Over the last few years, there has been a great deal of interest in Secondary Electron Emission (SEE) phenomena. SEE results from the interaction of materials with electrons, atoms, or ions. The amount of secondary emission depends on factors such as the bulk and surface properties of materials, the energy of incident particles, and their angle of incidence. When studying SEE, one is interested in determining the total secondary electron emission yield, which is defined as the number of secondary electrons produced per incident primary electron. The goal of our research is to identify novel materials (with a very low SEE yield coefficient) that will make highpower microwave devices more efficient and robust. To this end, we have employed a low-energy electron gun (5 eV to 2000 eV) to characterize different materials. These measurements have been performed in the DC regime, but pulsed mode measurements are planned for the future to supplement the data. In addition, in support of the experiments, ICEPIC (Improved Concurrent Electromagnetic Particle-in-Cell) simulations of SEE will be performed. Results obtained to-date will be presented.At the University of Maryland, we have been evaluating the use of gyroklystrons to drive RF accelerators. In order to use our 500 kV, 500 A system to energize a 17 GHz accelerator structure built by the Haimson Research Corporation', we need a large-signal gain of nearly 50 dB. To accomplish this goal, we have designed a six-cavity circuit in which the first three cavities operate near the cyclotron frequency and the last three cavities operate near the second harmonic. Adequate efficiencies are predicted via numerical simulations with the MAGYKL code2 even for average perpendicular to parallel velocity ratios as low as one. Efficiencies as high as 35% and output powers above 80 MW will be possible if an average velocity ratio of 1.4 can be achieved. Details of these simulations, along with cold test results for the microwave tube, will be presented.[1] J.

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At the University of Maryland, we have been evaluating the use of gyroklystrons to drive RF accelerators [1]. In order to use our 500 kV, 500 A system to energize a 17.136 GHz accelerator structure built by the Haimson Research Corporation (HRC) [2], we need to construct an interface that converts the gyroklystron TE02 output mode into two standard rectangular WR42 waveguide modes. This structure needs to support up to 80 MW of pulsed power (1 is pulses) in a high-vacuum (10-8 T) environment. The design and cold test results for this system will be presented. Design ResultsThe circular output waveguide is depicted in Fig. 1 as it will be connected to the HRC accelerator structure. The output power exits the gyroklystron (traveling right to left) and enters the large Dolph-Chebychev taper in the TE02 mode, shown on the lower, right-hand side of the figure. The taper is designed to maintain the mode purity to 0.01%, while reducing the waveguide radius to a point where only the two lowest radial modes can exist at the operating frequency. The power then enters a 4-period rippledwall converter, which converts the TE02 mode to TEo, mode with 99.9% efficiency at the operating frequency. A second nonlinear taper, integrated with the mode converter, further reduces the waveguide radius to where only the TEol mode can propagate at the operating frequency. A pumping cross then follows, since the system operates under high vacuum. This cross has hundreds of holes on the wall to allow for pumping while keeping the power loss under -50 dB. The next mode transducer transforms the TEOB mode into the TE20 mode in a waveguide of nearly square cross-section. This transducer configuration was developed by S. Tantawi. We scaled the design to our operating frequency and detailed the results elsewhere [3]. Linear tapers convert the waveguide crosssection to one which has the standard height for WR42 waveguide but twice the standard width. A bifurcation then divides this guide into two parts, and a sequence of bends, twists, and vacuum pumps is used to bring the power to the accelerator structure input windows. Because the accelerator has dual feeds and the phase balance is critical to proper operation, a phase shifter which operates by mechanically deforming the broad wall of the cavity (not indicated in the figure) will be used to fine tune the relative phase. The design requires many details to allow it to operate at full power at the required vacuum levels. These details are described in the paper, not just for the components already described, but also for the flange connections, bifurcation separator, and the high-power loads. SummaryThe design of the output waveguide / mode transducer has been completed and nearly 100% constructed. The measured results appear as predicted and the output waveguide will be connected to the gyrok-0-7803-9348-

The University of Maryland Gyroklystron Program has as a goal the production of 80 MW of peak power at 17 GHz, using a 500 kV, 500 A electron beam'. Frequency-doubling gyroklystron circuits that had 3-4 cavities and that were driven by a 150 kW, X-Band have to date only produced maximum powers of about 27 MW. The root cause of this inability to achieve the designed operating parameters has been non-uniform emission from the temperature-limited Magnetron Injection Gun (MIG). The azimuthal current density varies by more than + 50% due in large part to a 60°C temperature variation on the emitter surface. A new MIG with superior azimuthal current uniformity has been constructed by Calabazas Creek Research, Inc. and has been installed in the UM test bed2.In this paper we describe the design and hot test results for a four-cavity coaxial circuit being driven by this new MIG. The input cavity operates at 8.568 GHz in the TEOB, mode while the remaining cavities operate at twice the drive frequency in the TE021 mode. The tube is predicted to have an efficiency of 34% for an average perpendicular-to-parallel velocity ratio of 1.4. The large signal gain is about 46 dB at the desired operating point. Tube cold-test results, as well as the hot test characterization of tube dependence on voltage, current, velocity ratio, and magnetic field, will be given. Tube stability and output mode purity will also be assessed.Recently the regime of simultaneous excitation of several axial eigenmodes of the gyrotron cavity was studied in relativistic gyrotron with output power of hundreds of kW' 2. It was shown that such regimes exist predominantly with a proper reflector position, when the Q-factors of neighboring eigenmodes are close to each other. The paper is devoted to theoretical and experimental studies of this phenomena in 10-20 kV sub-relativistic gyrotrons.Experimental studies were carried out at the Kaband technological gyrotrons at the Forschungszentrum Karlsruhe (in the regime of 20 ms electron pulse duration) and the Institute of Applied Physics (CW operation), which were operated at the TE12 mode when the millimeter-wave output window was strongly mismatched and was used as external reflector.Two different situations were realized experimentally. In the first, when the Q-factors of axial modes were quite different, only the steady-state oscillations at two different frequencies were observed. In the second, when the Q-factors were close, the regime of two frequency oscillations was realized.These results are in a good agreement with "cold" electrodynamic studies and non-stationary PIC-code simulations. The possibilities of increasing the efficiency in the non-stationary regimes are discussed.