Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.     

A multigigawatt X-Band relativistic backward wave oscillator with a modulating resonant reflector

Effective generation regime with a high output pulse power has been experimentally realized in a relativistic backward wave oscillator (RBWO) with a resonant reflector and a slow-wave system having a diameter 1.6 times the radiation wavelength. At a guiding magnetic field of 4.5 T, the maximum peak power amounted to 4.3 GW at a frequency of 9.4 GHz, an efficiency of 31%, and a microwave pulse duration of 22 ns.     

A periodic pulsed relativistic backward wave oscillator mechanically tunable in an expanded frequency band

We have studied a periodic pulse train regime (1 s, 50 Hz) of a relativistic backward wave oscillator with a resonant reflector, which can be mechanically tuned from pulse to pulse within a frequency band of 9% on a level of ?3 dB of the maximum in the entire microwave peak power range in a magnetic field of 0.36 T. The maximum peak power in a pulse train amounted to 2.5 GW at a frequency of 3.6 GHz, an efficiency of 20%, and a microwave pulse duration of 20 ns.     

Generation of gigawatt 10-GHz pulses with stable phase

The generation of microwave pulses in a 10-GHz range has been studied in a nonstationary relativistic backward wave oscillator (BWO) operating at a pulse train repetition rate of up to 300 Hz. Regimes with a stabilized phase of the high-frequency filling of pulses with respect to the accelerating voltage pulse front have been observed at a BWO peak output power of ～1 and 3 GW. In pulse trains with a length of 10–100 s, the average output microwave power reached ～1 kW.     

Relativistic Gigawatt BWT microwave pulse duration increased upon treating the slow-wave structure surface with a low-energy high-current electron beam

A new method of insulator surface treatment, which allows the electric strength of the material to be increased, is proposed and verified. Using this method for treating the slow-wave structure of a relativistic backward wave tube (BWT), the microwave pulse duration was increased from ～6 to ～30 ns at an output BWT radiation power of 3 GW.     

A relativistic backward wave oscillator with a modulating resonance reflector

A high-efficiency relativistic microwave source based on a backward wave oscillator (BWO) with a resonance reflector has been studied by experimental and numerical methods. The BWO is capable of generating 12-ns pulses with a carrier frequency of 9.93 GHz at an output radiation power of 0.75 ± 0.11 GW. For the BWO pumped by a high-current electron beam from a SINUS-6K accelerator, the regime with relatively high pulse-to-pulse stability is characterized by a 40% efficiency of power conversion.     

High-current relativistic electron beam transport in a drift tube in the absence of an external magnetic field

The current of an intense relativistic electron beam emitted from a metal-dielectric multiedge cathode that passes through an anode grid and is transported in a smooth-wall drift tube in the absence of an external magnetic field has been measured at the collector. In these experiments, the electron flux was transported over a distance twice as large as the drift tube diameter. An optimum cathode and storage electrode configurations have been found that allowed the efficiency of the relativistic Cherenkov microwave oscillator without a guiding magnetic field to be increased up to 10% at a maximum output pulse radiation power of 1.3 GW and a working frequency of 4.03 GHz.     

High-efficiency subnanosecond microwave pulse generation in a relativistic backward wave tube

The regime of nonstationary oscillations with a short-time power burst, which is typical of the initial stage of a transient process developed when a beam current significantly exceeds the starting level, is studied in a relativistic backward wave tube (BWT) operating in the millimeter wavelength range. The results of numerical calculations allowed conditions to be established that provide for a nearly 90% efficiency of the power transfer from an electron beam with the parameters 300 keV, 2 kA, 1 ns to the microwave pulse with a duration of 8–10 periods of the microwave field oscillations. The experiments showed the possibility of generating such pulses with a width of 200–250 ps and a power of up to ∼400 MW at a central frequency of about 38 GHz.     

