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CLIC Technical Committee
Summary of meeting held on 15.12.2009
Present:
A. Andersson, Ed. Ciapala, R. Corsini, JP. Delahaye, R. Herr, JB. Jeanneret, M. Jonker,
G. Morpurgo, G. Riddone, H. Schmickler
Excused:
C. Hauviller, H. Mainaud-Durand, J. Osborne
1. Approval of minutes and outstanding actions
http://indico.cern.ch/materialDisplay.py?materialId=minutes&confId=54995
The list of actions is available in Indico:
http://indico.cern.ch/materialDisplay.py?materialId=2&confId=54995
2. Recurrent items
The list of feasibility issues is available in EDMS# 918791.
The updated list and schedule of CTC topics until September 2010 is available in Indico.
http://indico.cern.ch/materialDisplay.py?materialId=0&confId=54994
3. Consideration on the Conceptual Phase Stabilisation System – D. Schulte
http://indico.cern.ch/materialDisplay.py?contribId=6&materialId=slides&confId=54996
The concept for phase stabilization system of the drive beam generation and beam transport has
been presented. The aim is to synchronize the drive beam with respect to the main beam, and then
to synchronize the two main beams.
The main-beam to main-beam phase tolerance and the main-beam to drive beam tolerance have
been calculated. Integrated simulations have been performed with PLACET and GUINEA-PIG of
main linac, BDS and beam-beam.
To evaluate the impact of RF error in a misaligned machine, the configuration after ten days of
ground motion and one-to-one alignment were considered. The resulting luminosity from emittance
growth is comparable to the one caused by limited BDS bandwidth.
The relative gradient tolerance of the drive beam after it has been accelerated is 1x10-5, which is
very tight. Higher values were measured in CTF3. A solution which relaxes tolerances is needed.
The proposal is to mitigate this effect with a feedforward at the final turn-around. In this case, the
phase error can be cured.
In the central complex, external timing reference is assumed. Along the main linac, distributed timing
system is considered. One proposal would be to use the main beam as a reference. A second
proposal would be to use the FEL scheme.
Filtering and intra-pulse feedback are to be considered as a spare option.
The proposed strategy is to implement feedback to deal with slow variations and tuning to deal with
static or slow imperfections.
Open issues are the drive beam source design, the damping ring phase, energy and charge stability
and the relative phasing of the drive beam to the RF.
We have a concept for drive beam generation and transport complex that leads to acceptable
tolerances. For the main beam we need to decide whether it serves as a distributed timing reference
or not. The effective loop and transfer line lengths will be measured and can be corrected with
feedback.
The missing systems need to be studied and detailed layouts for the conceptual systems.
ACTION BPWG
Input from RF, phase stability and CTF3 are needed to establish that design is viable.
ACTION BPWG, G. Morpurgo, E. Jensen, CTF3
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We should review the module phase stability strategy.
ACTION BPWG and CMWG
4. The synchronization of the CLIC beams – G. Morpurgo
http://indico.cern.ch/materialDisplay.py?contribId=4&materialId=slides&confId=54996
For CLIC to work properly, each of the 24x2 drive beam trains must be tightly synchronized (~20
femtoseconds == 6 microns) with the main beam to whom they provide the acceleration energy.
This synchronization has to be achieved at the entrance of the deceleration/acceleration sectors. In
addition to this, the two main beams must be synchronized with each other to a precision of 70-100
femtoseconds (20-30 microns). The strategy to achieve the synchronization between the main beam
and the drive beam trains consists of several steps. First, the moment when the drive beam is
produced should be synchronized as best as possible with the moment when the main beam leaves
the Damping Ring. After this, all the effect which can spoil this synchronization should be kept under
control. Finally, the arrival times of the outgoing main beam and drive beam train are measured
before the turn-around which brings the drive beam train into the decelerating sector, and the two
beams are resynchronized by tuning the path length of the drive beam train ("feed-forward"). The
arrival times will be again measured after the correction, to be sure that the resynchronization
mechanism works, and to detect variations in the main beam path length going all the way to the
final turn-around and back; these variations can be corrected for next pulse ("feed-back"). The
picture is made more complicated by the structures of the drive beam and of the main beam, which
do not consist of a single bunch, but of a sequence of several bunches, ideally equally spaced and
with equal intensity and energy. In particular, the drive beam bunch distribution will certainly show a
modulation repeating every 24 bunches, due to the different paths in the recombination complex. In
addition, any variation in charge between bunches translates into an energy variation (due to the fully
loaded acceleration schema), and therefore into different paths in the bending sections. For the main
beam, one has to ensure that the bunches do not perform out-of-phase longitudinal oscillations
when they leave the Damping Ring, because the resulting errors will be probably impossible to
correct.
