The code POSINST was developed starting in 1994 at LBNL following the discovery of the photoelectron instability at the Photon Factory at KEK. The initial object of the code was to assess a similar potential instability at the PEP-II B Factory, then under design. In the 19 years since then, the code has been refined, augmented, benchmarked against other similar codes, and validated against experimental results at many storage rings worldwide.
POSINST belongs to a class of codes usually referred to as build-up codes. These codes make the approximation that the beam is a prescribed function of space and time, and therefore is non-dynamical. The electrons, on the other hand, are fully dynamical.
For a significant class of uses in modeling high-energy bunched-beam storage rings, the longitudinal motion of the electrons in the cloud is negligible compared to the transverse motion; hence the code makes the approximation that the dynamics is 2D. Simulations with POSINST thus allow a detailed analysis of the electron cloud build-up, dissipation, time scales and energy/angle distributions in a specific, short, portion of a ring (e.g., a given dipole magnet, or a field-free region).
There are regions in a storage ring, however, in which the longitudinal electron motion is essentially 3D, such as magnetic fringe fields, wigglers, etc. In such cases, 3D codes are required. However, for typical high-energy storage rings, such regions add up to a small fraction of the machine circumference, hence it is a useful and meaningful first approximation to assess of the electron cloud at various locations around the ring with 2D codes.
POSINST allows only a very limited study (via the computation of the dipole wake) of the effects that the cloud exerts back on the beam , and then only in very specific cases, but provides an accurate description of the local e-cloud density distribution. Another class of codes, namely 3D self-consistent codes, allow the study of the dynamics of the electron cloud and the beam under their mutual, simultaneous influence. These codes are vastly more computationally demanding than build-up codes.
The main strength of POSINST lies in a detailed model of secondary electron emission, embodied in the code. A detailed model of photoemission has recently been added. In a typical application, one provides as input to the code the required parameters to fully describe photoemission and secondary emission, along with a description of the beam. The code then provides, as output, the evolution of the electron-cloud intensity and spatio-temporal distribution. By comparing the results against experimental measurements, one can adjust the input parameters and iterate the process, thereby arriving at a detailed description of the electronic properties of the vacuum chamber surface.
POSINST embodies the following features:
- Detailed model for secondary emission.
- Detailed model for photoemission.
- Simplified model for ionization of residual gas by the beam optionally included.
- Simplified model for electron generation from stray beam particles striking the vacuum chamber walls optionally included.
- 2D beam-electron forces, including image charges from the beam and the cloud electrons
- 2D electron space-charge forces, including surface charges.
- Transverse and longitudinal beam profile selectable from a class of functions such as Gaussian, parabolic, etc.
- Transverse profile of the chamber may be elliptical or rectangular.
- Antechamber slots in the chamber optionally included.
- 3D electron kinematics (the 3rd dimension, along the direction of the beam, has been included with a future view to a fully 3D code).
POSINST is written in Fortran 90. All input and output files are in plain text format. The code makes use of the free library DCDLIB.
POSINST is often used together with WARP.
To Learn More…
These are the principal references on POSINST.
- M. A. Furman and G. R. Lambertson, “The Electron-Cloud Instability in the Arcs of the PEP-II Positron Ring” (PDF), LBNL-41123/CBP Note-246, PEP-II AP Note AP 97.27, November 25, 1997; Proc. Intl. Workshop on Multibunch Instabilities in Future Electron and Positron Accelerators (MBI-97), KEK, Tsukuba, Japan, 15-18 July 1997, KEK Proceedings 97-17, Dec. 1997 (Y. H. Chin, ed.), p. 170.
- M. A. Furman, “The Electron-Cloud Effect in the Arcs of the LHC” (PDF), LBNL-41482/CBP Note 247/LHC Project Report 180, May 20, 1998.
- M. A. Furman and M. T. F. Pivi, “Probabilistic Model for the Simulation of Secondary Electron Emission” (PDF), LBNL-49771/CBP Note-415, November 6, 2002; PRST-AB paper v5/i12/e124404 (2003). Erratum: PRST-AB 16, 069901(E) (2013)
- M. A. Furman and M. T. F. Pivi, “Simulation of Secondary Electron Emission Based on a Phenomenological Probabilistic Model” (PDF), LBNL-52807/SLAC-PUB-9912, June 2, 2003.
- M. A. Furman and M. T. F. Pivi, “Simulation of Secondary Electron Emission Based on a Phenomenological Probabilistic Model”, LBNL-52807/SLAC-PUB-9912, June 2, 2003 (http://mafurman.lbl.gov/LBNL-52807.pdf).
Technical correspondence on POSINST is handled by its developer, Miguel Furman.