A magnetic chicane also called a bunch compressor helps form dense bunches of electrons in a free-electron laser.[1][2] A magnetic chicane makes electrons detour slightly from their otherwise straight path, and in that way is similar to a chicane on a road.
A magnetic chicane consists of four dipole magnets, giving electrons at the beginning of a bunch a longer path than electrons at the end of the bunch, thereby allowing the lagging electrons to catch up.[3][4][5]
Free-electron laser
editA free-electron laser depends upon a beam of tightly bunched electrons. Short bunches of electrons are produced by a photoinjector, but they quickly grow, because electrons have negative charge and little mass, causing the bunch to expand. As the bunch is accelerated, the electrons gain mass and quickly approach the speed of light. After that, electrons at the end of the bunch cannot go any faster to catch up with electrons at the beginning of the bunch.
Chirp
editThis problem is solved by adjusting the phase of the driving electric field to more strongly add energy and mass to electrons at the trailing end of the bunch. This is called negative energy chirp, meaning the energy decreases along the direction of beam travel.[6] Because the beam is traveling at almost the speed of light, the trailing electrons gain mass, rather than velocity. This results in a correlation between mass and position in the bunch.
Chicane
editThe chicane gives lagging electrons time to catch up. More massive electrons are deflected less by the magnetic field than lighter electrons, and therefor take a shorter path through the chicane, resulting in a shorter bunch. A chicane consists of four dipole magnets with the following roles:
- Deflects the beam slightly away from the central axis of the accelerator, with lighter electrons deflected more than more massive electrons.
- Deflects the beam in the opposite direction, making it parallel to the central axis, but with an offset. The offset is greatest for lighter electrons.
- Deflects the beam back towards the central axis.
- Deflects the beam back in the direction of the central axis.
Limitations
editIn practice, bunch compression cannot be done a single step. To avoid beam emittance blowup, beam compression is usually done by using two chicanes.[7]
References
edit- ^ Pellegrini, C.; Marinelli, A.; Reiche, S. (2016). "The physics of x-ray free-electron lasers". Reviews of Modern Physics. 88 (1): 015006. Bibcode:2016RvMP...88a5006P. doi:10.1103/RevModPhys.88.015006.
- ^ "X-Ray free-electron lasers" (PDF). Retrieved November 21, 2022.
- ^ Hastings, J.; Pellegrini, C.; Marinelli, A. (2020). Physics of and Science with X-Ray Free-Electron Lasers. IOS Press. ISBN 9781643681337. Retrieved November 21, 2022.
- ^ "A magnetic chicane for bunch compression". ELBE Center for High-Power Radiation Sources. Retrieved November 21, 2022.
- ^ Nathan W. Ray; Vida-Michelle Nixon; Matthias Fuchs (2018). "Optimization of Magnetic Chicane for Maximum Electron Beam Compression". Retrieved November 21, 2022.
- ^ Emma, P.; Venturini, M.; Bane, K. L. F.; Stupakov, G.; Kang, H.-S.; Chae, M. S.; Hong, J.; Min, C.-K.; Yang, H.; Ha, T.; Lee, W. W.; Park, C. D.; Park, S. J.; Ko, I. S. (2014). "Experimental Demonstration of Energy-Chirp Control in Relativistic Electron Bunches Using a Corrugated Pipe". Physical Review Letters. 112 (3): 034801. Bibcode:2014PhRvL.112c4801E. doi:10.1103/PhysRevLett.112.034801. PMID 24484143.
- ^ Ultraviolet and Soft X-Ray Free-Electron Lasers. Springer Tracts in Modern Physics. Vol. 229. 2009. p. 131. doi:10.1007/978-3-540-79572-8. ISBN 978-3-540-79571-1. Retrieved November 24, 2022.
External links
edit- RF and Space Charge Emittance in Guns, a basic definition of emittance
- Space Charge Induced Beam Emittance Growth and Halo Formation
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