The microwaves induce the spin flip transitions and must be tuned very carefully to the frequency of the energy gap to maximize polarization. Unfortunately, as the target material accumulates radiation damage from the beam, this optimal frequency changes. "'The most important task of the target operators is to constantly monitor and tweak the microwave frequency to maximize polarization."' The microwaves are provided by the EIO tube, which allows the frequency of microwaves to be changed within limits by adjusting a bellows on the oscillation cavity. Wave guides carry the microwaves from the tube to a horn which shine on the target cups. The picture below shows the gold horn above the two target cups on the new target inserts. In g2p, two magnetic field strengths (5T and 2.5T) will be used and thus two nominal microwave frequencies will be used ( 140 and 70 GHz). This requires two different EIO tubes, but the operation will be similar.
More information on the microwave can be found here
The CPI EIO (Extended Interaction Oscillator) is a microwave generator used in DNP.