what is EPR saturation?

 Signal intensities do not depend solely on concentrations. They also depend
on the microwave power. If you do not use too much microwave power, the
signal intensity grows as the square root of the power. At higher power levels,
the signal diminishes as well as broadens with increasing microwave power
levels. This effect is called saturation. If you want to measure accurate line-
widths, lineshapes, and closely spaced hyperfine splittings, you should avoid
saturation by using low microwave power. A quick means of checking for the
absence of saturation is to decrease the microwave power and verify that the
signal intensity also decreases by the square root of the microwave power.

 https://www.cif.iastate.edu/sites/default/files/uploads/EPR/CW%20EPR%20Primer(XenonSoftware).pdf