Secondary Emission Yield Research Articles

Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory

This paper presents a statistical theory for the initial onset of multipactor breakdown in coaxial transmission lines, taking both the nonuniform electric field and random electron emission velocity into account. A general numerical method is first developed to construct the joint probability density function based on the approximate equation of the electron trajectory. The nonstationary dynamics of the multipactor process on both surfaces of coaxial lines are modelled based on the probability of various impacts and their corresponding secondary emission. The resonant assumption of the classical theory on the independent double-sided and single-sided impacts is replaced by the consideration of their interaction. As a result, the time evolutions of the electron population for exponential growth and absorption on both inner and outer conductor, in response to the applied voltage above and below the multipactor breakdown level, are obtained to investigate the exact mechanism of multipactor discharge in coaxial lines. Furthermore, the multipactor threshold predictions of the presented model are compared with experimental results using measured secondary emission yield of the tested samples which shows reasonable agreement. Finally, the detailed impact scenario reveals that single-surface multipactor is more likely to occur with a higher outer to inner conductor radius ratio.

Measurement of the secondary electron emission from CVD diamond films using phosphor screen detectors

Diamond-based photomultipliers have the potential to provide a significant improvement over existing devices due to diamond's high secondary electron yield and narrow energy distribution of secondary electrons which improves energy resolution creating extremely fast response times. In this paper we describe an experimental apparatus designed to study secondary electron emission from diamond membranes only 400 nm thick, observed in reflection and transmission configurations. The setup consists of a system of calibrated P22 green phosphor screens acting as radiation converters which are used in combination with photomultiplier tubes to acquire secondary emission yield data from the diamond samples. The superior signal voltage sampling of the phosphor screen setup compared with traditional Faraday Cup detection allows the variation in the secondary electron yield across the sample to be visualised, allowing spatial distributions to be obtained. Preliminary reflection and transmission yield data are presented as a function of primary electron energy for selected CVD diamond films and membranes. Reflection data were also obtained from the same sample set using a Faraday Cup detector setup. In general, the curves for secondary electron yield versus primary energy for both measurement setups were comparable. On average a 15–20% lower signal was recorded on our setup compared to the Faraday Cup, which was attributed to the lower photoluminescent efficiency of the P22 phosphor screens when operated at sub-kilovolt bias voltages.

Effect of secondary emission on the argon plasma afterglow with large dust density

A zero-dimensional, space-averaged model for argon plasma afterglow with large dust density is developed. In the model, three groups of electrons in the plasma afterglow are assumed: (i) thermal electrons with Maxwellian distribution, (ii) energetic electrons generated by metastable-metastable collisions (metastable pooling), and (iii) secondary electrons generated at collisions of ions with the electrodes, which have sufficiently large negative voltages in the afterglow. The model calculates the time-dependencies for electron densities in plasma afterglow based on experimental decay times for metastable density and electrode bias. The effect of secondary emission on electron density in the afterglow is estimated by varying secondary emission yields. It is found that this effect is less important than metastable pooling. The case of dust-free plasma afterglow is considered also, and it is found that in the afterglow the effect of secondary emission may be more important than metastable pooling. The secondary emission may increase thermal electron density ne in dust-free and dusty plasma afterglows on a few ten percentages. The calculated time dependencies for ne in dust-free and dusty plasma afterglows describe well the experimental results.

Effect of high-efficiency emitter on noise characteristics of electron amplifiers

Electron-optical devices with microchannel amplification have many advantages, except poor noise characteristics. In order to increase gain and reduce a noise factor a layer with increased secondary emission is deposited on the top of a contact conducting layer at the entrance of channels. Effects, arising in the channel amplifier with the high-efficiency emitter, are investigated in this work using the computational method, developed by the author. A computational method for simulation of stochastic processes of an electron multiplication is based on 3D Monte Carlo simulations and the theorems of serial and parallel amplification stages proposed by the author. The method provides a high calculation accuracy with minimal cost of computations. The computational model is used to investigate the effect on the noise factor of the incidence angle of the input electron beam, nonuniformity of the emitter surface, the depth and secondary emission yield of the high-efficiency emitter.

