6 Measurement at SITF
6.3 Experimental results for uncompressed bunches
6 Measurement at SITF: emittance measurements
for the projected bunch, but for the central slice of the bunch.
During the measurements, the last OTR screen in the FODO section was defect, which left 6 screens available for the projected emittance measurement using the multi-screen method, and 5 for the slice emittance measurement. For each beam size measurement, multiple images were recorded for statistical reason (20 single shot images for the multi-screen method, 10 for the quadrupole scan method). Each image was first subtracted with background images and then processed to reduce noise (see AppendixD). Since the beam profiles were in Gaussian shape for measurements with both the uncompressed and compressed bunches, the beam sizes are determined by using Gaussian fit to the transverse profiles. On the other hand, determination of beam sizes from Gaussian fit is more robust than rms values in the case that the OTR screens have very low signal to noise ratio. The errors given in the following sections include only statistical errors and are determined according to error propagation (see Eq.1.27).
During the slice emittance measurements, the bunch length is determined from Gaussian fit.
Each slice is defined with a width of 1/5 of the bunch length and the central slice as the one at the longitudinal mean position of the bunch. The same definition is used in both methods for consis-tency.
6.3.1 Projected emittance
During the measurement using the multi-screen method, the images taken with the 4th. OTR screen displayed two beamlets similar to the features of the OTR point spread function (see Fig.6.8) and therefore were omitted for the reconstruction of the emittance.
200 400 600 800
200 400 600 800
x(µm)
y(µm)
0 200 400 600
Figure 6.8:Example beam image taken with the 4th. OTR screen during the projected emittance measure-ment with uncompressed beam. Two beamlets are visible.
For each of the method, one measurement was performed. The results of the projected emittance measurements are presented in Fig.6.9, and the reconstructed normalized projected emittancesεx,y
together with the mismatch parametersMx,yare summarized in Table6.1. The results obtained with these two methods are comparable, but the normalized emittances derived using the multi-screen
6.3 Experimental results for uncompressed bunches
method are slightly larger than that using the single quadrupole scan method in both planes. Errors from optics mismatch are minimized in the single quadrupole scan method, while the mismatch pa-rameters of 1.13 and 1.07 in the multi-screen method indicate that there is still an error contribution stemming from the optics mismatch.
−1 0 1
−1 0 1
normalizedx
normalizedx′
−1 0 1
−1 0 1
normalizedy
normalizedy′
−1 0 1
−1 0 1
normalizedy
normalizedy′
−1 0 1
−1 0 1
normalizedx
normalizedx′
Figure 6.9:Fits of the beam ellipses using (left, solid lines) the multi-screen method and (right, dashed lines) the single quadrupole scan for the measurement of horizontal (blue) and vertical (red) projected emittance with uncompressed bunches. The lines represent each measured beam size. The results are presented in normalized coordinates as given in Eq.1.26.
Table 6.1:Summary of projected emittance measurements with uncompressed bunches.
multi-screen method single quadrupole scan
εx 487±8 nm 486±2 nm
Mx 1.13 1.00
εy 479±6 nm 458±3 nm
My 1.07 1.06
6 Measurement at SITF: emittance measurements
6.3.2 Slice emittance
The accelerator was operated with the same settings as for the projected emittance measurement.
Several matching iterations were performed to match the central slice to the design optics. The bunch length was determined from Gaussian fit and amounts to approximately 3 ps. Each slice was chosen to have a width of∼0.6 ps.
0.6 0.8 1 1.2
I(arb.units)
200 300 400 500 600
ε(nm)
0 0.5 1 1.5 multi-screen 2
M
0.6 0.8 1 1.2
I(arb.units)
−6 −4 −2 0 2 4 6
200 300 400 500 600
slice index
ε(nm)
0 0.5 1 1.5 quadrupole scan 2
M
Figure 6.10:Slice emittance measured at the first TDS RF zero-crossing with uncompressed bunches using (top, solid) multi-screen and (bottom, dashed) quadrupole scan method. The red lines represent the mis-match parameter with respect to the design optics. The grey lines (with filled areas) represent the longitu-dinal profile in arbitrary units. The head of the bunch is on the right hand side of the horizontal axis.
