Longitudinal Density Monitoring for LHC using synchrotron radiation

1. Longitudinal Density Monitoring for LHC using synchrotron radiation J.-F. Beche, J. Byrd, S. De Santis, P. Denes, M. Placidi, W. Turner, M. Zolotorev (LBNL) LARP Meeting - 27 April, 2006

2. Overview and History • Longitudinal density monitoring (LDM) is important for understanding a number of LHC accelerator physics issues, particularly diffusion of beam into the abort gap. • Involvement in LDM started at inception of LARP • Sophisticated optical-mixing technique not funded. • Study of Abort Gap Monitor using MCP-PMT funded. • New approaches: • Coupling SR to optical fiber for remote optical diagnostics. Longitudinal Density Monitoring - John Byrd

3. Diagnostics for measuring longitudinal parameters of the LHC beams LHC SPECIFICATIONS C. Fischer, LHC-B-ES-0005 - Abort Gap Monitor - Debunched Beam - Bunch Tails - Ghost Bunches • Bunch Core • (“real time”) Longitudinal Density Monitoring - John Byrd

4. Diagnostics for machine protection and accelerator physics • Abort Gap Monitor (AGM) to be included in the LHC interlock chain. • Monitoring the 3.3 ms long gap in the LHC fill pattern (abort kicker raise time. • Other applications (LDM) are for accelerator physics studies. • Even though a single device/technique could do the job, the above strongly points towards two independent instruments. Longitudinal Density Monitoring - John Byrd

5. AGM: deliverables for CY04 • Engineering feasibility study white paper. • Investigation of viable techniques based on: • Synchrotron RadiationWake Fields • • Gated PMT• BPMs • • APDs• Wall current monitor • • Streak camera Longitudinal Density Monitoring - John Byrd

6. MCP-PMT for the AGM Gate min. raise time: 1 ns <2.5 ns RF bucket spacing Gate voltage: 10 V Low voltage switching required Gain at –3.4 kV: 106 High gain < 10 dark counts/sec Low noise Max duty cycle: 1% 100 ns -> 100 kHz max sampling rate -> 3 ms to measure entire abort gap (w/o integration) Longitudinal Density Monitoring - John Byrd

7. Synchrotron light source for longitudinal diagnostics Extraction mirror • 5T SC undulator in IR4 (low energy) • Exit edge of D3 magnet (high energy) Available photon flux Longitudinal Density Monitoring - John Byrd

8. MCP-PMT has the required sensitivity and accuracy Conservative estimates (1% QE, 10% BW, etc.) point out that a MCP-PMT based AGM can easily detect the required charge densities at both injection and collision energy. The MCP-PMT could map the rest of the ring at the same time and measure the unbunched beam. Unbunched beam at injection generates “radiation flashes” during the ramp. This should be part of the protection system as well Longitudinal Density Monitoring - John Byrd

9. AGM hardware tests • An MCP-PMT has been tested at the ALS (dynamic range, photocatode saturation, noise properties) and on the Tevatron (unbunched and bunched beam). • The MCP-PMT will also be tested as a possible device for accelerator physics applications (LDM). Longitudinal Density Monitoring - John Byrd

10. ~120 ns gap (3.3 µs) Tests at the ALS 328 RF buckets 276+1 filled (280-620 ps) Bunch width ~50 ps (2.5 ns) Bunch spacing 2 ns (2808/35640) “Camshaft” pulse (LHC parameters) (no camshaft) Longitudinal Density Monitoring - John Byrd

11. Pinhole, ND filter (single photon counting) Experimental setup SROC Hamamatsu Streak Trigger Unit Stanford DG535 Delay 1.5 MHz ~100 kHz HP8114A Pulser 10 V Gate Synchrotron Light MCP PMT Tektronix TDS754D Hamamatsu C3360 HV -3 kV Longitudinal Density Monitoring - John Byrd

12. Empty buckets (gap) Regular bunches Parasitic bunch Camshaft …and more recent data No need to average data, at least at the ALS. Future experiments at the ALS will carefully evaluate the available photon flux and simulate the LHC parameters, if possible. Longitudinal Density Monitoring - John Byrd

