ANL, Argonne, USA
Euclid TechLabs, LLC, Solon, Ohio, USA
Illinois Institute of Technology, Chicago, Illinois, USA
The AWA Facility is presently undergoing several upgrades that will enable it to further study wakefield acceleration driven by high charge electron beams. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures as well as in metallic structures (iris loaded, photonic band gap, etc). Several facility upgrades are underway: (a) a new RF gun with a higher quantum efficiency photocathode will replace the RF gun that has been used to generate the drive bunches; (b) the existing RF gun will be used to generate a witness beam to probe the wakefields; (c) three new L-band RF power stations, each providing 25 MW, will be added to the facility; (d) five linac structures will be added to the drive beamline, bringing the beam energy up from 15 MeV to 75 MeV. The drive beam will consist of bunch trains of up to 32 bunches spaced by 0.77 ns with up to 100 nC per bunch. The goal of future experiments is to reach accelerating gradients of several hundred MV/m and to extract RF pulses with GW power level.
Muons, Inc, Batavia, USA
Old Dominion University, Norfolk, Virginia, USA
LBNL, Berkeley, California, USA
ANL, Argonne, USA
Fermilab, Batavia, USA
Voss Scientific, Albuquerque, New Mexico, USA
Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved, and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual gas and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of radiofrequency and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) has been built, and a series of detailed experiments is planned at the Argonne Wakefield Accelerator. These experiments will be followed by additional experiments using a second test cell operating at 402.5 MHz.
Yale University, Beam Physics Laboratory, New Haven, Connecticut, USA
ANL, Argonne, USA
Omega-P, Inc., New Haven, Connecticut, USA
Columbia University, New York, USA
Northern Illinois University, DeKalb, Illinois, USA
NSC/KIPT, Kharkov, Ukraine
Recent tests of a two-channel rectangular dielectric lined accelerator structure are described; comparison with theory and related issues are presented. The structure (with channel width ratio 6:1) is designed to have a maximum transformer ratio of ~12.5:1. It operates mainly in the LSM31 mode (~ 30GHz). The dielectric liner is cordierite (dielectric constant ~4.76). The acceleration gradient is 1.2 MV/m for each 10nC of the drive bunch for the first acceleration peak of the wakefield, and 0.92 MV/m for the second peak. The structure is installed into the AWA beam-line (Argonne National Lab) and is excited by a single 10-50nC, 14MeV drive bunch. Both the drive bunch and a delayed witness bunch are produced at the same photocathode. This is the first experiment to test a two-channel dielectric rectangular wakefield device where the accelerated bunch may be continuously energized by the drive bunch. The immediate experimental objective is to observe the energy gain and spread, and thereby draw conclusions from the experimental results and the theory model predictions. The observed energy change of the test bunch might be well explained*.
* G. V. Sotnikov, et al., Advanced Accelerator Concepts: 13th Workshop, Carl B. Schroeder, Wim Leemans and Eric Esarey, editors, AIP Conf. Proc. 1086), pp. 415–420 (AIP, New York, 2009).
Euclid TechLabs, LLC, Solon, Ohio, USA
ANL, Argonne, USA
CERN, Geneva, Switzerland
In the past decade, tremendous efforts have been put into the development of the CLIC Power Extraction and Transfer Structure (PETS), and significant progress has been made. However, one concern remains the manufacturing cost of the PETS, particularly considering the quantities needed for a TeV machine. A dielectric-based wakefield power extractor in principle is much cheaper to build. A low surface electric field to gradient ratio is another big advantage of the dielectric-loaded accelerating/decelerating structure. We are currently investigating the possibility of using a cost-effective dielectric-based wakefield power extractor as an alternative to the CLIC PETS. We designed a 12 GHz dielectric-based power extractor which has a similar performance to CLIC PETS with parameters 23 mm beam channel, 240 ns pulse duration, 135 MW output per structure using the CLIC drive beam. In order to study potential rf breakdown issues, as a first step we are building a 11.424 GHz dielectric-based power extractor scaled from the 12 GHz version, and plan to perform a high power rf test using the SLAC 11.424 GHz high power rf source.
Euclid TechLabs, LLC, Solon, Ohio, USA
ANL, Argonne, USA
We report on a collinear wakefield experiment using the first tunable dielectric loaded accelerating structure. Dielectric-based accelerators are generally lacking in approaches to tune the frequency after fabrication. However, by introducing an extra layer of nonlinear ferroelectric which has a dielectric constant sensitive to temperature and DC voltage, the frequency of a DLA structure can be tuned on the fly by controlling the temperature or DC bias. The experiment demonstrated that by varying the temperature of the structure over a 50°C temperature range, the energy of a witness bunch at a fixed delay with respect to the drive beam could be changed by an amount corresponding to more than half of the nominal structure wavelength.
Euclid TechLabs, LLC, Solon, Ohio, USA
ANL, Argonne, USA
Diamond has been proposed as a dielectric material for dielectric loaded accelerating (DLA) structures. It has a very low microwave loss tangent, the highest available thermoconductive coefficient and high RF breakdown field. In this paper we report the results from a wakefield breakdown test of diamond-loaded rectangular accelerating structure and development of a cylindrical diamond DLA. We expect to achieve field levels on the order of 100 MV/m in the structure using the 100nC beam at the Argonne Wakefield Accelerator Facility. Single crystal diamond plates produced by chemical vapor deposition (CVD) are used in the structure. The structure is designed to yield up to 0.5 GV/m fields on the diamond surface to test it for breakdown. A surface analysis of the diamond is performed before and after the beam test.
Euclid TechLabs, LLC, Solon, Ohio, USA
ANL, Argonne, USA
In a general sense, a high gradient is desirable for a TeV level linear collider design because it can reduce the total linac length. More importantly, the efficiency and the cost to sustain such a gradient should be considered as well in the optimization process of an overall design. We propose a high energy linear collider based on a short rf pulse (~22ns flat top), high gradient (~267MV/m loaded gradient), high frequency (26GHz) dielectric two beam accelerator scheme. This scheme is a modular design and its unique locally repetitive drive beam structure allows a flexible configuration to meet different needs. Major parameters of a conceptual 3-TeV linear collider are presented. This preliminary study shows an efficient (~7% overall ) short pulse collider may be achievable. As the first step, a dielectric based broadband accelerating structure is under development.