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PSB – Linac 4 Synchronization

Summary of discussions held with M.E. Angoletta, C. Carli, M. Chanel, A. Findlay, F. Pedersen. PSB – Linac 4 Synchronization. Chopper timing limitations: 20 ns < T NO BEAM < 1000 ns 45 ns < T BEAM

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PSB – Linac 4 Synchronization

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  1. Summary of discussions held with M.E. Angoletta, C. Carli, M. Chanel, A. Findlay, F. Pedersen PSB – Linac 4 Synchronization PSB with Linac 4 Working group meeting 24/04/2009

  2. Chopper timing limitations: 20 ns < TNO BEAM < 1000 ns 45 ns < TBEAM In other words: when the beam is not wanted towards the PSB, small bursts of 40 ns every 1000 ns will still be sent. In the Head or Tail Dump, it is not yet known if this 4% duty cycle is acceptable, but for the switching from one ring to another, this is considered as non acceptable The distributor rise time from one ring to the following is planned to be < 500 ns. If the rf (1 MHz) phase was erratic, the total “BEAM OFF” time could be as long as 1500 ns. This value cannot be reached by the chopper. Conclusion: The rf needs to be in a pre-programmed phase at injection and thus synchronized in some way. Chopper limitations and ring rf synchronization PSB with Linac 4 Working group meeting 24/04/2009

  3. PSB Injection scheme • The main dipolar field in the PSB will be ramping-up during injection (1.2 T/s). • The longest injection will take 100 μs in each ring • =>The field increase will be 1.2 G/100μs • => ΔE = 154 keV/100 μs if following the ΔR=0 frequency law (bucket height = 2.5 MeVP-P and σ of the Linac 4 energy distribution = 0.12 MeV) • => ΔR = -0.8 mm/100μs if following the Δp = 0 frequency law • Δp = -14.3 keV/100μs and ΔR = -0.86 mm/100 μs if staying at Δf = 0. • Δf = 379 Hz/100 μs if following the ΔR=0 frequency law PSB with Linac 4 Working group meeting 24/04/2009

  4. PSB Injection scheme • Operational constraint: • The hardware setup will be such as to allow different type of injections: • Injection in a stationary rf bucket (fixed rf frequency) • Injection in an accelerating bucket (rf frequency following a ΔR=0 law) • Injection with any type of rf frequency variation law . • Magnetic field increase + the duration of each injection (up to 100 μs) • A correction needs to be applied to get the proper “beam energy - field - trajectory” trio. PSB with Linac 4 Working group meeting 24/04/2009

  5. Field - Energy: • No field correction => Linac 4 energy increased by < 154 x 4 = 617 keV to keep the proper trajectory. This solution is abandoned as the longitudinal painting already uses the total dynamic range (+/- 1 MeV) of the transfer line from Linac 4 to PSB • BdL dipolar correction elements offset to cancel the field rise from one injection to the following. This way each ring will have the same “beam energy - field - trajectory” trio. Only the offset of the field being corrected, the Linac 4 energy will have to be increased by 154 keV/100μs (max ramping with a ΔR=0 frequency law). This will be the default type of operation. • Total Field increase correction. The feasibility has not been studied and might be impaired by the limited bandwidth of the BdL system. This type of correction would be perfect for a capture process at a fixed frequency and fixed energy • Conclusion: • The 4 rf are to be synchronized at injection • the magnetic field at injection is to be equal in all rings thanks to the BdL • The Linac4 energy is to be modulated to adapt to the acceleration during the capture process in each ring PSB Injection scheme PSB with Linac 4 Working group meeting 24/04/2009

  6. From the previous slides we learnt that the PSB rings should be synchronized to a reference at injection and that the Linac energy would have to be ramped up to cope with the magnetic field increase. The injection synchronization needs a reference frequency source. It could be an external generator of a standard CO type (Pentek). This unit is ppm programmable but cannot adapt in real time to a magnetic field change. Is it a problem having a fixed frequency reference instead of one being able to adapt to the real time measurement of the magnetic field? Can the Linac4 energy adapt to the real time measurement of B? Can the bucket filling timing adapt to the real time measurement of B? PSB Injection Synchronization PSB with Linac 4 Working group meeting 24/04/2009

