The synchronization of the CLIC beams Giulio Morpurgo

The problem The synchronization the CLIC beams Two couple of beams (DB1, MB1 and DB2, MB2) Synchronize DB1 (x 24) with MB1 Synchronize DB2 (x 24) with MB2 Synchronize MB1 with MB2

drive beam accelerator 2.38 GeV, 1.0 GHz combiner rings Circumferences delay loop 72.4 m CR m CR m CR1 CR2 delay loop 1 km e + injector, 2.4 GeV e - injector 2.4 GeV CLIC 3 TeV e + main linac e - main linac BC2 BC1 e + DR 365m e - DR 365m booster linac, 9 GeV, 2 GHz or 4 GHz ? decelerator, 24 sectors of 868 m IP1 BDS 2.75 km BDS 2.75 km 47.9 km CR2 delay loop drive beam accelerator 2.38 GeV, 1.0 GHz 1 km CR1 TA R= 120 m TA R= 120 m 245 m 83ps 45fs DB MB Stolen from B. Jeanneret MB

The Main Beam(s) : 312 bunches spaced by 15 cm (500 picoseconds) – Ideally all the bunches have the same charge and energy The Drive Beam(s) : 24 trains of 2904 bunches spaced by 2.5 cm (83.3 picoseconds) – Ideally all the bunches have the same charge (0.1%) – Error in charge implies error in energy (fully loaded acceleration) – Energy spread inside a bunch ~ 1% – Inside a train, an energy modulation with period 24 bunches and amplitude 0.1%. This is due to different paths (-> energy losses) in the recombination complex) A CLIC pulse

A CLIC pulse (continued) The Main Beams are centrally produced, then after some manipulation are sent to the end of the CLIC tunnels. At that point, with an energy of 9 GeV, they pass trough a turn-around and enter the Main Linac The Drive Beams are centrally produced and accelerated to 2.4 GeV. After some manipulation, the 24 trains are sent towards the end of the CLIC tunnels. But the trains do not reach it, as each one is deflected into a different turn-around, leading to one of the 24 “Decelerator Sectors”. These turn-around are shorter than the one for the Main Beam.

The MB-DB synchronization requirement The drive beam train (x 24 x 2) enters its decelerating sector at time t d. The main beam arrives later, at time t m. The optimal value for t d -t m is Δt opt. The real value is Δt real. The requirement is that | Δt opt – Δt real | < 20 femtoseconds* If we convert this to distances, at the speed of light 20 femtoseconds == 6 microns * the number may actually not be exactly this; but the order of magnitude is correct

Where does the MB-DB requirement come from If the drive beam(s) and the main beam have a wrong relative phase, the main beam – will not be accelerated properly, and – its emittance will blow up (reducing luminosity)

The MB-MB synchronization requirement, and where it comes from CLIC goal: collide two beams The two main beams should arrive simultaneously at the IP and collide where  is minimum, otherwise the luminosity will be reduced. The max. tolerable mismatch is of the order of femto-seconds (i.e. ~20-30 microns)

The goal of this presentation Present a synchronization strategy Identify difficulties and problems Identify areas where work is required Propose something to be done in the incoming years

Some general remarks In principle, we have a way (feed-forward) to correct the average time offset between the Main Beam and each Drive Beam train. The effects of small time errors in between the different bunches of a Drive Beam train may tend to compensate each other, as the accelerating field for the Main Beam bunches is influenced by many Drive Beam bunches The effects of small time errors in between the different bunches of the Main Beam would be more harmful, because they could not be individually corrected

The synchronization strategy Synchronize at the beginning Try to maintain the synchronization Identify effects which could spoil the synchronization Measure and correct in places where synchronization could have been lost Measure to check if we were able to do it

Synchronize at the beginning (1) The two drive beams are produced at the same time, and not too far from each other. The production mechanism is the same for both beams. It should not be too difficult to achieve a good synchronization. The two main beams are produced using two different mechanism. Perhaps they can be synchronized at the two pre- injector Linacs? The two main beams spend 20 msecs in the Pre-Damping rings, and 20 msecs in the Damping rings. A common RF-timing system there should give “perfect” synchronization.