Voltage-tuned relativistic backward wave oscillator

A relativistic backward wave oscillator for the 10-GHz range with the oscillation frequency tuned within about 5% by changing the accelerating voltage from 600 to 350 kV has been developed. Discrete variations in the voltage and the corresponding frequency tuning from pulse to pulse is rapidly performed by changing the anode-cathode distance in the vacuum diode without breaking vacuum in the working volume. During this, the electron beam power remains almost constant, while the output microwave power varies within 0.4–0.8 GW. The introduction of a dielectric cylinder into the accelerating gap provides a smooth voltage drop from 600 to 350 kV with the corresponding frequency tuning during a 20-ns pulse.     

Generation of chaotic microwave pulses in a ring system based on a klystron power amplifier and a nonlinear delay line on magnetostatic waves

The autonomous generation of stationary chaotic microwave pulse trains in a self-oscillating ring system with a multicavity klystron power amplifier operating in a small-signal regime and a wideband non-linear delay line on surface magnetostatic waves has been experimentally studied. It is established that the characteristics of generated chaotic microwave pulses can be controlled by varying the electron beam current and accelerating voltage in the klystron.     

Self-modulated generation observed in a delayed feedback relativistic microwave gyrotron

Self-modulated generation regimes were studied in a delayed feedback relativistic gyrotron operating on the H ₀₁ mode with a central frequency of 9.2 GHz. At a fixed electron beam energy of 230 keV, an increase in the electron beam current from 10 to 45 A led to the transition from a stationary to periodic self-modulated generation regime. The modulation period was about 16 ns, while the relative amplitude of the modulation increased in proportion to the beam current, reaching up to 90%. The microwave pulse duration exceeded 6 μs at an average power of up to 1 MW. The experimental data obtained agree well with the results of simulation using the PIC code KARAT.     

Relativistic Cherenkov microwave oscillator without a guiding magnetic field

A relativistic Cherenkov microwave oscillator without a guiding magnetic field has been designed, constructed, and tested in which a continuous cylindrical electron beam propagates in a short (L ≈ 3λ, λ being the radiation wavelength) resonant slow-wave structure. The electron beam is energy-modulated at the input of the interaction space, which provides conditions for the energy exchange at a wave phase velocity exceeding the particle velocity. The effective beam-wave coupling is provided by a nearly homogeneous profile of the longitudinal electric field component of the synchronous wave in the interaction space cross section. The efficiency of power conversion from high-current electron beam to electromagnetic radiation at E₀₁ mode is about 8% at a maximum output pulse radiation power of 1.2 ± 0.3 GW and a working frequency of 4.05 GHz.     

Microwave-energy extraction from a resonator via oversized interference switch

We have studied an interference switch based on an oversized H-plane rectangular waveguide T-junction. Conditions necessary for effective operation of the switch as an energy extractor are evaluated. Microwave pulses of 3.5-ns duration with 2.8-MW power at a gain of 17.5 dB have been obtained using a prototype microwave compressor for the 3-cm waveband with a 72 × 34 mm² waveguide resonator and a 58 × 25 mm² waveguide switch. It is shown that, using the proposed switch, it is possible to obtain RF pulses with a power of up to ～0.1 GW in the 3-cm band and ～1 GW in the 10-cm band.     

Synchronous extraction of microwave energy from cavities through a packet of interference switches

Synchronous extraction of energy from cavity resonators for X-band microwave radiation through a compact packet of five interference switches based on H tees has been experimentally analyzed. It is shown that switches can be completely synchronized and the synchronization conditions are determined. Microwave pulses have been generated upon synchronous extraction of energy from five single-mode cavities (power ～0.8 MW, gain ～12 dB, and width ～3.2 ns) and from one superdimensional cavity (power ～2.2 MW, gain ～16.5 dB, and width ～3.5 ns). The operation limits of X- and S-band microwave compressors with extraction of energy through a packet are estimated.     

Mechanically tuned relativistic backward wave oscillator

A high-power relativistic microwave oscillator with low magnetic field is created based on a backward wave oscillator (BWO) with resonance reflector. For fixed parameters of the slow-wave structure (SWS) and the electron beam, the oscillation frequency of this BWO can be mechanically tuned within a 12% band by moving the resonance reflector relative to the SWS. A maximum output pulse radiation power of 4±1 GW at a working frequency of 3.6 GHz is achieved with a magnetic field of 4.6 kOe.     