  7. The magnetic field measurement precision is claimed to be in the order of 100 ppm. The precision of the power supplies (set value to obtained value) is of the same order. Choosing the measured value or the set value seems to create the same potential error. For a 2311 Gauss injection field, this corresponds to a maximum error of 0.2 Gauss. If the rf reference frequency stays fixed and doesn’t correspond to the real magnetic field ( in terms of ΔR = 0 law), the 0.2 Gauss will correspond to an rf bucket being shifted by -2.4 keV (0.1% of the bucket height or 2% of the Linac4 one sigma energy spread) and an orbit offset of -0.14 mm. If the idea is to track the B-train with the rf reference (in case we don’t trust the control), the rf bucket will be shifted up by +25.6 keV for an extra two 0.1Gauss tics. This corresponds to 1% of the bucket height and 20% of the Linac4 one sigma energy spread. If the rf reference is updated, the calculated bucket filling phases (if not updated) can be wrong up to ΔφREV < 63 Hz x 400μs x 360o = 9o. (63 Hz corresponds to ΔB=0.2 Gauss and ΔR=0) PSB Injection Synchronization PSB with Linac 4 Working group meeting 24/04/2009

  8. Conclusion: Adapting the rf reference to the measured B field doesn’t give more precision (in terms of evaluation of the real field) than using the programmed value. This choice would imply to adapt in real time the Linac4 energy and to modify (also in real time) the turn-per-turn timing values of the chopper. This possible real time compensation of the Linac energy and bucket painting timing requires an investment which is not supported. Conclusion: The 4 rf are to be synchronized at injection to a pre-programmed reference source the magnetic field at injection is to be equal in all rings thanks to the BdL The Linac4 energy is to be modulated to adapt to the acceleration during the capture process in each ring PSB Injection Synchronization PSB with Linac 4 Working group meeting 24/04/2009

  9. Recap: While the magnetic field is ramping up at 1.2 T/s, different type of injections will be allowed: Injection in a stationary rf bucket (fixed rf frequency) Injection in an accelerating bucket (rf frequency following a ΔR=0 law) Injection with any type of rf frequency variation law . From the Linac4 point of view, the initial rf buckets in each ring will be similar but with a specific phase. How will the phase difference between rings be programmed by the application? Due to the 1000ns maximum OFF state of the beam and due to the 500ns rise time of the distributor, a dephasing of 500ns between rings would sound right. It is estimated here that the distributor rise time is identical even when ring 1 is being filled just after ring 4 (3 rings jump). Synchronization with Linac 4 PSB with Linac 4 Working group meeting 24/04/2009

  10. Let’s check if the 500 ns dephasing is fine in all cases. Let’s try the most critical scenario (not realistic?) in this context: Before switching to ring n-x, ring n bucket is filled from rf phase 0o to rf phase 10o; We then wait until the end of the revolution period and switch to the n-x ring. In ring n-x only the interval from rf phase 350o to 360o is being filled-up. This operation would means 340o (rev phase)+ 500 ns = 1444 ns of beam chopping. This is not achievable by the chopper!! In such a case the ring switching should be started before the end of the revolution in ring n (just after 10o in our case). Then, after the 500 ns of the distributor rise, the rev phase in ring n-x should be close to 350o where the bucket filling is being started. Conclusion: The application which determines the chopper timing for the bucket filling should also take care of the distributor timing. Synchronization with Linac 4 PSB with Linac 4 Working group meeting 24/04/2009

  11. The role of the chopper is to inhibit the beam in these different case: • When the distributor is in the head or tail dump position • During the switching time of the distributor (<500 ns) • During the PSB injection process where the beam is out of the bucket • During the PSB injection process where the beam density is required to be low (bunch shaping). • The chopper has its own constraints: • 20 ns < TNO BEAM < 1000 ns • 45 ns < TBEAM • These constraints mean that there is a “granularity” of 20 ns (2 % of rev) in the beam OFF state and of 45 ns (4.5 % of rev) in the beam ON state. Chopper timing PSB with Linac 4 Working group meeting 24/04/2009

  12. What is the amount of timing information that is required for every single machine turn? For a protocol with a sequence of words [TON , TOFF , TON ,…] , due to the TON - TOFF constraints, the maximum number of words for each revolution of 1000ns would be 30 { (1000ns x 2)/(20+45ns)}. Multiplying this value by the 100 possible turns; we would end up with 3000 words per ring in the worst case. If a time resolution of 1ns is chosen and the maximum value for the words TON and TOFF is 1000 ns, we end-up with a maximum of 3000 words of 10 bits for each ring. N.B.: As there no real need for a 1ns time resolution, it might be wise to relax the constraints for the electronic circuit by asking only for a 2ns resolution. The way the ON-OFF sequence will be digitally represented is to be decided by the designer after the topics has been discussed with a CO specialist. Chopper timing PSB with Linac 4 Working group meeting 24/04/2009

