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Modern electron linear accelerators are often designed to produce smooth bunch distributions characterized by their macroscopic ensemble-average moments. However, an increasing number of accelerator applications call for finer control over the beam distribution, e.g., by requiring specific shapes for its projection along one coordinate. Ultimately, the control of the beam distribution at the single-particle level could enable new opportunities in accelerator science. This review discusses the recent progress toward controlling electron beam distributions on the "mesoscopic" scale with an emphasis on shaping the beam or introducing complex correlations required for some applications. This review emphasizes experimental and theoretical developments of electron-bunch shaping methods based on bounded external electromagnetic fields or via interactions with the self-generated velocity and radiation fields., 78 pages, 60 figues; submitted to the Review of Modern Physics

(2006). Right to Choose Your Lawyer in Competition Investigations: Law Society of Ireland v Competition Authority. European Competition Journal: Vol. 2, No. 2, pp. 371-385.

We propose combining a transverse‐to‐longitudinal emittance exchange beam line and a conventional modulator to generate high harmonics with short‐wavelength free‐electron lasers. The key point of this scheme is the use of a multi‐slit mask to create a transverse‐shaped beam. In an emittance exchange beam line, the transverse‐shaped bunch will produce a longitudinal‐shaped one, resulting in separated energy bands. The modulator that follows this step will generate micro‐bunches containing high harmonics. A coherent synchrotron radiation effect is also considered in this paper; compared to the two‐modulator scheme, the method investigated here is more flexible and can be used to relax the coherent synchrotron radiation effect.

A single‐shot longitudinal phase space diagnostics is currently being assembled at the Argonne Wakefield Accelerator (AWA). The diagnostics consists of mapping the longitudinal phase space coordinates to the transverse position space plane using a beamline that incorporates two magnetic dipoles arranged as a dogleg followed by an rf deflecting cavity. The deflecting cavity streaks the beam vertically while the dipole magnets bend horizontally. In this paper, we analyze analytically and via numerical simulations the performances of such diagnostics and their limitations. Simulations done in IMPACT‐T shows good reconstruction of the longitudinal phase space.

In the Two Beam Accelerator (TBA), wakefield power extractors which extract high power RF from a high current beam are used to power high gradient accelerating structures. A dielectric‐based Wakefield Power Extractor (DWPE) is one option in addition to the metallic structures considered previously, like the CLIC PETS (Power Extraction and Transfer Structure). 7.8 GHz and 26 GHz DWPE prototypes have been successfully built and tested at the Argonne Wakefield Accelerator (AWA) facility. We are currently designing an X‐band version for a potential application with the CLIC beam. In this article, we report on test results of the 26 GHz DWPE and the preliminary design of the X‐band structure. Future plan and possible difficulties in the development of DWPEs are also discussed.

High‐brightness electron beam sources play a crucial role in many advanced acceleration schemes as well as linac‐based light sources such as Energy Recovery Linac (ERL) based light sources and FELs. Three varieties electron sources (photo, thermionic, and field emission) the photoinjector is especially attractive due to its innate ability to control the time structure of intense electron bunches with low transverse and longitudinal emittance. In all cases, the quality of the bunch begins with the intrinsic emittance and time response of the photocathode, and we discuss the trade‐offs involved between the various material choices. A variety of longitudinal laser pulse‐shaping techniques are currently being developed to linearize space charge forces or create trains of ultra‐short bunches. The emittance compensation technique mitigates the growth of the projected emittance due to the linear space charge force while the bunch is accelerated and compressed in an injector. While the normal conducting rf photoinjector is the workhorse of this field, the dc photoinjector is being pushed to its high‐voltage limits, and the year 2010 promises to be a critical year for the superconducting rf photoinjector. Parallel to the development of the hardware, rapid progress has also been made with modeling codes, theory, and bench marking of diagnostics. We attempt to give both a tutorial of photoinjectors and a review of the current state of the art in this rapidly developing field.

