M.A. Basten

Analysis of rectangular waveguide-gratings for amplifier applications

A slow-wave structure composed of a grating inside a rectangular waveguide is analyzed. This type of slow-wave structure is examined for use in a low-voltage amplifier application with a sheet electron beam. Dispersion curves, mode field profiles, and taper designs for the waveguide-grating are presented. The amplifier application places stringent requirements on the taper sections that match the smooth waveguide to the waveguide-grating with minimal reflection. >

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

Sheet electron beams are attractive for high-power microwave sources due to their ability to transport high current, at reduced current density, through thin clearance apertures and in close proximity to walls or RF structures. This paper reports on the theoretical investigation of magnetic quadrupole formation of elliptical sheet electron beams for use in high-power microwave devices. The beam envelope equations for an initially round beam passing through a physical non-symmetric quadrupole pair in the presence of space-charge, finite beam emittance, and under the effects of third-order field components and longitudinal velocity variations are presented. The presence of space-charge compensates for over-focusing in the thin beam-dimension and allows for the formation of highly elliptic sheet electron beams. As an example, the results of our study were applied to an existing Pierce gun source with a beam radius of 0.6 cm, beam energy of 10 keV and current density of 2.0 A/cm/sup 2/. We find that an elliptical beam with major radius r/sub a/=3.61 cm, minor radius r/sub b/=0.16 cm and ellipticity (r/sub a//r/sub b/) of 22.5 can be produced with only modest quadrupole gradients of 64 G/cm and 18 G/cm. Quadrupole formation of elliptical sheet-beams may be particularly suited for experimental research applications since existing round-beam electron guns may be used and changes in beam ellipticity may be made without breaking the vacuum system. >

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...

Demonstration via simulation of stable confinement of sheet electron beams using periodic magnetic focusing

Using a two and one-half dimensional (two-dimensional for fields, three-dimensional for particle velocities) particle-in-cell (PIC) code we simulate the dynamics of a highly-elliptic sheet electron beam focused by a periodically-cusped-magnetic (PCM) field array. For edge-focusing, a periodic-quadrupole-magnetic (PQM) array is placed along the sides. Very high-space-charge, low-voltage beams may be focused in this way, without disruptive diocotron instability. The PCM-PQM hybrid array appears convenient for two-plane matching of the beam to minimize beam ripple in both transverse dimensions.

Formation and transport of sheet-electron beams and multibeam configurations for high-power microwave devices

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 RF interaction circuit. Possible microwave device applications using sheet electron beams include sheet-beam klystrons, rectangular grating circuits, 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 multibeam transport and discuss results from an experimental investigation of the formation, stability and transport of PCM-focused sheet electron beams. This includes a laboratory method of forming an elliptical sheet beam using magnetic quadrupole pair and a round-beam Pierce gun.

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.

Analysis of 3-D phase space dynamics of pencil-to-sheet-electron-beam transformation in highly-non-paraxial quadrupole lens system

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 structure. Previous investigations have indicated the feasibility for laboratory formation of an elliptical sheet beam using magnetic quadrupoles and a Pierce gun pencil-beam source. The configuration exhibits several unique physical features of phase space evolution not observed in more conventional, paraxial beam transport systems. Previous numerical simulations using MAGIC3-D indicate the r.m.s. phase space volume occupied by the sheetbeam increased longitudinally, which contradicts the principle of phase space conservation for a conservative system (Liouvilles theorem). The explanation is that is Liouvilles theorem does not strictly apply to r.m.s. emittance calculations and some alternative phase space measure might provide closer agreement with Liouvilles theorem. The developed phase space measure is non-statistical and uses principles from mathematical morphology. We discuss and compare the results of numerical simulations using the TRACE3-D and MAGIC3-D codes. In addition, we discuss the morphological algorithm for computing phase space area and compare its results with that of the r.m.s. formalism for calculating phase space area.

High-perveance electron beams for high-power, slow-wave microwave devices

Summary form only given. One of the problems in the scaling of high-power vacuum microwave sources to higher frequencies is the need to transport beams with high space charge density, since the RF circuit transverse dimensions tend to decrease with wavelength. Research results establish that novel periodic magnetic focusing optics offer the opportunity to propagate much higher beam currents than previously achievable in slow-wave vacuum microwave devices. Theory and simulations establish that stable, edge focused transport of linear sheet electron beams can be realized with periodically-cusped magnetic (PCM) focusing for very high current density beams (>200 A/cm/sup 2/) at low beam voltages (10 kV). Edge focusing options include offsetting the poles of the planar PCM array or using periodic permanent quadrupole magnetic (PPQM) edge focusing. Both options provide stable edge focusing with the former method requiring fewer magnet pieces and the latter method providing superior beam matching capability. Using quadrupole magnetic optics to produce very large aspect ratio elliptical sheet electron beams is well suited for laboratory experiments. Simulation and experimental results with a 10 kV, 2 A electron beam confirm the feasibility of this approach. Round beam focusing by short period PPQM arrays was also studied.

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.