1. Harmonic Lasing for LCLS-II? Z. Huang 11/10/2013 1

2. Motivation LCLS HXR 5 keV barely reaches saturation at 150 m. Very small K (= 0.59) is a main problem Harmonic lasing 1 overcomes this problem by running fundamental wavelength at a much larger K 3D theory shows a fundamental gain length at 5 keV is 5.5 m, while harmonic theory 2 yields a 3 rd harmonic gain length of 3.3 m at 5 keV In case that HXR line is removed for cost reasons, SXR line can reach at least 3 times photon energy 2 1.Schneidmiller and Yurkov, PRSTAB 15, 080702 (2012). 2.Huang and Kim, PRE 62, 7295 (2000).

3. HXR at 5 keV 4 GeV, 1 kA, 0.4 µ m emittance, 500 keV E-spread 3 From Y. Ding Fundamental at 5 keV u=2.6 cm, K=0.59

4. HXR at 5 keV Retune K = 2.25, adjust phase shifters to suppress fundamental at 1.66 keV 3 rd harmonic lases at 5 keV and saturates around 70 m Fundamental at 1.66 keV u=2.6 cm, K=2.25 3 rd harmonic at 5 keV Steady State simulations so far. SASE requires more elaborated phase shifter schemes (combinations of 2pi/3 and 4pi/3)

5. SXR at 1.24 keV 4 GeV, 1 kA, 0.4 µ m emittance, 500 keV E-spread 5 From G. Marcus Fundamental at 1.24 keV u=3.9 cm, K=2.07

6. SXR harmonic lasing (at 3.7 keV) Same K (=2.07) but use phase shifters to disrupt fundamental at 1.24 keV 6 Fundamental at 1.24 keV 3 rd harmonic at 3.7 keV Steady State simulations so far. SASE requires more elaborated phase shifter schemes (combinations of 2pi/3 and 4pi/3)

7. Discussions SASE simulations are required to verify these preliminary steady state results (may see BW narrowed by a factor of ~3) Harmonic lasing allows HXR to reach 5 keV (and more) with shorter saturation length, hence allows self-seeding at 5 keV Harmonic lasing extends the wavelength reach of SXR. In case HXR is cut from cost, SXR line may be lengthened to reach 5 keV with harmonic lasing Phase shifters can be used in combinations with filters to suppress fundamental. A filtered harmonic lasing test is planned for LCLS (Ratner et al., FEL2013) 7