## 2 Flash Free-Electron Laser Facility
### Upgrade Plans
#### 2.2.2 New Bunch Compressors  
The current FLASH accelerator uses a long 6-dipole S-type chicane [61; 62; 63] as the last compression stage. Recently, an additional bunch compressor section was added to the FLASH2 beamline to apply more flexible compression schemes that reduce the peak current in the extraction section from the FLASH linac to the FLASH2 beamline. Therefore, the final compression for FLASH2 is performed just before the undulator section, which leads to better overall electron and SASE performance. As part of the upgrade plans, the S-type compression section is redesigned to improve the overall compression in the FLASH machine, increase the flexibility of compression schemes, and realize an additional second matching section.  
To accommodate the beam emittance and optics measurement devices, the S-type chicane will be replaced by a shorter 4-dipole D-type chicane (BC2 in Figure 1). The need to balance the request for moderately compressed bunches, with reduced micro-bunching, for seeding (\(\sim\)600-800 A) and stronger compression (\(\sim\)1-2 kA) with significant residual energy chirp for the ultra-short SASE bunches, requires a flexible chicane design which is capable of a variable momentum compaction R\({}_{56}\).  
With the design of the new chicane [64], it has also been considered to allow a correction of the existing intra-bunch transverse to longitudinal correlation. Such a correlation, deriving from wake-fields and coupler kicks, has been demonstrated to be detrimental to the FEL performance, and its effect is expected to be more evident with seeding. With a horizontally movable vacuum chamber, quadrupole/skew-quadrupole packs in the legs of the chicane will be used for correcting such longitudinal to transverse correlations [64; 65; 66]. The new bunch compressor chicane design foresees the two inner dipoles mounted on a girder, which can be moved horizontally on rails, therefore enabling bending angles ranging continuously from 0 to 6\({}^{\circ}\) or equivalently an R\({}_{56}\) from 0 to 100 mm. Microbunching instability has been demonstrated to be critical for the operations of seeded FEL. A non-controlled microbunching leads to strong reductions of the FEL intensity but also to a significant deterioration of the spectral quality. Due to such a high sensitivity to microbunching, existing seeded FEL have shown to better operate with beams compressed in a single bunch compressor. Both FERMI and the Shanghai soft X-ray user facility have experienced reduced FEL performance when operating the accelerator with two bunch compressors [67; 68]. To verify that the double stage compression scheme designed for FLASH2020+ will be capable of supporting external seeding, dedicated numerical and analytical studies of microbunching amplification along the linac have been performed [69; 70; 71].  
Our studies (Figure 2) show that with a small energy spread introduced by the laser heater, the microbunching gain at the end of the linac is limited to a few tens for the FLASH2020+ design and comparable to the one estimated for the typical configuration of the FERMI linac [72]. The simulation results also show that FLASH2020+ double compressor scheme does not exhibit the high gain expected at FERMI when operated with two bunch compressors [73].  
Figure 1: Sketch of the FLASH linac with planned laser heater (LH) and new bunch compressors (BC1, BC2) and the energy gain after the FLASH2020+ upgrade.