C.W. Baik

Study on velocity spread for axis-encircling electron beams generated by single magnetic cusp

The physical characteristics of an annular Pierce-type electron gun are investigated analytically. The electron gun is used in conjunction with a nonadiabatic magnetic reversal and adiabatic compression region to produce an axis-encircling beam. Typical magnetic field profiles that can generate zero velocity spreads are obtained from the analytical calculation, taking into account initial canonical angular momentum spreads at the cathode and the crossing of the beam trajectory and magnetic flux line before the magnetic cusp. Using this magnetic fields, a fairly low axial velocity spread of less than 1% is achieved by an electron trajectory program [W. B. Hermannsfeldt, Electron Trajectory Program (Stanford Linear Acceleration Center, Stanford, CA, 1979)], which agrees well with that by analytical estimation.

Experimental verification of low-velocity spread axis-encircling electron beam

We have experimentally demonstrated a low-velocity spread, axis-encircling electron beam using a comparatively simple Pierce-type electron gun and single cusp magnetic field based on a recent theory [Jeon et al., Appl. Phys. Lett. 80, 3703 (2002)] developed with the assumption of small orbit gyration before the magnetic cusp, in which every physical property has been analyzed and the possibility of zero percent axial velocity spread was concluded. The velocity ratio and the axial velocity spread were measured to compare the theory with varied operation conditions of the electron gun. These results agree well with our hypothesis.

1.5 octave wideband traveling-wave tube with heavily-loaded helical slow-wave structure

Summary form only given. A 1.5 octave wideband traveling wave tube (TWT) with a helical structure loaded by the thick dielectric support rods has been designed and fabricated for the frequency range of 6-18 GHz. Helical slow-wave structure (SWS) was modeled using three-dimensional HFSS code. The nonresonant perturbation measurement using a thin copper wire with 20 mm diameter was performed to verify the phase velocity and interaction impedance of the helical structure. The performance of TWT was predicted using one-dimensional (1-D) nonlinear theory involving a macro particle beam model. The harmonic effect was considered in this calculation. The measured performance of TWT using a beam voltage 4 kV and a beam current of 120 mA was shown. These results were compared with a 1-D nonlinear theory. The comparison showed that the measured power and gain were less than the predicted one but had a similar trend over the operating frequency range. The 2nd harmonic levels at the low frequency range of 6-8 GHz were nearly 0 dBc. This relatively high 2nd harmonic level might be attributed to the positive dispersion at the low frequency range due to the deformation of a barrel during the assembly process.

A study on broadband multi-hole directional coupler

The conventional theories of waveguide directional couplers, including Bethes theory, correction theory, and multi-hole array theory, can be a good start for designing very tight multi-hole couplers with square coupling apertures. Comparing with the conventional theory and HFSS, a modified Cohns empirical factor of thickness attenuation was obtained. A 0 dB-waveguide multi-hole coupler (X-band) is fabricated, and the calculated coupling and directivity are compared with HFSS simulation and measured values, showing a good agreement.

Matching of the helix with coaxial coupler

Helical slow-wave structure has been widely used to obtain broad-band in TWTs. Its excellent wide-band characteristic is applied continuously to commercial communications and military radar sources as known as helix-TWTs, however, the impedance matching of the helix circuit with input or output transmission lines is still an important issue to be operated with low reflection. In this paper, the matching is calculated using an equivalent circuit theory to obtain broad-band, and the analysis is presented in detail with the theoretical methods on characteristic impedance of helix and matching of the circuit to the coaxial transmission line with a vacuum window.

