Microwave Power Products, Inc. (MPP, previously a division of Communications and Power Industries) has designed, fabricated, and factory tested a dual-frequency gyrotron and matching optics unit capable of producing over 600 kW of continuous RF output at either 70 GHz or 105 GHz, by adjusting operating conditions to excite either the TE11,3 or the TE 14,5 mode in a cylindrical interaction cavity. Fabrication of two identical gyrotrons and two matching optics units is complete. The first gyrotron has undergone initial testing and has been shipped to the customer site. The second gyrotron is currently (February, 2025) in the initial stages of factory testing. In this paper, design analyses are summarized, and test results are presented.

Currently a large amount of debris, potentially lethal to resident space assets, is orbiting Earth. While a handful of countries keep track of it, a significant gap is emerging between the debris density, its growth, and the ability to track it and monitor it on a continuous basis. This article takes a close look at the key high-power radars that are entrusted with this difficult task and examines and assesses the high-power transmitter technology and its advancement over the past four decades.

MPP has recently developed a 50 kW peak power W-band gyro-TWT amplifier with a novel internal mode converter capable of efficiently transforming the TE01 operating mode to a high-quality Gaussian beam over the 92 – 100 GHz frequency band. The gyro-TWT was successfully operated at 48.8 kW peak output powers and duty factors up to 30%. Design details and the results of experimental demonstrations, including infrared images of the output beam, are described below.

Communications & Power Industries (CPI) has completed the detailed design of a dual-frequency gyrotron and matching optics unit capable of producing over 600 kW of continuous RF output at either 70 GHz or 105 GHz, by adjusting operating conditions to excite either the TE 11 , 3 or the TE 14 , 5 mode in a cylindrical interaction cavity. Fabrication of two identical gyrotrons and two matching optics units is nearly complete, and factory testing of the first gyrotron is anticipated to begin in the spring of 2024. In this paper, design analyses will be discussed, and preliminary test results, if available, will be presented.

The VGE-8071A gyrotron, currently in fabrication, is designed to produce 600 kW of continuous RF output power in a Gaussian output beam at either 70 GHz or 105 GHz. The gyrotron circuit employs the TE 11,3 interaction mode at 70 GHz, or the TE 14,5 interaction mode at 105 GHz. To facilitate conversion of either operating mode into a Gaussian output beam, a dual-frequency internal converter is employed, consisting of a numerically optimized dimpled-wall launcher, two phase correcting mirrors, and a final toroidal mirror to direct the RF beam through the gyrotron output window. An external 2-mirror matching optics unit couples either output beam to an external transmission line system, using frequency-specific swappable mirrors for precise alignment and optimal coupling of the beam to the transmission line.

CPI, partnered with Bruker, has been engaged in a long-standing program to develop gyrotrons for Bruker's commercial dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) spectrometers. The Bruker-CPI development program, started more than 15 years ago, has resulted in a family of robust and reliable continuous wave (cw) high-frequency gyrotrons. The design features and measured performance of the Bruker-CPI gyrotrons at 263, 395, 527, and 593 GHz, all of which have demonstrated over 50 W cw output power in high-quality Gaussian beams, will be described.

CPI is currently involved in the design and development of gyrotron oscillators and amplifiers for a variety of applications. A family of low-power continuous wave (cw) gyrotrons for commercial dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) spectrometer is currently in production. Key features, as well as results from the experimental demonstrations of low-power gyrotrons at 263 GHz, 395 GHz, 527 GHz, and 593 GHz for this application will be discussed. In addition, megawatt class gyrotrons at 117.5 and 140 GHz are in production for tokamak and stellarator electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) systems. Design philosophies and design challenges, operational experience, as well as the results of experimental demonstrations at the fusion facilities will be presented.

