LHC Beam Diffusion Dependence on RF Noise: Models And Measurements Page: 2 of 3
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ICC
r -e
Z,-mC~lzIofyS aaaB
4s i eFrequency (Hz)
Figure 3: Beam 2 cavity sum and RF reference-BPL OFF
Time (s)
Figure 6: Beam 1 bunch length and BPL error signal E with
time. BPL turned off at t 4000 seconds.
of the BPL. During these measurement, the LHC was op-
erating at 3.5 TeV, with non-colliding, single bunch of 9e9
intensity per ring. Figure 7 shows the cavity phase noise-6 -14
1_ -
a-124s
S-160
Frequency (Hz)IC -1S
Figure 4: Cavity phase noise for cavity 2B1 with 1.5 MV,
Q of 60k, 3.5 TeV beam.
Beam Phase Loop
The notch seen at the synchrotron frequency of about 23
Hz, is introduced by the Beam Phase Loop (BPL), a narrow
bandwidth loop that synchronizes the average beam phase
over a turn with the RF phase, via adjustments of the 400.8
MHz reference, as shown in the simplified block diagram
in Figure 1.
As such, it leads to a major performance improvement.
Figure 5 shows the substantial reduction of the phase er-
ror E at fs with the BPL closed. Furthermore, Figure 6
~ 0
PL Closed
1- - PL open
z -26
-u -
-2 -4-
-100 -86 -66 -46 -26 0 20 40 60 BO 100
Frequency (H Z)Frequency (Hz)
Figure 7: Cavity 6B2 phase noise spectral density with
BPL T-1
spectral density as a function of the BPL inverse time con-
stant r -1. Increasing the BPL r -1 clearly decreases the
noise at fs. It is interesting to note that noise around fs ac-
tually goes up, with no effect on beam dynamics though.
Therefore, the rms noise value is not a valuable metric
for beam dynamics performance. To show the effect near
GV14S-7 -
-mFrequency (Hz)
Figure 5: Beam 1 phase error with BPL on/off.
shows the significant change in bunch length growth when
the BPL was turned off at the later stage of this measure-
ment. Bunch length data used in this work were provided
by the Beam Quality Monitor [3].
Beam Diffusion Studies
Dedicated measurements were conducted to better quan-
tify the relationship between the sampled noise power and
the bunch length, and also to better understand the effectFigure 8: Cavity 6B2 noise spectral density with BPL r-i.
fs the image has been enlarged near the synchrotron fre-
quency (Figure 8).
The approximate bunch length growth rate duz/dt for
each w-1 setting is reported in Table 1 for Beam 1 and Ta-
ble 2 for Beam 2. An estimate of the total power sam-
pled by the beam is also shown. Unfortunately, since this
power is dominated by the power spectral density around
fs, the accuracy of this estimate is limited by the instru-
ment resolution, as seen in Figure 8. Still though, one can
see the clear correlation between noise power and longitu--13i
10 s-nzs
= 281
=14s
=zs
=sz -14s
-180
=s
m 1s
35
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Mastorides, T.; Rivetta, C.; Fox, J. D.; Van Winkle, D.; Baudrenghien, P.; Butterworth, A. et al. LHC Beam Diffusion Dependence on RF Noise: Models And Measurements, article, September 14, 2010; California. (https://digital.library.unt.edu/ark:/67531/metadc1012331/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.