Effective Secondary Electron Yield Research Articles

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.

Effects of fast atoms and energy-dependent secondary electron emission yields in PIC/MCC simulations of capacitively coupled plasmas

In most PIC/MCC simulations of radio frequency capacitively coupled plasmas (CCPs) several simplifications are commonly made: (i) fast neutrals are not traced, (ii) heavy particle induced excitation and ionization are neglected, (iii) secondary electron emission from boundary surfaces due to neutral particle impact is not taken into account, and (iv) the secondary electron emission coefficient is assumed to be constant, i.e. independent of the incident particle energy and the surface conditions. Here, we examine the validity of these simplifications under conditions typical for plasma processing applications. We study the effects of including fast neutrals and using realistic energy-dependent secondary electron emission coefficients for ions and fast neutrals in simulations of CCPs operated in argon at 13.56 MHz and at neutral gas pressures between 5 Pa and 100 Pa. We find an increase of the plasma density and the ion flux to the electrodes under most conditions when heavy particles are included realistically in the simulation. The sheath widths are found to be smaller and the simulations are found to diverge at high pressures for high voltage amplitudes in qualitative agreement with experimental findings. By switching individual processes on and off in the simulations we identify their individual effects on the ionization dynamics and plasma parameters. While the gas-phase effects of heavy particle processes are found to be moderate at most conditions, the self-consistent calculation of the effective secondary electron yield proves to be important in simulations of CCPs in order to yield realistic results.

Experimental and kinetic simulation studies of radio-frequency and direct-current breakdown in synthetic air

We report a combined experimental and modeling study of the breakdown of synthetic air under the influence of static (dc) and time-varying electric fields, up to radio-frequencies (RF). The simulations of the breakdown events are based on a simplified model that makes the problem computationally tractable: only electrons are traced by Monte Carlo simulation. In the dc case the simulation is used for the determination of the effective secondary electron emission coefficient, based on the measured dc Paschen curve. In the RF case we observe a transition between the modes (i) where reproduction of the charges takes place in the gas volume (at electron oscillation amplitudes smaller than the gap size) and (ii) where surface processes—secondary electron emission and electron reflection—play an important role, as most of the oscillating electrons reach the electrode surfaces. The RF (f = 13.56 MHz) simulations, assuming a constant effective secondary electron yield, reproduce well the breakdown curve obtained in the experiment.

Monte-Carlo method of computing multipactor threshold in microwave devices

In order to find a rapid and accurate numerical method to compute the multipactor threshold in microwave device, three enhanced Monte-Carlo (MC) methods are proposed which are single particle-multiple collision MC, multiple particle-single collision MC and multiple particle-multiple collision MC method. The three MC methods all give the random nature of the secondary electrons, including their initial energies, phases and angles. And in all of the methods, the electron trajectory is computed with Runge-Kutta method and the secondary electron yield (SEY) per collision is computed with Furman model. The effective SEY is taken as the criterion to judge whether multipactor occurs, the definition of which is a little different from those of the three MC methods. As a verification, the multipactor in a parallel plate transmission line is investigated with the presented MC methods and the traditional MC method. The numerical results of the four MC methods are compared with the results of the statistical theory. It is demonstrated that the single particle-multiple collision MC method has the smallest error and the best stability.

A fast single particle Monte-Carlo method of computing the breakdown threshold of multipactor in microwave device

To compute the breakdown thresholds of multipactor in microwave devices, a fast single particle Monte-Carlo (SP-MC) method is presented, which considers the random nature of secondary electrons and their initial energies, phases and angles. With Runge-Kutta method and Furman model, the motion of the electron and the secondary electron yield (SEY) of the wall of the device are computed. An effective SEY is regarded as a criterion to estimate whether multipactor occurs, which is computed by averaging the SEYs for all impacts. As an example, the multipactor in a transmission line composed of parallel plates is investigated with the presented SP-MC method, traditional Monte-Carlo method, statistical theory method and particle-in-cell method separately. The results obtained from the SP-MC method accord well with those from the statistical theory method and particle-in-cell method, including the results of the susceptibility zones, break thresholds on specific products of frequency and gap space. Moreover, the SP-MC method is more adaptive than the statistical theory method, more stable than the traditional Monte-Carlo method and much more efficient than the particle-in-cell method.

Suppression of secondary emission in a magnetic field using triangular and rectangular surfaces

The effect of surface roughness on the secondary electron emission from a sawtooth and isosceles triangular surface as well as a rectangular surface in a magnetic field under electron bombardment is investigated using a Monte-Carlo method. In all cases, some of the secondary electrons emitted from the surface return to the surface within their first few gyrations, resulting in a low effective secondary electron yield. Either sawtooth or isosceles triangle surface can significantly reduce the secondary emission yield in a magnetic field below the multipacting threshold with a weak dependence on the size of the surface and magnetic field. Rectangular surfaces can also reduce SEY below 1.0 but have a large sensitivity to the size of the grooves and the magnetic field. The impedance as well as other implementations of the grooved surfaces are also discussed.

Study of transient processes in nitrogen by measurements of dynamic breakdown voltages

In this paper the measurements of the dynamic breakdown voltages U b for linearly rising pulses in nitrogen at low pressure are presented. The measurements were carried out for the rates of voltage rise k up to $300\, {\rm V\,s}^{-1}$ . Dependence of the breakdown voltages, delay times and electron yields on the rate of rise were obtained experimentally and theoretically under different conditions. It was found that the overvoltage $\overline {\Delta U_b}$ and the mean effective electron yield $\overline {YP}$ is proportional to $\sqrt k$ ( Y is a number of generated electrons in the interelectrode space per second and P the breakdown probability), while the statistical time delay $\overline t_s$ is proportional to $1/\sqrt k$ . In the second part, the experimental breakdown voltage distributions were obtained, fitted by theoretical distributions and some breakdown parameters relevant to experimental conditions were determined. Based on the approximate analytical and numerical models, the dependence of the effective secondary electron yield γ on the overvoltage and on the rate of voltage rise were derived from these measurements. It was found that γ varies linearly with the overvoltage for a constant k , and the slope of γ is proportional to $\sqrt k$ .

