Research is continuing on the NRL K/sub u/-band ubitron amplifier after several major component modifications. Specifically, the input coupler has been modified for broader bandwidth and lower transmission loss, and the wiggler has been redesigned to generate a much more uniform field in the interaction region and lengthened. Steps have also been taken to remove discontinuities that excited oscillations at high wiggler fields. With a 250 kV, 100 A, 4 mm radius beam and these modifications, we anticipate saturated gain of 25-30 db, output power of 1-5 MW, efficiency greater than 15%, and bandwidth exceeding 20% in the 12.4-18 GHz band. Measurements will be compared with the 3-D nonlinear code ARACHNE.<>

: Utilizing a novel ultraviolet photolithography microfabrication technique involving embedded polymer monofilaments, the U.S. Naval Research Laboratory is demonstrating and developing millimeter-wave vacuum electronic traveling wave tube amplifiers at W- and G-band in the 10s to 100s of watts power range at several percent instantaneous bandwidth.

Summary form only given. An 18-beam, seven-cavity multiple-beam klystron ("MBK3") has been developed at the Naval Research Laboratory and is designed to produce a peak rf output power of >500 kW over a 400-MHz frequency band centered at 3.1 GHz. The amplifier operates in the TM01 fundamental mode, with the 18 beamlets clustered around the axis of the device in two concentric rings of 12 (outer) and six (inner) beamlets.We will present the results of recent experimental measurements on the MBK. The 18-beam electron gun is designed to operate at a nominal cathode voltage of 42 kV and a total beam current of 41.6 A.2 The gun operates in the space-charge limited regime and emission from the cathode is controlled by a modulating anode. The electron gun has met its perveance design goal of 4.8 X 10"6 AV"3/2 with very good beam transmission (>97% in the absence of rf). Preliminary rf testing has shown amplification across the full design frequency band of 2.9 to 3.3 GHz. The MBK was recently regunned to correct a high pressure gas problem in the cathode region. Further results of beam and rf performance will be presented, as available.

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 describe the results of recent experiments with an eight- beam, five-cavity multiple-beam klystron (MBK). The electron gun is of the same design as used in our previous MBK and operates at a nominal cathode voltage of -45 kV and a total current of 32 A. The electrodynamic circuit is comprised of a two-gap input cavity, a two-gap idler cavity, two additional single-gap idler cavities, and a two-gap output cavity and has a total length of 22 cm. All of the multi-gap cavities operate in the pi-mode. Some of the cavities were loaded with a lossy dielectric to reduce the Q. Output power is extracted from both of the output resonator gaps in four waveguide arms (two on each gap). The tube produces ~ 600 kW of peak output power at saturation with a corresponding electronic efficiency of 40%. Beam transmission is excellent: > 99% in the small-signal regime and > 93% at saturation. Despite a mismatch in the input circuit that reduces the gain in the upper portion of the band, the MBK has a measured 3-dB bandwidth of ~ 6%. We will describe efforts to improve the input circuit match and will compare measured data with the results of numerical modeling using the MAGIC 3D particle-in-cell code and the 2.5D large-signal code, TESLA.

Summary form only given. We investigate a 3-D generalized modal-expansion technique to model the fields and resonance characteristics of low-Q cavities and multi-cavity circuits found in broadband klystron and multiple-beam klystron amplifiers. Such cavities are often complex, multi-gap structures and full 3-D analysis is essential to their design. For broadband input and output circuits, that have low Qexternal, a traditional cavity eigenmode representation is more difficult to define due to the strong coupling of the cavity to an external waveguide. Usually, numerical solution requires addition of an absorbing boundary condition inside the waveguide port to determine a leaky eigenmode of the open system, or one must resort to computing many driven-frequency solutions across the frequency band and perform subsequent field analysis to determine the cavity stored energy dependence on frequency and hence extract the resonance frequency and Q. The analysis becomes more problematic when multiple overlapping resonance peaks occur inside the operating bandwidth, as is often the case. In our approach, we avoid the numerical solution of the open cavity-waveguide system, and introduce a new technique using only modes of a finite closed cavity, terminated at the waveguide aperture by short-circuiting either the tangential electric or magnetic field. By representing fields inside the cavity using a dual set of eigenmodes we determine the coupling to an external waveguide to determine the actual frequency and Q of the open cavity-waveguide system. We present the theory behind the analysis, and compare to existing methodology

