Anomalously high charging of dispersed particles by 25-keV electron beam

The phenomenon of ultrahigh charging of dust particles under the action of an electron beam has been studied. The electron energy was 25 keV and the characteristic size of dispersed aluminum oxide micro-particles was about 100 μm. The charge acquired by a dust particle amounted to 5 × 10⁷ e, which corresponds to a charge of 10⁶ e for a micron-sized particle and is greater by at least two orders of magnitude than the characteristic values of charge observed for particles of the same size in low-pressure gas discharge.     

Ion beam effects on dust grains: 2—Influence of charging history

Dust grains in space are charged by various processes. Impacts of energetic ions lead to deposition of positive charge on the grain, increasing the grain potential and, as a consequence, the electric field at its surface. The accumulated charge is spontaneously released as an emission current when the electric field reaches a threshold. This discharging current is usually attributed to field ionization of any gas surrounding the grain or to ion field emission and would thus be predominantly a function of the surface potential. However, preliminary studies [Velyhan A, Z?ilavý P, Pavl? J, S?afránková J, Něme?ek Z. Ion beam effects on dust grains. Vacuum 2004;76:447-55] using melamine formaldehyde spheres have shown that the discharging current depends strongly on the energy of primary ions. The present paper continues these investigations with the motivation to understand the whole charging/discharging process. The experiment is based on the capture of a single dust grain in an electrodynamic quadrupole. The trapped grain is exposed to an ion beam with different energies up to 5 keV and its charge and surface potential are estimated from the frequency of its oscillations in the quadrupole. The charging/discharging currents are determined from temporal changes of the grain charge. Our results suggest that the grain charge is accumulated in a thick surface layer of non-conducting samples. The thickness of this layer depends on the mass and energy of primary ions. On the other hand, the beam ions probably recombine on the metallic surfaces and create an adsorbed layer there. We believe that the main discharging process is field desorption complemented in this particular case with post-ionization.     

Dusting of Alumina and Electrostatic Charging

The dust content and electrostatic charge of aluminium oxide (alumina) has been measured with the Faraday cup and in an electrostatic deflection apparatus. The dust was made and released from a fluidized bed. After generation, the dust entered the measurement zone where it was separated according to charge. Large differences in the electrostatic charge and the dusting behavior were observed. The charge of the alumina dust is generally relatively neutral, but aluminas with highly charged particles were observed. The dusting profile varies considerably. Some powders released dust only in the beginning of the test, while others constantly released dust.     

Dusting of Alumina and Electrostatic Charging

The dust content and electrostatic charge of aluminium oxide (alumina) has been measured with the Faraday cup and in an electrostatic deflection apparatus. The dust was made and released from a fluidized bed. After generation, the dust entered the measurement zone where it was separated according to charge. Large differences in the electrostatic charge and the dusting behavior were observed. The charge of the alumina dust is generally relatively neutral, but aluminas with highly charged particles were observed. The dusting profile varies considerably. Some powders released dust only in the beginning of the test, while others constantly released dust.     

Bunch length measurement of picosecond electron beams from a photoinjector using coherent transition radiation

The bunch length of an electron beam derived from the UCLA Saturnus photoinjector has been measured using a 45° CTR foil. The sudden change of electrons boundary conditions cause them to radiate (transition radiation) with the spectral power entirely dependent upon the degree of coherency, which strongly relates to the beam size. A polarizing Michelson interferometer allowed measurement of the auto-correlation of the coherent transition radiation signal. An analysis method was developed to compensate for undetected low-frequency radiation and systematically extract the bunch length information for a specific beam model. This analysis allowed observation of pulse lengthening due to the space charge, as well as compression with the variation of the RF injection phase. The hypothesis of a satellite beam has been also tested using this analysis.     

