LHC Beam Instrumentation Further Discussion

Transcription

1 LHC Beam Instrumentation Further Discussion LHC Machine Advisory Committee 9 th December 2005 Rhodri Jones (CERN AB/BDI)

2 Possible Discussion Topics Open Questions Tune measurement base band tune & 50Hz mains ripple Coexistence with transverse damper Ion operation Instruments not covered in general session Schottky monitors Wire Scanners Abort Gap Monitor

3 PLL Tune Measurement System Collaboration with BNL as part of the US-LARP programme Advantages Provides a continuous tune measurement & can be used for feedback Precision a function of bandwidth (~10-5 for 1-10Hz) Necessary for continuous coupling and chromaticity measurements Problems Requires constant excitation For proton & heavy ion machines this implies small amplitudes. PLL functioning strongly linked to Chromaticity & Coupling Synchrotron sidebands can interfere with the main tune peak Large chromaticity can widen the main tune peak PLL can lose lock Large coupling can cause PLL to jump from H to V tune peaks Coexistence with transverse damping

4 Feedback using the PLL tune system Tune feedback Not trivial to go from tune measurement to feedback Requires a stable PLL tune measurement system Experience from RHIC Although feedback was available from an early stage there has been considerable difficulty in making it reliable under varying machine conditions. After 3 years feedback is still not used routinely during operation. Main problem is the stability of the PLL tune measurement when crossing transition with heavy ion beams. More stable with proton beams where there is no transition crossing. Can expect similar difficulties during LHC snap-back when all machine parameters change simultaneously Chromaticity feedback Not yet attempted on other high energy hadron machines RHIC will try to test this next year Two options for creating dp/p variations: Standard RF frequency modulation too slow? Fast RF phase modulation can PLL track the tune with this bandwidth?

5 SPS BBQ Transverse damper noise Damper system OFF LHC pilot, p +, 26GeV Damper system ON No explicit beam excitation

6 Mains ripple in the beam spectrum Placing the tune on a 50 Hz multiple increases beam oscillations! 50Hz No explicit excitation

7 Tevatron BBQ measurements H plane H plane V plane V plane BBQ front-end on its built-in power supply BBQ front-end on batteries Measurement by C.-Y. Tan (FNAL)

8 RHIC BBQ measurements - Collisions Store beginning 5 hours later (end of the store) H plane H plane V plane V plane

9 RHIC BBQ Mains ripple in the beam spectrum 180 Hz 360 Hz 720 Hz BBQ near transition Million turn BPM near transition The BBQ sensitivity was estimated to be better than 10 nm f [Hz] Measurement by P. Cameron (BNL) RHIC BBQ compared to a million turn BPM

10 4.86 mm gap SPS Q variations due to collimator jaw motion 3.86 mm gap 2.86 mm gap 40Hz 2.26 mm gap 1.96 mm gap No explicit excitation Tune variations 2 12 Hz clearly measured Resolution in the order of 10-5 of f rev achieved Resolution limited by the tune path width and the beam stability 1 bunch, p +, coasting (RF 270 GeV

11 The LHC Schottky System Accepted in October 2005 as part of US-LARP FNAL responsible for: Pick-up design underway - completion date set for end March 2006 Analogue electronic chain (x 4) underway Hardware Commissioning Software algorithms for determination of beam parameters CERN responsible for: Infrastructure Cables in place Pick-up manufacture Will attempt to be in-time for vacuum installation in Point 4 As for most beam instrumentation this is a very tight schedule Digital acquisition system Baseband detection using BBQ acquisition system

12 The LHC Schottky System Slotted Waveguide Pickup 4.7 GHz 60 x 60 mm aperture x 1.2 meter long

13 LHC Schottky Specifications Proposed LHC System Specifications 4.7 GHz center frequency 100 MHz bandwidth minimum allows bunch by bunch gating One Horizontal and Vertical tank each LHC ring Gating single or multiple bunch Double down conversion RF synchronous LO from 4.7GHz to 21.4MHz Fixed LO from 21.4MHz to the khz. Proposed LHC System Capabilities Non-invasive Tune measurement for each ring Non-invasive Chromaticity measurements Measurement of momentum spread Measurement of beam beam tune shift Continuous on-line emittance monitor Ability to measure individual or multiple bunches Down conversion utilizing RF source allows monitoring up the LHC Ramp Built in calibration system to monitor gain variation with time

