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  • 1
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrons ionized from tightly bound atomic states by a high-intensity laser pulse can gain energies from one to millions of electron volts dependent on the intensity of the pulse. We have currently been investigating hundreds of kilovolt to megavolt electrons produced by ionization of krypton and argon with terawatt laser pulses. Angular and energy distributions have been measured to determine the usability of this electron source as an injector for higher energy accelerators. Studies have included pressure dependence, angular ejection angle energy dependence, and polarization dependence. In particular, the energy-dependent ejection angle of electrons has been used to produce electron beams with energies peaked at 600 keV. Numerical simulations of these electrons show that 4 MV electron beams with excellent beam quality and femtosecond pulse widths can be produced from this electron source using higher power laser pulses. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 4258-4267 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An intense relativistic electron beam injected into dense gas characteristically propagates in a self-pinched mode but is susceptible to the resistive hose instability. This convective instability typically leads to large amplitude beam motion and the disruption of propagation. Theory and computation suggest that, although resistive hose cannot be completely suppressed, its convective growth can be reduced by varying the average betatron oscillation frequency from head to tail in the beam pulse. We report here on experiments designed to implement this variation by tailoring the beam emittance using an ion-focused regime "conditioning'' cell. Conditioning effectiveness is assessed by using measured beam quantities to evaluate a detuning parameter η(t). This information is correlated with beam propagation measurements to determine the optimum conditioning for resistive hose suppression.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 3580-3591 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Conducting tubes filled with neutral gas at pressures between 0.001 and 0.1 Torr can be used to transport, to center, and to reduce the transverse oscillations of high current ((approximately-greater-than)10 kA) electron beams. Electron impact ionization of the gas leads to partial neutralization of the beam space charge allowing self-focused beam transport and phase-mix damping of injected beam oscillations. In addition, the presence of conducting walls helps center the beam in the transport tube. High current beams, transported through a 1.3 m long tube, were centered to within one-tenth of the beam radius and input transverse oscillations were damped to submillimeter values without significant current loss or emittance growth. Beam transport properties are examined as a function of injected current, gas pressure, and cell geometry. Experimental results are compared with a theoretical model.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 5985-5994 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thin conducting foils focus charged particle beams through image charges induced on the foils. Such focusing has led to the suggestion that foils be used to transport intense, relativistic electron beams in high-energy accelerators. This paper examines some of the limitations of foil focusing including sensitivity to the beam parameters, emittance growth from anharmonic focusing, and beam stability in multifoil transport. The analysis is based on a thin-lens electrostatic treatment of paraxial beams.
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The interaction of intense proton beams with low-pressure (0.25 to 4 Torr) background gases is studied to evaluate beam-current neutralization during transport. Electrons to neutralize the beam are provided by beam-induced ionization of the gas. In experiments with 1 MeV, 1 kA/cm2 protons, net currents outside the beam envelope and electron densities within the beam envelope are measured for helium, neon, argon, and air. Net-current fractions are 2% to 8% and ionization fractions are 0.6% to 5% for 5 to 7 kA beams. Simulations of the experiments for helium and argon suggest that fast electrons play an important role in generating a significant fraction of the return current in a halo outside the beam. As a result, net currents inside the beam may be larger than inferred from magnetic-field measurements outside the beam. Ions at the head of the beam are observed to lose more energy than expected from collisional energy losses in the background gas.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4338-4354 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The resistive hose instability has long been recognized as the major impediment to the propagation of intense, relativistic electrons beams in dense gas. However, hose is a convective instability, and therefore its growth is limited by the length of the beam pulse, the local growth rate, and the speed at which the instability convects through the pulse. The convective speed and the growth rate depend on the beam and plasma parameters, and these vary strongly from beam head to tail. In this paper, hose theory is reformulated to incorporate these variations, and the reformulated model is then used to compute the maximum hose growth possible in a given beam pulse. In air, the model predicts that hose grows by many orders of magnitude when the beam current is less than 10 kA or has a rise time more than a few nanoseconds long. But the growth is predicted to be less than a factor of 20 if the current is 50 kA or more, the rise time is subnanosecond, and the beam radius is properly tapered from head to tail. The model is supported by extensive numerical simulations and is in general agreement with available experimental data. Many of the issues discussed here may have application to other instabilities as well. © 1995 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 4153-4165 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Relativistic electron beams propagating through dense gas are subject to the resistive hose instability, a virulent kink instability that restricts the effective range of high-current beams. Previous studies have shown that the instability can be suppressed by centering the beam and tailoring its emittance prior to injection into the gas. One means of centering and tailoring a beam is to use short "conditioning'' cells that operate in the low-pressure, ion-focused regime. In this paper, analytic models are developed to understand and assess the performance of such cells.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2696-2706 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theory is presented for the guiding of relativistic electron beams by rarefied gaseous channels. The analysis is based on analytic computations of the transverse force felt by a rigid-rod beam propagating off axis from a channel of reduced gas density. The density gradients produce an attractive channel force that can be surprisingly robust, even though it develops from relatively subtle gas chemistry properties. Static numerical calculations support the analytic work. Longitudinal beam coupling and effects that degrade channel guidance are discussed as well.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 3056-3073 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An electron beam injected into a conducting medium produces inductive electric fields. These fields drive conduction currents that, for stable beams, oppose the beam current and thereby reduce the total (net) current. For unstable beams that undergo large transverse displacements, the inductive fields can reverse direction and drive conduction current parallel to the beam current. An analytic and numerical treatment of the latter effect is presented to explain current enhancement as observed for electron beams propagating in gases at pressures above 10 Torr.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1431-1442 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma channels have been used to guide intense laser pulses over distances of many Rayleigh lengths. This paper investigates the possibility of using a short plasma channel to provide focusing or control of the spot size of a laser pulse at intensities far above the usual damage limits of conventional optical elements. Analytical models for the focal length and focused spot size of a single plasma channel lens and a nonconverging laser pulse are presented, and results are compared with the two-dimensional simulation code LEM [J. Krall et al., Phys. Rev. E 48, 2157 (1993)]. Several advanced thin lens configurations, including multiple lens transport systems, and both focusing and defocusing lenses for externally focused converging laser pulses are also analyzed. Experimental techniques for producing appropriate plasma profiles are reviewed, and evidence for plasma channel focusing in a capillary discharge guiding experiment is analyzed. Thick "overmoded" lenses offer a possible alternative if there are experimental difficulties in producing sufficiently thin plasma channels. A variety of potential applications exist for the various proposed lens configurations. © 2002 American Institute of Physics.
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