Time-dependent multimode simulation of gyrotron oscillators

Fliflet, A. W.; Lee, R. C.; Gold, Steven H.; Manheimer, W. M.; Ott, Edward

doi:10.1103/physreva.43.6166

Cited by 66 publications

(33 citation statements)

References 15 publications

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“…The interesting observation here is the frequency dynamics during the rise and fall of the beam voltage which chirps by about 300 MHz-well outside the notch filter region. The frequency evolution can only be fitted using only three exponential functions each corresponding to a different time constant, namely, thermal Ͼ charge Ͼ V where the time constant thermal is related to the thermal expansion of the cavity, charge is the time constant related to the space charge buildup, 17 and V is the time constant related to the beam voltage rise time in the gyrotron. These measurements enable us to determine the timing for switching in the VCVA attenuation to protect the receiver from radiation with frequencies outside the rejection band of notch during the ramping phases.…”

Section: A Gyrotron Frequency Dynamicsmentioning

confidence: 99%

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

Méo

Bindslev

Korsholm

et al. 2008

Review of Scientific Instruments

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The collective Thomson scattering ͑CTS͒ diagnostic installed on ASDEX Upgrade uses millimeter waves generated by the newly installed 1 MW dual frequency gyrotron as probing radiation at 105 GHz. It measures backscattered radiation with a heterodyne receiver having 50 channels ͑between 100 and 110 GHz͒ to resolve the one-dimensional velocity distribution of the confined fast ions. The steerable antennas will allow different scattering geometries to fully explore the anisotropic fast ion distributions at different spatial locations. This paper covers the capabilities and operational limits of the diagnostic. It then describes the commissioning activities carried out to date. These activities include gyrotron studies, transmission line alignment, and beam pattern measurements in the vacuum vessel. Overlap experiments in near perpendicular and near parallel have confirmed the successful alignment of the system. First results in near perpendicular of scattered spectra in a neutral beam injection ͑NBI͒ and ion cyclotron resonance heating ͑ICRH͒ plasma ͑minority hydrogen͒ on ASDEX Upgrade have shown evidence of ICRH heating phase of hydrogen.

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Section: A Gyrotron Frequency Dynamicsmentioning

confidence: 99%

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

Méo

Bindslev

Korsholm

et al. 2008

Review of Scientific Instruments

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“…Using the prescription developed in previously in Fliflet et al (1991), the mode amplitude and phase can be rewritten as:…”

Section: Vzmentioning

confidence: 99%

“…This becomes progressively more difficult to do as the fractional mode spacing shrinks, since 1) the intended mode may never start oscillation, or 2) it may be unstable at its optimum detuning in the presence of competition from nearby modes, or 3) the final state may be a multimode state. In this paper, we use a time-dependent multimode simulation code (Fliflet et al 1991) to examine the optimization of the gyrotron parameters for the 280 GHz TE 4 2, 7 -mode gyrotron experiment being set up by Kreischer and coworkers at the Massachusetts Institute of Technology (MIT), in the presence of competition from nearby TEm,7 and other TEm,t modes.…”

Section: Introductionmentioning

confidence: 99%

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Multimode Simulation of High Frequency Gyrotrons.

Gold

Fliflet

1991

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“…Figure 2a is the simulation result of time behaviour of two modes 26 , which are observed in the experiment, at B c = 6.67 T. Here, as initial powers, 1 kW and 1 mW are set for TE 31 Fig. 3.…”

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confidence: 99%

Achievement of robust high-efficiency 1 MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron

et al. 2007

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Gyrotrons are a high-power source of coherent microwave radiation 1 . Their oscillation mechanism is a cyclotron-resonance maser effect, in which a fraction of the rotational kinetic energy of a mildly relativistic magnetized electron beam is converted into electromagnetic energy. The most active area of gyrotron development is their potential use for heating magnetically confined fusion plasmas to the point of thermonuclear ignition. A major obstacle to this endeavour is that during high-power millimetre-wave operation 2-9 competing modes and mode shifts seriously degrade a gyrotron's stability and efficiency 10-13 . Here, we show that these problems can be overcome by active control of the electron-beam parameters during the oscillation. In doing so, we successfully demonstrate the robust steady-state operation of a 170 GHz gyrotron producing a continuous 1 MW output power with an unprecedented efficiency of over 55% in a hard-selfexcitation region. Moreover, we find that an adjacent resonant mode previously expected to compete with and adversely affect the principal operating mode does not in fact jeopardize but rather helps this mode as a result of nonlinear effects. The result improves the outlook for using these devices for heating and instability control in future experimental fusion reactors, such as ITER [14][15][16][17][18][19] .A basic configuration of the high-power gyrotron oscillator used in the experiment 7 is shown in Supplementary Information, Figs S1,S2. The nominal operation mode is TE 31,8 , in a cylindrical open resonator, whose radius is 17.9 mm. An annular electron beam of 9.13 mm in radius is injected into the resonator along the axial magnetic field to excite TE 31,8 . The oscillation millimetre-wave power P osci is converted to a gaussian-like beam using a quasi-optical launcher 20,21 attached to the resonator, and transmitted through an edge-cooled diamond window as P out . Here, P out ∼ 0.92P osci due to the ohmic loss and the diffraction loss P loss . The collector of the gyrotron is earthed. By applying a positive voltage V d.c. to the resonator section against the collector, the energy recovery of the spent electron beam is available to enhance the overall efficiency 22 .After a demonstration of 1 h oscillation at P out = 0.6 MW with fixed parameters, the operation parameters are actively controlled with a slow timescale to investigate the oscillation characteristics in the continuous-wave state. Figure 1a shows the dependence of the output power on the magnetic field in the resonator, B c . Here, V b ∼ −72.5 kV and V d.c. ∼ 25.5 kV. After the electron-beam I b had stabilized at ∼30 A completely, which takes ∼1 min, the B c scan started from 6.72 T. The frequency is ∼170 GHz. The power increases as the B c decreases, that is, the cyclotron resonance mismatch factor Δ = (1 − (f ce /γf )) increases. Here, f and f ce are oscillation and non-relativistic cyclotron frequencies, respectively, and γ is a relativistic factor of the initial electrons. The maximum power of 0.8 MW is obtained...

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Time-dependent multimode simulation of gyrotron oscillators

Cited by 66 publications

References 15 publications

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

Multimode Simulation of High Frequency Gyrotrons.

Achievement of robust high-efficiency 1 MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron

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