# Presentation: 100085

## Instrumentation for Linac-based X-Ray FELs

- Henrik Loos
- 12th Beam Instrumentation Workshop
- May 1-4, 2006

## Outline

- XFEL introduction
- LCLS overview
- Electron beam diagnostics
- Transverse Beam Properties
- Longitudinal Beam Properties
- Photon beam diagnostics

## X-Ray FEL Features

- ~1Å photon wavelength or ~10keV photon energy
- Uses SASE principle to amplify and saturate spontaneous radiation in ~100m of undulator
- Requires
- Multi GeV beam energy
- kA peak beam current
- Micron beam emittance to match photon beam phase space

## X-Ray FEL Parameters

| Electron Beam |  | LCLS | XFEL | SCSS |
| --- | --- | --- | --- | --- |
| Energy | GeV | 4.3-13.6 | 10-20 | 6.1 |
| Peak Current | kA | 3.4 | 5 | 3 |
| Bunch Charge | nC | 0.2-1 | 1 | 1 |
| Norm. Slice Emittance | μm | 1.2 | 1.4 | 0.85 |
| Bunch Length | fs | 70 | 80 | 80 |
| Slice Energy Spread | MeV | 1.4 | 2.5 | 0.25 |
|  |  |  |  |  |
| Photon Beam |  | LCLS | XFEL | SCSS |
| Saturation Length | m | 60-100 | 40-170 | 80 |
| Photon Energy | keV | 0.8-8 | 0.2-12.4 | 12 |
| Peak Power | GW | 4-8 | 22-135 | 3 |

## Linac Coherent Light Source

**Linac Coherent Light Source**

## LCLS Accelerator Layout

- SLAC linac tunnel

- research yard

**Linac-0**
*L *=6 m

**Linac-1**
*L *9 m
**rf  25°

**Linac-2**
*L *330 m
**rf  41°

**Linac-3**
*L *550 m
**rf  0°

**BC1**
*L *6 m
*R*56 39 mm

**BC2**
*L *22 m
*R*56 25 mm

**DL2  **
*L *=275 m
*R*56  0

**DL1**
*L *12 m
*R*56 0

**undulator**
*L *=130 m

**6 MeV**
***z * 0.83 mm
***** * 0.05 %

**135 MeV**
***z * 0.83 mm
***** * 0.10 %

**250 MeV**
***z * 0.19 mm
***** * 1.6 %

**4.30 GeV**
***z * 0.022 mm
***** * 0.71 %

**13.6 GeV**
***z * 0.022 mm
***** * 0.01 %

**Linac-*****X***
*L *=0.6 m
**rf= 

**21-1**
**b,c,d**

***...existing***
***linac***

**L0-a,b**

**rf**
**gun**

**21-3b**
**24-6d**

**X**

**25-1a**
**30-8c**

- Courtesy P. Emma

## LCLS Diagnostics Tasks

- Charge
- Toroids (Gun, Inj, BC, Und)
- Faraday cups (Gun & Inj)
- Trajectory & energy
- Stripline BPMs (Gun, Inj, Linac)
- Cavity BPMs (Und)
- Profile monitors (Inj), compare position with alignment laser
- Transverse emittance & energy spread
- Wire scanners
- YAG screen (Gun, Inj)
- OTR screens (Inj, Linac)

- Bunch length
- Cherenkov radiators + streak camera (Gun)
- Transverse cavity + OTR (Inj, Linac)
- Coherent radiation power (BC)
- Slice measurements
- Horizontal emittance
- T-cavity + quad + OTR
- Vertical Emittance
- OTR in dispersive beam line + quad
- Energy spread
- T-cavity + OTR in dispersive beam line

## Diagnostics Requirements

| Parameter | Method | Unit | Resolution |
| --- | --- | --- | --- |
| Current | Toroid, FC | % | 2 |
| Position | Stripline BPM | μm | 5 - 20 |
|  | Cavity BPM | μm | 1 |
| Beam Size | Wire Scanner | μm | 5 |
|  | YAG | μm | 15 – 30 |
|  | OTR | μm | 5 – 30 |
| Bunch Length | Streak Camera | fs | 300 |
|  | Transverse Cavity | Slices | 10 |
|  | BLM | % | 5 |

