Christopher Behrens

A multi-channel THz and infrared spectrometer for femtosecond electron bunch diagnostics by single-shot spectroscopy of coherent radiation

Abstract The required high peak current in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. In this paper, a frequency-domain diagnostic method is described that is capable of resolving structures in the femtosecond regime. A novel in-vacuum spectrometer has been developed for spectroscopy of coherent radiation in the THz and infrared range. The spectrometer is equipped with five consecutive dispersion gratings and 120 parallel readout channels; it can be operated either in short (5– 44 μ m ) or in long wavelength mode (45– 430 μ m ). Fast parallel readout permits the spectroscopy of coherent radiation from single electron bunches. Test measurements at the soft X-ray free-electron laser FLASH, using coherent transition radiation, demonstrate excellent performance of the spectrometer. The device is planned for use as an online bunch profile monitor during regular FEL operation.

Electron beam profile imaging in the presence of coherent optical radiation effects

High-brightness electron beams with low energy spread at existing and future x-ray free-electron lasers are affected by various collective beam self-interactions and microbunching instabilities. The corresponding coherent optical radiation effects, e.g., coherent optical transition radiation, render electron beam profile imaging impossible and become a serious issue for all kinds of electron beam diagnostics using imaging screens. Furthermore, coherent optical radiation effects can also be related to intrinsically ultrashort electron bunches or the existence of ultrashort spikes inside the electron bunches. In this paper, we discuss methods to suppress coherent optical radiation effects both by electron beam profile imaging in dispersive beamlines and by using scintillation imaging screens in combination with separation techniques. The suppression of coherent optical emission in dispersive beamlines is shown by analytical calculations, numerical simulations, and measurements. Transverse and longitudinal electron beam profile measurements in the presence of coherent optical radiation effects in non-dispersive beamlines are demonstrated by applying a temporal separation technique.

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 beams longitudinal phase space are discussed.

X-Ray Free-Electron Lasers: Technical Realization and Experimental Results

The physical and technological challenges of FELs become quite demanding with decreasing wavelength, but in recent years X-ray FELs with wavelengths in the Angstrom regime (1 A \(=\) 0.1 nm \(=\,10^{-10}\,\)m) have become a reality with the successful commissioning and operation of the “Linac Coherent Light Source” LCLS [1], the world’s first FEL providing atomic resolution. In this chapter, we discuss the most important aspects and challenges of X-ray FELs and present some of the excellent experimental results achieved at LCLS and the second facility of this kind, the “Spring-8 Angstrom Compact free-electron LAser” SACLA [2]. Low-gain FEL oscillators in the X-ray regime have been proposed [3] but not yet demonstrated, they are not considered here. Comprehensive information and technical details on X-ray FELs are found in the design reports of LCLS [4] and the European XFEL [5]. Further valuable information is presented in a review article by Huang and Kim [6].

Low-Gain FEL Theory

We assume the presence of an initial light wave with wavelength λ l which may be provided either by an external source such as an optical laser, or by the spontaneously emitted undulator radiation which is captured in the optical cavity. Following the terminology in laser physics, one speaks of an FEL amplifier if the lasing process is initiated by seed radiation, and of an FEL oscillator if the lasing process starts from spontaneous radiation. The bunches make very many passages through the undulator.

Energy Spread, Space Charge and 3D Effects

The realistic description of FELs operating in the high-gain regime has to be based on a three-dimensional theory. The dependencies of the electron beam current density and of the light wave on the transverse coordinates x and y must be taken into account. Betatron oscillations of the electrons and diffraction of the light wave play an important role.

The EUV and Soft X-Ray FEL in Hamburg

The idea to use a long linear accelerator (linac) for providing the drive beam for an X-ray free-electron laser was conceived at the Stanford Linear Accelerator Center SLAC. In the Linac Coherent Light Source LCLS (see Chap. 9) a 1 km long section of the SLAC electron linac, which has been the major facility for elementary particle physics at Stanford since 1965, delivers the beam needed in the FEL.

Applications of the High-Gain FEL Equations

In the first part of this chapter we want to exploit the third-order differential equation for the amplitude of the FEL wave in order to obtain a deeper understanding of the properties and peculiarities of high-gain free-electron lasers.

Self-Amplified Spontaneous Emission and FEL Seeding

For wavelengths in the extreme ultraviolet and X-ray regime the start-up of the FEL process by seed radiation is hampered by the lack of lasers with the desired wavelength. Seeding by a high harmonic of an optical or infrared laser is a possibility which has been realized in recent years, see Sect. 7.4, where also other seeding schemes are described. The process of Self-Amplified Spontaneous Emission (SASE) permits the start-up of lasing at an arbitrary wavelength, without the need of external radiation.