# Presentation: 248176

## 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

- 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.

## 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”)**

## Diagnostics for machine protection and accelerator physics

- Abort Gap Monitor (AGM) to be included in the _**LHC interlock chain**_.
- Monitoring the 3.3 s 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**.

## Engineering feasibility study white paper.

Investigation of viable techniques based on:

  Synchrotron Radiation	                   Wake Fields

 	 • Gated PMT			• BPMs
	 • APDs		        • Wall current monitor
	 • Streak camera

- Investigation of viable techniques based on:
***  ****Synchrotron Radiation***	****        **           *Wake Fields*
-  	 • Gated PMT			• BPMs
- 	 • APDs		        • Wall current monitor
- 	 • Streak camera

- AGM: deliverables for CY04

## 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)

## Synchrotron light source for longitudinal diagnostics

-  5T SC undulator in IR4 (low energy)
-  Exit edge of D3 magnet (high energy)

***Extraction mirror***

***Available photon flux***

## 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

## 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).

## Tests at the ALS

- Bunch spacing 2 ns

- Bunch width ~50 ps

- “Camshaft” pulse

**(LHC parameters)**

- 328 RF buckets
- 276+1 filled

- (2808/35640)

- (280-620 ps)

- (2.5 ns)

- (no camshaft)

## Experimental setup

- SROC

- Hamamatsu
- Streak Trigger Unit

- HP8114A
- Pulser

- MCP
- PMT

- Synchrotron Light

- Stanford DG535
- Delay

- 1.5 MHz

- ~100 kHz

- 10 V Gate

- Hamamatsu C3360
- HV

- -3 kV

- Tektronix
- TDS754D

## ...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.

## Some more interesting data

- Compare zero level with gate on and off

- Photocathode recovery is not an issue

## Tevatron Results

- Set up test at FNAL Tevatron using MCP-PMT to look at abort gap. Use existing edge radiation port as source.

- Tevatron Results

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

## 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

## LDM: laser-mixing technique- an optical sampling scope -

## Optical sampling by electro-optic modulation

*** ******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.***

*Data acquisition board (ADC)*

## 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.

## Main Characteristics of Optical Fibers

**• ****850 nm***       ~ 2.5 dB/Km***• 1310 nm***       ~ 0.4 dB/Km***• 1550 nm***       ~ 0.2 dB/Km** *

**Attenuation**

**Dispersion**

**• ****850 nm **(multimode fiber)*       6 GHz/100 m bandwidth**       **requires 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** *

*commercially available optical fibers*

- Dispersion at these wavelengths can be reduced by compensating fibers (DCF, D ≈ -100 ps/nm/Km)

## 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

## Radiation hardness of optical fiber

## 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.

## 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