One technique to enhance the transformer ratio beyond the ordinary limit of 2 in a collinear wakefield acceleration scheme is to use a ramped bunched train (RBT). The first experimental demonstration has been reported in [1]. However, due to the mismatch between the beam bunch length and frequency of the accelerating structure, the observed transformer ratio was only marginally above 2 in the earlier experiment. We recently revisited this experiment with an optimized bunch length using the laser stacking technique at Argonne Wakefield Accelerator (AWA) facility. A transformer ratio of 3.4 has been measured using two drive bunches. Attempting to use four drive bunches met with major challenges. In this article, measurement results and data analysis from these experiments are presented in detail.

This paper presents a progress report on the development and testing of X‐band dielectric‐loaded accelerating structures. Recent tests on several quartz DLA structures with different inner diameters are reported. Designs for gap‐free DLA structures are presented. Also, planned new experiments are discussed, including higher gradient traveling‐wave and standing‐wave structures and special grooved structures for multipactor suppression.

As the dimensions of accelerating structures become smaller and beam intensities higher, the transverse wakefields driven by the beam become quite large with even a slight misalignment of the beam from the geometric axis. These deflection modes can cause inter‐bunch beam breakup and intra‐bunch head‐tail instabilities along the beam path, and thus BBU control becomes a critical issue. All new metal based accelerating structures, like the accelerating structures developed at SLAC or power extractors at CLIC, have designs in which the transverse modes are heavily damped. Similarly, minimizing the transverse wakefield modes (here the HEMmn hybrid modes in Dielectric‐Loaded Accelerating (DLA) structures) is also very critical for developing dielectric based high energy accelerators. In this paper, we present the design of a 7.8 GHz transverse mode damped DLA structure currently under construction, along with plans for the experimental program.

The dynamics of the beam in structure‐based wakefield accelerators leads to beam stability issues not ordinarily found in other machines. In particular, the high current drive beam in an efficient wakefield accelerator loses a large fraction of its energy in the decelerator structure, resulting in physical emittance growth, increased energy spread, and the possibility of head‐tail instability for an off axis beam, all of which can lead to severe reduction of beam intensity. Beam breakup (BBU) effects resulting from parasitic wakefields provide a potentially serious limitation to the performance of dielectric structure based wakefield accelerators as well. We report on experimental and numerical investigation of BBU and its mitigation. The experimental program focuses on BBU measurements at the AWA facility in a number of high gradient and high transformer ratio wakefield devices. New pickup‐based beam diagnostics will provide methods for studying parasitic wakefields that are currently unavailable. The numerical part of this research is based on a particle‐Green’s function beam breakup code we are developing that allows rapid, efficient simulation of beam breakup effects in advanced linear accelerators. The goal of this work is to be able to compare the results of detailed experimental measurements with the accurate numerical results and to design an external FODO channel for the control of the beam in the presence of strong transverse wakefields.

We report on a design of a TM‐mode based metamaterial‐loaded waveguide. Network analyzer measurements demonstrated a left‐handed propagation region for the TM11 mode at around 10 GHz. A beamline experiment was performed with the metamaterial‐loaded waveguide. In this experiment, a 6 MeV electron beam passes through the waveguide and generates a wakefield, via the Cherenkov radiation mechanism. We detected a signal in the left‐handed frequency band at 10 GHz. This is an indirect demonstration of reverse Cherenkov radiation as discussed in several papers. Cherenkov radiation in artificially constructed materials (metamaterials, MTM) can provide unusual, engineered features that can be advantageous for particle detector design.

The discussions of the working group on beam and radiation generation, monitoring, and control (working group 6) at the 2008 advanced accelerator concepts workshop are summarized. The discussions concerned electron injectors, phase space manipulation, beam diagnostics, pulse train generation, intense beam physics, and radiation generation.

