Secondary Emission Coefficient Research Articles

Secondary electron emission coefficient for fission fragments with a fixed mass and kinetic energy

The experimentally obtained secondary emission coefficient of electrons from thin aluminum oxide films is described within the framework of effective-charge models and a velocity-dependent expression for fission-fragment energy losses. At the same time, it was shown that secondary emission is related to energy lost to electrons, and not to the ionic charge of the fragments. This semiempirical formula can be used to calculate energy losses from heavy ions in heavy-element materials. In our opinion, the work of Brunelle et al. [3] cannot serve as a basis for identifying ion charges that penetrate both surfaces of a target.

Low-energy secondary-electron spectroscopy of molybdenum

A theoretical interpretation of the fine structure of the secondary-electron emission spectra and the secondary-electron emission coefficient is given for molybdenum. The calculations are carried out with allowance for the energy dependence of the energy-band broadening, the electron-electron and electron-plasmon contributions to the state occupation function, and the isotropic component of the current from surface-scattered electrons. It is shown that the fine structure of the secondary emission spectra and the secondary emission coefficient is mainly attributable to the electronic structure of the final states into which electrons enter and from which electrons are emitted.

Secondary electron flow from the axially symmetric collector without magnetic field

Axially symmetric electron beam collector without magnetic field providing a low secondary electron flow is discussed. A simple formula for estimating the collector secondary emission coefficient is suggested. It is in good agreement with results of measurements. The suppression of the secondary electron flow from the collector down to values less than 1 × 10 −5 is experimentally obtained.

The effects of secondary electron emission on the operation of position sensitive anodes

Secondary electron emission plays a crucial role in the operation of position sensing anodes used in imaging microchannel plate (MCP) intensifiers. Secondary electrons emitted from the anode strongly influence both the size and shape of the collected charge cloud distribution and also mediate charge redistribution between the anode electrodes. The performance of position readouts using charge division can be severely limited by these mechanisms. We describe experiments designed to study the secondary electron emission process from a charge division position readout. An intermediate grid between the MCP and anode is used to establish an electric field above the anode to control the trajectories of the secondaries without disturbing the operation of the MCP. Tuning of the grid and anode voltages allows the measurement of a variety of physical processes occurring within the detector. We present measurements of the secondary emission coefficients of anode and grid materials, and show how both the initial spatial and energy distribution of primary electrons from the MCP may be measured. The usefulness of controlling secondary electrons to improve imaging performance is demonstrated.

Effective secondary emission coefficient in a high-pressure noble gas

The back-scattering of secondary electrons toward the cathode in a high-density noble gas and the resulting effective secondary (electron) emission coefficient, ESEC, is evaluated using a kinetic approach. The behaviour of the ESEC in a mixture of noble gases is also discussed. The obtained results can be applied to different sources of secondary electrons - ions, photons, etc. We provide a comparison of obtained expressions with experiments.

Plasma convection induced by toroidal asymmetries of the divertor plates and gas puffing

In the open field line region of the scrape-off layer (SOL), the plasma potential is to a considerable degree determined by the boundary conditions on the divertor plates. The idea is explored here that, by making the divertor plates slightly `wavy' in the toroidal direction, toroidally varying potentials can be produced throughout the SOL and thereby convective plasma motion induced. This motion should lead to a broadening of the SOL and to a reduction of the heat load on the divertor plates. An attractive feature of this technique is that it induces strong convection only on the open field lines without causing enhanced transport inside the separatrix. The possibility is also considered of inducing plasma convection by varying the composition of the divertor plate in the toroidal and radial directions, so as to affect the plasma potential by virtue of the toroidal variation of the secondary emission coefficient. Mutual biasing of adjacent elements of the plates is the third option. Finally, the use of toroidally asymmetric gas puffing to induce toroidal asymmetry in the potential distribution is discussed

Effect of random secondary delay times and emission velocities in electron multipactors

