An elegant and convenient rigorous approach for solving circular open-ended dielectric-loaded waveguide diffraction problems is presented. It uses the solution of corresponding Wiener-Hopf-Fock equation and leads to an infinite linear system for reflection coefficients (S-parameters) of the waveguide, the latter can be efficiently solved numerically using the reducing technique. As a specific example directly applicable to beam-driven radiation sources based on dielectric-lined capillaries, diffraction of a slow TM symmetrical mode at the open end of a circular waveguide with uniform dielectric filling is considered. A series of such modes forms the wakefield (Cherenkov radiation field) generated by a charged particle bunch during its passage along the waveguide axis. Calculated S-parameters were compared with those obtained from COMSOL simulation and an excellent agreement is shown. This method is expected to be very convenient for analytical investigation of various electromagnetic interactions of Terahertz (THz) waves (both free and guided) and charged particle bunches with slow-wave structures prospective in context of modern beam-driven THz emitters, THz accererators and THz-based bunch manipulation and bunch diagnostic systems.
On the base of general approach we obtain some results that can be useful in the process of tuning of nonunifrom disc-loaded waveguides. Our consideration has shown that simple values that characterize the detuning of the cells can be introduced only for the disc-loaded waveguide with parameters that change very slow. In general case it is needed to conduct full numerical simulation of specific disc-loaded waveguide and obtain all necessary coupling coefficients. After that one can start the tuning process on the base of bead-pull field distribution measurements.
The coupled cavity model for calculation characteristics of inhomogeneous disc-loaded waveguides with taking into account the rounding of the disk hole edges was developed. On the base of this model simulation of section tuning process with the field measurement method was conducted. Accuracy of this method under tuning of disc-loaded waveguides was evaluated. As it follows from our investigation one can use simple coefficients in tuning process, but the needed values of these coefficients must be obtained by calculation of DLW parameters.
On the base of more general model we assessed accuracy of method that is widely used for post- tuning accelerator sections. Our consideration has shown that for wide range of disk loaded waveguide (DLW) parameters using simple tuning coefficients gives appropriate characteristics of DLWs. For injector sections, in which phase velocity and amplitude change, there are some deviation from the desired characteristics. It is necessary to evaluate the influence of additional phase shift on beam characteristics at the injector exit.
The wakefields of a relativistic electron beam passing through a waveguide loaded with an active medium with weak resonant dispersion have been considered. For the calculations in this paper the parameters of the medium are those of a solution of fullerene (C60) in a nematic liquid crystal that exhibits activity in the X-band. It was shown that several of the TM accelerating modes can be amplified for the geometries under consideration; structures in which higher order modes are amplified exhibit essential advantages as PASERs. In particular, the amplification of the highest mode occurs in a s…
On the base of modified mode matching method we obtain some results that can be useful in the process of tuning of nonunifrom disk-loaded structures. Our consideration has shown that there are some parameters that depend only on the geometric sizes of sells and equals known values for the needed phase distribution of the amplitudes of field expansion. Changing the geometric sized in such way that these parameters will tend to a given values, we can numerically design the structure that have the given phase distribution of these amplitudes. We consider these tuning parameters as a good instrume…
A new approach to the description of inhomogeneous disk-loaded waveguides (chains of coupled resonators) is proposed. New matrix difference equations based on the technique of coupled integral equations and the decomposition method are obtained. Various approximate approaches have been developed, including the WKB approximation.
We consider the use of a dielectric-loaded structure to extract rf energy from a high current electron drive beam as a power source for a high energy two-beam accelerator. This represents an alternative technique which we show to have some significant advantages over the use of the currently proposed corrugated metal structures as power extraction devices. We discuss a particular design that will extract high power rf (0.1 -- 1 GW) from a high current drive beam. Rf generation and transport in this class of devices have already been demonstrated at lower frequencies. We discuss the design para…
In this work, a comparative study of the susceptibility chart in a partially dielectric-loaded rectangular waveguide and in its equivalent partially dielectric-loaded parallel-plate waveguide with the same height is performed. This study shows how the inhomogeneity of the electric field inside the rectangular waveguide modifies the edges of the multipactor susceptibility chart with respect to that predicted by the parallel-plate waveguide case. Moreover, comparisons of the evolution of the population and the trajectory of the effective electron and the DC field appearing in the waveguide in a …
We consider a semi-infinite open-ended cylindrical waveguide with uniform dielectric filling placed into collinear infinite vacuum waveguide with larger radius. Electromagnetic field produced by a point charge or Gaussian bunch moving along structure's axis from the dielectric waveguide into the vacuum one is investigated. We utilize the modified residue-calculus technique and obtain rigorous analytical solution of the problem by determining coefficients of mode excitation in each subarea of the structure. Numerical simulations in CST Particle Studio are also performed and an excellent agreeme…
The Disk Loaded Waveguide (DLW) is the mostly used high frequency structure for acceleration of lightweight particles - electrons in the high energy range. In some physical experiments acceleration of more heavy particles - muons to medium energies is required. DLW parameters are considered for particle velocity 0.04 < β< 1 both for the fundamental and the nearest backward spatial harmonics. Physical and technical restrictions for DLW application in the low βrange and lower frequency (the L-band range) are analyzed. Basing on particularities of acceleration with Traveling Wave (TW) mode, deep …
