Low Velocity Electron Research Articles

Scattering by a low-velocity charge with applied magnetic fields in a Laguerre–Gaussian beam

A method to analyze the scattering characteristics of a Laguerre–Gaussian (LG) vortex beam by a low-velocity charge in a steady applied magnetic field is investigated in this paper. Based on the plane wave angular spectrum representation and the electromagnetic radiation theory of a moving charge, the expressions of the scattered field and scattered power per unit solid angle are presented. Considering a low-velocity electron, the distribution of the scattered power per unit solid angle is numerically simulated. The effects of electron motion parameters, beam parameters, and applied magnetic field are discussed in detail. The results show that the longitudinal motion of the electron, which is parallel to the direction of the applied magnetic field, makes the distribution of the scattered power no longer maintain radial consistency but has a concave–convex distribution. Due to the periodicity of the circular motion of the electron in the applied magnetic field, the scattering power distribution pattern consists of many annular lobes. The spectral characteristics of the backscattered field are analyzed by fast Fourier transform. It is confirmed that the scattered field satisfies the rotational and the linear Doppler effects, and the accuracy of the formula provided in this paper is verified. This work lays a foundation for further study of the scattering characteristics of plasma on vortex electromagnetic waves and helps to promote the in-depth development of vortex beams in the field of plasma diagnosis.

Beam-based alignment of SRF cavities in an electron injector linac

Proper alignment of accelerating cavities is an important issue concerning beam quality of linear rf-accelerators. In particular, SRF cavities of injector linacs using high accelerating gradients on low-velocity electron beams can deteriorate the beam quality significantly when not aligned sufficiently. On the other hand, knowing the exact position of every cavity after several cool-down cycles of a cryomodule can be difficult depending on the particular module design. We will report on operational experience on the SC injector of the Darmstadt superconducting recirculating linac and ERL (S-DALINAC) showing unwanted effects on beam dynamics and beam quality due to cavity misalignment, such as transverse beam deflections by changing accelerating phases and a growth of transverse emittance. These effects could already be observed at small beam currents of a few nA, so space-charge effects were eliminated to be the reason for these observations. In this work we will report on the possibility to realign SRF cavities after cool-down by measuring the transverse deflection of the beam and compare results with beam dynamics simulations.

Faint solar radio structures from decametric observations

Aims. Decameter radio observations of the solar corona reveal the presence of numerous faint frequency drifting emissions, similar to “solar S bursts” which are reported in the literature. We present a statistical analysis of the characteristics of these emissions and propose a mechanism to excite the Langmuir waves thought to be at the origin of these emissions. Methods. The observations were performed between 1998 and 2002 with the Digital Spectro Polarimeter (DSP) receivers operated at the UTR-2 and Nancay decameter radio telescopes in the frequency range 15–30 MHz. Our theoretical explanation is based on Vlasov-Ampere simulations.Results. Based on the frequency drift rate, three populations of structures can be identified. The largest population presents an average negative frequency drift of -0.9 MHz s-1 and a lifetime up to 11 s (median value of 2.72 s). A second population shows a very small frequency drift of -0.1 MHz s-1 and a short lifetime of about 1 s. The third population presents an average positive frequency drift of +0.95 MHz s-1 and a lifetime of up to 3 s. Also, the frequency drift as a function of frequency is consistent with the former results, which present results in higher frequency range. No specific relationship was found between the occurrence of these emissions and the solar cycle or presence of flares. Assuming that these emissions are produced by “electron clouds” propagating the solar corona, we deduce electron velocities of about 3–5 times the electron thermal velocity. As previously shown, a localized, time-dependent modulation of the electron distribution function (heating) leads to low velocity electron clouds (consistent with observations), which, in turn, can generate Langmuir waves and electromagnetic signals by nonlinear processes.

Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

One-dimensional Eulerian Vlasov-Maxwell simulations are presented that show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. A low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single-hot-spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a Gauss-Hermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.

