Cryogenic magic-angle spinning continuous wave EPR and DNP spectroscopy at 7 T with a gyrotron

Dynamic nuclear polarization (DNP) relies on the transfer of electron polarization to nuclei through microwave irradiation and is typically performed under cryogenic magic-angle spinning (MAS) at high magnetic fields. Gyrotrons are commonly used microwave sources in DNP because of their ability to produce high-power microwaves over a broad frequency range. An important step towards a more in-depth understanding of DNP mechanisms and rational optimization of DNP performance is the access to instrumentation, which can provide information about the DNP process. Continuous wave (CW) electron paramagnetic resonance (EPR) can reveal important information on the electron spin system during DNP experiments. Here, we present a dual CW EPR/DNP spectrometer operated under MAS at 100 K and 7 T using a frequency-agile 198 GHz gyrotron. The measured sensitivity for CW EPR at 198 GHz using an MAS stator is 4 $\times$ 10¹³ spins/(G$\sqrt{Hz}$). To illustrate the electron and nuclear detection capabilities of our setup, DNP profiles and CW EPR spectra of a diamond powder and a trityl sample were recorded under the same conditions, specifically at 100 K and under MAS.