Traditionally, the modeling of rf driven electron cascades (multipactors) has employed a constant emission velocity and zero delay time for all secondaries. This paper introduces the effects from the random secondary emission velocities, as well as the random delay time in the secondary emission. By combining the emission related random phase kicks with the focusing rf action it is shown that the electron impact phase, though wandering erratically, nevertheless stays in the vicinity of a stable phase-locked multipactor phase; the rms phase deviation from phase locking is bounded and directly proportional to the rms fluctuation in the secondary emission. The random phase wandering around the locked phase reduces the nominal secondary emission coefficient /spl delta/ to an effective value /spl delta/* between /spl delta/>/spl delta/*/spl ges//spl delta//2 and depending not only on the impact energy of the primary but also on the thermal spread of the secondaries. Multipactor quenching from secondary spreads may occur through the reduction in /spl delta/*. For sufficiently small fluctuations, the modeling of a cold multipactor with zero thermal spreads is sufficient to determine the actual multipactor behavior in terms of only two control parameters; the normalized rf and magnetic field strengths.

Flat thin CRT based on controlled electron transport through insulating structures

The feasibility is demonstrated of a flat thin cathode ray tube with insulating supports inside the envelope to withstand the atmospheric pressure. The supports are actively used to guide the electrons from the cathodes to the phosphor screen. The electrons are transported through the insulating structures by a self-regulating secondary emission process, provided a sufficiently high voltage is applied between the entrance and the exit of the structures. The local potentials are automatically adjusted by charge deposition on the insulating walls until a situation is established characterized by the property that the local average secondary emission coefficient is equal to one everywhere in the structure. The flat thin CRT with these insulating internal supports has a thickness of about 1 cm for any display size and a relatively low mass. Large displays have been realized showing a good performance: brightness, contrast, colour purity and power dissipation are comparable to or better than obtained with conventional cathode ray tubes. Some other interesting applications based on the same phenomenon are briefly discussed: an electron ‘fibre’, an electron source with high current density and a beam switch.

Electrical characteristics of a low pressure wire discharge and an application to high current density electron gun

The electrical characteristics of a wire discharge plasma were investigated, in particular, taking notice of the effect of electron trapping by axial magnetic fields. It was found that the plasma property changed remarkably, when the magnetic fields were applied. This property may be used to control the wire discharge plasma. The wire discharge plasma plays an important part as an ion source of an ion-induced secondary emission electron gun. This type of electron gun was demonstrated. The maximum electron beam current obtained was 90mA/cm 2 at beam energy of 100keV. In our calculation model, the secondary emission coefficient γ is 12.0 for 100keV helium ion and the electron beam extraction efficiency β obtained is 0.35.

Secondary electron current loss in electron cooling devices

The efficiency of secondary electron capture in a recuperator with a longitudinal magnetic field is evaluated. To characterize this efficiency, the value of the collector secondary emission coefficient is introduced, for the calculation of which a simple formula is proposed. The effects determining the difference between the current losses in straight systems and devices with bending magnets are analyzed. It is experimentally shown that there is no unambiguous relation between the efficiency of the secondary electron capture by the collector and the current loss. The mechanism which determines the current loss in straight systems is suggested.

Investigations on the ignition of high-voltage glow discharges

The ignition of high-voltage glow discharges at voltages of several kilovolts cannot be described by the Townsend theory. Therefore a simulation code was developed, which allows one to estimate the influence of several elementary processes during the ignition process and to calculate ignition curves. One result of these simulations is that secondary electron emission by electron impact at the anode is very important for production of charged particles. Experimental measurements of ignition voltage yield that the similarity law Un=f(pd) is not valid. The ignition voltage decreases with increasing electric field, which can be related to changes in ionization cross sections and secondary emission coefficients due to the high electric fields of several kilovolts per centimetre.

Transition from a Townsend discharge to a normal discharge via two-dimensional modeling.

The transition from a Townsend discharge to a normal discharge is investigated using a two-dimensional numerical model and an approximate analysis. The numerical model is based on a fluid description of electron and ion transport coupled with Poisson's equation, with the ionization source depending on the local field strength or provided by a Monte Carlo simulation of the fast electrons. The model is applied to an argon discharge, for a product of pressure and gap length in the 1--10 Torr cm range. The proposed analytical model provides insight into the major physical phenomena observed experimentally in the subnormal glow region: the lateral constriction of the Townsend discharge with an increase of the current, the negative differential resistance of the discharge with a hysteresis loop in the current-voltage characteristics, and the appearance of current oscillations and their dependence on parameters of the external circuit. The field distortion is responsible for the constriction of the Townsend discharge provided that either the sign of the second derivative of the ionization coefficient \ensuremath{\alpha} with respect to the electric field strength E is positive or the secondary emission coefficient \ensuremath{\gamma} is an increasing function of E. A simple analytical description of nonlocal ionization is also suggested. Subnormal oscillations are treated as a two-dimensional phenomenon.