Experimental verification of low-velocity spread axis-encircling electron beam

We have experimentally demonstrated a low-velocity spread, axis-encircling electron beam using a comparatively simple Pierce-type electron gun and single cusp magnetic field based on a recent theory [Jeon et al., Appl. Phys. Lett. 80, 3703 (2002)] developed with the assumption of small orbit gyration before the magnetic cusp, in which every physical property has been analyzed and the possibility of zero percent axial velocity spread was concluded. The velocity ratio and the axial velocity spread were measured to compare the theory with varied operation conditions of the electron gun. These results agree well with our hypothesis.

Secondary electron emission of solids by impact of molecular ions and clusters

The impact of a cluster ion on a solid surface can be described as the simultaneous arrival of many atoms at close distances on the surface, which gives rise to a high energy deposit in a small volume. Among the various subsequent phenomena, kinetic electron emission can shed some light on the interaction processes involved. At low velocity ( E ≈ 1 eV/amu) electron emission below the kinematic threshold has already been observed as well as thermionic emission from the decay of highly excited rebounding clusters. At higher velocity ( E ≈ keV-MeV/amu) the cooperative effect of the projectile atoms leads to nonlinear electron yields as was first observed with small molecular ions and more recently with larger clusters.

Electron velocity space hole modes

In space plasmas the local electron distribution is formed by ballistic motion from different source regions. Since electrons with positive and negative parallel (υ∥) velocities have different sources and experience different interactions with the large‐scale electromagnetic fields, the electron distribution function near υ∥ = 0 should possess considerable structure, including the possibility of velocity space holes, i.e., regions where the phase space density is zero, and sharp discontinuities. A particular hole distribution, which may occur upstream of slow shocks in the distant geomagnetic tail, is shown to support a nonstandard plasma wave mode with frequencies between the ion and electron plasma frequencies and parallel phase speed above the ion, but well below the electron, thermal speed. The hole mode can be destabilized by a positive slope in the low parallel velocity electron distribution, and the growth rate is strongly enhanced if the electron parallel temperature exceeds the perpendicular temperature; the mode can also be excited by a hot, fast ion beam. The hole modes exist only for a narrow range of propagation angles about the magnetic field direction. A discontinuity in the integrated parallel distribution function also supports a nonstandard wave mode which is unstable if the value of the distribution function increases across the discontinuity.

Pioneers of Television — Philo Taylor Farnsworth

This is the third in the series of “Pioneers of Television,” written for the SMPTE Journal. Philo T. Farnsworth was one of the most prolific inventors in television history, with over 75 important inventions in his name. Born in 1906 in Utah, he conceived an all-electric system of television while only 15 years of age. He typified the school of inventors that not only conceived the ideas but built and implemented them. By the time he was 21 he had secured financing and was operating a laboratory in San Francisco, from which came a host of important inventions. Farnsworth was the first inventor in the world to build and operate an all-electric television system, in July 1929. This included an electric camera tube, the image dissector; a vacuum-tube scanning, blanking, and sync generator; and a magnetically focused cathode ray tube. His image dissector camera tube was the first to pick up “live” images in broad daylight. He was the inventor of the method for generating anode high voltage from the scanning currents. His research in camera tubes led him to patent the first camera tube to use a low-velocity electron beam that scanned a target at cathode potential. This led to the Orthicon and Image Orthicon camera tubes that made postwar television possible. Farnsworth's imagination knew no limits, and without his inventions no modern-day television system could operate. Yet in spite of these considerable achievements, he is relatively unknown and has been overshadowed by other pioneers in television history.

Analysis of a low magnetic field TEm11gyroklystron amplifier†

Abstract Large- and small-signal numerical calculations are presented for a two-cavity, low magnetic field gyroklystron amplifier operating in the TE m11 whispering-gallery mode. The gyroklystron system modelled consists of a bunching cavity and an output cavity separated by a drift tube. For operation of both cavities at TE511, gain and emission efficiency are studied for a high energy (γ = 1·6), low axial velocity (βVerbar; = 0·1) electron beam. Prebunching prior to the output cavity in the gyroklystron leads to an increase of the maximum efficiency by more than a factor of two over that obtained from operating the output cavity as an oscillator. Model calculations are presented which show that magnetic field tapering greatly reduces the effects of any initial axial velocity spread in the electron beam, allowing high gain (≳40 dB) and high efficiency (25-30%) to be achieved.

