OBSERVATION OF FEMTOSECOND BUNCH LENGTH USING A TRANSVERSELY DEFLECTING STRUCTURE Markus Hning, Andy

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OBSERVATION OF FEMTOSECOND BUNCH LENGTH USING A
TRANSVERSELY DEFLECTING STRUCTUREMarkus Hüning,
Andy Bolzmann, Holger Schlarb (DESY,
Hamburg),Josef Frisch, Douglas McCormick, Marc
Ross, Tonee Smith (SLAC, Menlo Park,
California),Jörg Rossbach (Universität Hamburg,
Hamburg)
Laser
Abstract The design of the VUV-FEL at DESY
demands bunch lengths in the order of 50 fs and
below. For the diagnostic of such very short
bunches a transverse deflecting RF structure
(LOLA) has been installed which streaks the beam
according to the longitudinal distribution. Tests
in the VUV-FEL yielded a rich substructure of the
bunches. The most pronounced peak in the has a
rms length of approximately 50 fs during FEL
operation and below 20 fs FWHM at maximum
compression. Depending on the transverse focusing
a resolution well below 50 fs was achieved.
Bunch Compressor 1 2
By-Pass Line Collimator Undulator
Dump
LOLA
Screen
CTR
RF-Gun
Figure 1 Schematic drawing of the VUV-FEL beam
line. At the end of the accelerating section
there is the transversely deflecting structure
LOLA. It streaks the beam vertically so that on a
downstream viewscreen the longitudinal profile of
the bunches can be studied.
Left and right Image of a LOLA streak during
SASE operation. The streak is vertical, the head
of the bunch is on top. The streak was 4 fs/pixel
(26 mm). During standard operation the beam size
at the diagnostic screen is not optimized for
minimum vertical beam size. Although in a
different regime the two settings delivered
comparable FEL intensity. Far right LOLA streak
during dedicated machine time. The streak was
2fs/pixel, and the vertical beam size minimized,
indicated by the blue line. The settings were
close to FEL operation, but may have deviated in
certain respects. Especially compression might
have been stronger.
Phase space simulation of the first bunch
compressor. Left weak compression, right
maximum compression.
Sequence of stepping ACC1 from on-crest to 12
off-crest, the maximum compression in the first
bunch-compressor is found at 14
THPP035