Muons, Inc, Batavia, USA
Enrico Fermi Institute, University of Chicago, Chicago, Illinois, USA
A new generation of large-area, low cost time-of-flight detectors with time resolutions ≤ 10 ps and space resolutions ≤ 1 mm is being developed for use in nuclear and particle physics experiments, as well as for medical and industrial applications. Such detectors are being considered for use in muon cooling channel tests. Designs and fabrication of prototype planes and associated readout electronics are described. Results of simulations of time and space resolutions are presented.
MIT/PSFC, Cambridge, Massachusetts, USA
We present the design of an overmoded photonic band gap (PBG) accelerator cavity, made from a 2D lattice of sapphire rods supported between copper plates, that operates in a TM02-like mode at 17 GHz. The cavity does not support the lower-frequency TM01-like mode. Higher-order modes are damped effectively by removing rods from the lattice so that only the operating mode is supported with a high quality factor. The TM02 cavity mitigates the high pulsed heating of the copper surface seen in some metal-rod TM01 PBG cavities, which may be an advantage for high-gradient operation. We discuss plans for testing a 17 GHz TM02 standing-wave cavity at gradients above 100 MV/m.
LANL, Los Alamos, New Mexico, USA
Niowave, Inc., Lansing, Michigan, USA
We present a design of a superconducting photonic band gap (PBG) accelerator cell operating at 700 MHz. It has been long recognized that PBG structures have great potential in reducing long-range wakefields in accelerators. Using PBG structures in superconducting particle accelerators will allow moving forward to significantly higher beam luminosities and lead towards a completely new generation of colliders for high energy physics. We designed the superconducting PBG cell which incorporates higher order mode (HOM) couplers to conduct the HOMs filtered by the PBG structure out of the cryostat. The accelerator characteristics of the cell were evaluated numerically. A scaled prototype cell was fabricated out of copper at the higher frequency of 2.8 GHz and cold-tested. The 700 MHz niobium cell will be fabricated at Niowave, Inc. and tested for high gradient at Los Alamos in the near future.
SLAC, Menlo Park, California, USA
FACET at SLAC provides high charge, high peak current, low emittance electron beam that is bunched at THz wavelength scale during its normal operation. A THz light source based coherent transition radiation (CTR) from this beam would potentially be the brightest short-pulse THz source ever constructed. Efforts have been put into building this photon source together with a user area, to provide a platform to utilize this unique THz radiation for novel nonlinear and ultrafast phenomena researches and experiments.
SLAC, Menlo Park, California, USA
SLAC, Menlo Park, California, USA
Stanford University, Stanford, California, USA
A grating-based design is a promising candidate for a laser-driven dielectric accelerator. Through simulations, we show the merits of a readily fabricated grating structure as an accelerating component. Additionally, we show that with a small design perturbation, the accelerating component can be converted into a focusing structure. The understanding of these two components is critical in the successful development of any complete accelerator.
MPI-P, München, Germany
LANL, Los Alamos, New Mexico, USA
UCLA, Los Angeles, California, USA
NSC/KIPT, Kharkov, Ukraine
Yale University, Physics Department, New Haven, CT, USA
Omega-P, Inc., New Haven, Connecticut, USA
A key parameter of wakefield acceleration is the transformer ratio T. For a dielectric wakefield accelerator, it has been suggested to use a ramped drive bunch train (RBT), or a multizone dielectric structure to enhance T. Here we show the possibility of greatly improving the RBT technique by the use of a numerical algorithm. We study a two-channel 28 GHz structure with two nested Alumina cylindrical shells (CDWA) which is to be excited by a train of four annular bunches having energy 14 MeV and axial RMS size 1mm; the total charge of bunches is 200 nC. For bunch charge and spacing chosen for optimum acceleration gradient, or for optimizing T using the standard method, we obtain T~3.6. We found that if the charge ratios are 1.0:2.4:3.5:5.0 and the spaces between the bunches are 2.5, 2.5, and 4.5 wakefield periods, then T~17. The RBT also can be used successfully in a high gradient THz CDWA structure.
