FFTB 98-03
Vertical position stability of the FFTB electron beam
measured by the KEK BSM monitor.
M.Woods, T. Kotseroglou, T. Shintake
Abstract
The vertical position jitter of the electron beam of the Final Focus Test Beam ( FFTB)
was measured during the December ’97 run to be y = 40 nm by the KEK Beam Spotsize
Monitor (KEK BSM). Two dedicated measurements were taken for this, and they gave
consistent results. This position jitter is a significant contribution to the 70 nanometer
spotsizes measured during this FFTB run.
1. Introduction
The Final Focus Test Beam ( FFTB) is used to focus the electron beam to nanometer
scale sizes in order to test the final focus optics of the Next Linear Collider (NLC). The
measurement of the transverse size of the electron beam is done by scanning the electron
beam position on the bright and dark fringes of an interferometer and then measuring the
modulation of the Compton scattered gammas [1]. The quality and stability of the fringes
of the laser interferometer were measured just prior to the December run, and were found
to be stable at the level of 20nm [2]. During the December ’97 FFTB run, we measured
the electron beam vertical position jitter from measurements of the intensity jitter of the
Compton-scattered gammas using the KEK BSM. For these measurements, the electron
beam rate is 30 Hz and the laser rate is 10Hz. This gives Compton collisions at 10Hz,
and the laser off pulses are used to determine the background in the gamma detector. In
this note, we summarize the electron beam jitter results and the implications on the beam
size measurement.
2. Electron beam position jitter
One can determine the position jitter of the electron beam on the fringe pattern from
the observed intensity jitter of the Compton signal. Fig.1 is a typical electron beam
vertical spot size measurement by scanning the electron beam on the two laser beam
interference fringe pattern. It shows the number of Compton scatters as a function of
electron beam position.
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Figure 1. Electron beam vertical position scan through the laser fringes produces
Compton scattered gammas , which number varies sinusoidally with the electron
position.
The jitter of the number of gammas detected at the inflection points ( signal ~ 180 counts
) is dominated by position jitter of the electron beam on the fringe pattern. The jitter at the
peak is dominated by Compton counting statistics and targeting instabilities ( signal ~ 300
counts). The data in this plot has the ‘laser off’ background subtracted. This background
is typically 30-35 adc counts and is quite stable (see Fig. 2).
For the data in Fig. 1, we can write the number of Compton scattered gammas, A, as
follows:
 y  y0

A  180  130 sin
 2 
 533

(1)
dA
130  2

 153
.
dy ( y  y 0)  0
533
(2)
So,
So, the calibration at y-y0=0 is
2
 y ( nm ) 
A
(3)
1.53
Figure 2. Background of Compton gammas ( laser off).
3.1 Data set #1
We took buffered data acquisition of 60 ‘laser on’ samples using the MCC SCP.
With the laser firing at 10Hz, this corresponded to 6 second samples. The distribution
of the Compton gamma signal is shown in Fig. 3, when the electron beam is located
on a bright fringe (the peak of the sinusoidal distribution in Fig. 1). Fig. 4 shows a
similar distribution when the electron beam is located at the inflection point.
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Figure 3. Electron beam position jitter distribution from data collected at the peak
Compton signal.
Figure 4. Electron beam position jitter distribution from data collected at the
inflection point
From the data in Fig. 3, we find that Compton counting statistics introduces a 14%
jitter in the gamma signal at the peak. Hence the expected jitter from Compton counting
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statistics would be 20% rms at the inflection point. Yet the observed jitter as noted in
Fig. 4 is 31%, indicating an extra contribution from position jitter of the electron beam
relative to the fringe pattern. We took a large number of buffered acquisition data
samples, both at the peak and at the inflection point. Fig. 5 is a histogram of the observed
peak jitter from these data samples. On average, the jitter at the peak was observed to be
14%. Fig. 6 is a similar histogram for the jitter observed at the inflection point, which on
average is 31%. We then corrected the observed jitter at the inflection point for the
expected 20% jitter contribution from Compton counting statistics, and used Equation 3
to convert the corrected intensity jitter to a position jitter. A histogram of the results from
doing this is presented in Fig. 7.
Figure 5. Electron beam position jitter distribution from data collected at the peak
Compton signal.
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Figure 6. Electron beam position jitter distribution from data collected at the
inflection point
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Figure 7. Electron beam position jitter distribution after correction for Compton
statistics etc.
From the data in Fig. 7, we conclude that the vertical position jitter of the electron
beam relative to the fringe pattern is 40nm rms.
3.2 Data Set #2
We used the correlation plot data acquisition facility to take a second set of data. A
typical vertical beam spotsize scan at the time this data was taken is shown in Fig. 8.
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Figure 8. Electron beam vertical position scan through the laser fringes at the time of
Data set #2 was collected.
Then with the electron beam positioned near a bright fringe, we took a correlation plot
of the Compton gamma signal vs time with 10 laser on samples per data point. This
is shown in Fig. 9, where the error bar represents the rms of the signal fluctuations. A
similar correlation plot is shown in Fig. 10, with the electron beam positioned near
the inflection point. Though for this plot, 20 laser on samples (2 seconds) are taken at
each data point.
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Figure 9. Correlation plot of the Compton gamma signal vs time with 10 laser on
samples per data point taken at the Compton peak. The error bar represents the rms of
the signal fluctuations.
Figure 10. Correlation plot of the Compton gamma signal vs time with 10 laser on
samples per data point taken at the inflection point. The error bar represents the rms of
the signal fluctuations
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From the data in Figs. 9 and 10, we find similar jitter at the peak and inflection points
as was observed in Data Set #1. Following the same procedure used there, we derive
the position jitter of the electron beam relative to the fringe pattern from the rms
fluctations observed in the inflection point data after making a correction for the
Compton counting statistics. A histogram of this result is plotted in Fig. 11. We find
from this data set that the position jitter of the electron beam relative to the fringe
pattern is 41nm in a 2 second time window.
Figure 11. Vertical position jitter of beam relative to the laser fringe pattern is
measured to be 41 nm in sigma ( after correction for Compton counting statistics and
other effects).
This is very consistent with the result noted using Data Set #1. We also find that the
jitter is dominated by pulse-to-pulse fluctuations rather than slow drifts. Slow drifts
are sometimes observed and are consistent with the results noted in Reference [2].
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SUMMARY
During the December ’97 FFTB run, we took two sets of data to measure the vertical
position jitter of the electron beam relative to the KEK BSM fringe pattern.
Consistent results were found for the two data sets with a relative jitter observed of
approximately 40nm rms. This jitter is dominated by pulse-to-pulse fluctuations.
Slow drifts are smaller, though consistent with 20nm drifts over 25 seconds as noted
in Reference [2].
References
1. T. Shintake, NIM A311, 453-464 (1992).
2. M. Woods et al., FFTB 98-02 (1998).
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