Synchronize at the beginning (2) The time interval between the production of the Main Beam and the production of the Drive Beam responsible for its acceleration is about 40 millisecs (the Main Beams stay 20 msecs in the Pre-Damping ring and 20 msecs on the Damping ring); probably too much to obtain a relative precision of a few femto- seconds. We should synchronize the production of the drive beam and the extraction of the main beam from the damping ring. We need a central “femto-timing” system covering this part of the CLIC accelerator

Synchronize after damping rings In the damping ring, the longitudinal position is determined by the buckets defined by the RF system. This offers good synchronization opportunities. However......has the amplitude of longitudinal bunch oscillations in the damping rings been considered? (i.e. How well centred is the bunch inside its bucket when we extract it?) Bucket size: 15 cm, bunch size 1.5 mm, tolerance 6 microns (~ 1/250 of bunch size) If different bunches had different oscillation phases, this could be a problem NEEDS TO BE INVESTIGATED

Maintain and correct the synchronization In principle, if the two beams are synchronized at some moment, to maintain the synchronization one just needs to have a perfect control on the lengths of the two beam paths. If we want to be able to correct the unavoidable errors, we must be able to measure the (difference in) the path lengths of the beams, and compensate these differences by tuneable chicanes.

Possible sources of errors Beam energy errors -> different paths in turn-around, bends, chicanes Magnetic field variations – Power supply stability – “Train” currents Ground expansion/contraction – Tides (moon) – Lake level / Rain Timing systems (if the two Main Beams are not extracted at same time)

Measure and correct where synchronization could have been lost Feed-forward: measure and take action on the same pulse Feed-back: measure this pulse, take action on the next one Post-mortem: measurements showing how well we managed to synchronize the beams Synchronization chicane: where the path length is modified Feed-forward will be used to fine-tune the synchronization between the Main Beam and each Drive Beam train Feed-forward could be needed to synchronize the two Main Beams (if they are not extracted simultaneously from the Damping Rings) A synchronization chicane somewhere will be needed to resynchronize the two Main Beams (and perhaps the Drive Beams) The “post-mortem” measurements will help in (re)setting the reference values for the feed-forward systems (feed-back)

Drive Beam – Main Beam synchronization Each Drive Beam train has to be synchronized with the Main Beam Before the entrance of each Drive Beam turnaround – Measure arrival time of outgoing Main Beam – Measure arrival time of specific Drive Beam train – Compare difference of arrival times against a reference – Correct the path length of DB train by setting a chicane at the end of the DB turnaround

e + injector, 2.4 GeV e - injector 2.4 GeV CLIC 3 TeV e + main linac e - main linac BC2 BC1 e + DR 365m e - DR 365m booster linac, 9 GeV, 2 GHz or 4 GHz ? decelerator, 24 sectors of 868 m IP1 BDS 2.75 km BDS 2.75 km 47.9 km drive beam accelerator 2.38 GeV, 1.0 GHz combiner rings Circumferences delay loop 72.4 m CR m CR m CR1 CR2 delay loop 1 km CR2 delay loop drive beam accelerator 2.38 GeV, 1.0 GHz 1 km CR1 TA R= 120 m TA R= 120 m 245 m 83ps 45fs DB MB Stolen from B. Jeanneret Drive Beam Main Beam resynchronization

The famous MB DB feed-forward in the tunnel Using a local femto-timing system, measure delay Δt real between the two outgoing beams DB turn- around MB to IP Out-going DB B B’ D D’ Sensitivity of δE measurement/correction? Intra-pulse corrections ? *Precision of the « phase » measurement *Stability of local timing C Stolen from B. Jeanneret At B&B’: compare Δt(MB DB) real with reference Δt (the difference is the “phase” error) Also measure Drive Beam energy error δE (use A&B) Compute correction (δE dependent) to “re-align” DB to MB using “C-D” chicane Use D&D’ to measure residual phase error MB/DB after feed-forward Use B&C to measure time to cover the Drive Beam turn-around Out-going MB *see work by Jonathan and Alexandra Issues: A

