Controlled longitudinal emittance blowup in LHC E Shaposhnikova
Controlled longitudinal emittance blow-up in LHC E. Shaposhnikova for BE/RF • HW and SW: M. E. Angoletta, M. Jaussi, J. Tuckmantel, J. Sanchez Quesada • P. Baudrenghien, A. Butterworth, T. Bohl, U. Wehrle, … • Operation team (G. Papotti + …)
Reminder • Longitudinal emittance of nominal beam in LHC Design Report: – 0. 7 e. Vs (inj. ) → 1 e. Vs (after filamentation) at 450 Ge. V – 2. 5 e. Vs at 7 Te. V (controlled emit. blow-up during ramp) IBS growth rates in longitudinal and transverse plane Longitudinal beam stability: to have the same thresholds as at 450 Ge. V with 0. 7 e. Vs and as at 7 Te. V with 2. 5 e. Vs → 1. 75 e. Vs was proposed at 3. 5 Te. V
Loss of Landau damping: during the ramp (1. 8 Te. V) B 1 - 1. 1 x 1011 B 2 - 1. 05 x 1011 1. 05 ns – 0. 35 e. Vs (450 Ge. V, 5 MV) BQM of G. Papotti
and on flat top B 1 - 1. 1 x 1011 • 450 Ge. V, 5 MV: 1. 25 ns - 1. 3 ns → 0. 5 e. Vs • 3. 5 Te. V, 8 MV: 0. 65 ns → 0. 5 e. Vs B 2 - 1. 05 x 1011 • 450 Ge. V, 5 MV: 1. 45 ns → 0. 6 e. Vs • 3. 5 Te. V, 8 MV: 0. 72 ns → 0. 6 e. Vs
Loss of Landau damping • Cause – low frequency inductive impedance. LHC DR: (Im. Z/n)eff = 0. 07 Ohm • Leads to undamped bunch oscillations: quadrupole in our case (dipole are damped by phase loop). Observed in other machines (SPS, Fermilab MR, …) – “dancing bunches” • Weak dependence on voltage (~ V 1/4) – observed (5 -12 MV) • Strong dependence on longitudinal emit. (~ ε 5/2 ) • Energy dependence 1/Es → emittance blow-up: ε ~ Es 2/5 (factor 2. 3 from 450 Ge. V to 3. 5 Te. V, 0. 35 e. Vs → 0. 8 e. Vs)
Loss of Landau damping: threshold during the cycle (old voltage program, 5 - 8 MV) loss of Landau damping Z/n=0. 06 Ohm
Longitudinal emittances from SPS o Practically no loss on FB for bunch length below 1. 4 (? ) ns, initial bunch length growth …, growth rate is reducing with bunch length increase o Some losses for bunch length > 1. 7 ns (after injection and filamentation in 3. 5 MV) due to full bucket – maximum injected emittance of 0. 65 e. Vs o Higher voltage on FB is less matched to the shape of the SPS bunches → tails, more loss on FB → Emittance of 0. 6 e. Vs from SPS is a good compromise to minimise losses on FB and ease the controlled emittance blow-up during the ramp in LHC → controlled emittance blow-up in the SPS
When: longitudinal beam parameters during present cycle momentum synch. freq. spread for emit. of 0. 6 e. Vs 1 st blow-up “new” 400 MHz voltage program momentum filling factor for emittance of 0. 6 e. Vs
Longitudinal emittance in LHC • Emittance of 0. 7 e. Vs will be sufficient for longitudinal bunch stability at 3. 5 Te. V, but there are other limitations (IBS)… • Controlled blow-up is more difficult for smaller synchrotron frequency spread (small bucket filling factor) • Small filling factor is required to avoid particle losses during blow-up and ramp • Relative incoherent synchrotron frequency shift for intensity 10 11 and short bunches (0. 8 ns) is ~ 0. 03 → comparable to the spread at high energies (without blow-up) Comparison to the SPS emittance blow-up : - much smaller synch. frequency spread (SPS: ~100 Hz with 800 MHz on) - much slower ramp (SPS: blow-up during 2 s)
Emittance blow-up during the ramp bunch length for emittance of 0. 6 e. Vs
Commissioning steps • The same method and noise generation as in SPS: band-limited noise inside synchrotron frequency band injected through phase loop • Flat bottom, one bunch, emit=0. 4 e. Vs, 3 tests on 7. 06. 2010 ü 2 x 1010, 2 beams ü 1011 , 1 beam • Ramp with 2 x 1010, than 1011, emit=0. 4, 0. 6 e. Vs - this night • Instrumentation: ü bunch length (BQM) ü BCT (DC and FBCT) – abort gap monitor – not yet available
Limited band-width phase noise application Longitudinal emittance, 450 Ge. V, 3. 5 MV Synchrotron frequency distribution 450 Ge. V, 3. 5 MV
Test 1: 2 x 1010, large bandwidth (32 -44 Hz), different amplitudes (φrms=0. 1, 0. 2, 0. 3, 0. 4 deg)
Test 2: smaller bandwidth (36 -42 Hz and 34 -42 Hz), larger amplitude (φrms=0. 4, 0. 6 deg), 2 x 1010
Test 3: nominal intensity 1. 1 x 1011 small bandwidth (36 -42 Hz), large amplitude (φrms=0. 6 deg) The same frequency bandwidth worked for high intensity bunch
Summary • The noise generation hardware and software work as expected • Successful first tests (at 450 Ge. V) of controlled blow-up to a given value by controlling – the frequency spectrum – the amplitude of the applied phase noise – time of noise application →Next step – emittance blow-up during the ramp (tonight): phase noise with a bandwidth corresponding to the fixed relative synchrotron frequency spread (fixed filling factor) for low and high intensity bunches
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