General accelerator physics to be investigated:

  • space charge effects, especially in the low energy parts
  • ion trapping
  • beam break up
  • coherent synchrotron radiation
     

Coherent Synchrotron Radiation (CSR) and wake fields

One of the important features of ERL-based light sources is the possibility to produce short electron bunches with the length of down to 3 micron (or duration of 10 fs). However, so short bunches radiate coherently at the wavelengths above approximately the bunch length. The power of coherent radiation of such a beam can be very high, if the average current is high:


Here Q is the bunch charge, R is the bending radius in dipole magnets, l0 is the bunch length, the power from all magnets (full turn of 360°) is taken into account, numerical coefficient can appear depending on the longitudinal electron distribution in the bunch . Estimations show, e.g. for a 100 fs full charge (77 pC) bunches with full filling (1.3 GHz bunch repetition rate) in bERLinPro average CSR power would be about 30 kW, that is a serious threat for the accelerator components and vacuum system.

Other beam-induced fields in an accelerator are e.g. the higher oscillating modes in the accelerating cavities, fields of the so called “mirror” charges and currents in the vacuum chamber walls, etc. These fields lead to the beam energy or quality losses too and should be avoided. Careful modelling of the CSR and other beam-induced electromagnetic fields is important for the understanding of the limits of the achievable bunch parameters in the future accelerators.

Fig.: example of "CSR wake" induced energy modulation along a bunch, longitudinal phase space in a.u., white: initial distribution, red: final distribution


 References

[1]  E.L. Saldin, E.A. Schneidmiller, M.V. Yurkov, “On the coherent radiation of an electron bunch moving in an arc of a circle”, Nuc. Instrum. Methods Phys. Res. A, 398, pp. 373–-394, (1997).

[2]  S. Heifets and G. Stupakov, Beam instability and microbunching due to coherent synchrotron radiation, SLAC-PUB-8767, 1/2001