Alexei Kanareykin

Experimental observation of energy modulation in electron beams passing through terahertz dielectric wakefield structures.

We report the observation of a strong wakefield induced energy modulation in an energy-chirped electron bunch passing through a dielectric-lined waveguide. This modulation can be effectively converted into a spatial modulation forming microbunches with a periodicity of 0.5-1 ps and, hence, capable of driving coherent terahertz radiation. The experimental results agree well with theoretical predictions.

Observation of an anomalous correlation between permittivity and tunability of a doped (Ba,Sr)TiO3 ferroelectric ceramic developed for microwave applications

Anomalous dependencies of the dynamic (pulse) and static tunability [k(U)=C(0)/C(U)] as a function of permittivity (e) were observed in ferroelectric varactors based on doped paraelectric state (Ba,Sr)TiO3 ceramics. The reduction of the relatively high permittivity value from e≅810 down to e≅260 by introducing various proportions of a Mg2TiO4 additive resulted in a 20% increase in tunability. Furthermore, ceramics with this additive have demonstrated dynamic tunability noticeably higher than the static tunability, also unexpected for this type of material.

Experimental demonstration of wakefield effects in a THz planar diamond accelerating structure

We have directly measured THz wakefields induced by a subpicosecond, intense relativistic electron bunch in a diamond loaded accelerating structure via the wakefield acceleration method. We present here the beam test results from the diamond based structure. Diamond has been chosen for its high breakdown threshold and unique thermoconductive properties. Fields produced by a leading (drive) beam were used to accelerate a trailing (witness) electron bunch, which followed the drive bunch at a variable distance. The energy gain of a witness bunch as a function of its separation from the drive bunch describes the time structure of the generated wakefield.

Experimental demonstration of energy-chirp compensation by a tunable dielectric-based structure.

A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.

Planar ultrananocrystalline diamond field emitter in accelerator radio frequency electron injector: Performance metrics

A case performance study of a planar field emission cathode (FEC) based on nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in an RF 1.3 GHz electron gun. The FEC was a 100 nm (N)UNCD film grown on a 20 mm diameter stainless steel disk with a Mo buffer layer. At surface gradients 45–65 MV/m, peak currents of 1–80 mA (equivalent to 0.3–25 mA/cm2) were achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD surface with (1) the beam emittance of 1.5 mm × mrad/mm-rms and (2) longitudinal FWHM and rms widths of non-Gaussian energy spread of 0.7% and 11% at an electron energy of 2 MeV. Current stability was tested over the course of 36 × 103 RF pulses (equivalent to 288 × 106 GHz oscillations).

Fast Switching Ferroelectric Materials for Accelerator Applications

Fast switching (<10 nsec) measurement results on the recently developed BST(M) (barium strontium titanium oxide composition with magnesium‐based additions) ferroelectric materials are presented. These materials can be used as the basis for new advanced technology components suitable for high‐gradient accelerators. A ferroelectric ceramic has an electric field‐dependent dielectric permittivity that can be altered by applying a bias voltage. Ferroelectric materials offer significant benefits for linear collider applications, in particular, for switching and control elements where a very short response time of <10 nsec is required. The measurement results presented here show that the new BST(M) ceramic exhibits a high tunability factor: a bias field of 40–50 kV/cm reduces the permittivity by a factor of 1.3–1.5. The recently developed technology of gold biasing contact deposition on large diameter (110 cm) thin wall ferroelectric rings allowed ∼few nsec switching times in witness sample experiments. The ferroelectric rings can be used at high pulsed power (tens of megawatts) for X‐band components as well as at high average power in the range of a few kilowatts for the L‐band phase‐shifter, under development for optimization of the ILC rf coupling. Accelerator applications include fast active X‐band and Ka‐band high‐power ferroelectric switches, high‐power X‐band and L‐band phase shifters, and tunable dielectric‐loaded accelerating structures.Fast switching (<10 nsec) measurement results on the recently developed BST(M) (barium strontium titanium oxide composition with magnesium‐based additions) ferroelectric materials are presented. These materials can be used as the basis for new advanced technology components suitable for high‐gradient accelerators. A ferroelectric ceramic has an electric field‐dependent dielectric permittivity that can be altered by applying a bias voltage. Ferroelectric materials offer significant benefits for linear collider applications, in particular, for switching and control elements where a very short response time of <10 nsec is required. The measurement results presented here show that the new BST(M) ceramic exhibits a high tunability factor: a bias field of 40–50 kV/cm reduces the permittivity by a factor of 1.3–1.5. The recently developed technology of gold biasing contact deposition on large diameter (110 cm) thin wall ferroelectric rings allowed ∼few nsec switching times in witness sample experiments. The ferr...

Transformer Ratio Enhancement for Structure-Based Wakefield Acceleration

A limiting factor in the efficiency of wakefield accelerators is the fact that the transformer ratio R, the parameter that characterizes the energy transfer efficiency from the accelerating structure to the accelerated electron beam, is less than 2 for most technologically realizable beam‐structure configurations. We are planning an experiment to study transformer ratio enhancement in a 13.625 GHz dielectric wakefield structure driven by a ramped bunch train. In this paper we present an experimental program for the demonstration of this Enhanced Transformer Ratio Dielectric Wakefield Accelerator (ETR‐DWA).

Observation of multipactor suppression in a dielectric-loaded accelerating structure using an applied axial magnetic field

Efforts by a number of institutions to develop a Dielectric-Loaded Accelerating (DLA) structure capable of supporting high gradient acceleration when driven by an external radio frequency source have been ongoing over the past decade. Single surface resonant multipactor has been previously identified as one of the major limitations on the practical application of DLA structures in electron accelerators. In this paper, we report the results of an experiment that demonstrated suppression of multipactor growth in an X-band DLA structure through the use of an applied axial magnetic field. This represents an advance toward the practical use of DLA structures in many accelerator applications.

Diamond‐Based Dielectric Loaded Accelerating Structures

Diamond is an excellent material for dielectric loaded accelerating structures. It has a high breakdown threshold, very low dielectric losses and an extremely high coefficient of thermal conductance. In this paper we review progress on the design and development of diamond—based accelerating structures in the GHz—THz frequency range.

A high gradient test of a single-cell superconducting radio frequency cavity with a feedback waveguide

The most severe problem of the international linear collider (ILC-type) is its high cost, resulting in part from the enormous length of the collider. This length is determined mainly by the achievable accelerating gradient in the RF system of the collider. In current technology, the maximum acceleration gradient in superconducting (SC) structures is determined mainly by the value of the surface RF magnetic field. In order to increase the gradient, a superconducting traveling wave accelerating (STWA) structure is suggested. Utilization of STWA structure with small phase advance per cell for future high energy linear colliders such as ILCs may provide an accelerating gradient 1.2–1.4 times larger [1] than a standing wave structure. However, STWA structure requires a feedback waveguide for power redirecting from the end of the structure back to the front end of accelerating structure. Recent tests of a 1.3 GHz model of a single-cell cavity with waveguide feedback demonstrated an accelerating gradient comparable to the gradient of a single-cell ILC-type cavity from the same manufacturer [2]. In the present paper, high gradient test results are presented.