High-frequency high-power DNP/EPR spectrometer operating at 7 T magnetic field

Area of Science:

• Magnetic Resonance Spectroscopy
• Physical Chemistry
• Materials Science

Background:

• Pulse Dynamic Nuclear Polarization (DNP) requires high-power, coherent millimeter-wave (mm-wave) pulses.
• Existing technology limits the generation of mm-waves at frequencies relevant to high-resolution Nuclear Magnetic Resonance (NMR) spectrometers.

Purpose of the Study:

• To develop and integrate a high-power mm-wave pulse amplifier for pulse DNP.
• To enhance the performance of a 7 Tesla (T) NMR/DNP/Electron Paramagnetic Resonance (EPR) spectrometer.
• To characterize the performance of the upgraded system for various DNP experiments.

Main Methods:

• Integration of a custom-built Extended Interaction Klystron (EIK) pulse amplifier (140 W at 197.8 GHz) with a 7 T NMR/DNP/EPR spectrometer.
• Utilizing a Thomas Keating, Ltd. quasioptical bridge and a homebuilt DNP probe with photonic bandgap (PBG) resonators.
• Employing three-pulse electron spin echo nutation experiments to measure electronic B1e fields.
• Conducting "gated" mode DNP experiments with 20 μs mm-wave pulses and continuous wave (CW) DNP experiments.

Main Results:

• Achieved <9 ns π/2 pulses at ~50 W EIK output with a PBG resonator quality factor of Q≈300.
• Demonstrated solid-effect DNP with ¹³C gains up to 20 for polystyrene-BDPA samples.
• Observed comparable ¹³C enhancement in gated DNP mode for HPHT diamond as in CW mode, with no DNP for polystyrene-BDPA in CW mode.
• Achieved ³¹P Overhauser DNP enhancements of 7-14 in liquid samples.

Conclusions:

• The integration of a high-power EIK amplifier and PBG resonators significantly advances pulse DNP capabilities.
• The upgraded spectrometer enables efficient DNP experiments in both gated and CW modes for various samples.
• The system demonstrates versatility for solid-state and liquid-state DNP applications, enhancing NMR sensitivity.