Fig. 9.

¹H DNP signal enhancement as a function of microwave power at the end of the probe waveguide measured on 0.1 M U-¹³C-¹⁵N-proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio). ¹³C CPMAS experiment with 8 scans, 10 s recycle delay, 1 dummy scan, ω_r/2π = 8 kHz, 97 K sample temperature, microwaves on continuously. 64 scans were acquired for the microwave off measurement and the error bar on the DNP signal enhancement is ±1%. The gyrotron electron beam current, cathode voltage, and cavity temperature were varied to cover the full power range with constant frequency. The microwave power was measured with the transmission line directional coupler and calorimeter, calibrated with the water load at the end of the probe waveguide.