High-peak power Ka-band gyrotron oscillatorexperiments with slotted and unslotted cavities

A K_a-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage (~900 keV) and high current (~500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TE_m2 type, allowing stable operation in a linearly polarized TE₁₃ mode. A peak power of 35 MW has been observed at 6% efficiency     

Axial mode locking in a harmonic-multiplying, inverted gyrotwystron

When in microwave sources, an open waveguide operating at frequencies close to cutoff is used as a resonator, a number of modes with the same transverse structure, but different axial distribution can be excited by an electron beam. The width of the resonance curves of these modes broadens as the number of axial variations grows. This leads to overlapping of these curves at large axial indexes. As a result, phase locking of such modes may occur. A theory describing axial mode locking in gyrodevices is developed. The results of theoretical analysis are compared to a harmonic-multiplying, inverted gyrotwystron experiment. It is shown that such phase-locked operation in a set of modes with overlapping resonance curves can significantly enlarge the bandwidth of gyrodevices. Furthermore, this technique may be broadly applicable to other devices, which employ cavities with overlapping modes     

Phase locking and bandwidth in a gyrotron oscillator

For imaging radar and for satelitte and space communication (e.g. NASA's deep space network), it is important that the bandwidth be as large as possible. Here we derive a formalism for computing the phase locking bandwidth that can be achieved in a gyrotron oscillator while varying the beam voltage. As an example, a second harmonic TE_02/03 gyrotron is considered. For this device, the effective bandwidth can be increased by a factor of about 3 compared with the fixed voltage case by allowing the beam voltage to change together with the input locking signal.     

The gain and efficiency improvement of a free-electron laser by anoptical klystron configuration

The feasibility of a high-grain amplifier in an optical klystron configuration at wavelengths of 1-2 mm was investigated using a computer simulation. A one-dimensional single-particle model for the free-electron laser (FEL) interaction was used in the numerical studies. It was found that for a three-cavity optical klystron gain of at least 30 dB and relatively high efficiency is possible for relatively modest electron-beam parameters (V⩽500 kV, J≈100 A/cm²)     

Mode competition, suppression, and efficiency enhancement in overmoded gyrotron oscillators

Gyrotron oscillators are of great interest as sources of high power mm wave radiation for electron cyclotron resonance heating and current drive in magnetic fusion research devices. Gyrotrons capable of efficiently generating cw power 1 megawatt will be required in future magnetic fusion studies. However, as gyrotron power approaches the megawatt level, a very large, overmoded cavity must be employed in order to keep ohmic power losses in the cavity at an acceptable level, and the problem of mode overpopulation becomes severe. Also, it becomes increasingly important to optimize gyrotron efficiency for a number of important reasons including minimizing the problem of collecting the electron beam energy. In the present paper, a detailed experimental and theoretical study of mode competition and mode locking in an overmoded gyrotron is presented. Efficiency enhancement (to 60%) and high peak power (430 kW) were achieved in the TE_2,4,1 mode using magnetostatic profiling in the cavity. With selective mode suppression, peak power of 475 kW was generated in the TE_0,4,1 mode.This work was performed at the Naval Research Laboratory. Some of the authors have affiliations with other institutions, as indicated:     

Gyrotron travelling wave amplifier: II. Effects of velocity spread and wall resistivity

The small signal gain and bandwidth of the gyrotron travelling-wave amplifier (gyro-TWA) operating at the fundamental cyclotron harmonic are examined. The analytical and numerical studies focus on the effects of velocity spread in the electron beam and of a distributed wall resistivity in the waveguide. It is found that wall resistivity reduces the forward gain of the amplifier only by an amount approximately equal to 1/3 of the corresponding cold tube loss. Significant increase in bandwidth may result, under certain conditions, from the presence of wall resistivity. A moderate amount (5–10%) of velocity spread does not reduce the peak gain significantly in general, but may reduce the bandwidth by an amount depending on the applied magnetic field. Based on considerations of such factors as power, stability, gain, bandwidth, and efficiency, operating parameters for the gyro-TWA are suggested.     

Experimental study of a Ka-band gyrotron backward-waveoscillator

A gyrotron backward-wave oscillator (gyro-BWO) has been operated and magnetically tuned over the frequency range 27-32 GHz. Tuning by varying the electron beam voltage was effective over a smaller frequency range (Δf~1 GHz). Output power was as large as 7 kW, corresponding to a device efficiency of 19%. This large efficiency value was unexpected, and related analysis indicates it may be associated with the nonuniform magnetic field profile in the interaction region