An experimental demonstration of a tunable Dielectric Loaded Accelerating (DLA) [1] structure is planned using a nonlinear ferroelectric with temperature‐ or voltage‐controllable permittivity. We designed and tested two prototype Ka‐band double layer ferroelectric‐ceramic structures (cylindrical and planar) consisting of linear ceramic layers (dielectric constant of 6.8) and BST(M) composite ferroelectric layers of 400–800 μm thickness and dielectric constant of 450–550. The frequency shift by temperature variation of the cylindrical Ka‐band tunable DLA of 14 MHz/° K has been demonstrated leading to an overall DLA structure frequency tuning range of 140–280 MHz with 10–20° K temperature variation. The Ka band prototype DLA structure demonstrated a 6 MHz frequency tuning range for a dc bias field design at 25 kV/cm field strength.

A “multiple beamlet” experiment aimed at investigating the transverse space charge effect was recently conducted at the Argonne Wakefield Accelerator. The experiment generated a symmetric pattern of 5 beamlets on the photocathode of the RF gun with the drive laser. We explored the evolution of the thereby produced 5 MeV, space‐charge dominated electron beamlets in the 2 m drift following the RF photocathode gun for various external focusing. Two important effects were observed and benchmarked using the particle‐in‐cell beam dynamics code Impact‐T. In this paper, we present our experimental observation and their benchmarking with Impact‐T.

High frequency, high power rf sources are needed for many applications in particle accelerators, communications, radar, etc. In this article we present a design of a 26 GHz high power rf source based on the extraction of wakefields from a relativistic electron beam. The extractor is designed to couple out rf power generated from a high charge electron bunch train traversing a dielectric loaded waveguide. Using a 20 nC bunch train (bunch length of 1.5 mm) at the Argonne Wakefield Accelerator (AWA) facility, we can obtain a steady 26 GHz output power of 148 MW. The extractor has been fabricated and bench tested, with the first high power beam experiments to be performed in the coming year.

We report on recent progress in a program to develop an RF‐driven Dielectric‐Loaded Accelerating (DLA) structure, capable of supporting high gradient acceleration. Previous high power tests revealed that the earlier DLA structures suffered from multipactor and arcing at the dielectric joint. A few new DLA structures have been designed to alleviate this limitation including the coaxial coupler based DLA structure and the clamped DLA structure. These structures were recently fabricated and high power tested at the NRL X‐band Magnicon facility. Results show the multipactor can be reduced by the TiN coating on the dielectric surface. Gradient of 15 MV/m has also been tested without dielectric breakdown in the test of the clamped DLA structure. Detailed results are reported, and future plans discussed.

This paper presents a progress report on the development of a dielectric-loaded test accelerator in the Magnicon Facility at the Naval Research Laboratory (NRL). The accelerator will be powered by an 11.4-GHz magnicon amplifier that provides up to 25 MW of output power in a {approx}250-ns pulse at up to 10 Hz. The accelerator includes a 5-MeV electron injector originally developed at the Tsinghua University in Beijing, China, and can incorporate dielectric-loaded accelerating (DLA) structures of up to 0.5 m in length. The DLA structures are being developed by Argonne National Laboratory and Euclid Techlabs, and shorter test structures fabricated from a variety of dielectric materials have undergone rf testing at NRL at accelerating gradients up to 15 MV/m. The first stage of the accelerator, including the 5-MeV injector, has recently begun operation, and initial operation of the complete dielectric-loaded test accelerator, including injector, DLA test structure, and spectrometer, should take place within the next year.

In this paper, we present updated results on power extraction testing of a 7.8 GHz dielectric loaded waveguide power extractor using both high charge single bunches and bunch trains. We have generated a 1.7 ns radio frequency (rf) pulse with 30 MW of power with a single 66 nC electron bunch. Then we have generated a pulse train of electron beam for rf generation of 10 ns and 22 ns rf pulses.

This paper presents a progress report on a joint project by the Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL), in collaboration with the Stanford Linear Accelerator Center (SLAC) and Euclid Techlabs, LLC, to develop an X-band dielectric-loaded test accelerator in the NRL Magnicon Facility. The magnicon is a high-power 11.4-GHz amplifier tube that can produce up to 25 MW of power in 200-ns pulses at up to 10 Hz. The dielectric accelerating structures are developed by ANL and Euclid Techlabs, and tested at NRL and SLAC. The accelerator will include a 5-MeV electron injector developed by the Accelerator Laboratory of Tsinghua University, Beijing, China. SLAC has develped a means to combine the two magnicon output arms, in order to drive an injector and accelerator with separate control of the power ratio and relative phase. The initial operation of the injector should take place later this year, and the first test with a DLA structure, and a spectrometer should take place within the next year.

