کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
5407384 | 1393214 | 2007 | 29 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
250Â GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
شیمی
شیمی تئوریک و عملی
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چکیده انگلیسی
In this paper, we describe a 250Â GHz gyrotron oscillator, a critical component of an integrated system for magic angle spinning (MAS) dynamic nuclear polarization (DNP) experiments at 9Â T, corresponding to 380Â MHz 1H frequency. The 250Â GHz gyrotron is the first gyro-device designed with the goal of seamless integration with an NMR spectrometer for routine DNP enhanced NMR spectroscopy and has operated under computer control for periods of up to 21 days with a 100% duty cycle. Following a brief historical review of the field, we present studies of the membrane protein bacteriorhodopsin (bR) using DNP enhanced multidimensional NMR. These results include assignment of active site resonances in [U-13C, 15N]-bR and demonstrate the utility of DNP for studies of membrane proteins. Next, we review the theory of gyro-devices from quantum mechanical and classical viewpoints and discuss the unique considerations that apply to gyrotron oscillators designed for DNP experiments. We then characterize the operation of the 250Â GHz gyrotron in detail, including its long-term stability and controllability. We have measured the spectral purity of the gyrotron emission using both homodyne and heterodyne techniques. Radiation intensity patterns from the corrugated waveguide that delivers power to the NMR probe were measured using two new techniques to confirm pure mode content: a thermometric approach based on the temperature-dependent color of liquid crystalline media applied to a substrate and imaging with a pyroelectric camera. We next present a detailed study of the mode excitation characteristics of the gyrotron. Exploration of the operating characteristics of several fundamental modes reveals broadband continuous frequency tuning of up to 1.8Â GHz as a function of the magnetic field alone, a feature that may be exploited in future tunable gyrotron designs. Oscillation of the 250Â GHz gyrotron at the second harmonic of cyclotron resonance begins at extremely low beam currents (as low 12Â mA) at frequencies between 320 and 365Â GHz, suggesting an efficient route for the generation of even higher frequency radiation. The low starting currents were attributed to an elevated cavity Q, which is confirmed by cavity thermal load measurements. We conclude with an appendix containing a detailed description of the control system that safely automates all aspects of the gyrotron operation.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Journal of Magnetic Resonance - Volume 189, Issue 2, December 2007, Pages 251-279
Journal: Journal of Magnetic Resonance - Volume 189, Issue 2, December 2007, Pages 251-279
نویسندگان
Vikram S. Bajaj, Melissa K. Hornstein, Kenneth E. Kreischer, Jagadishwar R. Sirigiri, Paul P. Woskov, Melody L. Mak-Jurkauskas, Judith Herzfeld, Richard J. Temkin, Robert G. Griffin,