25 ns performance - longitudinal plane T. Argyropoulos, E. - PowerPoint PPT Presentation

Other means to increase the SPS 25 ns performance - longitudinal plane T. Argyropoulos, E. Shaposhnikova, Jose E. Varela LHC Performance Workshop (Chamonix 2014) 22-25 September 2014 Acknowledgements: H, Bartosik, T. Bohl, F. Caspers, H. Damerau, A. Lasheen, E. Montesinos, J. E. Muller , D. Quartullo, H. Timkó, C. Zannini T. Argyropoulos, LIU Day 2014

Outline  Status of the 25 ns LHC beam in the SPS before LS1 (2012)  Main performance limitations in the SPS for HL-LHC parameters  RF power  Emittance blow-up due to longitudinal instabilities  Possible additional means to increase the 25 ns beam intensity in the SPS after approved LIU upgrades  At SPS – LHC transfer  During acceleration ramp  Summary

25 ns beam in SPS before LS1 Achieved in the last 2012 MD at 450 GeV/c: 4 batches with intensity of 1.35x10 11 p/b (double RF and Q20 optics) and bunch length ~ 1.7 ns  To avoid losses bunch length required for transfer to LHC: τ 4 σ ≤ 1.7 ns (BQM max 1.9 ns)  This result is used as a reference point for scaling to the higher intensities Beam transmission  High losses for injected intensities above 1.4x10 11  The 200 MHz RF voltage during the ramp was increased  close to the limit of 7 MV (from beam loading)  Longitudinal instabilities J. Esteban Muller et al.

HL-LHC: limitations for SPS-LHC transfer  HL-LHC request: 2.4x10 11 p/b at SPS flat top with τ 4 σ ≤ 1.7 ns , but 1/2 ) larger longitudinal emittance is needed for beam stability ( 𝜁 𝑚 ∝ 𝑂 𝑐   limited RF voltage due to beam loading and potential well distortion 𝑄𝑋𝐸 ∝ 𝜐 −3 ) 2 and 𝑊 ( 𝑊 ∝ 𝜁 𝑚 𝑗𝑜𝑒 After 200 MHz upgrade (2020): 2x4 + 4x3  After upgrade we can reach RF voltage at transfer to LHC • 2.7 A (2.1x10 11 p/b) without performance degradation • ~10 MV should be available for 3 A (2.3x10 11 p/b)  But 12.5 MV are required Ref. point V ind for τ = const (LD & PWD) Solutions:  Increase acceptable longitudinal emittance 𝜻 𝒎  Reduce longitudinal blow-up Note: single bunch scaling for LD & PWD from (impedance) present experimental results (ref. point)

Uncontrolled emittance blow-up: possible impedance source SPS longitudinal impedance model: RF cavities (200 MHz + HOM, 800 MHz), BPMs, kickers, resistive wall, unshielded pumping ports, Y – chamber, beam scrapers Search for high frequency impedance  Measurements at flat bottom with long bunches (25 ns) and RF off Peak at 1.4 GHz  Vacuum flanges (different types, ~500 in the ring)

Uncontrolled emittance blow-up: microwave instability?  Simulations of a single bunch on the SPS flat top as a function of intensity using the SPS impedance model (including the vacuum flanges)  compare with measurements Single bunch at the SPS flat top (meas. from AWAKE MD in 2012) Q20 – Double RF – V 200 = 2 MV (low voltage before bunch rotation)  Good agreement of these measurements with particle simulations  Signs of microwave (mw) instability  Main contribution from the 1.4 GHz resonant impedance from the vacuum flanges from simulations: N th = 2x10 11 MD to find the mw instability threshold

Uncontrolled emittance blow-up: multi-bunch case Simulations for 6 bunches (25 ns spacing) at SPS flat top Intensity threshold as a function of bunch length for 1 & 6 bunches Q20 – Double RF – V 200 = 7 MV Qualitative agreement of simulations with measurements:  N th of 6 bunches is ~ twice lower than of single bunch  Only a few bunches are coupled, no coupled bunch modes  indeed in measurements 25 ns and 50 ns spaced bunches are coupled, but batches spaced by 225 ns are decoupled  N th increases with emittance MD for coupled bunch instability threshold 1 batch & different number of bunches

Large longitudinal emittance at SPS flat top  Large longitudinal emittance at flat top (> 0.55 eVs or τ 4 σ > 1.8 ns)  problem for losses in the LHC  Three solutions are considered: 1) Bunch rotation on the SPS flat top 2) New SC 200 MHz RF system in the LHC 3) Reduce uncontrolled emittance blow-up by impedance identification and reduction

Bunch rotation at flat top (1/3)  Already tried successfully for single high intensity ( ~ 2.5 - 3.0 x10 11 ) bunches (MD for AWAKE)  but for very small emittance ( ε l ~0.3 eVs) Single bunch MD for AWAKE in 2012 N = 2.8x10 11 Bunch length (ns)  Maximum needed voltage available only in 2020  Larger bunch tails  more beam loss in the LHC ?