Field programmable gate arrays (FPGAs) offer a powerful alternative to application specific integrated circuits (ASIC) or general purpose processors in accelerator control applications. Software development for these devices can be awkward and time consuming, however, when using low level hardware design languages. To facilitate the use of FPGAs in control systems we are developing a library of software tools based on ImpulseC, a high level subset of the C language specifically designed for FPGA programming. Development and testing of the software will be performed on a Xilinx Virtex-4 FPGA demo board. Here we present initial results of algorithms of relevance to controls applications implemented in Impulse C.

We describe the Argonne Wakefield Accelerator Facility (AWA), pointing out its present capabilities and goals. We present recent measurements on beam loading observed in our photocathode RF gun. Wakefield measurements in dielectric loaded structures are also reported. Our most recent wakefield structure operates at 15 GHz, and has been excited by single electron bunches and also by sets of two closely spaced bunches. When driven by 43 nC bunches, the accelerating gradient in this structure reached 23 MV/m. No signs of electric breakdown have been observed. This report ends with a brief discussion on the next activities to take place at the AWA facility.

A 15 GHz dielectric loaded wakefield structure has been used to extract energy from a 13 MeV electron beam. Short electron bunches (13 ps FWHM) of up to 43 nC traversed the structure, generating an accelerating field of ∼23 MV/m. The structure consists of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively. The 102 mm long dielectric cylinder is inserted into a cylindrical copper waveguide. Short metallic cylinders, with the same inner and outer radii as the ceramic, are inserted on both ends of the copper waveguide, making it a standing wave structure. A field probe present near the outer edge of the dielectric samples the RF field generated by the electron bunches. This signal is then sent to a mixer circuit, where the 15 GHz signal is down converted to 5 GHz and sent to an oscilloscope. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step in these experiments, another structure, with an output coupler, has been designed and is presently being fabricated.

Dielectric loaded wakefield structures have potential to be used as high gradient accelerator components. Using the high current drive beam at the Argonne Wakefield Accelerator Facility, we employed cylindrical dielectric loaded wakefield structures to generate accelerating fields of up to 86 MV/m, at 10 GHz. Short electron bunches of up to 86 nC are used to drive these fields, either as single bunches or as bunch trains. The structures consist of cylindrical ceramic tubes (cordierite) with a dielectric constant of 4.76, inserted into cylindrical copper waveguides. These standing‐wave structures have a field probe near the outer diameter of the dielectric, in order to sample the RF fields generated by the electron bunches. Monitoring the field probe signal serves to verify the absence of electric breakdown in the structures. MAFIA simulations are used to calculate the amplitude of the fields generated by the traversing electrons bunches.

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).

We present a practical method for achieving a transformer ratio (R) greater than 2 with any collinear wakefield accelerator - i.e. with either plasma or structure based wakefield accelerators. It is known that the transformer ratio cannot generally be greater than 2 for a symmetric drive bunch in a collinear wakefield accelerator. However, using a ramped bunch train (RBT) where a train of n electron drive bunches, with increasing ('ramping') charge, one can achieve R=2n after the bunch train. We believe this method is feasible from an engineering standpoint. We describe a proof of principle experiment using an disk-loaded waveguide, of frequency 13.65 GHz, driven by a RBT of 4 electron bunches. We expect to achieve R>6 using 4 electron bunches. Details of the simulation and experimental design are presented.

We give a summary and a analysis of the wakefields in the dielectric channels. We found that using a conventional description of the electromagnetic wave propagation in dielectric channel, we can derive the power flow and wakefield generation. Detailed analysis of the higher order HEM II mode is also given including separation of TM and TE dominated modes. Numerical calculations of the longitudinal and transverse Wakefields in multimode structures are given.

