High harmonic generation in a large-volume capillary for seeding of free-electron lasers

Siew Jean Goh

2

Motivation

High Harmonic Generation Setup

Result

Overview and summary

Outline

3E. Allaria et al, Nature Photonics Vol 6, pg 699–704 (2012)

Why Seed a Free-Electron Laser ?

Shorter FEL Wavelengths?

4

• Single order pulse energy ~100 nJ

• Seed wavelength range (30-40 nm)

• High pulse energy stability (< 10 %)

• High pointing stability (< 20 % of beam size)

High Harmonic Source for Seeding

0.3 mmSeed Beam

e-Beam15 m

Seeded Operation down to 5-6 nm

High Harmonic Setup

5

6

Large Capillary (Motivation)

• Assist phase matching with waveguiding

• Selective enhancement via pulse shaping

• Self-phase-modulation & self-compression

• Stabilize beam pointing by waveguiding

• Output scales with volume

7

Typical High Harmonic Spectrum

• Drive laser energy = 6.5 mJ, pressure: 4 mbar• Spectrum determined by filter edge,cut-off and phase-matching

8

Typical High Harmonic Beam Profile

Excellent spatial quality at 4 - 6mbar

20 shots integrated beam profile

5 mbar

1 mm

•Near gaussian and elliptical

•Energy content

•Center of gravity

•Standard deviation

Beam pointing and divergence

𝐸= ඵ 𝐼ሺ𝑥,𝑦ሻ∞−∞ 𝑑𝑥 𝑑𝑦

Divergence (x-axis)= 2σx / L= 3 mm / 9 m = 0.3 mrad

9

High Harmonic Energy

From 20 consecutive measurements of 20 shots integrated beam profile

10

Energy Jitter

From 20 consecutive measurements of 20 shots integrated beam profile

11

DivergenceL = 9 m

From 20 consecutive measurements of 20 shots integrated beam profile

12

Pointing Stability

From 20 consecutive measurements of 20 shots integrated beam profile

13

Harmonic Selective Enhancement

• Selective enhancement of 2.5 (at 37 nm)

14

Overview of ResultsStandard Capillary 150 m; 1.0 mJ,

53 mbar (Ar)(100 x single shot)

Large Capillary 508 m; 6.5 mJ

6 mbar (Ar)(20 x 20 Shots)

Seed Requirement

FERMI@Elettra

Wavelength 35 nm 32 nm 30 - 40nm

Divergence 1.5 mrad (x)1.6 mrad (y)

0.5 mrad (x)0.4 mrad (y)

< 10%

Pointing Stability

6% (x)17% (y)

11% (x) 9% (y)

< 20%

Energy Jitter 18 % 34 % < 10%

Energy 0.2 nJ 0.6 nJ ~100 nJ

15

Output Energy

Possible problems Reabsorption in differential pumping section Absorption in oxide layer on Al filters Absorption in carbon layer deposit on XUV camera

Next steps Reduce gas flow, cleaning, absolute calibration Add amplitude shaping

16

Summary

Large-volume capillary (d= 500 m)

Phase matching at 5 - 6 mbar

Selective enhancement (2.5-fold at 37 nm)

Beam pointing stability (~10%)

17

Acknowledgements

17

Acknowledgement

Thank you for your attention!