2. Impact of the current debuncher limitations (…can we get rid of longitudinal painting?)
E. Benedetto
LIU-PSB injection meeting, 15/9/2016

3. 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

4. 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:

5. 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

6. 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

7. 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!

8. 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?

9. 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

10. 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

11. 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

12. 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

13. 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 !!!

14. 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

16. Peaks in line density & filamentation
V. Forte

17. Beamlets overlap