As of January 2002, DESY has also been operating a small linear accelerator in Zeuthen. This Photo Injector Test Facility, also known as PITZ, is used for the development and optimization of laser-driven radio-frequency electron sources such as those that will be needed for the European XFEL and the International Linear Collider ILC.
PITZ – test stand for electron sources
Such facilities require electron beams of the highest quality. In practice, this means that the electron bunches in the beam must be very short and also possess an extremely low emittance. The latter property, which depends on the size and the divergence of the beam, is an important measure of the quality of a particle beam: the lower the emittance, the better the beam can be focused.
At the PITZ test facility, work is under way to develop and test electron sources that produce a particle beam of the requisite high quality. In addition to developing computer simulations and refining theoretical approaches, the scientists at PITZ are also investigating the processes involved in the generation, acceleration and shaping of the electron bunches. Their aim is to optimize beam quality and the operating parameters, such as reliability, of the particle sources.
- Test facility with a linear accelerator at DESY in Zeuthen
- Used for the development and optimization of electron sources
- Length: approx. 12 metres
- Commissioning: 2002
In order to produce electron beams with the requisite quality for the European XFEL, the scientists at PITZ use a laser to fire high-precision pulses of ultraviolet light at a photocathode located at the head of a cavity resonator. Due to the photoelectric effect, the laser pulses release electrons from the cathode, which are drawn off and accelerated by a radio-frequency electromagnetic field.
The main factor impairing the quality of the resulting electron bunches is the fact that all the particles repel one other because they all have the same charge. The only way to satisfactorily collimate the particles is by accelerating them rapidly and applying an external magnetic field that acts like a lens. The particles then leave the cavity resonator as a dense bunch and fly along the beam pipe, where their properties are precisely measured.
In this way, the scientists can build up an exact picture of the physical processes involved in their generation, which in turn enables them to enhance the quality of the electron source. Measurements made include the temporal length, energy distribution and spatial extension of the electron bunches. In addition, the scientists determine the extent to which the particles’ trajectories are parallel to one another. The better these parameters can be optimized, the greater the intensity of the light that will later be generated in the undulators of the European XFEL.
At the end of 2008 a new photocathode laser developed by the Max Born Institute in Berlin was installed at PITZ. It delivers special ultraviolet pulses – so-called flat-top pulses – that are specifically tailored to the beam dynamics of the injector. The facility was also equipped with a new electron source, the copper resonator of which displays much improved vacuum properties thanks to a special dry-ice cleaning process developed in Hamburg. In particular, the photo injector’s dark current is considerably less than that of the previous source. As a result, it is possible to produce electron beams with a very low transverse emittance – i.e. beams which have an extremely small diameter and do not diverge – as required for the European XFEL.