Franklin N. Wood

A gyrotron-traveling-wave tube amplifier experiment with a ceramic loaded interaction region

The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

A TE/sub 11/ K/sub a/-band gyro-TWT amplifier with high-average power compatible distributed loss

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.

Experimental investigation of a high power, two-cavity, 35 GHz gyroklystron amplifier

Experiments on a two-cavity gyroklystron amplifier are performed to demonstrate high power coherent radiation amplification at 34.95 GHz. Experiments show a saturated efficiency of 37%, a bandwidth of 0.36%, and a gain of 23.6 dB corresponding to peak radiation output power of 210 kW. Experimental results are in good agreement with large signal simulations. Calculations also show that a stagger-tuned three-cavity circuit increases the bandwidth to more than 0.9%.

Demonstration of an S-band, 600-kW fundamental-mode multiple-beam klystron

We present initial experimental results from the successful operation of a 600-kW peak, fundamental-mode multiple-beam klystron (MBK). The eight-beam device operates at a cathode voltage of /spl sim/45 kV and a total beam current of /spl sim/32 A with an axial guiding magnetic field of 1.8-2.2 kG. In the absence of radio-frequency (RF) drive, the measured beam transmission is in excess of 99%; at a driven frequency of 3.25 GHz, the measured beam transmission at saturation is /spl ges/97%, where the four-cavity circuit generates a peak power of /spl sim/600 kW with an electronic efficiency of 40%. The measured beam transport and RF performance are in excellent agreement with predictions made by the three-dimensional gun/collector code, MICHELLE, and the large-signal klystron code, TESLA. The accuracy of the design codes enabled the achievement of a working device in a single hardware design pass.

Experimental studies of a four-cavity, 35 GHz gyroklystron amplifier

The experimental results from a four-cavity, Ka-band gyroklystron amplifier operating in the(TE)/sub 011/ cylindrical mode at the fundamental of the cyclotron frequency are presented. A peak output power of 208 kW at 31.90 GHz, with a 3-dB bandwidth of 178 MHz (0.5%), an electronic efficiency of 30%, and a saturated gain of 53 dB was obtained with a 72-kV, 9.6-A electron beam at a magnetic field of 12.95 kG and a measured beam velocity ratio of 1.36. The magnetic field was found to have a strong influence on the power-bandwidth tradeoff. At 13.35 kG, a peak output power of 174 kW at 31.90 GHz, with a 3-dB bandwidth of 240 MHz (0.7%), an electronic efficiency of 25%, and a saturated gain of 56 dB was obtained with the same beam parameters. The gyroklystron amplifier was unconditionally stable at these operating points and stability studies are reported. Experimental results were found to be in excellent agreement with large signal simulations.

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.

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.

Experimental Study and Analysis of an S-Band Multiple-Beam Klystron With 6% Bandwidth

We present experimental results and analyses of an eight-beam five-cavity multiple-beam klystron (MBK) operating at a center frequency of ~3.2 GHz. The device met its performance goals in its first hardware implementation, generating a peak RF output power of 600 kW and a 3-dB bandwidth of ~6%. The circuit was modeled with TESLA, a 2.5-D large-signal klystron/MBK code that was extended to enable simulations of the low- Q multiple-gap cavities used to increase the bandwidth. Details of the model and underlying theory are described, and the simulation results are compared with experimental measurements. The good agreement between the model and the experiment provides a validation for our tools and techniques that will be used in the design of future devices.

3.4: Sheet beam stick for low-voltage W-band extended interaction klystron (EIK)

The stable transport of high-perveance, low-voltage sheet electron beams is a key requirement for the successful development of compact, high-power sheet beam amplifiers. We describe a beam stick to demonstrate the transport of such a beam (19.5 kV, 3.5 A) in a solenoidal magnetic field of about 8.5 kG. The beam stick consists of a novel sheet beam gun having single-plane convergence of a factor of ∼30, a permanent magnet solenoid, a 1.8-cm-long × 5 mm wide × 0.4 mm high beam tunnel, and an isolated collector. The engineering design was based closely upon MICHELLE and MAGIC-3D simulations of a W-band extended interaction klystron.