Waterbag Bunches

High-brightness electron beam applications such as single-shot Ultrafast Electron Diffraction and X-ray Free Electron Lasers require ultra short, pulsed electron beams with extremely high charge densities. These high charge densities give rise to very strong space-charge forces which generally lead to non-linear and irreversible expansion of the electron bunches, thus spoiling the beam brightness. For many years this was considered an unavoidable problem, which could only be controlled to some degree by slowing the Coulomb explosion by accelerating bunches to relativistic speeds as fast as possible.  There is however one class of charged-particle distributions: the hard-edged, uniformly filled, ellipsoidal bunches, a.k.a. ‘waterbag’ bunches, which are characterized by space-charge fields which are linear functions of position. As a result the Coulomb expansion of these ideal objects is fully self-similar and therefore reversible with linear external electromagnetic fields, i.e. regular charged particle optics. Until recently waterbag bunches were considered purely theoretical textbook idealizations. In 2004 we showed that they can be realized in practice to a very good approximation by femtosecond photoemission with properly shaped laser pulses [1] In 2008 Musumeci et al. demonstrated experimental realization of these objects at relativistic speeds [2]. An important consequence is the fact that it now becomes possible to create high-brightness, space-charge dominated beams at sub-relativistic speeds as well, which is essential for applications such as single-shot Ultrafast Electron Diffraction. We are now working on the full 6D phase space characterization of waterbag bunches at sub-relativistics speeds.

Keywords: High-brightness electron beams, space-charge dominated beams, ultrafast electron diffraction, femtosecond lasers.

People involved: The following people are or have been involved in this project:

PhD studentPeter Pasmans
Former PhD studentsThijs van Oudheusden and Willem op 't Root
MSc studentsBarry van den Ham
Former studentsArjan Klessens
TechniciansEddy Rietman, Jolanda van de Ven, Ad Kemper, Harry van Doorn, and Iman Koole
Staff membersBas van der Geer, Marieke de Loos, and Jom Luiten


[1] O.J. Luiten, S. B. van der Geer, M. J. de Loos, F. B. Kiewiet, and M. J. van der Wiel, Phys. Rev. Lett. 93 094802 (2004)

[2] P. Musumeci, J. T. Moody, R. J. England, J. B. Rosenzweig, and T. Tran, Phys. Rev. Lett. 100, 244801 (2008).