Initial Axial Velocity Research Articles

2D expansion of the low-density interelectrode vacuum arc plasma jet in an axial magnetic field

The two-dimensional expansion of a current carrying plasma jet in the interelectrode gap of a vacuum arc with an axial magnetic field is analysed by finding the steady state solution of the fully ionized plasma in the hydrodynamic approximation. Two models are presented: (1) expansion into a duct with known geometry and (2) free jet expansion. The first approach models the plasma jet expansion with a conical shape. In the second model the geometric position of the free boundary was determined by the free hydrodynamic jet expansion into vacuum without and with the influence of a magnetic field. In the case of plasma expanding into a conical guide, it was found that the flow field in the near-axis region does not depend on the cone angle for cone angles . The radial velocity becomes comparable to the axial velocity due to the expansion, depending on the cone angle and the initial axial velocity. A model of the free boundary plasma expansion was developed, based on the jet-like (i.e. axial velocity larger than the radial velocity) plasma flow in the vacuum arc near the cathode spot. The free jet boundary was calculated by solving the equations for the normal and tangential velocity components at the free boundary. It was found that the plasma jet had a conical shape, and for axial distances 3 - 4 times greater than the initial jet radius, the radial velocity becomes comparable with the axial velocity if no magnetic field is imposed. Imposition of a magnetic field reduces the radial component of the plasma velocity. The streamline angle is about for a 0.001 T magnetic field and about for a 0.01 T magnetic field. The plasma remains quasi-neutral in all regions except in the space charge boundary layer, where an outward directed electric field appears for low magnetic fields, and an inward directed field is present for strong magnetic fields.

Design of multimegawatt millimeter-wave converters for operation at high gyroharmonics

The authors discuss a feasibility study for the production of multimegawatt RF power from a spatiotemporally modulated gyrating relativistic electron beam at a high harmonic of the frequency of modulation. Parameters are given for optimizing the production of power at 148.5 GHz, the thirteenth harmonic of an 11.424-GHz cyclotron autoresonance traveling wave interaction that accelerates the beam. With an injected 16-A 200-kV beam, to which 7.2 MW is added in the accelerator, it is predicted that a power output of 2.2 MW at 148.5 GHz can be realized, after taking into account ohmic wall losses. This requires an injected laminar beam with small initial axial velocity spread, and an output structure that effectively suppresses competing modes. In this device a tapered guide magnetic field no stronger than 8.2 kG is required. Means to produce the required high-quality electron beam, and to allow only the TE/sub 13.1/ design mode to propagate in the output structure are discussed. A similar design for a fifteenth harmonic converter at 171.4 GHz is also described.

Initial velocities of positive and negative protein molecule-ions produced in matrix-assisted ultraviolet laser desorption using a liquid matrix

The initial axial velocities for the positive and negative cytochrome c molecule-ions desorbed from a liquid matrix (3-nitro-benzyl alcohol) using a 17 ns ultraviolet laser (266 nm) have been determined. The method employed was based upon measurement of ion flight times through a field-free path co-axial with the ion optical axis. The possible interferences, such as energy deficits, due to the presence of electric fields were avoided by using a grid-electrode placed at a very short distance from the sample stage. The potential of this electrode was maintained at the same potential as the sample stage, thereby creating a narrow field-free region within which ion generation took place. Systematic variations of the potential at a second grid-electrode caused a gradual shift of the ion flight times, which could be related to the initial ion velocities. The ion velocities were obtained by correlating the measured time-shifts with values from theoretical analysis. The positive and negative molecule-ions were found to have axial velocities of 840±70 and 750±40 ms −1, respectively.

Modelling of droplet dynamic and thermal histories during spray forming—II. Effect of process parameters

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomised droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomisation and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomisation; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.

Ion ring propagation in a magnetized plasma

The propagation of an ion ring in a 2.5 m long, 0.30 m diam magnetized plasma column was studied using an axial array of magnetic probes and fast proton detectors, a microwave interferometer, and a grating spectrometer. The ring propagated with an initial axial velocity 3×106 m/sec, about four times the Alfvén speed in the plasma, and excited damped magnetosonic waves whose peak amplitude depended on the plasma return current. The plasma electrons were heated by about 1 eV in response to the ring, as measured by the Hγ line-to-continuum intensity ratio. This heating is primarily attributed to classical Coulomb collisions, although about 1%–10% can be attributed to the collective ring–plasma interaction. The collective interaction had no measurable effect on the dynamics of the ion ring, consistent with theoretical models, given the beam, plasma, and magnetic-field parameters in the experiment.

Analysis of the deflection system for a magnetic-field-immersed magnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM/sub 110/ mode. The length of each cavity is pi nu /sub z// omega , and that of the drift space is pi nu /sub z// omega /sub c/, where omega is the RF frequency, omega /sub c/ is the relativistic gyrofrequency in the guide field, and nu /sub z/ is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where omega /sub c// omega =2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The dynamics of ion rings in highly conductive plasmas

A numerical study employing a hybrid computer code (particle ions/fluid electrons) is made of the dynamics of translating ion rings in a highly conductive plasma. The translational kinetic energy of such rings is lost through the generation of Alfvén waves. When the initial axial velocity of the ring is greater than the local Alfvén speed in the plasma, a typical slowing down length is zA =(ar/4ζ)v̄4z0/(v2θv2A), where ar is the ring radial width, ζ=‖δB/B0‖, δB is the ring self-magnetic-field strength on axis, B0 is the strength of the axial magnetic field, v̄z0 is the initial translational velocity of the ring, vθ is the average toroidal ring particle velocity, vA=B0/(4πρ)1/2 is the Alfvén velocity, and ρ is the plasma mass density. Also discussed is the merging of an ion ring with a spheromak.

