4 Measurement at LCLS
4.4 Experimental results
(C)OTR Observation angle
Detector Lens Vacuum window
In-vacuum
mirror Scintillator
Crystal angle Electron beam
Figure 4.6:Observation geometry of the high-resolution profile monitor developed for the SwissFEL. The crystal angleαis 8.1○and the observation angleβis 15○. The CCD sensor is slightly tilted to fulfil the Scheimpflug principle.
4.3.2 Installation at LCLS
One exemplar of the high-resolution profile monitor has been installed upstream of the undulator section at the LCLS. Since LCLS has been suffering from strong COTR intensities, neutral density filters with optical density of 2 (transmission of 1%) are mounted onto the vacuum window to protect the camera.
The imaging system installed at LCLS has a demagnification of 1.8∶1, which corresponds to an effective pixel size of 11.63µm×11.63µm. The Ginzburg-Frank formula is not valid for the con-figuration of the high-resolution profile monitor at LCLS, since the far-field condition is violated. A generalized equation (see Eq.C.4) is applied for the estimation of the angular distribution of the TR intensity and is shown in Fig.4.7. In comparison to the peak value, the measurable intensity at the detector, which is placed at an angle of 23.1○(∼0.4032 rad) away from the backward axis of COTR, is expected to be reduced to 2.05 · 10−6and 1.65 · 10−6at a beam energy of 4.2 GeV and 13.1 GeV, re-spectively. Due to the quadratic dependence onN, which is in the order of 109, suppression of COTR could fail depending on the coherence level of the electron bunch and has to be tested experimentally.
4.4 Experimental results
Since the scintillation light is emitted incoherently, the total radiated intensity is expected to have a linear dependency on the bunch charge provided that COTR is suppressed. During each
measure-4 Measurement at LCLS: suppression of COTR
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035
0 0.2 0.4 0.6 0.8 1
θ(rad)
intensity(arb.units)
far-field,E=4.2 GeV near-field,E=4.2 GeV far-field,E=13.1 GeV near-field,E=13.1 GeV
0.4026 0.4034 0
0.5 1 1.5
2 · 10−6
Figure 4.7:Angular distribution of TR in (solid lines) far-field and (dotted lines) near-field. The inset plot shows the TR intensity around the angle of 23.1○(∼0.4032 rad), at which the camera is installed at LCLS.
ment scan, the bunch charge is kept constant, while the transverse or longitudinal bunch structures are changed. The integrated camera counts in the beam area of each image, which is a measure for the radiation intensity, is calculated. Constant camera counts for each scan are expected for incoherent beam imaging diagnostics.
The experiments were performed at three different machine settings (see Table.4.1) covering low to high charge and energy for soft and hard X-rays operations. Both cases with the laser heater turned on and off are investigated. The case of laser heater on is the nominal operation setting for LCLS, while the case of laser heater off is explicitly chosen to generate intense COTR.
Table 4.1:Machine settings for the spatial separation experiments at the LCLS.
unit A B C
Charge pC 20 20 150
Energy GeV 4.2 13.1 13.1
4.4.1 Compression scan
As indicated by Eq.4.2, the coherence level of the COTR intensity depends on the longitudinal form factorF(k), which is then determined by the longitudinal structures inside the bunch. The compres-sion settings of the machine are accordingly varied from under-comprescompres-sion to over-comprescompres-sion in order to change the longitudinal structures of the bunch. Two configurations of the laser heater are
4.4 Experimental results
studied at all three machine settings: (i) the laser heater is turned off (laser energy 0µJ) and (ii) the laser heater is turned on with a laser energy of 42µJ (corresponding to a heated rms energy spread of 45 keV [HBD+10]).
0.8 0.9 1 1.1
1.2 A h= −41.2 keV/fs
intensity(arb.units)
0.8 1 1.2
1.4 B h= −51.1 keV/fs
intensity(arb.units)
−70 −65 −60 −55 −50 −45 −40 −35
0 2 4 6
8 C h= −66.4 keV/fs
chirph(keV/fs)
intensity(arb.units) laser heater off
laser heater on
Figure 4.8:Compression scan at the settings A, B and C with the laser heater turned off (blue) and on (red).
During the compression scan at each setting, the RF amplitude and phase of the L2-linac (see Fig.4.5) are changed simultaneously to provide different chirpsh(energy per length) to the electron bunch while keeping the beam energy constant. Different chirps of the electron bunch lead to different compressions in the downstream bunch compressor BC2 (see Fig.4.5). The dashed vertical lines mark the compression settings which were used during the measurements presented in Figs.4.9,4.11and4.13.