The deflecting power of the TDSPTDSwas kept at a moderate value complying with the following restrictions: (i) The beam size of the streaked bunch on the OTR screens should be relatively small so that there will be still enough light emission from the OTR screens, (ii) The longitudinal resolution is still enough to resolve the slices. The TDS powerPTDSremained constant for all the measurements using the multi-screen method (PTDS=0.07 MW) and quadrupole scan method (PTDS =0.11 MW), so that influence from the TDS induced effects can be excluded in the reconstruction of emittance.
Due to the significant differences in the spatial resolutions and light yields of the OTR and high-resolution screens, as well as the accelerator optics, an identical TDS power for both methods was not possible. As expected from Fig.6.4, the measured longitudinal resolution on the first OTR screen was only 1/2 of the bunch length and therefore not usable for slice emittance measurement.
6.3 Experimental results for uncompressed bunches
0.6 0.8 1 1.2
I(arb.units)
200 300 400 500 600
ε(nm)
0 0.5 1 1.5 multi-screen 2
M
0.6 0.8 1 1.2
I(arb.units)
−6 −4 −2 0 2 4 6
200 300 400 500 600
slice index
ε(nm)
0 0.5 1 1.5 quadrupole scan 2
M
Figure 6.11:Slice emittance measured at the second TDS RF zero-crossing (i.e. with 180○phase shift compared to Fig.6.10) with uncompressed bunches using (top, solid) multi-screen and (bottom, dashed) quadrupole scan method. The red lines represent the mismatch parameter with respect to the design optics. The grey lines (with filled areas) represent the longitudinal profile in arbitrary units.
Figure6.10shows the normalized horizontal slice emittance and slice mismatch parameter ob-tained using (top) multi-screen and (bottom) quadrupole scan method with the TDS operated around the RF zero-crossing. The grey lines (with filled areas) represent the current in each slice. The re-constructed slice emittance from these two methods shows the same behaviours along the slices:
constant emittance for the slices with positive indices and increasing emittance values towards the negative indices. The slice mismatch parameter from these two methods show the same feature as well. The slice emittance values obtained with the multi-screen method are in general larger than those obtained with the quadrupole scan method.
In order to investigate the influence of the TDS streak and the initial bunch correlation in(y,z) and(y′,z)on the reconstructed longitudinal distribution, the slice emittance measurement was re-peated at the other TDS RF zero-crossing (see Fig. 6.11), i.e. with 180○ phase shift compared to Fig.6.10. The consistency of the reconstructed slice emittance and slice mismatch parameter de-rived for these two TDS RF zero-crossings excludes the influence of an initial bunch correlation and further confirms the reliability of the measured results. The fact of measuring larger slice emittance from the multi-screen method than the quadrupole scan method is still observed.
6 Measurement at SITF: emittance measurements
Table 6.2:Summary of the central slice parameters measured with uncompressed bunches.
unit multi-screen quadrupole scan 1st. TDS phase
εx nm 369±11 314±7
Mx 1.10 1.19
βx m 6.58±0.32 5.73±0.18
αx −0.95±0.05 −0.90±0.02
2nd. TDS phase
εx nm 353±13 321±7
Mx 1.18 1.12
βx m 6.23±0.31 6.19±0.21
αx −1.03±0.04 −0.86±0.02
βDesign,x m 9.43
αDesign,x −1.02
Since the design Twiss parameters at the reconstruction point are the same in the measurements using these two methods, they can be compared as well. Table6.2summarizes the reconstructed parameters for the central slice, which was the one matched during the matching iterations. Al-though the slice emittance measured with the multi-screen method is larger than that measured with quadrupole scan method, the Twiss parameters measured with both methods and at both TDS RF zero-crossings show good consistency. In all cases, the measured beta-function has a large deviation of∼40% to the design value, while the alpha-function is matched within∼10% to the design value.
Better matching could not be achieved during the matching iterations at the beginning of the mea-surement series. According to Figs.6.6and6.7, the relative deviation of the reconstructed emittance (measured at the initial optics ofβ ≈6 m andα ≈ −1) from the real value is estimated to be below 2% for both methods, and the relative standard deviation of the reconstructed emittance below 10%.
It cannot fully explain the discrepancy of the emittance of∼15% measured using the two methods.
Possible systematic errors resulted from the screen resolution are discussed in Section6.5.