Electron Beam Diagnostics

two-dimensional CCD image of a single electron bunch

Top: two-dimensional CCD image of a single electron bunch whose time profile is translated into a horizontal coordinate on an observation screen. The head of the bunch is at the left side. Bottom: a computed temporal charge profile as function of time. One observes a sharp peak at the head with a full width at half maximum of 65 femtoseconds, followed by a long tail. The sharp peak contains about 20 percent of the bunch charge; only here the local charge density is high enough to obtain a large gain in the SASE process.

Pulse shape of single electron bunches

Pulse shape of single electron bunches measured with the spectral decoding method. Left column: wrong RF phase in the first accelerator module. The bunches develop a double peak structure. Right column: correct RF phase, leading to optimum compression of the bunch.

The requirements on electron beam quality are very demanding and in some cases at the limit of present-day technology. High-resolution diagnostic instruments are essential for a detailed understanding of the physical principles of emittance preservation, bunch compression and lasing in the SASE mode. We restrict ourselves here to a description of two techniques permitting single-shot direct visualization of longitudinal electron bunch profiles: Transverse Deflecting Structures (TDS) and Electro-Optic (EO) detection systems.

In the TDS the temporal profile of the electron bunch is transferred to a spatial profile on a view screen by a rapidly varying electromagnetic field, analogous to the sawtooth voltage in conventional oscilloscope tubes. The TDS at FLASH is a 3.6-m-long traveling-wave structure operating at 2.856 GHz, in which a combination of electric and magnetic fields produces a transverse kick for the electron bunches. The bunches pass the TDS near zero crossing of the RF field and receive no net deflection but are streak­ed in the transverse direction. A single bunch out of a train can be streaked. With a fast k­ick­er magnet, this bunch is deflected towards a view screen that is photographed by a CCD camera. The other electron bunches are not affected.

The electro-optic effect offers the possibility to measure the longitudinal charge distribution in the electron bunches with 50- to 100-femtosecond resolution. The principle is as follows: The electric field of the relativistic bunch induces an optical birefringence in a crystal such as gallium-phosphide, which is then probed with femtosecond titanium-sapphire laser pulses. One EO experiment is installed in the FLASH linac between the last accelerator module and the undulator. The linearly polarized laser pulse acquires an elliptical polarization in the crystal which is converted into an intensity modulation. Single-shot measurements of individual bunches are possible by spectral, temporal or spatial decoding methods. These data are very useful for accelerator diagnostics. For instance, wrong parameters in the bunch compression scheme are
immediately visible from the reconstructed bunch shape.

The EO experiment has a lower time resolution than the TDS but has the advantage of being non-destructive: The same bunch which has been analyzed with the EO system can be used to generate FEL radiation downstream. Moreover, the EO signals can be utilized as arrival time signals of the FEL pulses in pump-and-probe experiments.

Detailed information on how the precise timing of femtosecond pump-and-probe pulses is achieved at FLASH is presented in the next chapter which describes the user facility in the experimental hall, while the then following chapter on the first experimental results includes an elegant experiment showing that the two extremely short pulses do in fact overlap in space and time.