Limits of low-energy electron optics

High-resolution beams of low-energy (20–2000 eV) electrons are obviously attractive because of the very compact volume of interaction between the beam and sample, and because the associated high secondary emission coefficient minimizes charging of insulating samples. Previous work showed that values of aberration coefficients can be scaled down with voltage and minimum values are achieved by maximizing the focusing field. Here we derive some very simple expressions for the minimum beam diameter and show some experimental results. For a magnetic lens with a constant field of 1 T operating at a large demagnification, the limiting beam diameter is set by chromatic aberration (with an energy spread of 1 eV) and diffraction, and is approximately dm=126 V−1/2 nm, where V is the electron energy in eV; the dependence on magnetic field strength B is B−1/2. For a retarding field lens operating at a large demagnification and with the final landing voltage V much less than the accelerating potential, the limiting value of beam diameter is given by de=17 V−1/4 nm; the dependence on electric field strength E is E−1/2. Experimentally, preliminary results were obtained with a small, (permanent) magnetic lens that approximates the constant field case and can be inserted into the work chamber of a conventional scanning electron microscope (SEM). This added immersion lens effectively extends the range of operation of an ordinary SEM down to low energy, and has so far achieved 40 nm resolution at 300 V.

Nonlinear self-consistent theory for crossed-field devices.

A closed, nonlinear set of fluid equations that is based on the electron guiding-center orbits and is generally applicable to the analysis of crossed-field, slow-wave devices, is developed. The equations are used to model the behavior of the crossed-field amplifier. The dielectric response from the spoke charge is self-consistently included. A mean-field approximation is introduced to express the effect of the spoke charge on the rf mode profile. The dielectric modifications are then parametrized by an average amplitude factor \ensuremath{\Lambda}^ and an average phase shift \ensuremath{\psi}^ from the vacuum values. In the synchronous with the rf signal frame of reference the streamlines follow the equipotential surfaces of the transformed fields. In the steady state, the flow is incompressible. A uniform-density, constant-height electron hub feeds the current spokes. The secondary electron production at the cathode is computed self-consistently through the secondary-emission coefficient and the average impact energy. The spoke current is determined by the difference of the E\ifmmode\times\else\texttimes\fi{}B drift at the top of the hub from the rf phase velocity. At small space-charge density relative to the Brillouin density, the dielectric corrections enter as a rotation ${\mathit{e}}^{\mathit{i}\mathrm{\ensuremath{\psi}}\mathrm{^}}$ of the complex growth rate relative to the growth without spoke self-fields. The numerical solutions for arbitrary space charge are compared with previous results without spoke-field effects. An increase in the rf gain and in the anode dc current is observed at any given operation point, despite a small drop in the efficiency. It is concluded that the increase in the rf field strength in the anode-cathode space, caused by the spoke self-field, compensates for the detuning effects from the modifications on the rf mode profile. In reentrant devices, the recycling of the space charge further increases the output power and the level of noise generated by the output-input feedback.

Effects of secondary and thermionic electron emission on the collector and source sheaths of a finite ion temperature plasma using kinetic theory and numerical simulation

The region between a Maxwellian plasma source and an absorbing surface that emits cool electrons is modeled numerically with dynamic, electrostatic particle simulation and theoretically with a static, kinetic plasma-sheath model. Steady-state emission results are applied specifically to secondary electrons that are induced by either incident ions or electrons, but are also valid for thermionic and photoelectrons. The ratio of the emitted electron current to incident electron current is varied up to and beyond the critical emission coefficient (ratio) that causes electric field reversal at the collector. Results from these models agree very well over the range from zero to five times the critical emission coefficient. Increasing the secondary emission coefficient is found to reduce the collector potential and decrease the ion energy deposited, yet increase the total energy flux to the collector. In the simulation, some heating of the secondary electron stream is observed to gradually evolve over many Debye lengths, possibly because of a beam–plasma interaction. This heating increases potential fluctuations but causes only small deviations from the predictions with static theory.

