brighter and shorter - slac.stanford.edu · brighter and shorter nora norvell september 30th 2016...

Brighter and Shorter

Nora Norvell September 30th 2016

Current LCLS operational setups and diagnostics for super short pulses

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The Accelerator Control Room

Experimental Halls

x2151

Short pulses are great! How do we measure them?!

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Short pulses are great: How do we measure them?!

• Idea: measure photon pulse indirectly from the “Parent” electron bunch from Yuantao Ding in 2011

• Look at electron bunch energy spread noninvasively after photons are sent on to the hutches

• Can get shot by shot photon pulse duration calculations!

Ding, Y. et al. “Femtosecond x-ray pulse temporal characterization in free-electron lasers using a transverse deflector” Physics Review ST 120701 (2011).

e-

sz

Horizontally‘streaked’ bunch

Vertical benddipole magnet

X-band Transverse RF Deflecting Cavity

(XTCAV)

Undulator

X-rays

z

x

yVertically

dispersed bunch

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Diagnostic Layout

XTCAV: Resolving the e-bunch t-E phase space

Schematic courtesy of Tim Maxwell

• Use X-band transverse deflector (XTCAV) to give a time vs X relation• Use bend magnet as energy spectrometer to give energy vs Y relation

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Diagnostic Layout

Image courtesy Patrick Krejcik

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XTCAV amplitude vs streaking effects

Simulated example: 1.5 kA, 4 GeV (1 keV) beam

1) Suppress lasing or “lasing off”, collect images of distribution in the absence of lasing.

Slides courtesy of Tim Maxwell

T. Maxwell et al. " Femtosecond-scale x-ray FEL diagnostics with the LCLS X-band transverse deflector ", Proc. SPIE 9210, X-Ray Free-Electron Lasers: Beam Diagnostics, Beamline Instrumentation, and Applications II, 92100J (September 5, 2014); doi:10.1117/12.2065252;


2) Unsuppress lasing or “lasing on”, compare. Two things happen in time slices were SASE/lasing occur:

Slices lose energy to create x-raysWidth of lasing slices increase


3) Compute slice-statistical properties of the images


4) Apply formulas to get the x-ray power profile

Equation 1

Equation 2


4) Apply formulas to get the x-ray power profile

x

xet eV

Ef gb

ep

srfrf2

1=

Time resolution

Result:1 fs rms @ SXR2 fs rms @ HXR

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Changes in electron time vs energy phase space can infer the photon pulse width

Electron beam kicked in undulatorhall so lasing OFF

Example: 4.7GeV electrons/ 1keV photons, 150 pC, 1.5mjs

Lasing ON: Producing Xrays!

Reconstruct the photon profile from the time dependent energy loss

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• FEL on images from same 8 keV data set

• Shots with uneven lasing identified

FEL off FEL on FEL on

Slide courtesy of Tim Maxwell

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XTCAV Pros:• Once setup, non

interceptive to photon delivery

• ~1-3 femtosecond resolution

• Camera 120 hz capable. Pulse by pulse duration profiles!

• Usable at all photon energies

• Savable in the LCLS DAQ

XTCAV Cons:• Takes about ~10 mins of

invasive time to get background images calibration etc.

• Increases Machine Protection System (MPS) trips less orbit stability and resulting charge loss in the dump

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Now we can see how long pulses are! How do we make super short pulses?General idea:• The peak power (photons that are emitted for each

electron) for what LCLS can produce remains constant• Compress the electron bunch as much as possible• If needed: take head and tail electrons out of the

lasing process until desired bunch length is achieved

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How do we make super short pulses?

Three main ways to change photon pulse duration1. Increase electron beam energy spread (Chirp) through

bunch compressor chicanes2. Lower the number of electrons we send down the

machine3. Slotted foil

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Nominal Pulse Duration Control: Electron Chirp

• Chirping the electron beam• Chirp: creating a energy spread dependent on time• In practice: Give the head of the bunch less energy than

the tail, send chirped electron beam through two chicanes with four magnets.

• How we create this energy spread: Put some RF stations off phase!

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L1S L1X

L3BC2BC1L1 DL2

DL1 BLM1 BLM2 Undulator

L2E-BPMs

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Photon Pulse Duration Control: Lowering the Charge

• Nominal charge setup 2016: 250pC chopped down to 180 pC

• Two ways to lower the charge:• Change electrons released from the cathode:

• Current LCLS charge range 20pC- 250pC• Change drive laser size • Change drive laser phases • Retune the entire machine

• Quicker option, close collimators in BC1 for smaller charge changes such as 250pC-100pC.

• Use metal collimators as selective electron dumps• Ideal for much quicker configuration changes

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Photon Pulse Duration Control: Slotted Foil

• Emittance spoiling foiling inserted in the second Bunch Compressor Chicane

• Impacted electrons will not lase• Easy to insert, easy to control pulse

width with foil motor Change photon

pulse by changing how far

the foil is inserted!

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Photon Pulse Duration Control: Slotted Foil

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Short Pulse: Shift Case study September 21st, 2016

• Setting: SLAC Accelerator Control Room• Personnel: LCLS Ops and Machine Physicist • Photon energy: 5.88 keV

• Nominal setup before any pulse shortening:• 4.99 mJ• 50fs x-ray pulses

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Short Pulse Shift Case Study: September 21st, 2016

Nominal Setup beam profile: 6keV Photons

Charge: 185 pCBC1: 210 ABC2: 3500 A

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Move the slotted foil into the beam path in the 2nd bunch compressor chicane

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Move the slotted foil into the beam path in the 2nd bunch compressor chicane

• 10 - 25 fs pulses with 0.35 - 3 mJ respectively• Total time: As long as it takes to push a button and wait for a motor (seconds)

• 6-15 fs pulses required changing the e-compression scheme for signs of lasing. 0.2 mJ –1.4 mJ

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Next change charge 250 pC à 100 pC off the gun• Total time to change charge off gun: ~45 mins

Best case scenario:• 2.09 mJ• ~15 fs pulses• Charge: 67 pC• BC1: 90 A• BC2: 4000 A• 6 keV photons

brighter and shorter - slac.stanford.edu · brighter and shorter nora norvell september 30th 2016...

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