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Linac 4 Timing

Summary of discussions held with M.E. Angoletta, P. Baudrenghien, C . Carli, M. Chanel, A. Findlay. Linac 4 Timing.

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Linac 4 Timing

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  1. Summary of discussions held with M.E. Angoletta, P. Baudrenghien, C. Carli, M. Chanel, A. Findlay Linac 4 Timing L4-PSB working group meeting 25/06/2009

  2. During it’s 37 years long history, transverse space charge at low energy has been a dominant cause of intensity (Isolde, CNGS, AD) and brightness (LHC) performance limitations in the PSB. • One very important parameter is the bunching factor at low energy: • Make the bucket long (reduce fs), add a second harmonic RF • Make bunches as long as possible: fill the bucket as much as possible (but without losses) • Avoid bunch shape oscillations and associated oscillations in bunching factor • Modify the potential well to make bunches flatter (= add second harmonic RF) • Modify the injected phase space distribution in the Linac or prior to capture in PSB. • Modified potential well. • During the h = 5 era, it was first studied with a h = 10 prototype in 19801 and all 4 C16 systems were added in 19822. • During the LHC injector conversion project in 97/983, the dual harmonic operation was retained as we entered the h = 1 & 2 era in the PSB to maximize the brightness of the LHC beams in the PSB 1) Shaping of Proton Distribution for Raising the Space-Charge Limit of the CERN PS Booster, XIth Int. Conf. on High Energy Accelerators, CERN, Geneva, 1980 or CERH/PS/BR 80-14 2) A Second Harmonic (6-16 MHz) RF System with Feedback-Reduced Gap Impedance for Accelerating Flat-topped Bunches in the CERN PS Booster http://cern.ch/AccelConf/p83/PDF/PAC1983_3499.PDF 3) Conversion of the PS Complex as LHC Proton Pre-Injector, http://cern.ch/AccelConf/pac97/papers/pdf/9W011.PDF Justification for Longitudinal Painting in PSB 1/3 L4-PSB working group meeting 25/06/2009

  3. Modified phase space distribution (hollow distribution, depressed central density) • The concept was proposed analytically as a sum of two elliptical momentum distributions in 19794 • Machine studies were done in 79/801 by using the 400 MHz debuncher of Linac2 with 180 degree phase change as well as empty bucket deposition using PSB 3 MHz rf. • Instabilities associated with closing of the phase loop prevented this from being used operationally; mechanism not well understood at that time. • In the h = 1&2 era, this method was further pursued with machine studies in 995 using the 16 MHz C16 system to inject an empty bucket into the coasting beam prior to RF capture (requires dB/dt = 0 at injection). • Numerical computations of longitudinal bunched beam transfer functions of bunches with depressed central density by Shane Koscielniak (TRIUMF) contributed significantly to the understanding of the mechanism of the instability. 4) Bunches with Local Elliptic Energy Distributions http://cern.ch/AccelConf/p79/PDF/PAC1979_3526.PDF 5) New Technique for Bunch Shape Flattening http://cern.ch/AccelConf/p99/PAPERS/TUBL7.PDF Justification for Longitudinal Painting in PSB 2/3 L4-PSB working group meeting 25/06/2009

  4. Longitudinal painting in the Linac4 era • The increased injection energy (160 MeV) reduces the transverse space charge by a factor of 2 • The 3 MeV chopper and H- charge exchange injection permits loss-free injection into a moving bucket at full dB/dt which means reduced time at low energy. • Turn by turn energy modulation and turn-by-turn timing control of the 3 MeV chopper timing permits to inject a longitudinal phase space distribution very close to being the stationary distribution such that harmful bunch shape oscillations and losses are avoided and obtain a good bunching factor at low energy which improves both brightness and intensity • The digital chopper timing control system with associated application and controls software layers must be conceived and planned to have turn-by-turn timing control ready from day one of operation of PSB with Linac4 • This permits the LHC (brightness, reduced filling time) and other CERN users (intensity) to rapidly obtain maximum profit from the Linac4 upgrade • To fully profit from this longitudinal painting (depressed central density, intensity control) several chopper on/off events must be foreseen within a single turn • The PSB will then finally have the longitudinal phase space density control tool we have always wished we had for the last 37 years!! 5) Linac4, a New Injector for the CERN PS Booster http://cern.ch/AccelConf/e06/PAPERS/TUPLS057.PDF Justification for Longitudinal Painting in PSB 3/3 L4-PSB working group meeting 25/06/2009

  5.  2μs Pre-chopper +LEBT Debuncher RF Feedforward Distri 4*rf From F. Gerigk Linac 4 - PSB 180 m Source 45 keV Chopper Amplitude modulated for energy modulation (painting) from P. Baudrenghien RF feed-forward, energy modulator, debuncher, distributor and rf have to be in phase with the Chopper L4-PSB working group meeting 25/06/2009

