Passive Linearization of the Magnetic Bunch Compression Using Self-Induced Fields

In linac-driven free-electron lasers, colliders, and energy recovery linacs, a common way to compress the electron bunch to kiloampere level is based upon the implementation of a magnetic dispersive element that converts particle energy deviation into a path-length difference. Nonlinearities of such a process are usually compensated by enabling a high harmonic rf structure properly tuned in amplitude and phase. This approach is however not straightforward, e.g., in $C$-band and $X$-band linacs. In this Letter we demonstrate that the longitudinal self-induced field excited by the electron beam itself is able to linearize the compression process without any use of high harmonic rf structure. The method is implemented at the FERMI linac, with the resulting high quality beam used to drive the seeded free-electron laser during user experiments.

Figure 1

FERMI linac layout including the rf photocathode gun, linacs L00, L01, L02 (SLAC-type traveling wave accelerating structures), L03 and L04 (high-impedance BTW accelerating structures), the high harmonic cavity (i.e., $X$-band rf section), and the two magnetic bunch compressors BC1 and BC2. The high energy deflecting cavity (HERFD) and the diagnostic beam dump (DBD) are placed after L04.

Figure 2

LiTrack simulation results. (a) Current profile of a 700 pC bunch at the FERMI injector exit at about 100 MeV (blue line) and after BC1 (red line), which is set with $R_{56,\mathrm{BC}1}=-41\mathrm{mm}$. (b) The wake potential in L03 induced by the electron bunch compressed in BC1 when L03 is composed of BTW sections (blue) or TW SLAC-type sections (red). Current distribution (c) and longitudinal phase space (d) at the end of the linac obtained with (blue line) and without (red line) inclusion of the longitudinal wakefields in the L02 and L03 sections. The head of the bunch is on the left.

Figure 3

(a) Longitudinal phase space of a 700-pC electron bunch measured by stretching the electron beam with the rf deflector and energy dispersing it with the DBD spectrometer. (b) Elegant simulation results in the same condition. The head of the bunch is on the left.

Figure 4

Single‐shot FEL spectrum at 25.9 nm obtained in the optimized configuration. The inset shows the FEL gain curve at 25 nm measured progressively detuning each radiator undulator.

Figure 5

(a) LiTrack simulated current profiles after BC1 for different compression factors and (b) the corresponding induced voltage in the PSI-DLW device. (c) Beam temporal profiles measured after BC2 as a function of ${\varphi }_{\mathrm{L}01}$ and ${\varphi }_{\mathrm{L}02}$, keeping constant the bunch length after BC2. (d) Measured longitudinal phase space of a shorter electron bunch compressed to about 700 A. In all cases the charge per bunch is 700 pC and the head of the bunch is on the left.