Electron heating caused by the ion-acoustic decay instability in a finite-length system

The ion-acoustic decay instability is investigated for a finite-length plasma with density somewhat below the cutoff density of the electromagnetic driver (n∼0.7nc). For this regime, the heating in a very long system can overpopulate the electron tail and cause linear saturation of the low phase velocity electron plasma waves. For a short system, the instability is nonlinearly saturated at larger amplitude by ion trapping. Absorption can be significantly increased by the large-amplitude ion waves. These results compare favorably with microwave experiments.

A New Magnetic-Focus Static-Deflection System for a Sincle-Tube Color Camera

The need for high quality electron optical systems for small size color camera has remarkedly increased since the advent of compact VTRs. A new magnetic-focus static-deflection system has been developed under a program of exploration of possibilities of designing vidicon tubes. The objective is to make a focusing and deflection system which will have high quality electron optical characteristics with simple construction and require low power consumption. The main characteristics required are small focus spot size, scan linearity and normal beam landing. The beam landing angle affects shading, resolution, lag and flicker of camera tubes which employ low-velocity electron beam [1]. The normal beam landing performance is essential especially for a single tube color camera system in order to read out coded color signals with accuracy.

Calculations of Image Lag in Vidicon-Type Camera Tubes. I. Beam Discharge Lag and Total Image Lag, Including Photoconductive Lag

Image lag in vidicon-type camera tubes is analyzed, taking scanning processes with a low-velocity electron beam into consideration. As the beam landing characteristics, not only usual form of ib=Ibexp (av) but also newly invented one of ib=Ib/{1+exp (-av)} are employed. The latter shows much better agreement to the measured characteristics than the former, but the difference between them is negligible at the regions where image lags become troublesome. First, by expressing target surface potentials and target output currents in exponential forms, the relationship between them is obtained analytically and presented graphically as the target surface potential curves under steady state conditions. Then, the fundamental expressions for beam discharge lag or capacitive lag are obtained for both rise and decay by applying the above-mentioned relationship to transient cases. Finally, the total image lag, including photoconductive lag, is analyzed. From the results obtained by computer calculation, the general features of image lag and the lower limit of image lag are known.

An Electron Beam Method for Measuring High Sheet Resistances of Thin Films

Measurement of sheet resistances of high resistance (up to 1016 Ω/□) thin films using conventional methods is both difficult and subject to error. We have developed a method for making sheet resistance measurements in which a low velocity electron beam is used to inject charge into the film. The injected charge drifts under the influence of its self-induced field to metal electrodes evaporated on the film, where it is collected and measured externally as a time dependent current. We give examples of thin GaAs films for which the measured current can be fit with a current predicted by a simple model over many orders of magnitude by adjusting only one parameter, the sheet resistance of the film. The good agreement between experimental and theoretical currents provides reliable values for the sheet resistance. This allows an estimation of a lower limit on the carrier density native to our GaAs films.

The Interaction between Low Velocity Electron Beams and Insulating Surfaces under Electron Bombardment and Photo Induced Excitations