We present a practical method for achieving a transformer ratio (R) greater than 2 with any collinear wakefield accelerator—i.e. with either plasma or structure based wakefield accelerators. It is known that the transformer ratio cannot generally be greater than 2 for a symmetric drive bunch in a collinear wakefield accelerator. However, using a ramped bunch train (RBT) where a train of n electron drive bunches, with increasing (“ramping”) charge, one can achieve R=2n after the bunch train. We believe this method is feasible from an engineering standpoint using existing technology and an experiment to be preformed at the Argonne Wakefield Accelerator (AWA) is planned.

We report on the experimental results of a new wakefield acceleration scheme. The multibunch driven, multimode, dielectric wakefield accelerator was demonstrated at the Argonne Wakefield Accelerator (AWA). In this experiment, a bunch train of 4, 5 nC electron bunches, separated by 760 ps, was passed through a 60 cm long dielectric-lined cylindrical waveguide. The separation was chosen to match the net acceleration wavelength of the multimode structure. By carefully measuring the energy spectrum of the 4 beams after they passed through the waveguide, we demonstrated that the wakefield is indeed enhanced by a train of periodically spaced electron bunches. The analysis of the multimode structure driven by a bunch train was done by a trivial extension to the existing theory since we are operating in the linear regime. This work represents the first experimental demonstration of this concept and also shows that multibunch operation of wakefield accelerators is worthy of further investigation.

We report on the experimental results of the dielectric based two beam accelerator (step-up transformer). By using a single high charge beam, we have generated and extracted a high power RF pulse from a 7.8 GHz primary dielectric structure and then subsequently transferred to a second accelerating structure with higher dielectric constant and smaller transverse dimensions. We have measured the energy change of a second (witness) beam passing through the acceleration stage. The measured gradient is >4 times the deceleration gradient. The detailed experiment of set-up and results of the measurements are discussed. Future plans for the development of a 100 MeV demonstration accelerator based on this technique is presented.

The Argonne Wakefield Accelerator has been successfully used for conducting wakefield experiments in dielectric loaded structures and plasmas. Although the initial wakefield experiments were successful, higher drive beam quality would substantially improve the wakefield accelerating gradients. For this reason we have built a new 1-1/2 cell L-band photocathode RF gun. This gun is expected to produce 10 - 100 nC bunches with 2 - 5 ps rms pulse length and normalized emittance less than 100 mm mrad. The gun will initially have a Cu photocathode, which will soon be replaced by a high quantum efficiency CsTe one, allowing the generation of a train of high charge bunches. The beam energy at the exit of the gun cavity will be in the range 7.5 - 10 MeV. A standing-wave linac structure operating at the same frequency (1.3 GHz) will increase the beam energy to about 15 MeV. This beam will be used in high gradient wakefield acceleration experiments and other high intensity electron beam applications. Travelling wave dielectric loaded structures, operating at 7.8 and 15.6 GHz, will be excited by the propagation of single bunches or by trains of up to 32 electron bunches, reaching gradients in excess of 100 MV/m over distances of the order of 1 meter.

We report on the experimental results of the dielectric based two beam accelerator (step-up transformer). By using a single high charge beam, we have generated and extracted a high power RF pulse from a 7.8 GHz primary dielectric structure and then subsequently transferred to a second accelerating structure with higher dielectric constant and smaller transverse dimensions. We have measured the energy change of a second (witness) beam passing through the acceleration stage. The measured gradient is >4 times the deceleration gradient. The detailed experiment of set-up and results of the measurements are discussed. Future plans for the development of a 100 MeV demonstration accelerator based on this technique is presented.

The dielectric wakefield transformer (DWT) is one route to practical high energy wakefield-based accelerators. Progress has been made in a number of areas relevant to the demonstration of this device. In this article we describe recent bench measurements and beam experiments using 7.8 and 15.6 GHz structures and discuss some remaining technical challenges in the development of the DWT.