Agust Valfells

Space-Charge Modulation in Vacuum Microdiodes at THz Frequencies

We investigate the dynamics of a space-charge limited, photoinjected, electron beam in a microscopic vacuum diode. Because of the small nature of the system it is possible to conduct high-resolution simulations where the number of simulated particles is equal to the number of electrons within the system. In a series of simulations of molecular dynamics type, where electrons are treated as point charges, we address and analyze space-charge effects in a micrometer-scale vacuum diode. We have been able to reproduce breakup of a single pulse injected with a current density beyond the Child-Langmuir limit, and we find that continuous injection of current into the diode gap results in a well-defined train of electron bunches corresponding to THz frequency. A simple analytical explanation of this behavior is given.

100 years of the physics of diodes

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the ...

Molecular Dynamics Simulations of Field Emission From a Planar Nanodiode

High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission in planar nanodiodes. The effects of space charge and emitter radius are examined and compared to previous results concerning transition from Fowler-Nordheim to Child-Langmuir current. The Fowler-Nordheim law is used to determine the current density injected into the system and the Metropolis-Hastings algorithm to find a favourable point of emission on the emitter surface. A simple fluid like model is also developed and its results are in qualitative agreement with the simulations.

Tunability of the terahertz space-charge modulation in a vacuum microdiode

Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It ...

Synchronization in Arrays of Vacuum Microdiodes

Simulations have shown that space-charge effects can lead to regular modulation of photoemitted beams in vacuum diodes with gap sizes on the order of 1 μm and accelerating voltage on the order of 1 V. These modulations are in the terahertz regime and can be tuned by simply changing the emitter area or accelerating vacuum field. The average current in the diode corresponds to the Child-Langmuir current, but the amplitude of the oscillations is affected by various factors. Given the small size and voltage of the system, the maximum radiated ac power is expected to be small. In this paper, we show that an array of small emitters produces higher frequency signals than a single large emitter of the same area and how these emitters may be synchronized to produce higher power signals.

Molecular Dynamics Simulations of Field Emission From a Prolate Spheroidal Tip

High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission from a prolate spheroidal tip. The space charge limited current is several times lower than the current calculated with the Fowler-Nordheim formula. The image-charge is taken into account with a spherical approximation, which is good around the top of the tip, i.e. region where the current is generated.

Susceptibility diagram of multipactor discharge on a dielectric-effects of an external magnetic field and oblique RF electric field

Summary form only given. Monte-Carlo simulations have recently been used to construct the susceptibility diagram of multipactor discharge on a dielectric, under the assumption that the RF electric field is parallel to the dielectric surface. This diagram, constructed from kinematic considerations, turns out to be extremely useful in the prediction of the saturation level. We generalize the susceptibility diagram in two aspects: inclusion of an external magnetic field and the effects of an oblique RF electric field. It is found that the presence of an external magnetic field does not qualitatively change the susceptibility diagram, regardless of the orientation of the magnetic field. This statement holds for all magnetic fields simulated, up to those values whose cyclotron frequency is on the order of the RF frequency.

Synchronization of THz space-charge oscillation in arrays of vacuum microdiodes

Summary form only given. Simulations have shown that space-charge effects can lead to regular modulation of photoemitted beams in vacuum diodes with gaps size on the order of 1μm and accelerating voltage on the order of 1V1. These modulations are in the THz regime and can be tuned by simply changing the emitter area or accelerating vacuum field2. The average current in the diode corresponds to the Child-Langmuir current, but the amplitude of the socillations is affected by various factors. Given the small size and voltage of the system, the maximum radiated AC power is expected to be small. In this work we show that an array of small emitters produces higher frequency signals than a single large emitter of same area, and how these emitters may be synchronized to produce higher power signals. We also present some results on beam structure from such arrays and possibility of how to control it.

Molecular dynamics simulations of field emission from a planar nanodiode and prolate spheroidal tip

Summary form only given. High resolution molecular dynamics simulations with full Coulomb effect are used to investigate field emission in planar nanodiodes and from a prolate-spheroidal tip. The effects of space charge and emitter radius in a planar nanodiode are examined and compared to previous results concerning transition from Fowler-Nordheim to Child-Langmuir current1,2. Similarly, different geometries for the prolate-spheroidal tips are investigated to determine the influence of space-charge and Coulomb effects on emission current and beam quality from the tip. The Fowler-Nordheim law is used to determine the current density injected into the system and the Metropolis-Hastings algorithm to find a favourable point of emission on the emitter surface.

Vacuum microdiodes as tunable THZ oscillators

Summary form only given. Vacuum diodes with gap spacing of the order 1μm and gap voltages of the order 1V exhibit space-charge induced current modulation in the THz regime. The frequency can be tuned by simply altering the applied DC vacuum electric field, and the frequency range corresponding to the range of the applied field can be set by selection of the size of the emitter area on the cathode. The current in a microdiode of this type will typically range from tens to hundreds of microamperes, thus limiting the THz power that can be drawn from a single microdiode. Coupling between microdiodes is a problem of interest as it could offer an avenue to generate higher power levels by using an array of emitters. We present an overview of work done on vacuum microdiode oscillators, with some new results concerning beam quality in a single microdiode and interaction between beams from neighboring emitters.