J.M. Taccetti

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.

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.

Design and performance calculations for a Ka-band CHI wiggler ubitron amplifier

Abstract Design and performance calculations for a coaxial hybrid iron (CHI) wiggler free-electron laser configuration are presented. The capability of generating high fields at short periods, as well as good beam focusing properties, make it a desirable configuration for high power coherent radiation sources in relatively compact systems. In addition to a description of the geometry, numerical calculations detailing the magnetostatic wiggler fields, the beam dynamics, and the interaction of the beam with the electromagnetic waves in the K a -band (26–40 GHz) will be presented. Key considerations for the experimental design will be outlined and discussed.

Theory of the free-electron laser based upon a coaxial hybrid wiggler

Abstract A 3D nonlinear FEL formulation 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 nonferrite spacers) along the axis of a solenoidal. Relatively high field strengths can be achieved with the CHI wiggler at shorter wiggler periods than is possible in many other conventional wiggler designs. The formulation is a slow-time-scale analysis of the interaction of an annular electron beam with the CHI wiggler in a coaxial waveguide. Simulations are presented for W-band operation. The results indicate that operation over a wide bandwidth is practical, and that the uniform- and tapered-wiggler bandwidths are comparable.

Design study of a G-band FEL amplifier for application to cyclotron resonant heating in magnetic fusion reactors

Abstract A G-band (140–150 GHz) free-electron laser is described using a coaxial hybrid iron (CHI) wiggler. The CHI wiggler is produced by insertion into a solenoid of a central rod and an outer ring composed of alternating ferrite and nonferrite spacers. The position of the spacers is such that the ferrite (nonferrite) spacers on the central rod are opposite the nonferrite (ferrite) spacers on the outer ring. The field is cylindrically symmetric and exhibits minima in the center of the gap providing for enhanced beam focusing. We describe a tapered wiggler amplifier for plasma heating applications. Preliminary design studies using a nonlinear simulation indicates that output powers of 3.5 MW are possible using a 690 kV/40 A electron beam for a total efficiency of 13%. It is important to note that no beam loss was observed even for realistic values of beam energy spread.

Simulation of an annular beam free-electron laser

A nonlinear analysis of an annular beam FEL with a helical wiggler and axial guide field is presented. An annular beam has the advantage of reduced DC self-fields, facilitating beam transport in short period wigglers. A 55 kV/5 A annular beam interacting with the TE11 cylindrical waveguide mode is considered. The inner and outer beam radii are 0.27 and 0.33 cm, respectively. The wiggler amplitude is 250 G and the period is 0.9 cm. Axial guide fields up to 3 kG are studied. The ARACHNE slow-time-scale simulation code shows that efficiencies of 10%, corresponding to gains >40 dB, are possible for grazing incidence with the TE11 mode in Ku-band. In addition, the 3 dB instantaneous bandwidth is found to be greater than 20%.

Operation of a Ka-band CHI wiggler ubitron amplifier

Summary form only given, as follows. The status of a 35 GHz CHI (Coaxial Hybrid Iron) wiggler ubitron amplifier experiment being performed at the Naval Research Laboratory is presented. The purpose of the experiment is to study the CHI wigglers potential of generating high wiggler fields at short wiggler periods as well as its excellent beam focusing and transport properties. The wiggler is immersed in an applied axial field which acts to both create the wiggler field and enhance the interaction. This enhancement near autoresonance is also investigated, for both Group I and II orbits. The wiggler period is 6.4 mm. The operation point is chosen at grazing incidence (at 35 GHz) of beam and RF uncoupled dispersion curves, resulting in a wide instantaneous bandwidth. A SLAG klystron gun has been modified to produce an annular 100 kV, 10 A beam. Focusing of the beam from the gun into the CHI structure is provided by a system of three DC magnet coils. Broadband (/spl ap/20%) RF input and output couplers have been designed to couple from a rectangular TE/sub 10/ to the coaxial TE/sub 01/ interaction mode. Simulations using the above parameters have shown an intrinsic untapered efficiency of 6.75% when operating at an applied field of 7.7 kG. The instantaneous bandwidth was /spl ap/25%, with a gain of 28.3 dB, saturating at a distance of 63 cm for 100 W drive power. Results to date are reported, along with measurements of the performance of the separate components.

The NRL Ka-band CHI wiggler ubitron amplifier

Summary form only given. A 35 GHz CHI (Coaxial Hybrid Ion) wiggler ubitron amplifier experiment is under construction at the Naval Research Laboratory. The purpose of the experiment is to study the CHI wiggler configurations potential of generating high wiggler magnetic fields at short periods with excellent beam focusing and transport properties. The nominal design parameters of the experiment are a center frequency of 35 GHz, wiggler period of 7.5 mm, and beam voltage of approximately 150 kV. Calculations have shown an intrinsic (untapered) efficiency of /spl ap/7% when operating at 6.3 kG axial field (wiggler field, B/sub w//spl ap/1270 G). The calculated gain was 36 dB, saturating at a distance of 46 cm. These parameters yield an instantaneous amplifier bandwidth of /spl ap/25%. There appears to be room for further improvement in efficiency, a matter that will be scrutinized more closely in the final design. Components of the experiment include a modified SLAC klystron electron gun (its beam converted from solid to annular), and the input and output RF couplers. Extensive work was performed on the design of the annular beam gun. This includes different techniques used in the development of the electron beam focusing coils, since the beam is immersed in a magnetic field from the cathode onward. In addition to the design of the components themselves, their placement in relation to each other in the gun-wiggler region is critical due to space limitation and beam transport issues. The techniques used for beam focusing were also used to study the transport of the beam from the gun output to the wiggler input.

A CHI wiggler ubitron amplifier experiment: wiggler characterization

A Ka-band (26-40 GHz) coaxial hybrid iron (CHI) wiggler ubitron amplifier experiment under construction at the Naval Research Laboratory is described. The principal goal of the experiment is to investigate the performance tradeoffs involved in the CHI configuration for high frequency amplifiers operating at low voltages with small wiggler periods. The nominal design parameters are a center frequency of 35 GHz, wiggler period of 0.75 cm, and beam voltage of approximately 150 kV. Nonlinear simulations of the interaction are presented, along with the results of magnetic field measurements performed on a prototype version of the CHI wiggler.

Design and application of coaxial wigglers in free-electron lasers

The Naval Research Laboratory is investigating innovative magnetic wigglers to reduce beam energy requirements for millimeter wave FELs and to enhance the gain and efficiency. Recent work has focused on coaxial designs. The advantages of this are twofold. First, annular configurations are advantageous for propagating high current beams. The annular geometry permits use of the central structure to enhance the wiggler field, hence, allowing shorter wiggler periods. One such wiggler is referred to as the Coaxial Hybrid Iron (CHI) wiggler, and employs a solenoid enclosing periodic arrays of ferromagnetic and nonferromagnetic material arranged as an outer ring and an inner rod. A second wiggler uses both outer and inner bifilar helical current windings. Both wiggler designs result in substantial enhancements in the wiggler field experienced by the electron beam as compared with the fields in the absence of the central structure. A prototype CHI wiggler is discussed along with a 35 GHz amplifier experiment which is under construction. Preliminary performance calculations for a two helix wiggler system are discussed. This will include both orbit theory and a fully 3D nonlinear simulation of the interaction.