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In the design of a lens the most important parameter is the paraxial focal length. Though, most focal length measurement methods are not measuring the paraxial focal length, but a focal length influenced by aberrations. We have developed a method to determine the paraxial focal length of strong focusing lenses using experimental ray tracing in a 2D cross section measurement. The method developed by us gives a much higher accuracy in measuring the paraxial focal length than the compared methods according to the German Institute for Standardization and Neal et al.. It shows an accuracy of up to 0.15% in measurement.

We investigate the interaction between a single atom and a light field in the strong focusing regime. Such a configuration is subject to recent experimental work not only with atoms but also molecules and other atom-like systems such as quantum dots. We derive the scattering probability for photons by such a microscopic object modeled by a two-level system, starting with a Gaussian beam as the spatial mode of the light field. The focusing by an ideal lens is modeled by adopting a field with spherical wave fronts compatible with Maxwell equations. Using a semi-classical approach for the atom-field interaction, we predict a scattering probability of photons by a single atom of up to 98% for realistic focusing parameters. Experimental results for different focusing strengths are compared with our theoretical model.

We propose and demonstrate a method employing ferroelectric monomolecular layers, by which it is possible to precisely measure the planar light field polarization in the focus of a lens. This method allowed us to establish for the first time to our knowledge, the perpendicularly oriented field that is anticipated at high apertures. For a numerical aperture 1.4 oil immersion lens illuminated with linearly polarized plane waves, the integral of the modulus square of the perpendicular component amounts to (1.51 r 0.2) % of that of the initial polarization. It is experimentally proven that depolarization decreases with decreasing aperture angle and increases when using annular apertures. Annuli formed by a central obstruction with a diameter of 89 % of that of the entrance pupil raise the integral to 5.5 %. This compares well with the value of 5.8% predicted by electromagnetic focusing theory; however, the depolarization is also due to imperfections connected with focusing by refraction. Besides fluorescence microscopy and single molecule spectroscopy, the measured intensity of the depolarized component in the focal plane is relevant to all forms of light spectroscopy combining strong focusing with polarization analysis.

1407_44An experiment to evaluate the feasibility of the spiral line induction accelerator (SLIA) as a compact high current accelerator is currently underway at Pulse Sciences, Inc. The experiment involves injecting a 3.5 MeV, 10 kA electron beam into a strong focusing racetrack spiral transport line with two passes through two 1.5 MV induction accelerating units, giving a nominal 9.5 MeV output. The status of hardware fabrication and full-scale component tests are summarized with emphasis on the electrical design of the injector and accelerating cells. Initial experimental results at 1 MeV are reported on beam matching and transport in stellarator fields, and on the design and testing of the shielded (coaxial) accelerating gaps to minimize the beam breakup (BBU) instability.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

1407_47Since the previously reported successful demonstration of acceleration of electrons to $OM 12 MeV, higher trapped beam current and longer beam lifetime have been obtained. The improved confinement has led to a higher peak energy $OM 18 MeV, and is the result of increasing the strong focusing and toroidal magnetic fields, reducing magnetic field errors, and optimizing the betatron flux condition. Modifications to the electromagnet buswork are in progress to further reduce field errors, and a fast pulsed magnet system to provide dynamic stabilization of the cyclotron resonances is being studied.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The lifetime of the circulating electron beam in the NRL modified betatron has been substantially increased from a few sec to over 700 sec by the addition of strong focusing windings to the device. The injected beam consistently spirals from the injector to the minor axis and is trapped. The s0. 5 kA trapped electron ring has been accelerated to 12 MeV from the injection energy of MeV. The beam acceleration has been confirmed not only by the x-ray attenuation technique but also with the detection. of photoneutrons from the reaction D(''y n)H. The experimental observations show that during acceleration the electrons cross initially higher order cyclotron resonances. Eventually when the ratio of the vertical B to toroidal magnetic field B0 is 1/9 i. e. at I 9 a substantial fraction of the ring electrons is lost. The remaining electrons are lost at higher B that is at lower values. Experiments are in progress to locate and eliminate the field error(s) that excite the resonances and thus to accelerate the electron ring to even higher energy. 1.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Existing methods of matching the low-energy Hbeam from the ion source into the Radio. Frequency- Quadrupole (RFQ) accelerator using gas focusing and magnetic lenses suffer from beam loss emittance growth and inadequate control capability. It is shown that an electrostatic quadrupole (ESQ) system is an attractive alternative providing strong focusing and good control as well as preventing plasma build*up and associated problems. A new conceptual design of an ESQ system for matching a 120 keV 120 mA beam with a normalized 4xrms emittance of 3 x i0 m-rad into a 425-MHz RFQ is developed using laminarflow and linear-optics codes and a modified PARMILA particle-in-cell simulation code which includes the nonlinear effects of the external fields and space charge. Simulation results show a 100 transmission and low emittance growth. 1.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A Hamiltonian-theoretic analysis of emittance growth during high current electron beam transport, coupled with an envelope equation analysis of the induced transverse beam oscillations, may be applied to develop scaling laws for the emittance growth suffered during such events as beam acceleration, propagation past irises or constrictions in the waveguide wall, beam aperturing, and axial variation in the magnetic guide field strength. Criteria may be developed for preventing excessive emittance growth by avoiding abrupt axial variations and providing sufficiently strong focusing forces. Designed variations in the waveguide wall shape and in the strength of the magnetic guide field may be introduced to greatly reduce emittance growth during events such as beam acceleration.

The generation of intense, high-brightness H- beams requires solutions of several physics and technological problems in the low-energy beam transport (LEBT) section of the accelerator to avoid particle losses and emittance growth. Existing accelerators and experiments use space-charge neutralization, usually in combination with magnetic lenses, to focus the beam. However, the demands for high beam brightness and intensity require a better understanding and control of gas focusing and associated instabilities or the use of alternate methods of transporting the beam. In this paper we present results of a new theoretical treatment of the charge-neutralized beam-plasma steady state. The second part of the paper presents computer results for a novel alternate focusing scheme using electrostatic quadrupole lenses in which no plasma buildup and charge neutralization occurs. Such quadrupole lenses provide efficient strong focusing in the LFRT line and, more importantly, good control of the beam matching both into the electrostatic column and into the RPO.