## LCLS Injector Diagnostics

- Wire Scanner

- YAG, FC

- Cherenkov

- OTR

- OTR

- YAG

- T-Cavity

- YAG

- Phase Monitor

- Toroid

- Toroid

- Toroid

## LCLS Linac Diagnostics

- SLAC linac tunnel

- research yard

**Linac-0**
*L *=6 m

**Linac-1**
*L *9 m

**Linac-2**
*L *330 m

**Linac-3**
*L *550 m

**BC-1**
*L *6 m

**BC-2**
*L *22 m

**LTU**
*L *=275 m

- undulator

- 135 MeV

- 250 MeV

- 4.30 GeV

- 13.6 GeV

**Linac-*****X***
*L *=0.6 m

- 21-3b
- 24-6d

- 25-1a
- 30-8c

- 6 MeV

- L0-A,B

**rf**
**gun**

- X

- T-Cav

- T-Cav

**Dump**

- 21-1b
- 21-1d

- Spect.

- OTR

- BLM

- WS

- OTR

- OTR

- WS

- BLM - Bunch Length Monitor

- WS - Wire Scanner

## Beam Profile Monitors (YAG & OTR)

- YAG requirements
- Use 100μm thick crystals to meet resolution
- GTF measurements show feasibility
- OTR requirements
- Optimize yield to enable beam profile measurement at 0.2nC
- Provide sufficient depth of field for imaging of 45° foil
- Simulation shows 1mm DOF for f/# of 5 within 20μm resolution
- Match direction of reflection with axis of dispersion or T-CAV deflection
- Foil is aluminum to optimize TR yield and 1μm thick to minimize radiation

| OTR yield for 100mrad angular acceptance |  |  |
| --- | --- | --- |
| Energy (MeV) | QE (%), 450-650 nm | QE (%), 400-750 nm |
| 135 | 0.44 | 0.75 |
| 4300 | 0.98 | 1.68 |
| 13500 | 1.17 | 1.99 |

## Optics Layout

- Used for all standard YAG/OTR screens
- Telecentric lens
- 55mm focal length
- >100 line pairs/mm
- Magnification up to 1:1
- Stack of 2 insertable neutral density filters
- Beam splitter and reticule for in situ calibration
- Megapixel CCD with 12bit and 4.6μm pixel
- Radiation shielding required in main linac tunnel

## OTR/YAG Optics Design

- Courtesy V. Srinivasan

## OTR Imager with Tilted Geometry

- Need wide field of view in focus for measurements in spectrometer beam line
- Tilt OTR screen and CCD by 5 degrees in 1:1 imaging
- 10um resolution

- B.X. Wang et al. PAC05

## Simulation of OTR Beam Size Measurement

**Simulation of OTR Beam Size Measurement**

- Simulation of CCD image
- Include 0.5% TR yield, photon shot noise, and typical CCD parameters for quantum efficiency, read out noise, pixel size, digitizer gain
- Calculation of beam size
- Generate beam profile with 10σ bounding box
- Compare rms width of profile with original Gaussian beam size
- Simulation agrees well with OTR measurement at GTF
- Error of 5% in beam size for beam of 0.1nC, 260μm at 10μm resolution

- Q = 0.1nC

- E = 135 MeV

## Longitudinal Diagnostics

- Gun region
- Cherenkov radiator & streak camera
- Bunch length and slice emittance
- Transverse cavity
- Longitudinal feedback loop
- Integrated power from coherent radiation

## Cherenkov Radiators

- Located in gun region for temporal diagnostics of 6 MeV beam from gun
- Convert electron beam time structure into light pulse for streak camera measurement
- Cherenkov light suitable at low beam energies
- Design requirements
- Match time resolution of radiator to streak camera (Hamamatsu FESCA-200, < 300fs)
- Generate and transport a sufficient # of photons for 200pC beam to streak camera in laser room (10m away)

## Cherenkov Radiator Design

- Fused silica
- n = 1.458, θCR = 46.7°
- Total internal reflection
- Frosting of back surface
- NΦ = 7.5/e/mm/50nm @400nm
- Temporal and spatial resolution
- Thickness of 100μm
- Δt = 375fs
- Δx = 190μm

- Courtesy D. Dowell

## Optical Transport Layout

- 1:1 relay imaging from radiator to streak camera
- Assume 1% efficiency from frosting to scatter into 100mrad
- 6% acceptance through tube for source of 5mm x 100mrad
- 1.5·105 photons on slit of streak camera for 200 pC

- Courtesy D. Dowell

## Transverse Cavity

- Translates longitudinal into transverse beam profile when operating at RF zero crossing
- Parasitic operation with kicker and off-axis screen
- Single shot absolute bunch length measurement
- Temporal resolution limited by unstreaked spot size

## Transverse Cavity Measurement at TTF

- Scans at high power ~16MW

- 1 picosecond

- 1 picosecond

- 13 femtosecond FWHM spike!

