12/6/2014
AWAKE Performance Meeting
5th AWAKE Performance Meeting
Present: Ans PARDONS, Christoph HESSLER, Christophe ALANZEAU, Edda
GSCHWENDTNER, Eduard FELDBAUMER, Silvia CIPICCIA, Alexey PETRENKO,
Michele BATTISTIN, Jeremie BAUCHE, Mikhail MARTYANOV, Chiara BRACCO,
Steffen DOEBERT, Thanasis MANOUSOS, Vasilis VLACHOUDIS, Valentin
FEDOSSEEV.
These minutes and the documents shown or referred to are available on indico at
https://indico.cern.ch/event/323750/
Electron beam dump (E. Feldbaumer)
Studies on dumping the electron beam in the floor of the electron tunnel were performed. The
FLUKA geometry is on the same level, so the beam was dumped into the wall in the
simulations. The beam parameters are the following: pencil electron beam with 20 MeV/c
kinetic energy, bunch charge of 1 nC, repetition rate of 10 Hz and an intensity of 2.25E14
electrons/hour.
The results show that the dose equivalent rate in mSv/hr is above the limit of supervised
radiation area. Therefore access to TT41 and TCC4 is not possible. The access and
interlock system must be adapted to that.
The particle fluence spectra show that there are only electrons and photons. Steffen
comments that in practice one could have a hole in the wall with shielding around. Edi
mentions that this should not improve to situation a lot.
In addition, the effect of stopping the beam inside the electron source area with a 4 cm long
Aluminium cylinder of 0.5 m radius has been studied. The simulations results are shown with
dose per hour. Note that this is a worst-case scenario, as the beam certainly would be stopped
after some minutes. In any case, the results show that shielded doors for the electron area
are required. At the same time the 30 cm concrete shielding around the source is OK.
Energy deposition on the fast valve (E. Feldbaumer)
The layout of the fast valve and material was provided by VAT. The valve has a guiding plate
of 2 mm plus a valve plate of 3.9 mm, both made of stainless steel. The beam parameters of
400GeV/c, beam-size of =200m and ultimate intensity of 3.5E11 p/bunch were used in the
simulations.
The results show a maximum energy deposition on the fast valve of 10.03 GeV/cm3/pp,
which translates into a maximal temperature raise of 158.5 K per bunch. Action  EN/STI
will calculate the subsequent thermal stresses. Action  Edi to give EN/STI the input
parameters.
Action  Edda to check with Erdem/Patric about the temperature of the valve during
operation as the plasma is heated to 200 deg C. Is there a problem of deformation due to the
mechanical stress and heat?
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Action  Edi will do the same calculations to study the effect, when the electron beam hits
the fast valve.
It is noted that hitting the fast valve is an accident scenario only for the proton beam. This
should be protected through interlock systems.
The question was raised whether it would make sense to install the laser dump after the
electron merging point – instead of some meters downstream the laser merging point. There
is an issue of space and also it is not clear whether this is really needed.
 As baseline the laser dump stays in the area between the laser merging point and the
first following dipole.
The laser stopper could either be fast (10Hz) or we stop already the production of electrons at
the laser (easier solution).
The possibility of pulsing the electron dipole was discussed. Action  Jeremie needs more
details on the beam/magnet specifications. A fast rising time would be needed even when the
warning early signal would be used for switching the dipole.
RP studies for the laser beam block (S. Cipiccia)
Two different beam block positions have been studied: 1.5 m downstream the plasma cell and
20 m before the separation wall.Three different beam block materials were used: 2mm Al,
4mm SiO2 and 4mm SiO2 + 2mm Al. Aluminium is meant to block the laser, whereas Quarz
is installed when a small fraction of the laser beam should be extracted for diagnostic
purposes. For the simulations 3E11 protons per bunch at 400 GeV/c were assumed.
In addition the simulations were done with two different beam characteristics: a) ‘collimated
beam’, i.e. proton beam passing to the laser block without going through a plasma and b)
‘Alexey beam’, i.e. proton beam passing through the plasma cell experiencing selfmodulation-instabilities.
Assuming a temperature recovery between two consecutive beam shots, the temperature rise
from the energy deposition in the different laser beam blocks is about 1-2 degrees C for the
‘Alexey beam’ and about 10-15 degrees C for the ‘collimated beam’. Action  EN/STI to
study the thermal and mechanical stresses due to the temperature rise. Action  Silvia to
give EN/STI the input parameters.
The effect on the prompt dose is hardly different between Alexey and collimated beam,
which is assumed to be also similar for the high-energy hadrons and the low energy neutron
flux. The results show that the presence of the beam block affects the radiation environment,
which needs to be taken into account when placing electronic devices and racks. The results
are above the recommended limit of hadrons. The calculations were done assuming 4 weeks
continuous running at 3E11 at 400 GeV/c. For low energy neutron flux 8 weeks of running
were assumed.
Compressed proton beams (A. Petrenko)
Alexey showed the results of the angular spread of the proton beam downstream the plasma
cell. This has an important impact on the design of the vacuum tube.
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The simulations were done with the baseline beam (z = 400 ns), a 2x compressed beam (z
= 200 ns) and a 3x compressed beam (z = 134 ns). For these different beams, both the
plasma cell length was changed accordingly, i.e. 10 m, 5 m, 3.3 m as well as the plasma
density. The reason for the different plasma cell lengthes is that the defocusing part of the
protons comes from the SMI effect inside the plasma cell, which happens anyway only in the
first meters.
The energy gain of electrons gained in the wakefield driven by the three different beams was
shown (~1.3GeV for baseline beam, ~2 GeV for 2x compressed beam, ~2.3 GeV for 3x
compressed beam). In addition the acceptance of the plasma wakefield was presented; for the
2x compressed beam a better focused electron beam (transversally by factor ~2) is needed
(x,y from 250 m to 100 m).
The results show that the angular spread of the proton beam after the plasma cell is at the
order of 1 mrad, quite independent from the proton beam compression. The reason is that the
angular spread comes only from the SMI effect.
It is noted that the spectrometer adds another 1 mrad to the angular spread from the plasma.
The proton beam envelope 20 m downstream the plasma exit is therefore +4 cm/-6 cm
(considering the additional ‘kick’ from the spectrometer).
It must be also stressed that the electron beam envelope downstream the plasma cell is much
bigger than the proton beam envelope (+/-3cm at 5 m downstream the plasma cell). However,
these electrons will be stopped at the spectrometer, ~5m downstream the plasma exit. Alexey
showed the angular spread of the electrons in the AWAKE collaboration meeting in April
2014 at http://indico.cern.ch/event/308579/session/2/contribution/9/material/slides/1.pdf .
Action  Alexey to provide the electron envelope to Jan.
Action  Chiara to check whether the OTRs in transfer lines see beams with comparable
beam parameters (beam-size, etc…)
E. Gschwendtner, 13 June 2014
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