Operating Modi BESSY II
Standard Fill Pattern
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  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 . 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
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.
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
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|
< 100mA (injection 7am, 3pm)
< 15mA (injection 11pm)
|Camshaft (Pos 201)||4.0mA||
Current = as other Bunches
3 x 4.0mA
Current = as other Bunches
|low current (Pos 225)||---||30μA|
|Lifetime||>5.0h||>10h / >20h|
|Purity (Camshaft)||>1000 (Range +/-40ns)||>1000 (Range +/-40ns)|