1. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 1 2010-01-12 LHC Q Stability Revisited M. Gasior & Ralph J. Steinhagen, BE-BI for the BI-QP team: A. Boccardi, M. Gasior, S. Jackson, R. Jones, R. Steinhagen

2. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 2 Outline Effects/features discovered so far: –Residual tune stability RQT[D/F] circuit current ripple Other sources?? –Broad frequency “hump” driven beam excitation → emittance blow-up Amplitude calibration B1-B2 correlation –Beam-Beam coupling effect @1.17 TeV N.B. will skip particularities of transverse damper & res. 8 kHz line issues → found to be related to UPS (courtesy BE-RF)

3. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 3 Residual LHC Tune Stability Example: 3. ramp (2009-11-30 @00:15, HP-filtered, Q-FB 'off'): –Residual tune stability ΔQ ≈ 5·10 -4 no particular frequency dependence → 'white noise' Since it scales with energy → checked relevant quadrupole circuits...

4. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 4 Residual LHC Tune Stability – Source #1: RQT[D/F] Circuit Noise Tune trim ↔ circuit correlation as used by LSA & Q-FB: For comparison: –e.g. ΔQ H = 10 -4 @450 GeV → ΔI RQTF = 2.2 mA & ΔI RQTD = 0.4 mA –Specified nominal current stability (over 0.5 h): ΔI = 10 -5 · 600 A = 6 mA –Some RQT circuits have short-term stabilities of ~ 1-2 mA Why do some other jitter by up to 10 mA? Is this an effect of the parallel (protection) resistor? Would 'σ(I RQT[D/F] ) < 2 mA' feasible or is it too ambitious? → being investigated/addressed by our PC experts Hampers Q'-tracker operation with targeted mod. amplitudes of ~10 -5 –increase dp/p modulation to 10 -4 in order to compensate for this effect? → many not-so-nice side-effects and implications for every-day operation!!

5. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 5 BBQ spectra with absolute amplitude scaling

6. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 6 BBQ spectra with absolute amplitude scaling

7. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 7 Shifting Frequency Dune → LHC's 'Hunchback' Initially identified has 'hump' but actually a fast frequency shifting oscillation with the mean drifting slowly between 0.25...0.32 f rev Beam 1 vs. Beam 2 Example: Q v being shifted onto the 'hump' Time-resolved 'hump' structure:

8. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 8 Shifting Frequency Dune → LHC's 'Hunchback' Correlation and Frequency Characteristics Hump on Beam 1 is correlated with the one in Beam 2: correlation factor = 0.895207 frequency change spectra 1/f reference BBQ frequency res. limit for 1024 turns @10Hz

9. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 9 Shifting Frequency Dune → LHC's 'Hunchback' The 'hump' became more apparent around 2009-11-28 – 2009-12-03

10. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 10 Shifting Frequency Dune → LHC's 'Hunchback' On the question 'Whether we see the 'hump' at 1.17 TeV': –tune spectra before (450 GeV) and after (1.18 TeV) the ramp #6: –Central frequency shifted down –Amplitude seems to approximately scale with energy (-8dB reduction)

11. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 11 Beam-Beam Effect @1.18 TeV Tune kick in B2 was also seen in B1! –an indication that we were colliding two beams... –surprising/interesting: B1-B2 coupling is about 0.15 N.B. n b ≈ 3·10 -9 p/bunch

12. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 12 Summary 'Hump' issue remains: –predominantly seen in the vertical plane, –beam gets resonantly excited if tune in the vicinity of this frequency → emittance blow-up as nicely documented by the BSRT –To 1 st order unlikely effects causing the 'hump' (tested with beam): ADT, MSI, CODs,...? Some other remaining questions: –Can the MQT[D/F] corrector circuit stability be improved? –Why does the beam oscillate with um amplitudes at the tune frequency? –Origin of the non-8kHz lines?

13. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 13

14. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 14 ADT calibration tones @ 0.16 & 0.18 N.B. hor. signals smaller due to the lower β functions (front-end selection)

15. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 15 Third LHC Ramp to 1.18 TeV 2009-11-30

16. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 16 Residual LHC Tune Stability @450 GeV Using chrip-type excitation for improved S/N ratio: –Residual tune stability ΔQ ≈ 5·10 -4 issue for Q'-PLL operation that targeted operation within a few 10 -5 No particular frequency dependence ↔ white noise Though small → could it contribute to emittance blow-up?

17. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 17 Residual LHC Tune Stability without excitation

18. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 18 Third LHC Ramp to 1.18 TeV 2009-11-30 – Fourier Spectra of Residual Tune Ripple Additional frequency measurements taken at 10Hz confirm that the tune ripple spectrum is flat up to at least 5 Hz

19. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 19 BBQ Noise Floor with (blue) and without beam (red)

20. LHC Beam Commissioning, Ralph.Steinhagen@CERN.ch, 2009-12-04Ralph.Steinhagen@CERN.ch 20 BBQ Noise Floor with (blue) and without beam (red) - ZOOM