J. Joe

Wave dispersion and growth analysis of low‐voltage grating Čerenkov amplifiers

A theoretical and computational investigation of an electron sheet beam propagating over a grating structure in a rectangular waveguide is carried out. Regimes for low voltage Cerenkov amplifier operation are sought by examining the complex dispersion relation for hybrid waveguide modes in the slow wave structure, which includes sheet beam space‐charge effects. A computer code is developed to examine the complex dispersion relation and growth rates for the wave modes. Mode competition is considered and methods to reduce it are presented. Briggs’ criteria is utilized to examine absolute and convective wave growth for the forward wave, backward wave, and transition mode regimes of operation as a function of the beam, hybrid mode, and slow wave grating characteristics. An examination of the effects of beam spread on absolute and convective wave growth to determine regimes for amplifier operation is carried out. A modest Maxwellian beam spread is found to yield a regime of effective backward convective amplif...

Experimental and theoretical investigations of a rectangular grating structure for low-voltage traveling wave tube amplifiers

A periodic, rectangular grating slow-wave-structure is considered for forward and backward wave low-voltage (⩽10 kV) Ku-band traveling wave tube (TWT) amplifiers. For forward wave operation, it is required that the ratio of groove depth, d, to grating period, p, be large (i.e. d/p⩾5) while small values of d/p allow backward wave operation. For large d/p, skin effect losses in the grating slots are large and can substantially reduce the growth rate produced by the beam-slow wave interaction. Phase and amplitude measurements of the grating structures utilizing a slotted line and a fast Fourier transform (FFT) analysis have been carried out. The results show that the measured dispersion relations for both shallow (d/p= 0.446) and deep groove gratings (d/p= 7.43) agree very well with the theoretical dispersion relations. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solen...

Nonlinear analysis of wiggler taper, node competition, and space-charge effects for a 280-GHz free electron laser

The nonlinear characteristics of a 280-GHz free electron laser (FEL) are simulated by numerical computation. The three-dimensional set of coupled nonlinear differential equations is solved for a set of TE and TM modes valid for the high gain Compton regime. The use of a nonlinear taper for efficiency enhancement, the sensitivity of gain to competing mode power levels, space-charge effects, and the effect of electron beam source distributions on gain and efficiency are examined for a 10-MeV, 3-kA beam. It is found that the nonlinear taper greatly enhances the gain and efficiency and makes the saturation power levels relatively insensitive to the competing mode power levels. The efficiency is increased to 48% by means of a nonlinear taper in which the 3-D and wiggler-averaged codes are compared and the effects of space charge are found to reduce the efficiency to a level of 32%. The effect of beam quality in terms of the four volume phase space is examined and found to have an observable effect at this wavelength. >

Waveguide coupled rectangular grating periodic structure for low-voltage amplifiers

Summary form only given, as follows. The rectangular grating periodic structure is a slow wave device which offers higher power, low voltage, compact Ku band amplifiers, with the advantage of sheet beam compatibility. Experiments and theoretical modelling have been done for two types of gratings, one with a large groove depth to grating period (d/p=7.43) called the deep groove grating and the other with a smaller ratio (d/p=0.446) called the shallow groove grating. Phase measurements are obtained utilizing a probe signal in a slotted line. This is mixed with the reference signal and fast Fourier transformed (FFT) to yield the experimental, dispersion relation. The transmittance and growth analysis has provided information about the beam interaction for shallow and deep groove gratings. The amplifier measurements are done using a round probe beam of 1 mm diameter from a 10 kV, 0.25 A, electron gun, confined by a 1 kG focussing solenoidal magnetic field. The microwave power has been coupled to the grating system utilizing loops. The observed gain yields single particle amplification. To observe the linear gain regime for the amplifier, the loop coupling has been upgraded to a waveguide coupling scheme. A waveguide Tee section is used for launching the microwave power without affecting the beam launching. This enhances the power injected in the TE/sub Z10/ waveguide mode into the amplifier system which converts to the TE/sub X10/ hybrid mode in the grating periodic structure. The phase and linear gain measurements for the shallow groove backward wave amplifier with waveguide coupled power launching are presented. We have also begun PIC simulation of the beam grating amplifier system.

Formation and transport of low-voltage, space-charge dominated sheet electron beams for high-power microwave devices

Summary form only given, as follows. Sheet electron beams have the potential to make possible higher power sources of microwave radiation due to their ability to transport high currents, at reduced current densities, through a single narrow RF interaction circuit. We will discuss experimental investigations of the formation of an elliptical sheet beam using magnet quadrupoles and a round-beam electron gun. Features of this configuration include a low-cost, commercially available Pierce gun (10 kV, 2 A, r/sub b/=0.3 cm) and a lens system consisting of four tunable magnetic quadrupoles with modest field gradients (B/sub q//R,/sub q/<60 G/cm). Three-dimensional envelope and particle-in-cell simulations indicate that this method can generate highly elliptic output beams (a=2.7 rm, b=0.1 cm), with variability in final beam size, for laboratory experiments on sheet beam transport. We also will present the results of particle-in-cell simulations of the transport of sheet beams in long-period offset-pole periodic magnet arrays. While the stability of sheet beams in short-period arrays has previously been established, the extension to longer magnet periods indicate that side-focusing of space-charge dominated sheet beams is more problematic than beam stability. However, long-term (>20 periods) stable transport is demonstrated for /spl lambda//sub m/=1 cm for a 2 A, 10 kV elliptical beam with a=2.7 cm and b=0.05 cm.