The space-charge distribution of the electromagnetic field excited by point charge moving with a constant velocity on the axis of the circular disc-loaded waveguide was analyzed. The energy losses per structure period numerically were calculated. With increasing charge energy main features of the waveguide modes reach some finite values and, so way, define the limit radiation spectrum. The saturation of the amplitudes of modes (which belongs to the lower part of the limit spectrum) was observed. The analytical techniques and results of calculations were discussed with the use of translation sy…
An electron bunch passing through dielectric-lined waveguide generates $\check{C}$erenkov radiation that can result in high-peak axial electric field suitable for acceleration of a subsequent bunch. Axial field beyond Gigavolt-per-meter are attainable in structures with sub-mm sizes depending on the achievement of suitable electron bunch parameters. A promising configuration consists of using planar dielectric structure driven by flat electron bunches. In this paper we present a three-dimensional analysis of wakefields produced by flat beams in planar dielectric structures thereby extending th…
A rigorous approach for solving canonical circular open-ended dielectric-lined waveguide diffraction problems is presented. This is continuation of our recent paper [1] where a simpler case of uniform dielectric filling has been considered. Here we deal with the case of an open-ended circular waveguide with layered dielectric filling which is closer to potential applications. The presented method uses the solution of corresponding Wiener-Hopf-Fock equation and leads to an infinite linear system for reflection coefficients (S-parameters) of the waveguide, the latter can be efficiently solved nu…
A problem of diffraction of a symmetrical transverse magnetic mode $ \text{TM}_{0l} $ by an open-ended cylindrical waveguide corrugated inside is considered. A depth and a period of corrugations are supposed to be much less than the wavelength and the waveguide radius. Therefore a corrugated waveguide wall can be described in terms of equivalent boundary conditions, i.e. a corresponding impedance boundary condition can be applied. Both vacuum case and the case of uniform dielectric filling of the waveguide is considered. The diffraction problem is solved using the modified tayloring technique …
Dielectric-loaded accelerating (DLA) structures are being studied as an alternative to conventional disk-loaded copper structures to produce the high accelerating gradient. This paper presents the design, fabrication and low-power RF measurement of an externally-powered X-band DLA structure with a dielectric constant epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. A dielectric matching section for coupling the RF power from a circular waveguide to an X-band DLA structure consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degree, resulting in a br…
We numerically investigate the acceleration and bunch compression capabilities of 20 mJ, 0.6 THz-centered coherent terahertz pulses in optimized metallic dielectric-loaded cylindrical waveguides. In particular, we theoretically demonstrate the acceleration of 1.6 pC and 16 pC electron bunches from 1 MeV to 10 MeV over an interaction distance of 20mm, the compression of a 1.6 pC 1 MeV bunch from 100 fs to 2 fs (50 times compression) over an interaction distance of about 18mm, and the compression of a 1.6 pC 10 MeV bunch from 100 fs to 1.61 fs (62 times) over an interaction distance of 42 cm. Th…
The interaction of short relativistic charged particle bunches with waveguides and other accelerator system components is a critical issue for the development of X-ray FELs (free electron lasers) and linear collider projects. Wakefield Cherenkov losses of short bunches have been studied previously for resistive wall, disk-loaded, corrugated and dielectric loaded waveguides. It was noted in various publications [1] that if the slowdown layer is thin, the Cherenkov loss factor of a short bunch does not depend on the guiding system material and is a constant for any given transverse cross section…
A linear theory of a wakefield excitation in a plasma-dielectric accelerating structure by a drive electron bunch in the case of an off-axis bunch injection has been constructed. The structure under investigation is a round dielectric-loaded metal waveguide with a channel for the charged particles, filled with homogeneous cold plasma. Derived theory was used to investigate numerically the spatial distribution of the bunch-excited wakefield components, which act on both the drive and witness bunches.
It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This paper presents a novel design of matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. It consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degree, resulting in a broadband coupling at a low RF field which …
We have developed the mode matching technique that is based on the using the eigenmodes of circular cavities and the eigenwaves of circular waveguides as the basic functions for calculation the properties of nonuniform disc-loaded waveguides. We have obtained exact infinite systems of coupled equations which can be reduced by making some assumptions. Under such procedure we can receive more exact parameters of nonuniform equivalent circuits by solving the appropriative algebraic systems. These parameters of equivalent circuits are functions both geometric sizes and frequency. Moreover, under s…
We investigate the radiation from a bunch of relativistic electrons moving along the cylindrical waveguide axis, assuming that the waveguide is partially loaded by a medium with periodic dielectric permittivity and magnetic permeability. The spectral distribution of the radiation energy flux through the cross section of the waveguide is studied at large distances from the medium. The analysis is based on the corresponding exact solution of Maxwell equations for the case of a single electron moving along the waveguide axis. The results of numerical calculations are presented in the special case…
Modern trends in beam-driven radiation sources involve interaction of Cherenkov wakefields with open-ended circular waveguide structures having complicated dielectric lining, with a three-layer dielectric capillary recently offered for reducing the radiation divergency being a representative example \cite{Jiang2020}. This paper presents rigorous approach allowing analytical description of electromagnetic processes occuring when the described structure is excited by single waveguide mode. In other words, corresponding canonical waveguide diffraction problem is considered in rigorous formulation…
