Millimeter-wave (mmW) traveling-wave tube (TWT) devices are of interest for applications requiring amplifier output power in the >100 W range over instantaneous bandwidth of several GHz, but fabrication of the sub-wavelength slowwave structures has been a persistent challenge. We discuss the development of TWT device design and fabrication methods to advance the output power and bandwidth capability and improve the SWaP-C (size, weight, power, cost) of devices in the Ka-band to W-band frequency ranges.

The accelerator community is making the transition to inductive output tube (IOT) technology for a number of high-power UHF and L-band applications as a result of their inherent benefits. Scientists, funded by the Office of Naval Research and Naval Research Laboratory, are investigating the physics of the beam-wave interaction of the IOT. The time-domain electrostatic PIC code MICHELLE [1], in conjunction with the Analyst® [2] suite of electromagnetic codes, were used to model the cathode-grid-anode structure that comprises the input cavity. Our investigation has led to the discovery of a delay mechanism responsible for intra-bunch charge formation, as evidenced by IOT X-ray generation with energies significantly higher than the cathode accelerating potential, increasing with RF output power. Time-domain PIC results of this effect will be shown. We will also present simulation results of the large-signal beam wave interaction in the output cavity using the code TESLA [3, 4]. Examples of single beam and multiple-beam (MB) IOTs will also be discussed.

This paper describes the latest advances in code development targeted to model the beam wave interaction of inductive output tubes. At the core of this effort is the modeling and simulation of the input cavity, which requires the utilization of finite-element (FE) electromagnetic (EM), magnetostatic and electrostatic time-domain (ESTD) PIC codes. Key to meaningful design of the input cavity is the incorporation of an electron beam loading model, which is particularly challenging when using an ESTD-PIC code; our starting point for modeling this effect will be described. Another feature of this development is the collection of particle data, which is passed to the large signal code for simulation of the beam-wave interaction of the output cavity, and is returned back to the ESTD PIC code for collector design and optimization. The latest results of this effort will be shown.

We present the results of experimental measurements on an 18-beam S-band multiple-beam klystron. The electron gun has met its perveance design goal of 4.8 μPerv with very good beam transmission (>97% in the absence of rf). Preliminary rf testing has shown amplification across the full 400-MHz band (2.9 to 3.3 GHz). The MBK was recently re-gunned to correct a high pressure gas issue; further results of beam and rf performance will be presented, as available.

The extended interaction klystron (EIK) is a compact amplifier that is well-suited to millimeter-wave and submillimeter-wave operation, having achieved state-of-the-art performance of 9 W CW at 218 GHz with a tightly focused 12-kV round beam. By utilizing an EIK circuit in a sheet-beam topology, our goal is to produce more than an order of magnitude higher power than can be generated with a round beam. This talk describes the design of a four-cavity 220-GHz EIK circuit, completed with the code MAGIC-3D based on our sheet-beam EIK cavity topology. This circuit includes an input cavity to initiate beam velocity modulation, two synchronously tuned idler cavities to enhance beam modulation, and an output cavity for RF power extraction. The input and output circuits are nearly identical with eight gaps each and an external coupling waveguide, which yields a Qext, ~ 300. The idler cavities have seven gaps with no external coupling. All cavities operate in the 2p-mode. The circuit is driven by a 520 mA, 16.5 kV sheet beam with an aspect ratio of 19. The beam is transported using a 9 kG solenoidal magnetic field. With these beam parameters and the large number of gaps, this circuit configuration effects a very efficient beam modulation. Output power of 453 W is achieved in MAGIC-3D with an input power of 25 mW, corresponding to an electronic gain of 41.6 dB in a circuit length of about 1.2 cm. Our sheet-beam EIK cavity topology also facilitates fabrication. Two sets of 220-GHz OFHC copper cavities have been fabricated with conventional machining techniques for evaluation purposes. The surface finish and precision appear to be within specified tolerances. This is particularly impressive in consideration of the fact that the width of the interaction gaps is only slightly larger than a typical human hair (~ 75 microns). Cold testing is currently underway at NRL. Cold test results will be presented at the conference. Details on beam formation and transport will also- be presented and discussed.

We develop a methodology for the design of multiple-cavity klystron interaction circuits. We demonstrate our approach with the detailed design of a collector and a four-cavity circuit for a multiple-beam klystron (MBK) operating in the fundamental mode at a center frequency of 3.27 GHz (S-band). These elements are designed to be used with a 32-A 45-kV magnetically shielded eight-beam electron gun currently under fabrication . Upon integration of the gun, circuit, and collector, the MBK will be used for beam transport and beam-wave interaction studies and to validate developmental design codes and design methodologies. The device has a predicted gain of 33 dB at a peak pulsed output power of 750 kW with a corresponding electronic efficiency of 52%. For the present design, broad bandwidth is not a design objective, and the 3-dB bandwidth is 2.5%. Downstream of the output cavity, the magnetic field profile and the interior surface profile of the collector are carefully shaped to minimize the space-charge potential depression at the entrance to the collector, minimizing reflected electrons. The maximum calculated instantaneous power density on the walls of the collector is approximately 55 kW/cm/sup 2/; at low duty cycles (<1.8%), the average power density is well within the limits for liquid cooling for pulse lengths up to 1.3 ms.

This paper describes the detailed design of an eight-beam electron gun for use in S-band multiple-beam amplifiers operating in the fundamental mode. The gun operating voltage is 45 kV with a total beam current of 32 A, evenly divided among the beamlets. Each individual beam has a perveance of 0.42 mpervs making a total beam perveance of 3.35 mpervs. The optimized electron gun is singly convergent using a four-fold symmetry with the four inner and four outer emitters interlaced 90/spl deg/ apart. The emitter current density has been kept below 10 A/cm/sup 2/ (space-charge limited). The cathode is magnetically shielded and the longitudinal magnetic field in the interaction region is in the range of 1.1-1.8 kG. The design of the magnetic focusing system minimizes beam corkscrewing as well as electron interception on the tunnel walls. Beam optics simulations of the gun indicate excellent beam transport characteristics with a final beam-to-tunnel radial fill factor of less than 0.45. The primary computational tools used in the design process were the three-dimensional gun code MICHELLE, and the magnetostatics code MAXWELL-3D.

The simulation code MAGY developed at the University of Maryland and Naval Research Laboratory is able to describe the self-consistent nonlinear interaction between the electromagnetic fields of simply connected axisymmetric structures and electron beams. The code has been used effectively primarily for the design and simulation of gyro-devices. The main advantage of the MAGY code is its capability to find both steady state and slowly evolving dynamic solutions with minimal computational effort. The goal of the present work is to develop the formulation to be capable to model vacuum electronic devices with external cavities while keeping all the useful features of the MAGY code.