An investigation of the applicability of permanent magnet quadrupole arrays for high-power microwave tubes

Summary form only given, as follows. Magnetic quadrupoles have long been used in accelerator research for focusing high-energy particle beams. The advent of high-field, rare-earth permanent magnet materials has also spurred interest in permanent magnet quadrupoles (PMQs) since they are more lightweight and compact than their electromagnet counterparts and do not require power supplies or extraneous cooling. We study the potential use of closely-packed periodic permanent magnet quadrupoles for use in microwave tubes. In the most basic configuration, PMQs require four separate magnets in any given transverse plane, hence they are more expensive and complicated to assemble than conventional PPM stacks. Despite this fact, the stronger focusing available from PMQs may make them attractive for high-perveance beams, even at the low beam voltages generally of interest for compact slow-wave devices. Being iron-free systems, PMQ arrays may also lead to reductions in tube weight. We will compare and contrast PMQ and PPM stacks in terms of the beam focusing and beam quality that each provide and in terms of the comparative cost and difficulty of assembly. Results from two-dimensional beam envelope calculations and particle-in-cell simulations will be discussed.

Magnetic quadrupole formation of low-voltage sheet electron beams for high-power microwave devices

Summary form only given. Sheet electron beams have the potential to make possible higher power sources of microwave radiation due to their ability to transport high currents, at reduced current densities, through a single narrow RF interaction circuit. Possible microwave device applications using sheet electron beams include sheet-beam klystrons, grating TWTs, and planar FELs. One difficulty with the experimental investigation and implementation of sheet beams is the lack of a satisfactory source for large aspect-ratio beams. An attractive solution is the use of magnetic quadrupoles to transform an initially round beam from a conventional Pierce gun into a highly eccentric elliptical beam. Both 2-D envelope simulations and 3-D envelope and PIC code simulations indicate that this is a viable method of sheet beam formation, particularly for experimental investigations where flexibility and low-cost fabrication is desired. We are currently constructing a system to experimentally test this method. Features of the experiment include a low-cost commercially available Pierce gun (10 kV, 2 A, r/sub b/=0.3 cm), a four quadrupole sheet beam-forming system (R/sub q/=4 cm, B/sub q//R/sub q/<60 g/cm, L/sub q/=3 cm), and an highly elliptical output beam (a=2.7 cm, b=0.1 cm). Results of the 3-D PIC simulations of the beam and 3-D magnetostatic finite-element simulations of the quadrupole fringe fields will be discussed. Details of the experimental design and initial experimental measurements will be presented.

Stable transport and side-focusing of sheet electron beams in periodically cusped magnetic field configurations

Sheet electron beams and configurations with multiple electron beams have the potential to make possible higher power sources of microwave radiation due to their ability to transport high currents, at reduced current densities, through a single narrow RF interaction circuit. Possible microwave device applications using sheet electron beams include sheet-beam klystrons, grating TWTs, and planar FELs. Historically, implementation of sheet beams in microwave devices has been discouraged by their susceptibility to the diocotron instability in solenoidal focusing systems. However, recent theoretical and numerical studies have shown that stable transport of sheet beams is possible in periodically cusped magnetic (PCM) fields. The use of an offset-pole PCM configuration has been shown analytically to provide side-fields for 2D focusing of the beam, and this has been recently verified with PIC code simulations. We will present further theoretical studies of sheet and multi-beam transport and discuss experimental measurements of an offset-pole PCM array which is currently being constructed.

Cold Test, Spontaneous Emission And Gain In A Rectangular Cerenkov Amplifier.

The authors present experimental results for the rectangular Cerenkov grating amplifier. This research is being carried out to develop a Ka-band (35 GHz), low voltage (10 kV), moderate power (10 kW) source. They have constructed a Ku-band grating structure to study a scaled version of this source. The tapered grating consists of two tapered Ku-band smooth wave guide sections and two 3.5-inch sections of five-step-tapered gratings. Both tapered and untapered grating structures have been cold tested utilizing the network analyzer measurements. They find that their taper design reduced the reflection coefficient from {minus}5 dB to less than {minus}20 dB over a 12--15 GHz bandwidth. Spontaneous emission results resulting from passing the circular electron beam from a Litton thermionic gun over the grating structure will be presented. They have theoretically investigated the sheet beam interaction with hybrid modes in a deep groove rectangular grating waveguide. A complex dispersion relation, which includes a finite axial energy spread of the beam, describing the interaction has been solved. The authors find that the instability is always convective in the forward wave mode regime.

Sheet Beam Slow-wave Amplifiers

Sheet electron beams used in conjunction with slow-wave (Cerenkov) structures are a promising way to realize higher average power millimeter-wave amplifiers. For example, a sheet beam with a meander line structure is proposed to obtain a 100 watt W-band power booster amplifier. A sheet beam with a tapered grating structure is also being considered as a wideband ({approximately} 10--20% instantaneous bandwidth) Ka-band amplifier with approximately 10 kW of average output power. The authors describe results of research that examine critical technological issues relevant to the realization of the proposed devices. The method of forming a sheet beam using magnetic quadrupole lenses and focusing it using periodically-cusped magnetic (PCM) fields are discussed. A pencil beam from a 10 kV, 0.25 A Pierce electron source is used for the initial investigations. The EGUN simulations with the measured magnetic field indicates that a thin (2 mm dia.) beam is available at the interaction region. Beam characterization has been performed using current density probes and an electrostatic velocity spread analyzer. Numerical modeling and cold test measurements of a tapered slow-wave structure together with the simulations and measurements of small-signal gain and bandwidth are also presented.