Table 2.

Comparison of some XLO and XFELO parameters at LCLS-II

Parameter	XLO	XFELO
Gain per pass	Up to ${\mathrm{10}}^6$	1.2 to 1.5
Cavity length, m	$\sim$10	$>$260
Lasing medium size, m	$3\times 10^{-4}$	$\sim$100
Angular tolerance, $\mu$rad	1	$\sim$0.01
Number of photons (max)	$5\times 10^{10}$	$10^{10}$
Peak power, MW	$\sim$270	$\sim$4.7
Pulse length, fs	37.4	530
FWHM $\mathrm{\Delta }t\mathrm{\Delta }\omega$, fs$\cdot$eV	1.8	4.4

FWHM, full-width at half-maximum.