Bunch-Picking, Single Bunch Operation and Low-Alpha Mode
The time structure of the photon pulses emitted from the storage ring is given by the time structure of stored electrons in the ring. As BESSY II is based on 500 MHz RF-cavities the maximum number of bunches is 400 which travel close to light velocity along the ring circumference of 240 m. Hence, the arrival frequency of an individual bunch is 1.25 MHz and the distance between adjacent bunches is 2 ns. The whole bunch train can be 800 ns long, which is also the time an electron needs to arrive again after one turn in the ring.
Photons from BESSY II’s Single Bunch Operation
In single bunch mode, which is available 2 weeks/semester, the photon pulses are emitted from 1 single bunch in the ring filled with 20 mA which corresponds to a huge electron bunch charge of 100 nC. The X-ray pulses of high temporal purity >104 arrive with a repetition frequency of 1.25 MHz at the experiment. There is no emission over a “dark” period of 800 ns duration, a feature which is important for several time-resolved X-ray methods like time-of-flight spectroscopy and laser-pump x-ray-probe studies. However, the available time resolution is limited in this case by the current-dependent bunch length of the electrons and is < 100 ps.
X-Ray Pulse Picker
Time-resolved methods require isolated pulses of X-rays but the available temporal patterns are determined by the fill pattern of electrons in the ring. There are several methods to pick certain pulses out of the complicated temporal pattern of the X-rays in regular user mode. One relatively easy way is to use mechanical choppers in the beamline’s and some of them have been indeed routinely equipped with choppers to shape ns-wide macro pulses with kHz repetition rate out of the pulse train pattern. However, the ultimate choppers for storage rings should be able to pick out photons from 1 individual bunch (the hybrid bunch) out of the pulse train with MHz repetition rate. In this case, such a beamline is mimicking a single bunch pattern in regular multibunch operation. We currently follow this route in the framework of a national collaboration.
Picosecond X-Ray Pulses from the BESSY Low-Alpha Mode
Employing the so-called low-alpha mode of the storage ring, which is scheduled for one week per semester as regular user operation, bunch lengths of 2-3 ps (rms) are available on all beamlines at expense of total intensity (storage ring current 15 or 45 mA instead of 300 mA). Since each bunch out of the 351 bunches in the fill pattern has this shortened length, the x-ray pulses can be used to probe at very high duty cycle, up to the storage ring HF of 500 MHz. At the FEMTOSPEX facility, pump-probe experiments at 6 KHz using the synchronization of the hybrid bunch and the slicing laser are routinely performed on systems where the physics is not faster than 10 ps.