Two-Color I-SASE

A. Marinelli, J. Wu, C. PellegriniLCLS2 Meeting

SLAC 1/30/2013

• It has been suggested that a wide, 1-2% wide X-ray spectrum, might be an advantage for coherent diffraction imaging.

• A possible alternative is a two colors spectrum with narrow lines separated by 1-2%.

• A two color spectrum has recently been observed at LCLS while doing the iSASE studies.

• A dedicated 2-color iSASE experiment is being planned

two colors 1/8/13

Diffraction imaging

Science with 2-Color iSASE

Soichi Wakatsuki et al.

Use of 2-color iSASE for SAD/MAD Phasing on XPP

Two proposals already submitted

Request for separation from0.1% to 2%

Science with 2-Color iSASE

Soichi Wakatsuki et al.

Use of 2-color iSASE for SAD/MAD Phasing on XPP

Two proposals already submitted

Request for separation from0.1% to 2%

~within FEL bandwidth

Proof of Principle Demonstration of iSASE

Proof of principle demo: use detuned undulators at LCLS

Delay line introduces longer coherence legnth

Machine layout:First 5 undulator sections on-resonant From 6th on, even number: 6, 8, …, 30, and 32 largely detuned (can either be random or form a separate spectrum line two color)From 6th on, odd number: 7, 9, …, 31, and 33 on resonant

Perform proof-of-principle experiment on LCLS for an improved SASE (iSASE)

Electron bunch: 150 pC, compressed to 3 kA8.45 keV FEE HXSSS13.825 GeV electron energy

two colors 1/8/13

LCLS iSASE experimental setup

Experimental Results

Observed a reduction of linewidthby ~ 3.4

Two-Color iSASE

Alternating K value-> One wavelength gains, the other is delayed->Two colors with narrower bandwidth than SASE

Observed During iSASE Experiment

Observed During iSASE Experiment

Line separation not consistent with detuning!

1-D TheoryEffect of high-gain FEL on the collective variables described by 3x3 matrix

Two Color I-SASEDefine detuning d with respect to average resonant frequency (w1+w2)/2

Two Color I-SASEFor large detuning:

Two Color I-SASEFor large detuning:

Undulator dispersion

Undulator delay

Equivalent to iSASE transfer matrix…

Example: Delta = 6Sidebands appear at the frequency given by undulator periodicity.

Relative intensity of the 4 peaks strongly dependent on Delta(weaker dependence for longer periodicity)

Period = 2 Lg

Period = 4 LgPeriod = 8 Lg

Genesis SimulationsGenesis simulations for ideal beam show same trends. The sideband separation is consistent with our understanding.

Can we control this structure?

DeltaK = 0.6% DeltaK = 0.9%

DeltaK = 1.1%

Power VS Time

Gain Curve

Comparison with Alberto’s Experiment

2-color experiment at LCLS worked beautifully…What does our method do that hasn’t been done yet?

1) Perfect synchronization of two colors (good for imaging) BUT no tunable delay (actually tunable within slippage length...)

2) Each color has the i-SASE bandwidth (tunability within the SASE bandwidtdh...).

3) Control of sidebands allows extra degree of freedom on the spectral structure.

Comparison

Similar Power Level (a little higher for the two undulator method)

A Few Words on LCLS2

-The greater range of tunability of LCLS2 allows even more flexibility in tayloring the spectrum.

-Mimic ~10% bandwidth with more than two colors?

Open Questions 1) Are we over-complicating the analysis?

The chain matrix multiplication method is direct but does not give intuitive closed form formulae

Maybe some more intuitive picture is possible…

2) In practice, how much control do we have over the sidebands for short undulator periodicity?Can we use them for fine-tuning of the spectrum within the SASE bandwidth?

Open Questions

What happens with a realistic beam distribution?

Working on GENESIS simuations right now…Experiment will tell us the answer!

Experimental Plan:Study spectrum VS K separation looking for:1) Wavelength separation VS Delta K2) Sideband structure manipulation changing periodicity3) XPP spectrometer for overall spectrum FEE spectrometer for single-shot study of sidebands