Arc and Chicane Bunch Compression Schemes for Hard and Soft X-Ray Free Electron Laser Facilities: A Comparison

Abstract

X-ray free-electron laser (XFEL) facilities require progressive compression of electron bunches as they are accelerated from an injector to the undulators. This is necessary to achieve the peak currents required for efficient lasing, without compromising transverse brightness. In the present generation of XFELs, high peak currents are achieved by means of a sequence of four-dipole bunch compression chicanes. It is well known that these systems are not ideal in that they allow projected emittance dilution at the percent level, and they exhibit amplification of microbunching, which typically must be controlled through the otherwise unwanted addition of slice energy spread by use of a laser heater. Both emittance dilution and microbunching are mediated through coherent synchrotron radiation that occurs within a bunch compression chicane. In this paper we introduce a new option for bunch compressors, that of full arc compression, and compare it to the standard four-dipole chicane and to a recently proposed variant, the five-dipole CSR mitigating chicane. It is shown that the arc compressor and the five-dipole chicane are able to give greatly improved XFEL performance compared to the standard four-dipole chicane, both in soft and hard X-ray regimes. This is demonstrated in the context of two proposed XFELs, SXL at MAX-IV, Sweden, and UK-XFEL. It is further shown that the optimal choice of compression option depends on the particular FEL scheme. This means that a simultaneous multi-FEL facility, such as UK-XFEL, must implement both arc and five-dipole methods and must be able to select between them on a bunch-by-bunch basis scheme. This means that a simultaneous multi-FEL facility, such as UK-XFEL, must implement both arc and five-dipole methods and must be able to select between them on a bunch-by-bunch basis.

Figures (20)

• Figure 1: Configurations of three different bunch compressor schemes. Left: symmetric C-chicane. Middle: five-dipole chicane. Right: arc (achromat) with optics balance. Red arrows show the direction and magnitude (not to scale) of the CSR-induced changes in horizontal momentum of particles. The CSR kicks would approximately cancel each other in the five-dipole chicane and double-bend achromat.
• Figure 2: Layout of linac with arc (top), symmetric C-chicane (middle), and five-dipole chicane (bottom) bunch compressors. The arc bunch compression scheme uses the proposed SXL beamline at MAX IV. Existing Short Pulse Facility (SPF) and diagnostics (TDC) beamlines at MAX IV are also shown.
• Figure 3: Longitudinal phase space distributions of electron bunches from arc (top), symmetric C-chicane (middle), and five-dipole chicane (bottom) compression schemes for bunch charges: $100\,pC$ (left) and $10\,pC$ (right).
• Figure 4: Current (top), slice energy spread (middle) and horizontal slice emittance (bottom) of electron bunches from arc (blue), symmetric C-chicane (red), and five-dipole chicane (green) compression schemes for bunch charges: $100\,pC$ (left) and $10\,pC$ (right).
• Figure 5: The FEL pulse energy (top left), duration (top right), spectral brightness (bottom left), and relative bandwidth (bottom right) as a function of distance $s$ along the undulator. Results from GENESIS simulations using $100\,pC$ bunches are shown, with bunches from the arc (blue), symmetric C-chicane (red), five-dipole chicane (green) and arc-2 (magenta) compression schemes. The thick solid lines shows the average of $5$ random shots.