Abstracts Brochure - CERN

MOPCH012
MOPCH013
MOPCH014
26-Jun-06 16:00 - 18:00 MOPCH — Poster Session
Influence of Ambient Magnetic Field Changes on SASE
The VUV-FEL at DESY in Hamburg (Ger-
H. Kapitza, P. Göttlicher, N. Heidbrook, H. Schlarb (DESY)
many) is mostly located inside the circular
accelerator PETRA which serves as an injector
for the electron proton collider HERA. SASE was regularly lost in the VUV-FEL when protons were ramped to the
injection energy in PETRA. This effect was mediated by magnetic field changes in the order of 1 microtesla, caused by
time-dependent uncompensated magnet currents of more than 800 A which made PETRA act like a large current loop.
The resulting beam displacements of several hundred micrometers in the undulators proved to be enough to make
SASE fail. This serious disturbance of user runs was eliminated by introducing an improved compensation scheme
which further limits residual currents in PETRA during proton injection. The consequences of this observation for
the design of the XFEL are briefly discussed.
Slice Emittance Measurements at the VUV-FEL
The SASE process in Free Electron Lasers
M. Roehrs, C. Gerth, M. Huening, H. Schlarb (DESY)
mainly depends on time-sliced parameters
of charge density, energy spread and transverse
emittance. At the VUV-FEL at DESY, electron bunches are compressed longitudinally in two magnetic chicanes
in order to achieve high peak currents. The compression causes considerabe variations in slice emittance along the
bunches. The vertically deflecting rf-structure LOLA, which is in operation at the VUV-FEL since early 2005, allows
to resolve longitudinal variations in horizontal slice width for single bunches. The horizontal slice emittances can be
determined by additionally varying the strengths of the quadrupoles upstream of LOLA. Results of slice emittance
measurements using different bunch compression schemes are presented.
Energy-time Correlation Measurements Using a Transversely Deflecting RF Structure
To initiate the lasing process in SASE-based
M. Roehrs, C. Gerth, O. Grimm, M. Huening, H. Schlarb (DESY) Free Electron Lasers, electron bunches with
high peak currents are necessary. At the
VUV-FEL at DESY, high peak currents are produced by bunch shortening in magnetic chicanes induced by a linear
energy-time gradient. The residual uncorrelated time-sliced energy width after compression is a crucial parameter for
the lasing process. The final energy-time correlation provides important information about the compression process.
This paper presents a measurement of slice energy spread and energy-time correlation using a vertically deflecting
rf-structure (LOLA). The structure allows to map the time delay of bunch slices to the vertical axis of a screen. After
dispersing the bunches horizontally with a dipole, the energy-time correlation can be directly obtained in a single
shot measurement. Results for different bunch compression schemes are presented. The measured bunch length in
case of a non-compressed beam is compared to streak camera measurements.
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