  13. The ON-OFF sequence of the chopper will be decided by the operation. The control hardware (ISC: Injection Sequencing Control) will create the Beam ON-OFF windows as required by the operation insuring that the (20 ns < TNO BEAM < 1000 ns) and (45 ns < TBEAM ) conditions are fulfilled. The programmability of the chopper timing should allow to create many different bunch filling schemes (hollow bunches …) Note that the chopper amplifier needs a control pulse of 5V on 50 Ω. The control pulse length will correspond to the Kick length (Beam-OFF) including? a time distortion (the output pulse length is not exactly what is requested - less than1 ns) that should be taken into account. When the distributor is in the head or tail dump position, the chopper is expected to stay in the BEAM-OFF state with the minimum beam duty cycle. This topics is still to be discussed as there might be a request from the Linac4 rf crew to have some beam available before the first injection. This demand comes from the worry that the first Linac bunches might be perturbed by beam loading transients effects in the accelerating chain. Chopper operation PSB with Linac 4 Working group meeting 24/04/2009

  14. Chopper operation Courtesy Mauro Paoluzzi PSB with Linac 4 Working group meeting 24/04/2009

  15. The ISC: Injection Sequencing Control will receive the reference rf on which all rings are synchronized. The Beam ON-OFF timing values (including? the negligible time distortion of the chopper amplifier) calculated by an application will take this reference rf to create the Beam ON-OFF sequence. The application will predict the buckets position and shape assuming the proper value of the rf voltages, h1-h2 phase, initial injection synchro phase, type of frequency law during the capture process and flight time modulation due to the L4 energy modulation. The application will also take care of the filling-up of the buckets, taking into account the expected distribution, by creating a Linac4 energy modulation signal with its corresponding debuncher phase modulation. The application will also calculate the distributor timing values to allow having a Beam OFF duration bellow 1000ns as described in slide 10. The application could also calculate a feed-forward function to be applied to the rf chain. Chopper operation PSB with Linac 4 Working group meeting 24/04/2009

  16. Debuncher Pre-chopper +LEBT Energy modulation Source Distri 4*rf Chopper Linac 4 Synchronization with Linac 4 180 m Beam OFF Window 45 keV Linac rf feed-forward Injection Sequencing control Voltage modulation Phase modulation BIXi.SDIS Timing CO BIXi.SInjChop BIXi.SInjRF Rev Inj. Ref Source h2 1/2 h1 SP2T Inj. rf reference (h1 or h2) ΔE Linac, Δφ debuncher Application BIXi.SDIS TON-OFF Chopper BIXi.RF_PHASE RF feed-forward PSB with Linac 4 Working group meeting 24/04/2009

  17. Where should the ISC (Injection Sequencing Control) be located ? Inputs to the ISC: Register values from CO network Start pulse from standard CO modules Rev reference from C0 module Output from ISC: TOFF chopper window to chopper amplifier Voltage modulation to last 2 PIM cells RF feed-forward Phase modulation to debuncher Linac4 signal(s) interesting for the operation of the Beam control: PU SUM signal (if the Linac4 micro-bunches are still discernible) or fast beam transformers (able to discriminate 20ns pulses)from each of the 4 injection lines (after the distributor but before the ring) ISC PSB with Linac 4 Working group meeting 24/04/2009

  18. Where should the ISC (Injection Sequencing Control) be located ? The Beam-OFF signal controlling the chopper is supposed to create a window having a 1 ns precision. Having such a precision requires a fast rise time and/or a very stable threshold level. Considering the possibility of having the ISC far from the chopper, with a possible common mode signal impairing the DC level of the signal and thus the threshold point, it would be convenient to have a 1 ns rise time of the control window. Such a rise time corresponds to a bandwidth of 1 GHz. A 200 m long Flexwell 3/8 has an attenuation of 17 dB at 1 GHz . The temperature dependence of the cable electrical length is also an issue to be considered Conclusion: The ISC could well be close to the chopper amplifier where the control network is available. The decision can be left to the team in charge of the development. ISC PSB with Linac 4 Working group meeting 24/04/2009

  19. The 4 PSB rf will be synchronized at injection to a ppm programmable CO type signal generator (Pentek). This signal will be the reference both for the ISC and for the 4 rf . The BdL correctors will be used to equalize the dipolar field in all rings during each individual capture process The Linac4 energy will be modulated to adapt to the acceleration during the capture process in each ring The debuncher phase will be modulated to adapt to the Linac4 energy modulation The chopper will be activated taking into account the flight time changes due to the Linac4 energy modulation, taking into account the ramping-up frequency during capture, taking into account the painting scheme and taking into account the switching particularities of the chopper amplifier. The Injection Sequencing Control module developed by the rf team will be located at the position they find most convenient. A pick-up sum signal or a fast transformer signal issued from the 4 PSB individual injection lines need to be available for monitoring in the BOR as a diagnostic tool for all the upstream Linac4 system. Summary PSB with Linac 4 Working group meeting 24/04/2009

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