Plasma Scattering Measurement using a Submillimeter Wave Gyrotron as a Radiation Source

Abstract

Plasma scattering measurement is effective technique to observe low frequency density fluctuations excited in plasma. The spatial and wave number resolutions and the S/N ratio of measurement depend on the wavelength range, the size and the intensity of a probe beam. A well-collimated, submillimeter wave beam is suitable for improving the spatial and wave number resolutions. Application of high frequency gyrotron is effective in improving the S/N ratio of the measurement because of its capacity to deliver high power. Unlike the molecular vapor lasers, the gyrotrons generate diverging beam of radiation with TEmn mode structure. It is therefore necessary to convert the output radiation into a Gaussian beam. A quasi-optical antenna is a suitable element for the conversion system under consideration since it is applicable to several TE 0n and TE 1n modes. In order to apply the gyrotron to plasma scattering measurement, we have stabilized the output (P=110W, f=354GHz) of gyrotron up to the level (∆P/P<1 %, ∆f<10 kHz). The gyrotron output can be stabilized by decreasing the fluctuation of the cathode potential.

Spectral Characteristics of a 140-GHz Long-Pulsed Gyrotron

“…Linewidth and lineshape measurements of the gyrotron are very important in applications that are utilizing a frequency shift by scattering, for example, radar [26] and CTS [5]- [7]. DNP/NMR also require a narrow linewidth to match the exact frequency in the EPR spectrum for maximum signal enhancement in NMR [3].…”

“…In a number of recent successful applications using gyrotrons, such as dynamic nuclear polarization enhanced nuclear magnetic resonance [dynamic nuclear polarization (DNP)/nuclear magnetic resonance (NMR)] to investigate the structures of bio-molecular systems [3], [4] and collective Thomson scattering (CTS) to diagnose the plasmas [5]- [7], an improved knowledge of the source spectral characteristics and cleaner more stable frequency operation are of critical importance. To satisfy these challenging requirements, Massachusetts Institute of Technology (MIT) has been developing gyrotrons [8]- [12] and measurement systems [13], [14].…”