Department Accelerator Operation and Technology

Operating Modi BESSY II

The filling pattern consists of a main train, a single bunch at 4 mA in the center of the 56 ns wide ion clearing gap, two so-called PPRE-bunches of variable transverse excitation at 3 mA.  On top of the main bunch train there are 7 slicing bunches. In total 373 out of 400 possible buckets in the storage ring are filled and topped up. Image: HZB

Standard User Filling Pattern

BESSY II provides Top-Up operation with a complex bunch filling pattern.

The main bunch train provides for average brightness / average brilliance users.

In addition, there are the slicing bunches for ultrafast experiments at the Femtoslicing facility and x-ray pump-probe applications with a single bunch in a filling gap.
A new feature are two transversally excited high current bunches in the main train to support time-resolved Photoelectron- and X-ray spectroscopy based on the PPRE-technique.

Single Bunch

In Single Bunch Operation only one bucket (no 201) is populated with electrons. The photon pulse derived from this bunch returns every 800 ns  (1.25 MHz) according to the ring energy and the circumference. The purity of this single bunch, i.e. the ratio of electrons in the bunch to the number of electrons in other bunches, is 10⁴. Single bunch operation is a unique tool for some time-resolved experiments. We offer two weeks of Single bunch operation twice a year with a bunch current of 13 mA in Top Up Mode.

Few Bunch

To extend the range for time-resolved experiments, BESSY offers a Few Bunch Mode of 4 bunches every sunday during the Single Bunch Operation weeks. The bunches are stored on the bucket positions 1, 101, 201 and 301 providing a photon pulse repetition rate of 5 MHz (200 ns). The total current of 30 mA is equally distributed on all four bunches.

Low-alpha User Operation

Low-alpha User Operation

In the Low-alpha User Operation the momentum compaction factor alpha is modified to smaller values by a special setting of the electron optics of the storage ring. Thus, the lengths of the electron bunches are decreased giving rise to coherent synchrotron radiation in the THz spectral region. Also, the photon pulse lengths are shortened compared standard mode. This provides an option for time-resolved studies by using the Camshaft Bunch (see table below). Currently, we offer low-alpha operation during two weeks of a year. Within a low-alpha week, two sub-modes are possible:

Mode A (on demand): 15 mA ring current, resulting in 45 μA per bunch; injections at 11pm.
Mode B (default): 100 mA ring current, resulting in 285 μA per bunch; injections at 7am and 3pm. Here, an additional short bunch (≤ 30 µA; approx. 7 ps bunch length FWHM) is added for time-resolved experiments.

Overview: filling patterns

* streak camera measurements
For detailed information on parameter range and stability refer to your local contact or beamline scientist.