  6. PSB beam demand = some number of Linac 4 bunches (352.2 MHz). Intensity precision: No jitter, 3 bunches doesn’t mean 3 +/- 1 bunch(es). Time precision: One L4 bunch at time t doesn’t mean 1 bunch at time t + TL4_RF This one Linac 4 rf period uncertainty is only accepted as a global shift of the one-revolution bunch train. Absolute Time reference: Could be time zero of the entire injection process (start injection re-synchronized with the revolution reference) Could be “start injection” + each revolution reference tic Reference clock: These constraints imply that the chopper should use the L4_RF or a (sub) multiple. Chopper + Feed-Forward timing L4-PSB working group meeting 25/06/2009

  7. Chopper + Feed-Forward timing Single absolute time reference = start injection Pros: No intensity jitter No time jitter (just a single jitter of one L4 rf period for the global filling pattern) Cons: any error is integrated over the entire injection process duration (≠ FECs to be well synchronized when changing L4 fRF or PSB fINJ REF) The entire injection process lasts < 400 μs = 140k TL4-RF = 18 bit value Time Value Event Number L4-PSB working group meeting 25/06/2009

  8. Chopper + Feed-Forward timing Dual absolute time reference = start injection + revolution reference => L4 rf re-synchronized at each revolution Pros: No intensity jitter Any timing program error is effective only within one pair of revolutions Cons: One L4 rf period jitter for the global filling pattern within one revolution Need to treat a pair of revolution periods for each revolution filling pattern as the actual rf overlaps the reference during acceleration L4-PSB working group meeting 25/06/2009

  9. Chopper + Feed-Forward timing Triple absolute time reference = start injection + revolution reference + multiple of the reference => L4 rf re-synchronized at each Beam ON-OFF Pros: Any timing program error is effective only within one revolution Cons: One L4 rf period jitter for any Beam ON-OFF event (intensity jitter) Most complicated L4-PSB working group meeting 25/06/2009

  10. The function generator will provide the set point value to the energy modulator (Voltage amplitude modulation) and to the debuncher (phase and ? amplitude modulation). The typical signal is a triangle with a >20 μs period. Functions for the Modulator + debuncher Requirements : New value every revolution. Both the Modulator and the debuncher need to have the same final response. Staircase signal acceptable when both modulator and debuncher have a high BW (staircase response also = flat line painting). When one or both of the response times are long (>10ns) the homogeneity of the painting will be affected by parasitic exponentials. This would require additional interpolated values. L4-PSB working group meeting 25/06/2009

  11. Functions for the Modulator + debuncher Requirements : New value every revolution. Both the Modulator and the debuncher need to have the same final response. Staircase signal acceptable when both modulator and debuncher have a high BW (staircase response also = flat line painting). When one or both of the response times are long (>10ns) the homogeneity of the painting will be affected by parasitic exponentials. This would require an addition of interpolated values to obtain a smooth ramp. Time precision: one L4 rf period (although not critical) Absolute Time reference: Time zero of the entire injection process (start injection re-synchronized with the revolution reference) Reference clock: Injection revolution reference L4-PSB working group meeting 25/06/2009

  12. Block diagram  2μs Pre-chopper +LEBT Energy modulation Debuncher Distri 4*rf Source Chopper Linac 4 160 m 20 m Linac rf feed-forward Function Generator FMC3 (CO) 45 keV Chopper Control Application BIXi.RF_PHASE + dF/dT TON-OFF Chopper L4 rf BIXi.SDIS CTRV Timing CO BIX.SInjChop BIXi.SInjRF Rev Inj. Ref Source h2 1/2 h1 SP2T Inj. rf reference (h1 or h2) BIXi.SDIS L4-PSB working group meeting 25/06/2009

  13. Block diagram  2μs Pre-chopper +LEBT Energy modulation Debuncher Distri 4*rf Source Chopper Linac 4 20 m 160 m 45 keV Linac4 Low level RF, timings …. PSB Low level RF and standard timing modules Trigger(s), possible RF train(s) and E-modulation Application Provides data for chopper & E-modulation well in advance (different data for different USER’s for PPM) Slide from C. Carli Aim of these discussions: discuss and agree on an interface between these three parties L4-PSB working group meeting 25/06/2009

  14. Security issues Requirements : The Linac 4 beam should not reach high (Beam-OFF at the chopper level) energy when: the Distributor targets the Head Dump the Distributor is in a transition state the Distributor targets the Tail Dump The Tail Dump position of the distributor should be removed, at the kicker level, as the tail Dump is unlikely to withstand a long L4 pulse. L4-PSB working group meeting 25/06/2009

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