Joon Y. Choe

Theory of Gyrotron Amplifiers in a Disk or a Helix Loaded Waveguide

The gyrotron amplifiers in two slow wave structures, disc and helix-loaded waveguides, are investigated for wideband applications, For each structure, properties of the vacuum waveguide mode are examined in connection with the gyrotron application, and the gyrotron dispersion relation is obtained. For the disc-loaded waveguide, it is found that the group velocity of the waveguide mode can be easily varied by the disc parameters, and the gain and the bandwidth are at least comparable to those of the ordinary gyrotron. In the helix-loaded waveguide configuration, the gyrotron can be very broad in its bandwidth. Moreover, the hybrid mode operation, using the helix mode as a carrier, shows possibility of fast wave wideband amplification by contouring the waveguide.

Properties of the electromagnetic wave propagation in a helix‐loaded waveguide

Properties of the electromagnetic waves propagating through a helix‐loaded waveguide are investigated, including the important influence of the outer conducting wall on the dispersion properties. A closed algebraic dispersion relation for the eigenfrequency ω and the axial wave number k is obtained for arbitrary azimuthal harmonic number. It is shown that in the limiting case, where the outer conducting wall approaches close to the helix, this dispersion relation is reduced to three distinctive modes. These are the transverse electric mode, the transverse magnetic mode, and the helix mode, which can be further simplified to straight lines in the (ω, k) parameter space. Numerical investigation of the dispersion relation is also presented.

Analysis of the wide band gyrotron amplifier in a dielectric loaded waveguide

The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocities vph : one fast wave, the long wavelength mode (LWM, vph ≳c), and two slow waves, the intermediate (IWM, c≳vph ≳ce−1/2) and the short (SWM, vph <ce−1/2) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM) at a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (<5%), the slow wave (IWM) is preferable for the wide band operation,...

Operation of cusptron at fundamental and harmonic cyclotron frequencies

Microwave radiation at the fundamental and harmonic electron cyclotron frequencies generated by a cusptron oscillator is reported. A low-energy, axis-rotating beam of 28-30 kV, 0.8-3.5 A, 4 mu s, and 60 pps interacts with a single RF mode, both in a circular cavity and in a six-vane circuit by the negative mass instability. In fundamental and second-harmonic frequency generation with a circular circuit, the independently excited modes are TE/sub 11/s and TE/sub 21/s, with radiation power of more than 1.8 kW and an electronic efficiency of approximately 7.5%. Employing a six-vane circuit, microwave radiation of 6.0 GHz (sixth harmonic) and 3.9 GHz (fourth harmonic) is also independently generated with more than 10.4 and 4.0 kW radiation power, respectively. Corresponding electronic efficiencies are approximately 10.0 and 9.5%. >

Gyrotron amplifier in a helix loaded waveguide

Stability properties of the cyclotron maser instability in a relativistic annular electron beam propagating through a cylindrical waveguide loaded with a sheath helix are investigated, in connection with application to the gyrotron amplifier. The stability analysis is carried out, assuming that the beam is thin and tenuous. A closed form of the dispersion relation for the cyclotron maser instability is obtained, including the influence of the sheath helix on stability behavior. One of the most important features in this analysis is that the bandwidth of the gyrotron amplifier for a helix mode is very broad. Moreover, it is also shown that for an appropriate choice of the helix radius and pitch angle, the maximum gain of the gyrotron amplifier for a hybrid wave is twice that for an ordinary waveguide.

Mixed slow‐wave operation of a wide‐band dielectric gyrotron

The bandwidth broadening effect of mixing two slow‐wave modes in a dielectric loaded gyrotron is investigated for a small axial momentum spread. The usual intermediate wavelength mode (IWM) integrates with the short wavelength mode (SWM) by reducing the electron beam radius, and creates a mixed slow wave. The resulting bandwidth for the mixed mode gyrotron becomes at least twice broader than that for the pure IWM at the small velocity spread (≲1%) with a substantial contribution of the SWM. Moreover, the presence of the SWM reduces considerably the troublesome heat dissipation in the dielectric layer. Therefore, the mixed mode operation can allow up to several times more power than the usual IWM mode operation.

Theory of gyrotron amplifier in a waveguide with inner dielectric material

Properties of the gyrotron interaction in a waveguide with inner dielectric rod are investigated for a hollow electron beam, including influence of azimuthally asymmetric perturbations. Stability analysis is carried out for an electron distribution function in which all electrons have the same value in the energy and canonical angular momentum but a lorentzian distribution in the axial momentum, A closed algebraic dispersion relation for a gyrotron amplifier with a centre rod of dielectric is obtained assuming that the hollow electron beam is thin. From numerical analysis of this dispersion rulation, it is shown that, stability behaviour of thes fust wave mode is remarkably independent of the dielectric constant. With a small axial momentum spread (Δ≲0.005), a super bandwidth is shown to be attainable by a mixed mode operation.

Experimental Results of Cusptron Microwave Tube Study

A cusptron microwave tube generates microwave radiation at high harmonics of the electron cyclotron frequency. This device utilizes the negative mass instability for the resonant interaction of an axis rotating electron beam and the modes in a conducting boundary with a multivane structure. A 17 kV, 3 Usec and 60 pps electron beam passed through a cusped magnetic field, produces radiation at the sixth and twelfth harmonic frequencies with the six and twelve vane structures, respectively. In the sixth harmonic case, the grazing condition is obtained at B = 250 G and approximately 500 W radiation is produced at 4.36 GHz. In the twelfth harmonic case, however, it is operated in a continuously tunable frequency range of 7.4 - 8.2 GHz in B = 210 - 220 G. The radiation power level is substantially reduced to few watts without the grazing condition satisfied. Since it is operated at high harmonic frequencies it holds promise as an efficient, compact, and tunable microwave tube suitable for many practical applications.

Theory of Gyrotron Amplifier in a Tape Helix Loaded Waveguide

Stability properties of the cyclotron maser instability in an annular electron beam propagating through a cylindrical waveguide loaded with a tape helix are investigated, in connection with applications on the gyrotron amplifier. Closed form of the dispersion relation for the cyclotron maser instability is obtained, including influence of the tape helix in stability behavior. It is shown that the bandwidth of the gyrotron amplifier for a tape helix is narrow in comparison with results for a sheath helix. However, the growth rate of the tape helix gyrotron is comparable to that of the sheath helix gyrotron.

Design Study of Cusptron Amplifier for Accelerators

Research and development of an RF source for next generation electron-positron colliders is proposed based on the newly developed cusptron microwave tubes. The cusptron generates high harmonics of the electron cyclotron frequency by resonant interaction of an axis-encircling electron beam and the modes in a multi-vane conducting wall structure. The strong coupling through the negative mass instability, the relatively low magnetic field requirement, the high current capability, and multi-feed possibility are some of the features of the cusptron that are especially suited for the accelerator applications. The intended parameters for prototype cusptron are P = 100 MW peak power at f = 8.5 GHz using P = 240 MW beam. In order to reach this ultimate goal, first the scaling law of the beam and field parameters for the cusptron is examined, and the RF circuit design in conjunction with these beam parameters are presented. The design parameters for the scaled down experiments, intended as intermediate steps and tests for the scaling laws, are also given.