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Brighter and Shorter
Nora Norvell September 30th 2016
Current LCLS operational setups and diagnostics for super short pulses
5
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
6
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
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;
Simulated example: 1.5 kA, 4 GeV (1 keV) beam
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
Simulated example: 1.5 kA, 4 GeV (1 keV) beam
4) Apply formulas to get the x-ray power profile
Equation 1
Equation 2
Simulated example: 1.5 kA, 4 GeV (1 keV) beam
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
14
Short pulses are great: How do we measure them?!
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
16
Short pulses are great: How do we measure them?!
• 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
17
Short pulses are great: How do we measure them?!
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
18
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
19
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
20
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!
21
Nominal Pulse Duration Control: Electron Chirp
L1S L1X
L3BC2BC1L1 DL2
DL1 BLM1 BLM2 Undulator
L2E-BPMs
23
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
24
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!
26
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
27
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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Short Pulse Shift Case Study: September 21st, 2016
Move the slotted foil into the beam path in the 2nd bunch compressor chicane
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Short Pulse Shift Case Study: September 21st, 2016
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
30
Short Pulse Shift Case Study: September 21st, 2016
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