maximum throughout the series.
- Synchrotron sources. Bending magnet, wiggler, undulator and elliptic undulator are calculated internally within xrt. There is also a legacy approach to sampling synchrotron sources using the codes ws and urgent which are parts of XOP package. Please look the section Comparison of synchrotron source codes for the comparison between the implementations. If the photon source is one of the synchrotron sources, the total flux in the beam is reported not just in number of rays but in physical units of ph/s. The total power or absorbed power can be opted instead of flux and is reported in W. The power density can be visualized by isolines. The magnetic gap of undulators can be tapered. Undulators can be calculated in near field. Undulators can be calculated on GPU, with a high gain in computation speed, which is important for tapering and near field calculations.
- Shapes. There are several predefined shapes of optical elements implemented as python classes. The inheritance mechanism simplifies creation of other shapes. The user specifies methods for the surface height and the surface normal. For asymmetric crystals, the normal to the atomic planes can be additionally given. The surface and the normals are defined either in local (x, y) coordinates or in user-defined parametric coordinates. Parametric representation enables closed shapes such as capillaries or wave guides. It also enables exact solutions for complex shapes (e.g. a logarithmic spiral) without any expansion. The methods of finding the intersections of rays with the surface are very robust and can cope with pathological cases as sharp surface kinks. Notice that the search for intersection points does not involve any approximation and has only numerical inaccuracy which is set by default as 1 fm. Any surface can be combined with a (differently and variably oriented) crystal structure and/or (variable) grating vector. Surfaces can be faceted.
- Energy dispersive elements. Implemented are crystals in dynamical diffraction, gratings (also with efficiency calculations), Fresnel zone plates, Bragg-Fresnel optics and multilayers in dynamical diffraction. Crystals can work in Bragg or Laue cases, in reflection or in transmission. The two-field polarization phenomena are fully preserved, also within the Darwin diffraction plateau, thus enabling the ray tracing of crystal-based phase retarders.
- Materials. The material properties are incorporated using three different tabulations of the scattering factors, with differently wide and differently dense energy meshes. Refraction index and absorption coefficient are calculated from the scattering factors. Two-surface bodies, such as plates or refractive lenses, are treated with both refraction and absorption.
- Multiple reflections. xrt can trace multiple reflections in a single optical element. This is useful, for example in ‘whispering gallery’ optics or in Montel or Wolter mirrors.
- Non-sequential optics. xrt can trace non-sequential optics where different parts of the incoming beam meet different surfaces. Examples of such optics are poly-capillaries and Wolter mirrors.
- Global coordinate system. The optical elements are positioned in a global coordinate system. This is convenient for modeling a real synchrotron beamline. The coordinates in this system can be directly taken from a CAD library. The optical surfaces are defined in their local systems for the user’s convenience.
- Beam categories. xrt discriminates rays by several categories: good, out, over and dead. This distinction simplifies the adjustment of entrance and exit slits. An alarm is triggered if the fraction of dead rays exceeds a specified level.
- Portability. xrt runs on Windows and Unix-like platforms, wherever you can run python.
- Examples. xrt comes with many examples; see the galleries, the links are at the top bar.
numpy, scipy and matplotlib are required. If you use OpenCL for calculations on GPU or CPU, you need AMD/NVIDIA drivers, Intel CPU only OpenCL runtime (these are search key words), pytools and pyopencl.
Python 2 and 3
The code can run in both Python branches without any modification.