Self-Seeding on LCLS-II

Juhao Wu Michael Rowen

Paul EmmaDec. 08, 2010

Hard x-ray @ 13 keV– In real simulation, we take 0.95 Å FEL– Charge: 20 pC, electron peak current: 3 kA,

normalized emittance: 0.4 mm-mrad, centroid energy 12.5 GeV, slice energy spread sg = 2.8

– Undulator period 3.2 cm, K = 2.26, each segment 3.4 m (magnetic length only), b-function 33 m

– Notch filter seeding (Transmitted light in Bragg Geometry): G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010

HXR (13 keV) SASE FEL• Power along the undulator in left, and energy

spread along the undulator in the right

– Take 1.5 GW onto C(400) seed can up to 10 MW– This is at 60 m magnetic length into undulator, the

energy spread is sg = 4.8

Single crystal monochromator• We take C(400) with absorption, symmetric

Bragg geometry. The bandwidth is close to 1.0E-5.

Single crystal monochromator

• FEL spectrum after the single-crystal monochromator

Single crystal monochromator• FEL after the single-crystal monochromator

Park the electron bunch here

HXR (13 keV) Self-seeded FEL• Electron slice relative energy spread is taken as sg = 4.8, with

other parameters not changed– Seed is now 10 MW.

– Saturation is around 45 m magnetic length total of about 105 m magnetic length.

Future Implementation• The proposed LCLS-II hard x-ray undulator is

~100m total magnetic length.• Could seed up to 13keV with mini-chicane and

crystal at ½ of proposed undulator length.• Just reach saturation in seeded beam in 2nd ½

undulator @ 13keV.– Would get some increased power from tapering after

saturation at lower energies.– Add undulator segments to increase seeded power at

all energies.

Soft x-ray @ 2 keV

– In real simulation, we simply take 6 Å FEL– Charge: 250 pC, electron peak current: 3 kA,

normalized emittance: 0.6 mm-mrad, centroid energy 12.5 GeV, slice energy spread sg = 2.8

– Undulator period 5.5 cm, K = 4.91, each segment 3.4 m, b-function 15 m

– Use monochromator: Y. Feng, J. Wu, M. Rowen, P. Heimann, J. Krzywinski, J. Hastings, et. al. LCLS-II Physics meeting 8/4/2010

SXR (2 keV) SASE FEL• Power along the undulator in left, and energy spread along the

undulator in the right

– Take 2 GW onto VLS gratings (efficiency 5.0E-5) seed 0.1 MW

– This is at 42.24 m into undulator, the energy spread is sg = 8.5334

SXR (2 keV) Self-seeded FEL• Electron slice relative energy spread is taken as sg = 8.5334,

with other parameters not changed– Seed is now 0.1 MW

– Saturation is around 55 m total of about 100 m

Future Implementation

• Add shorter 2nd undulator and chicane:– Seeding SASE undulator ~45m total magnetic length.– Chicane, electron optics and mono ~30m.– The proposed LCLS-II undulator is 78m long. – Space for polarizing after burners 10m.

• Totals to 163m. – Requires the full length of the undulator hall.

Possible experiment at LCLS

• Following closely Geloni’s calculation, but with more practical consideration for 8 keV– Low charge 20 pC, 0.4 mm-mrad emittance,

slice energy spread 1.3 MeV– May consider taking the section 15 undulator

out to implement the chicane and single crystal

SASE FEL at exit of 13 undulators

• Even though we will plan to take out the 15th undualtor, here we assume the SASE FEL from the exit of 13 undulator on the single crystal– This can be regarded as safety consideration not to

have the 14th into consideration yet

Single crystal monochromator• We take C(400) ideal case with absorption,

symmetric Bragg geometry. The bandwidth is about 1.0E-5.

Single crystal monochromator

• FEL spectrum after the single-crystal monochromator

Single crystal monochromator• FEL after the single-crystal monochromator

Self-seeded FEL at exit of 10 undulators

• There are 18 undualtors after the monochromator– FEL at the exit of 10 undulator

2.8E-5