= Solution
Lower <energy> changes the source <Frequency spectrum>[spectrum] as well as emittance. At fixed bending geometry, bending-radiation critical <energy> scales as $\gamma^3$, so it falls by $10^{3/2}$; for fixed undulator period and deflection parameter the fundamental wavelength grows as $\gamma^{-2}$, by ten. A smaller <electron> emittance therefore does not automatically improve brightness at the original 10 nm wavelength: the device may no longer radiate there on its fundamental, flux and <harmonic> operation change, and diffraction remains. Lattice redesign can retain beam <energy> and target <Frequency spectrum>[spectrum] while reducing emittance, which is why <energy> reduction alone is not a generally satisfactory substitute.
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