Operating Modi BESSY II

Standard Fill Pattern

MultiBunch Fillpattern

The new filling pattern consists of a Hybrid (or Camshaft) bunch at 4 mA (Chopper) in the center of the 200 ns wide ion clearing gap followed by the so-called PPRE-bunch of variable transverse excitation at 3 mA and 84 ns later. Together with the usual multibunch filling and the 3 slicing bunches on top of the multibunch train, now 302 out of 400 possible buckets in the storage ring are filled and topped up. Image: HZB

Standard Fill Pattern (Multi Bunch Hybrid)

Since July 2015 BESSY II has been providing a new bunch filling pattern in Top-Up mode. It will open new opportunities especially for research teams dealing with time-resolved x-ray experiments. It is of significant importance for us and the community anticipating BESSY VSR.

Apart from ultrafast experiments at the Femtoslicing facility (slicing bunches) and x-ray pump-probe applications with the hybrid (or camshaft) bunch, now also time-of-flight experiments with the ARTOF and other instruments that use the pulse selection of the MHz-Chopper [1]  can be carried out in normal mode.

The new additional bunch in the dark gap arriving 84 ns after the chopper bunch can be transversally excited to support time-resolved Photoelectron- and X-ray spectroscopy based on the PPRE-technique [2]. Having such time-resolved methods at hand in the regular usermode we are even now able to take a glimpse into future operation modes at BESSY VSR.

Single Bunch & Few Bunch Operation

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 Multi Bunch


Low-alpha Multi Bunch Hybrid Mode

Low-alpha Multi Bunch Hybrid Mode

In the Low-alpha Multi Bunch Hybrid Mode the momentum compaction factor 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, 3ps (7ps FWHM) compared to 15 ps (35ps FWHM) in normal modes, providing an option for time-resolved studies by using the hybrid bunch. Currently, we offer low-alpha operation two weeks a year. Each week two modes are provided in alternating shifts:
Mode A:   15 mA results 50 μA per bunch,
Mode B: 100 mA results 300 μA per bunch

Standard MB vs Low Alpha

  MB-Hybrid Top Up Low Alpha
Ringcurrent 300mA

< 100mA (injection 7am, 3pm)

<   15mA (injection 11pm)

Camshaft (Pos 201) 4.0mA

Current = as other Bunches

(max 4.5ps)


3 x 4.0mA

(separation 12ns)



(Pos 243)

Current = as other Bunches

(Pos 177)

low current (Pos 225) --- 30μA
Lifetime >5.0h >10h / >20h
Purity (Camshaft) >1000 (Range +/-40ns) >1000 (Range +/-40ns)
Gap 200ns 128ns