Predictive 3D theory of microbunching gain through compressors
In plain words
Engineers estimate how much the density ripples grow, but measured growth often differs from predictions. A theory that predicts it reliably in three dimensions is missing.
Precise statement
For a linac with laser heater, chicanes and transport, predict the microbunching gain spectrum $G(\lambda)$ for $\lambda\sim 0.1\text{ to }100\,\mathrm{micron}$ including 3D coherent synchrotron radiation, longitudinal space charge, intrabeam scattering and transverse-longitudinal coupling. An answer reproduces measured bunching spectra (for example via coherent optical transition radiation) within a factor of two at each wavelength.
What would settle it
Start-to-end predictions agreeing with measured gain spectra on at least two XFELs without fitted parameters.
Status in the literature
1D linear gain theory with laser-heater suppression works for gross trends; quantitative 3D agreement is lacking.