Influence of slow wave structure explosive emission on high-power surface wave oscillator

As the working frequency of a vacuum electron device reaches the terahertz frequency band, the cross section of the surface wave oscillators (SWO) becomes very small, and the micro-fabrication precision of the device cannot be guaranteed, at the same time, because the electromagnetic field of SWO concentrates on the inner surface in slow wave structure, when the working voltage of surface wave oscillator is very high, the explosive emission probability of the slow wave structure increases greatly, and the explosive emission can influence the working characteristic of the device. This paper analyses the distributing property of the electrical field in the slow wave structure of 0.14 THz SWO. Parameters of the SWO under study are as follows: working voltage is 312 kV, explosive emitted current is 1.67 kA, periodic length of the slow wave structure is 0.7 mm, width of the slot is 0.4 mm, and the height of it is 0.3 mm; cold-test results indicate that the amplitude of the electrical field in the slow wave structure varies sinusoidally; the amplitude of the electrical field reaches a maximum value in the middle of the slow wave structure near its inner surface, and the explosive electron emission can occur most possibly in this position, because the electrical field in the slow wave structure varies with very high working frequency. The explosive emitted electron may bombard back the slow wave structure, and the secondary electrons will be emitted at a certain probability, for which the formula proposed by Vaughan is used to compute the secondary emission yield, and this formula is implemented in the self-developed particle-in-cell code UNIPIC; while the code is used to simulate 0.14 THz SWO with explosive emission in the slow wave structure. In the simulation, the slow wave structure multipactor discharge induced by electrons is also considered; the phase space of the electrons emitted from the slow wave structure shows that the energy of secondary electrons is below 5 keV, so the validity for secondary electron yield is affirmed. Numerical simulation results indicate that because the emitted electrons from the slow wave structure change the distribution character of the electrical field in the slow wave structure, especially the amplitude of the electrical field in the middle of the slow wave structure, the beam-wave interaction is weakened, and as a result, output power decreases from about 22.6 megawatts to only 1.89 megawatts.

Studies on the pressed yttrium oxide-tungsten matrix as a possible dispenser cathode material

Yttrium oxide was chosen as the secondary emission substance based on calculation results through first principle theory method. A new kind of pressed yttrium oxide-tungsten matrix dispenser cathodes are prepared by a sol–gel method combined with high temperature sintering in dry hydrogen atmosphere. The results show that the growth of the grains is hampered by the pinning effect of Y2O3 distributing uniformly between the tungsten particles, resulting in the formation of small grain size. It is found that Y2O3 improves the secondary electron emission property, i.e., the secondary emission yield increases with the increase of Y2O3 content in the samples. The maximum secondary emission yield δmax of the cathode with 15% amount of Y2O3 can reach 2.92. Furthermore, the cathode shows a certain thermionic emission performance. The zero field emission current density J0 of 4.18A/cm2 has reached at 1050 °Cb for this kind of cathode after being activated at 1200 °Cb, which are much higher than that of rare earth oxide doped molybdenum (REO-Mo) cathode reported in the previous work.

Influence of the oblique magnetic field on the secondary electron emission from the plasma facing materials in fusion reactor

The effect of the magnetic field inclination on the energy and angular distribution of electrons impinging on the first wall is modeled. It demonstrates the necessity to take into account the dependences of average energy and incident angle of electrons bombarding the wall on the magnetic field inclination in calculation of coefficient and the yield of secondary electron-electron emission.

Multipactor suppression by micro-structured gold/silver coatings for space applications

The secondary electron emission (SEE) from materials used in high power RF devices in space is the main trigger and sustaining mechanism of the resonant avalanche electron discharge known as the multipactor effect. It limits the attainable power of those devices. During recent decades, some scientific research has been focused on material properties for obtaining anti-multipactor coatings of low secondary emission yield (SEY). The European Space Agency (ESA) is leading a technological research on a new approach based on surface roughness that might perform as a kind of blackbody or Faraday cage effect. A multilayer coating structure was adopted for fulfilling the stringent requirements of the space. The surface of a standard silver plating was modified by a two-step treatment. First, a wet chemically etching process created a roughness of high aspect ratio, in the scale of microns. Secondly, the surface was coated with a protective 2 μm overlayer of gold, using magnetron sputtering. This anti-multipactor coating has been tested on several types of Ku-band WR75 waveguide transformers and band-pass filters, with excellent results. The multipactor effect was suppressed for two waveguides, even when applying the maximum available power levels. As for the other final four, the increase of multipactor power level was in the range of 4–6 dB. These results were obtained after more than one year of air exposure. In spite of the strong roughness, the insertion losses were diminished by 25% with respect to the values attained in the tests of the standard anti-multipactor coating, Alodine.

