D.B. McDermott

High-power harmonic gyro-TWT's. I. Linear theory and oscillation study

A linear theory using Laplace transforms which is applicable to both gyrotron traveling wave amplifiers (gyro-TWTs) and gyrotron backward-wave oscillators (gyro-BWOs) is presented. The validity of the linear theory is verified by comparing it with an existing nonlinear self-consistent theory based on a different approach. In conjunction with a time-dependent multimode particle simulation code, the linear theory is applied to study the stability of harmonic gyro-TWTs. It is shown that a harmonic gyro-TWT can be made stable to all forms of spontaneous oscillations by employing a multistage interaction structure and that it can generate power levels far in excess of those possible for a fundamental gyro-TWT. The linear bandwidth of a second-harmonic gyro-TWT amplifier is also calculated. >

Marginal stability design criterion for gyro-TWTs and comparison of fundamental with second harmonic operation

Abstract Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.

Operation of a Millimeter-Wave Harmonic Gyrotron,

The operation of a millimeter-wave harmonic gyrotron is described in which the interaction is between large-orbit axis-encircling electrons and cylindrical cavity TEnll modes. Efficiencies up to 15% have been measured for moderate harmonic interactions and multi-kW power levels have been attained at the tenth harmonic of the cyclotron frequency. The concept allows the magnetic field of the gyrotron to be reduced by an order of magnitude, thereby making a submillimeter-wave gyrotron feasible.

High-power harmonic gyro-TWT's. II. Nonlinear theory and design

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with alpha =1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability. >

Dielectric-loaded wideband gyro-TWT

The bandwidth of a gyro-TWT (traveling-wave tube) can be widened by employing a dielectric-loaded waveguide to reduce the circuits dispersion. Fast wave interaction allows the requirements on the beams quality to be relaxed compared with slow wave interaction. A low- alpha ( identical to nu /sub perpendicular to // nu /sub z/) electron beam is chosen to avoid the absolute instability and minimize the possibility of dielectric charging. This device is investigated using a self-consistent single-mode, large-signal simulation based on a slow time scale formulation. Simulation results show that a constant drive bandwidth of 20% can be achieved for a 100 kV, 5 A electron beam with a velocity ratio of alpha =0.59 and an axial velocity spread of 2.0%. The growth rate is relatively low because of the low alpha of the electron beam. The design of a proof-of-principle experiment is described. The tube is expected to deliver a power of 80 kW from 9 to 11 GHz with 15% efficiency and a saturated gain of 30 dB. The performance of a single-anode magnetron injection gun designed to produce the required high-quality electron beam has been studied through simulation. >

Production of relativistic, rotating electron beams by gyroresonant rf acceleration in a TE111 cavity

The rf acceleration of moderate current (≲1 A) electron beams to 500 keV in a TE111 cylindrical cavity resonator is described. Experimental results are compared with theory. rf to beam energy conversion efficiencies in excess of 50% have been observed. The resultant axis‐encircling beam with a large ratio of perpendicular velocity to longitudinal velocity is ideal as a driver for a high‐harmonic gyrotron.

Design of a W-band second-harmonic TE/sub 02/ gyro-TWT amplifier

A harmonic gyrotron traveling-wave tube (gyro-TWT) amplifier is described that can stably deliver high peak and average power in the low-loss TE/sub 02/-mode at 91.4 GHz. The single-stage second-harmonic TE/sub 02/ gyro-TWT is predicted to produce a peak power of 600 kW with an efficiency of 24%, a saturated gain of 30 dB, and a 3-dB bandwidth of 2.7%. The amplifier employs a 100-kV, 25-A electron beam emitted from a magnetron injection gun with v/sub /spl perp///v/sub /spl par//=1.2 and /spl Delta/v/sub /spl par///v/sub /spl par//=8%. The device is based on the proven concept that the electron beam current can be much higher in a stable harmonic gyro-TWT amplifier than at the fundamental due to the relatively weaker strength of the harmonic interaction. The TE/sub 02/ overmoded interaction waveguide is sufficiently large to handle an average power of 60 kW and provides considerable clearance for the high current electron beam. An innovative mode-selective interaction circuit prevents the amplifier from oscillating in undesired modes.

Large-signal characteristics of a wide-band dielectric-loaded gyro-TWT amplifier

The bandwidth of a gyrotron traveling wave amplifier (gyro-TWT) has been significantly increased by partially filling the interaction waveguide with dielectric to reduce the circuits dispersion. The proof-of-principle experiment was designed for X-band, and employs the fundamental mode of rectangular waveguide loaded with dielectric slabs along the narrow sidewalls. The amplifier yields a peak output power of 55 kW with 11% efficiency, 27 dB saturated gain, and an unprecedented untapered gyro-TWT constant-drive bandwidth of 11% and saturated bandwidth exceeding 14%. The single-stage amplifier is completely zero-drive stable. The 95-kV 5-A electron beam was produced by a single-anode magnetron injection gun with p/sub /spl perp////spl upsi//sub z/=0.6, as determined by the EGUN code, and /spl Delta//spl upsi//sub z///spl upsi//sub z/=4%, determined as the best fit to the gyro-TWT large-signal simulation data. Simulation studies predict that by lowering the velocity spread to /spl Delta//spl upsi//sub z///spl upsi//sub z/=2%, the amplifier performance will be further enhanced to a constant-drive bandwidth of 20% with 15% efficiency.

Nonlinear analysis of high-harmonic slotted gyro-TWT amplifier

A nonlinear self-consistent simulation code is employed to investigate the behavior of the slotted gyrotron traveling-wave amplifier (gyro-TWT), in which an axis-encircling electron beam synchronously interacts with a high-order azimuthal mode in a magnetron-type waveguide. The efficiency of a fourth-harmonic device with an ideal 60 kV, 5 A beam is shown to reach 30% for alpha identical to nu /sub perpendicular to // nu /sub z/=2. The growth rate for the pi mode is roughly 25% larger than for the 2 pi mode. The efficiency increases for lower voltage and the device is found to be moderately sensitive to the radial spread of the beams guiding center position and extremely sensitive to the axial velocity spread. For an ideal 60 kV, 5 A beam with alpha =1.5, the efficiency of a second-harmonic gyro-TWT is 42% and falls to 10% for an eighth-harmonic device. The design of a 35 GHz, 60 kV, 5A, alpha =1.5, eight-vane, fourth-harmonic gyro-TWT with 7% axial velocity spread is presented. It is predicted that this design will yield a peak output power of 90 kW, a peak efficiency of 30%, and 6.3% saturated bandwidth. >

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%.