Cryogen-free superconducting magnet system for multifrequency electron paramagnetic resonance up to 12.1T

Abstract

Multifrequency and high field/high frequency ͑HF͒ electron paramagnetic resonance ͑EPR͒ is a powerful spectroscopy for studying paramagnetic spin systems ranging from organic-free radicals to catalytic paramagnetic metal ion centers in metalloproteins. Typically, HF EPR experiments are carried out at resonant frequencies = 95-300 GHz and this requires magnetic fields of 3.4-10.7 T for electronic spins with g Ϸ 2.0. Such fields could be easily achieved with superconducting magnets, but, unlike NMR, these magnets cannot operate in a persistent mode in order to satisfy a wide range of resonant fields required by the experiment. Operating and maintaining conventional passively cooled superconducting magnets in EPR laboratories require frequent transfer of cryogens by trained personnel. Here we describe and characterize a versatile cryogen-free magnet system for HF EPR at magnetic fields up to 12.1 T that is suitable for ramping the magnetic field over the entire range, precision scans around the target field, and/or holding the field at the target value. We also demonstrate that in a nonpersistent mode of operation the magnetic field can be stabilized to better than 0.3 ppm/ h over 15 h period by employing a transducer-controlled power supply. Such stability is sufficient for many HF EPR experiments. An important feature of the system is that it is virtually maintenance-free because it is based on a cryogen-free technology and therefore does not require any liquid cryogens ͑liquid helium or nitrogen͒ for operation. We believe that actively cooled superconducting magnets are ideally suited for a wide range of HF EPR experiments including studies of spin-labeled nucleic acids and proteins, single-molecule magnets, and metalloproteins.

A CW and pulse EPR spectrometer operating at 122 and 244 GHz using a quasi-optical bridge and a cryogen-free 12 T superconducting magnet

“…A part of the 29.5 GHz signal is mixed with the 1 GHz offset signal (5) generated by a voltage-controlled oscillator (VCO) (0.8-1.0 GHz) through ah up-converter (6). The resulting 30.5 GHz signal is then amplified (7) to 1-2 W (amplifier by Spacek) and multiplied (8) to 122 GHz (150 mW) using two high-power doublers (from Virginia Diodes).…”

“…The main coil is equipped with a thermo-switch allowing persistent mode operation. The magnet system has been described in detail by Smimov et al in a recent publication [6]. The main advantages of the cryogen-free system are the possibility to continuously sweep the magnet (0.3 T/min) over wide field ranges without the costs of helium boil off and the "graceful" recovery of the system after a magnet quench.…”

“…To date only a few pulse HF-EPR (frequency higher than 95 GHz) spectrometers have been described in the literature. Smimov et al [6] and Gerfen et al [7] are using a 130 GHz continuous-wave (CW) and pulse EPR bridge developed by V. Krymov using IMPATT amplifiers and heterodyne quadrature detection. In the group of R. G. Griffin an instrument was developed for EPR, ENDOR, and DNP at 140 GHz [8,9]; Prisner et al [10] described a 180 GHz pulsed instrument, while Schmidt et al [11,12] developed a pulse EPR spectrometer operating at 270 GHz.…”

Experimental study from linear to chaotic regimes on a terahertz-frequency gyrotron oscillator

“…The superconducting 10 T magnet (SCM), manufactured by Cryogenics Ltd. (London, UK) consists of a He-free cryostat 32 with a nominal operating temperature of the NbTi superconducting solenoid of 3.7 K. The magnet warm-bore diameter is 75 mm. Two independent copper coils in the cathode region allow the independent control of the cathode magnetic field and its gradient.…”

High-Frequency EPR Instrumentation

“…Although this leads to a somewhat higher static boil-off, the operation of the magnet is greatly simplified. Recently, cryogenfree magnet technology has been introduced in the field of EPR [95]. The great advantage of these systems is the ability to continuously sweep the main coil without the associated helium costs.…”