Presented By: Nuclear Engineering & Radiological Sciences
PhD Defense: Steven Exelby
Design, Development, and Experiments on the Recirculating Planar Crossed-Field Amplifier
Title: Design, Development, and Experiments on the Recirculating Planar Crossed-Field Amplifier
Chair: Prof. Ronald Gilgenbach
Abstract: The Recirculating Planar Crossed-Field Amplifier (RPCFA) is an adaptation of the Recirculating Planar Magnetron (RPM) that has been the focus of research at the University of Michigan’s Plasma, Pulsed Power and Microwave Laboratory for nearly a decade.1 The RPM is a high power microwave (HPM) source that utilizes an innovative geometry to overcome the high power generating limitations of existing magnetrons while maintaining the characteristic high efficiencies of these devices. The large planar cathode emits higher currents, leading to higher power, than a rod cathode at charge limited densities due to its increased emitting surface area. The recirculating interaction space effectively recycles the electron beam, which preserves the electronic efficiency of the device. The RPCFA utilizes a geometry similar to the RPM and is likewise expected to hold the same advantages over traditional CFAs that the RPM holds over traditional magnetrons. Additionally, the planar amplifying structure permits the injection and extraction ports be placed physical far away. This allows the Brillouin hub to demodulate over a long distance which minimizes feedback, a primary power limiting mechanism in cylindrical CFAs. The focus of this experiment is to design and test a prototype RPCFA which demonstrates these features.
The slow wave structure (SWS) is the key component to a magnetron and CFA which is responsible for promoting the synchronous interaction between the RF wave and the Brillouin hub. The SWS in a CFA is designed with the following criteria: First, the SWS must act as a transmission line over the band of frequencies (typically around 10% bandwidth) the device is expected to amplify. The transmission should minimize radiative and resistive losses and minimize reflection and any point in the RF circuit. Thus, the SWS should be inherently low Q. Second, the SWS must generate periodic fringing RF fields in response to an excitation in the amplification band. Finally, the phase velocity of these fringing fields must be slowed to the speed of the Brillouin hub, in this application that will be approximately 0.2 to 0.3c. Using Ansys HFSS driven modal simulation, a SWS was designed that satisfied the criteria listed. This SWS was reproduced using the particle-in- cell code, MAGIC, to simulate operation.2 MAGIC simulation showed the device could generate amplification of a 1.3 MW signal at 3 GHz to a steady state output power of 29 MW, a gain of nearly 13.5 dB. MAGIC simulation also predicted approximately 13% bandwidth, zero drive stability (meaning output power was dependent on the presence of an input signal), and nearly linear amplification in the range of testable input powers.
The RPCFA was then prototyped, employing the "lost wax" additive manufacturing technique to build the intricate SWS, and traditional machining techniques to fabricate the supporting components. The RPCFA was tested experimentally to reproduce the results of MAGIC simulation. Zero-Drive stability was observed and a minimum amplifiable power was established to be around 100 W. Moderate power (10’s of kW) input signals showed moderate levels of amplification, 7.87 dB with high levels of variability, σ = 2.74 dB. At moderate power the bandwidth was measured to be 15%, slightly exceeding the bandwidth predicted by simulation. At high power input drive (150+ kW) the RPCFA demonstrated much more consistent amplification and increased mean gain of 8.71 dB with σ = 0.63 dB. The RPCFA consistently reached peak output powers around 5 MW before exhibiting RF breakdown of the RF circuit. This RF breakdown is characterized by the abrupt termination of the transmitted RF signal. This RF breakdown represents the current limit for output power on the RPCFA. Future experiments will make attempts to remove this limitation, redesigning the S-band structure to accommodate high RF fields or moving to lower frequencies where features will be inherently larger.
[1] R.M. Gilgenbach, Y.Y. Lau, D.M. French, B.W. Hoff, J. Luginsland, and M. Franzi, "Crossed field device," U.S. Patent US 8 841 867B2, Sep. 23, 2014.
[2] Developed by Alliant Techsystems
Chair: Prof. Ronald Gilgenbach
Abstract: The Recirculating Planar Crossed-Field Amplifier (RPCFA) is an adaptation of the Recirculating Planar Magnetron (RPM) that has been the focus of research at the University of Michigan’s Plasma, Pulsed Power and Microwave Laboratory for nearly a decade.1 The RPM is a high power microwave (HPM) source that utilizes an innovative geometry to overcome the high power generating limitations of existing magnetrons while maintaining the characteristic high efficiencies of these devices. The large planar cathode emits higher currents, leading to higher power, than a rod cathode at charge limited densities due to its increased emitting surface area. The recirculating interaction space effectively recycles the electron beam, which preserves the electronic efficiency of the device. The RPCFA utilizes a geometry similar to the RPM and is likewise expected to hold the same advantages over traditional CFAs that the RPM holds over traditional magnetrons. Additionally, the planar amplifying structure permits the injection and extraction ports be placed physical far away. This allows the Brillouin hub to demodulate over a long distance which minimizes feedback, a primary power limiting mechanism in cylindrical CFAs. The focus of this experiment is to design and test a prototype RPCFA which demonstrates these features.
The slow wave structure (SWS) is the key component to a magnetron and CFA which is responsible for promoting the synchronous interaction between the RF wave and the Brillouin hub. The SWS in a CFA is designed with the following criteria: First, the SWS must act as a transmission line over the band of frequencies (typically around 10% bandwidth) the device is expected to amplify. The transmission should minimize radiative and resistive losses and minimize reflection and any point in the RF circuit. Thus, the SWS should be inherently low Q. Second, the SWS must generate periodic fringing RF fields in response to an excitation in the amplification band. Finally, the phase velocity of these fringing fields must be slowed to the speed of the Brillouin hub, in this application that will be approximately 0.2 to 0.3c. Using Ansys HFSS driven modal simulation, a SWS was designed that satisfied the criteria listed. This SWS was reproduced using the particle-in- cell code, MAGIC, to simulate operation.2 MAGIC simulation showed the device could generate amplification of a 1.3 MW signal at 3 GHz to a steady state output power of 29 MW, a gain of nearly 13.5 dB. MAGIC simulation also predicted approximately 13% bandwidth, zero drive stability (meaning output power was dependent on the presence of an input signal), and nearly linear amplification in the range of testable input powers.
The RPCFA was then prototyped, employing the "lost wax" additive manufacturing technique to build the intricate SWS, and traditional machining techniques to fabricate the supporting components. The RPCFA was tested experimentally to reproduce the results of MAGIC simulation. Zero-Drive stability was observed and a minimum amplifiable power was established to be around 100 W. Moderate power (10’s of kW) input signals showed moderate levels of amplification, 7.87 dB with high levels of variability, σ = 2.74 dB. At moderate power the bandwidth was measured to be 15%, slightly exceeding the bandwidth predicted by simulation. At high power input drive (150+ kW) the RPCFA demonstrated much more consistent amplification and increased mean gain of 8.71 dB with σ = 0.63 dB. The RPCFA consistently reached peak output powers around 5 MW before exhibiting RF breakdown of the RF circuit. This RF breakdown is characterized by the abrupt termination of the transmitted RF signal. This RF breakdown represents the current limit for output power on the RPCFA. Future experiments will make attempts to remove this limitation, redesigning the S-band structure to accommodate high RF fields or moving to lower frequencies where features will be inherently larger.
[1] R.M. Gilgenbach, Y.Y. Lau, D.M. French, B.W. Hoff, J. Luginsland, and M. Franzi, "Crossed field device," U.S. Patent US 8 841 867B2, Sep. 23, 2014.
[2] Developed by Alliant Techsystems
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