Three-wave interaction in a free-electron laser with a circularly polarised electromagnetic wave wiggler and an axial guide field

The interaction of the pump wave, the scattered wave and the space-charge wave in a free-electron laser with an electromagnetic wave wiggler and an axial guide field is analysed, where both the scattered wave and pump wave are circularly polarised. The linear dispersion and non-linear growth of these three waves are derived in the laboratory frame. The possibility of explosive instabilities for a fast electromagnetic wave pump propagating counter to the electron beam is discussed and its critical condition is ( omega p+kp nu z0)- omega b/ gamma zo< omega c<(kp nu z0), where omega p and kp are the pump wave frequency and wave number, omega b, nu zO and gamma z0 are the plasma frequency, initial axial velocity and Lorentz factor of the relativistic electron beam, respectively, and omega c is the electron cyclotron frequency in the axial guide field. Group II orbits ( omega c> omega p+kp nu z0) may no longer give positive gain in the process of three-wave interaction.

Initial velocity distributions of secondary ions in electronic sputtering of biomolecules

Beams of fast heavy ions from the Uppsala EN-tandem accelerator and a time-of-flight mass spectrometer have been used to investigate the initial axial velocity distributions of secondary ions from samples of biomolecules of different sample thickness. The initial “axial” energies, the energy corresponding to the centroid of the initial axial velocity distribution, of desorbed H + and H + 2 ions were found to increase linearly with the sample thickness in the region 0 to 400 Å, whereas the initial “ axial” energy for other secondary ions, including molecular ions (M + H) + of peptides, seem to be independent of the sample layer thickness. An approximation to the time-of-flight equation at high acceleration voltage has been used to “energy” calibrate time-of-flight spectra of high mass biomolecular ions. The initial “axial” energy distribution of molecular ions of bovine insulin is presented.

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.

Axial flow in trailing line vortices

Axial flow in the core of a laminar steady trailing vortex from the tip of a semi-infinite wing is analyzed assuming small departure of the axial velocity from the free-stream velocity. It is further assumed that the axial pressure gradient is determined by the swirl velocities of an ideal infinite line vortex in which the radial and the associated axial velocity variations are neglected in the equation for the angular momentum. The axial and lateral variations of the axial velocity depend on the strength of the vortex and initial axial velocity distribution which must be specified at some station behind the wing except at the virtual origin of the vortex where a nonintegrable singularity exists. Numerical solutions for the axial velocity are obtained using the axial pressure gradient given by the line vortex and analytical solutions are obtained using an equivalent axial pressure gradient with good agreement between the two sets of axial velocity distributions. Resolution of the previous uncertainties in this field is given which were due to the unrecognized singularity at the virtual origin of the vortex. Using the calculated axial velocity the neglected radial and the associated axial fluxes of angular momentum are determined and the limits of validity of the theory presented here in terms of a suitably defined vortex Reynolds number and a nondimensional distance measured from the virtual origin of the vortex are given.

Application of the method of spatial characteristics to the solution of axially symmetric problems relating to the propagation of elastic waves

We present a difference scheme, based on the method of spatial characteristics, for solving axially symmetric dynamical problems of the theory of elasticity. Consideration is given to the possibility of solving a Cauchy problem and a problem for a solid or a hollow cylinder which takes boundary conditions into account. It is suggested that linear problems may be solved by this method. An example is given in which the parameters characterizing the stress-deformation state of a semiinfinite cylinder are calculated, the points of the end of the cylinder being given an initial axial velocity. The calculation of these parameters was carried out on the BESM-6 computer.

Stopping of an Electron Ring by Induced Image Currents in Resistive Wire Loops

The possibility of trapping an electron ring in the Maryland ERA by use of a few resistive wire loops was investigated numerically. It was found that a resistive wire of 45 ohm resistance and 404 nh self-inductance was able to stop electron rings of 1013 electrons with initial axial velocities up to szo = 0.05. A scheme, with two resistive wires and a small local mirror field, allowed the trapping and focusing of electron rings of small axial dimension. Electron rings with relatively large axial lengths were also studied.

Fast-Acting Gas Valve

The design and operation of an electrodynamically driven valve, which is employed to deliver rapidly a gas pulse (0.6 cc STP) with a sharp front to the annulus of a coaxial plasmoid source of small aspect ratio (2-mm annulus at 2-cm radius), is described. The spring loaded piston holds the plenum closed against pressures up to 2 atm. The piston is driven open by eddy currents induced by discharging a capacitor through a coil surrounding the plenum. A piston velocity of 5 cm/msec is achieved in less than 20 μsec and is maintained for 7 mm of travel. The arrival of a gas front (air) at the annulus was detected in 100 μsec after firing, employing hot-wire anemometry. The subsequent axial position and velocity of this front in the source annulus were determined as functions of time and position, respectively. It was found that the initial axial velocity of the front exceeded the stagnation sound speed (though not the vacuum escape speed) and remained constant until the displacement thickness from wall friction reached the half-annulus spacing.