A.K. Ganguly

Microstrip Excitation of Magnetostatic Surface Waves: Theory and Experiment

A model is developed for excitation of magnetostatic surface waves (MSSW) with microstrip transmission lines. Energy carried away in MSSW propagating perpendicular to the microstrip is related to electromagnetic (EM) energy propagating along the microstrip line by an equivalent radiation resistance. Supporting experimental results are in excellent agreement with predictions derived from this model.

Methods of Efficiency Enhancement and Scaling for the Gyrotron Oscillator

It is shown that a gyrotron oscillator operating in a slightly tapered magnetic field can attain an efficiency of ~78 percent, approximately 1.7 times higher than that obtainable in a constant magnetic field. Extensive numerical data have been tabulated and a convenient parameter is introduced to generate numerical efficiency scaling relations through which optimum operating conditions are clearly exhibited. Conditions for reaching the high efficiency operating regime are also studied and numerically illustrated.

Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of /spl sim/10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWTs with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (/spl alpha/), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies /spl plusmn/30/spl deg/ from fitted linear phase line. >

Limiting current in gyrotrons

Analytical expression for the space charge limited current is derived on the basis of a simplified model. Numerical data for wide ranging beam and geometrical parameters are presented.

Large-signal theory of a two-stage wideband gyro-TWT

A self-consistent large-signal theory is used to obtain gain and efficiency of the two-stage tapered gyro-TWT in a nonuniform external magnetic field. Calculations show that it is possible to design a device having 45 percent bandwidth at 45-dB gain and 25 percent efficiency if the axial velocity spread of the beam electrons is zero. It is found that the gain and bandwidth are very sensitive to the degree of beam velocity spread, the external magnetic field profile, and the various beam and circuit parameters. We also derive an expression for the efficiency under quasi-linear approximation in order to get a physical understanding of the device performance.

Complex Radiation Impedance of Microstrip-Excited Magnetostatic-Interface Waves

A technique is described for computing the radiation reactance of microstrip-excited MSSWs from a Hilbert transform of a previously derived expression of the radiation resistance. The series combination of the radiation reactance, radiation resistance, and an inductive reactance corresponding to a shorted section of microstrip line forms an equivalent circuit characterizing the excitation process. Supporting impedance measurements are presented and limitations of the model are discussed.

Experimental study of efficiency optimization in a three-cavity gyroklystron amplifier

An experimental study quantifying the effects of both penultimate cavity tuning and magnetic field profiling on the saturated efficiency of a three-cavity gyroklystron amplifier has been performed at a frequency of 4.5 GHz. As predicted by theory, it is observed that the penultimate cavity detuning in the gyroklystron is in the opposite direction from a conventional klystron for enhanced efficiency operation. This is a result of the opposite energy dependence of the bunching mechanism for the two interactions. Magnetic field profiling has also been shown to have a very strong effect on the amplifier efficiency, both through beam loading effects in detuning the penultimate cavity and through enhanced energy extraction in the output cavity. The measured efficiency of the gyroklystron is observed to improve from 5% to 21% by profiling the magnetic field along the device, and then from 21% to 29% by penultimate cavity detuning with an optimized magnetic field profile. >

A high power, 600 μm wavelength free electron laser☆

We report high power emission (18 MW) at a wavelength of λ = 640 μm and a bandwith Δλ/λ < 0.04 from a superradiant free electron laser (FEL) excited by a 2 MeV, 1 kA electron beam. Comparison of the experiment with a nonlinear simulation yields good agreement. Theoretical extrapolation to a tapered wiggler experiment shows that power levels of ∼ 140 MW could be achieved with an efficiency of ∼ 7%.

Review of two-stream amplifier performance

The basic concept of the electron-electron two-stream instability and its application in building a microwave/millimeter-wave amplifier are reviewed from the perspective of the general use of high-frequency devices. A historical review of the relevant literature is presented, with additional discussion of unpublished laboratory notebooks from the US Naval Research Laboratory. A summary of the theoretical background and its extension to the bounded three-dimensional case is given, along with results from one-dimensional nonlinear particle simulation, which is compared to nonlinear theory. The advantages, limitations, and shortcomings of a two-stream amplifier are discussed in comparison with other currently available devices that work in the same range of parameters. >

A Continuously Tunable Ka-Band Gyrotron

An experimental study was carried out to test the concept of using a gyrotron with a tapered circuit in a uniform magnetic field as a high-power tunable source of millimeter-wave radiation. It was demonstrated that the device can be used as either an oscillator or an amplifier with the operating frequency tunable by varying the magnetic field. In the oscillator mode, the tube produced a peak output power of 500-750 W with the frequency continuously tunable from 30 to 39 GHz. As an amplifier mode, the tube demonstrated an electronic gain of ~23 dB at a magnetically tunable center frequency with an instantaneous bandwidth of ~5 percent. The stability characteristics of the amplifier were also investigated.