Summary form only given. We present the design of a broadband multiple-beam klystron (MBK) capable of generating >700 kW of peak RF power with a 3-dB instantaneous bandwidth of >11% at a center frequency of ~3.3 GHz. The MBK will operate in the TM01 fundamental-mode and is designed to be powered by an 18-beam, 42 kV, 41.6 A electron gun that is currently in fabrication. The gun employs a concentric beam-packing topology of 6 inner and 12 outer beamlets, each with an emitter current density of ~11 A/cm2. The electrodynamic circuit is comprised of six cavities with an overall length of 22 cm. Three of the six cavities are of a two-gap design, with inherently higher R/Q's as compared to single-gap cavities, thus enabling broader bandwidth operation and higher power and gain in a shorter interaction length. We will describe our design methodology, including the development of analytic design aids, discuss design trade-offs, and present final circuit parameters along with the results of 3-D particle-in-cell simulations of realistic circuit topologies

Summary form only given. The phase sensitivity of a high power amplifier to fluctuations in operational parameters such as power supply voltage is an important characteristic for applications in advanced radar. We present the results of experimental measurements of the phase sensitivity (pushing factor) of a four-cavity multiple-beam klystron (MBK) operating in S-band. This MBK has recently been shown to generate ~600 kW peak RF power with an electronic efficiency of 40% and a 3-dB instantaneous bandwidth of ~2% at a center frequency of ~3.3 GHz. The nominal operating parameters of the eight-beam electron gun are 45 kV, 32 A (8times4 A). The measured pushing factor is in the range of 0.0134deg/volt to 0.015deg/volt and is in excellent agreement with analytic theory and large-signal simulation. We will discuss the experimental results, describe the measurement technique, and discuss the analytic model and simulations

Summary form only given. We present recent experimental progress with the NRL 8-beam, 4-cavity multiple-beam klystron. The electron gun and circuit were designed by an NRL/ATK MRC/beam-wave research team with mechanical design and fabrication support from CPI. The MBK is designed to operate at a frequency of ~3.25 GHz, producing >600 kW of peak RF power with a saturated gain of ~33 dB and an instantaneous 3-dB bandwidth of ~2%. The gun is Brillouin focused and is operated at a cathode-anode potential of -45 kV, producing a total beam current of 32 A (8times4 A); individual emitter loading is ~10 A/cm2. The beam current measured during low-voltage gun tests (-1250lesVak les-200 volts) was in excellent agreement with particle simulations using the 3-D gun code, MICHELLE, confirming that (i) the electron emission is space-charge limited; and (ii) fabricated dimensions and component alignment are within specified values (zero or minimal interception in the gun region). High-power beam transport tests conducted at the full beam voltage and current and a solenoidal axial magnetic field of 1.8 kG are in excellent agreement with the design values, with a measured beam transmission of ges99% for the duration of the pulse flat-top and a total beam perveance very close to the predicted value of 3.35times10-6 A/V3/2. RF amplification testing is currently in progress

Details of the amplifier physics design are given in [7], but a summary of the major design issues is given here. Given the accelerating voltage and desired frequency range, gain, and output power, the various tradeoffs for the interaction region can be explored. The lengths of the loaded and unloaded sections of the interaction circuit are selected to satisfy stability constraints, overall amplifier gain, and efficiency requirements. The loss section needs to be sufficiently long and with sufficient loss for the desired gain and zero-drive stability, but not too long or velocity spread will compromise performance. It must especially load the spurious modes. In our case, the loss design must also be high-average power capable. A short unloaded section is added for nonlinear gain and high power. Its length must be less than the critical length for start oscillation in the given magnetic field. A too long section will also compromise bandwidth due to beam velocity spread. The appropriate magnetic field must be chosen for near grazing to slightly above grazing with the waveguide mode for the given waveguide size and beam voltage. The magnetic field is tapered where possible in regions of uniform diameter waveguide in order to ensure no oscillations. For this design, the field is tapered at the input coupler and at the output at the unloaded section and uptaper. A nonlinear uptaper is used to couple RF from the interaction circuit to the outside world with miminal reflection of the operating mode and minimal conversion to other modes. For each set of beam and field values, the RF input power is chosen for flattest response. The implementation of these design considerations is described below. The wall geometry, magnetic field profile, and calculated power for this amplifier design are shown in Fig. 1 for the operational parameters: kV, A, mm,

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain /spl sim/60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in K/sub a/-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE/sub 11/ mode and for stabilization employs a high-average power compatible diffractive loading technique.

