Showing papers on "Perveance published in 1994"

Nonlinear resonances and chaotic behavior in a periodically focused intense charged-particle beam

Abstract: It is shown that beam self-field effects induce nonlinear resonances and chaotic behavior in the envelope oscillations of an intense charged-particle beam propagating through a periodic focusing field. The resonance condition is derived and expressed in terms of the vacuum phase advance and the beam perveance. Certain correlations are found between such resonant and chaotic behavior and well-known instabilities in periodically focused high-current ion beams. The predicted nonlinear resonances and chaotic behavior are expected to be observable in beam transport experiments in which there is a mismatch between the beam and the periodic focusing field.

Demonstration and evaluation of carbon-carbon ion optics

Abstract: The principal life-limiting component of ion thrusters is the ion optics set used to electrostatically accelerate ionized propellant from the thruster's discharge chamber An ion optics set consists of two or three thin, closely spaced, multiaperture grids, usually made of molybdenum Grid lifetime is limited by sputter erosion from ion impingement Data in the literature show that carbon is about five times more resistant to sputter erosion than molybdenum Carbon in the form of graphite is used for laboratory ion optics, but graphite is too fragile for spacecraft use Use of carbon-carbon composites may enable fabrication of rugged, thermomechanically stable optics with greater lifetime than molybdenum This article describes a laboratory demonstration of a flat, twogrid, 10-cm-diam carbon-carbon ion optics set, with an evaluation of selected performance characteristics relative to a molybdenum optics set of roughly similar geometry The carbon-carbon optics delivered stable operation at a cold grid gap of 02 mm, an average current density of 22 mA/cm, and an inferred voltage gradient of 6400 V/mm Differences observed in the maximum perveance condition, the electron backstreaming limit, and the defocusing limit between the two optics sets were consistent with the known geometrical differences

Development of a new type of Cs plasma ion gun for application in a heavy ion beam tokamak diagnostic

Abstract: A new type of high-density caesium plasma ion source is described, which is capable of producing Cs+ beam currents of up to 300 mu A. The ion emitting surface in the caesium plasma conforms automatically to the accelerator geometry to give perveance match, providing that the ion density is correctly adjusted with respect to the required beam energy. Ready operation of the source at different energies with constant perveance is facilitated in this way. Consequently, the beam divergence is practically independent of the beam energy. The plasma density is maintained at the required level by controlling the temperature of the source chamber walls on which atomic caesium is deposited. The ion source and the beam transport line have been developed at Culham and deliver a low-divergence 1.5 mu A Cs+ beam at the gun output, with an energy of up to 25 keV. The apparatus now serves as the injection part of the heavy ion beam deflection analyser of the ISTTOK tokamak in Lisbon.

Phase advance for an intense charged particle beam propagating through a periodic quadrupole focusing field in the smooth-beam approximation

Abstract: The envelope equations for a uniform‐density Kapchinskij–Vladimirskij (KV) beam equilibrium are used to derive a transcendental equation for the phase advance of an intense charged particle beam propagating through a periodic quadrupole focusing lattice, κq(s)=κq(s+S). The analysis is carried out for the case of a matched beam in the smooth‐beam approximation, and precise estimates of the phase advance are obtained for a wide range of system parameters and choices of lattice function κq(s). Introducing the quadratic measure, σ20/S2 = 〈[∫s0sds κq(s)]2〉, of the average quadrupole focusing field squared, a detailed analysis of the transcendental equation for the phase advance σ is used to quantify the range of validity of the approximate estimate of the phase advance obtained from the simple quadratic equation (σ/S)2+(K/e)(σ/S)=(σ0/S)2. Here, σ=eS/r2b is the phase advance for a circular beam with average radius rb, e is the unnormalized beam emittance, S is the periodicity length of the lattice, and K is the self‐field perveance. For σ0≲30°, it is found that the (approximate) quadratic expression for σ gives an excellent estimate of the phase advance over the entire range of KS/e, and the quadratic estimate for σ is accurate to within 5% for values of σ0 approaching σ0=60°.

Study of beam dynamics for an intense, high brightness H- beam to design an efficient low-energy beam transport using ESQ lenses

Abstract: With an aim of transporting an initially diverging high-perveance (generalized beam perveance 2I b I 0 β 3 γ 3 = 0.003 ), high-brightness (normalized brightness ∼1011 A/(m rad)2) H− beam and finally focusing it without any significant emittance dilution, a detailed simulation scheme has been set up incorporating the various nonlinear forces due to the beam and the external focusing elements, e.g., due to space charges, geometrical and chromatic aberrations. The analysis is done following a particular hierarchy to identify the mechanism of emittance growth; this procedure is used to optimize the lens parameters. A combination of six electrostatic quadrupole lenses is configured to deliver a satisfactory solution. The estimated emittance growth is a factor of about 1.6, and this is mainly due to chromatic aberrations. A relatively small group of particles is found to be responsible for the emittance growth. The analysis highlights a number of important issues, e.g., sensitivity to the beam distribution, beam current, lens misalignments, etc. An ESQ LEBT system with some novel features in terms of compactness and mechanical rigidity is developed, and its essential characteristics are described.

A gridless, variable perveance Pierce electron gun.

Abstract: ii This thesis covers the design and development of a modulated Pierce electron gun used in the construction of experimental travelling wave tube (TWT) amplifiers. The gun incorporated an open aperture switching electrode, positioned mid-way between anode and cathode, to pulse the beam. This method of modulation did not have the same adverse effects on electron trajectories as in the case of a conventional mesh grid, but rather the electrode could be used to alter the focus conditions within the gun and subsequently improve certain beam characteristics. Ion focusing effects could also be eliminated with the electrode, allowing dual mode operation of the guns without the complications normally associated with such a practice. The switching electrode was simulated to ascertain its effect on electron trajectories within the gun, using finite element analysis as well as an electron optics design program. A test gun was constructed in a glass envelope in order to investigate the performance of the new design. The glass gun allowed a beam analysis to be performed, as well as thermal measurements to be made. Results from this gun compared favourably with earlier simulations. The results of two metal/ceramic construction TWTs are presented, showing the beneficial effects of the switching electrode on the performance of the tubes as a whole, and the electrode 's potential to compensate for constructional anomalies. The joining of metals to ceramic using active brazing techniques is also an important aspect tackled by the thesis, wit h several innovative ideas being implemented in the construction of the devices. A simple yet reliable electrical feed-through was developed for those guns having a ceramic envelope. Extensive work was also performed on the manufacture of impregnated tungsten \ cathodes for use in the electron guns. Several test diodes, including a water-cooled demountable test vehicle, were constructed to test the performance of the cathodes. An analysis was performed on the patchy behaviour of some of the initial cathodes to improve the preparation methods used in the laboratory. The emission results obtained from the cathodes are documented, as is the successful incorporation of several of them into the new modulated gun design.

Producing magnetic fields in working gaps useful for irradiating a surface with atomic molecular ions

Abstract: A deflection apparatus for high perveance ion beams, operating at 20Hz fundamental and substantially higher order harmonics, has a magnetic structure formed of laminations (72) with thickness in range between 0.2 and 1 millimeter. A compensator with similar laminated structures (71,75) with resonant excitation circuit, operating at 20 Hz or higher, is in phase locked relationship with the frequency of the previously deflected beam. Other features with broader applicability to produce strong magnetic field in magnetic gap are shown.