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Department Accelerator Operation and Technology

Operating Modi BESSY II

Standard Fill Pattern

MultiBunch Fillpattern - enlarged view

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

LowAlphaFillpattern - enlarged view

Low-alpha Multi Bunch Hybrid Mode

Low-alpha Mode

In the Low-alpha Multi Bunch Hybrid Mode 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 offered in alternating 12 hour shifts:

Mode A: 15 mA ring current, resulting in 45 μA per bunch; injections at 11pm.
Mode B: 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: Standard MB vs Low Alpha


Standard MB-Hybrid
(Top Up operation)

Low Alpha

Mode A

Mode B

Ring current (mA)

≤ 300

≤ 15

≤ 100


bunch length (FWHM)*

0.9 mA

45 ps

45 μA

7 ps

285 μA

16 ps

Camshaft bunch


Camshaft Gap

Purity in Range +/-40 ns

bunch length (FWHM)*

4.0 mA


200 ns

> 1000

 80 ps

≤ 43 μA


128 ns

> 1000

 7 ps

≤ 300 μA


128 ns

> 1000

 16 ps

Slicing bunches


bunch length (FWHM)*

7 x 3.0 mA

379, 385, 391, 397, 3, 9, 15

80 ps


not offered

PPRE bunch


bunch length (FWHM)*

3.0 mA


70 ps

≤ 45 μA


7 ps

≤ 300 μA


16 ps

low current” bunch


bunch length (FWHM)*


not available

≤ 30 μA


7 ps

Ring current Lifetime

> 5.0 h

> 20 h

> 10 h

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