Feedback-induced noise in crossed field devices

A guiding center fluid model involving a single RF mode of slowly varying amplitude and phase is used to study the long-time behavior of a distributed emission, reentrant crossed-field amplifier. The space charge recirculation through the drift space provides the main coupling mechanism between the output and the input. The output fluctuations among successive tube passes in the RF power, phase shift and hub density generate a narrow bandwidth around the carrier. By fixing the external control parameters and varying the secondary emission coefficient or the recirculating hub charge fraction, the pass-to-pass fluctuations exhibit transitions from steady-state to periodic limit cycles and finally to unpredictable, chaotic behavior. It is shown that the development of chaotic output patterns is connected with slow, large-amplitude hub density oscillations during a single pass. Analysis shows that the nonlinear and time-delay effects characterizing the hub density evolution cause unstable, oscillatory behavior. The influence of excess space charge accumulation on the tube intrinsic noise is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Determination of the noncharging electron-beam energy on insulators

A new method allowing accurate determination of the primary electron energy corresponding to the zero charge deposition on insulators is presented. It consists in evaluating the surface voltage variations under an electron-beam charge deposition, by means of a classical secondary electron spectrometer. Experimental data on lithography photoresists are reported. Comparison is made with classical metrology measurements, i.e., imaging profiles, and a good agreement is found. Experimental results show the angular dependence of the highest noncharge energy. A formulation of the secondary emission coefficient angular dependence is proposed and discussed versus experimental data.

Guiding center theory and simulations of the crossed-field amplifier

Electrons in crossed-field devices execute complicated orbits in crossed static electric and magnetic fields, driven at the same time by slow rf waves. The dynamics is considerably simplified by averaging the fast-time-scale, periodic gyromotion. The reduced equations derived here follow the slow guiding center drift in the synchronous frame of reference moving at the phase velocity of the wave. In that frame, the streamlines follow the equipotential surfaces of the transformed fields. In steady-state, the flow is incompressible. A closed, nonlinear set of equations, based on the guiding center flow, is developed to model the crossed-field amplifier behavior. The effects of the electron hub surrounding the cathode are included. The secondary production at the cathode is computed self-consistently through the secondary emission coefficient and the average impact energy. The equations are implemented in a numerical algorithm that is much faster and more efficient than existing particle codes and shows excellent agreement with experimental results for characteristic curves, tube gain, and efficiency, over a wide parameter regime.

Anomalous voltage-current characteristics in the sputtering of YBaCuO

The I–V characteristics of magnetron glow discharges with a Y 1Ba 2Cu 3O 7−δ superconductor cathode are studied and found to have anomalously low operating voltage, negative resistance and strong hysteresis effects. The energy dependence of the ion induced secondary electron and secondary ion emission coefficient of Y 1Ba 2Cu 3O 7−δ was measured and found to have a minimum at 350 V, a strong increase below this value and a steady increase above this value. The characteristics of the discharge are explained in terms of the secondary species emission coefficient.

Energy distributions and emission coefficients of secondary ions ejected from AIIIBV group semiconductors

The energy distributions of Ga+ and Ga2+ secondary ions sputtered from the surface of single crystals of GaAs and GaP by Ar+ (E = 18 keV, j = 1·10−6 A/cm2) ions were studied. Integral emission coefficients for secondary ions from four semiconductors (GaAs, GaP, InAs, and A10.35Ga0.65As) bombarded by Ar+ ions (E = 2 keV, j0 = 1·10−6 A/cm2) were also measured. It is shown that the energy distribution of the Ga+ secondary ions are broad and qualitatively similar to distributions of secondary ions sputtered from metal oxide surfaces. Very high integral emission coefficients (up to 20–30%) for secondary ions are also observed, exceeding analogous coefficients for clean metal surfaces. These facts (the significant number of low energy secondary electrons and high secondary ion emission coefficients) are due to the lower density of free electrons in comparison with metals, which ensures a high degree of survival of sputtered ions, and the partially ionic character of the bonds in the semiconductors under study.