Joseph Santoru

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. >

Electromagnetic-wave absorption in highly collisional plasmas

An experimental and theoretical investigation of basic plasma‐physics processes relating to the absorption of electromagnetic waves in collisional plasmas is described. One‐way absorption of 63 dB at 4 GHz was demonstrated in a section of plasma‐loaded rectangular C‐band waveguide. The ultraviolet photoionization plasma‐production technique employed permits independent variation of the plasma‐density profile and electron‐collision frequency. A theoretical model for the absorption and scattering processes, which includes scattering contributions from the plasma‐vacuum interface, and partial reflections and collisional absorption in the bulk plasma, is in reasonable agreement with the experimental results.

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. >

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.

Recent progress in the development of plasma-filled traveling-wave tubes and backward-wave oscillators

Summary form only given. The presence of a controlled amount of background plasma inside microwave tubes can possibly lead to improvement in their characteristics beyond what is available in evacuated devices. In particular, recent results clearly demonstrated that the presence of plasma can significantly increase the bandwidth, efficiency and power handling capabilities of non-relativistic microwave oscillators and amplifiers and allow operation without a guiding magnetic field. In the present paper recent scientific advances in this field, both theoretical and experimental, are reviewed with emphasis on basic processes. We review some fundamental physical issues such as the formation of hybrid waves in plasma-filled slow-wave structures and the role of these modes in improving the beam/wave coupling; the beneficial role of AC space charge in plasma-filled devices and the effect of plasma on electron beam transport; and efficiency and bandwidth enhancement due to the presence of the plasma. Also reviewed are recent experimental results on plasma-loaded TWT amplifiers and BWO oscillators. Plasma-loaded microwave devices have the potential to advance the technological and scientific base of microwave tubes, and also to have an impact on commercial and industrial applications through the development of commercially viable technologies.

Short-pulse generator with a CROSSATRON/sup (R/) switch

We have developed a compact, high-voltage, short-pulse generator that employs an 8459H CROSSATRON/sup (R/) switch as the electronic switch. The generator provides 9-kV pulses at 3.3 kHz for 5-s bursts with pulse risetimes /spl les/20 ns and pulse widths /spl les/20 ns. Shot-to-shot stability of /spl les/10 ns over a 20-s burst has been demonstrated. For low repetition rates, a magnetic shockline can be used at the generator output to produce sharpened pulse risetimes of 550 ps.

High-repetition-rate, short-pulse generator using a planar CROSSATRON(R) switch

Summary form only given. We have developed a high-repetition-rate, high-voltage short-pulse generator that employs a new planar CROSSATRON(R) switch to discharge a low-inductance capacitor array into a load. Typical pulse parameters are 20-kV capacitor-charge voltage, 50-ns pulse width, 20-40-ns pulse risetime, and up to 120-kHz pulse-repetition rate in bursts of ten pulses. The burst length is limited by the capacitor charging power supply, not the intrinsic characteristics of the tube. High-repetition rate operation is enabled by a new planar CROSSATRON switch that uses a flat cathode, anode and grids, rather than the cylindrical electrode geometry used in other CROSSATRONs. CROSSATRON switches are plasma-discharge devices that conduct high current with low forward-voltage drop at high speeds. CROSSATRON switches also feature rapid recovery times that permit operation at high pulse-repetition frequencies. In fact, when configured in closing-only applications, as in the short-pulse generator, a maximum closing rate of di/dt=10/sup 11/ A/s can be obtained with voltage recovery times of <100 /spl mu/s.