Picosecond high-voltage pulse measurements.

This paper presents capacitive sensors based on oxide dielectric substrates that provide a high attenuation factor of up to 30 · 103 with a transient time of ∼38 ps. The sensors made it possible to significantly reduce the number of attenuators and increase the bandwidth of the measuring path. The presented sensors have been used successfully for recording high voltage to MV pulses in the time range from a few nanoseconds to tens of picoseconds. The use of the sensors as the point receiving antennas for recording radio pulses in the GHz frequency range with high electric fields is also discussed, along with the trough directional coupler as a unit for additional attenuation of the voltage of the sensor response. The couplers have a high attenuation factor of 1-5 · 103 with a transient time of ∼38 ps. Using the couplers as attenuators for recording waveforms further improves the parameters (bandwidth, transient time) of the measuring circuit. A discussion of some features of the operational mode of this type of sensor and coupler with a focus on wave processes is also presented. This discussion can be used to analyze the general approaches to the improvement of the sensor and coupler as components of the measuring circuit. In this paper, the authors also propose methods for calibrating and calculating the attenuation factor of the sensors.

Four-channel generator of 8-GHz radiation based on gyromagnetic non-linear transmitting lines.

Test results of high-voltage one- and four-channel radio-frequency (RF) generators based on the coaxial gyromagnetic ferrite-filled nonlinear transmission lines (NLTL) with external magnetic bias and RF-modulation frequency of a high-voltage pulse envelope of ∼8 GHz are presented. Electrical strength of oil-isolated NLTLs was tested in a compact version of one-channel generator based on the RADAN driver at a repetition rate of 100 pps. In case of a stationary setup, 5-ns pulse with -500 kV amplitude was split into 4 channels with individual NLTLs. Gyromagnetic line output pulses had fast damped RF-modulation with a maximum modulation depth more than 50% and the peak amplitude of -200 kV. Independent control of a delay time in each channel was realized by the coaxial spiral lines with a central biased ferrite core. The coherent summation possibility of RF fields in the free space radiated by a 4-channel system of conical dielectric antennas was demonstrated.

Phase-Imposing Initiation of Cherenkov Superradiance Emission by an Ultrashort-Seed Microwave Pulse.

For the first time, we demonstrate experimentally the possibility of Cherenkov superradiant generation with a phase imposed by an ultrashort seed microwave pulse. The phases of seed and initiated Ka-band microwave pulses were correlated with the accuracy of 0.5-0.7 rad for the power ratio down to -35 dB. Characteristics of such a process were determined in the frame of a basic theoretical model that describes both spontaneous and stimulated emission of an electron beam moving in corrugated waveguides. The obtained results open up opportunities of reaching extremely high radiation power density in phased arrays of short-pulse coherently operating microwave generators.

Four channel high power rf source with beam steering based on gyromagnetic nonlinear transmission lines.

The synchronized operation of four gyromagnetic nonlinear transmission lines (NLTLs) was tested with a pulse repetition frequency up to 1 kHz during 1 s bursts. High voltage pulses with a duration of ∼5 ns from the solid state driver S-500 were split into four 48 Ω channels reaching about -200 kV in each channel with ∼10% variation in the amplitude. The maximum peak voltage at the NLTL output was within 220-235 kV with the maximum modulation depth of decaying oscillations up to 90% at the center frequency near 2.1 GHz. The relative delay between channels reached the half-period of the center frequency of oscillations. The associated beam steering by four element array of conical helical antennas was demonstrated in a horizontal plane at 17°. The effective potential of radiation reached 360 kV at the radiation axis. The effect of ferrite temperature on the shock wave velocity in gyromagnetic NLTL is observed.

Energy compression of nanosecond high-voltage pulses based on two-stage hybrid scheme.

Test results of high-voltage subnanosecond pulse generator with a hybrid, two-stage energy compression scheme are presented. After the first compression section with a gas discharger, a ferrite-filled gyromagnetic nonlinear transmitting line is used. The offered technical solution makes it possible to increase the voltage pulse amplitude from -185 kV to -325 kV, with a 2-ns pulse rise time minimized down to ∼180 ps. For the small output voltage amplitude of -240 kV, the shortest pulse front of ∼85 ps was obtained. The generator with maximum amplitude was utilized to form an ultra-short flow of runaway electrons in air-filled discharge gap with particles' energy approaching to 700 keV.

Generation of Subterahertz Superradiance Pulses Based on Excitation of a Surface Wave by Relativistic Electron Bunches Moving in Oversized Corrugated Waveguides.

