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  • 1
    Electronic Resource
    Electronic Resource
    Coupled moment expansion model for the dynamics in a beam–plasma system (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4729-4747 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new technique is developed to study beam dynamics in an overdense plasma. This technique is an alternative to, and extension of, the well-known envelope equation formalism, and it can be used in systems with nonlinear forces. It is based on a systematic moment expansion of the Vlasov equation. In contrast to the envelope equation, which is derived assuming constant rms beam emittance, this coupled moment expansion (CME) model allows the emittance to vary through coupling to higher order moments. The CME model is implemented in slab geometry in the absence of return currents. The evolution of the low order moments of the distribution function are followed with ∼100 ordinary differential equations. The CME simulation yields rms beam sizes,velocity spreads, and emittances that are in good agreement with particle-in-cell simulations for a wide range of system parameters. New analytical results for the second-order moments in the phase-mixed state are obtained. Unlike previous results, these connect the final values of the second-order moments with the initial beam mismatch. These analytical estimates are in good agreement with the CME model and particle-in-cell simulations. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1314624
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  • 2
    Electronic Resource
    Electronic Resource
    Stability of highly asymmetric non-neutral plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 19-30 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stability of non-neutral plasma equilibria subjected to external, azimuthal–asymmetric potentials is studied. The system exhibits a wide range of complex, symmetry-dependent, bifurcation phenomena. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873299
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  • 3
    Electronic Resource
    Electronic Resource
    Laser wake-field acceleration and optical guiding in a hollow plasma channel (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 310-318 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The accelerating and focusing wake fields that can be excited by a short laser pulse in a hollow underdense plasma are examined. The evacuated channel in the plasma serves as an optical fiber to guide the laser pulse over many Rayleigh lengths. Wake fields excited by plasma current at the edge of the channel extend to the center where they may be used for ultrahigh gradient acceleration of particles over long distances. The wake field and equilibrium laser profiles are found analytically and compared to two-dimensional (2-D) particle-in-cell (PIC) simulations. Laser propagation is simulated over more than ten Rayleigh lengths. The accelerating gradients on the axis of a channel of radius c/ωp are of order of one-half of the gradients in a uniform plasma. For present high-power lasers, multi-GeV/m gradients are predicted. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871107
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  • 4
    Electronic Resource
    Electronic Resource
    Frequency shifting in free-electron lasers (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 157-166 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Frequency shifting in free-electron laser (FEL) oscillators and amplifiers is investigated theoretically and numerically. The analysis includes frequency shifts from the resonant FEL interaction and the nonresonant beam dielectric. Expressions for the frequency shift in a microwave amplifier with time-dependent beam energy and current are derived and found to be in good agreement with experiments. The theory shows that temporal changes in the detuning are the dominant factor in determining the frequency shift. Electron energy fluctuations produce frequency shifts in the Compton regime, while both current and energy variations are significant in the Raman regime. The effect is particularly important for high-power microwave drivers proposed for high gradient accelerators, where the phase of the radio-frequency radiation is subject to significant constraints. FEL oscillator response to variations in beam energy is examined. It is shown that in a low-gain oscillator which experiences a sudden jump in beam energy the FEL creates spikes at the head and tail of the beam which are at the shifted frequency. The shifting is generated by time dependence in dielectric function which arises from slippage and finite lengths of the electron or optical pulse. The propagation diffusion equation is shown to describe the propagation of the spikes into the main body of the pulse.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870546
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  • 5
    Electronic Resource
    Electronic Resource
    Pure electron plasmas in asymmetric trapsf| (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2378-2386 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Pure electron plasmas are routinely confined within cylindrically symmetric Penning traps. In this paper the static and dynamic properties of plasmas confined in traps with applied electric field asymmetries are investigated. Simple analytical theories are derived and used to predict the shapes of the stable noncircular plasma equilibria observed in experiments. Both analytical and experimental results agree with those of a vortex-in-cell simulation. For an l=1 diocotron mode in a cylindrically symmetric trap, the plasma rotates as a rigid column in a circular orbit. In contrast, plasmas in systems with electric field asymmetries are shown to have an analog to the l=1 mode in which the shape of the plasma changes as it rotates in a noncircular orbit. These bulk plasma features are studied with a Hamiltonian model. It is seen that, for a small plasma, the area enclosed by the orbit of the center of charge is an invariant when electric field perturbations are applied adiabatically. This invariant has been observed experimentally. The breaking of the invariant is also studied. The dynamic Hamiltonian model is also used to predict the shape and frequency of the large amplitude l=1 and l=2 diocotron modes in symmetric traps.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860721
