Electron cloud meeting #14, 23-24/07/2014
Participants: H. Bartosik, R. Cimino, J. Hulsman, G. Iadarola, K. Li, E. Métral, L.
Mether, A. Romano, G. Rumolo, M. Taborelli
Excused:
Matters arising and general information (G. Rumolo)
Follow up from the minutes of last meeting and new information:
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The PhD project for multipacting in RF structures was prepared and
financing (50% HL-LHC and 50% LIU-SPS) has been requested. A synergy
between scrubbing studies on the test bench and the study of electron
cloud in crab cavities has been established and the project should attack
both under the LIU-SPS and HL-LHC umbrellas.
Lotta Mether will give a talk on “Two-stream instabilities in low emittance
rings” at the LER Workshop in Frascati (15-17 September, 2014). A
rehearsal is foreseen within the electron cloud meeting framework
sometime early in September.
LESEY laboratory measurements and SEY parametrisation (R. Cimino)
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Two methods are used for measuring the SEY of a sample in laboratory.
The common set up consists of an electron gun shooting electrons on a
sample and the ratio between emitted and impinging electrons on the
sample needs to be determined. Both methods rely first of all on the
measurement of the electron current from the electron gun used to
bombard the sample. This is usually done with a Faraday cup. Then, in
one case the current from the sample is measured, which is the incoming
current minus the current of emitted electrons. With the second method,
the emitted electrons are directly measured after reaching a
hemispherical biased collector surrounding the sample. Measurements
are done at different energies by varying the energy of the electrons
emitted by the gun. All these SEY measurements are done with low
electron currents to avoid scrubbing the sample while different energies
are scanned through.
The main difficulties of measuring SEY at low energy are the
electromagnetic perturbations on the electrons before they land on the
sample (e.g. the influence of the earth magnetic field or the field from ion
pumps to have UHV conditions around the sample), the divergence of the
low energy electron beam and the gun capability of producing electron
beams in the few eV range with low energy spreads. One of the
advantages of the first measurement method is that the gun can be placed
closer to the sample. This reduces the influence of spurious
electromagnetic fields as well as the divergence of the electron beam
before it reaches the sample. The problem of the gun performance in the
low energy range is usually circumvented by producing higher energy
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electrons with the gun and then slowing them down with a negative bias
before they hit the sample.
From the measurements published in 2004 in PRL, it seemed that low
energy electrons tend to have a high probability of elastic reflection. More
recently Kaganovich et al. (ECLOUD’12) have suggested that both
calculations and old measurements show actually that SEY tends to
vanish for low energy electrons.
SEY measurements were repeated on an atomically clean Cu sample with
careful setting of the energy scale. Since measurements are done on a
biased sample, obviously electrons with energies below eVbias will not
reach the sample resulting into an SEY equal to 1. This is to be interpreted
as a feature of the measurement technique. Besides, there is also a
blurred zone for electrons with energy around eVbias because the
measurement suffers from the existence of an energy spread for the
incoming electron beam. Here part of the electrons will be repelled by the
bias voltage while others will reach the sample and produce secondaries.
With these measurements, the very low SEY at low energy is recovered on
the atomically clean sample. Also the width of the band of transition to the
expected SEY=1 at very low energy is consistent with the energy spread
from the electron gun and scales with it. Correcting the measurements in
this transition band, the convergence to values close to zero of the
measured SEY is recovered in the low energy range for this specific
sample.
However measurements on the “as received” sample show that real
measurable SEY curve does not converge to zero for this type of surface
and actually plateaus already for energies higher than the bias energy
(below 50 eV). This behavior has been shown to have a significant impact
in the predicted electron cloud build up thresholds in the LHC arc dipoles,
for instance. Practically, there is a big difference between the behavior of
a pure metal and a technical surface.
Repeated measurements on a sample scrubbed in laboratory have shown
that the low energy part of the curve seems not to change its shape with
the scrubbing (i.e. while the SEY is lowered by the electron
bombardment). This result is still preliminary.
Using the measurements above one could think of either using a new
parametrization for the low energy behavior of the SEY curve, or
alternatively feed experimental curves directly into future PyECLOUD
simulations.
Update on current activities (A. Romano)
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The PS electron detector in MU98 was further analyzed by simulations
The radial position of the beam was scanned in order to find out for which
position we may expect to see a signal and when the signal could even
disappear due to stripe formation outside of the region covered by the
monitor. The idea will be then to study the electron signal while doing
radial steering and measure the expected effect on the signal.
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Scanning the beam position by +/- 3cm around the nominal orbit, the
total electron flux through the chamber is found not to be much affected
by the position of the beam in the chamber, but obviously only on the SEY.
The electron flux through the holes depends on both the SEY and the
beam radial position. In particular, depending on the SEY, the signal could
quickly disappear if the beam is between -3 cm and its nominal orbit.
For an SEY of 1.6 (typical PS values from past experience), the signal
drops when the beam is moved more than 1 cm inwards (i.e. away from
the detector), while it is again recovered when the beam is moved back
towards its nominal orbit or even farther outwards.
Next step will be to perform both bunch intensity and length scans to
predict what we could expect at different points in the 25 ns beam
production cycle as well as with different intensities injected from the
PSB.
For the moment the electron signals are still displayed in e/m, but this
will be soon updated in uA (the conversion is not trivial due to the
geometry of the detector)
AOB
None.
Adjournment
Next electron cloud meeting will be taking place in August 2014.
GR, 29/07/2014