What exactly has to be synchronized? The main beam consists of 312 bunches spaced by 500 picoseconds (15 cms) – Are these intervals all equal? To which precision? – Are the energies of the individual bunches all identical? To which precision? – In principle earlier errors should be cancelled by the damping ring; only concern could be longitudinal oscillations around bucket centre NEEDS TO BE STUDIED – How stable is the average energy of the Main Beam (path length in the final turn-around)? Is a compensation mechanism required?

What exactly has to be synchronized? (1) A train of the drive beam consists of 2904 bunches spaced by 83.3 picoseconds (2.5 cms) – Are these intervals all equal? To which precision? – Are the energies of the individual bunches all identical? To which precision? – Are the currents of the individual bunches all identical? To which precision* (current error energy error) *CTF %, without any specific emphasis on this point NEEDS TO BE STUDIED

What exactly has to be synchronized ?(2) The bunches of the drive beam lose their energy in the PETS, and this energy produces the accelerating field for the main beam. Every bunch of the main beam is accelerated by the field – produced by n (~60) bunches* of the drive beam – (perturbed by the previous n/6 main beam bunches) * If the filling time of the RF cavities is 5 ns

What exactly has to be synchronized?(3) What is the effect on the MB of a regular energy oscillation pattern among the Drive Beam bunches? – Different paths in the recombination complex -> different energy losses due to synchrotron radiation (although only 0.1 %, while energy spread within a bunch ~ 1%) – Different path lengths in the DB turn-around

What has to be measured? Main Beam – The bunches have to be aligned as precisely as possible to the 500 picosecond intervals. – The system should work also when the pilot main beam is used (ideally should be able to work also with just the arrival time of the first bunch) Drive Beam – Over how many bunches do we measure? – Do we have to do several measures (and corrections) over a train ?

Questions related to the feed-forward Initial setting of the references Range of the feed-forward (how large phase errors should it be able to correct?) Speed: 1 correction per train or intra-train correction? On how many drive-beam bunches should it average? Timing stability (<20 femtoseconds over 140 µ secs) Compatibility with Machine Protection: it must do very quick corrections, therefore its inertia must be much smaller than the 2 millisecs specified by the MPS. The range of the corrections should be such that no dangerous situation can be produced.

What is not corrected by the feed-forward: classes of errors and symptoms After measuring the arrival time of the outgoing beams, comparing them with the Reference, and correcting the drive beam path length, we should have achieved a perfect synchronization, and this should be confirmed by time arrival measurements at D and D’. If the measurements at D and D’ show lack of synchronization, we have to understand what is wrong, and take action. Let’s assume that the problem is not a measurement one, and try to identify what can cause it.

Classes of errors, and error patterns (1) (Initial setup: Reference not yet established: change it) Some initial bunches of one of the two beams are lost somewhere after BB’ (error in time, not in phase, which should go modulo 2.5 cm) – A loss of one or more bunches will not go undetected – Main Beam: error at all DD’ points closer to IP – Drive Beam: error only at one DD’ point The path length of the Main Beam (from B’ to D’) has changed – One should look for a correlated effect (not necessarily identical) at all the 24 DD’ synchronization points – If the path length change comes only from the final Main Beam turnaround, the effect should be identical at all 24 DD’ points – Check also IP synchronization between the two Main Beams; it should be visible there (unless change is identical on both sides).

