1,721,089 research outputs found
Emission from ferroelectric cathodes versus temperature and the relevant physics
No full textWe show that electron emission from a relaxer ferroelectric cathode varies with the temperature as the dielectric constant. Consequently, there is no variation when the material makes the antiferroparaelectric phase transition. In addition, we have found that the emission increases with the polarizability of the material. The emission is originated by an induced change of the normal and/or spontaneous polarization. The maximum efficiency of the process, that is the ratio between the emitted charge of energetic electrons and the charge supplied to the sample by the excitation pulse, resulted in about 20%. The good emission of energetic electrons from a ceramic cathode at few hundred degrees temperature, makes this kind of cathode a proper electron injector for ion sources
Ion source improvement by electron injection from a ferroelectric cathode
No full textWith increasing rf power, the electron concentration in the plasma of electron cyclotron resonance (ECR) ion sources is decreasing in comparison to the ion concentration, so that the plasma is charging up positively. When reviewing the basic performance requirements of ECR sources it becomes evident that the direct injection of electrons into the ECR plasma is increasing the electron charge density and the ion current yield. Ferroelectric ceramics was used as very robust, electron emitting cathode material under the heavy-duty conditions inside the plasma chamber of an ECR ion source. The electron emission from the ferroelectric cathode is turned on by a high repetition-rate bipolar pulse of +/-1.2 to 1.6 kV amplitude to the electrodes deposited on both sides of the disk-shaped cathode. Lead-barium-zirconium-titanate (30/70/30) cathodes doped with 2 mol % Bi2O3 were installed and tested in the Ar-ion plasma of the ECR ion source CAESAR at INFN-LNS, Catania. The aim was to visibly increase the yield of the ion output current by electron injection. The plasma was heated by rf in continuous mode, while the ferroelectric cathode was pulsed at a repetition rate of 50 to 250 Hz. Pulsed electron injection at 250 Hz led to a noticeable increase of 30% of the Ar-8+ output current at a rf power level of 400 W. In addition, magnetohydrodynamic instabilities were damped during and after electron injection. These results strongly encourage further research and development on the application of ferroelectric cathodes for increasing the ion current yield from hot plasma ion sources
NIR luminescence and laser parameters of Ca3Sc2Ge3O12 garnet host crystals activated with Tm3+ and Ho3+
No full textWe have investigated the optical properties of Ca3Sc2Ge3O12 doped with Tm3+ and Ho3+, a new potential laser material for the 2 mum region. We present a room temperature characterization of both singly and co-doped crystals, including the absorption spectra in the visible and NIR regions, the NIR fluorescence spectra and the decay time measurements. In particular, attention has been focused on the H-3(4) <-- H-3(6) (Tm3+) absorption transition lying in the 800 nm region and suitable for diode laser pumping and on the Tm3+-Ho3+ energy transfer process in order to improve the efficiency of the Ho3+ emission. From the experimental data we have calculated some parameters that deeply affect the performance of a possible NIR laser, i.e. the emission cross sections in the 2 mum region and the value of the transfer coefficient between the Tm3+ and Ho3+ ions inside this crystal
A shaper for providing long laser target waveforms
No full textWe discuss the features of a shaping device consisting of a spectral amplitude modulator followed by a pair of diffraction gratings arranged as a linear dispersive stretcher. The proposed system is aimed at transforming input short pulses into long target pulses. The basic of the shaper is the construction of an amplitude profile in the frequency domain and the reproduction of the profile in the time domain. The foreseen characteristics of mechanical simplicity, stability to parameters perturbations and the capability of providing long rectangular pulses with fast rise-time make the device interesting for laser systems devoted to irradiation of radiofrequency electron gun photocathodes. (C) 2004 Elsevier B.V. All rights reserved
Operation of ceramic cathodes with bipolar pulse excitation
No full textThe emission from ferroelectric cathodes excited by bipolar pulses does not suffer the hysteretic behavior shown with monopolar pulse excitation. This is because the bipolar pulse forces a neat charging-discharging cycle of the whole surface of the sample, and, therefore, the flipping of the polarization. The emission becomes definitely stable, whereas it was erratic in current amplitude, pulse shape and starting time with monopolar excitation. The emission yield of energetic electrons from ferroelectric cathodes of doped lead-titanate-zirconate-lanthanum increased from a few hundreds of milliamperes to some amperes (with current density of 30-50 A/cm(2)) in passing from monopolar to bipolar voltage pulse excitation
