Active Bragg Compressor of 3-cm Wavelength Microwave Pulses

We present the results of studies of the active compressor of 3-cm wavelength microwave pulses, which uses a high-Q storage Bragg resonator excited at the H₀₁ mode and new types of plasma switches. Phase variation during a compressed pulse and phase correlation of the input and compressed microwave pulses are studied both experimentally and theoretically. Using a single-channel compressor excited at the megawatt power level by the magnicon radiation with frequency 11.4 GHz, a power amplification factor equal to 9 was reached for an output-pulse duration of 40–50 ns and a peak power of up to 25 MW. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 597–616, July 2008.     

Two-channel 100-MW microwave compressor for the three-centimeter wavelength range

We present the results of the studies of a 100-megawatt active two-channel compressor of microwave pulses for the three-centimeter wavelength range. The compressors of the transit and reflector types are excited at the TE₀₁ mode of a circular waveguide, and the energy is output by using resonance plasma switches. The channels of the compressors are connected to a microwave oscillator and a load via a special quasioptical 3-dB directional coupler with enhanced electric strength. The use of the quasioptical coupler allows one to ensure the decoupling of the input microwave line (by about 20 dB) and combine coherently the pulses compressed in each of the compressor channel. High-and low-power tests of the compressors have been performed and the wave phase in the compressed pulses has been measured. The pulses obtained for an incident power of 5 MW have a power of 40–53 MW, a duration of 40–60 ns, and a power gain greater than 10. The compression efficiency amounts to 55%. The stability and good reproducibility of the amplitude and frequency characteristics of the radiation from a two-channel compressor make it promising for use in the linear accelerators of charged particles. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 8, pp. 660–674, August 2008.     

Method for achievement of a multigigawatt peak power by compressing microwave pulses of a relativistic backward-wave oscillator in a helical waveguide

We show theoretically that the use of a circular oversized waveguide with a helically corrugated internal surface as a dispersive medium ensures efficient compression of frequency-modulated microwave pulses up to peak powers of about 10 GW. According to the calculations, a pulse with the required frequency modulation can be obtained in a relativistic backward-wave oscillator operated in the 3-cm wavelength range and producing an output power of hundreds of megawatts. In a demonstration experiment, a 80-ns pulse of kilowatt power with frequency modulation in a 5% band was compressed to a 1.5-ns pulse with a 25-fold power amplification. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 40–53, January 2007.     

Summation of signals during energy extraction from 3-cm range oversized cavities

Data on the summation of signals from two synchronously operating resonant microwave (3-cm) compressors with oversized cavities are described. The energy was extracted with interference switches. The summation was performed in a waveguide tee and a 3-dB three-slot bridge. For a signal duration of ∼30 ns at a level of 0.5, a peak power of ∼1 MW, and an efficiency of ∼30%, a gain was found to approach ∼11 dB.     

Relativistic backward wave oscillator using a selective mode converter

A new version of the relativistic backward wave oscillator (BWO) is proposed and investigated experimentally, where the cutoff (for the working mode mode) taper at the cathode end is replaced by a selective Bragg-type mode converter. In the experimental BWO model, which operates in the three-centimeter range and is equipped with a mode converter based on a slightly corrugated waveguide, a radiated power of 700 MW in pulses of duration up to 100 ns with an output spatial structure similar to a Gaussian wave beam is obtained at an accelerating potential of 0.8 MV and a focusing magnetic field of 7 kOe. Zh. Tekh. Fiz. 69, 102–105 (November 1999)     

Experimental investigation of a 25-MW microwave (3-cm range) compressor prototype

An experimental study of a resonance microwave (3-cm-range) compressor with gas insulation and energy output through a superdimensional coaxial interference switch is reported. The rated parameters of the compressor are output signal power ∼25 MW, signal duration 2 ns, gain 26 dB, and efficiency ∼57%. A gain of 20 dB was achieved at a peak output signal power of 12 MW, signal duration of 2 ns, efficiency of ∼24%, and traveling wave power of 24 MW.     

Testing a thermoacoustic sensor of high-power microwave pulses

We present the results of testing a new thermoacoustic sensor designed to detect microwave pulses having durations from 3 to 120 ns at wavelengths of 0.8 and 3 cm. Operation of the sensor is based on the effect of generation of acoustic signals during absorption of microwave pulses in a radiotransparent substrate–absorber–liquid layered structure . A thin nanometer-thick film deposited on a substrate is used as an absorber. Microwaves are converted to an acoustic pulse in the film and the adjacent liquid. The pulse is received by a wideband acoustic receiver and then recorded by a digital oscilloscope. It is shown that for a pulse duration of 120 ns, the shape of the signal recorded by the thermoacoustic sensor completely corresponds to the signal of a tube-diode detector of microwave pulses. The response of the thermoacoustic sensor to shorter pulses (3 and 5 ns long) is a pulse with a duration of 18 ns which is determined by a limited frequency band of the acoustic receiver.     