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain similar to60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in K.-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE11 mode and for stabilization employs a high-average power compatible diffractive loading technique.

Summary form only given. In multiple-beam amplifiers, beamlets are transported in individual beam tunnels but interact with RF fields in a common interaction region. This approach enables the designer to have the best of both worlds: individual beamlets can have low perveance which is conducive to efficient bunching and beam transport, leading to higher gain, electronic efficiencies, and average power, while the aggregate beam current can be high, facilitating high beam and RF power, and also broad bandwidth. We will report on the design of a beam forming system for high-average-power broadband S-band multiple-beam amplifiers to be developed at the Naval Research Laboratory (NRL). These amplifiers will utilize eight individual electron beams and operates in the fundamental TM/sub 01/ mode. The singly convergent electron gun topology has fourfold symmetry with four inner and four outer emitters, interlaced 90/spl deg/ apart. The operating voltage is 45 kV with a total beam current of 32 A, evenly divided among the beamlets. Each individual beam perveance is 0.42 micro-pervs, for a total beam microperveance of 3.35. Cathode loading is kept below 10 A/cm/sup 2/ (space charge limited) for lifetime considerations. The cathode is magnetically shielded and the magnetic field in the interaction region is 1.1-1.8 kG. A key design feature is the magnetic focusing system, designed to ensure minimal beam corkscrewing, which limits the beam clearance in the beam tunnel. The primary computational tools used in this design were the 3-D gun code, MICHELLE, and the magnetic code, MAXWELL-3D. Beam optics simulations of the gun design demonstrate excellent beam transport characteristics with a final beam-to-tunnel fill factor less than 40%.

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.

A very attractive class of amplifiers that has the potential to provide high RF power at low beam voltage uses multiple electron beams rather than the single beam of conventional devices. These multiple beam amplifiers (MBAs) typically have individual beam channels with a common RF interaction region. A multiple beam klystron (MBK) illustrates the concept. In such a scheme, individual beamlets have parameters similar to those in conventional, single-beam amplifiers, while multiplying the output power by approximately the number of beamlets. The important result is that the operating voltage and space charge limitations for each beamlet are essentially unchanged, yet the output power can be much greater than that of a corresponding single-beam device. Furthermore, the overall high perveance characteristic of the MBA permits large bandwidth operation, while the low perveance of the individual beamlet enables high efficiency operation.

High power millimeter wave instrumentation radars have a number of important applications ranging from defense missions to basic scientific studies At the Naval Research Laboratory, a new high power 94 GHz radar named WARLOC has been developed. This radar employs a high power gyro-klystron as the final power amplifier and was developed during 1996-2001. The WARLOC radar has been integrated as a transportable system, using the 100 kW peak, 10 kW average power gyro-klystron amplifier, a low-loss transmission line, a quasioptical duplexer, and a Cassegrain antenna. The transmitter operation and waveguide system is the subject of this paper.

Summary form only given. A 35 GHz CHI (Coaxial Hybrid Ion) wiggler ubitron amplifier experiment is under construction at the Naval Research Laboratory. The purpose of the experiment is to study the CHI wiggler configuration's potential of generating high wiggler magnetic fields at short periods with excellent beam focusing and transport properties. The nominal design parameters of the experiment are a center frequency of 35 GHz, wiggler period of 7.5 mm, and beam voltage of approximately 150 kV. Calculations have shown an intrinsic (untapered) efficiency of /spl ap/7% when operating at 6.3 kG axial field (wiggler field, B/sub w//spl ap/1270 G). The calculated gain was 36 dB, saturating at a distance of 46 cm. These parameters yield an instantaneous amplifier bandwidth of /spl ap/25%. There appears to be room for further improvement in efficiency, a matter that will be scrutinized more closely in the final design. Components of the experiment include a modified SLAC klystron electron gun (its beam converted from solid to annular), and the input and output RF couplers. Extensive work was performed on the design of the annular beam gun. This includes different techniques used in the development of the electron beam focusing coils, since the beam is immersed in a magnetic field from the cathode onward. In addition to the design of the components themselves, their placement in relation to each other in the gun-wiggler region is critical due to space limitation and beam transport issues. The techniques used for beam focusing were also used to study the transport of the beam from the gun output to the wiggler input.