It is a pleasure to inform you that on Tuesday 22nd of February, 2000, the
shift crew succeeded to observe first lasing of the TTF FEL. The observed
wavelength is 109 nm. The
increase in intensity into the coherent angle compared to the spontaneous radiation is about 2
orders of magnitude. The width of the radiation cone is approximately
300 microradian as compared to 3 milliradian for the spontaneous radiation.
The intensity of the radiation shows a strong dependency on the bunch
charge. The observations are in agreement with what is expected for SASE.
In the following we provide you as a member of the TTF/TESLA collaboration
with detailed information about the observed
signals and with results from the first week of lasing. Please feel free to
contact us for more information.
The first SASE seen by the photodiode downstream of the undulator
The picture is a snapshot of the screen of the console. Shown is the signal of
the photodiode placed about 15 m downstream of the undulator. The signal is
the voltage of the detector and therefore the photon intensity in arbitrary
units. The polarity of the signal is negative which means higher intensities are larger negative numbers. Until February 22nd, 4:47 a.m. the signal was
typically around -0.5, then the signal suddenly came to saturation of the
detector/amplifier. The layout of the TTF Linac is shown on the next picture.
The signal is fluctuating not because of machine instabilities but because of
the physics of the SASE process.
Schematic layout of the TTF Linac during the first SASE operation
The TTF Linac consists of the laser driven RF gun delivering about 1 nC of
bunch charge at an energy of approx. 4 MeV. The beam is then accelerated in the
superconducting capture cavity. Beam diagnostics proofs the electron beam
quality. Acceleration in the first module (8 s.c. cavities at approx. 14 MV/m)
yields 120 MeV in the bunch compressor. There the bunch is compressed to a
length of less than 0.5 mm. Further acceleration yields a total energy of
230 MeV. This energy does not correspond with the maximum possible gradient in
the s.c. cavities. The gradient was reduced in order to take advantage of
the relaxed beam quality requirements for SASE.
The linac was operated with only a few bunches per macro pulse (1 to 10).
The repetition rate was 1 Hz. Bunch charges of 0.1 to 1.0 nC were used.
According to previous measurements the projected emittance downstream of
the second linac module was below 20 pi mm mrad. The slice
emittance was better. Since several days the beam was close to axis when passing the
undulator. A correction in the displacement and the injection angle resulted
in the first SASE.
Layout of the photon beam diagnostics at the TTF FEL
The electron beam as well as the photon beam enter the setup from the right.
The electron beam is deflected by the dipole magnet into the beam dump.
The photon beam passes a variable and movable aperture (5 to 0.3 mm) and its intensity
is then measured by photodiodes / detectors of different sensitivity.
A deflecting mirror allows measuring the photon beam spectrum using a 1 m NIM
spectrometer and a CCD detector.
The first photon beam spectrum taken had its maximum at about 109 nm. The
image taken with the spectrometer's CCD is shown as well as the photon
intensity vs. wavelength. The width of the distribution is around 1%.
The measurement was taken over 1 minute and therefore is the integral over
60 photon pulses produced by 60 electron bunches. The aperture used
in front of the photodiode was 5 mm.
A variation in the steerer settings in front of the undulator yields a
beam displacement in front of the undulator. In the undulator a betatron
oscillation with an amplitude of the same order of magnitude will be the
The picture shows the photon intensity vs. the beam position at the
entrance of the undulator. The beam was displaced in steps of about
0.1 mm. Several bunches were measured for each steerer setting. According
to the staticstics of the SASE process bunch intensities between zero
and the maximum measured value were seen. The width of the distribution
is larger than expected and not a consequence of beam jitter. The beam
jitter is seen as the width of the 'vertical lines' in the distribution
(approx. 0.05 mm). The result needs more analysis, especially in terms of
a detailed error discussion.
Photon intensity for two different levels of bunch charge
The photon intensity was measured vs. time. At a certain moment (330 seconds, i.e. after 330 bunches) the bunch charge was changed from 0.2 nC to 0.8 nC.
The photon intensity increases by a factor of approx.150.
The more detailed measurement is the photon intensity as a function of bunch
charge. The measurement was carried out without correcting the beam optics
which depend on the bunch charge. The beam movement as a function of the
varied bunch charge was found to be small. The displacement was below 0.1 mm.
The photon intensity increases exponentially just as expected for SASE
amplification. Spontaneous emission would give a linear increase as a function
of the bunch charge. The plotted curve proportional to (Q)^1/3 represents a
simple, one-dimensional, steady state FEL description.
The figures compare the transversal
photon beam profiles measured in the past and now with SASE.
The photodiode with an aperture in front of it was scanned in both
directions perpendicular to the beam. To avoid saturation of the
amplifier of the photodiode, the signal in the SASE case was attenuated
using a 36 times smaller aperture. The distance between the photodiode
and the end of the undulator is approx. 12m. In the past, observing only
spontaneous emission, the signal was about a factor of three wider. This is
in good agreement with the the 1/gamma opening cone for the power density
distribution of spontaneous emission. For SASE in saturation we would
expect an even smaller distribution than we presently measure, so that
we assume that only part of the electron bunch is strongly radiating, i.e.
the effective source size is smaller, hence the divergence is bigger.
Thi figure shows the spectrum we measured in the past (top)
with no SASE while the bottom graph shows the spectrum with SASE. The spectra
were recorded with a horizontally focusing mirror in front of the vertical
monochromator entrance slit, thus it shows a convolution with the wavelength
distribution in the horizontal direction. It has to be noted that the bump on
the low wavelength side of the SASE spectrum is an artefact due to the readout
The figure shows the photon intensity signal from 7 out of 10
bunches of a bunchtrain. It was measured with a PtSi photodiode and a
0.5 mm diameter aperture in front of it to avoid saturation of the
amplifier behind the photodiode. One clearly sees that each individual bunch
shows a different intensity of the corresponding photon beam. That is an
indication of the statistics in the SASE process starting from shot noise.