Y. Hadas

Pulsed plasma electron sources

There is a continuous interest in research of electron sources which can be used for generation of uniform electron beams produced at E≤105 V/cm and duration ≤10−5 s. In this review, several types of plasma electron sources will be considered, namely, passive (metal ceramic, velvet and carbon fiber with and without CsI coating, and multicapillary and multislot cathodes) and active (ferroelectric and hollow anodes) plasma sources. The operation of passive sources is governed by the formation of flashover plasma whose parameters depend on the amplitude and rise time of the accelerating electric field. In the case of ferroelectric and hollow-anode plasma sources the plasma parameters are controlled by the driving pulse and discharge current, respectively. Using different time- and space-resolved electrical, optical, spectroscopical, Thomson scattering and x-ray diagnostics, the parameters of the plasma and generated electron beam were characterized.

Plasma dynamics during relativistic S-band magnetron operation

Results of a time- and space-resolved spectroscopic research studies of an S-band relativistic magnetron generating 1.5×108 W microwave power at f=2950 MHz, powered by a linear induction accelerator (450 kV, 4 kA, 150 ns), are presented. The cathode plasma electron density and temperature were obtained by analyzing Hα and Hβ hydrogen Ballmer series and carbon C II and C III ion spectral lines and the results of collision radiative modeling. It was shown that the microwave generation is accompanied by a significant increase in plasma density and ion temperature, up to ∼5×1016 cm−3 and ∼8 eV, respectively. The plasma electron temperature was found to be ∼8 eV. It was shown that the plasma expansion velocity in the axial direction reaches ∼107 cm/s and does not exceed ∼2×105 cm/s in the radial direction. In addition, it was shown that the plasma is not uniform and consists of separate plasma spots whose number increases during the accelerating pulse. Estimates of the plasma transport parameters and its inter...

Characterization of multicapillary dielectric cathodes

Parameters of the plasma and electron beam produced by a multicapillary cathode in a diode powered by a ∼200kV, ∼300ns pulse are presented. It was found that the source of electrons is the plasma ejected from the capillaries. Inside the capillaries this plasma obtains electron density and temperature of ∼8×1015cm−3 and ∼5eV, respectively. In the vicinity of the cathode, the density and temperature of the plasma electrons were found to be 2×1014cm−3 and 4.5eV, respectively, for electron current density of ∼40A∕cm2. It was shown that the plasma expansion velocity is in the range of (1–2)×106cm∕s for current density of >12A∕cm2.

Drastic improvement in the S-band relativistic magnetron operation

The superior operation of a S-band relativistic magnetron powered by a Linear Induction Accelerator with ≤400 kV, ≤4 kA, and ∼150 ns output pulses was revealed when the magnetron was coupled with a resonance load and a part of the generated microwave power stored in the resonator was reflected back to the magnetron. It is shown that, under optimal conditions, the efficiency of the magnetron operation increases by ∼40% and the generated microwave power reaches the power of the electron beam.

S-band relativistic magnetron operation with an active plasma cathode

Results of experimental research on a relativistic S-band magnetron with a ferroelectric plasma source as a cathode are presented. The cathode plasma was generated using a driving pulse (∼3 kV, 200 ns) applied to the ferroelectric cathode electrodes via inductive decoupling prior to the beginning of an accelerating pulse (200 kV, 150 ns) delivered by a linear induction accelerator. The magnetron and generated microwave radiation parameters obtained for the ferroelectric plasma cathode and the explosive emission plasma were compared. It was shown that the application of the ferroelectric plasma cathode allows one to avoid a time delay in the appearance of the electron emission to achieve a better matching between the magnetron and linear induction accelerator impedances and to increase significantly (∼30%) the duration of the microwave pulse with an ∼10% increase in the microwave power. The latter results in the microwave radiation generation being 30% more efficient than when the explosive emission cathod...