General Study of Multipactor Between Curved Metal Surfaces

An analytical study of the electron trajectories between two opposite electrodes having curved surfaces is undertaken for the case when the electron transit time exceeds the RF period. The analysis is based on a statistical approach, which makes it possible to calculate the width of an electron bunch after a number of electron transits taking into account the spread of electron emission velocity, and the spatial nonuniformity of the RF field using the concept of the ponderomotive force. The results are used to estimate the multipactor threshold in terms of a value of the secondary emission yield, which is necessary to balance electron losses. Based on the model, it is predicted that multipactor is impossible inside the realistic configuration of a helix antenna where four electrodes are placed on the same cylindrical surface.

Tracking multiple generation and suppression of secondary electrons on periodic triangular surface

To research the dynamic course of multipactor suppression on the periodically patterned surface, tens of electron collision processes are tracked by numerical calculation. The influences of microwave frequency, amplitude of RF electric field, slope angle, the local field enhancement, and the tilted incident electric field on the multipactor suppression are studied by tracking multi-generation electrons' trajectories, hopping and flight time, collision energy, and secondary emission yield. Meanwhile, the dynamic processes of secondary electrons on the periodic surface are analyzed by particle-in-cell (PIC) simulation. The PIC results are consistent with the analytical results in which the electrons fly reciprocatingly between the slopes and impact on the slopes; the methods of increasing the slope angle, enlarging the RF field, and lowering the frequency in a certain range are helpful to enhance the multipactor suppression steadily and persistently.

Magnetic dipole discharges. III. Instabilities

Instabilities in a cross-field discharge around a permanent magnet have been investigated. The permanent magnet serves as a cold cathode and the chamber wall as an anode. The magnet is biased strongly negative and emits secondary electrons due to impact of energetic ions. The electrons outside the sheath are confined by the strong dipolar magnetic field and by the ion-rich sheath surrounding the magnet. The electron energy peaks in the equatorial plane where most ionization occurs and the ions are trapped in a negative potential well. The discharge mechanism is the same as that of cylindrical and planar magnetrons, but here extended to a 3-D cathode geometry using a single dipole magnet. While the basic properties of the discharge are presented in a companion paper, the present focus is on various observed instabilities. The first is an ion sheath instability which oscillates the plasma potential outside the sheath below the ion plasma frequency. It arises in ion-rich sheaths with low electron supply, which is the case for low secondary emission yields. Sheath oscillations modulate the discharge current creating oscillating magnetic fields. The second instability is current-driven ion sound turbulence due to counter-streaming electrons and ions. The fluctuations have a broad spectrum and short correlation lengths in all directions. The third type of fluctuations is spiky potential and current oscillations in high density discharges. These appear to be due to unstable emission properties of the magnetron cathode.

Extended lifetime MCP-PMTs: Characterisation and lifetime measurements of ALD coated microchannel plates, in a sealed photomultiplier tube