The results of the compression scans are shown in Fig.4.8. At all settings, the scans with laser heater off (blue line) show notable variations of the intensity. The enhancements of the intensities all occur at the high compression setting with high peak currents. In the worst case (setting C with high charge and high energy), fluctuation of the beam shape in the camera images is observed from shot
4 Measurement at LCLS: suppression of COTR
to shot. The intensity increases up to a factor of 7, which clearly indicates the presence of COTR.
In the case of the laser heater turned on (red line), the intensity remains constant within a varia-tion of less than 10% for setting A and B. For setting C, the intensity measured with laser heater on reaches an enhancement of a factor of 2. A closer look at the single images taken at the compression setting where the maximum intensity enhancement is observed indicates disturbances from an other radiation source. As can be seen in Fig.4.9, the images exhibit on the one edge some stripes pattern resembling the structure on the chamfer of the in-vacuum mirror (see Fig.4.10). Since the mirror is located at a close distance of nominally 3.73 mm to the beam axis, there is coherent optical diffraction radiation (CODR) generated on the edge of the mirror and reflected back to the camera. This CODR pattern overlaps with the beam image and results in a slight gradient in the background intensity (see Fig.4.9). The fact that CODR is produced when the beam is present makes it very difficult to be separated from the beam image for data analysis. Mirrors with sharper edges are considered in the future upgrade of the high-resolution profile monitor [Isc].
1 2 3 4
1
2 3
x(mm)
y(mm)
0 400 800 1200
0 1 2 3 4
0 1 2 3 · 104
x(mm)
intensity(arb.units)
1 2 3
2000 400600 1000800
Figure 4.9:(Left) Single image containing stripes pattern taken at the machine setting C with the chirph=
−66.4 keV/fs (referred to the dashed line in Fig.4.8C), where the intensity enhancement is maximum. The laser heater was turned on. (Right) Horizontal profile with a slight gradient in the background intensity.
4.4.2 Beam size scan
In the beam size scans, the transverse beam size of the bunch is varied by changing the quadrupole strength. The transverse beam size could have an influence on the transverse coherence level. The other purpose of the beam size scan is to investigate the performance of the scintillator crystal for different electron densities. When the electron bunch is transversely focused to a small area, the high electron density may induce saturation of the luminescence centres in the scintillator crystal.
As a result, the intensity of the emitted scintillation light is not in linear dependence of the electron density any more.
4.4 Experimental results
mirror face
chamfer
Figure 4.10:Microscopic photo of the in-vacuum mirror. The structures on the chamfer induces CODR and are imaged together with the beam by the camera. Photo courtesy of Rasmus Ischebeck, PSI.
The first scan was performed with low-compression scheme at the machine setting A (referred to the dashed line ath= −41.2 keV/fs in Fig.4.8A) and is presented in Fig.4.11A. While the beam sizes are changed by a factor of 10, the intensity stays constant within a variation of less than 10%.
Another scan was performed with high-compression scheme at the machine setting B (referred to the dashed line ath = −51.1 keV/fs in Fig.4.8B). At this high-compression setting, the intensity measured with the laser heater turned off exceeds generally that with the laser heater on, regardless of the transverse beam sizes. A small decrease of the intensity of about 5% with the laser heater on is observed for one particular beam size. Figure4.12shows the vertical beam sizes of the bunches with different beam areas measured at the setting B with the laser heater turned on. At the beam area of 0.55 · 104µm2, where the intensity drops about 5%, the vertical beam size is in waist and the vertical profile has some flat-top feature (see the inset plot of Fig.4.12), which is a typical evidence for saturation in the scintillator crystal.
4.4.3 Laser heater energy scan
It has been shown in the previous two scans that the laser heater system has drastic influence on the mitigation of the micro-bunching instabilities and effectively reduced the coherence level in the emission of OTR. One scan of the laser heater energy was performed with the high-compression scheme at the machine setting B (referred to the dashed line ath = −51.1 keV/fs in Fig.4.8B). The results are presented in Fig.4.13. With increasing laser heater energy, COTR is quickly suppressed.
Starting from 17.4µJ, the variation of the measured intensity remains within 1%.
4 Measurement at LCLS: suppression of COTR
0.9 0.95 1 1.05
1.1 A
intensity(arb.units) laser heater on
0 5000 10000 15000 20000 25000 30000
0.9 1 1.1 1.2 B
beam area (µm2)
intensity(arb.units) laser heater on
laser heater off
Figure 4.11:Beam size scan at the settings A and B with the compression-scheme referred to the dashed lines in Fig.4.8A (ath= −41.2 keV/fs) and B (ath= −51.1 keV/fs), respectively. The beam sizes are changed by varying the quadrupole field strength. The beam area is defined as the product of the beam sizes in the xandyplanes.