B.G. Danly

High efficiency operation of a 140 GHz pulsed gyrotron

Abstract The operating characteristics of a 140 GHz pulsed gyrotron are reported. Total efficiencies of 36% and output powers of 175 kW have been obtained in single-mode operation. Measurements of power and efficiency have been made for a variety of modes between 120 and 160 GHz, and these results are in good agreement with predictions based on non-linear theory. The best results have been obtained with isolated asymmetric modes, such as the TE4, 2, 1 (127·3 GHz), TE2, 3, 1,(136·7 GHz) and TE3, 3, 1(155·6 GHz). Although mode competition was found to prevent the TE0, 3, 1, mode (139·5 GHz) from reaching the optimum operating conditions, an output power of 138 kW and total efficiency of 29% were achieved with this mode. A variety of new and highly accurate diagnostic techniques that have been developed to measure the power, frequency and mode content of the output radiation are reviewed. In addition, the operating characteristics of both laminar and non-laminar magnetron injection guns are compared. The hig...

35 GHz cyclotron autoresonance maser amplifier

Studies of a cyclotron autoresonance maser are presented. The measurements are carried out at a frequency of 35 GHz using a mildly relativistic electron beam (1.5 MeV, 260 A) generated by a field emission electron gun followed by an emittance selector that removes the outer, hot electrons. Perpendicular energy is imparted to the electrons by means of a bifilar helical wiggler. Measurements give a small signal gain of 90 dB/m and a saturated power output of 10 MW. The corresponding electronic efficiency is 3%. Computer simulations are also presented.

Optimization of gyroklystron efficiency

In this article, the optimization of gyroklystron efficiency is investigated by employing a two–step procedure. As a first step, the prebuncher is analyzed using a small signal approximation, since the cavity(ies) here serve mainly to modulate the velocities of the electrons slightly, which will be bunched in the field‐free drift section(s). It is found that the electrons entering the energy extraction cavity can be characterized entirely by only two dimensionless parameters: a bunching parameter q and a relative phase ψ. The numerical simulation of the extraction cavity, based on the nonlinear pendulum equations describing the interaction between the electrons and the rf field, supplemented by the initial conditions specified by q and ψ, constitutes the second step. The final result of this two‐step analysis is the efficiency, η⊥,opt optimized with respect to q, ψ, and the magnetic detuning parameter Δ. This efficiency depends only on the normalized cavity length μ and the normalized rf field F of the en...

Frequency pulling and bandwidth measurements of a 140 GHz pulsed gyrotron

Abstract Accurate measurements of the emission frequency and bandwidth of a pulsed 140 GHz gyrotron have been made using a harmonic mixer system. This system has been used to measure the bandwidth of individual 1 μs 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 non-linear 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 non-linear 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 millimetre ...

Multimegawatt relativistic harmonic gyrotron traveling-wave tube amplifier experiments

The first multimegawatt (4 MW, /spl eta/=8%) harmonic (/spl omega/=s/spl Omega//sub c/, s=2,3) relativistic gyrotron traveling-wave tube (gyro-twt) amplifier experiment has been designed, built, and tested. Results from this experimental setup, including the first ever reported third-harmonic gyro-twt results, are presented. Operation frequency is 17.1 GHz. Detailed phase measurements are also presented. The electron beam source is SNOMAD-II, a solid-state nonlinear magnetic accelerator driver with nominal parameters of 400 kV and 350 A. The flat-top pulsewidth is 30 ns. The electron beam is focused using a Pierce geometry and then imparted with transverse momentum using a bifilar helical wiggler magnet. The imparted beam pitch is a /spl alpha//spl equiv//spl beta//sub /spl perp////spl beta//sub /spl par///spl ap/1. Experimental operation involving both a second-harmonic interaction with the TE/sub 21/ mode and a third-harmonic interaction with the TE/sub 31/ mode, both at 17 GHz, has been characterized. The third-harmonic interaction resulted in 4-MW output power and 50-dB single-pass gain, with an efficiency of up to /spl sim/8% (for 115-A beam current). The best measured phase stability of the TE/sub 31/ amplified pulse was /spl plusmn/10/spl deg/ over a 9-ns period. The phase stability was limited because the maximum RF power was attained when operating far from wiggler resonance. The second harmonic, TE/sub 21/ had a peak amplified power of 2 MW corresponding to 40 dB single-pass gain and 4% efficiency. The second-harmonic interaction showed stronger superradiant emission than the third-harmonic interaction. Characterizations of the second- and third-harmonic gyro-twt experiments presented here include measurement of far-field radiation patterns, gain and phase versus interaction length, phase stability, and output power versus input power.