The first experiments on the observation of short pulsed superradiant (SR) emission with the excitation of a surface wave by a relativistic electron bunch moving in an oversized corrugated waveguide were performed. Subterahertz SR pulses with a central frequency of 0.14 THz, an ultrashort duration of 150 ps, and an extremely high peak power of 50-70 MW were generated. The experiments were based on a theoretical consideration including the quasioptical approach and direct particle-in-cell simulations.

Generation of electromagnetic fields of extremely high intensity by coherent summation of Cherenkov superradiance pulses.

We demonstrate both theoretically and experimentally the possibility of correlating the phase of a Cherenkov superradiance (SR) pulse to the sharp edge of a current pulse, when spontaneous emission of the electron bunch edge serves as the seed for SR processes. By division of the driving voltage pulse across several parallel channels equipped with independent cathodes we can synchronize several SR sources to arrange a two-dimensional array. In the experiments carried out, coherent summation of radiation from four independent 8-mm wavelength band SR generators with peak power 600 MW results in the interference maximum of the directional diagram with an intensity that is equivalent to radiation from a single source with a power of 10 GW.

High repetition rate multi-channel source of high-power rf-modulated pulses.

This paper presents the results of testing a high voltage pulse generator based on parallel gyromagnetic nonlinear transmission lines filled with saturable ferrite. The generator is capable of producing almost identical stable rf-modulated nanosecond high voltage pulses in each of the two, or four, parallel output channels. The output voltage amplitude in each channel can reach -285 or -180 kV, respectively, with a rf modulation depth of up to 60%. Drive pulses were produced as the packets of duration 1-5 s at a pulse repetition frequency of 800 Hz using a driver equipped with all-solid-state switches. Splitting the driver pulse provided electric field strengths in the channels which were below the breakdown field strength of the transmission lines. As a result, the use of nonlinear transmission lines of reduced diameter made it possible to increase the center frequency of the excited rf oscillations to ∼2 GHz.

Current waveform reconstruction from an explosively emissive cathode at a subnanosecond voltage front.

We describe the methods of registration and reconstruction of an envelope of explosive electron emission current from the edge of a cylindrical cathode, which provides a picosecond time reference of the emitted electron beam with a subnanosecond voltage front applied to the accelerating gap. Variation of the front steepness allows one to determine the beam onset time in the experiments, where a collector-type current probe can be used. The advanced method of dynamic time domain reflectometry provides exact data on electron beam current rise and track changes in the cathode emission from pulse to pulse with a precision of less than 10 ps.

Time-domain reflectometry of high-voltage nonlinear loads with picosecond resolution.

The construction and characterization of a measuring module for high voltage time-domain reflectometry with picosecond resolution is presented. The device provides registration of low-distortion reflections from the load at conditions of irregular shape and pronounced instability of an incident probing pulse with amplitude of hundreds of kilovolts and a width/front up to ~2 ns. This is achieved with registration of differential reflecting signals by using two voltage probes, whose original signals are shifted in time and have a calibrated delay. Specially designed voltage dividers based on coaxial coupled lines with equalized dielectric constant of insulation meet the requirements of voltage probes identity. Reflectometry allows one to study dynamic changes of the loads for a specific time range of ~10(-10) s and faster. An example of reflectometry-based analysis for variations in breakdown development rate of atmospheric discharge gap supplied with the pulses of ~100-kV amplitude and a front slope of ~4 × 10(14) V/s is presented.

Observation of the avalanche of runaway electrons in air in a strong electric field.

The generation of an avalanche of runaway electrons is demonstrated for the first time in a laboratory experiment. Two flows of runaway electrons are formed sequentially in an extended air discharge gap at the stage of delay of a pulsed breakdown. The first, picosecond, runaway electron flow is emitted in the cathode region where the field is enhanced. Being accelerated in the gap, this beam generates electrons due to impact ionization. These secondary electrons form a delayed avalanche of runaway electrons if the field is strong enough. The properties of the avalanche correspond to the existing notions about the runaway breakdown in air. The measured current of the avalanche exceeds up to an order the current of the initiating electron beam.

Generation of powerful subnanosecond microwave pulses by intense electron bunches moving in a periodic backward wave structure in the superradiative regime.

Experimental results of the observation of coherent stimulated radiation from subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating wave are presented. The subnanosecond microwave pulses in Ka and W bands were generated with repetition frequencies of up to 25 Hz. The mechanism of microwave pulse generation was associated with self-bunching, and the mutual influence of different parts of the electron pulse due to slippage of the wave with respect to the electrons; this can be interpreted as superradiance. The illumination of a panel of neon bulbs resulted in a finely structured pattern corresponding to the excitation of the TM01 mode. Observation of rf breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the absolute peak power as high as 60 MW for the 300-ps pulses at 38 GHz. These results are compared with numerical simulations. The initial observation of 75-GHz, 10-15-MW radiation pulses with a duration of less than 150 ps is also reported.