Equilibrium phase instability in the double RF system for Landau damping

A longitudinal coupled bunch instability in an electron storage ring was suppressed by the Landau damping in a double rf system composed of a second harmonic rf cavity. The damping became ineffective, however, above a beam current of 30 mA; the beam bunch slipped out of the optimum phase of the total rf voltage for the damping, which accompanied a simultaneous deformation of the total voltage. The unexpected phenomenon of the phase slip is explained by the concept of equilibrium phase instability of the beam bunch based on a rigid bunch model. The phase slip of the bunch was suppressed by introducing a phase feedback loop, resulting in an improvement of the maximum beam current for the damping. Discussions are made on various conditions of the equilibrium phase instability, including another possibility for avoiding the phase slip.     

Longitudinal space charge in final-focus systems for linear colliders

In the final-focus system of a linear collider, the effect of longitudinal space charge can be significant. A diverging or converging ultra-relativistic beam experiences a longitudinal space-charge force which is independent of the beam energy. This force, though weak and often neglected, causes an energy variation across the bunch which depends on the beam size, beam-pipe radius and bunch population and this may affect the chromatic correction of a beam line. Because of the inherent large chromaticity of a typical final-focus system, a very small energy variation, induced in the final focus, can lead to an intolerable increase of the spot size at the interaction point. The space-charge force, thus, gives rise to a limit on bunch intensity beyond which the resulting spot-size increase will degrade the collider performance. In this paper, we evaluate the effect of the longitudinal space charge and derive intensity limits for the three existing or proposed final foci.     

Improvement of beam lifetime and vacuum system of the PF-AR

Successive improvements have been performed on the vacuum system for the Photon Factory Advanced Ring (PF-AR) at the High Energy Accelerator Research Organization (KEK). The main purpose is to prolong electron beam lifetime for stable operation as an intense pulsed X-ray source. In the past three years, a total of 61 sputter ion pumps (SIPs) were additionally installed, and the increased effective pumping speed amounts to 13% of total. Comparison between calculated and observed beam lifetimes indicates that the lifetime is restricted mainly by the residual gas scatterings and that improvement of the vacuum will realize still longer lifetime. Sudden beam lifetime drops caused by dust trappings have been investigated for many years. The frequency of the lifetime drops has decreased as operation time elapsed after a large-scale reconstruction. Effect of distributed ion pumps (DIPs) on the lifetime drops has also been investigated experimentally.     

Lattice and beam optics design for suppression of CSR-induced emittance growth at the KEK-ERL test facility

The lattice and beam optics of the arc section of the KEK-ERL test facility, having an energy of 200 MeV, were optimized to efficiently suppress emittance growth based on a simulation using a particle-tracking method taking coherent synchrotron radiation effects into account.The lattice optimization in the arc section was performed under two conditions: a high-current mode with a bunch charge of 76.9 pC without bunch compression, and a short-bunch mode with bunch compression, producing a final bunch length of around 0.1 ps. The simulation results showed that, in the high-current mode, emittance growth was efficiently suppressed by keeping a root-mean-square (rms) bunch length of 1 ps at a bunch charge of 76.9 pC, and in the short-bunch mode, emittance growth was kept within permissible limits with a maximum allowable bunch charge of 23.1 pC at an rms bunch length of 0.1 ps.     

SCLC diode thin film rate monitor

The injection of a vapour beam into a thermionic vacuum diode in a Kingdom cage type configuration modifies the diode characteristics considerably. The anode current increases on injection, the increase being dependent on the anode voltage. This is attributed to the partial neutralization of the space charge by the presence of positive ions in the vapour beam (produced by the electron bombardment in the Kingdon cage configuration). The increase in the plate current in the space charge region of the diode is linearly proportional to the number of atoms in the vapour beam for various metal evaporants at low rates of evaporation. A theory to explain the neutralization effect has been developed. The observed effect lends itself to a useful device to monitor the vapour beam density and, therefore, the evaporation and deposition rates of materials. Vapour beam densities as low as 10¹³/cm² sec and hence deposition rates as low as 0.1 Å/sec can be measured. Unlike the ionization gauge type of monitor, this device is not seriously affected by the presence of background (ambient) gases during vacuum evaporation.     

