Robert W. Schumacher

High-power microwave source based on an unmagnetized backward-wave oscillator

A unique, high-power microwave source, called PASOTRON (Plasma-Assisted Slow-wave Oscillator), has been developed. The PASOTRON utilizes a long-pulse E-gun and plasma-filled slow-wave structure (SWS) to produce high-energy microwave pulses from a simple, lightweight device that utilizes no externally-produced magnetic fields. The novel E-gun employs a low-pressure glow discharge to provide a stable, high current-density electron source. A high-perveance, multi-aperture electron accelerator produces an E-beam that is operated in the ion-focused regime; where the beam-produced plasma filling the SWS space-charge neutralizes the beam, and the self-pinch force compresses the beamlets to provide propagation through the SWS. The PASOTRON E-gun has produced beams with voltages of up to 220 kV and currents in excess of 1 kA for pulse lengths of over 100 /spl mu/sec. The PASOTRON HPM source normally operates in the TM/sub 01/ mode, and a unique mode converter has been developed to efficiently produce a TE/sub 11/ output mode with fixed polarization, The PASOTRON also has the ability to directly produce TE-mode radiation with a rotating output polarization, PASOTRON HPM sources have operated in L, S, C and X-bands, and have produced output powers in the 1 to 5 MW range in C-band at about 20% efficiency with pulse lengths of over 100 /spl mu/sec. >

High-current, low-pressure plasma-cathode electron gun

Plasma-cathode electron gun structures capable of operation in low-pressure, e.g., <5×10-3 Torr, ionizable gas environments are disclosed. They utilize a thermionic emitter within an enclosure with a partially transparent electrode defining a plasma face. Spaced anodes are disposed adjacent the electrode to extract an electron beam from the plasma face. A magnetic system forms an inward directed field, and a portion of the plasma electrons are directed through this field to enhance ionization efficiency.

Performance and pulse shortening effects in a 200-kV PASOTRON/sup TM/ HPM source

The PASOTRON/sup TM/ is a unique, high-power microwave source that uses a long-pulse (/spl les/100 /spl mu/s) plasma-cathode electron-gun and plasma-filled slow-wave structure (SWS) to produce high-energy microwave pulses. The device utilizes no externally-produced magnetic fields; relying on a beam-generated plasma channel in the SWS to transport the beam. Peak powers of up to 5 MW were previously reported in C-band using a rippled-wall waveguide SWS. Scaling experiments indicated that increasing the beam voltage above the 90 kV C-band operation produces significantly higher peak powers. We report results from an L-band PASOTRON/sup TM/ BWO designed to operate at 200 kV. The plasma-cathode E-gun built for this device generated beams with voltages of up to 225 kV and currents in excess of 1 kA for pulse lengths of up to 200 /spl mu/s. The L-band PASOTRON/sup TM/ BWO produced 10-20 MW of peak power in the TM/sub 01/ mode, which was converted in the output to a TE/sub 11/ mode with fixed polarization. The PASOTRON/sup TM/ also directly produced TE-mode radiation in the 5-10 MW power range with a rotating output polarization, the rate of which can be controlled externally. The peak power and poise width was limited by the stability of the plasma channel at high peak powers and excessive plasma generation in the SWS during the long pulse length.

PLASMA-ANODE ELECTRON GUN

The plasma‐anode electron gun (PAG) is an electron source in which the thermionic cathode is replaced with a cold, secondary‐electron‐emitting electrode. Electron emission is stimulated by bombarding the cathode with high‐energy ions. Ions are injected into the high‐voltage gap through a gridded structure from a plasma source (gas pressure ≤50 mTorr) that is embedded in the anode electrode. The gridded structure serves as both a cathode for the plasma discharge and as an anode for the PAG. The beam current is modulated at near ground potential by modulating the plasma source, eliminating the need for a high‐voltage modulator system. During laboratory tests, the PAG has demonstrated square‐wave, 17‐μs‐long beam pulses at 100 kV and 10 A, and it has operated stably at 70 kV and 2.5 A for 210 μs pulse lengths without gap closure.

High-power, single-beam plasma wave tube

Short-pulse, ultra-broadband sources of RF radiation are needed for a variety of new applications. To meet this demand, we have developed and optimized a single-beam Plasma Wave Tube (PWT), The PWT is a unique microwave/millimeter-wave source which utilizes the interaction between beamexcited electron plasma waves to generate kilowatt-power (/spl sim/10 kW) radiation at microwave to millimeter-wave frequencies with a beam-to-radiation conversion efficiency of /spl ges/0.4%. In a single-beam PWT, an electron beam (/spl les/40 kV, /spl ges/200 A, 5-to-20-/spl mu/s pulse width) is injected into a gas-filled (e,g., hydrogen) cylindrical waveguide. The beam first ionizes the gas to generate a plasma, and then nonlinearly interacts with the plasma to generate radiation from 6-to-60 GHz. Slew rates of up to 7 GHz//spl mu/s have been measured during a single beam pulse. The radiation has a wide instantaneous bandwidth, typically 10 GHz or wider. Electron-beam transport through the waveguide is accomplished with no externally applied magnetic fields because the beam space charge is cancelled by the background plasma. >

