Multimode Oscillation and Mode Competition in High-Frequency Gyrotrons

Kreischer, K.E.; Temkin, R.J.; Fetterman, H. R.; Mulligan, W. J.

doi:10.1109/tmtt.1984.1132711

Cited by 79 publications

(25 citation statements)

References 18 publications

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“…In this case, the rf signals of two modes simultaneously present in the gyrotron were fed into the mixer, which generated a signal corresponding to the difference frequency of the two modes. Details of this work can be found in a recent paper (Kreischer et al 1984a). …”

Section: Discussionmentioning

confidence: 99%

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Frequency pulling and bandwidth measurements of a 140 GHz pulsed gyrotron

Kreischer

Danly

Woskoboinikow

et al. 1984

International Journal of Electronics

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Accurate measurements of the emission frequency and bandwidth of a pulsed 140GHz gyrotron have been made using a harmonic mixer system. This system has been used to measure the bandwidth of individual, 1 Asec pulses of the gyrotron, to determine the dependence of the operating frequency on the cathode voltage and resonator magnetic field, to detect and identify second harmonic radiation, and to study multimode operation.Bandwidths as low as 3 MHz, which is the instrumental limit, have been observed. In addition, frequency pulling has been measured and compared with predictions based on linear and self-consistent nonlinear theory. It was found that linear theory is inadequate for describing the frequency characteristics of a gyrotron operating well above the starting current, while self-consistent nonlinear theory was in reasonable agreement with the experimental results. The small bandwidths and stable operating frequencies that were measured confirm the viability of the gyrotron as a millimeter and submillimeter source for plasma diagnostics.2

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Section: Discussionmentioning

confidence: 99%

“…The harmonic mixer system has also been used in conjunction with mode competition and multimode oscillation studies (Kreischer et al 1984a). In this case.…”

Section: Ghz) Modesmentioning

confidence: 99%

Frequency pulling and bandwidth measurements of a 140 GHz pulsed gyrotron

Kreischer

Danly

Woskoboinikow

et al. 1984

International Journal of Electronics

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“…Simultaneous oscillation of multi modes was observed in a 140 GHz gyrotron [5]. The main operation mode of this gyrotron was relatively low-mode TE 03 at the fundamental resonance.…”

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confidence: 89%

Observation of Simultaneous Oscillation of Multiple Modes in a CW 300 GHz Gyrotron

Saito

Nakano

Mitsudo

et al. 2007

Plasma and Fusion Research

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Multi-mode oscillation was observed in a 300 GHz fully CW gyrotron. It has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a submillimeter-wave material processing system. This gyrotron delivers 1.75 kW/CW at maximum. The radiation pattern is a Gaussian beam when the magnetic field strength B c at the cavity is properly adjusted. However, within a range of B c values, simultaneous oscillation of competing modes is observed, manifesting in radiation of the output power in multiple directions. Medium-power millimeter/submillimeter waves have numerous applications in various fields. Among those applications, ceramic sintering and material processing are most promising. Recently, since a strong non-thermal effect is expected for higher frequency power, use of 20 -30 GHz gyrotrons has become popular [1]. A CW gyrotron with a frequency of hundreds of GHz has not yet been realized as a power source for practical applications, even though a stronger non-thermal effect can be expected. Thus, realization of a gyrotron with this frequency range and kW order power is strongly desired.Mode competition is a serious problem in the design and operation of a medium to high power, high frequency gyrotron because the mode density inevitably becomes very high owing to the extremely oversized cavity. Usually, the designated mode with the smallest starting current grows by suppressing competing modes until finally, single-mode oscillation is realized.However, there is a chance of multi-mode oscillation depending on the operation condition. We have observed this phenomenon on a fully CW 300 GHz gyrotron called FU CW I. This is very interesting from the viewpoint of the gyrotron physics. Moreover, understanding of multi-mode oscillation is a basis for the development of medium-power CW gyrotrons with further higher frequencies.This gyrotron with kW order output power has been developed according to the above requirements and installed in the Research Center for Development of FarInfrared Region, University of Fukui. It is the first gyrotron having these parameters for practical applications [2]. FU CW I works at the fundamental electron cyclotron resonance in a 12 T liquid helium-free superconducting magnet. It is operated at a relatively low cathode voltage of author's e-mail: saitot@fir.fukui-u.ac.jp 15 kV. The maximum beam current is 1 A.The designated cavity mode of FU CW I is TE 22,8 . The radius of the flat region of the cavity R c is 8.39 mm, and the resonance frequency of the TE 22,8 mode is almost 300 GHz. The length of the flat region of the cavity is 15 mm and the total Q value including the Ohmic loss is about 6000 [3]. The gyrotron is equipped with a mode converter composed of a Vlasov-type radiator and three beamshaping mirrors. The output power is delivered through the vacuum window in a Gaussian beam. The vacuum window is made of BN and its diameter is 80 mm. This gyrotron can be operated in either CW mode or pulse mode.A performance t...

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“…4), for many magnetic field values, the simultaneous oscillation of two or more modes occurred. The existence of multimode oscillation at the fundamental of the cyclotron frequency is an important problem and has been discussed by several authors (Moiseev and Nusinovich 1974, Nusinovich 1981, Kreischer et. al.…”

Section: (Kxre)mentioning

confidence: 99%

Harmonic emission from high-power high-frequency gyrotrons

Byerly

Danly

Kreischer

et al. 1984

International Journal of Electronics

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The results of a study of second harmonic emission from a gyrotron designed for high power, high frequency operation at the fundamental of the cyclotron frequency are presented. Stable, very narrow bandwidth second harmonic cavity emission from 209 GHz to 302 GHz has been observed.An output power of 25 kW and efficiency of 6.5% in the TE 1 1 ,2, 1 mode at 241 GHz is reported; this represents the highest power obtained to date from a high frequency (>100 GHz) harmonic gyrotron. These experiments have been carried out in a cavity for which the mode density is very high; the cavity diameter is approximately six free-space wavelengths for emission at the second harmonic. Mode competition between fundamental and second harmonic modes is discussed. It is also shown that, in general, gyrotrons designed for high power, low 0 operation in overmoded cavities at the fundamental will also have high efficiencies and strong emission in second harmonic modes. Prospects for high frequency harmonic gyrotrons for plasma diagnostics and other applications are described.

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Multimode Oscillation and Mode Competition in High-Frequency Gyrotrons

Cited by 79 publications

References 18 publications

Frequency pulling and bandwidth measurements of a 140 GHz pulsed gyrotron

Frequency pulling and bandwidth measurements of a 140 GHz pulsed gyrotron

Observation of Simultaneous Oscillation of Multiple Modes in a CW 300 GHz Gyrotron

Harmonic emission from high-power high-frequency gyrotrons

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