- Beam without and with BC 3 (second bunch compressor)

- Courtesy J. Frisch

## TCAV in LCLS after BC2

- Short 70fs bunch length requires full RF power for cavity
- Parasitic measurement with beam optics optimized for SASE
- Resolution 20fs sufficient for length measurement

## Bunch Length Monitor

- Relative bunch length measurement used for longitudinal feedback
- Non-intercepting, calibrated with interceptive TCAV measurement
- Based on integrated power from coherent radiation source (C*R)
- Single electron radiation spectrum W1(ω) depends on radiation source
- Bunch length determined bylong wavelengths λ » 2πσrms
- BC1: 1cm – 1mm
- BC2: 1mm - .1mm

- BC1

- BC2

## Radiation Sources

- Wide range of bunch lengths from 25um to 300um
- Diode detectors work well below 300GHz
- Pyroelectric detectors work well above 300GHz
- Long bunches
- Couple radiation from ceramic gap in beam pipe into waveguides with different diode detectors
- Short bunches
- Extract coherent radiation from bend magnet with hole mirror and send to a pyroelectric detector

## CER Detector Layout

- Edge rad. dominates over synchrotron and diffraction
- Near field calculation necessary for radiation spectrum at detector

## Bunch Length Sensitivity of Detector Signal

- Detection efficiency includes diffraction, vacuum window, water absorption, pyroelectric detector response, and bunch form factor.
- Introduce high and low pass filters at 10cm-1 and 20cm-1.

## Gap Radiation Detector

**Gap Radiation Detector**

- Expect 2uJ radiation energy from 2cm gap for 1nC, 200um bunch (Calculation J. Wu)
- Energy density of 1.6nJ/mm2
- Diode sensitivity ~0.1pJ/mm2
- Disperse pulse in 20cm waveguide to keep diodes in linear range
- Diodes paired to reduce dependence on beam position

- Courtesy S. Smith

## Photon Beam Diagnostics

- Measure spontaneous radiation for undulator commissioning
- Measure FEL photon beam for SASE commissioning
- Nondestructive measurements of beam properties for user operation

## LCLS FEE Schematic

**LCLS FEE Schematic**

- Solid
- Attenuator

- Gas
- Attenuator

- High-Energy
- Slit

- Start of Experimental
- Hutches

- 5 mm diameter collimators

- Muon Shield

- FEL Offset mirror system

- Total
- Energy
- Calorimeter

- WFOV Direct Imager

- Spectrometer / Indirect Imager mirror

- Windowless Ion Chamber

- Windowless Ion Chamber

- e-

- Diagnostic Package

- FEL Spectrometer and Direct Imager in NEH

- Courtesy R. Bionta

## Wide Field of View Direct Imager

**Wide Field of View Direct Imager**

**Single shot measurement of *****f(x,y), x, y ,u***** **

**Camera**

**Photoelectrons generated by 0.01% FEL**

**Scintillators**

- Courtesy R. Bionta

## Indirect Imager

**Single shot measurement of *****f(x,y), x, y, u***
**Multi shot measurement of***** *********

- Angle selects energy and attenuation

**Indirect Imager**

- B4C/SiC Test Multilayers Fabricated

- Courtesy R. Bionta

## Total Energy Calorimeter

- CMR Sensor array 100 pixels

- Cold Si substrate

- Cooling ring

- Xray Beam

**Single shot measurement of *****f(x,y), x, y, u***** **

**Nd****0.8****Sr****0.2****Mn****O3**

- t = 0

- t = 100 s

- t = 300 s

- T

- T, ms

- 0

- 5

- Thermal diffusion calculations performed

- Courtesy R. Bionta

## Ion Chamber

- Segmented cathodes for position measurement

**Single shot, *****non destructive*****, measurement of**
** *****x’, y’, x, y ,u***** **

- Imaging of optical emission for position measurement

- 1 torr

- Courtesy R. Bionta

## Summary

- Electron beam diagnostics based on proven methods
- Photon beam diagnostics needs development of new techniques which are difficult to test due to the lack of a photon source comparable to an X-FEL
**Acknowledgements**
- Thanks to many colleges from the LCLS collaboration