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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. Overview and History.

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longitudinal density monitoring for lhc using synchrotron radiation

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

overview and history
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

diagnostics for measuring longitudinal parameters of the lhc beams
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

diagnostics for machine protection and accelerator physics
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

agm deliverables for cy04
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

mcp pmt for the agm
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

synchrotron light source for longitudinal diagnostics
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

mcp pmt has the required sensitivity and accuracy
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

agm hardware tests
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

tests at the als

~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

experimental setup

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

and more recent data

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

some more interesting data

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

tevatron results
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

slide15

Tevatron Results

Measure integrated signal for each RF bucket in gap during injection process.

Longitudinal Density Monitoring - John Byrd

agm summary
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

ldm laser mixing technique an optical sampling scope

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

optical sampling by electro optic modulation

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

present status of the optical sampling technology
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

main characteristics of optical fibers
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

measure coupling of sr to fiber at als
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

radiation hardness of optical fiber
Radiation hardness of optical fiber

Longitudinal Density Monitoring - John Byrd

ldm summary
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

larp task proposal
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