Third harmonic frequency multiplication of a two-stage tapered gyro-TWT amplifier

The third harmonic frequency multiplication is experimentally verified using a two-stage tapered gyrotron traveling-wave tube amplifier in low-power operation. The third harmonic frequency multiplication is predicted and investigated using a self-consistent large-signal theory and a particle-in-cell code simulation. Both results show a good agreement in frequency multiplication and power amplification. The interaction between a 30 kV axis-encircling electron beam with a drive signal in the tapered waveguide of the input stage modulates the electron beam at the fundamental cyclotron harmonic, then the third harmonic component of the modulated beam current is chosen to be extracted in the tapered output stage. In the proof-of-principle experiment, X-band drive signals from 10.6 to 12 GHz are multiplied by three times to be Ka-band output frequencies from 31.8 to 36 GHz, showing consistent results with theoretical predictions when a 30 kV, 160 mA electron beam is used.

Experimental verification of frequency multiplication in two-stage tapered gyro-TWT

Summary form only given, as follows. A Ka-band harmonic multiplying two-stage tapered gyroTWT has been developed. The third harmonic interaction of electron cyclotron frequencies is used to multiply X-band signals to Ka-band. A previous similar experimental demonstration at NRL showed a 20 % bandwidth at saturated gain of 25 dB with 16 % efficiency when the predicted velocity spreads of 4 % were assumed at Ka-band with the fundamental harmonic interaction. This work showed the linearly tapered rectangular waveguide at both stages could produce enhanced gain, gain uniformity, and efficiency.

Proof of Principle Experiment on Photonic Crystal Reflex Klystron

Our research group has designed a photonic crystal reflex klystron which employs a high-order-mode resonator and multi-electron beams. Here a photonic crystal resonator with TM330 mode is employed in a reflex klystron for X-band proof-of-principle experiment. For the electron beam generation, carbon nanotube cathode is used as multi-beam (3 times 3 beams) source.

Study on Ballistic Bunching in Frequency Multiplying Distributed Interaction Gyroklystron Amplifier

We re-classify this two-stage and highly tapered gyro-device as distributed interaction two-cavity gyroklystron since this device has a finite interaction length at each operating frequency in a highly tapered waveguide. Here the ballistic bunching theory previously driven is used for the estimation of the maximum bunching distance for each operating frequency. The third harmonic multiplying scheme is applied to two-stage tapered waveguides with a rectangular cross-section. The large-orbit electron beam is adopted operating at the beam voltage and current of 30kV and 1A, respectively. Theoretically predicted maximum bunching length is compared with the drift length from the Lareg-Signal-Theory code simulation. And they are compared with the interaction length of the circuit

Frequency-multiplying gyrotron traveling-wave tube amplifier

Summary form only given. The frequency multiplication was theoretically predicted and verified by experimental observation in a two-stage tapered gyrotron traveling-wave tube amplifier. The electron cyclotron maser interaction at harmonic frequencies was utilized between traveling electromagnetic waves in the two-stage tapered waveguide and the helical electron beam, under the presence of a reduced guiding magnetic field by harmonic operation. A 30 kV, 1 A axis-encircling electron beam from a cusp gun was chosen for both the frequency multiplication and the power amplification. For the theoretical predictions, a small-signal theory was used to investigate the spatial growth rate of the device as a guideline for large-signal simulations. Device performances such as the saturated output power, harmonic frequency multiplication, and electron bunching behavior were numerically analyzed using the large-signal theory and compared with those from particle-in-cell (PIC) code simulations. The dependencies of a bunching parameter on a beam velocity ratio, an interaction length, and an input drive power were analytically derived using a ballistic bunching theory. These theoretical analyses on the frequency-multiplying gyro-TWT were the basis of the experimental development. In the proof-of-principle experiment, the theoretical prediction of frequency multiplication from an X-band drive signal to a Ka-band output signal through the third harmonic electron cyclotron maser interaction in the two-stage tapered gyro-TWT was verified. The measured Ka-band output frequencies from 31.8 to 36.0 GHz were three times of the input drive frequencies from 10.6 to 12.0 GHz. The measured output power was about 20 dBm due to a reduced beam current of 160 mA. The power amplification, however, is expected when the beam emission is improved, and it is now underway.