Reversible electron beam heating for suppression of microbunching instabilities at free-electron lasers

The presence of microbunching instabilities due to the compression of high-brightness electron beams at existing and future x-ray free-electron lasers (FELs) results in restrictions on the attainable lasing performance and renders beam imaging with optical transition radiation impossible. The instability can be suppressed by introducing additional energy spread, i.e., heating the electron beam, as demonstrated by the successful operation of the laser heater system at the Linac Coherent Light Source. The increased energy spread is typically tolerable for self-amplified spontaneous emission FELs but limits the effectiveness of advanced FEL schemes such as seeding. In this paper, we present a reversible electron beam heating system based on two transverse deflecting radio-frequency structures (TDSs) upstream and downstream of a magnetic bunch compressor chicane. The additional energy spread is introduced in the first TDS, which suppresses the microbunching instability, and then is eliminated in the second TDS. We show the feasibility of the microbunching gain suppression based on calculations and simulations including the effects of coherent synchrotron radiation. Acceptable electron beam and radio-frequency jitter are identified, and inherent options for diagnostics and on-line monitoring of the electron beam’s longitudinal phase space are discussed.

Figure 1

Layout of a reversible electron beam heater system including two transverse deflecting rf structures located upstream and downstream of a magnetic bunch compressor (BC) chicane, and longitudinal phase space diagnostics using screens and synchrotron radiation monitors. Parameters related to the reversible beam heater system are denoted in curly brackets.

Figure 2

Relevant accelerator optics (Twiss parameters) and positions of the transverse deflecting rf structures used to numerically demonstrate the reversible beam heater system.

Figure 3

Simulation of the longitudinal phase space after removing the correlated energy chirp: (a) Upstream of the first TDS, (b) directly downstream of the first TDS, (c) directly downstream of the bunch compressor and upstream of the second TDS, and (d) downstream of the second TDS. The axes scales change from (b) to (c) when bunch compression takes place. The bunch head is on the left, i.e., where $z/c<0$.

Figure 4

Simulations without CSR effects on the impact of the reversible heater system on projected emittances, core energy spread, and slice energy spread: (a) Projected emittances (normalized) and core energy spread, and (b) slice energy spread for $V_2$ at minimum emittance [see Fig. 4a]. The longitudinal coordinate is normalized to the bunch length.

Figure 5

Simulation on the impact of the reversible beam heater system on projected emittances, core energy spread, and slice energy spread: (a) Projected emittances (normalized) and core energy spread, and (b) slice energy spread for $V_2$ at minimum emittance [see Fig. 5a]. CSR effects are included by means of the 1-dimensional model in elegant [35].

Figure 6

Simulation of the normalized slice emittance for both the vertical and horizontal upstream of the first and downstream of the second TDS. CSR effects are included.

Figure 7

Simulation on suppression of microbunching instabilities due to an initial density modulation, i.e., simulating CSR-driven microbunching. The entire longitudinal phase space, after removing the correlated energy chirp, is shown.

Figure 8

Simulation on suppression of microbunching instabilities due to an initial energy modulation, i.e., simulating longitudinal space charge driven microbunching. For the sake of clarity, only the core of the longitudinal phase space, after removing the correlated energy chirp, is shown.