* C.Jing et.al., Phys. Rev. Lett. 98 144801, (2007)
** C. Wang, et.al. Proc. PAC 2005. IEEE, 2005, p.1333.
*** G. Sotnikov et.al. PRST-AB, 061302 (2009).
University of Dundee, Nethergate, Dundee, Scotland, United Kingdom
USTRAT/SUPA, Glasgow, United Kingdom
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
UAD, Dundee, United Kingdom
The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laser-plasma accelerators for the production of ultra-short electron beams as drivers of incoherent and coherent radiation sources from plasma and magnetic undulators. Here we report on the latest laser wakefield accelerator experiments on the University of Strathclyde ALPHA-X accelerator beam line looking at narrow energy spread electron beams. ALPHA-X uses a 26 TW Ti:sapphire laser (energy 900 mJ, duration 35 fs) focused into a helium gas jet (nozzle length 2 mm) to generate high quality monoenergetic electron beams with central energy in the range 80-180 MeV. The beam is fully characterised in terms of the charge, transverse emittance, energy spread and bunch length. In particular, the energy spectrum (with less than 1% measured energy spread) is obtained using a high resolution magnetic dipole imaging spectrometer.
Euclid TechLabs, LLC, Solon, Ohio, USA
ANL, Argonne, USA
IIT, Chicago, Illinois, USA
Significant progress has been made in the development of high gradient rf driven dielectric accelerating structures (DLA). One principal effect limiting further advances in this technology is the problem of multipactor. The fraction of the power absorbed at saturation in DLA experiments was found to increase with the incident power, with more than 30% of the incident power per unit length being absorbed. We studied a possibility of multipactor mitigation by introduction of surface grooves (transverse and longitudinal) to interrupt the resonant trajectories of electrons in the multipactor discharge. Four DLA structures based on quartz tubes with transverse and longitudinal grooves of various dimensions were designed. In this paper we report simulation results and plans for high-power tests of these structures.
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
We report here on the development of THz diamond wakefield structures produced using Chemical Vapor Deposition (CVD) technology*. The diamond structures would be used in a THz generation experiment at the new FACET facility at SLAC. We consider a dielectric based accelerating structure to study of the physical limitations encountered driving >GV/m wakefields in the cylindrical and planar geometries of a dielectric wakefield accelerator (DWA). In a DWA, an ultrashort drive bunch traverses the evacuated central region of the structure, creating Cherenkov wakefields in the dielectric to accelerate a witness bunch. A diamond-based DWA structure will allow a sustained accelerating gradient exceeding breakdown threshold demonstrated with the FFTB experiments**. The electrical and mechanical properties of diamond make it an ideal candidate material for use in dielectric rf structures: high breakdown voltage, extremely low dielectric losses and the highest thermoconductive coefficient available for removing waste heat from the device.
*R. J. Barker et al., Modern Microwave and Millimeter-Wave Power Electronics, IEEE Press/Wiley-Interscience, Piscataway NJ 2005, Chapter 7
**M.C. Thompson et al. Phys. Rev.Lett.100:214801, 2008.
Fermilab, Batavia, USA
LBNL, Berkeley, California, USA
Tech-X, Boulder, Colorado, USA
Control of injection into a high gradient laser-plasma accelerator is presented using the beat between two ’colliding’ laser pulses to kick electrons into the plasma wake accelerating phase. Stable intersection and performance over hours of operation were obtained using active pointing control. Dependence of injector performance on laser and plasma parameters were characterized in coordination with simulations. By scanning the intersection point of the lasers, the injection position was controlled, mapping the acceleration length. Laser modifications to extend acceleration length are discussed towards production of tunable stable electron bunches as needed for applications including Thomson gamma sources and high energy colliders.
LBNL, Berkeley, California, USA
University of Nevada, Reno, Reno, Nevada, USA
UCB, Berkeley, California, USA
A technique has been developed to accurately align a laser beam through a plasma channel by minimizing the shift in laser centroid and angle at the channel outptut. If only the shift in centroid or angle is measured, then accurate alignment is provided by minimizing laser centroid motion at the channel exit as the channel properties are scanned. The improvement in alignment accuracy pro- vided by this technique is important for minimizing electron beam pointing errors in laser plasma accelerators.
