R.H. Jackson

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.

Study of gain, bandwidth, and tunability of a millimeter‐wave free‐electron laser operating in the collective regime

Frequency-resolved measurements of the emission of a collective free-electron laser operating at millimeter wavelengths have shown emission spectra that agree with theoretical predictions for the collective free-electron laser instability. Broad tunability, moderate emission linewidths, and high single frequency gain have been observed. In addition, adjusting the axial field in the end region of the interaction has been found in some cases to cause a large increase in measured power and efficiency.

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...

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.

Nonlinear theory and design of a harmonic ubitron/free-electron laser

Abstract A fully three-dimensional nonlinear analysis of the harmonic Ubitron/Free Electron Laser is discussed which is valid for arbitrary harmonic number. The analysis has been performed for a configuration consisting of a beam propagating through a loss-free rectangular waveguide in the presence of a planar wiggler field. The wiggler model includes an adiabatic entry taper to model the injection of the beam into the wiggler, parabolic pole pieces to provide additional focussing in the plane normal to the wiggler, and an amplitude taper downstream from the entry region for efficiency enhancement. The advantage of harmonic operation is that relatively high operating frequencies may be obtained with relatively modest beam energies; however, this occurs at the expense of a greater sensitivity to beam thermal effects. In addition to enhancing the extraction efficiency, a tapered wiggler has been shown to reduce the sensitivity of the interaction to thermal effects. Thus, the tapered wiggler is designed to counter the increased thermal sensitivity of the harmonic interaction. Specific design criteria for a third harmonic experiment operating at 15 GHz with a 55 keV electron beam are discussed.

A Ka-band CHI-wiggler free-electron maser: experimental results

Abstract Due to the nature of the Coaxial Hybrid Iron wiggler, a resonance between the transverse wiggler and cyclotron motions of the beam exists which can enhance the Free-Electron Maser interaction. The difference in behavior on either side of this resonance is presented. Amplification with a bandwidth on the order of a few percent was achieved. In spite of beam propagation problems, the gain was about 5 dB for operation below gyroresonance, and 7 dB above it, when wideband grazing intersection operation was achieved. A gain as high as 13.5 dB was obtained for cutting intersection between the beam line and the TE 01 dispersion curve.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith-Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith-Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith-Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.