Electron cloud meeting #10, 16/05/2014
Participants: G. Arduini, H. Bartosik, R. Cimino, G. Iadarola, K. Li, E. Métral, L.
Mether, F. Petrov, A. Romano, G. Rumolo
Excused:
Matters arising and general information (G. Rumolo)
Follow up from the minutes of last meeting and new information:
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There was an LBOC presentation by S. Popescu, in which the new tool for
the online monitoring of the heat load of Sector 12 was presented. This
can be used at the start up as well as on old data. Next on the list is the
tool for the heat load in the ITs. This will be very handy for the Summer
student joining the team in July.
G. Iadarola, L. Mether and G. Rumolo visited Cornell for measurements of
electron cloud in quadrupoles and fast beam ion instability at Cesr-TA.
During their stay two measurements sessions on FBII and one on EC took
place. During the FBII measurements, few meters of accelerator were
purposefully degraded pressure-wise to excite a fast beam ion instability.
For the electron cloud measurements a time resolved measurement of the
electron cloud in quadrupole over successive turns with trains of 20
bunches and a witness “cleaning” bunch placed at variable distances from
the last bunch of the train were used for assessing the importance of
electron trapping in quadrupoles.
The Summer student, Johannes Hulsman, selected to work on the
reconstruction of the heat load history in the LHC triplets will start on 7
July, 2014 and will stay at CERN slightly more than two months.
Electron Cloud Multi-bunch Wakefield Simulations (F. Petrov – TUDarmstadt)
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The observed electron cloud induced emittance blow up along LHC trains
is the main motivation of this study. Bunch-to-bunch coupling and related
instabilities are good candidate to be the origin of this behavior.
Physically, an offset bunch kicks differently electrons close to the
chamber wall according to their positions. Since the bunch kick imparts
different energies to the electrons according to their positions, their
multipacting behavior can be different due to the shape of the SEY curve.
Build up simulations with circular chambers (20mm radius) and one
offset bunch in a train were used to try to quantify this effect.
Offsetting a bunch (0.5 to 3 mm) has a larger effect on the electron cloud
build up in a dipole (H plane) than in a field-free region.
Two different bunch intensity values were simulated (0.7 and 1.3e11
ppb), so that the energy kick given to bunches close to the chamber wall
could be below or above the one of the peak of the SEY curve (250 eV).
While the vertical offset does not imply any memory extending to the next
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bunch, the horizontal one is longer and is basically “forgotten” after
another bunch passage for the high intensity, while it continues to be
there for much longer in the case of the lower intensity.
Fixing the bunch intensity to 1e11 ppb and considering two different
E_max (150 and 350 eV), the long memory in the horizontal plane is
found in both cases but the wake has different sign.
At CERN, a possible example of horizontal coupled bunch instability is the
one observed at 26 GeV in the PS , so this model could applied there to
explain the observations. However, a combined build up/beam dynamics
code is needed to study the effect.
News on e-cloud calculations (A. Romano, G. Iadarola, L. Mether)
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Annalisa implemented an electron cloud detector in PyECLOUD, which
produces in output the signal obtained by the flux of electrons going
through holes in the chamber wall (modeled as perfect absorbers). The
final goal is to study the new PS electron cloud detectors, but to first
benchmark the results of the simulations, the case of the SPS strip
monitors was considered. For this monitor data are available for a wide
range of magnetic fields, so that the result of the simulation can be
directly benchmarked against experimental data.
