Thomas A. Spencer

Results from gyrotron backward wave oscillator experiments utilizing a high-current high-voltage annular electron beam

We report the first gyrotron-backward-wave oscillator experiments which utilize a high-current (1-4 kA) high-voltage (300-500 keV) annular electron beam. The experiment was designed to operate in the TE/sub 01/ backward wave mode. Radio frequency extracted power was 0.1-4 MW, with pulselengths of 80-500 ns. Experimental results suggest the possibility of competition with the TE/sub 21/ backward wave mode. Frequency spectrum measurements have shown a wide content of frequencies during the voltage pulse, an undesirable result for use in narrow-band devices.

Recent progress in the Hard-Tube MILO experiment

The Hard-Tube MILO (Magnetically Insulated transmission Line Oscillator) is a gigawatt-class L-band high power microwave tube driven by a 500 kV, 60 kA electron beam. It is nearly identical to the MILO reported by Calico et al., with the principle difference being that the Hard-Tube MILO has been constructed using conventional-tube brazing techniques for the rf joints, while the earlier version of MILO used finger-stock connections for the rf joints. This paper reports on recent experimental improvements to the tube that have allowed us to generate 1.5 - 2.0 GW pulses of 175 ns duration; a 2.5 times improvement in the pulse width over the original tube. In addition, we report on experiments to identify the breakdown mechanism in the vacuum radome for the Vlasov antenna used to radiate the microwave pulse. Finally, details of an optimized version of the Hard-Tube MILO that should allow us to generate over 3.0 GW are presented.

Pulse shortening in the magnetically insulated line oscillator (MILO)

The Magnetically Insulated Line Oscillator (MILO) is a cross field tube that has been studied analytically and experimentally by researchers in several laboratories. The tube is remarkable in that it requires no externally imposed magnetic field, but rather it can be designed to provide a sufficient self field in the relativistic electron beam to guide the electrons. The MILO can be made to operate at high power in the power range above 100 MW. It has been observed that the tube experiences a diminution in pulse width when operated at successively higher powers. This phenomenon, called variously pulse shortening or pulse tearing is also observed in conventional tubes designed for lower power. The process of conditioning commercial tubes is a costly part of the production of high power tubes for applications including particle accelerators. In the case of high power microwave tubes operating in excess of 100 MW, it presents a limitation on the energy that can be extracted from these tubes. This paper describes work performed at the Phillips Laboratory on a relatively high power MILO and discusses the phenomena that may account for this behavior.

Dynamics of the space charge limiting current in gyro-type devices

Summary form only given, as follows. The Air Force Phillips Lab is investigating the dynamic change interactions which occur between the growing electromagnetic waves generated in a gyro-type device, and an electron beam with non-zero perpendicular velocity operating near the space-charge limiting current. The 2-1/2D code MAGIC is being used to perform simulations of the TE/sub 01/ mode, and results will be presented. Initial results show that beams of this nature can suffer from a different kind of pulse shortening than typically found in other devices, thus placing an upper limit on the power and pulse length of gyro-type devices.

Investigation of lower order mode suppression in a high current, high voltage gyro-BWO experiment

Summary form only given, as follows. The Air Force Phillips Laboratory gyro-BWO experiment is utilizing the RAMBO pulser, with electron beam parameters of: V/sub D/=300-800 kV; I/sub D/=1-50 kA; pulselength=1-3 /spl mu/s. An annular electron beam of /spl sim/1-3 kA is produced by an annular aluminum cathode. The interaction cavity is designed to radiate in the frequency range of 4.2-5.5 GHz in a TE/sub 01/ mode. The interaction cavity has a radius of 4.37 cm and a length of 15 cm. Diode and interaction magnetic fields are used together to provide a magnetic compression of the electron beam. C-Band bevel-cut antennas located at the diode end of the experiment are used to extract the backward wave. Experiments have shown evidence of mode competition existing as two different frequency values appearing at the same time. A helical slotted cavity has been designed, in an effort to suppress the TE/sub n1n1/ modes, n/spl ne/0. Analysis and numerical simulations from the 3-D code HFSS will be presented, as well as the latest experimental results.