LLNL, Livermore, California, USA
UCLA, Los Angeles, California, USA
We describe the Inverse Free Electron Laser (IFEL) accelerator currently under construction at LLNL in collaboration with UCLA. This project combines a strongly tapered undulator with a 10 Hz repetition rate, Ti:Sapphire laser to produce > 200 MeV/m average accelerating gradient over the 50 cm long undulator. The project goal is to demonstrate IFEL accelerator technology that preserves the input beam quality and is well suited for future light source applications. We discuss the accelerator design focusing on issues associated with the use of 800 nm, 100 fs laser pulses. Three-dimensional simulations of the IFEL interaction are presented which guide the choice of laser and electron beam parameters. Finally, experimental plans and potential future developments are discussed.
NRL, Washington, DC, USA
ANL, Argonne, USA
Euclid TechLabs, LLC, Solon, Ohio, USA
Icarus Research, Inc., Bethesda, Maryland, USA
A joint program is under way to study externally driven X-band dielectric-loaded accelerating (DLA) structures and CLIC-type power extraction structures. The structures are designed and fabricated by Argonne National Laboratory and Euclid Techlabs and tested at up to 20 MW drive power using the X-band Magnicon Facility at the Naval Research Laboratory, with additional tests carried out at SLAC. Thus far, tests have been carried out on a large variety of structures fabricated from quartz, alumina, and MCT-20, and the principal problems have been multipactor loading and rf breakdown.* Multipactor loading occurs on the inner surface of the dielectric in a region of strong normal and tangential rf electric fields; rf breakdown occurs principally at discontinuities in the dielectric. Gap-free DLA structures have been tested at 15 MV/m without breakdown. New tests are being prepared to address these two issues. New gap-free structures will make use of a metallic coating on the outer surface of the dielectric in order to permit tapering both the inner and outer diameters for rf matching, while new multipactor studies will examine the use of grooved surfaces to suppress multipactor.
* C. Jing, W. Gai, J.G. Power, R. Konecny, W. Liu, S.H. Gold, A.K. Kinkead, S.G. Tantawi, V. Dolgashev, and A. Kanareykin, IEEE Trans. Plasma Sci., vol. 38, pp. 1354–1360, June 2010.
Northern Illinois University, DeKalb, Illinois, USA
Tech-X, Boulder, Colorado, USA
Wakefields with amplitude in the 10's MV/m range can be routinely generated by passing electron beams through dielectric-loaded structures. The main obstacle in obtaining high field amplitude (in the GV/m range) is the ability to focus the high-peak-current electron beam in the transverse plane to micron level, and to maintain the focusing all the way along the dielectric structure. In this paper we explore the use of a flat, high-peak current, electron beams to be produced at the Fermilab's NML facility to drive dielectric loaded structures. Based on beam dynamics simulation we anticipate that we can obtain flat beams with very small vertical size (under 100 microns) and peak current is in excess of 1 kA. We present simulations of the wakefield generation based on theoretical models and PIC simulations with VORPAL.
SLAC, Menlo Park, California, USA
Stanford University, Stanford, California, USA
We present results for the fabrication of a silicon woodpile accelerator structure. The structure was designed to have an accelerating mode at 3.95 μm, with a high characteristic impedance and an accelerating gradient of 530 MeV/m. The fabrication process uses standard nanofabrication techniques in a layer-by-layer process to produce a three-dimensional photonic crystal with 400 nm features. Reflection spectroscopy measurements reveal a peak spanning from three to five microns, and are show good agreement with simulations.
* Sears, PRST-AB, 11, 101301, (2008).
** Cowan, PRST-AB, 11, 011301, (2008).
*** McGuinness, J. Mod. Opt., vol. 56, is. 18, pp. 2142, (2009).
**** Lin, Nature, 394, pp. 251 (1998).
SLAC, Menlo Park, California, USA
Photonic band gap (PBG) fibers with hollow core defects are being designed and fabricated for use as laser driven accelerators because they appear capable of providing gradients of several GeV/m at picosecond pulse lengths. While we expect to have fiber down to 1.5-2.0 micron wavelengths we still lack a viable means for efficient coupling of laser power into these structures. The reasons for this include the very different character of these TM-like modes from those familiar in the telecom field and the fact that the defect must function as both a longitudinal waveguide for the accelerating field and a transport channel for the particles. We discuss the status of our coupling work in terms of what has been done and the options we are pursuing for both end and side coupling. In both basic coupler types, the symmetry of the PBG crystal leads to significant differences between this and the telecom field. We show that side coupling provides more possibilities and is preferred. Our motivation is to test new fiber for gradient, mode content and throughput on the NLCTA at SLAC.