To implement the initial synchronization and the final feed-forward correction, very precise "femtosecond" timing systems will be required.
The synchronization between the two main beams could be in principle initially achieved at the
Damping Rings, by having them sharing the same RF system and timing (and power supplies); but this
is somehow complicated by the fact that in the present schema the two main beams do not leave
the damping ring at the same time, as they are accelerated by the booster Linac one after the other.
An interleaved acceleration schema would be preferable from the synchronization point of view, but
it is currently not envisaged for other reasons. One should evaluate the additional cost of making a
second parallel booster Linac, as the beam synchronization remains anyway one of the most critical
issues. After the two main beams are split and sent to the opposite sides of CLIC, any attempt of
resynchronization becomes difficult; the only possibility would be to have a global femto-second
timing system over the entire CLIC complex (or several local systems resynchronized with the
central one?), use it to measure the arrival time of the main beams before the final turn-arounds, and
implement correction chicanes after the turn-around.
Because of the very tight requirements, all sort of effects (tides, lake level, train currents, etc.) have
to be studied; experience could be gained by the work done at LEP by the energy calibration group.
As far as the activity for next years is concerned, a complete feed-forward system should be installed
at CTF3, using the combined ring as a turn-around. The set-up should be equipped with double
instrumentation, to gain experience on the possible sources of measurement errors and on the level
of precision that can be achieved. A femto-timing system covering the CTF3 complex should be put
in place. As soon as a main beam is available, one should start studying the synchronization between
the production of the two beams. Once the femto-timing system is in place, one could study the
resynchronization between local timing system, by logically splitting into two parts and comparing
the globally distributed timing information with the resynchronized one.
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5. Precise and Fast Timing Network – a first view – JB. Jeanneret
http://indico.cern.ch/materialDisplay.py?contribId=5&materialId=slides&confId=54996
The Main Beam (MB) and its Drive Beam (DB) must be synchronized down to a phase tolerance of
δφ=0.2o, with all contributions included. As for the transmission error along the network, a smaller
value (1/3) must be considered. Therefore, δφnetwork=0.07o or equivalently converted to distance and
time δz = 5 μm and δt = 15 fs as for a coherent error. The same values are twice smaller for the
synchronization of the two MB’s.
While a measurement system which feeds a delay system made of a chicane with RF-kickers at every
DB turn-around along the main tunnel was already proposed (D.Schulte et al. CLIC Note 598 , A.
Andersson and J. Sladen, EPAC06-TUPCH086, and PAC07-MOPAN066), a global timing network is
essential for diagnostics and corrections.
The network must comply with the above-mentioned tolerances over long distances, i.e. up to 2
kms. Standard copper cabling cannot be used, because of errors associated to thermal expansion,
see slides. The optical fiber approach developed for XFEL must be considered. There is presently no
in-house expertise about this sophisticated technology. A collaboration with Bilkent University in
Ankara is considered, together with contacts in DESY and PSI.
A tentative network schematic is presented in figure 1, which is an improved version of the one
shown in the slides. An open point is about the need, or not, of a fiber system along the main linac,
which would add to the use of the 9 GeV beam line as a timing source. The presence of the fiber
line would help to diagnose any error associated to the long memory storage between the passage
of the 9 GeV beam and of the accelerated beam and to possible path-length errors in the Main Beam
turn-around.
It remains to work-out the segments of the network which need be precise only, i.e. used for intercycle corrections, and those one which must be both precise and fast, i.e. for intra-cycle corrections.
In parallel, timing errors affecting either the measurements or the beams (vibrations in curved beam
lines, pulsed kicker errors) must be explored.
Figure 1: Updated CLIC fast and precise timing network scheme (B. Jeanneret)
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Next Meeting
The next meeting is scheduled on 26.01.2010
Agenda
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Approval of minutes
Work planning and budget needs for CTC related 2010 activities
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Workpackges to be prepared during 2010 – G. Riddone, H. Schmickler
Follow-up of open actions and future meetings
The dates for the next CTC meetings are available in Indico:
http://indico.cern.ch/categoryDisplay.py?categId=1795
ANNEX 1: Action item lists
Minutes written by JB. Jeanneret, G. Morpurgo, G. Riddone