Marat R. Ul'masculov

Stability of Injection of a Subnanosecond High-Current Electron Beam and Dynamic Effects Within Its Rise Time

The stability of the injection of short electron beams and the dynamic processes that occur during their transport were experimentally studied. Beams of energy 200-300 keV, current of 1-1500 A, and duration of 0.05-3 ns with a current rise time of 30-300 ps were formed in a cold-cathode electrode gap. The distribution of the accelerating electric field was highly nonuniform. The cases of vacuum and air insulation of the electron diode were considered. The shortest beams with currents of a few amperes were generated in the mode of continuous acceleration of electrons in atmospheric air. For measuring beam currents, special collector probes were used which ensured a picosecond resolution.

Coherent Summation of Emission From Relativistic Cherenkov Sources as a Way of Production of Extremely High-Intensity Microwave Pulses

For relativistic Cherenkov devices, we investigate the process of high-power microwave pulse generation with its phase correlating to the sharp edge of an e-beam current pulse. Our theoretical consideration is referred to quasi-stationary and superradiative (SR) generation regimes when spontaneous emission of the e-beam edge serves as the seed for the development of further coherent oscillations. Phase correlation of the excited microwave pulses with the characteristics of the current pulse front and/or an initial external electromagnetic pulse has been additionally confirmed by particle-in-cell simulations. Pulse-to-pulse stability of the radiation phase within several percents of the oscillation period makes it possible to arrange multichannel schemes producing mutually coherent microwave pulses. In the experiments that have been carried out, the cathodes of independent generators were powered by identical accelerating pulses from strictly synchronized voltage modulators, or by splitting the pulse from a single powerful modulator. For the 2-ns regime with the power of each Ka-band backward-wave oscillator about 100 MW, we demonstrate quadratic growth of the power density in the interference maximum of the directional diagram. In a short pulse SR regime, with the peak power of 600 MW in a single channel, for a four-channel 2-D array, we attained a 16-fold radiation intensity gain.

Microwave Breakdown of Air by Nanosecond and Subnanosecond Ka-Band Pulses

The initial stage of atmospheric air breakdowns in Ka-band pulsed microwave fields was observed for the microwave power increasing to a maximum within a characteristic time of ~300 ps. Pulsed microwaves were produced by relativistic BWOs with output powers of ~170 and ~500 MW (4-ns and 300-ps FWHMs, respectively). The effects of air breakdown in the fields of microwave pulses of this type are pronounced when the radiation is extracted through the vacuum window of a horn antenna and when it is channeled in passing through waveguides and quasi-optical lines. Of particular interest are modes in which repetitive pulsed microwaves are generated.

Magnetically insulated coaxial diode non-linear properties

Summary form only given. Recently developed method of dynamic time-domain reflectometry (DTDR, [1]) provides a picosecond time reference of the emitted electron beam with a subnanosecond voltage front applied to the accelerating gap of magnetically insulated coaxial diode (MICD) which is widely applicable in relativistic backward wave oscillators (BWOs). Our additional motivation of presented investigation is related with DTDR capabilities in determining of time-dependent rise of electron current emission. The last opportunity makes one possible to formulate physically-correct initial conditions for the unsolved task of non-stationary MICD where emissivity of the explosive electron emission (EEE) cathode is finite. Above problems are urgent to create phase-stable excitation of multi-channel microwave oscillators operating in X-band and Ka-band [2,3]. In the report we are planning to suggest information regarding the macroscopic electric field strength at the cathode emissive edge which is sufficient for transition of field emission to the EEE. This knowledge is important to provide a stable, rapid-response operation of the cathode, which is typically made of graphite. Similar data are necessary to eliminate spurious emissions at a high-potential metallic cathode holder. DTDR provides an opportunity to track changes in the cathode emission from pulse to pulse with a precision of less than 10 ps when cathode aging. Also, dynamic changes of the MICD impedance will be analyzed for the cathodes operating in various magnetic fields, and when graphite dispersiveness of such a cathodes differs from sample to sample. Finally, the particle-in-cell model of relativistic BWO will be presented, where real emissivity of the cathode will be considered.