14 LHC Wire Scanners Baseline Scenario Use of existing movement system Too slow to meet specs on bunch intensity measurable at 7TeV Position determination not accurate enough to meet specs Use of new electronics Bunch to bunch capability using fast BCT electronics (integration at 40MHz) Parallel development New motorisation system Allows movement of wire at up to 2m/s Intensity limit becomes 2 nominal batches at 7 TeV Improved position measurement system Special electronics based on logarithmic amplifiers for tail scans Open Issues Wire heating due to RF modes Measurements to be performed on prototype in Jan 2006 RF coupling to parking cavity minimised via small slots Parking cavity fitted with ferrites for damping of RF modes Once validated production of the final modules can start

15 LHC Wire Scanners 40μm Carbon wire

16 Instrument Performance for Ion Operation BPMs Arc button BPMs have operating threshold of ~ cpb ~17% nominal bunch intensity Inner Triplet stripline BPMs For proton running signal level attenuated by ~6dB for compatibility with arc BPM system electronics Possible to remove this for ion running & gain a factor 2? 5% Percentage Error w.r.t. Half Radius [%] 4% 3% 2% 1% 0% -1% -2% -3% -4% Linearity - High Sensitivity Linearity - Low Sensitivity Noise - High Sensitivity Noise - Low sensitivity Pilot Nominal Pb ion Nominal Ultimate -5% 1E+08 1E+09 1E+10 1E+11 1E+12 Number of Charges per Bunch

17 Instrument Performance for Ion Operation Synchrotron light monitor BSTR With current undulator light emission is in IR (950nm-2.3μm) Fast camera replaced by an IR-Sensitive InGaAs video camera VME frame-grabber for 3 Hz beam size, tilt and postmortem Single bunch light emission below 1 TeV is only just above detector sensitivity limit BSRT is not the main instrument for Ions. Rest Gas Ionisation Profile Monitor (BGI) Main monitor foreseen for continous ion profile measurement Signal produced goes as Z 2 Nominal Pb ion bunch gives more signal than ultimate proton bunch Absolute beam size At injection better than 10% ~16% at 7 TeV will require cross calibration to reduce this value

18 Instrument Performance for Ion Operation BLM response to ion losses Study overseen by I-LHC project for dispersion suppressor dipoles Concluded that ratio of energy deposition in the dipole coils to ionisation chamber response is similar to that for protons BLM quench thresholds determined for protons should therefore also be applicable to ions Simulations still to be done for the collimator region Abort Gap monitor (BSRA) Insufficient visible light at injection energy peak emission at 1.9um No photo-cathodes available for gated MCP detector No alternative detector found! must be fast, sensitive and gated Possible solution with short period undulator?

19 Instrument Performance for Ion Operation BCT response to ions BCTDC Range Sampling rate Full Scale ADC Acquisition location 1 bit equivalent Expected resolution 1ms 20ms 1s High Resolution 50Hz 1A 21 bit + sign front end (tunnel) 500nA X 7µA 1µA Low Resolution 1kHz 10mA 11 bit + sign back end (surface) 5µA 300µA 7µA _ Circulating beam (early scheme 60 bunches of charges ~ 650μA) 1% resolution 6μA (OK for BCTDC) BCTFR response Single pass similar to proton pilot bunches for nominal ion bunch intensity 10% i.e charges at limit for BCTFR Circulating Beam Limit at around cpb

20 Instrument Performance for Ion Operation 592 nominal bunches 60 nominal bunches Visibility threshold on arc BPM Guaranteed functioning of arc BPM

21 Abort Gap Monitor (BSRA) Protons OK, 30x 100ns measurement slots across abort gap Abort level ~ 50 photons/ 100ns Will need Look Up Table for energy dependence Ions Insufficient visible light at injection energy peak emission at 1.9um No photo-cathodes available for gated MCP detector No alternative detector found! must be fast, sensitive and gated Possible solution with short period undulator?