13. Parasitic bunch Gate signal on Gate signal delayed 28 ns Gate signal on Parasitic bunches Some more interesting data Photocathode recovery is not an issue Compare zero level with gate on and off Longitudinal Density Monitoring - John Byrd

14. Tevatron Results Set up test at FNAL Tevatron using MCP-PMT to look at abort gap. Use existing edge radiation port as source. Longitudinal Density Monitoring - John Byrd

15. Tevatron Results Measure integrated signal for each RF bucket in gap during injection process. Longitudinal Density Monitoring - John Byrd

16. AGM Summary • - Can be easily switched at the required speed • - S/N ratio seems adequate • - Fast photocathode recovery (~100’s ps) • MCP-PMT available in different bands. Which is the most suitable for LHC ? • Time resolution inadequate for LHC specs. • Low duty cycle (1%) inadequate for LHC spec on update rate. MCP-PMT looks promising Longitudinal Density Monitoring - John Byrd

17. 1+2 = visible wavelength Mixed Photons 1+2 Bunch Synchrotron Photons 1 PMT Non-linear crystal [BBO] Filter(1+2) Laser Photons 2 laser pulse length << bunch length Too good for the LHC: ~fs resolution ! LDM: laser-mixing technique- an optical sampling scope - Longitudinal Density Monitoring - John Byrd

18. Electro-optic modulator PMT Fiberoptic Fiberoptic Fiberoptic coupler Fast pulser PC Optical sampling by electro-optic modulation Data acquisition board (ADC) • Highly reliable technology (commonly used in large bandwith data networks). • Synchrotron light transmitted on fiber optics allows for the instrument to be easily placed away from high-radiation areas (ground level ?). • Needs an RF pulser capable of generating 50 ps long pulses at 40 MHz repetition rate. • Coupling of synchrotron light into fiber optics needs to be investigated. Longitudinal Density Monitoring - John Byrd

19. Present status of the optical sampling technology • Though much slower than the laser-mixing scheme, it is still adequate for satisfying the LHC specifications. • We are presently running bench tests with a 10 Gb/s modulator. • Coupling of synchrotron radiation into an optical fiber has been characterized. Longitudinal Density Monitoring - John Byrd

20. Main Characteristics of Optical Fibers commercially available optical fibers Dispersion Attenuation • 850 nm (multimode fiber) 6 GHz/100 m bandwidthrequires sampling electronics in tunnel• 1310 nm (non dispersion shifted fiber) - nominal zero dispersion at 1310 nm - total delay for 10% BW ≈ 2 ps/100 m • 1550 nm(zero-dispersion shifted fibers are no more manifactured) - D ≈ 17 ps/nm/Km - total delay for 10% BW ≈ 260 ps/100 m • 850 nm ~ 2.5 dB/Km• 1310 nm ~ 0.4 dB/Km• 1550 nm ~ 0.2 dB/Km Dispersion at these wavelengths can be reduced by compensating fibers (DCF, D ≈ -100 ps/nm/Km) Longitudinal Density Monitoring - John Byrd

21. Measure coupling of SR to fiber at ALS • Two available beamlines: • 7.2 has a streak camera, but the optics are defective for our application. Phase variations on the wavefront which results in low coupling efficiency. • 3.1 is good, we might temporarily move the streak camera there. • Experiment run at cost zero (i.e. with the components we have). • Main objective: optimizing the coupling of synchrotron radiation into the fiber (10% BW). • We are using 880 nm, rather than 1310. • Results indicate ~50% efficiency Longitudinal Density Monitoring - John Byrd

22. Radiation hardness of optical fiber Longitudinal Density Monitoring - John Byrd

23. LDM Summary • Useful diagnostic for understanding LHC longitudinal dynamics. • Leverages experience with SR diagnostics from light source community. • Strong interest from the accelerator physics point of view. Longitudinal Density Monitoring - John Byrd

24. LARP Task Proposal • Task: Synchrotron light longitudinal density monitor • John Byrd (PI), Stefano De Santis • Goal: Develop SR diagnostic techniques for understanding LHC changes in LHC longitudinal bunch structure • Deliverables and Schedule: • Work in progress. Basic setup ready for early LHC operation. • Resources needed: • Travel: 5 k$/year • Labor: 0.35 FTE Scientist/year • $100k total to provide detector and fiber coupler Longitudinal Density Monitoring - John Byrd