As the bunch intensity increases, finding the source of The Hump becomes more cirtical. We have seen from various reports that it causes a vertical spreading of the bunch that steadily reduces luminosity. When it hits the tune frequency it can lead to beam losses and can even trigger a dump. Its varying frequency and unpredictability make it very hard to work around. The information we see in the reports is limited so it is hard to gauge what is being done but they may not consider it safe to proceed to higher intensity beams until The Hump is resolved.
Here is some info from a report on beam stability at the beginning of May (
http://cdsweb.cern.ch/record/1267395/fi ... 10-008.pdf)
The source of the broad-band excitation signal around
the nominal vertical tune working point (0.31) visible in
Figure 1 remains elusive. Similar to the 8 kHz lines, once
this perturbation is in the vicinity of the tune, the beam
gets resonantly excited, subsequently decreasing beam lifetime.
While the effect has been observed on both beams, it
is more dominant in the vertical plane of B2. A higher
temporal analysis revealed that the broad-frequency distribution
is actually caused by a narrow-band single frequency
with the same shifting mean frequency for both
beams, as shown in Figure 10. The central frequency of
the hump shifted typically between 0.15 and 0.45 frev over
a duration of a few minutes to hours. The B1 to B2 hump
frequency correlation factor is about 0.896. Nevertheless,
the ’hump’ is visible independent of whether there are one
or two beams circulating in the machine. The magnitude
spectrum of the hump frequency shift as a function of time
revealed a 1/f dependence. As visible in Figure 1, the
amplitude of the ’hump’ corresponding to a few hundred
nano-metre is extremely small and – provided it is caused
by a single dipolar-type kick – corresponds to a deflection
angle in the order of a few nano-rads. Due to the huge
number of elements in the machine that a priori could potentially
create these minuscule deflections, the identification
and location of the true perturbation source proved to
be extremely difficult. Based on switching ’off’ given accelerator
elements, the orbit correctors, transverse damper
exciter, injection septa, transfer lines, pre-injector accelerators,
experimental magnets and higher-ordermagnets could
be ruled out as the cause for the ’hump’. Despite a series
of investigations, the true source of the ’hump’ remains unknown,
and its investigation and mitigation is a priority of
the ongoing LHC commissioning.
CONCLUSIONS
[...] While the initial performance is
sufficient for present operation, some effects such as the
residual un-explained tune oscillations, the large tune jitter
during injection and the ’hump’ will need to be addressed
in view of nominal LHC operation.
It is indeed worrying that they have not been able to identify the source after so much time. Yes the LHC is complicated with many components, but there are engineers who know it very well. Why can they not identify the cause given its peculiar frequency characteristics?
They have not said if they have considered using sensors around the tunnel to detect vibrations or EM interference. As time goes on and more components are eliminated, the likelihood that the source comes from outside the LHC is increasing. Switching bits off and on may not be enough to find it.
