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. 2006:6373:63730C.
doi: 10.1117/12.686436.

Continuous-wave Submillimeter-wave Gyrotrons

Seong-Tae Han  1 , Robert G Griffin, Kan-Nian Hu, Chan-Gyu Joo, Colin D Joye, Ivan Mastovsky, Michael A Shapiro, Jagadishwar R Sirigiri, Richard J Temkin, Antonio C Torrezan, Paul P Woskov
Affiliations

Continuous-wave Submillimeter-wave Gyrotrons

Seong-Tae Han et al. Proc SPIE Int Soc Opt Eng. 2006.

Abstract

Recently, dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) has emerged as a powerful technique to obtain significant enhancements in spin spectra from biological samples. For DNP in modern NMR systems, a high power continuous-wave source in the submillimeter wavelength range is necessary. Gyrotrons can deliver tens of watts of CW power at submillimeter wavelengths and are well suited for use in DNP/NMR spectrometers. To date, 140 GHz and 250 GHz gyrotrons are being employed in DNP spectrometer experiments at 200 MHz and 380 MHz at MIT. A 460 GHz gyrotron, which has operated with 8 W of CW output power, will soon be installed in a 700 MHz NMR spectrometer. High power radiation with good spectral and spatial resolution from these gyrotrons should provide NMR spectrometers with high signal enhancement through DNP. Also, these tubes operating at submillimeter wavelengths should have important applications in research in physics, chemistry, biology, materials science and medicine.

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Figures

Figure 1
Figure 1
Schematic drawing of a gyrotron in cross-sectional view (not shown to scale). The basic components of the gyrotron include a magnet, and an electron gun, and a vacuum tube which consists of a beam tunnel, a resonator, and a mode converter and collector.
Figure 2
Figure 2
Radiation intensity pattern from the 460 GHz gyrotron; (a) burn image on thermal paper with absorbing material on the back side and (b) image recorded by a pyroelectric camera
Figure 3
Figure 3
Summary of stable CW operation of the 460 GHz gyrotron over 24 hours: (a) Diode voltage proportional to the output power. (b) Pressure on the ion pumps located at the collector side. (c) Water-cooled collector temperature. (d) Cathode heater current. (e) Collector current. (f) Body current.
Figure 4
Figure 4
Statistical analysis of power fluctuations from the 460 GHz gyrotron over one day long operation.
Figure 5
Figure 5
Frequency shift over the duration of the pulse in the 140 GHz gyrotron operation. Solid dots represent measured data and the solid line is a curve fitted to an exponential function. For this measurement, the gyrotron operates at 12.9 kV of voltage and 30 mA of beam current. The spectrum analyzer is set to 14 ms of sweep time and 30 kHz of resolution bandwidth.
Figure 6
Figure 6
Spectral linewidth of the 140 GHz gyrotron in the IF domain, in which the LO frequency was locked at 19.99400 GHz and its 7th harmonic was mixed with the gyrotron frequency. For this measurement, the spectrum analyzer was set to 10 kHz of resolution bandwidth and 100 ms of sweep time. Sweeping was delayed by 10 s to wait until the frequency was stabilized after the onset of radiation.
See this image and copyright information in PMC

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