Free-electron lasers with electromagnetic standing wave wigglers

A detailed analysis of the electromagnetic standing wave wiggler for free-electron lasers (FELs) is conducted for both circular and linear wiggler polarizations, following a single-particle approach. After determination of the unperturbed electron orbits in the wiggler field, the single-particle spontaneous emission spectrum and subsequently the gain in the low gain Compton regime (using the Einstein coefficient method) are explicitly calculated. This analysis results in a clear understanding of the resonance conditions and the coupling strength associated with each resonance of this type of FEL. In particular, a striking feature obtained from this investigation is that the electromagnetic standing wave wiggler FEL, under certain circumstances, exhibits a rich harmonic content. This harmonic content is caused by the presence of both the forward and backward wave components of the standing wave wiggler field. In addition, the nonlinear self-consistent equations for this type of FEL are also presented, permitting further investigation of it by the theoretical techniques and numerical codes developed for conventional FELs.

Long‐pulse millimeter‐wave free‐electron laser and cyclotron autoresonance maser experiments

Experimental results on high‐power long‐pulse free‐electron laser (FEL) and cyclotron autoresonance maser (CARM) experiments are summarized. Single‐mode operation of a free‐electron laser oscillator at 27.4 GHz with a Bragg resonator has been obtained, with an output power of 990 kW for a beam energy of 320 keV and transmitted current of 30 A, corresponding to an efficiency of 10.3%. Free‐electron maser (FEM) amplifier operation at 35 GHz has yielded a gain of 26 dB with an output power of 800 kW, corresponding to an efficiency of 8.6%. CARM oscillator experiments at 32 GHz with a different electron gun have yielded lower powers because of poor beam quality; planned CARM experiments are discussed.

High-power operation of a 170 GHz megawatt gyrotron

Recent gyrotron oscillator experiments have achieved record powers at 170 GHz. Single mode emission with a peak output power of 1.5 MW and an efficiency of 35% has been measured. The experiment is based on a resonant TE28,8,1 cylindrical cavity situated in a 6.7 T magnetic field. Microwaves are generated in the cavity by an 83 kV annular electron beam produced by a triode-type magnetron injection gun that is capable of currents up to 50 A. Megawatt power levels with efficiencies between 30%–36% have been measured over a wide range of operating parameters for the TE28,8,1 mode. Similar results were also achieved in the neighboring TE27,8,1 mode at 166.6 GHz, and the TE29,8,1 mode at 173.5 GHz. The high output power is the result of a carefully designed electron gun with low perpendicular velocity spread (6%–10%) and a novel cavity with an output iris that is less prone to mode competition. These results are in good agreement with nonlinear multimode simulations.

High-power millimeter-wave Bragg free-electron maser oscillator experiments

Abstract Single-mode, high-power operation of a free-electron maser (FEM) oscillator with a Bragg cavity is reported. The beam is produced by a 0.27 μperv thermionic electron gun which was successfully operated up to 580 kV and 120 A. compressed to a nominal radius of 4 mm with a measured axial energy spread of Δγ 1 /γ 11 , transported through the interaction region by a 2.35 kG axial magnetic field, and pumped by a 500 G amplitude. 30 mm period permanent-magnet helical wiggler. A 10-period-long adiabatic introduction ensures high-quality group-I operation. The Bragg resonator is designed to provide feedback in the TE 11 mode at frequencies considerably higher than cut-off ( ω/2π 2 7.5 GHz and ω c /2π=11 GHz , respectively) to avoid beam interception and allow high-power operation. The FEM oscillator generated 990 kW (±0.5 dB ) of microwave power in a single axial mode at 27.47 GHz with a beam voltage of 320 kV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. The starting current of this mode was measured to be 4 A, and a peak efficiency of 12.5% was observed for I t = 23 A and P μ w = 920 kW . The system was operated with three of the four axial modes of the Bragg resonator with an observed nonlinear frequency pulling of about 70 MHz. Far-field radiation pattern measurements confirm the TE 11 operation.

Long pulse high gain 35 GHz free-electron maser amplifier experiments

Results of the design and operation of a 35 GHz high gain, long pulse free-electron maser (FEM) amplifier are presented and discussed. The electron beam used in this experiment is produced by a thermionic electron gun which was successfully operated up to 580 kV and 120 A. with a measured perveance of 0.27 μperv, in excellent agreement with the design value. The beam is compressed to a nominal radius of 4 mm and guided by a 2.35 kG axial magnetic field. A 500 G amplitude, 30 mm period SmCo permanent magnet helical wiggler with a 10 period long adiabatic introduction provides the perpendicular momentum to the electrons and ensures high quality group 1 orbits in the interaction region. The measured energy spread before injection in the wiggler is Δγ∥/γ∥ < 0.5%; the spread in the interaction region inferred from the FEM performance is Δγ∥/γ∥ ≅ 1.0% The amplifier operates in the TE11 mode, with a cylindrical waveguide radius a = 8 m. and a corresponding cutoff frequency ωc/2π = 10.98 GHz. The amplifier was voltage-tuned from 18 GHz to 40 GHz in excellent agreement with TE11 linear theory. An instantaneous low gain bandwidth of more than 12 GHz was observed. In the high gain regime, a total output power of 800 kW was observed for an input power of 2 kW, yielding 26 dB of gain. The FEM was operating at an energy of 310 keV and a transmitted current of 30 A with a corresponding electronic efficiency of 8.6%.