Principles of high current electron beam acceleration

High current pulsed electron accelerators operate with beam currents exceeding 1 kA, pulse lengths from 20 ns to 1 μs, and output energies up to 50 MeV. Potential applications include pulsed radiography, intense microwave generation, free electron laser drivers, directed energy for defense, and industrial radiation processing applications. This paper gives a tutorial on the principles of high current electron accelerators. It is divided into four sections: (a) high current sources, (b) space charge dominated extractors, (c) beam transport with strong self-fields, and (d) methods of high power acceleration. In addition to discussions of conventional technology, such as the linear induction accelerator, promising new approaches to beam generation and acceleration are outlined. These include laser driven photocathodes, ion channel focusing, and high power rf accelerators.     

Resonance S-band relativistic backward wave oscillator based on a submicrosecond pulsed high-voltage generator

We present the results of investigations of a resonance S-band relativistic backward wave oscillator (RBWO) that employs a submicrosecond pulsed high-voltage source based on a Marx generator scheme with a water-filled pulse-forming line. It is shown that the spontaneous limitation of the microwave pulse duration in the RBWO is caused by the emission of charged particles from plasma generated at the surface of a slow-wave system under the action of intense high-frequency fields. An increase in the electric strength of the electrodynamic system of the resonance RBWO, which was achieved by processing the surface with a high-current low-energy electron beam, allowed the pulse energy to be increased to 250 J at a peak output radiation power of about 3 GW.     

Generation of powerful narrow-band 75-GHz radiation in a free-electron maser with two-dimensional distributed feedback

A planar generator based on a free-electron maser (FEM generator) is developed employing a relativistic sheet electron beam (0.8 MeV/1 kA/4 μs) formed by an ELMI accelerator. A hybrid two-mirror resonator consisting of a two-dimensional upstream and a one-dimensional downstream Bragg reflectors is used as the electrodynamic system. The use of two-dimensional distributed feedback implemented in the upstream Bragg structure has made it possible to achieve stable well reproducible single mode generation regime with transverse dimensions of the system of ～25 × 2.5 wavelengths. Experiments carried out at a frequency of 75 GHz have yielded narrow-band radiation with spectrum width of ～20 MHz in pulses with a duration of ～100–200 ns and power of 30–50 MW.     

Dynamics of subnanosecond electron beam formation in gas-filled and vacuum diodes

The dynamic characteristics of a subnanosecond pulsed electron beam formation in the accelerating gap of a gas-filled or evacuated diode have been studied at a time resolution ～10^?11 s. In the air-filled gap, the electron beam pulse with a current amplitude of several amperes is formed up to about one hundred picoseconds earlier than the analogous pulse under vacuum conditions, and the measured pulse duration (～10^?10 s) is close to the electron flight time across a diode gap in the continuous acceleration regime. It is shown that a nanosecond prepulse plays an important role by initiating the emission of electrons that are subsequently accelerated by the high-voltage pulse with a subnanosecond front.     

Study of microwave components for an electron cyclotron resonance source: Simulations and performance

A high power (2 kW, CW) magnetron-based microwave system operating at 2.45 GHz has been designed, tested, characterized, and used to produce plasma. The system consists of a microwave source, an isolator, a directional coupler, a three-stub tuner, a high voltage break, a microwave vacuum window, and a microwave launcher. These microwave components were simulated using microwave studio software. The low power and full term characterization of the microwave system has been done using vector network analyzer. The system was tested for 2 kW continuous wave of microwave power using glass-water load. The microwave system has been developed to study the microwave interaction with plasma at different operation regimes (Gases: Nitrogen, argon and hydrogen; Gas pressure : 10^?5–10^?3 mbar; Microwave power : 300–1000 W; Magnetic field: 875–1000 G) and to extract the proton beam current with hydrogen produced plasma. A plasma density ～5 × 10¹¹ cm^?3 and average electron temperature of ～13 eV was obtained. This article describes various aspects of the microwave system including design, fabrication, characterization and performance studies of the microwave components.