M.R. Ulmaskulov

Experimental observation of wiggler superradiance under group synchronism condition

Abstract The first results of the observation of superradiance from a single, subnanosecond, high current, electron bunch passing through a wiggler immersed in a guide magnetic field are presented. The 300–500 ps microwave pulses were generated in the high gain regime for both the conventional and reverse directions of the guide magnetic field. The dependence of the radiation power on the interaction length as well as the absolute value of the power, 100–200 kW, were related with the development of self-bunching and consequently with coherent emission.

Phase Control in Parallel Channels of Shock-Excited Microwave Nanosecond Oscillators

The theoretical premises and experimental results of phase control in high-power microwave oscillators with nanosecond pulse duration are presented. In experiments, two-channel backward wave oscillators (BWOs) for both steady state (100-150 cycles) and super-radiance (SR) mode operation (10-20 cycles) are discussed. For the phase control, the shift of the moment with fastest current rise is provided in the sections of nonlinear transmission lines with axially biased ferrites. The voltage pulse sharpening and shift of group velocity depend on the dc axial magnetic field. In SR mode, two-channel source is capable of producing 2 × 0.3 GW pulses with duration of 2 ns and the center frequency of 10 GHz. The source operates at the repetition rate up to 100 pps with electronic control of the phase in one channel relative to another. The last experiment is carried out using two synchronized compact RADAN-type drivers with two parallel Ka-band BWOs (100 MW, 2 ns, 37 GHz). The controllable shift of interference picture is a proof of the coherency in the aggregated radiation. At the maximum of the pattern in the far zone, the detector indicateds fourfold increase in power density over that measured from single channel.

Nanosecond hybrid Modulator for the fast-repetitive driving of X-band, gigawatt-power microwave source

Results of tests of a fast-repetitive nanosecond modulator with a peak output power of 3 GW are presented. It comprises of a type S-5N charging driver, a power compressor based on a pulsed forming line, and a gas spark gap. The modulator has been designed for the formation of high-current electron beams and high-power microwave generation in a pulsed-repetitive regime. Excitation of 10-GHz microwave pulses /spl sim/1 ns long in a relativistic backward-wave oscillator with an elongated periodic slow-wave system has been studied. Optimization of the regime of interaction between electromagnetic fields and particles provided an average power of microwave radiation in the burst-repetitive mode (1 s; /spl sim/700 Hz) of up to 2.5 kW at a focusing magnetic field (/spl sim/0.6 T) below the cyclotron resonance region. The peak output power of the oscillator exceeded 2 GW.

Highly efficient generation of subnanosecond microwave pulses in Ka-band relativistic BWO

This paper presents the results of investigations of the mode of nonsteady-state oscillations with a short-time power burst which is characteristic of the initial stage of the transient process in a backward wave oscillator when the beam operating current is far in excess of the starting current. Numerical simulations have yielded the conditions under which the efficiency of the power transfer from an electron beam with a particle energy of 300 keV, a current of 2 kA, and a duration of 1 ns into a microwave pulse containing 8-10 high-frequency field periods approaches 90%. Experimentally, it has been demonstrated that the production of pulses like these with a duration of 200-250 ps, a power of up to /spl sim/400 MW, and a central frequency of about 38 GHz is feasible.

Compact Ka-Band Backward-Wave Generator of Superradiative Pulses Operating at Reduced Guiding Magnetic Field

A compact repetitive superradiative Ka-band BWO is described, and the results of a study of its characteristics are presented. For an electron beam transported in a magnetic field of 2 T, which is less than the value corresponding to a cyclotron resonance, the efficiency of the conversion of electron beam power to pulsed electromagnetic radiation is close to unity. The peak power of an electromagnetic superradiative pulse of 250-ps duration is ~ 500 MW.

Experimental observation of Cherenkov superradiance from an intense electron bunch

Abstract Experimental results are presented of the observation of stimulated Cherenkov radiation when intense subnanosecond electron bunches were moving through a dielectric-loaded waveguide. The ultrashort duration microwave pulses (up to 400 ps which is equivalent to ∼10 periods of the radiation) in combination with specific dependencies of peak microwave power on interaction length demonstrate that this emission can be interpreted as coherent superradiant (SR) emission. The peak power associated with the SR emission was ∼1–2 MW. The spectrum of radiation belongs to the frequency range 38–48 GHz. The experimental results are compared with theoretical analyses.

Control of the Operation Mode of a Relativistic Ka -Band Backward-Wave Oscillator

Variations in the transient time and in the microwave pulse duration and peak power have been demonstrated for a relativistic subgigawatt Ka -band backward-wave oscillator (BWO) with the slow-wave structure geometry, the accelerating voltage, and the electron beam current remained unchanged. This was done taking into account that the BWO operation mode depends on the starting-current-to-beam-current ratio. Efficient variation of the starting current was attained by changing the beam path in view of a significant difference between the times of the guide pulsed magnetic field diffusion into the electron injection region and beam-wave interaction space.

Compact repetitive generator of high-power broadband electromagnetic pulses

Using a subnanosecond high-voltage modulator with a pulse repetition frequency of up to 100 pps, we have carried out experimental research into the excitation of ultrabroadband TEM antennas. The modulator consisted of a RADAN-303B nanosecond driver and a pulse sharpener based on gas spart gaps. The device was capable of generating single-polarity and monocycle pulses with readjustable duration and amplitude. The amplitude and power of the pulses supplied to the antenna ranged up to 100 kV and 200 MW, respectively. Preliminary data have been obtained on the stability of modulator subnanosecond pulse parameters under conditions where the amplitude dispersion of nanosecond-driver pulses is on the order of 5%. Results are provided for the electric strength of the matching junction between the modulator and the antenna. The ultrabroadband pulse was recorded at distances of up to 25 m. Its characteristics permitted spatial resolution of reflections from conductive objects with a shape nonuniformity scale of no more than 25 cm. Radiators with increased directivity in the E or H (E and H) plane were explored. Such radiators incorporates two (four) TEM antennas energized by a split modulator pulse. Consideration is given to versions of phased and opposite-phase antenna connection.

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.

Correction of shape and energy compression of high-voltage nanosecond pulses by compact converters with a traveling wave mode

The results of investigations of converters ensuring correction of the shape and energy compression of high-voltage (∼200 kV) pulses with durations of several nanoseconds and the formation of pulses with subnanosecond rise times across a load of ∼50 Ω are described. The converters are assembled on the basis of forming and transmitting coaxial lines and a high-pressure nitrogen spark gap. Correcting devices are open-circuit strip-line stubs connected to different points of the transmitting or forming line. Depending on the stub connection point, a series or parallel discharge of the forming line and stub to the load is initiated after switching of the spark gap. As a result, the pulse may become either longer or shorter; in the latter case, its amplitude increases. The design of the stubs built into the insulating gap of the coaxial line does not increase the size of the device at all.