Abstracts Brochure - CERN

THPLS — Poster Session 29-Jun-06 16:00 - 18:00
*A. Zholents and M. Zoloterev, PRL 76 (1996), 912.
Bunch Shape Diagnostics Using Femtoslicing
Laser-energy modulation of relativistic electron
bunches as needed for the BESSY femtosecond
(fs) x-ray source is accompanied by
the emission of fs THz pulses*. The total THz
K. Holldack, T. Kachel, S. Khan, T. Quast (BESSY GmbH) R. Mitzner
(Universität Muenster, Physikalisches Institut)
intensity probes the square of the longitudinal particle density within a slice of ∼50 fs length (fwhm). The bunch
shape can be directly monitored while sweeping the time delay between laser and bunch clock. The method is
demonstrated for bunch lengths between 3 and 30 ps (rms) in different operation modes of BESSY II. The use of THz
signals from successive turns and the influence of periodic bursts of coherent synchrotron radiation, which lock to the
laser pulse under certain conditions, are discussed. The method is used for setting up and stabilizing the temporal
overlap between a fs-laser and a relativistic electron bunch.
*K. Holldack et al., Phys. Rev. Lett. (2006), accepted Dec. 2005.
Orbit Stability in the ’Low Alpha’ Mode of the BESSY Light Source
Running the light source during dedicated
shifts in the so-called ’low alpha’ mode, R. Müller, J. Feikes, P. Kuske, G. Wuestefeld (BESSY GmbH)
BESSY serves two major user groups: THz
experiments take advantage of intense, coherent synchrotron radiation (CSR) generated by the short bunches. Time
resolved experiments appreciate the very short, high intensity VUV and x-ray pulses in the ps range that help, e.g.,
prepare the high resolution, low intensity fs-slicing experiments. In the ’low alpha’ mode, the sensitivity of the storage
ring with respect to energy and horizontal orbit is increased by orders of magnitude while the user experiments
require the same beam stability as in ’normal’ mode. In this paper an overview of the operational conditions of
this specific user mode, the stabilization measures taken, observations and available diagnostic results as well as the
achievements and shortcomings of the adapted slow orbit feedback are given.
FLUKA Calculations of Neutron Spectra at BESSY
The synchrotron light source BESSY consists
of a 50 MeV microtron, a full energy synchro- K. Ott (BESSY GmbH)
tron and a 1.9 GeV storage ring. The electron
losses during injection causes electromagnetic cascades within the stainless steel of the vacuum system and the aluminum
chambers of the undulators. The cascade-produced neutrons result from giant resonances, quasi-deuteron
fissions and photo-pion productions. The cross sections of the evaporation reactions of neutrons are an order of
magnitude higher than the cross sections of the latter two reaction channels. The energy distribution of the giant
resonance neutrons has a maximum at about 1 MeV in comparison with 100 - 200 MeV of the high energy neutrons.
At electron accelerators outside the shielding wall, half of the neutron dose is often determined by the more penetrating
high energy part of the neutron fluence. We used the particle interaction and transport code FLUKA for
the calculations of the energy distribution of both the fluence and the dose inside and outside the shielding wall for
different realistic scenarios. From the integrated spectra we get the calibration factor to determine the total neutron
dose from the measurements directly.
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