Atomic layer deposition (ALD) coating of microchannel plates (MCP) has been shown to offer significant performance advantages in MCP-PMTs (MCP Photomultiplier Tube). ALD is a chemical vapour process used to deposit atomic mono-layers on a substrate. A process has been developed to deposit a surface with improved secondary emission yield (SEY) on to an MCP substrate. The principal advantage of a higher SEY is the ability to achieve significantly higher gain at the same operating voltage across a single MCP. Further to this, it is suspected the atomic mono-layers deposited by ALD coating prevent desorption of gaseous contaminants in the MCP glass. The ions produced during desorption are widely believed to be a direct cause of photocathode ageing in MCP-PMTs, leading to the hope that ALD coating can improve the MCP-PMT lifetime. To fully characterise the performance of ALD coated MCPs, two MCP-PMTs were manufactured, one ALD coated and the other uncoated to be used as a reference. Each detector's gain, DQE, pulse shape and timing jitter were measured followed by a life test of the tubes. The ALD coated tube was found to have a higher gain at the same operating voltage, whilst being equivalent to a standard MCP in other performance characteristics. ALD coating gave a dramatically improved life time, after 5.16Ccm−2 total charge extracted, there was no measurable effect on the photocathode QE, although the MCP gain dropped by approximately 35%.

Numerical Calculation of an Equilibrium Dust Grain Potential in Lunar Environment

The interaction of plasma particles with dust grains leads to their charging. An equilibrium grain potential depends on a plasma environment, as well as on the grain composition, size, shape, and charging history. We present results of calculations of the equilibrium potential of the grain immersed in the plasma simulating a lunar environment. In calculations, we apply a modified model of the secondary electron emission from dust grains, which takes into account grain sizes, their material, and surface roughness. Since this model describes the increase in the secondary emission yield caused by a finite dimension of the dust grain, the calculations provide a realistic estimation of the dust grain charge in the near-Earth environment. We show that the grain surface potential is a descending function of the grain size and this effect can even lead to opposite polarities of small and large grains.

Y2O3–Lu2O3 co-doped molybdenum secondary emission material

A new kind of RE2O3–Mo cathodes co-doped with Y2O3 and Lu2O3 were prepared by a solid–liquid method combined with the spark plasma sintering technique. The secondary electron emission property and secondary emission mechanism of the cathode have been studied. The results show that Y2O3–Lu2O3 co-doped Mo cathode exhibits better secondary emission property than single Y2O3 doped molybdenum cathode, and the maximum secondary emission yield (δmax) can reach 4.616 after being activated at 900°Cb. The Y2O3–Lu2O3 co-doped Mo cathode has high penetration depth of primary electrons and moderate escape depth of secondary electrons. Furthermore, the work function of Y2O3–Lu2O3–Mo cathode (2.41eV) is lower than that of Y2O3–Mo cathode (3.59eV). These phenomena are responsible for its excellent secondary emission property.

Sputtering of Al nanoclusters by 1–13 keV monatomic or polyatomic ions studied by Molecular Dynamics simulations

Molecular Dynamics (MD) simulations were employed to study sputtering of freestanding and supported spherical Al nanoclusters with 2–10nm diameters under bombardment by Al1 and Al13 projectiles with energies of 1–13keV at normal and oblique incidence. Both monatomic and clustered yields of secondary emission are found to be larger than those for (111) flat surface of the bulk Al (at equal irradiation conditions). In some events, target nanocluster receives a backward momentum and therefore its major part (more than 1/2 of the mass) is ejected due to produced secondary emission mainly towards the substrate direction. This “recoil effect” is found more pronounced under the impact of cluster projectiles and its probability decreases with increase of the target cluster size. A restricted number of MD simulations were performed to verify whether this “recoil effect” is strong enough to desorb a 4nm Al nanocluster off an Al (111) substrate. Desorption was observed under oblique Al13 impact within the impact parameter range of 0.6–0.9.

Secondary electron yield of Cu technical surfaces: Dependence on electron irradiation

The secondary emission yield (SEY) properties of colaminated Cu samples for LHC beam screens are correlated to the surface chemical composition determined by x-ray photoelectron spectroscopy. The surface of the as-received samples is characterized by the presence of significant quantities of contaminating adsorbates and by the maximum of the SEY curve (${\ensuremath{\delta}}_{\mathrm{max}}$) being as high as 2.1. After extended electron scrubbing at kinetic energy of 10 and 500 eV, the ${\ensuremath{\delta}}_{\mathrm{max}}$ value drops to the ultimate values of 1.35 and 1.1, respectively. In both cases the surface oxidized phases are significantly reduced, whereas only in the sample scrubbed at 500 eV the formation of a graphitic-like C layer is observed. We find that the electron scrubbing of technical Cu surfaces can be described as occurring in two steps: the first step consists in the electron-induced desorption of weakly bound contaminants that occurs indifferently at 10 and at 500 eV and corresponds to a partial decrease of ${\ensuremath{\delta}}_{\mathrm{max}}$; the second step, activated by more energetic electrons and becoming evident at high doses, increases the number of graphitic-like C-C bonds via the dissociation of adsorbates already contaminating the as-received surface or accumulating on this surface during irradiation. Our results demonstrate how the kinetic energy of impinging electrons is a crucial parameter when conditioning the surfaces of Cu and other metals by means of electron-induced chemical processing.

