Complex sentence: For an undulator with N periods, the brightness can be up to N ^ more than a bending magnet, because the intensity is enhanced up to a factor of N at harmonic wavelengths due to the constructive interference of the fields emitted during the N radiation periods.
Split into simpler sentences: For an undulator with N periods, the brightness can be up to N ^ more than a bending magnet. The first factor of N occurs because the intensity is enhanced up to a factor of N at harmonic wavelengths due to the constructive interference of the fields emitted during the N radiation periods.