Recapture of secondary electrons by the target in a DC planar magnetron discharge

In this paper we describe a simple two-dimensional model that allows the study of the individual secondary electron orbits in a DC planar magnetron discharge. Emphasis is on the recapture of secondary electrons by the target, which is enabled by their small initial energy, because this reduces the effective secondary electron yield as seen by the discharge. This reduction depends strongly on both the position along the race track and the gas pressure and it can be substantial for typical planar magnetron operating conditions. Our simple model allows to conclude that because of the sensitivity of the discharge on the secondary electron yield, the current–voltage characteristic, the spatial distribution as well as the pressure dependence of the planar magnetron discharge will be influenced by recapture.

Effective Secondary Electron Yield of a Cathode for Plasma Display Panel

In order to improve the discharge characteristics of a plasma display panel (PDP), the availability γ′/γ of the secondary electrons from the PDP cathodes is investigated, which is determined by the back-diffusion effect in the discharge gas. We carried out a Monte-Carlo simulation (MCS) under conditions which were varied: the gas composition, the initial energy distribution of emitted electrons due to ions and metastable atoms incident on the metal or insulator cathode, and the electron reflectivity at the cathode. The calculated values of γ′/γ are 0.1–0.95 for ions and metastable atoms under the typical operational condition of the AC- and DC-type PDPs. It is found that the low-energy electrons emitted by Xe ions and metastable atoms have considerable availability even from low reduced electric field E/p. The data obtained in this work may be useful for the discharge simulation and the design of PDP.

The effective secondary electron yield in the space-charge limited condition

The dependence of the effective secondary electron yield on the potential distribution and the target materials was investigated by means of electron transport simulation in the sheath with the potential distribution calculated for the different emission characteristics of Be ( Z=4), C ( Z=6), Mo ( Z=42) and W ( Z=74) as the target materials. For lower primary energy incidence the effective secondary electron yield in the space-charge limited condition (SCLC) is markedly suppressed due to the prompt return of the secondary electrons. For higher energy electrons, the yield is independent of the potential distribution in the sheath and the energy distribution of the emitted electrons is shifted to higher energy direction. This effect would increase the sheath potential resulting in the increase of both ion energy flux to the surface and sputtering yield. Because of their high reflection coefficients this result should be very important for high Z materials but not for low Z materials.

Plasma doping dosimetry

Plasma immersion ion implantation (PIII) is a large-area doping technique that can provide very high implant current at very low implant energy. Multiple ion species, plasma conditions, and implanter current-voltage waveforms in PIII lead to an exponential implant profile which is different from conventional implant profiles. A methodology is developed for using in situ measurements of the implanter current (I) and implant voltage (V) to derive an energy spectrum for a single implant pulse. The advantage of this technique is that a per-pulse profile may be determined experimentally, even in the presence of substrate etching, without any need for an implant model. If the ion species concentrations in the plasma are known, the energy spectrum found from the ion current and voltage waveforms can be used to construct a per-pulse implant profile. If the ion species distribution is not known a priori for a multispecies plasma, secondary ion mass spectroscopy (SIMS) data from an implanted sample can be used to estimate the ion species distribution and calibrate the IV-generated profile within a factor of two. Data from 1-5 kV, 2.5-5 kHz BF/sub 3/ PIII implants are used to demonstrate the concept. The implant profile for a single pulse can then be used to project the final implant profile and total implanted dose as a function of implant time, Pm pulse frequency, and substrate etching. In this work, an estimated secondary electron yield function is used to separate the total implant current into ion and secondary electron current components. The 25% dose variation error introduced by this effective secondary electron yield function could be avoided in a system which can measure ion current directly.

Secondary electron emission from beryllium irradiated by plasmas in a magnetic field

A Monte-Carlo simulation model of secondary electron emission from beryllium is combined with the kinetic equation analysis for the Lorentz force perpendicular to an oblique magnetic field and the acceleration due to the sheath electric field normal to the surface. The emphasis is put on the energy distribution of secondary electrons entering into an edge plasma after their transport in the sheath, as well as the effective secondary electron yield. Some of the secondary electrons emitted from the surface return to the surface within their first gyration, resulting in low effective secondary electron yield. The higher the energies of the secondary electrons or the lower the temperature of the edge plasma electrons, the more the secondary electrons return to the surface. Lowering of the yield is accompanied by a low-energy shift of the energy distribution.

Cathode Ray Tube Screen Charging and Conditions Leading to Positive Ion Deterioration

Screen charging in standard cathode ray tubes is related to the effective yield of secondary electrons. This yield depends strongly on the nature and geometry of the screen surface. If the surface is smooth, a sticking potential can be defined, above which the screen potential does not rise; if the surface is rough, as with a standard powder layer, field dependence of effective secondary electron yield causes more complex charging effects. Dependence of screen charging on phosphor thickness, coverage, particle size and shape, and on gas pressure in the tube is considered.Ion‐focussing field configurations, associated with negative screen charging in tubes of different shapes, are correlated with observed patterns of phosphor deterioration by positive ions; production of these ions is considered. Effects of raster geometry, anode materials, and the magnitude of the focussing field are mentioned. Observations relevant to x‐burn in commercial television tubes are summarized.