Study of Active Microwave Compressors Excited by Magnicon Radiation at a Frequency of 11.4 GHz

We present the results of a study of single-channel and two-channel compressors of microwave pulses, operated at a frequency of 11.4 GHz, in which a high-Q factor storing resonator and a novel plasma switch are used. In the single-channel compressor excited by magnicon radiation at a frequency of 11.4 GHz, 24-MW pulses have been obtained for a compression ratio equal to 8. In the two-channel compressor, coherent composition of pulses with the use of a 3-dB directional coupler has been demonstrated. Compressed pulses with powers of 9 to 11 MW and durations of 50 to 60 ns have been obtained at the incident-power level 1–1.5 MW, which corresponded to the compression ratio k = 8–9.3.     

Generation of high-power nanosecond pulses in a relativistic backward-wave oscillator in the regime of spatial accumulation of energy

We study the generation of electromagnetic pulses with a carrier frequency of 3.7 GHz in a relativistic backward-wave oscillator with a long slow-wave system in the superradiance regime of super-radiation for a magnetic induction of 0.2 T (below the cyclotron resonance). To decrease transverse velocities of the electrons, we use decompression of a hollow electron beam. Decompression in combination with a sharp leading edge of the high-voltage pulse (460 kV) applied to the explosive-emission cathode are used for increasing the cathode lifetime and improving the azimuthal uniformity of the beam. As a result, the achieved peak power of the microwave radiation amounts to 800 MW for a pulse duration of 2.5 ns and a repetition rate of 100 Hz. The uninterrupted operation in such a regime determined by the lifetime of the explosive-emission cathode is increased up to 10⁵–10⁶ pulses. The efficiency of conversion of the electron-beam power into the electromagnetic-wave power is increased up to 50%, The possibility of locking the electromagnetic oscillations phase by a sharp edge of the high-voltage pulse at the cathode was observed for the first time in such a relativistic generator. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 10, pp. 837–842, October 2006.     

Performance and pulse shortening effects in a 200-kVPASOTRONTM HPM source

The PASOTRON^TM is a unique, high-power microwave source that uses a long-pulse (⩽100 μs) 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^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 μs. The L-band PASOTRON^TM BWO produced 10-20 MW of peak power in the TM₀₁ mode, which was converted in the output to a TE₁₁ mode with fixed polarization. The PASOTRON ^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     

Investigation of the ozone formation process in a nanosecond microwave discharge in air and oxygen

Results are presented from an investigation of the dynamics of ozone formation in a freely localized discharge generated by a periodic train of 3-cm nanostructured microwave pulses in oxygen and air at a pressure p=3–30 Torr. Conditions for the efficient generation of ozone in air with the minimum production of nitrogen oxides are demonstrated experimentally. It is shown that when the microwave pulse repetition frequency is high, heating of the gas and the buildup of nitrogen oxides in the discharge in air reduce the ozone formation efficiency while under prolonged exposure the ozone formed initially is destroyed. The energy cost of forming ozone in oxygen and air is determined as a function of the microwave pulse length and repetition frequency and the gas pressure. The lowest energy cost of forming a single ozone molecule in these experiments is 16 eV per molecule for a discharge in air and 4 eV per molecule in oxygen. It was observed that circulating the gas through the discharge zone enhances th ozone formation efficiency. It is shown that there are optimal conditions for ozone formation as a function of the reduced electric field in the plasma. Zh. Tekh. Fiz. 67, 9–18 (March 1997)     

光子晶体高功率微波模式转换器设计

研究了传输线中非截面排列金属光子晶体电磁特性,并利用其作为移相器提出了一种紧凑型TEM—TE11模式转换器的设计方法.利用电磁软件cst microwave studio优化设计了一套L波段TEM—TE11模式转换器,在中心频率1.58 GHz上转换效率为98%.在1.56—1.625 GHz频率范围内,模式转换器转换效率大于90%,对应带宽4.1%.模式转换器功率容量为GW级,适用于高功率微波源系统.结合磁绝缘线振荡器开展了粒子模拟研究工作,发现模式转换器性能与设计结果相符,并且其引入不影响高功率微波器件的正常工作.     

High-pulse-energy high-efficiency mid-infrared generation based on KTA optical parametric oscillator

A high-energy, high-efficiency mid-infrared KTA OPO at 3.47 μm intracavity pumped by a Nd:YAG laser is presented. The maximum output energy is 31 mJ at the repetition rate of 10 Hz with a V-shaped cavity, corresponding to the absolute optical-to-optical conversion efficiency of 4.76% from the diode and the photon conversion efficiency of 87% from the fundamental to mid-infrared energy. The pulse width is 5.8 ns at the maximum output energy and the peak power reaches higher than 5 MW. The line width of the mid-infrared wave is about 1.1 nm (or 0.9 cm⁻¹ in wave number). The output energy demonstrates good stability. To our knowledge, these are the highest pulse energy and conversion efficiency of mid-infrared OPOs using bulk nonlinear crystals in the 3–5 μm range.     