The SPS electron cloud strip monitor is made of several rows of holes
tilted in the longitudinal direction. Simulating 7 different configurations is
enough to cover the full cases of the detector (for symmetry and
periodicity). Magnetic fields up to 900 G were considered and the
simulation step was different in the different regions in order to resolve
the more complex behavior for magnetic fields below 200-300 G. It can be
seen that the presence of the holes strongly affects the electron cloud in
the chamber and in fact, for magnetic fields above 200 G there is almost a
full suppression of the electron cloud at the hole positions, so that the
electrons that are measured are those close to the edges of the holes. It
can be seen that for low magnetic fields, the SEY threshold for
multipacting is strongly affected by the presence of the holes (it moves
from 1.3 without holes to 1.5 with holes!), but for high values of magnetic
fields it hardly moves. Even the saturation density for high fields is
basically the same in the two cases with and without holes, because when
the holes are present the electrons can multipact to high density values
between holes due to weaker electron space charge. It was also
demonstrated that for low magnetic fields, the threshold moved gently
from the values with holes to the values without holes while gradually
removing holes from the simulation.
The total current through the holes as a function of the SEY can be found
for all the different magnetic fields. Close to the multipacting thresholds,
the results found with PyECLOUD are very similar to those previously
found with ECLOUD (2009), which matched very well the measurements
done at the SPS by C. Yin-Valgren.
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Next steps are: 1) Apply model for the new PS ecloud detectors in MU98
and predict 2014 measurements; 2) Apply model to PS shielded pick up in
SS98 and compare with 2011/2012 time resolved data.
Gianni presented the results of his detailed numerical analysis of the
electron cloud build up in quadrupoles. The reason for this study was the
fact that an anomalous dependence of the electron cloud behavior on the
quadrupole magnetic field was found in simulations (increasing
thresholds with increasing gradient), which could not be explained either
intuitively nor looking into the details of the simulations results.
First, a study of the convergence of the simulation results with the time
step chosen for the simulation was conducted at a given gradient value.
This immediately revealed that the algorithm so far used for solving the
electron equations of motion (semi-analytical), which was checked to
converge for electron motion in dipole fields, performs poorly in presence
of a magnetic gradient. The reason is that the semi-analytic method used
in PyECLOUD to track the electrons (inherited from ECLOUD) allows
using time steps longer than the cyclotron radius, but is not suitable for
long term tracking, as it lacks symplecticity. This seems to be acceptable
for dipoles and drifts, where electrons lifetimes are limited to few ns, but
not in quadrupoles , where electrons can be trapped in the magnetic
gradient and survive for tens of ns or even more.
This fact unveils a need for long term accuracy in the build up simulations
in quadrupoles. The Boris tracker, known from plasma physics, seems to
be suitable for these cases. Though slower than the semi-analytical
method, the Boris tracker can be sped up with Python optimization and
then f2py.
The positive impact of the new tracker on convergence was immediately
evident, although unfortunately simulations become significantly overall
more time consuming due to much smaller time steps required.
The multipacting threshold for the LHC arc quadrupoles seems even
lower with the accurate algorithm. After scrubbing most probably they
will be the dominant contribution to heat loads and integrated electron
density along the ring (with the updated threshold values, full
suppression has become quite unlikely). This has a few implications. First,
Inner triplets studies need to be refined with the new tracking method.
The general conclusions will stay the same, but SEY estimates could be
different. Second, the effect of the electron cloud in the quads on the beam
dynamics should be assessed, because a likely scenario is that 25 ns
beams in the LHC will always create electron cloud in the quads. This
subject will be again subject of a presentation during the next electron
cloud meeting (scheduled tentatively on 27 June)
To study possible electron cloud formation in the FCC (main collider as
well as High Energy Booster in the same tunnel as the collider), Lotta
studied the electron cloud build up in the dipoles at injection energy for
different chamber radii. With 25 ns beams (bunch population of 1e11
ppb), the worst case is found for radii between 2 and 3.75 cm, with SEY
thresholds for multipacting as low as 1.1. For 50 ns beams (bunch
population 2e11 ppb), the SEY thresholds for multipacting are very high
(i.e. above 1.8) for radii below 2 cm, while they go monotonically down to
about 1.25 for a 1 cm radius.
AOB
None.
Adjournment
Next electron cloud meeting will be taking place on 27 June 2014.
GR, 05/06/2014