Four-channel source of synchronously modulated subgigawatt voltage pulses

Summary form only given. Investigations preceded the development of subgigawatt rf source based on a single, shock-excited ferrite-loaded nonlinear transmitting line (NLTL) demonstrated that the frequency (Frf) and power (Prf) of rf generation are determined by numerous parameters (see Ref. [1] and citation therein). The most critical are the properties of ferrite insert, radial and longitudinal dimensions of NLTL, as well as the amplitude (Vp) and the rise time of the feeding voltage pulse. When aiming simultaneous increase in Frf, Prf and pulses repetition frequency (PRF), one undoubtedly meet the problem of breakdown strength of NLTL. Refurbished version of an all-solid-state modulator [2] used in presented investigation produces at 50-Ohm terminal a pulses (~3 ns, FWHM) with the amplitude of (325-450) kV which depends on PRF (up to 800 Hz). Utilizing the power of such pulses (Pv > 2 GW) for rf excitation at Frf ~2 GHz and at highest PRF could be arranged by splitting feeding pulse to several identical NLTL-channels producing synchronous modulation of decreased in amplitude voltage pulses at identical Frf. In the prospect, such a multi-channel rf array may provide sharpening of the radiation pattern which also could be scanned somewhat with the electronic variation of the NLTLs delays. Above listed objectives of our investigation determine the stage, when technical features of separate components should be found prior full-scale experiments. We increased step-by step the quantity of NLTL channels (one, two, or four). For a single channel option, empty terminals of the modulator were loaded with absorbing water lines. In a single-channel version it was found that all tested NLTLs having overall diameter 40 mm and different in length sets of ferrite rings are resistant against breakdowns of both ferrite body as well as oil insulation pressurized up to 6 atm. In the limited PRF of 1 Hz, the voltage pulse top (Vp ~180 kV) was modulated at Frf ~2.25 GHz, and peak-to-peak rf amplitude attained Vrf ~100 kV. The rise of PRF to 800 Hz provided the following: Vp ~130 kV; Frf ~2.1 GHz; Vrf ~80 kV. In the options with two and four channels we tested correlations of Frf and Vrf instability with variations of Vp, as well as interchannel mishmash of above parameters and the reasons for that.

Summation of emission from superradiant sources as a way to obtain extreme power density microwaves

A theoretical model that covers both spontaneous and stimulated Cherenkov emission from an extended electron bunch has been developed. The initiation is described of the generation of superradiant pulses [1-3] by emission from the leading edge of the electron bunch. In combination with the proven experimentally picosecond stability of explosive emission from a cold cathode [4], it provides the possibility for strong correlation of phase of the SR pulses with respect to the leading edge of the electron pulse [5].

Advances in coherent power summation of independent Ka-band HPM oscillators

Report demonstrates the results of coherent power summation from two nanosecond-width relativistic Ka-band BWOs with the peak power above 200 MW in each channel. A peculiar feature of the experiment was in supplying the explosive electron emission cathodes with ultimate-stable split voltage pulse from an all-solid-state SOS modulator. Enhanced rate of the voltage rise provided stable emission of electron beams and minimized standard deviation of the microwaves phases between channels in the time scale down to 0.5 ps.

The concepts of in-phase multichannel Ka-band HPM oscillators

The prospects of development of in-phase Ka-band HPM oscillators based on relativistic BWOs without electrodynamic coupling are considered. Efficient RF power summation is possible if the phase spread in each channel will be at least ten times less than the period of carrier microwave frequency. Two approaches on construction of parallel, in-phased relativistic BWOs are compared. The system involving two independent high voltage PDFL generators with a gas discharge switches triggered by splitting electron beam appears to be not profitable energetically as compare to the version where electron injectors of two BWOs are supplied by splitting pulse from a single, more powerful high-voltage generator. For the last option it has been demonstrated already that mutual stability of current fronts of parallel e-beams attains units of picoseconds. This fact we used earlier in the experiments with two X-band BWOs [1] where phasing spread of microwave pulses did not exceed 2 ps. Report presents future options of similar experimental Ka-band devices being under development now.

Repetitive Generation of X-Band Superradiation at 3-GW Peak Power

Two versions of modernized experimental setup (i.e., nonstationary relativistic X-band BWO) were used for the generation of subnanoseeond-width, gigawatt-range pulses of microwave superradiation (SR).