C.K. Chong

Large-signal operation of a third-harmonic slotted gyro-TWT amplifier

The large-signal characteristics are reported for a slotted third-harmonic gyrotron traveling-wave tube (gyro-TWT) amplifier. The slotted interaction circuit yields strong harmonic interaction for relatively low-energy axis-encircling electrons. The circuit was sliced to suppress competing modes by interrupting their wall currents. The experiment was conducted in the X-band as a scaled test of a 95-GHz amplifier design developed in collaboration with CPI/Varian. A gyroresonant RF accelerator was employed to produce the required axis-encircling electron beam. The two-stage slotted third-harmonic gyro-TWT with a beam current equal to 60% of the design value was stable and yielded an output power of 6 kW with 5% efficiency and 11-dB saturated gain over a bandwidth of 3%.

Experimental investigation of a broadband dielectric-loaded gyro-TWT amplifier

The bandwidth of a gyrotron travelling wave amplifier has been broadened by incorporating a dielectric-loaded interaction waveguide to reduce the circuits dispersion. This proof-of-principle experiment was designed for the X-band frequency range and operates in the fundamental mode of a rectangular waveguide loaded with dielectric Macor. The amplifier is zero-drive stable and demonstrates a peak output power of 55 kW, 11% efficiency, 27 dB saturated gain with an unprecedented untapered gyro-TWT constant-drive bandwidth of 11% and a saturated bandwidth of 14%. Its performance can be further enhanced by reducing the beams axial velocity spread as shown by previous simulation studies.

Stability of a 95-GHz slotted third-harmonic gyro-TWT amplifier

A low-magnetic-field moderate-voltage gyrotron amplifier has been designed for stable high-performance operation at 95 GHz. A slotted interaction circuit is utilized to achieve strong amplification near the third cyclotron harmonic frequency. The start-oscillation conditions were determined by an analytical theory and confirmed by a multimode particle-in cell simulation code. The dominant threat to the amplifiers stability is from a third-harmonic peniotron backward-wave interaction. A slow-timescale particle-tracing simulation code predicts the three-section slotted third-harmonic gyro-TWT, which utilizes an 11.6-kG magnet and a 50-kV 3-A /spl upsi//sub /spl perp////spl upsi//sub z/=1.4 axis-encircling electron beam with an axial velocity spread of 6% will yield an output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB, and a constant-drive bandwidth of 2%.

Broadband linearly polarized beat-wave TE/sub m1//TE/sub 11/ mode converters

Broadband linearly polarized waveguide mode converters have been developed to transform the high-order cylindrical TE/sub m1/ output wave from harmonic gyrotron amplifiers into the more useful TE/sub 11/ fundamental waveguide mode. The converters corrugation period is equal to the beat between the two waves and the bandwidth is predicted to be inversely proportional to the number of periods. Four-period TE/sub 31//TE/sub 11/ and TE/sub 41//TE/sub 11/ converters with an azimuthal perturbation of m/sub c/=4 and m/sub c/=5, respectively, have yielded a peak conversion efficiency of 98% with a bandwidth greater than 3% and a one-period beat-wave converter has been designed to yield 12% bandwidth. However, it has been observed in measurements that the strong coupling in a short converter can lead to a shift of the center-frequency with an accompanying reduction of the efficiency and bandwidth. A two-period TE/sub 41//TE/sub 11/ converter with 5% bandwidth displayed a 5% frequency shift and a conversion efficiency of only 86%.

Stable 1 MW, third-harmonic gyro-TWT amplifier

The concept that the relatively weak harmonic gyro-TWT interactions allow high values of electron beam current for stable operation has been extended to design two extremely high power, 140 GHz, third-harmonic TE/sub 31/ gyro-TWT amplifiers. One device is driven by an axis-encircling electron beam from a cusp gun and the other employs a magnetron injection gun (MIG). These devices are predicted by a self-consistent nonlinear numerical simulation code to yield, respectively, output powers of 775 kW and 937 kW with 15.5% and 18.7% efficiency, saturated gains of 27 dB and 30 dB, and saturated bandwidths of % and 6.5%. The stability of the amplifiers is ensured by limiting the length of the interaction section(s) to the shortest starting oscillation length as determined by linear theory. The cylindrical waveguide circuits of both amplifiers have been sliced to suppress modes without a threefold azimuthal symmetry. The amplifier utilizing a MIG yields superior performance because the dominant competing interaction is minimized for the choice of the beams guiding center radius. The advantages as well as limitations of this approach for high power microwave generation are also addressed. >

Slotted third-harmonic gyro-TWT amplifier experiment

The first operation of a slotted third-harmonic gyrotron traveling-wave amplifier is reported. The low-magnetic-field moderate-voltage gyrotron amplifiers 62-keV 2.5-A /spl upsi//sub /spl perp////spl upsi//sub /spl par//=1.2 axis-encircling electron beam was supplied by a gyroresonant RF accelerator. The 10-GHz 1.3-kG single-section slotted third-harmonic amplifier is stable and yielded 12.5 dB of small signal gain with a bandwidth of 2.5%. The experiment was performed as a scaled proof-of-principle test of the 95-GHz multisection slotted amplifier under development at CPI (formerly Varian).

