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Published byShannon Jennings Modified over 6 years ago
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Impact of the current debuncher limitations (…can we get rid of longitudinal painting?)
E. Benedetto LIU-PSB injection meeting, 15/9/2016
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Outline New debuncher parameters Impact on beam production LHC
High Intensity (w. longitudinal painting) Mitigations (if no upgrade) Can we avoid longitudinal painting? Conclusions and next steps
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New achievable parameters
Max energy swap: +/-0.8MeV (it was +/-1.2MeV) Min period to complete one full swap: 40us (it was 40us, but with the larger excursion…) Min and possible range of DeltaE that can be guaranteed during the energy swap (~120keV was chosen at the time). Same Max pulse time (can we do ~700us, i.e. 4x150turns+h+t?): no problem From of A.Lombardi, 13/6/16:
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LHC beams (no Longitudinal painting)
Not a problem! Inject a “rectangle” form Linac4 (~20 to 30 turns) Required longitudinal characteristics: dE~400keV rms Chopping factor ~63% (~630ns bunch length) V. Forte t=0ms t=10ms t=50ms See also V. Forte’s presentation 6/10/15 at this WG meetings
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HI beams (w. Longitudinal painting)
C. Carli, R. Garoby See also CERN-ATS-Note PERF The scheme Swap of +/-1.2MeV 20 turns period (initially) RMS deltaE=120keV, i.e. 1/2 of Energy difference between two beamlets
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HI beams (w. Longitudinal painting)
Initial proposal, CC+RG: +/-1.2MeV 20us (20turns) full swap deltaE=~120keV In ~2011: +/-1.2MeV 40us (40turns) full swap deltaE=~120keV (same) Validated (and used as baseline) in simulations by ABP and ABT Now: +/-0.8MeV 40us (40turns) smaller swap DeltaE=~120keV (same) Is it still fine? No!
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HI beams (w. Longitudinal painting)
We do not fill all the available phase-space (main purpose of the painting): Filamentation & peaks of line density bad for Space Charge Equivalent of injecting a “rectangle” (as for LHC beams) which leads: more freedom in the choice of deltaE+chopping less complications Energy difference between 2 consecutive beamlets smaller and smaller: Is DeltaE=120keV still a good value?
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Mitigations: to paint bucket properly
Voltage reduced to V1=5.5kV, V2=4.5kV, dphi=pi+0.80 Fill 80% of the acceptance Total bunch length 0.69 ns (Bf=0.456), Emittance 1.01eVs Bucket height: ~0.8/1.2 smaller Bunch length ~almost the same (was ns, Bf=0.473), OK for Space Charge Smaller deltap/p, but dispersion contribution small for large emittance beams (from DeltaQ=0.57 to DeltaQ=0.61) (+) Minimize filamentation & peaks of line density, OK for space charge (-) Do not fully profit of available voltage (even larger w. Finmet cavities) (-) Smaller longitud emittance, requires larger Blowup at high energy To be checked: Impact of keeping deltaE=120keV, beamlets overlap
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Mitigations: profiting of the Finemet system
Idea!!! Let’s imagine we can use Finemet cav. in triple harmonics h=1+2+3 to flatten bunch profile and mitigate space charge (S. Albright, private communication) other interesting features for us: smaller bucket height more ~rectangular shape We can paint wanted phase-space One step further…(to be checked!) Smaller filamentation in case a “rectangle” is injected from L4 Preliminary …can we get rid of longitudinal painting? 50% acceptance filled Emi=1.1eVs Bl=670ns V1=9, V2=9, V3=4, dphi12=3.6, dphi13=0.9
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Mitigations: profiting of the Finmet system
Preliminary simulations (without space charge): S. Albright Preliminary V(h=1)=8kV, V(h=2)=8kV V(h=1)=9kV, V(h=2)=9kV, V(h=3)=4kV Not directly comparable (power is different), but it seems to go in the right direction… Smaller bucket height More rectangular
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Avoiding longitudinal painting
Longitudinal painting +/-1.2MeV Injection into h=1+2+3 bucket Complicated Much easier! Lengthy optimization Fast optimization, common to ~all beams Need to adjust painting function according to # of injected turns…that we regularily change No need to change anything if #turns in modified OP cruise control should transfer all the info in PPM: Chopping table start/end (per turn) Energy modulation period Less info to be trasmitted to Linac4, only 2 parameters: Chopping factor DeltaE Needs precise control of DeltaE while Mean energy is varying More flexible ~100kCHF Free, unless modif to PSB LLRF are needed Is it feasible to run in h=1+2+3? Operation in Double harmonics well known Lengthy optimization for RF: Voltage and phase for the 3 harmonics Perhaps smaller longitudinal emittance will require more blowup before extraction TBC
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Conclusions (1/2) Debuncher limitation not a problem if no longitudinal painting (LHC beams) Smaller energy excursion & longer energy ramp rate for painting Reduced painting area cannot fill uniformly the phase space Mean energy difference between beamlets gets smaller, is deltaE=120keV still fine? Mitigations: Reduce the voltage (down to ~5kV for h=1,h=2) OR: Use Finmet cavities in Triple harmonics (h=1+2+3) H=1+2+3: Preliminary settings identified (thanks S. Albright) Reduced bucket height can paint properly More ~rectangular bucket can reduce filamentation if no painting Tempting to avoid long. painting for high intensity beams as well !!!
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Conclusions (2/2) Next steps: check option h=1+2+3 & no longitudinal painting LLRF: is it feasible? What are the implications? Beam Dynamics simulations including space charge (transverse and longitudinal) to identify possible showstoppers and quantify advantages Compare the different scenario here listed: h=1+2 w/wo painting vs. h=1+2+3 w/wo painting option of reduced energy escursion Find optimum parameters to maximize longitud emittance and minimize line density and filamentation Study larger blowup during the ramp
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Peaks in line density & filamentation
V. Forte
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Beamlets overlap
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