The electron absorption coefficient (1 − δ̄) of an insulating surface held at an average potential V̄s between zero and the first crossover potential is observed to increase substantially when the insulator is subject to excitation from a separate electron beam or light source. This effect is apparently caused by the migration of holes to the vacuum interface under influence of the applied field. A vidicon gun in the low-velocity scanning mode was used to establish a potential V̄s of the exposed surfaces of thin films of As2S2 and As2S3 up to 5 μ in thickness which were evaporated onto electron-permeable or light-transparent conducting substrates. The average insulation excitation current ĪT was measured as a function of IB, the low velocity beam current, for fixed values of VT, the potential of the conducting substrate, under constant excitation conditions from either a light source or a 20 keV electron beam extended area source. The average electron absorptivity (1 − δ̄) of the insulator is determined as a function of V̄s, using the equation ĪT = (VT − V̄s)/R = (1 − δ̄)IB on the assumption that resistance R of the insulating film is independent of the field gradient applied across it, for a fixed value of electron bombardment (or light) excitation. Observations also include measurements on changes in (1 − δ̄) after evaporating thin layers of different, metal and insulating films over the exposed surfaces of the insulating films.

Biological effects of low velocity electrons

A low velocity (0–1000 V) electron gun for use in biological and biochemical research is described. Results of experiments on the bombardment of bacteria and yeast with electrons accelerated by potentials ranging from 0 to 750 V show that these organisms are killed at threshold electron velocities. Staphylococcus albus, which has an average size of 0.7–0.9 μ, requires electron accelerating potentials of approximately 25 V and over to produce a lethal effect, while Saccharomyces cerevisiae, with an average diameter of 50 μ, is killed only by electrons which have been accelerated by potentials in excess of 300 V, for the entire energy range tested. The experiments also establish the fact that the energy required to kill yeast cells is approximately 12 times that required to kill S. albus.

A dynamic electron image information store

A cascade image intensifier with low velocity electron image storage is being developed. The device has been used successfully as a framing camera: more than 108 frames per sec have been recorded. It is shown that the time delay introduced in the electron image could be used in nuclear physics to give resolving times of a few nanoseconds. The possibility of using the device as the fast store in an opto-electronic computer system is being investigated. It is further shown that the device may be adapted to extend sampling oscilloscope techniques in order to display transient phenomena.

反射電子線回折における補助低速電子銃の応用

反射電子線回折における補助低速電子銃の応用

A developmental tricolor vidicon having a multiple-electrode target

Color television cameras which are now widely used require three separate camera tubes to supply the simultaneous primary color information transmitted by the compatible system. This paper describes a developmental tricolor camera tube of the vidicon type for use in a single-tube color camera. The ability to generate the three simultaneous signals is achieved in the tricolor vidicon by means of a multiple-electrode target structure having three interlocking groups of color-sensitive strips connected to separate output terminals for each primary color. A single low-velocity electron beam scans the photoconductive target. No special requirements are made on the beam with respect to focus or scanning accuracy. Registry of the three signals is inherent in the design of the target. The performance of the developmental cameras which have been constructed to date does not equal the three-tube image orthicon camera from the standpoint of sensitivity, color fidelity, and uniformity. In its present state, the tube is potentially useful for industrial and scientific purposes where sufficient light is available and structural defects do not obscure the information desired. To extend its range of application, the development of more sensitive photoconductors and improved methods of fabrication are required.

Permachon—A storage pickup tube

The Permachon is a pickup tube having storage capabilities and multi copy readout characteristics. The target is a new solid state film having unusual electrical properties, particularly as applied to the memory of its own previous electrical and optical history. More specifically, the target under the stimulus of low velocity electron bombardment can remember and retain previously applied potentials. A direct and very significant consequence of the unusual properties of the Permachon is the ability to hold previously written information by a low velocity scanning operation, and at the same time to read this information out on a multi copy basis. It has been demonstrated that more than 104 copies may be made while scanning at the 30 frame per second rate. The image retention or hold feature can be used simultaneously with light input, thus greatly enhancing the sensitivity by integration.

The transient response of photoconductive camera tubes employing low-velocity scanning

The signal which results from the characteristics of a low-velocity electron beam and the capacitance of the scanned surface has been calculated. A very slow of the hyperbolic type can result if the scanned surface is stabilized at a retarding field potential. The calculations have been verified by tests on a 6198 Vidicon, which was taken as an example. For this tube, the relative contributions of the electronic transient and the photoconductive decay to the observed response were determined. They make comparable contributions to the total transient.