Bunch rotation at flat top (2/3)  Starting rotation from V 200 = 5 MV and assuming V 200 = 10 MV available at flat top (2.3x10 11 p/b and Q20 optics)  Simulations with the SPS impedance + FF and FB in the 200 MHz RF  bunch position variation along the batch agrees very well with measurements Bunch position variation along the batch Measurements with half intensity N = 1.3x10 11 p/b Bunch rotation for LHC beam can be tested in the SPS with limited 200 MHz RF voltage

Bunch rotation at flat top (3/3) LHC capture with 6 MV: simulated bunch position variation in the LHC Buckets Bunch 36 Bunch 72 Bunch 1 J. Esteban Muller Particle Losses less than 1.5 % per bunch  follow the beam loading effect of the SPS 200 MHz RF system  pessimistic estimations Avg. bunch length, τ mean = 1.45 ns Possible MD on SPS – LHC transfer

New SC 200 MHz RF system in the LHC (more in talks of R. Calaga and R. Tomas, Session 6 - HL-LHC)  Clean transfer between SPS and LHC  Double RF system in the LHC  better stability?, flat bunches, …  Additional impedance in the LHC, reliability issues  Double RF system operation in the LHC with all the complications (phase control,…)

Impedance identification (1/2) Efforts during the last 2 years to identify the impedance sources in the SPS ring  Beam measurements Long bunches with RF off  resonant impedance Synchrotron frequency shift  inductive part 1.4 GHz  Measurements and electromagnetic simulations of impedance for different devices/structures in the SPS ring Vacuum flanges Non-Shielded, enamelled BPH – QF ≈ 39 Enamelled QF – MBA ≈ 97

Impedance identification (2/2)  Vacuum flanges are the best candidate with strong peak at f r = 1.4 GHz (observed also from beam measurements) with R/Q = 9 k Ω (different types,~ 500) Confirmed by particle tracking simulations: N th increases by a factor of 2 without the impedance of vacuum flanges More studies for confirmation as the main source of mw instability Group II Group I

Impedance reduction  Reducing the longitudinal impedance will reduce the V ind and uncontrolled emittance blow-up  most robust solution  Preliminary ideas of reducing the impedance of the SPS vacuum flanges  Partial shielding + damping  R/Q reduction factor 8 could be achieved.  Only Group I (half) could be acted upon.  15-30 weeks of work  Flange redesign  Minimum impedance . R/Q reduction factor 20.  All flanges could be changed (≈550).  15 – 30 weeks of work  Higher cost (new elliptical bellows, …)

Limitation during the ramp  The situation after the upgrade of the SPS 200 MHz RF system (2x4 + 4x3) is assumed  For beam stability from certain energy (depending on intensity, emittance and optics) we need to have controlled longitudinal emittance blow-up Power/cavity for 2.2x10 11 Voltage correction for PWD 4 sections power limit 3 sections power limit ε l = (0.4 – 0.7 eVs) ε l = (0.4 – 0.7 eVs)  Optimistic scenario based on single bunch instability considerations  maximum emittance 0.7 eVs scaled from single bunch

Longer acceleration cycle Power/cavity for 2.5x10 11 - Nominal Power/cavity for 2.5x10 11 - Longer 4 sections power limit 3 sections power limit ε l = (0.4 – 0.7 eVs) ε l = (0.4 – 0.7 eVs) Possible improvement by redesign of the magnetic cycle  With twice longer acceleration cycle we can accelerate HL-LHC intensity but  Dedicated LHC filling is 30% longer  Average power increase in SPS  More time for instability to grow

Intermediate transition energy Q22  If power limitation is still an issue with Q20  Beam stability is an issue for Q26  Intermediate transition energy with γ t = 20 (Q22) (see talk of H. Bartosik) Beam stability (single bunch simulations at SPS flat top) Double RF – V 200 = 7 MV For same emittance Q22 provides:  Better stability compared to Q26  Worse stability compared to Q20

Intermediate transition energy Q22 Power consideration for longer acceleration cycle Power/cavity for 2.5x10 11 – Longer cycle  Controlled emittance blow-up will be still needed for stability  Longer cycle also necessary due to power limitations 4 sections power limit during ramp 3 sections power limit  More margin in power compared to Q20 ε l = (0.425 – 0.8 eVs)