Classes of errors, and error patterns (2) Drive beam intensity not perfectly stable – Error in energy -> error in turn-around – But it should be known by A-B measurement, and compensated – Remark: if all drive beam trains had, for example, a bigger intensity, i.e. lower energy, and the energy compensation correction did not work correctly, we should see the same synchronization error at all 24 DD’ points Magnet power supplies not stable (i.e. Path length not stable) – Measure – If instability in final MB turn-around, correlated error; if in DB turn-around(s) uncorrelated errors

Classes of errors, and symptoms “Time constant” of the error: it will be important to have models; if the “errors” can be predicted, References could be adjusted even in advance – Days/Hours (lake level, tides): change References MAY BE LEP EXPERIENCE (energy calibration) CAN BE REUSED – Hours/Minutes (“train currents”): change References MAY BE LEP EXPERIENCE CAN BE REUSED; measurements will be needed

Post-mortem: How do we know that everything was well synchronized? It could be that our references are wrong: we think we synchronize well, but in reality we do not. Can we measure the Main Beam size and energy at the end of every accelerating sector?

Main Beams synchronization The two Main Beams have to be synchronized, to arrive simultaneously at the IP A chicane to correct the path length will be installed just before the entrance of the Main Linac Depending on the extraction schema from the damping rings, a feed-forward mechanism might be applicable/(needed?).

Damping rings Ideally the two beams should leave the damping rings at the same time. To minimize differences: Use common Power Converters Use common timing However this might not be possible, as the two beams share the same Booster Linac, where they are accelerated from 2.4 to 9 GeV

Two ways the Booster Linac could work 1.First accelerate a complete main beam, then accelerate the other beam OR 2.Accelerate alternated bunches from the two beams

Schema 1 (currently envisaged) The two beams are not extracted from the damping rings at the same time The two transfer lines from the end of the Booster to the turn-around before the main Linac must have a length difference of about 50 meters (at least) Is this difference given by an additional 25x2 meters straight line, or by a bend?

Schema 2 The two beams are extracted at the same time An additional transfer line of a few centimetres (half of a bucket) is added between one of the damping rings and the Booster, and the same is done after the Booster to the other beam Advantages from the synchronization point of view – The two beams see the same timing (and same jitter) in the damping rings – The two beams see the same power supplies (and ripples) in the damping rings – The two beams “do the same things” after the booster

Main Beam feed-forward resynchronization (schema 1) (see next slide) Precision timing Measure arrival time difference of the two beams just after the Booster Linac Compare with a reference Send correction value to be applied to the “resynchronization chicane” located just before the entrance of the main Linac Update references for all 24 local MB-DB resynchronization systems (is this possible?) BUT THIS WOULD NOT HELP WITH ANY ERROR AFTER THE EARLY SYNCHRONIZATION POINT.

e + injector, 2.4 GeV e - injector 2.4 GeV CLIC 3 TeV e + main linac e - main linac, 12 GHz, 100 MV/m, km BC2 BC1 e + DR 365m e - DR 365m booster linac, 9 GeV, 2 GHz or 4 GHz ? decelerator, 24 sectors of 868 m IP1 BDS 2.75 km BDS 2.75 km 47.9 km drive beam accelerator 2.38 GeV, 1.0 GHz CR1 CR2 delay loop 326 klystrons 33 MW, 139  s 1 km CR2 delay loop drive beam accelerator 2.38 GeV, 1.0 GHz 326 klystrons 33 MW, 139  s 1 km CR1 TA R= 120 m TA R= 120 m 245 m Measure time difference Compensate delay Send Corrections Synchronization of the two Main Beams Feed-back after collision

More sophisticated feed-forward at MB turn-around ? (global timing system) Basic idea: – each of the two beam should enter and leave the final turn-around at a pre-determined Absolute Reference Time – In reality the entrance time will have an error – After the turn-around a chicane could compensate the initial error This system would require a global femto-second timing system covering the entire CLIC site, to be used also by the MB-DB feed- forward systems Can such a system be implemented over 50 kms? Perhaps we could have local timing systems, which resynchronize often enough by “simultaneously” exchanging time information over the same cable? The benefits of such a system have been described in Daniel’s slides

Main Beam turn-around timing system It would be interesting to have a local femto- timing timing system to measure precisely how long does it take for the Main Beam to travel along the turn-around. One could probably use the timing system of the first Drive Beam feed-forward

Additional complication It is being envisaged to split each main beam into two parts (to balance more the damping ring occupation). These two parts will be somehow recombined into the final pulse between the damping ring extraction and the Booster Linac. This would potentially create differences between the different bunches of a main beam, and the sources of errors should be carefully evaluated.

