H.P. Freund

Field theory of a traveling wave tube amplifier with a tape helix

A self-consistent relativistic field theory for a helix traveling wave tube (TWT) is presented for a configuration in which a magnetized pencil beam propagates through a tape helix enclosed with a loss-free well. A linear analysis of the interaction is solved subject to the boundary conditions imposed by the beam, helix, and wall. The wave equation for the fields within the electron beam corresponds to the Appleton-Hartree magnetoionic wave modes that are of mixed electrostatic/electromagnetic polarization. Hence, the determinantal dispersion equation that is obtained implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. This dispersion equation includes azimuthal variations and all spatial harmonics of the tape helix. Solutions that correspond to both the extraordinary (X) and ordinary (O) solutions for the Appleton-Hartree modes are found numerically. >

Amplifier performance of the NRL ubitron

Abstract Operation of the Naval Research Laboratory K u -band ubitron has successfully demonstrated a high power/efficiency and broad bandwidth. This device employs a helical wiggler/axial guide field configuration. Performance levels achieved at 16.6 GHz can be summarized as a peak power of 4.2 MW for an efficiency of 17.5% and a gain of 29 dB, and an instantaneous bandwidth of 22%. Substantial beam loss was observed. The specific loss rate was correlated with output power, and reached a level of 50% beam loss at the 4.2 MW level. Nonlinear simulations of the experiment are in good agreement with these observations.

Nonlinear theory of the free‐electron laser based upon a coaxial hybrid wiggler

A three‐dimensional nonlinear formulation of a free‐electron laser based upon a coaxial hybrid iron (CHI) wiggler is described. The CHI wiggler is created by insertion of a central rod and an outer ring [composed of alternating ferrite and dielectric spacers in which the ferrite (dielectric) spacer on the central rod is opposite to the dielectric (ferrite) spacer on the outer ring] along the axis of a solenoidal. An analytic model of the CHI wiggler is developed which is in good agreement with the Poisson/Superfish group of codes. The free‐electron laser (FEL) formulation is a slow‐time‐scale analysis of the interaction of an annular electron beam with the CHI wiggler in a coaxial waveguide. The electromagnetic field is represented as the superposition of the vacuum transverse electric (TE), transverse magnetic (TM), and transverse electromagnetic (TEM) modes of the waveguide, and a set of nonlinear second‐order differential equations is derived for the amplitudes and phases of these modes. These equation...

The coaxial hybrid iron (CHI) wiggler

Abstract A wiggler design has been developed which is scalable to small periods with high field amplitude, high beam current acceptance, and excellent transverse focusing and beam propagation properties. The coaxial hybrid iron (CHI) wiggler design consists of a coaxial arrangement of alternating ferromagnetic and non-ferromagnetic rings with the central portion of the coax shifted by one half period. The entire arrangement is immersed in a solenoidal field which results in a cylindrically symmetric periodic field. FEL configurations using this wiggler design have the potential for high power, high frequency coherent generation in relatively compact systems. Analytic and simulated characteristics of the CHI wiggler are discussed.

The nonlinear analysis of self‐field effects in free‐electron lasers

A model of the self‐fields associated with the charge density and current of the electron beam is incorporated into three‐dimensional nonlinear formulations of the interaction in free‐electron lasers for both planar and helical wiggler configurations. The model assumes the existence of a cylindrically symmetric electron beam with a flat‐top density profile and a uniform axial velocity, and the self‐electric and self‐magnetic fields are determined from Poisson’s equation and Ampere’s law. Diamagnetic and paramagnetic effects due the electron beam interaction with the wiggler field are neglected; hence, the model breaks down when the wiggler‐induced transverse displacement is comparable to the beam radius. The nonlinear formulations are based upon the arachne and wigglin codes, which represent slow‐time‐scale formulations for the evolution of the amplitudes and phases of a multimode superposition of vacuum waveguide modes. The electron dynamics in these codes are treated by means of the complete three‐dimen...

Nonlinear theory of collective effects in helix traveling wave tubes

A time-dependent collective nonlinear analysis of a helix traveling wave tube including fluctuating (ac) space-charge effects is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading of the helix are included, as is efficiency enhancement by tapering the helix pitch, and external focusing by means of either a uniform solenoidal magnetic field or a periodic field produced by a periodic permanent magnet stack. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. Vane loading describes the insertion of an arbitrary number of vanes running the length of the helix. The electromagnetic field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. The propagation of each wave in vacuo, as well as the interaction of each wave with the electron beam, is included by allowing the amplit...

Nonlinear theory of helix traveling wave tubes in the frequency domain

A nonlinear helix traveling wave tube (TWT) analysis in the frequency domain is presented for a sheath helix surrounded by a conducting wall. Dielectric- and vane-loading of the helix are included as is circuit tapering and external focusing by either a solenoid or a field produced by a periodic permanent magnet (PPM) stack. The electromagnetic field is treated as a superposition of waves where the amplitudes and phases vary slowly in z. The field equations are solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Beam space-charge waves are included using a superposition of solutions of the Helmholtz equation. The dc (direct current) self-fields of the beam are also included. The simulation is compared with linear theory as well as a time-domain simulation, each of which is applied to an example of a tube built at Northrop-Grumman Corp. The frequency- and time-domain simulations are in substantial agreement. The advantage of the frequency-domain formulatio...

A high power, 600 μm wavelength free electron laser☆

We report high power emission (18 MW) at a wavelength of λ = 640 μm and a bandwith Δλ/λ < 0.04 from a superradiant free electron laser (FEL) excited by a 2 MeV, 1 kA electron beam. Comparison of the experiment with a nonlinear simulation yields good agreement. Theoretical extrapolation to a tapered wiggler experiment shows that power levels of ∼ 140 MW could be achieved with an efficiency of ∼ 7%.

A free‐electron laser for cyclotron resonant heating in magnetic fusion reactors

A G‐band free‐electron laser designed for plasma heating is described using a coaxial hybrid iron (CHI) wiggler formed by insertion into a solenoid of a central rod and an outer ring of alternating ferrite and nonferrite spacers positioned so that the central ferrite (nonferrite) spacers are opposite the outer nonferrite (ferrite) spacers. The CHI wiggler provides for enhanced beam focusing and the ability to handle intense beams and high‐power continuous wave radiation. Simulations indicate that a power/efficiency of 3.5 MW/13% are possible using a 690 kV/40 A beam. No beam loss was found in simulation.

Improved amplifier performance of the NRL ubitron

Abstract Improved amplifier performance of the NRL Ku-band ubitron is reported following several experimental modifications. The major modification is the substitution of a higher-current (100 A), higher-quality electron gun for the original modified SLAC klystron gun (250 kV, 37 A). The experimental configuration is otherwise unchanged: a solid, uniform-density electron beam propagating through a helical wiggler/axial guide field configuration, interacting with a co-propagating circularly polarized TE 11 rf wave. With these changes, small-signal gains of 23 dB have been observed in the 12.6–17.5 GHz frequency range. Good agreement between measured and calculated gain in the Raman regime has been obtained using a three-wiggler model in the 3D nonlinear FEL code ARACHNE.