High dielectric constant ceramics for ion-electron sources
No full textFerroelectric disks, coated with proper electrodes, can easily produce a dense plasma cloud when excited with a high-voltage pulse. This plasma can be a source of either electrons or ions depending on the sign of the extracting field set in front of the disk. We present the behavior of the disks operating at high frequency as emitters of both electrons and ions in two experimental configurations: (a) without and (b) with two screening grids. These two screening grids are inserted when the plasma must be confined within the cathode region, The system is capable of providing ion pulses of a few hundred milliamperes, whose length can range from a hundred nanoseconds to dozen microseconds. The electron pulses of energetic electrons have typically an amplitude higher than a couple of amperes. Tests at MHz repetition rate were positive as for stable operation
Dielectric-spontaneous polarization versus electron emission in perovskite cathodes
No full textStrong electron emission in ferroelectric cathodes requires fast polarization change. We present the correlation between the emission of energetic electrons and the polarization change obtained experimentally. The shape of the emitted current pulse and its level depend on the amplitude and the rate of the polarization variation. The polarization variation showed a fast and a slow component, related to fast and slow nanodomain relaxation time respectively: The slow rate polarization of the relaxor samples used in our tests showed a relaxation time of a few microseconds. The emission in presence of the sole fast polarization naturally depends on the exciting pulse characteristics. The emission in presence of also the slow polarization depends strongly on the relaxation dynamics: The relatively long decay time leads to a freezing of screening electrons and consequently to a low emission level
Polarizability model of emission from ceramic cathodes
No full textWe show that properly electroded ceramic disks are strong and robust electron emitters when excited with short voltage pulses. They can operate in two different regimes: (I) a regime at low exciting voltage, similar to0.7 kV per millimeter thickness and (II) a regime at high exciting voltage, similar to2 kV/mm. In the first regime the excitation pulse must be bipolar, positive-negative, the output current results in several tens of milliamperes per cm(2). The first positive semiwave charges the cathode surface and the negative semiwave expels the previously accumulated electrons. This behavior of ceramic electroded disks, before as sink and next as source of electrons, is due to the succession of an attractive and then repulsive electron field on the unelectroded zones of the front surface. The two opposite oriented fields are generated by the voltage applied at the rear electrode. In the second regime, the excitation pulse can be either monopolar or bipolar, the emission process is governed by the building up of a plasma sheet as a consequence of the excitation pulse. The high-voltage exciting pulse initiates a discharge, and so the plasma, at the triple metal-insulator-vacuum point because of the strong longitudinal component of the electric field. The plasma cloud expands over the front surface becoming a dynamical electrode. Screening electrons accumulate in front of the sample, either in the plasma sheet or on the front surface of the material. The component of the electric field perpendicular to the surface (due to either the negative semiwave of a bipolar pulse or to the space charge of the huge amount of electrons no longer attracted by bound positive charge) pushes screening electrons out from the cathode
Polarization dynamics in a ferroelectric relaxor excited by short pulses
No full textPolarization dynamics in a relaxor ferroelectric is investigated by exciting samples, set at different temperatures, with short high voltage (approximate to1 kV) rectangular pulses (approximate to200 ns) and then by measuring the charge carried to the sample, i.e., the polarization. Further important information is extracted from the trailing edge of the charge signals, which detect the relaxation of the polarization. Polarization dynamics of zero-field-cooled samples show a logarithmic increase during the voltage pulse plateau and a logarithmic decay after the pulse. The slopes depend on the field amplitude and temperature. The model of reorientable noninteracting polar clusters adequately describes the experimental behaviors
Rectangular pulse formation in a laser harmonic generation
No full textIn a harmonic generation process the temporal profile of the up-converted pulse undergoes significative changes depending on the input profile and crystal length. A simple theoretical treatment and the corresponding physical view are presented. The matter and the properties of two shaping systems are investigated in view of producing rectangular up-converted pulses, as required by laser driven radiofrequency electron sources
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