TW FEL “Death-Ray“ Studies

Josef Frisch, Zhirong Huang, Yi Jiao

FEL Optimized for Imaging Many imaging experiments are similar

Hard X-rays ~1 Angstrom, minimum focus size, maximum flux.

Liquid or gas target injection Pump laser CCD arrays surrounding sample

Use a special purpose imaging chamber designed for rapid change of samples

FEL optimized for maximum flux at 1 Angstrom, ~10-30fsec

Conversations with experimenters suggest that 2TW in 10fs is required for imaging of single bio-molecules

Diamond crystal band-stop filter

Seeding undulator

Gain Undulator

Energy extraction taper

Band stop filter produces narrow band signal in tail

Chicane delays beam so that it interacts with tail of filtered pulse

A “notch” filter produces a narrow band peak in the transmitted spectrum

Narrow band seed power

Idea invented by E. Saldin at DESY

Self Seeding Improves FEL Efficiency

Minimal mirrors (maybe just KB set)

High Energy Operation - Estimates Vary electron voltage while keeping operating wavelength fixed at 1

Angstrom. Assume that peak current increases as the 4th root of beam energy.

Undulator wavelength changes, keep same peak B field as LCLS undulator so "K" changes

Roughly matches what we can use in LCLS_I, is probably conservative Assume 0.6um slice emittance and 1MeV energy spread (before

spontaneous energy spread is added). Run Ming Xie formula with spontaneous energy spread (Saldin et al,

“Design Formulas for VUV and X-RAY FELs”) calculated for 20 gain lengths (iterate).

Allow Beta function to optimize from 10 to 100 Meters

No Taper~60GW at 28 GeV

0.4 emittance, higher peak current, more agressive parameters

90GW at 28 GeVUsable to 45 GeV

500eV FEL can use high beam energy as well !

~25 GeV beam with existing LCLS_I undulator should work at 25 KeV

TW FEL Genesis Simulations by Zhirong Huang

27 GeV, 0.6um Emittance, 5KA peak current, 1.4 MeV espread, 4.5cm Undulator, K=4.95, 30M beta function

4TW (!!)700M undulator(!!)

NOT OPTIMAL!

Need Simulations• The simulation above is based on a guess at beam

parameters from Ming-Xi calculations. It may be VERY far from optimal

• 700 M undulator is huge – but not completely insane– Undulators are $200K/M, this is $140M for an undulator, LCLS_II

is $400M, XFEL is >$1B• E-beam parameters are reasonable, but R&D might improve

emittance or peak current.• Energies up to 45 GeV in principal available from the SLAC

linac. (probably can’t use that much energy)• High K helical undulators (Superconducting) may be possible.• Want to explore the design phase space.

Simulations• Use Genesis in CW beam mode to evaluate the performance of various

accelerator and undulator configurations– Slice simulation takes ~15 seconds on a PC – can afford to run a LOT of

simulations.• Use various electron beam energies, emittance, peak current.• Use linear and helical undulators, set for 1 angstrom output wavelength

with maximum practical K at each undulator wavelength.• Optimize output power at various undulator lengths (each 100M) for beta

function and K (2 parameters).• Note: need to apply physics to constrain parameters and to aid

optimization. • Create a large “library” of simulations to explore the phase space

– >106 simulation possible – probably 104 optimized conditions.• Generate an empirical model or software tool to predict the performance

of an FEL give input e-beam parameters (energy, emittance, peak current), and undulator parameters (length, max B-field for PM, or SC).

Automatically optimizing Varying one input parameter while keep

others constantGenerate the corresponding input file for

Genesis steady-state simulationRead the Genesis output, record interested

quantities, e.g. power, transverse radiation size, etc.

Find out the optimal input parameterAll these jobs are integrated into a MATLAB

code.

Scan x, y, at the undulator entrance and quadrupole gradient for a higher radiation power

Before scanning : x,= 11.6 m, y

= 9.1 m, K = 26.7 T/m, power = 2.63 TWAfter scanning : x,= 9.3 m, y = 9.3 m, K = 22.4 T/m, power = 2.83 TW

0 10 20 30 40 50 60

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

x (m)

Pm

ax (

TW

)

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

2

2.5

3

Quad gradient (T/m)

Pm

ax (

TW

)

0 10 20 30 40 50 601.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

y (m)

Pm

ax (

TW

)

Other Issues• Transverse mode quality may degrade for long tapered undulators

– Juhao is working on this?• Full 3-d effects may be important

– Will need full 3-d genesis runs for some interesting cases• Do we trust Genesis in this regime?

– Need to cross-compare against other FEL simulations• Experiments

– Want to compare prediction with the HXRSS seeding experiment at LCLS late this year.

– Need results soon enough to predict, not post-dict!• Need to do this quickly!

– SLAC is designing LCLS_II, working on Sector 0 test facility. – Will soon determine what parts of the LINAC are available in the future– Knowing possible future uses of the LINAC and undulator hall may affect decisions

now!– Around the world $5B of FEL projects proposed or under construction – these

results might affect their decisions.

Benefit to SLAC Imaging experiments are an important application of

FELs. Provides a unique capability at SLAC: No other lab will

build a 30 GeV accelerator for an FEL. Not made obsolete by XFEL or any other proposed FEL

project

LCLS Publicity image shows single molecule imaging, but this is NOT POSSIBLE with present day FELs!