Summary of required femto-timing systems A system covering the beam generation/recombination system and the damping rings, for the initial synchronization 24 x 2 local systems at the Drive Beam turn-around, for the feed-forward systems A system at the IP, to verify the MB-MB synchronization Possibly a system before or after the MB Booster Linac, to check the MB-MB synchronization Possibly a global system, or resynchronization of the local systems, for absolute Main Beam turn-around resynchronization

Summary of required correction chicanes 24 x 2 chicanes at the Drive Beam turn-around, for the feed-forward systems At least one chicane after one of the two MB turn- arounds, to resynchronize the two MBs. Two chicanes are needed if “independent” resynchronization based on a “global” timing is used.

Data analysis of timing information All the different feed-forward and measurement systems must send their data to a central processing unit, whose tasks are to analyse the synchronization errors and eventually produce a new set of References for the feed-forwards. The data should include – Arrival time intervals (i.e. Differences between two beams) – Corrections – Travel times in the different turn-around – Power supplies measurements at the time of the passage of the beam – Beam energy measurements

Proposals for next years (1) (only to stimulate discussion) Install a complete feed-forward system (like the final one) to correct predefined “errors” on the drive beam -Replace Main Beam arrival time by predefined value -Compute and send corrections -Measure the effect of the correction -Duplicate the “arrival-time” BPMs, to study errors and reproducibility -The chicane should be able to correct at least one order of magnitude more than the errors from the timing measurements Could the first time-arrival monitors be installed in the combiner ring, the last half-turn of it used as the turn-around, and the chicane be installed on the transfer line leaving the combiner ring?

Proposals for next years (2) (only to stimulate discussion) As soon as anything like a Main Beam is available, set up a “central timing system” to study the synchronization of the production of the Drive Beam and the Main Beam Study if it is possible to achieve a unique global timing system, either a genuine one, or one composed by several local systems with a resynchronization mechanism

Reserve slides

Post-mortem How do we know that everything was well synchronized ? IP measurement (arrival time of two beams) Main beam size ? Dispersion measurements after every sector? At the feed-back systems: arrival time after MB-DB correction All these measurements are very useful, also to modify the reference values for next pulse

e + injector, 2.4 GeV e - injector 2.4 GeV CLIC 3 TeV e + main linac e - main linac, 12 GHz, 100 MV/m, km BC2 BC1 e + DR 365m e - DR 365m booster linac, 9 GeV, 2 GHz or 4 GHz ? decelerator, 24 sectors of 868 m IP1 BDS 2.75 km BDS 2.75 km 47.9 km drive beam accelerator 2.38 GeV, 1.0 GHz combiner rings Circumferences delay loop 72.4 m CR m CR m CR1 CR2 delay loop 326 klystrons 33 MW, 139  s 1 km CR2 delay loop drive beam accelerator 2.38 GeV, 1.0 GHz 326 klystrons 33 MW, 139  s 1 km CR1 TA R= 120 m TA R= 120 m 245 m 83ps 45fs DB MB CLIC_WSHOP_2009,BJ,WG5

Constant errors Ex. Different lengths of the damping rings Damping ring length = 365m; RF frequency 2GHz The main beams spend about 20 msecs in the damping rings. They travel for Km == times the max. tolerance. They do about turns. If the lengths of the two damping rings differs just by 10 microns, the overall path length difference of the two beams would be 16 centimetres. But the two lengths could be kept identical if the same RF frequency is used (same RF clock).