LOW-PRESSURE PLASMA OPENING SWITCHES

Plasma switch devices such as thyratrons, spark gaps, and ignitrons have enjoyed great success in closing-switch applications where high currents must be switched from high voltage on short time scales with low forward-voltage drop. However, plasma devices are not usually considered as repetitive fast opening switches. Although they can be opened by commutation (or by counterpulse techniques), commutation requires significant power, and recovery times are generally long: ≳ 10 μs. Ideally, a plasma switch is desired which can perform both closing and opening functions at high speed and high repetition rates under low-power control, like a hard-vacuum tube, but without the large forward-voltage drop and low thermionic-cathode current restrictions which are associated with hard tubes.

Microwave/millimeter-wave generation in a counterstreaming-beam-plasma system

Radiation from 8 to 60 GHz is generated by injecting counterstreaming, high‐power (up to 50 keV and 4 A) electron beams into an unmagnetized, plasma‐loaded waveguide. The radiation is emitted at twice the plasma frequency and is amplitude modulated on the ion plasma frequency time scale. The beam‐to‐millimeter‐wave power conversion efficiency at 31 GHz is ≳0.04%. A turbulence theory model is consistent with the radiation characteristics and power scalings.

PASOTRON high-energy microwave source

The authors describe the operation and performance of a high-energy microwave source called the PASOTRON (plasma-assisted, slow-wave oscillator). The PASOTRON is a unique combination of a novel electron gun, and plasma-filled slow-wave structure which creates a source capable of generating 100- mu s-long RF pulses maintained at power levels of a few megawatts without the use of any magnetic focusing fields. A Hughes hollow-cathode-plasma electron gun is used to produce long, high-power beam pulses from which energy is efficiently extracted and converted into electromagnetic radiation. The authors present results which show that RF output power is in the 1-to-5 MW range, for RF pulse lengths up to 120 mu s from a PASOTRON tube designed to operate in the C-band frequency range. The integrated RF energy per pulse is up to 500 J, and the electron-beam to microwave-radiation power-conversion efficiency is approximately 20%. Instantaneous bandwidth measurements confirm that, for the long RF pulse duration, the PASOTRONs oscillation center frequency is maintained in a narrow line <3 MHz.<<ETX>>

PASOTRON™ high-energy microwave source

A unique, high-energy microwave source, called PASOTRON for Plasma-Assisted Slow-wave Oscillator has been developed. Similar to the Backward Wave Oscillator (BWO), the PASOTRON spontaneously generates microwave radiation by efficiently converting electron beam energy into electromagnetic radiation. The PASOTRON, however, utilizes a novel E-gun and plasma-filled slow-wave structure (SWS) to produce and propagate very long, high-power beam pulses which require no axial magnetic fields for transport. The long electron beam pulses are obtained from a Hughes Hollow-Cathode Plasma (HCP) E-gun, which employs a low-pressure glow discharge to provide a stable, high current-density electron source. Electrons from this source are accelerated through a multi-aperture array to produce a large area, high-current beam consisting initially of many individual beamlets. Since the device is operated in the ion focused regime, the plasma filling the SWS, space-charge neutralizes the beam, and the Bennett self-pinch compresses the beamlets and increases the beams current density. Experimental results from a PASOTRON tube designed to operate in a TM01 mode at C-band frequencies when driven by a 50-to-100-kV, 50-to-250 A electron beam are reported. Results show output power is in the l-to-4 MW range, for rf pulselengths up to 100 μsec, corresponding to an integrated energy per pulse of up to 300 J. Calculations show the E-beam to microwave-radiation power-conversion efficiency is ~20%. Instantaneous bandwidth measurements further reveal that for the duration of the long ripulse the PASOTRONs oscillation center frequency maintains a narrow line <;3MHz.

Low voltage drop plasma switch for inverter and modulator applications

A low forward voltage drop plasma switch has been developed for high‐efficiency inverter and modulator applications. The switch, called the HOLLOTRON, is based on a grid‐controlled, thermionic hollow‐cathode discharge. A low forward voltage drop (10–20 V) is achieved by operating the hollow‐cathode discharge in a static gas pressure of xenon. The dense plasma generated in the Ba‐oxide dispenser hollow cathode is spread over a relatively large control grid area by a diverging magnetic field superimposed on the discharge. Interruption of the discharge current at high current densities (≳4 A/cm2) over the grid area is achieved by biasing the control grid sufficiently negative with respect to the plasma. The HOLLOTRON switch has demonstrated voltage stand‐off of up to 20 kV, switching times of ≤0.3 μs, and pulse repetition frequencies of 20 kHz at 50% duty.