Reversible beam heater for suppression of microbunching instability by transverse gradient undulators

The microbunching instability driven by beam collective effects in a linear accelerator of a free-electron laser (FEL) facility significantly degrades the electron beam quality and FEL performance. A conventional method to suppress this instability is to introduce an additional uncorrelated energy spread by laser-electron interaction, which has been successfully operated in the Linac Coherent Light Source and Fermi@Elettra, etc. Some other ideas are recently proposed to suppress the instability without increasing energy spread, which could benefit the seeded FEL schemes. In this paper, we propose a reversible electron beam heater using two transverse gradient undulators to suppress the microbunching instability. This scheme introduces both an energy spread increase and a transverse-to-longitudinal phase space coupling, which suppress the microbunching instabilities driven by both longitudinal space charge and coherent synchrotron radiation before and within the system. Finally the induced energy spread increase and emittance growth are reversed. Theoretical analysis and numerical simulations are presented to verify the feasibility of the scheme and indicate the capability to improve the seeded FEL radiation performance.

Figure 1

Layout of the proposed reversible beam heater based on TGUs. The first TGU (T1) is located between the first and the second rf linacs (L1, L2) and the second one (T2) is located downstream of the bunch compressor (BC). The layout also includes several quadrupoles (Q1—Q4), a harmonic cavity (LX) and a sextupole (S1). According to Eq. (6), D1—D4 indicate the relative lengths where Q1 and Q2 are switched off.

Figure 2

Longitudinal phase space (slice energy spread) with energy chirp removed: (a) at the entrance of the reversible system, upstream of the first linac, (b) upstream of the bunch compressor, (c) downstream of the bunch compressor, upstream of the second TGU, (d) at the exit of the system, downstream of the second TGU. Energy chirp is removed up to the second order, and the bunch head is on the right-hand side.

Figure 3

The initial distributions: (a) the energy modulation amplitude for the case of the initial energy modulation, (b) the current profile for the case of the initial density modulation.

Figure 4

Smearing out the density modulation and energy modulation due to the heated energy spread and transverse-to-longitudinal coupling upstream of the bunch compressor. Energy chirp in Fig. 4 is removed up the 2nd order.

Figure 5

The centroid of the horizontal distribution with respect to the longitudinal position before and after the second TGU.

Figure 6

Suppression of the microbunching instabilities due to the initial density modulation and the initial energy modulation, i.e., simulating the initial collective effects driven modulation and the internal collective effects driven modulation. Current profiles (a, d) and longitudinal phase space (b, c, e, f) are presented. Energy chirps up to the 2nd order are removed.

Figure 7

The effects of the rf phase jitter on the electron beam in the system. (a) Normalized projected emittances in transverse phase space, (b) slice energy spread and compression factor in longitudinal phase space.