John Pasour

Intense Sheet Electron Beam Transport in a Uniform Solenoidal Magnetic Field

In this paper, the transport of intense sheet electron beams in a uniform solenoidal magnetic field in high-power vacuum electronic devices is theoretically examined with the 3-D beam optics code MICHELLE. It is shown that a solenoidal magnetic field can be an effective transport mechanism for sheet electron beams, provided the beam tunnel is matched to the beam shape, and vice versa. The advantage of solenoidal magnetic field transport relative to periodic magnetic transport resides in the feasibility of transporting higher current density beams due to the higher average field strength achievable in practice and the lower susceptibility to field errors from mechanical misalignments. In addition, a solenoidally transported electron beam is not susceptible to voltage cutoff as in a periodic magnetic focusing system; hence, device efficiency is potentially higher.

Modeling Investigation of an Ultrawideband Terahertz Sheet Beam Traveling-Wave Tube Amplifier Circuit

Extensive numerical analysis has demonstrated that a terahertz (H-band) sheet beam traveling-wave tube (TWT) amplifier circuit, composed of a staggered double grating array waveguide, has very broad bandwidth (~30%) of the fundamental passband (TE mode) with a 7:1 aspect ratio sheet beam without excitation of n = 1 space harmonic backward-wave modes. Particle-in-cell (PIC) simulations utilizing MAGIC3D and CST PS predict that the designed circuit produces ~150-300-W output power, corresponding to ~3%-5.5% intrinsic electronic efficiency (~35-38-dB saturated gain from 50-mW input driving power), over ~25% bandwidth, which is in good agreement with CHRISTINE 1-D code predictions. Simulations, using a perfectly matched layer boundary (~ -30-dB return loss), show that the circuit stably operates without noticeable oscillation. With a more realistic matching condition (~ -9.5-dB return loss), it becomes unstable. However, simulations show that the incorporation of an attenuating sever with tapered conductivity suppresses the instability in tube operation.

High-power four-cavity S-band multiple-beam klystron design

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.

Demonstration of a 100-kW Solenoidally Focused Sheet Electron Beam for Millimeter-Wave Amplifiers

The design and development of a high-perveance sheet electron beam is reported. A beamstick that employs a novel sheet beam gun together with a permanent magnet solenoid has been fabricated and tested up to 4.8 A at 22 kV. At the nominal operating point of 19.5 kV and 3.3 A, this beamstick has transported 98.5% of the emitted electron current through a 0.4 × 5 mm beam tunnel over a distance of 20 mm in a uniform 8.5-kG field. The beamstick collector has been depressed to -9 kV with very little effect on the beam transport. The performance very well agrees with simulations. This beamstick will be the basis for a high-power (~10-kW) W-band extended-interaction-klystron amplifier that is currently under development.

Electron gun design for fundamental mode S-band multiple-beam amplifiers

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.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ~17% has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Modulating electron beams for an X band relativistic klystron amplifier

A large diameter thin annular intense relativistic electron beam was modulated at a frequency of 10 GHz. The electron beam propagated inside a narrow annular drift tube in which gaps feeding radial cavities were inserted. An external source injected microwave power of 180 kW into a single input cavity followed by a simple idler structure. A stable 10 kA rf current modulation was induced on the 400 keV electron beam.

Demonstration of a Wideband 10-kW Ka-Band Sheet Beam TWT Amplifier

A sheet-beam coupled-cavity traveling wave tube has produced over 10 kW of peak power at a center frequency of 34 GHz, with a 3-dB bandwidth of almost 5 GHz. The power of this amplifier is an order of magnitude higher than state-of-the-art conventional amplifiers of comparable frequency, bandwidth, and operating voltage (<;20 kV). This unprecedented performance is made possible by a unique, Naval Research Laboratory (NRL)-developed sheet electron beam along with a novel slow-wave interaction structure. High-current, low-voltage operation provides high gain per unit length and allows an interaction structure<;5-cm long to be used to achieve the desired gain of 15 dB at saturation. Measured performance agrees well with 3-D particle-in-cell simulations.

1.4: Design of a high-gain wideband high-power 220-GHz multiple-beam serpentine TWT

A new concept for a compact high-gain multiple-beam TWT amplifier is introduced. To illustrate the concept, we present the design of a three-beam 220-GHz serpentine TWT amplifier. MAGIC-3D particle-in-cell code simulation results predict that the device should be capable of a peak power of 73 W and saturated gain of 42 dB over an instantaneous bandwidth of 50 GHz (23%), when powered by three 100 mA, 20 kV, well-focused electron beams. This performance can be achieved in a very compact circuit length of only 1.5 cm - a significant advantage for THz electron beam devices where issues of fabrication tolerances, beam alignment, and electron interception are of critical importance.

Design of a G-band sheet-beam Extended-Interaction Klystron

The preliminary design of a four-cavity G-band sheet-beam Extended-Interaction Klystron (EIK) Circuit is presented. The circuit design has been performed with the self-consistent particle-in-cell (PIC) code MAGIC-3D. All cavities operate in the 2-π mode. The circuit is powered by a 520 mA, 16.5 kV sheet-electron beam. Output power of 453 W is achieved with an input power of 25 mW, corresponding to an electronic gain of 41.6 dB in a circuit length of approximately of 1.2 cm.