First electron-cloud studies at the Large Hadron Collider

During the beam commissioning of the Large Hadron Collider (LHC) [LHC Design Report No. CERN-2004-003-V-1, 2004 [http://cds.cern.ch/record/782076?ln=en]; O. Bruning, H. Burkhardt, and S. Myers, Prog. Part. Nucl. Phys. 67, 705 (2012)] with 150, 75, 50, and 25-ns bunch spacing, important electron-cloud effects, like pressure rise, cryogenic heat load, beam instabilities, or emittance growth, were observed. Methods have been developed to infer different key beam-pipe surface parameters by benchmarking simulations and pressure rise as well as heat-load observations. These methods allow us to monitor the scrubbing process, i.e., the reduction of the secondary emission yield as a function of time, in order to decide on the most appropriate strategies for machine operation. To better understand the influence of electron clouds on the beam dynamics, simulations have been carried out to examine both the coherent and the incoherent effects on the beam. In this paper we present the methodology and first results for the scrubbing monitoring process at the LHC. We also review simulated instability thresholds and tune footprints for beams of different emittance, interacting with an electron cloud in field-free or dipole regions.

Prediction of Multipactor Breakdown for Multicarrier Applications: The Quasi-Stationary Method

A new prediction algorithm for multipactor breakdown determination in multicarrier signals is presented. This new algorithm assumes a quasi-stationary (QS) model based on the nonstationary theory for single-carrier signals. It determines the worst case, i.e., the combination of signal phases that yields the lowest breakdown level per carrier, using multipactor electron growth models. It considers the secondary emission yield properties of the material and the time-varying value of the multicarrier signal envelope.

Simulations of electron-cloud heat load for the cold arcs of the CERN Large Hadron Collider and its high-luminosity upgrade scenarios

The heat load generated by an electron cloud in the cold arcs of the Large Hadron Collider (LHC) is a concern for operation near and beyond nominal beam current. We report the results of simulation studies, with updated secondary-emission models, which examine the severity of the electron heat load over a range of possible operation parameters, both for the nominal LHC and for various luminosity-upgrade scenarios, such as the so-called ``full crab crossing'' and ``early separation'' schemes, the ``large Piwinski angle'' scheme, and a variant of the latter providing ``compatibility'' with the (upgraded) LHCb experiment. The variable parameters considered are the maximum secondary-emission yield, the number of particles per bunch, and the spacing between bunches. In addition, the dependence of the heat load on the longitudinal bunch profile is investigated.

Secondary electron emission yield (SEY) in amorphous and graphitic carbon films prepared by PLD

AbstractHigh secondary emission yield (SEY), and the subsequent build‐up of a secondary electron cloud, may severely limit the stability of high‐intensity particle beams inside particle accelerators. One of the best candidates of beam pipe coating for reduced SEY has been amorphous carbon (a‐C) produced by direct current (D.C.) magnetron sputtering. Here we used pulsed laser deposition (PLD), to prepare a‐C films from a pure carbon target at substrate temperatures ranging from 300 K to 773 K. The ablating laser was a Nd:YAG system operating at 1064 nm wavelength. With increasing temperature the optical band gap dropped from about 2.1 eV to 1.0 eV. This trend indicates transition from predominantly a‐C films to films with more graphitic content, which was also confirmed by Raman measurements. SEY spectra were taken upto 1732 eV of primary electron energy. The maximum SEY value decreased from 1.9 in a‐C films down to 1.4 in highly graphitic films deposited at higher temperatures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)