Update on injection studies of LHC beams from Linac4

V. Forte (BE/ABP-HSC)

Acknowledgements:J. Abelleira, C. Bracco, E. Benedetto, S. Hancock, M. Kowalska

LIU-PSB Injection Meeting - 06/10/2015

Outline

• 2 options of multi-turn injection• Brightness curves• RMS emittances evolution

2 options of simulated multi-turn injectionTransverse painting

(injection at x=-35 mm and y= 3 mm) + KSW modulation for betatron mismatch

On-axis(injection at x=-35 mm and y= 0 mm) +

constant KSW until multi-turn is over

ZOOMNo undershoot to -9.18 mm considered in

the next simulations on-axis (does not matter for LHC beams, which are small)

2 options of multi-turn injectionTransverse painting emittance at I=3.42e12 p. at (Qx,Qy)=(4.43, 4.6)

~1.2

~0.6

Brightness curves after 10e3 turns• The “infamous” Giovanni’s tables (EDMS – 1296306)

Brightness curves after 10e3 turns• On-axis simulations for the nominal working point for two initial bunch lengths

(47% and 61% chopping factors)

Brightness curves after 10e3 turns• Moving the working point up-right to (4.43, 4.60) -> gain in brightness

Brightness curves after 10e3 turns• Comparison with Elena’s results (IPAC15 – THPF088) for 61% chopping factor

(optimal choice)

A deeper look…• On-axis vs. transverse painting (61% chopping factor) -> tr. painting gives worst

results in the first 10e3 turns (~10 ms).

These cases will be analysed from now on…

550 ns 570 ns

603 ns 600 ns

Longitudinal profiles after 10e3 turns

dp/p=1.35e-3

Matched area = 1.27eVs

• The matched area is computed as the area of the iso-Hamiltonian (without s.c.) starting from the maximum bunch phase (obtained through a foot-tangent algorithm). S. Hancock suggestion: use RMS bunch length which is sort of bunch shape-independent

Longitudinal profiles after 10e3 turns• The current profiles of 609 ns – 403 keV rms are similar to the solutions adopted by Elena et al. in IPAC15- THPF088 . There, the initial

emittance of 1.17 eVs leads to slightly longer bunches

Current (initial 1.1 eVs rectangle area) – after 10000 turns

Elena (initial 1.17 eVs rectangle area) – after 7000 turns

~620 ns

~600 ns

Longitudinal profiles after 10e3 turns• The current profiles of 609 ns – 403 keV rms are similar to the solutions adopted by Elena et al. in IPAC15- THPF088 . There, the initial

emittance of 1.17 eVs leads to 620 ns bunches after ~7 ms. The second option of creating a very long bunch of 1.48 eVs initial area is attractive (very high brightness) but risky, because leads to fill 94% of the acceptance with already some losses at the edges…

Current (initial 1.1 eVs rectangle area) – after 10000 turns

Elena (initial 1.48 eVs rectangle area) – after 7000 turns

~600 ns – 1.27 eVs

~680 ns – 1.6 eVs

This solution gives better results in terms of brightness but it is very

close to the acceptance (the bunch occupies 94% of the bucket area)

A deeper look…• The two best cases will be now analysed in more detail

These cases will be analysed from now on…

RMS emittances vs. Intensity…

On-axis or transverse painting?

• The transverse painting preserves the shape of the distribution (it starts from higher emittances!)

• The on-axis solution is completely dominated by space charge but could be a bet!

On-axis after 10e3 turns

Transv. Painting after 10e3 turns

Halo

On-axis or transverse painting?

• The transverse painting permits a “controlled” initial blow-up through the betatron mismatch

• The on-axis solution is completely dominated by space charge

Going to absurd -> extrapolated constant linear growth induced by the vertical integer resonance. But in 40 ms from injection bg2 increases of 27% -> 27% less s.c. tune shift!

On-axis or transverse painting?

• The on-axis solution , after 10 ms, has smaller emittance and 0.59,0.73 s.c. tune shift• The transverse painting solution, after 10 ms, has bigger emittance, leading to

0.50,0.57 s.c. tune shift, and almost a steady state situation in rms emittance

On-axis at 3.421e12 p.

Transverse painting at 3.421e12 p.

Summary and next steps• Long term (10 ms) multi-turn injection simulations have been performed for the CERN PSB with PTC-Orbit

• The multi-turn injection scheme has been implemented without energy modulations using Elena’s inputs . In particular:• Quadrupolar errors at the chicane magnets + Eddy currents + Compensation QDE3, QDE14 (time varying)• Beta-beating (mostly in vertical) corrected• Excitation of half-integer corrected• Excitation of the integer line• Foil scattering

• Brightness curves have been simulated for different cases (on-axis and transverse painting injection, different working points and bunch lengths). They still confirm a brightness increase bigger than a factor 2. This margin is useful because these simulations don’t take into account other error sources (like distributed quadrupolar field errors and misalignments).

• A longitudinal injection scheme at 403 keV rms (optimal from IPAC15 MOPJE042) and 609 ns injected per turn (60.9% chopping factor) has been chosen as optimal at 40 mA unchopped from the Linac4, as it guarantees a matched area of ~1.3 eVs (75% of total acceptance) and 600 ns bunch length over 10 ms

• A different matched area method has been implemented (S. Hancock-like).

• Bigger chopping factors (at this energy spread) can lead to longitudinal losses even if beneficial for space charge blow up reduction (IPAC15 – THPF088)

• The on-axis injections:- have dynamics which are completely dependent on space charge (very small initial emittances).- show good agreement with the previous results by E. Benedetto et al. ( IPAC15 – THPF088). - Option to not be discarded for commissioning because give always better brightness than the transverse painting ,

but not yet steady state in 10 ms (risk of transverse halo formation!).

• The transverse painting injections- show a quick suppression of the tune spread to be far from the integer resonances.- preserve the single particle profiles.

Appendix

Scan with working point

E. Benedetto (SC meeting 19/3/15)