Rectangular-cross-section high-power gyrotron

Experiments are underway to generate high power, long-pulse microwaves by the gyrotron mechanism in rectangular-cross- section interaction tubes. Long-pulse electron beams are generated by MELBA (Michigan Electron Long Beam Accelerator), which operates with parameters: -0.8 MV, 1 - 10 kA diode current, and 0.5 - 1 microsecond pulselength. Multimegawatt range microwave power levels have been generated. Adjustment of the solenoidal magnetic field is being studied for polarization control. Polarization power ratios up to a factor of 15 have been achieved. Electron beam dynamics, i.e. beam alpha (the ratio of the beam perpendicular velocity to the parallel velocity, vperp/vpar, are being measured by radiation darkening on glass plates. Computer modelling utilized the MAGIC Code and a single particle orbit code into which are injected a distribution of electron angles or energies. Both small- orbit and large orbit (rotating) e-beams are being investigated.

Issues for generation of long pulse, intense annular relativistic electron beams for relativistic klystron devices

Design and testing of a magnitized, vacuum diode for use in a relativistic klystron amplifier is presented. This diode is to provide approximately 7 GW of electron beam power for 1 microsecond for partial conversion to rf energy by a 2 or 3 cavity klystron amplifier system. The present configuration will be shown. Results of calculations for the magnetic beam optics, electrostatic potential structure, and self-consisitent emission PIC simulations will be presented. Experimental results will be shown of the vacuum diode performance.

Microwave and electron-beam diagnostics of a rectangular cross section gyrotron

Diagnostic experiments and operation of multi-megawatt gyrotrons utilizing rectangular cross-section resonant cavities are currently under investigation. The goal of the experiments is to achieve gyrotron generation of high power- long pulse microwaves. A rectangular cross-section resonant cavity has the advantage that it is a linearly polarized source of microwaves. The output microwaves have a high degree of polarization control by changing the magnetic field in the interaction region. Diagnostics include cold tests of the microwave cavity, heterodyne mixer measurements of operating frequencies, and beam alpha (Vperp/Vparallel)/spatial distribution measurements using glass witness plates. Operation at microwave powers greater than 10 MW and high polarization power ratios (vertical/horizontal) greater than 100 have been observed in experiments. The electron beam was produced by MELBA (Michigan electron long beam accelerator) with the following parameters: -0.8 MV, 1 - 10 kA diode current, 0.5 - 1.0 microsecond pulse length.

Progress in elimination of pulse shortening in narrowband high-power microwave tubes

The problem of pulse shortening was raised at this conference two years ago, and in the intervening time, efforts to address the problem have been initiated. There is already some significant progress at our laboratory and at some others. This paper reviews the progress and describes the directions of research and industrial efforts underway at this time. The subject is an exciting one in view of the potential applications that would benefit from the extension of pulse length in Gigawatt class microwave tubes. The paper discusses some of the implications of success in this regard.

Investigation of lower-order mode suppression in a high-current high-voltage gyro-BWO experiment

The Air Force Phillips Laboratory Gyro-BWO experiment is utilizing the RAMBO pulse, with electron beam parameters of: VD equals 300 - 800 kV; ID equals 1.50 kA; pulselength equals 1 - 3 microsecond(s) . An annular electron beam of approximately 1 - 3 kA is produced by an annular aluminum cathode. The interaction cavity is designed to radiate in the frequency range of 4.2 - 5.5 GHz in a TE01 mode. The interaction cavity has a radius of 4.37 cm and a length of 15 cm. Diode and interaction magnetic fields are used together to provide a magnetic compression of the electron beam. C-Band bevel- cut antennas located at the diode end of the experiment are used to extract the backward wave. Experiments have shown evidence of mode competition existing as two different frequency values appearing at the same time. A helical slotted cavity has been designed, in an effort to suppress the TEnl modes, n not equal to 0. Analysis and numerical simulations from the 3D code HFSS will be presented, as well as the latest experimental results.