Levi Schächter

Dielectric laser accelerators

We describe recent advances in the study of particle acceleration using dielectric near-field structures driven by infrared lasers, which we refer to as Dielectric Laser Accelerators. Implications for high energy physics and other applications are discussed.

Advances in cold cathode physics and technology

We review recent progress in the physics and technology of cold cathode electron emitters. The characteristics of emission from field emitter arrays, photocathodes, and ferroelectrics are presented, together with a summary of the understanding of the physics involved. The paper concludes with a description of L-band micropulse gun, based on secondary emission in an RF cavity. Emphasis is placed on cathode development for electron guns to drive microwave tubes and RF accelerators.

Use of partially oxidized SiC particle bed for microwave sintering of low loss ceramics

Abstract A 1 kW hybrid microwave system was fabricated using a partially oxidized SiC powder bed and used to sinter small components of low-loss insulating ceramics. Samples of 3% yttria stabilized zirconia/20% alumina (3YZA) and 99% alumina were sintered to final densities of 99%(3YZA) and 95%(99% alumina). Partially oxidized silicon carbide (β-SiC) powder was used as susceptor (preheater). By comparison, unoxidised β-SiC powder, which couples well with microwaves at room temperature, exhibited thermal runaway above 400°C. It could be possible that at high oxidation levels the connectivity between SiC particles in β-SiC powder bed might become depercolated, and a model similar to one proposed for silicon nitridation under microwaves could be appropriate in understanding the microwave absorption phenomenon. Stable temperature measurements at various positions in the system confirmed that the maximum temperature was at the center of the 3YZA sample. The plot of the relative temperature difference between the sample and its surroundings as a function of sample temperature resulted in a bell-shaped curve with a clear maximum at around 800°C, associated with the rapid increase in radiation heat transfer above this temperature. Experiments also confirmed that sintering occurred at lower temperatures in a microwave field when compared to conventional sintering.

Electron‐beam diodes using ferroelectric cathodes

A new high current density electron source is investigated. The source consists of a polarized ceramic disk with aluminium electrodes coated on both faces. The front electrode is etched in a periodic grid to expose the ceramic beneath. A rapid change in the polarization state of the ceramic results in the emission of a high density electron cloud into a 1 to 10mm diode gap. The anode potential is maintained by a charged transmission line. Some of the emitted electrons traverse the gap and an electron current flows. The emitted electron current has been measured as a function of the gap spacing and the anode potential. Current densities in excess of 70 A/cm2 have been measured. The current is found to vary linearly with the anode voltage for gaps <; 10 mm, and exceeds the Child-Langmuir current by at least two orders of magnitude. The experimental data will be compared with predictions from a model based on the emission of a cloud of electrons from the ferroelectric which in turn reflex in the diode gap.

Analytic expression for triple-point electron emission from an ideal edge

The electric field in the vicinity of a metallic edge attached to a dielectric half-space is calculated analytically. The resulting electric field is used to evaluate the current emitted from the edge using the Fowler-Nordheim formula. It is shown analytically that the emitted current is proportional to the dielectric coefficient of the material.

Analysis of a diode with a ferroelectric cathode

It has been shown experimentally that electron current densities of more than 30 A/cm2 can be achieved from a cathode made of ferroelectric ceramic, when applying a field of order 0.1 MV/m. This current exceeds the Child–Langmuir current by two orders of magnitude. The current in the diode varies linearly with the applied voltage, provided that the latter is positive. In this theoretical study we show that the ferroelectric material plays a crucial role in the emission process. When a voltage is applied to the ferroelectric, the internal polarization field varies and the amount of screening charge required decreases. As a result, the electrons distribution near the cathode changes, forming a cloud which fills part of the diode gap. If now a positive voltage is applied to the anode, electrons are readily transferred through the diode gap. The qualitative and quantitative results of the theory are in good accordance with the experiment.

Smith–Purcell oscillator in an exponential gain regime

A Smith–Purcell oscillator with a thick electron beam is analyzed in its exponential gain regime. A general expression is obtained for the dispersion relation of the oscillator, and the threshold current for self‐sustained oscillations is found to be less than 1 A for radiation of 1‐mm wavelength.

influence of the guiding magnetic field on the performance of a Smith-Purcell amplifier operating in the weak Compton regime

We determine the general expression for the power exchanged between a thin beam of electrons, TE and TM waves in the presence of an open slow-wave structure, and a guiding magnetic field. When the system is illuminated only with a TM wave, the gain due to the longitudinal oscillations is identical to that in the case when an infinite magnetic field is present but, in addition, the transverse oscillation contributes to the absorbed power; the optimal magnetic field that should be applied is established. The maximal gain in the TE case is found to increase linearly with the intensity of the guiding field. When both fields are present there is a magnetic field that maximizes the additional power that emerges.

Bragg reflection waveguides with a matching layer

It is demonstrated that Bragg reflection waveguides, either planar or cylindrical, can be designed to support a symmetric mode with a specified core field distribution, by adjusting the first layer width. Analytic expressions are given for this matching layer, which matches between the electromagnetic field in the core, and a Bragg mirror optimally designed for the mode. This adjustment may change significantly the characteristics of the waveguide. At the particular wavelength for which the waveguide is designed, the electromagnetic field is identical to that of a partially dielectric loaded metallic or perfect magnetic waveguide, rather than a pure metallic waveguide. Either a planar or coaxial Bragg waveguide is shown to support a mode that has a TEM field distribution in the hollow region.

Electron emission from lead–zirconate–titanate ceramics

We report extensive experimental data on electron emission from lead–zirconate–titanate ferroelectric ceramics. A 1–2 MV/m pulse is applied to a gridded ferroelectric cathode and diode currents of up to 120 A/cm2 are measured across an A–K gap of 5×10−2 m, with the anode at 35 kV. Both the current and the anode voltage pulse duration may extend to several microseconds. The measurements extend previously reported data by nearly two orders of magnitude in the diode voltage and by a factor of more than 3 in the diode spacing. Two major regimes of operation were identified. In the first ∼1 μs the ferroelectric cathode controls the electron flow through the diode. Beyond this time plasma effects dominate the current flow. The results are of importance to the development of novel cathodes for high current electron beam generation.