H. Riege

Time‐dependent electron emission from ferroelectrics by external pulsed electric fields

Results of experiments investigating the influence of the amplitude, the growth rate, and the repetition rate of the exciting electric field pulses on the electron current and charge density emitted from ferroelectrics are reported. The behavior of two types of lead‐lanthanum‐zirconium‐titanate (PLZT) ceramics, 2/95/5 and 8/65/35, was studied. The temporal shift between the applied HV pulses and the emitted electron current pulses was different for the two materials. Regular electron emission was observed at repetition rates of up to 2 MHz in the PLZT‐2/95/5 material, showing that the recovery of the emitting sample back to the original state may happen in less than a microsecond.

Electron emission from ferroelectrics - a review

Abstract The strong pulsed emission of electrons from the surface of ferroelectric (FE) materials was discovered at CERN in 1987. Since then many aspects and properties of the method of generation and propagation of electron beams from FE have been studied experimentally. The method is based on macroscopic charge separation and self-emission of electrons under the influence of their own space-charge fields. Hence, this type of emission is not limited by the Langmuir-Child law as are conventional emission methods. Charge separation and electron emission can be achieved by rapid switching of the spontaneous, ferroelectric polarization. Polarization switching may be induced by application of electrical-field or mechanical-pressure pulses, as well as by thermal heating or laser illumination of the ferroelectric emitter. At higher emission intensities plasma formation assists the FE emission and leads to a strong growth of emitted current amplitude, which is no longer limited by the FE material and the surface properties. The most attractive features of FE emission are robustness and ease of manipulation of the emitter cathodes which can be transported through atmospheric air and used without any problems in vacuum, low-pressure gas or plasma environments. Large-area arrangements of multiple emitters, switched in interleaved mode, can produce electron beams of any shape, current amplitude or time structure. The successful application of FE emission in accelerator technology has been demonstrated experimentally in several cases, e.g. for triggering high-power gas switches, for photocathodes in electron guns, and for electron-beam generators intended to generate, neutralize and enhance ion beams in ion sources and ion linacs. Other applications can be envisaged in microwave power generators and in the fields of electronics and vacuum microelectronics.

Copious electron emission from PLZT ceramics with a high zirconium concentration

Abstract We describe the experimental method and report the results of electron emission from PLZT-2/95/5 ceramics subjected to rectangular HV pulses. The influence of HV amplitude, of gas pressure, and of temperature on electron beam current-density and charge was investigated. The kinetic energy was estimated by applying a decelerating field. Emitted current-density pulses of several A/cm2 and charges of 1 μC/cm2 are measured. A tentative interpretation is given.

Low‐pressure hollow cathode switch triggered by a pulsed electron beam emitted from ferroelectrics

A new type of low‐pressure gas switch is described. The switch is triggered by an electron beam that is emitted from the surface of a ferroelectric sample. The electron beam is generated within the hollow cathode and ejected through a hole of arbitrary shape into the main gap of the switch. The beam current and the electron energy can be chosen such that breakdown is achieved with small jitter. The switch with its ferroelectric trigger requires neither heating nor an auxiliary gas discharge. The fast spontaneous polarization change ΔPs, which is the cause of electron emission, is induced by a high‐voltage pulse from an electronic switching circuit.

Fast polarization changes in ferroelectrics and their application in accelerators

Abstract Several mechanisms are described which change the polarization in ferroelectric material. Provided the change is too rapid for the related surface charges to be screened or neutralized, the high charge density can lead to strong electric fields. The fields may possibly be used for emission and acceleration of electrons. First results of experiments are reported in which fast spontaneous polarization changes by reversal or phase transition have been demonstrated. Electrons of 25 keV energy have been observed, emitted from triglycine sulfate (TGS) crystals during phase transition while being slowly heated across the Curie temperature. With fast polarization changes, electron beams of even higher energy and density are expected.

FEATURES AND TECHNOLOGY OF FERROELECTRIC ELECTRON EMISSION

Spontaneous electrical polarization of ferroelectric materials can be changed either by reversal or by phase transition from a ferroelectric into a nonpolar state or vice versa. If spontaneous polarization changes are induced at a submicrosecond time scale, strong uncompensated surface charge densities and related fields are generated, which may lead to the intense self-emission of electrons from the negatively charged free surface areas of the ferroelectric cathode. The nature of this self-emission differs essentially from other methods of ferroelectric electron emission and from conventional electron emission in that the latter methods are only achieved by extracting electrons with externally applied electric fields. When electron guns are constructed with ferroelectric cathodes, new design criteria have to be taken into account. The intensity, the energy, the temporal and spatial distribution and the repetition rate of the emitted electron beams can be adjusted within wide limits. The advantages of fer...

Pulsed electron emission from PLZT ceramics

Abstract New experimental results on electron emission from PLZT ceramics of composition 2/95/5, subjected to rectangular high-voltage pulses are reported. The dependence of the emitted electron charge on the electric field strength and temperature was investigated. The energy of the emitted electrons was estimated by applying a decelerating potential to an auxiliary grid electrode. Emitted current densities of several A/cm2 and emitted charges up to ∼ 1 μC/cm2 (∼1013 electrons/cm2) were observed.

Intense laser‐induced electron emission from prepoled lead‐lanthanum‐zirconium‐titanate ceramics

A sample of lead‐lanthanum‐zirconium‐titanate (PLZT 9/65/35) has been exposed to 6‐ns‐long laser pulses of 266 nm wavelength. The maximum output pulse energy of the laser beam was 300 μJ, the output power density on the sample 5×105 W/cm2, and the beam diameter 3 mm. By applying a moderate extraction voltage of several kilovolts, intense electron beam pulses are emitted from the free sample surface. Their time structure corresponds to the time structure of the laser pulse. Electron beam current intensities of up to 0.1 A and 2 A/cm2 and total charges of 1 nC (corresponding to 20 nC/cm2 ) were measured with a simple Faraday cup. In the range where the parameters of laser intensity and of extraction voltage could be varied their influence on the emitted electron beam current amplitude was determined.

Intense laser-induced self-emission of electrons from ferroelectrics

Abstract We report on laser-induced electron emission (LIEE) from ferroelectrics (FE) at 266, 355 and 532 nm wavelength without any extraction voltage. Emitted charges of up to 4 nC/cm 2 with kinetic energies of up to 10 KeV, currents exceeding 100 mA, and emitted current densities of several A/cm 2 were observed with PLZT ceramics at laser pulse energies of 1 to 2 mJ (10 6 W/cm 2 ) and a pulse width of 5 ns FWHM. The driving electric field is generated by the switching of the spontaneous FE polarization P s in a thin surface layer at the time scale of 1 ns. The extraction-field-free emission can be maintained at a constant level, when the polarization switching of the FE cathode is regularly repeated. Reliable long-term LIEE operation of FE cathodes, emitting nC electron charges, has been demonstrated.

Field-excited electron emission from lanthanum-doped, barium-strontium-titanate ceramics

Abstract The field-excited electron emission from lanthanum-doped barium—strontium—titanate ceramics was studied. Emitted charge densities up to 100 nC/cm2 were measured. The results from field-excited emission and from dielectric measurements, obtained with the same samples, support the hypotheses of the existence and of the reversibility of fast-switching ferroelectric microdomains. These ceramics have properties, such as electrical conductivity, dielectric constant, thermostimulated currents, which are typically related to ferroelectric relaxors. It is shown that these properties can be modified by adequate thermal and electrical treatments.