Non-self-maintained discharge supported by a proton beam for studying stretched dust structures

The main parameters of non-self-maintained discharge supported by a proton beam in inert gas are obtained. The discharge is used to study dust structures in nuclear excited plasma. Its main features include low gas pressure (～133 Pa) and low proton beam current density (～10^?6 A/cm²). It is shown that a trap for negatively charged dust particles is formed in the discharge near-collector area at a sufficiently large negative voltage on the collector (?100 V), which is caused by a large negative potential jump in the Langmuir electrode layer whose width increases with distance from the center of the discharge. The possibility of generating stretched dust structures in non-self-maintained discharge is considered.     

Numerical simulation of rarefied dusty plasma in a normal glow discharge

A self-consistent model of a glow discharge, burning in a normal current density mode, with dust particles placed in it is presented. The model of a normal glow discharge is based on a diffusion-drift approximation. Within the framework of this model, the regions of a spatial charge are taken into account. The motion of dust particles in the glow discharge region is calculated using the method of molecular dynamics. The particle charge is estimated based on qualitative conceptions on the process of charging of solid particles. The proposed model is studied numerically, and the corresponding results are presented.     

A technique of measuring and correcting emittance dilutions due to accelerator structure misalignments

This paper describes a method of reducing the transverse emittance dilution in linear colliders due to transverse wakefields arising from misaligned accelerator structures. The technique is a generalization of the Wake-Free [T.O. Raubenheimer, Nucl. Instr. and Meth. A 306 (1991) 61] correction algorithm. The structure errors are measured locally by varying the bunch charge and/or bunch length and measuring the change in the beam trajectory. The structure errors can then be corrected by varying the trajectory or moving the structures. The results of simulations are presented demonstrating the viability of the technique.     

尘土颗粒带电特征的研究

使用密立根方法,对尘土颗粒的带电特性进行了研究。结果表明:尘土颗粒所带电荷的变化趋势可用一元三次多项式表示,电荷分布呈现带状区域,其所带的平均正电荷高于所带的负电荷。尘土颗粒的等效表面积和表面形貌是影响尘土带电的重要因素。环境相对湿度影响尘土颗粒的带电量,相对湿度越大,尘土带电荷越少。适当地控制机电元件运行的环境湿度,以及进行防尘处理,可以有效地降低故障率。     

Electrically charged filter materials

Filters made from ordinary textile fibres are too coarse to remove micrometre-sized dust particles, which are responsible for much respiratory disease. If, however, an electric charge is applied to the fibres, a filter can provide a very good respiratory protection. Fibres can be charged by triboelectric exchange, by corona or by induction; and charge levels can give rise to electric fields in the interstices of the filter, approaching the breakdown field of the air. Some of the effect of the charge is lost as the filters become loaded with dust, but charge stability during storage is high, with shelf lives of years being attainable. The author discusses the capture of particles by electric forces and electrically charged filter materials and their production methods. The measurement of electric charge on filters, charge stability during storage, filter performance when loaded with dust and the advantages and disadvantages of such filters, are also discussed     

An upgraded high-velocity dust particle accelerator at Concordia College in Moorhead, Minnesota

In 1975, Concordia College in Moorhead, Minnesota acquired a 2 MeV dust particle accelerator from NASA/GSFC that was used to test the lunar ejecta and micrometeorite (LEAM) experiment flown on Apollo 17. This high-speed dust particle accelerator is still fully functional and is currently being upgraded. Improvements to the electronic detection system have been undertaken including a computer-based, data acquisition system and new particle detection sensor electronics. These sensors have additional amplifiers that extend the range of charge detection to 1×10⁻¹² C allowing for the detection of larger particles. Improvements to the vacuum system have also been made. The accelerator beam line is now pumped with an oil-free, turbomolecular pump reducing possible problems with hydrocarbon contamination. In this work, we describe the facility, and outline some of the recent improvements to the dust particle accelerator and discuss its capabilities and limitations. We also summarize some of the recent experiments conducted using the facility.     