Tech-X, Boulder, Colorado, USA
LBNL, Berkeley, California, USA
The use of colliding laser pulses to control the injection of plasma electrons into the plasma wake of a laser-plasma accelerator is a promising approach to obtain reproducible and tunable electron bunches with low energy spread and emittance. We present recent particle-in-cell simulations of colliding pulse injection for parameters relevant to ongoing experiments at LBNL. We perform parameter scans in order to determine the best conditions for the production of high quality electron bunches, and compare the results with experimental data. We also evaluate the effect of laser focusing in the plasma channel and of higher order laser mode components on the bunch properties.
UCLA, Los Angeles, California, USA
PBPL, Los Angeles, USA
Manhattanville College, Purchase, New York, USA
UCLA, Los Angeles, California, USA
Laser-Driven Ion Acceleration in thin foils has demonstrated high-charge, low-emittance MeV ion beams with a picosecond duration. Such high-brightness beams are very attractive for a compact ion source or an injector for RF accelerators. However in the case of foils scaling of the pulse repetition rate and improving shot-to-shot reproducibility is a serious challenge. CO2 laser-plasma interactions provide a possibility for using a debris free gas jet for target normal sheath acceleration of ions. Gas jets have the advantage of precise density control around the critical plasma density for 10 um pulses (1019 cm-3) and can be run at 1-10 Hz. The master oscillator–power amplifier CO2 laser system at the UCLA Neptune Laboratory is being upgraded to generate 1 J, 3 ps pulses at 1Hz. For this purpose, a new 8 atm CO2 module is used to amplify a 3 ps pulse to ~10 GW level. Final amplification is realized in a 1-m long TEA CO2 amplifier, for which the bandwidth necessary for 3 ps pulses is provided by the field broadening mechanism. Modeling of the pulse amplification shows that ~0.3 TW power is achievable that should be sufficient for producing 1-3 MeV H+ protons from the gas plasma.
UCLA, Los Angeles, California, USA
CO2 laser-plasma interactions provide a unique parameter space for using a gas jet for Target Normal Sheath Acceleration (TNSA) of ions instead of a thin foil target. The generation of 1-5 MeV protons from the interaction of a 3 ps TW CO2 laser pulse with a gas target with a peak density around the critical plasma density (1019 cm-3) has been studied by 2D particle-in-cell simulations. The proton acceleration in the preformed plasma, having similar to the gas jet symmetric, linearly ramped density distribution, occurs via formation of a sheath of hot electrons on the back surface of the target. The maximum energy of the hot electrons and, hence net acceleration of protons is mainly defined by Forward Raman scattering instability in the underdense part of the plasma. This mechanism of an additional heating of electrons is strongly affected by nonlinear laser-plasma interactions and results in the proton energy enhancement by more than an order of magnitude in comparison with the regular ponderomotive force scaling of TNSA. Forward directed ion beams from a gaseous target can find an application as a high-brightness ion source-injector.
Yale University, Beam Physics Laboratory, New Haven, Connecticut, USA
Yale University, Physics Department, New Haven, CT, USA
Omega-P, Inc., New Haven, Connecticut, USA
An axially-asymmetric cavity to support several modes at harmonically-related frequencies is predicted to sustain higher RF breakdown thresholds than a conventional pillbox cavity, when driven by two or more external RF phase-locked harmonic sources. Experimental efforts are underway at Yale Beam Physics Lab to study RF breakdown in a bimodal asymmetric cavity. Such a cavity could be a basic building-block for a future high-gradient warm accelerator structure.
* S.Yu. Kazakov, S.V. Kuzikov, Y. Jiang, and J.L. Hirshfield, PRSTAB, 13, 071303 (2010).
** S.V. Kuzikov, S.Yu. Kazakov, Y. Jiang, and J.L. Hirshfield, PRL 104, 214801 (2010).
MIT/PSFC, Cambridge, Massachusetts, USA
SLAC, Menlo Park, California, USA
LLNL, Livermore, California, USA
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.