Publications (5)20.49 Total impact

  • X J Wang · H P Freund · D Harder · W H Miner · J B Murphy · H Qian · Y Shen · X Yang
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    ABSTRACT: We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser using a tapered undulator. Output and spectra in the fundamental and third harmonic were measured versus distance for uniform and tapered undulators. With a 4% field taper over 3 m, a 300% (50%) increase in the fundamental (third harmonic) output was observed. A significant improvement in the spectra with the elimination of sidebands was observed using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.
    Physical Review Letters 10/2009; 103(15):154801. DOI:10.1103/PHYSREVLETT.103.154801 · 7.51 Impact Factor
  • H. P. Freund · W. H. Miner
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    ABSTRACT: The enhancement of the efficiency in free-electron lasers (FELs) through the use of a tapered wiggler is well known. The physics of the tapered wiggler interaction has been studied in theory and simulation, and large efficiency enhancements have been observed in the laboratory in oscillators and seeded amplifiers. In this paper, we study the differences in the tapered wiggler interaction between seeded amplifiers and in FELs that start up from noise and grow to saturation in a single pass through the wiggler. This configuration is commonly referred to as self-amplified spontaneous emission (SASE). In comparison with seeded amplifiers, SASE FELs exhibit shot-to-shot fluctuations due to random phase noise in the electron bunches, and our purpose in this paper is to determine the effect of this phase noise on the tapered wiggler interaction. To this end, we study the interaction numerically using the MEDUSA simulation code for seeded and SASE FELs operating in the infrared regime. The results of the simulations indicate that the overall efficiencies of the seeded and SASE FELs are comparable for a uniform wiggler but that the output spectrum for the SASE FEL is much broader than for the seeded case. For a tapered wiggler, the efficiency enhancement in the SASE FEL is less than that found in the seeded example due to the broader excited spectrum that detunes the tapered wiggler interaction.
    Journal of Applied Physics 07/2009; 105(11-105):113106 - 113106-6. DOI:10.1063/1.3131630 · 2.18 Impact Factor
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    ABSTRACT: We describe a procedure for the simulation of free-electron-laser (FEL) oscillators. The simulation uses a combination of the MEDUSA simulation code for the FEL interaction and the OPC code to model the resonator. The simulations are compared with recent observations of the oscillator at the Thomas Jefferson National Accelerator Facility and are in substantial agreement with the experiment.
    Physical Review Letters 07/2009; 102(24):244802. DOI:10.1103/PhysRevLett.102.244802 · 7.51 Impact Factor
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    H. P. Freund · L. Giannessi · W. H. Miner
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    ABSTRACT: The problem of radiation start up in free-electron lasers (FELs) is important in the simulation of virtually all FEL configurations including oscillators and amplifiers in both seeded master oscillator power amplifier (MOPA) and self-amplified spontaneous emission (SASE) modes. Both oscillators and SASE FELs start up from spontaneous emission due to shot noise on the electron beam, which arises from the random fluctuations in the phase distribution of the electrons. The injected power in a MOPA is usually large enough to overwhelm the shot noise. However, this noise must be treated correctly in order to model the initial start up of the harmonics. In this paper, we discuss and compare two different shot noise models that are implemented in both one-dimensional wiggler-averaged (PERSEO) and non-wiggler-averaged (MEDUSA1D) simulation codes, and a three-dimensional non-wiggler-averaged (MEDUSA) formulation. These models are compared for examples describing both SASE and MOPA configurations in one dimension, in steady-state, and time-dependent simulations. Remarkable agreement is found between PERSEO and MEDUSA1D for the evolution of the fundamental and harmonics. In addition, three-dimensional correction factors have been included in the MEDUSA1D and PERSEO, which show reasonable agreement with MEDUSA for a sample MOPA in steady-state and time-dependent simulations.
    Journal of Applied Physics 01/2009; 104(12-104):123114 - 123114-11. DOI:10.1063/1.3040689 · 2.18 Impact Factor
  • H.P. Freund · Miner, W.H., Jr · J. Verboncoeur · Yanxia Li · E. Wright
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    ABSTRACT: In this paper, we discuss the development of a 3-D time-domain formulation and simulation code for modeling inductive output tubes (IOTs). This formulation relies on the integration of equivalent circuit equations in time coupled with the Lorentz force equations for particle trajectories. In the case of IOTs and klystrons, the equivalent circuit is a simple inductance-resistance-capacitance model. The same formulation using the equivalent circuit equations for Curnow cavities has been used to simulate coupled-cavity traveling-wave tubes. The connection between the equivalent circuit equations and the forces on the electrons used in the Lorentz force equations is through a scaling of a radio-frequency (RF) field model in which the amplitude is proportional to the cavity voltage. The RF field model can be obtained analytically [as derived in two dimensions by Kosmahl and Branch] or by means of a field map generated by electromagnetic structure simulators. The electron trajectories are integrated in these RF fields as well as using magnetostatic focusing fields and the space-charge fields. The space-charge fields are obtained by mapping the charge to a grid and then solving Poisson's equation. The new code is called NEMESIS, and we discuss the presently implemented IOT model and future development plans. Data on an IOT under development at Communications and Power Industries (CPI; K5H90W-2) have been provided for benchmarking the simulation. Comparison of NEMESIS with both anticipated performance predictions developed using scaling laws developed in-house at CPI and with actual tube performance has been good, and these comparisons are discussed in detail.
    IEEE Transactions on Plasma Science 09/2007; 35(4-35):1081 - 1088. DOI:10.1109/TPS.2007.902137 · 1.10 Impact Factor