# Presentation: 402153

## Remote Synchrotron Radiation-Based Longitudinal Beam Diagnostics

*J. Byrd, S. De Santis, A. Ratti, M. Zolotorev, Yian Yin*

- LBNL - november 1st, 2005

## Specifications for longitudinal measurements of the LHC beam

**Specifications for longitudinal measurements of the LHC beam**

**LHC SPECIFICATIONS**
***C. Fischer, LHC-B-ES-0005***

**- Abort Gap Monitor**

**- Debunched Beam**
**- Bunch Tails**
**- Ghost Bunches**

** ****Bunch Core**
**   ****(“real time”)**

## Fiberoptic-based Synchrotron Radiation Diagnostics

*...**coupling the synchrotron radiation emitted by the beam into an optical fiber*

- • Synchrotron light can be taken to an easily accessible location, at a distance from the diagnostics port, inexpensively.• Negligible attenuation and dispersion(0.5 dB/Km,  0.1 ps/nm2 Km)• Reliable components of common use in the telecommunication industry• No electronic component has to be near the light port or in other inaccessible areas.

- • Losses due to coupling of  	synchrotron radiation into the optical fiber (~ 3 dB ?)• Might need compensation of  transverse beam motion• ~10% bandwidth is easily achievable. Larger bandwidths require special care (acromatic lenses, dispersion compensation, etc.)

**PROS**

**CONS**

## 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)

## Available photon fluxes from the LHC longitudinal diagnostics light port

- Shaded areas show 10% bandwidth around 850, 1310 and 1550 nm wavelengths.**At top energy** the three bands are roughly equivalent (but 1310 nm is intrinsically dispersion free).

**At injection energy** the emission intensity is dominated by the SC undulator. The maximum flux is at 850 nm,  5 times higher than 1310 nm and 30 times higher than 1550 nm. Maximum flux is anyway a factor of 40 lower than at 7 TeV.

## Optical Fiber of Choice

***~ 0.4 dB/Km***

- Attenuation

- Dispersion

*- nominal zero dispersion at 1310 nm**- total delay for 10% BW ≈ ****2 ps/100 m**** *

*commercially available 1310 nm single-mode, non dispersion shifted fiber*

-  D() L

**This fiber, of standard use in telecommunications, seems to be the optimal solution, due to its excellent characteristics. It is available in a radiation hard variety as well.**

## Available photon fluxes from the LHC longitudinal diagnostics light port

- (Facchini)

- The shaded area shows a 10% bandwidth around the 1310 nm wavelength.At injection energy, the intensity is only five times lower than its maximum at 850 nm.There is a factor of ~100 reduction in intensity between 7 TeV and 450 GeV.

## Streak camera - Measurement of the beam longitudinal profile, bunch phase with picosecond resolution. (current ALS experiment)
Optical sampling scope - Mapping of charge density around the machine. 
Photodiode - Accurate measurement of bunch charge (integrating over many turns, may need cooling).
Gated Photomultiplier - Used as abort gap monitor at the Tevatron. (PAC ‘05)

- Optical sampling scope - *Mapping of charge density around the machine.** *
- Photodiode - *Accurate measurement of bunch charge (integrating over many turns, may need cooling).*
- Gated Photomultiplier - *Used as abort gap monitor at the Tevatron. **(PAC ‘05)*

**Uses of synchrotron Radiation for Longitudinal Diagnostics of the Beam**

- By using the synchrotron light transported on an optical fiber, it is possible to use many different devices on the same port. Instruments can be easily swapped and maintained, according to the particular situation.

## Example: ghost bunches measurement at 7 TeV

- 5 105 protons emit ~30 photons/turn in a 10% bandwidth.
- The electro-optic modulator/fast pulser combination can map the entire LHC ring, with the required resolution, every 500 orbits.
- In the allowed integration time, every single 50 ps-long region is sampled 200 times.
- A 70% QE photodiode would accumulate >4000 counts.
- We can estimate a total of -6/8 dB from the coupling into the optical fiber and the various insertion losses.
- Main noise sources are the modulator extintion ratio (~ 3 10-3) and the photodiode dark current (~ nA)

## Present State of the ALS Experiments

- 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.
- Preliminary results indicate ~50% efficiency