New simplified coupling scheme for the delivery of 20 MW Nd:YAG laser pulses by large core optical fibers

The feasibility of transmitting 20 MW, 5 ns laser pulses from a frequency doubled Nd:YAG laser through a standard 1500 μm multi mode optical fiber is demonstrated. A new coupling scheme employing an optical homogenizer prevents breakdown in air without the use of a vacuum chamber. At the same time a very homogeneous flat top beam profile on the fiber surface is achieved. The new scheme therefore clearly simplifies fiber coupling of high power laser pulses. Experiments on the delivery of more than 20000 pulses with 110 mJ mean energy without fiber damage have been performed. Received: 2 August 2000 / Revised version: 18 August 2000 / Published online: 22 November 2000     

Generation of high peak power excimer laser radiation by pulse shortening

Techniques for shortening the pulses of a commercial oscillator-amplifier excimer laser have been investigated. By a combination of a H₂-Raman cell and a saturable absorber dye jet the oscillator pulses are shortened from about 25 ns to typically 1.5 ns. Upon amplification pulses around 3.5 ns with peak powers of more than 100 MW for KrF (248 nm) and 25 MW for XeCl (308 nm) are obtained.     

Generation of high-energy pulses from an all-normal-dispersion figure-8 fiber laser

We propose and study numerically an all-normal-dispersion Ytterbium-doped figure-eight fiber laser scheme for generation of high-energy pulses. The monotonous pulse stretching that takes place in the fiber under the combined actions of normal dispersion and nonlinear Kerr effect is compensated by the amplitude modulation effect of a bandpass filter inserted in the ring section of the laser. The Nonlinear Optical Loop Mirror (NOLM) also contributes to shorten the pulses. An output coupler with a large output coupling ratio is inserted at the amplifier output in order to extract the maximal energy from the laser. A short segment of Ytterbium-doped fiber compensates for the losses. Stable single-pulse operation is predicted over a wide range of values of the laser parameters. If the laser parameters (ring and NOLM length, dispersion, filter bandwidth, output coupling ratio) are optimized, pulses with several tens of nanojoules energy are readily obtained, with picosecond duration and a large positive chirp which is linear near the peak. If small-signal gain is large enough, the use of very large output coupling ratios opens the way to pulse energies close to 100 nJ and, after dechirping outside the laser, to durations of ˜50 fs and peak powers of 1 MW.     

Effect of intermode coupling on the compression of radio pulses in an oversize cavity with an interference switch

The results of an experimental investigation of the effect of intermode coupling at the exit window of a cavity on the compression of radio pulses in an oversize cylindrical cavity with an interference switch, operating on H _01(n) modes, are reported. The effect of the intermode coupling at the exit window of a cavity on the energy extraction process is analyzed in a simple model in which the interacting modes are represented in the form of a system of two coupled cavities. The results of the analysis are compared with the experimental data. Zh. Tekh. Fiz. 69, 84–86 (February 1999)     

Electron kinetics in a discharge plasma produced by a focused microwave beam in free space

A study is made of the electron kinetics in a discharge plasma produced by a high-power beam of electromagnetic radiation in the centimeter-wave region under conditions approaching free space, when the dimensions of the chamber are much greater than the wavelength of the microwave radiation. Two regimes of discharge production are investigated: the regime of short microsecond pulses at a repetition rate of 200 Hz, and a single millisecond pulse regime. It is shown that at threshold values of the microwave energy flux density the electron density in the initial stages of discharge formation reaches the critical value, and that the average energy of the electrons is of the order of 1.5–3 eV. Zh. Tekh. Fiz. 67, 10–14 (June 1997)     

Increase in the energy efficiency of a pulsed-periodic relativistic backward wave oscillator with a modulating resonant reflector

An efficient microwave oscillator (320 MW and 7.9 GHz) that generates microwave pulses with a duration of 90 ns is developed using optimization of an electron-wave system and decompression of the longitudinal magnetic field with a maximum induction of 0.62 T in the region of an explosive-emission cathode and a lower field (0.36 T) with respect to cyclotron resonance in the slow-wave structure. In a packet (up to 10 ns) repetitively-pulsed (100 Hz) regime, the maximum conversion efficiency of the electron-beam power to microwave radiation is 27%. The mean energy of the radiation pulse (23 J) is about 18% of the pulse energy of high-voltage oscillator.     

X波段短脉冲相对论返波管设计与初步实验

 对基于短电子束脉冲超辐射机理的X波段相对论返波管进行了优化设计和粒子模拟,结果表明：在超辐射机理作用下,该器件能实现高峰值功率和高功率转换效率的微波辐射。在小型Tesla脉冲源基础上设计了阻抗变换段、二极管、磁场系统等装置,建立了一套小型窄脉冲电子加速器,以此为实验平台在低磁场条件下进行了器件的初步实验研究。在磁场0.73 T、束压约380 kV、束流约4.5 kA、脉宽3.1 ns条件下,实验获得的微波脉冲峰值功率约360 MW,脉宽1.10 ns,上升沿800 ps,频率9.15 GHz,功率转换效率为21%。