High-harmonic slotted gyroklystron amplifier: linear theory and nonlinear simulation

A fluid theory and two self-consistent particle-tracing simulation codes are described for designing low voltage, s/sup th/-harmonic slotted gyroklystron amplifiers, in which axis-encircling electron beams are in resonance with the s/sup th/-order azimuthal modes of a series of magnetron-type cavities, allowing the gyrotron amplifiers required magnetic field to be reduced by a factor of s. The linear fluid theory yields a convenient closed-form expression for gain and the nonlinear simulation code determines the large-signal device performance, while the much faster linear simulation code allows thorough, multi-dimensional parameter searches to be performed quickly. The simulation codes self-consistently account for shifts of the cavitys resonant frequency and quality factor due to beam loading. The three theoretical approaches, which agree in the small-signal regime for weak beam loading, were used to design a 95 GHz, three-cavity, slotted twelve-vane, sixth-harmonic gyroklystron amplifier utilizing a 70 kV, 10 A, v/sub /spl perp///v/sub z/=2, axis-encircling beam and a 6.1 kG magnet. The nonlinear self-consistent simulation code predicts that the sixth-harmonic gyrotron amplifier with an ideal beam will yield an electronic efficiency of 20% and a saturated gain of 37 dB, while the more realistic device with a 10% axial velocity spread will generate a peak output power of 84 kW with 12% efficiency, a saturated gain of 27 dB and a 0.2% constant-drive bandwidth. >

Gyro-TWT amplifier development at UCD: (1) 200 kW second-harmonic, (2) wideband and (3) slotted third-harmonic gyro-TWTs

Summary form only given. Our second-harmonic TE/sub 21/ gyro-TWT with an 80 kV, 20 A, v/sub /spl perp//v/sub /spl par//=1.0 MIG electron beam has yielded 210 kW at 16 GHz with 13% efficiency, 16 dB saturated gain, 2% saturated bandwidth and zero-drive stability. It remained stable because the dominant gyro-BWO modes, the second-harmonic TE/sub 11/ and third-harmonic TE/sub 31/, had been suppressed by slicing the cylindrical waveguide with two cuts separated in azimuth by 90/spl deg/. Harmonic gyro-TWTs not only require a substantially weaker magnetic field and offer nearly the same high efficiency as fundamental gyro-TWTs, but also have the potential to stably generate significantly higher levels of power because the threshold beam current level for oscillation is raised dramatically due to the relatively weaker harmonic interaction. The design for a 1 MW third-harmonic TE/sub 31/ gyro-TWT utilizing a MIG will also be presented.

Slotted third-harmonic peniotron forward-wave oscillator

An experiment has been built to test the novel, efficient peniotron interaction at the third harmonic in a slotted eight-vane waveguide by using the axis-encircling electron beams produced by a gyroresonant rf accelerator. By configuring the oscillator so that the peniotron is excited as a forward wave at the cutoff frequency of a travelling-wave circuit, the peniotron can be made to dominate over the usually stronger gyrotron interaction. The 10 GHz peniotron will be driven by a 70 kV, 3.5 A axis-encircling electron beam with (alpha) equals v(perpendicular)/VZ equals 1.3 and is predicted by a nonlinear, self-consistent, multi-mode PIC code to yield 110 kW with 45% efficiency. The travelling-wave circuit is terminated on the upstream end by a load and by a transition to a TE41/TE11 mode converter on the collector end. PIC code simulation results and a description of the assembled experiment are presented.

Slotted third-harmonic gyro-TWT

Summary form only given. A low magnetic field, low voltage gyro-TWT has been designed in collaboration with CPI/Varian for operation at 95 GHz and tested in a scaled experiment at 10 GHz. A slotted interaction circuit is utilized to achieve strong interaction at the third harmonic. Analytical theory was used to check for oscillation from gyro-BWO and the absolute instability. A self-consistent slow-timescale simulation code predicts the three-section, slotted third-harmonic gyro-TWT, which utilizes a 11.6 kG magnet and a 50 kV, 3 A, v/sub /spl perp//v/sub z/=1.4, axis-encircling electron beam with an axial velocity spread of 6%, which yields an output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB and a constant-drive bandwidth of 2%.