High-current electron beam generation in a diode with a multicapillary dielectric cathode

Results of high-current electron beam generation in an ∼200kV, ∼250ns diode with a multicapillary dielectric cathode (MCDC) assisted by either velvet-type or ferroelectric plasma sources (FPSs) are presented. Multicapillary cathodes made of cordierite, glass, and quartz glass samples were studied. It was found that the source of electrons is the plasma ejected from capillaries. The plasma parameters inside capillary channels and in the vicinity of the cathode surface were determined during the accelerating pulse using visible range spectroscopy. It was shown that glass multicapillary cathodes are characterized by less surface erosion than the cordierite cathodes. Also, it was found that multicapillary cathodes assisted by a FPS showed longer lifetime and better vacuum compatibility than multicapillary cathodes assisted by a velvet-type igniter. Finally, it was found that quartz glass MCDC assisted by FPS is characterized by almost simultaneous formation of the plasma in a cross-sectional area of the diele...

Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator has been investigated at submicrosecond duration of a high-current electron beam generated in a planar diode. The experiments were performed at accelerating voltages of les550 kV and diode currents of up to 17 kA using a radio-frequency cavity with a wide coupling window between its two sections. Three types of cathodes have been studied, namely, metal-dielectric, carbon fiber, and velvet cathodes. The main features of the operation of the vircator using each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached ~250 ns at the peak power level of ~100 MW; with the velvet cathode, a duration of ~400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.

Thomson scattering diagnostics of the plasma generated in a hollow anode with a ferroelectric plasma source

Thomson scattering of a laser beam was applied to study the plasma parameters inside a hollow anode having a ferroelectric plasma source incorporated in it. This method allowed avoiding difficulties related to spectroscopical measurements in the case of unknown electron energy distribution. It was found that the electron density and energy of the ferroelectric plasma are ∼1015cm−3 and ⩽5eV, respectively, and the density of the hollow anode bulk plasma is ∼6×1013cm−3. Applying an accelerating pulse for electron extraction from the bulk plasma leads to an increase in the electron density and energy of the ferroelectric plasma up to 6×1016cm−3 and ⩽20eV, respectively.

Plasma parameters of an active cathode during relativistic magnetron operation

The results of time- and space-resolved spectroscopic studies of the plasma produced at the surface of the ferroelectric cathode during the operation of an S-band relativistic magnetron generating ∼50 MW microwave power at f=3005 MHz and powered by a linear induction accelerator (LIA) (150 kV, 1.5 kA, 250 ns) are presented. The surface plasma was produced by a driving pulse (3 kV, 150 ns) prior to the application of the LIA accelerating high-voltage pulse. The cathode plasma electron density and temperature were obtained by analyzing hydrogen Hα and Hβ, and carbon ions CII and CIII spectral lines, and using the results of nonstationary collision radiative modeling. It was shown that the microwave generation causes an increase in plasma ion and electron temperature up to ∼4 and ∼7 eV, respectively, and the plasma density increases up to ∼7×1014 cm−3. Estimates of the plasma transport parameters and its interaction with microwave radiation are also discussed.

Observation of Plasma at the Quartz Rod Inside Annular Electron Beam Produced From a Knife-Edge Cathode in a Magnetic Field

Time-resolved light emission imaging was used to observe the plasma formation at the surface of a dielectric rod serving as a slow-wave supporting structure in an antenna-amplifier Cherenkov maser configuration. Experiments were performed using a quartz rod inserted into the hollow knife-edge cathode of a magnetically insulated foilless diode generating an annular electron beam of 0.9-1.7-kA current. The accelerating voltage pulse was delivered to the diode by a linear induction accelerator; the voltage amplitude ranged from 260 to 380 kV at the full pulse duration of ~ 200 ns. It was shown that the plasma at the rod surface is produced in the vicinity of the cathode edge plane corresponding to the location of the strong tangential electric field component. This plasma appears later than the explosive emission plasma at the cathode edge, and the intensity of light from this plasma increases, while the voltage rises. At voltages below 300 kV, the measured side-view light intensity drastically decreases, which indicates a significant decrease in the plasma density. It has been found that the surface plasma does not propagate along the rod; yet, the presence of plasma at a distance of 5-10 cm from the cathode was registered sporadically at high voltages. The electron emission from the surface plasma was observed as well.