A dual-frequency gyrotron capable of operation in the TE28,7 cavity interaction mode at 140 GHz, or in the TE22,5 mode at 104 GHz, has been developed for use in electron cyclotron heating in the W7-X stellarator at IPP Greifswald. The gyrotron incorporates an internal converter design that has been numerically optimized to convert either of the two operating modes into a high-quality Gaussian output beam. During short-pulse factory testing, the gyrotron produced 900 kW at 140 GHz, and 520 kW at 104 GHz. After delivery to IPP, the gyrotron was conditioned to long-pulse operation at 140 GHz, demonstrating 30-minute pulses at several power levels up to 811 kW, and producing ten consecutive ten-minute pulses at 811 kW as well. After long-pulse capabilities were demonstrated at 140 GHz, IPP requested an analysis of the feasibility of operating the gyrotron (without internal modification) in an additional mode with a frequency near 175 GHz. Several potential interaction modes were evaluated to determine the required operating parameters for excitation of these modes, and to assess the expected interaction efficiency, output power, internal diffraction losses, and output beam quality. The most promising modes appear to be the TE33,9 (173 GHz) and the TE34,9 (176 GHz), which should generate 400-500 kW of RF in a suitable magnet capable of producing the necessary 7.1 T field required for operation at these higher frequencies. Because the existing gyrotron’s internal converter was not optimized for these modes, however, internal losses are expected to be higher than usual (up to 7%), and the output beam pattern will require external phase-correction in order to produce a Gaussian beam. A feasibility analysis for such external phase correction has been performed, demonstrating that a high-quality beam can be recovered using numerically synthesized external phase-correcting mirror surfaces.

In megawatt-class gyrotrons, the residual energy of the electron beam must be dissipated in the collector without compromising the vacuum integrity of the device, necessitating the use of various methods for lowering the peak power density to acceptable levels. Secondary electron emission from the collector surface can redistribute the power deposition profile of the incident (primary) beam, which can either be beneficial or detrimental. Secondary electrons may also be re-accelerated toward the gyrotron body, and, if not magnetically reflected, may interfere with the operation of the interaction circuit. Here, simulations of a megawatt-class gyrotron collector, including the effects of voltage depression, magnetic sweeping, and secondary/reflected electrons, are presented, to assess the potential impact on gyrotron operation and longevity.

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) spectroscopy is a popular and growing application for high-frequency, continuous wave (cw) gyrotrons that produce powers in the range of tens to hundreds of watts at millimeter-wave and terahertz frequencies. As an original equipment manufacturer of gyrotrons for Bruker Biospin DNP systems, CPI has developed a series of high frequency gyrotrons, the most recent of which is a 50 W, 593 GHz device for a 900 MHz NMR spectrometer. Design and performance details for the 593 GHz gyrotron will be discussed.

Advances in both solid-state and vacuum-electron-based sources at frequencies greater than 200 GHz have been a key factor in the recent improvements in solid-state dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) instrumentation. The current state of the art in solid-state sources and vacuum-electron devices (VEDs), such as extended interaction oscillators (EIOs), extended interaction klystrons (EIKs), and gyrotrons for DNP applications are described. The key features and design aspects of gyrotrons, which are presently the most promising DNP sources for high-field NMR systems, are detailed. In addition, the current capabilities of high-performance DNP gyrotron sources are illustrated. The status of ongoing research efforts in DNP gyrotrons and future directions are discussed.

The ECH system on DIII-D is continuing to be upgraded, while simultaneously being operated nearly daily for plasma experiments. The latest major hardware addition is a new 117.5 GHz gyrotron, which generated 1.7 MW for short pulses during factory testing. A new gyrotron control system based on Field Programmable Gate Array (FPGA) technology with very high speed system data acquisition has significantly increased the flexibility and reliability of individual gyrotron operation. We have improved the performance of the fast mirror scanning, both by increasing the scan speeds and by adding new algorithms for controlling the aiming using commands generated by the Plasma Control System (PCS). The system is used for transport studies, ELM control, current profile control, non-inductive current generation, suppression of MHD modes, startup assist, plasma density control, and other applications. A program of protective measures, which has been in place for more than two years, has eliminated damage to hardware and diagnostics caused by overdense operation. Other activities not directly related to fusion research have used the ECH system to test components, study methods for improving production of semiconductor junctions and materials, and test the feasibility of using ground based microwave systems to power satellites into orbit.

Advances in dynamic nuclear polarization (DNP) instrumentation and methodology have been key factors in the recent growth of solid-state DNP NMR applications. We review the current state of the art of solid-state DNP NMR instrumentation primarily based on available commercial platforms. We start with a general system overview, including options for microwave sources and DNP NMR probes, and then focus on specific developments for DNP at 100K with magic angle spinning (MAS). Gyrotron microwave sources, passive components to transmit microwaves, the DNP MAS probe, a cooling device for low-temperature MAS, and sample preparation procedures including radicals for DNP are considered.

