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Field emission is strongly coupled to the breakdown problem. A series of experiments is being carried out at Argonne Wakefield Accelerator Facility (AWA) using an L-band photocathode gun. Cathodes with different shapes have been tested and a dark current imaging system has been set up. Initial experiment results are presented. [ABSTRACT FROM AUTHOR]

We report on dark current and Schottky-enabled photoemission from a copper photocathode surface. Field-emitted dark current is a major gradient-limiting factor in RF cavities. Field emission is generally attributed to geometrical projections on the bulk surface whose field enhancement factor (β) and the emitting area (Ae) can be extracted from the Fowler-Nordheim (FN) plot. Measurements were made at Tsinghua S-band RF gun facility in two separate experiments. Using the traditional FN formula for RF fields we discovered that field enhancement factor (β) alone cannot explain the full data set. Instead, we found that a low work function at some sites is required. In addition, surface analysis of the cathode after the experiment shows that geometric β indicated would be < 10. Thus we conclude that low work function sites with a small β are responsible for dark current emission and subsequent breakdown in high-gradient structures. The origin of these sites is unclear but could be due to defects or impurities. [ABSTRACT FROM AUTHOR]

Double triangular current profile (DT) gives a high transformer ratio which is the determining factor of the performance of collinear wakefield accelerator. This current profile can be generated using the emittance exchange (EEX) beam line. Argonne Wakefield Accelerator (AWA) facility plans to generate DT using the EEX beam line. We conducted start-to-end simulation for the AWA beam line using PARMELA code. Also, we discuss requirements of beam parameters for the generation of DT. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Transverse-to-longitudinal emittance exchange has promising applications in various advanced acceleration and light source concepts. A proof-of-principle experiment to demonstrate this phase space manipulation method is currently being planned at the Argonne Wakefield Accelerator. The experiment focuses on exchanging a low longitudinal emittance with a high transverse horizontal emittance and also incorporates room for possible parametric studies e.g. using an incoming flat beam with tunable horizontal emittance. In this paper, we present realistic start-to-end beam dynamics simulation of the scheme, explore the limitations of this phase space exchange. [ABSTRACT FROM AUTHOR]

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. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

High power tests are currently being conducted on RF-driven dielectric-loaded accelerating (DLA) structures to determine their viability as traveling-wave accelerators. These tests are a collaborative effort between Argonne National Laboratory (ANL) and the Naval Research Laboratory (NRL). In a previous high power test, single-surface multipactor was reported to be capable of absorbing more than half of the RF power incident on an alumina-based DLA structure. In this paper, we report on the most recent set of high power tests that are attempting to further understand multipactor and eventually suppress it. Several methods were employed to suppress multipactor including: the use of a magnetic field; a TIN surface coating; and a different dielectric material (Magnesium-Calcium-Titanate based). The effectiveness of these three methods are presented and discussed in the paper. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

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). © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper, we report experimental results on a series of high-power rf tests for dielectric-loaded accelerating (DLA) structures using a high power X-band Magnicon at the Naval Research Laboratory. The dielectric material loaded into this DLA structure is a commonly used high-Q ceramic, MgxCa1-xTiO3 (MCT), with a dielectric constant of 20. The purpose of these experiments is to study high-power phenomena in the DLA structure. Two important phenomena have been observed during these experiments. First, multipactor effects are strongly dependent on the dielectric material used in the DLA structure. In this case, the multipactor-induced power absorption threshold and trend to higher power differ when MCT is used instead of alumina. Second, although we did not observe dielectric breakdown in the bulk dielectric, breakdown occurred at the butt-joint between adjacent dielectric sections in the MCT structure. This occurs because of manufacturing imperfections of the joint that cause large, local field enhancements. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Beam dynamics estimations of a 15.6GHz ceramic RF power generator have been performed for an experiment planned at the upgraded AWA facility at ANL. Theoretical maximum peak power that can be generated exceeds 100–150 MW. Analytical and numerical calculations address the following issues: 50A+ heavy beam loading in linac, slow-wave structures, coupler and damper; beam breakup and dipole mode suppression, end-to-end beam transport, and generated RF waveform and spectrum. Comparison is made with an earlier 21GHz experiment in a DULY/CERN/ANL collaboration. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a method to vary the resonant frequency of a dielectric loaded structure (driven either by the wakefield of a beam or an external rf source). It consists of a thin layer of a ferroelectric material backing a layer of conventional ceramic. A DC bias voltage is used to vary the permittivity of the ferroelectric layer and thus rune the overall frequency of the accelerating structure. In this paper, a detailed model is given for this device and an experimental test is proposed. Employing this scheme would compensate for frequency shifts in the accelerating structure caused by ceramic waveguide machining tolerances and dielectric constant heterogeneity. We have identified a suitable material for this application: a BST ferroelectric-oxides compound with a dielectric constant of 300-500. A tuning range of (2-4)% could be achieved for a (11-13) GHz dielectric accelerating structure. [ABSTRACT FROM AUTHOR]

Significant progress in both the study of new acceleration structures and in the production and experimental testing of a variety of structures has taken place since the last Conference. Staging of laser accelerator sections and acceleration of particles by a microwave inverse FEL was demonstrated for the first time, fabrication of 92 GHz structures compatible with high power was demonstrated, new experimental results on dielectric based acceleration concept were presented, and detailed analysis of a new accelerator scheme, the inverted medium accelerator, was discussed. Novel structure development continued apace, with twisted waveguide structures, resonantly driven dielectric wakefield accelerators, and evanescent-mode capillary guide laser accelerators all being introduced. Power sources present and future and their impact on structure development were examined together with the need for renewed vigor in studying rf breakdown at higher frequencies. The needs and potential applications for table top radiation sources were discussed, with the expectation that development efforts in this area would directly impact the design of a future collider. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on the construction, numerical modeling and experimental testing of a traveling-wave acceleration structure based on a dielectric-lined circular waveguide. This type of structure has similar acceleration properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication, suppression of parasitic wakefield effects, and having no dark current. Efficient coupling of external RF power to the cylindrical dielectric waveguide is a technical challenge, particularly to structures loaded with very high dielectric constant materials. We have designed and constructed an 11.4GHz structure loaded with ceramic with dielectric constant of 20, to be powered by an external RF power source. High efficiency RF coupling has been achieved using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench tests using a network analyzer have demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, and vacuum tests have also shown that this dielectric loaded structure can be operated in an ultra-high vacuum environment. Thus, this work provides a necessary basis for construction of an accelerator using this kind of structure. We have also simulated the parameters of this structure using MAFIA. Within the limits of the approximations used, the results are in agreement with the bench measurements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL), in collaboration with the Stanford Linear Accelerator Center (SLAC), are developing a dielectric-loaded accelerator (DLA) test facility powered by the 11.424-GHz magnicon amplifier that was developed jointly by NRL and Omega-P, Inc. Thus far, DLA structures developed by ANL have been tested at the NRL Magnicon Facility without injected electrons, including tests of alumina and magnesium calcium titanate structures at gradients up to ∼8 MV/m. The next step is to inject electrons in order to build a compact DLA test accelerator. The Accelerator Laboratory of Tsinghua University in Beijing, China has developed a 5-MeV electron injector for the accelerator, and SLAC is developing a means to combine the two magnicon output arms, and to drive the injector and an accelerator section with separate control of the power ratio and relative phase. Also, RWBruce Associates, working with NRL, is developing a means to join ceramic tubes to produce long accelerating sections using a microwave brazing process. The installation and commissioning of the first dielectric-loaded test accelerator, including injector, DLA structure, and spectrometer, should take place within the next year. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]