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental and theoretical studies of a 35 GHz cyclotron autoresonance maser amplifier (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1755-1765 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental and theoretical studies of a cyclotron autoresonance maser (CARM) amplifier are reported. The measurements are carried out at a frequency of 35 GHz using a mildly relativistic electron beam (1.5 MeV, 130 A, 30 nsec) generated by a field emission electron gun followed by an emittance selector that removes the outer, hot electrons. Perpendicular energy is imparted to the electrons by means of a short bifilar helical wiggler. The entire system is immersed in a uniform axial magnetic field of 6–8 kG. With an input power of 17 kW at 35 GHz from a magnetron driver, the saturated power output is 12 MW in the lowest TE11 mode of a circular waveguide, corresponding to an electronic efficiency of 6.3%. The accompanying linear growth rate is 50 dB/m. When the system operates in the superradiant mode (in the absence of the magnetron driver) excitation of multiple waveguide modes is observed. A three-dimensional simulation code that has been developed to investigate the self-consistent interaction of the copropagating electromagnetic waveguide mode and the relativistic electron beam is in good agreement with the experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859694
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  • 7
    Electronic Resource
    Electronic Resource
    The linear theory of the circular free-electron laser (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 2209-2220 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A small signal theory of a free-electron laser (FEL) with a rotating electron beam in a uniform axial magnetic field and in an azimuthal wiggler field (the "circular'' FEL) is developed. The analysis includes the low and high gain regimes and the influence of longitudinal space-charge forces. It is found that the circular FEL instability has two regimes: a strong pump regime and a negative mass dominated regime. The negative mass regime replaces the weak pump (Raman) regime found for the usual FEL geometry in which the electron beam propagates in the axial direction (the "linear'' FEL). The dispersion relation is evaluated, and the resulting growth rates are compared with those of the linear FEL. For a cold beam, at fixed output frequency, the growth rate in the strong pump regime is larger, by a factor of γ2/3, in the circular FEL. The negative mass instability is shown to increase the growth rate and modify the bandwidth of the circular FEL in the strong pump regime. However, the circular FEL performance is found to be more sensitive to the energy spread than the linear FEL.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866155
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  • 8
    Electronic Resource
    Electronic Resource
    Waveguide mode deformation in free-electron lasers (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 2073-2084 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The free-electron laser (FEL)-induced modifications to the vacuum waveguide modes are calculated for low-frequency FEL's. Typically, the mode modifications are large and exhibit complicated axial behaviors. In addition to the wave component with a near vacuum wavenumber, components at two upshifted wavenumbers must be analyzed. Electron beam surface charges and currents are also important. At low gain, effects from all three roots of the FEL dispersion relation must be included. The dominant modification is due to the electron beam space-charge wave.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859072
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  • 9
    Electronic Resource
    Electronic Resource
    Prebunching in a collective Raman free-electron laser amplifier (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 401-406 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments, theory, and simulations are reported on the effects of electron prebunching in a mildly relativistic, low-current (200 kV, 1 A) free-electron laser amplifier operating in the collective (Raman) regime at a frequency of ∼10 GHz. Prebunching is established by injecting an electromagnetic wave into a bifilar helical wiggler and then transporting the bunched beam into a second magnetic wiggler region. The wave growth rate, Γ≡(1/P)(dP/dz), is deduced from measurements of the radiation intensity as a function of interaction length. Observations show that prebunching can increase the radiation growth rate Γ manyfold as compared with a system without prebunching.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859328
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  • 10
    Electronic Resource
    Electronic Resource
    Depletion and diffraction of an electromagnetic wiggler field during the free electron laser interaction (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1307-1315 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The self-consistent interaction of an electron beam with an electromagnetic wiggler and a radiation field is analyzed. Equations are derived for the depletion and diffraction of the wiggler wave. The formalism yields local conservation laws for the system energy flux and photon flux. Using a one-dimensional approximation, pump depletion is examined for pulsed and steady-state electron beams. A three-dimensional analysis shows that the wiggler diffraction is dominated by the nonresonant interaction of the electrons with the wiggler wave. This is studied in detail for a specific electron pulse shape. It is concluded that wiggler depletion and diffraction should not substantially degrade the free electron laser interaction for experimentally obtainable system parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858955
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