CPI has been developing megawatt-class gyrotrons for fusion plasma heating and current drive for a range of frequencies in the mm-wave regime. Gyrotrons at 110 GHz, 117.5 GHz, 140 GHz, and 170 GHz have been designed and fabricated, and have undergone initial factory testing. Some systems have been delivered for commissioning at fusion research facilities, while several await modification following initial test results. Design features and available test data for each design will be presented.

A 140 GHz gyrotron capable of producing output powers up to 900 kW for 1000-second pulses has been developed at CPI. Factory testing demonstrated 1000-second operation at the 25 A CW current limit of the test facility, at which the output power was about 500 kW, and demonstrated 900 kW operation for short pulses. The gyrotron was then shipped to Hefei, China, and installed as part of the electron cyclotron heating and current drive system for the EAST tokamak. Commissioning of the gyrotron at the EAST site is currently in progress, with the goal of demonstrating operation at full parameters (900 kW for 1000-second pulses).

A 140 GHz gyrotron capable of producing output powers up to 900 kW for 1000 second pulse durations has recently been developed and demonstrated at CPI. In July of 2014, the gyrotron was shipped to Hefei, China for use in the electron cyclotron heating and current drive (ECH&CD) system at the EAST tokamak. Installation of the gyrotron at EAST is complete and initial operation is underway. Design features of the gyrotron, results of the experimental demonstration at CPI, and the status of demonstrations in China will be discussed.

Two megawatt-class gyrotrons at frequencies of 117.5 GHz and 170 GHz have recently been fabricated and tested at CPI. The 117.5 GHz gyrotron was designed to produce up to 1.8 MW, for 10-second pulses. The 170 GHz gyrotron is specified as a 500 kW CW system, but has been designed with the goal of generating up to 1 MW CW. Thus far both gyrotrons have achieved the peak power goals in initial testing. Further work is aimed at extending operation to the required pulse length.

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) spectroscopy has emerged as a new and growing application for high-frequency, continuous wave (cw) vacuum electronic devices capable of producing powers in the range of tens to hundreds of watts at millimeter-wave and terahertz frequencies. As an original equipment manufacturer of gyrotrons for Bruker Biospin DNP spectrometers, CPI has developed continuous wave (cw) gyrotrons at 263 GHz, 395 GHz, and 527 GHz. Design and performance details for the second-harmonic 395 and 527 GHz gyrotrons will be presented.

A gyrotron operating at the second cyclotron harmonic in a magnetic field of 9.7 T has produced a CW output power of 25 W at 527 GHz. The gyrotron is now being employed in Dynamic Nuclear Polarization (DNP) Applications. The gyrotron design and results of the tests are presented.

An upgrade of the electron cyclotron heating system on DIII-D to almost 15 MW is being planned which will expand it from a system with six I MW 110 GHz gyrotrons to one with ten gyrotrons. A depressed collector 1.2 MW 110 GHz gyrotron is being commissioned as the seventh gyrotron. A new 117.5 GHz 1.5 MW depressed collector gyrotron has been designed, and the first article will be the eighth gyrotron. Two more are planned, increasing the system to ten total gyrotrons, and the existing 1 MW gyrotrons will subsequently be replaced with 1.5 MW gyrotrons.Communications and Power Industries completed the design of the 117.5 GHz gyrotron, and are now fabricating the first article. The design was optimized for a nominal 1.5 MW at a beam voltage of 105 kV, collector potential depression of 30 kV, and beam current of 50 A, but can achieve 1.8 MW at 60 A. The design of the collector permits modulation above 100 Hz by either the body or the cathode power supply, or both, while modulation below 100 Hz must use only the cathode power supply.General Atomics is developing solid-state power supplies for this upgrade: a solid-state modulator for the cathode power supply and a linear high voltage amplifier for the body power supply. The solid-state modulator has series-connected insulated-gate bipolar transistors that are switched at a fixed frequency by a pulse-width modulation regulator to control the output voltage. The design of the linear high voltage amplifier has series-connected transistors to control the output voltage, which was successfully demonstrated in a proof-of-principle test at 2 kV. The designs of complete power supplies are progressing.The design features of the 117.5 GHz 1.5 MW gyrotron and the solid-state cathode and body power supplies will be described and the current status and plans are presented. (C) 2013 Elsevier B.V. All rights reserved.