High-intensity multi-bunch beam generation by a photo-cathode RF gun

A multi-bunch photo-cathode RF gun system has been developed as an electron source for the production of intense quasi-monochromatic X-rays based on inverse Compton scattering. The desired multi-bunch beam is 100 bunches/pulse with a total charge of 500 nC and a bunch spacing of 2.8 ns. We modified the gun cavity of a ‘BNL-type IV’ RF gun to allow a CsTe cathode plug in the end plate. The system uses a four-dipole chicane beam line to allow the injection of laser light normal to the cathode surface. We compensate the gun cavity beam loading caused by the high-intensity multi-bunch electron beam by injecting the laser pulse before RF power has filled the cavity. We have achieved a total intensity of 220 nC in 100 bunches with a bunch-to-bunch energy spread under 1.3% (peak-to-peak). This paper concentrates on experiments to generate the high-intensity multi-bunch beam with compensation of the bunch-to-bunch energy spread due to heavy beam loading.     

Wake-field characteristics of a high-intensity, multibunched beam

A systematic experimental study on the wake-field characteristics of a high-intensity, multibunched electron beam was carried out in detail at the primary electron section of the KEK positron generator linac. The observed energy spectrum, which indicated the energy variation of one bunch from another, was explained fairly well by the multibunch effect of a longitudinal wake field; the importance of introducing a self-wake loss and a bunch-length effect into calculations of the energy loss due to a longitudinal wake field is noted. On the other hand, a transverse motion of each bunch, showing a peculiar behavior when the transverse instability occurred, was in good agreement with the results of a numerical calculation based on a multibunch version of Wilson's two-particle model. In both cases, the wake field for our cavity was estimated using a computer code called TBCI.     

Group velocity effect on resonant, long-range wake-fields in slow wave structures

Synchronous wake-fields in a dispersive waveguide are derived in a general explicit form on the basis of a rigorous electro-dynamical approach using Fourier transformations. The fundamental role of group velocity in wake-field propagation, calculation of attenuation, amplitudes, form-factors and loss-factors is analyzed for single bunch radiation. Adiabatic tapering of the waveguide and bunch density variation is taken into account analytically for the time-domain fields. Effects of field “compression/expansion” and group delays are demonstrated. The role of these effects is discussed for single bunch wake-fields, transient beam loading, BBU and HOMs. A novel waveguide structure with central rf coupling and both positive and negative velocities is proposed. It can be used effectively in both high-energy accelerators and single-section linacs.     

Determination of low-Z elements in individual environmental particles using windowless EPMA

The determination of low-Z elements such as carbon, nitrogen, and oxygen in atmospheric aerosol particles is of interest in studying environmental pollution. Conventional electron probe microanalysis technique has a limitation for the determination of the low-Z elements, mainly because the Be window in an energy-dispersive X-ray (EDX) detector hinders the detection of characteristic X-rays from light elements. The feasibility of low-Z element determination in individual particles using a windowless EDX detector is investigated. To develop a method capable of identifying chemical species of individual particles, both the matrix and the geometric effects of particles have to be evaluated. X-rays of low-Z elements generated by an electron beam are so soft that important matrix effects, mostly due to X-ray absorption, exist even within particles in the micrometer size range. Also, the observed radiation, especially that of light elements, experiences different extents of absorption, depending on the shape and size of the particles. Monte Carlo calculation is applied to explain the variation of observed X-ray intensities according to the geometric and chemical compositional variation of individual particles, at different primary electron beam energies. A comparison is carried out between simulated and experimental data, collected for standard individual particles with chemical compositions as generally observed in marine and continental aerosols. Despite the many fundamental problematic analytical factors involved in the observation of X-rays from low-Z elements, the Monte Carlo calculation proves to be quite reliable to evaluate those matrix and geometric effects. Practical aspects of the Monte Carlo calculation for the determination of light elements in individual particles are also considered.